Embed Size (px)
Citation preview
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 1
Development of Complex Curricula for Molecular Bionics and Infobionics Programs within a consortial framework
Consortium leader
PETER PAZMANY CATHOLIC UNIVERSITYConsortium members
SEMMELWEIS UNIVERSITY DIALOG CAMPUS PUBLISHER
The Project has been realised with the support of the European Union and has been co-financed by the European Social Fund
Molekulaacuteris bionika eacutes Infobionika Szakok tananyagaacutenak komplex fejleszteacutese konzorciumi keretben
A projekt az Euroacutepai Unioacute taacutemogataacutesaacuteval az Euroacutepai Szociaacutelis Alap taacutersfinansziacuterozaacutesaacuteval valoacutesul meg
PETER PAZMANY
CATHOLIC UNIVERSITY
SEMMELWEIS
UNIVERSITY
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2
Peter Pazmany Catholic University
Faculty of Information Technology
Ad hoc Sensor Networks
Multiple channel access
wwwitkppkehu
Eacuterzeacutekelő mobilhaacuteloacutezatok
Toumlbbszoumlroumls hozzaacutefeacutereacutes
Dr Olaacuteh Andraacutes
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3
Ad hoc Sensor Networks Multiple channel access
Lecture 5 reviewbull Signal space representationbull Optimal detection of signal in AWGN (Bayesian decision)bull Probability of error (BER and SER)bull Demodulation and detection for modulation schemesbull BER in fading channelbull Channel equalization
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 4
Outlinebull The goal of medium access controlbull Types of wireless networksbull Duplexing techniquesbull Multiple Accessbull Random Accessbull MAC for Wireless Sensor Networks
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 5
Ad hoc Sensor Networks Multiple channel access
Structure of a wireless communications link
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 6
Ad hoc Sensor Networks Multiple channel access
Layered Communication Approaches
2 Data Link layer it handles access to the unerlyingchannel and defines the data format It is split into twosublayersndash Logical Link Control it manages frames to upper and
lower layer (encapsulation decapsulation) and it enforceserrror control (checksum and parity bits)
ndash Medium Access Control (MAC) it coordinatestransmission between users sharing the spectrum Inwireless systems it must address the hidden terminalproblem and must exercise power control Goals preventcollisions while maximizing throughput and minimizingdelay
Physical
OSI reference model
Data link
Network
Transport
Middleware
Application
Recall from Chapter 1
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 7
Ad hoc Sensor Networks Multiple channel access
Medium Access Control
bull Thus the wireless spectrum (frequency band) is a very precious and limited resourcewe need to use this resource very efficiently
bull We also want our wireless system to have high user capacity with QoS constraintsbull The algorithms and protocols that enables this sharing by multiple users and
controlscoordinates the access to the wireless channel (medium) from differentusers are called MEDIUM ACCESS or MEDIA ACCESS or MULTIPLE ACCESSprotocols techniques schemes etchellip)
Designbull Goal Function Maximum Utilization
(eg throughput user capacity hellip)bull Constraints QoS
(eg delaylt10ms BERlt10-5hellip)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 8
Ad hoc Sensor Networks Multiple channel access
Medium Access Control (contrsquo)bull Multiple Access (Channel Partitioning or Coordinated Schemes)
ndash Techniques TDMA FDMA CDMA SDMAndash Examples GSM 3Ghellip
bull Random Access (or Random Schemes)ndash Techniques MACA MACAW Aloha 80211 MAChellipndash Examples Wifi WSNhellip
bull Polling based schemesndash Access is coordinated by a central nodendash Examples Bluetooth BlueSkyhellip
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 9
Ad hoc Sensor Networks Multiple channel access
bull Infrastructured networksndash base stations are the bridgesndash a mobile host will communicate with the
nearest base station (or access point)ndash handoff is taken when a host roams from one
base to anotherndash Medium access control assisted by BS
bull Ad hoc networksndash infrastructureless no fixed base stationsndash without the assistance of base stations for
communicationndash Due to transmission range constraint two
wireless nodes need multi-hop routing forcommunication
ndash quickly and unpredictably changing topology
Two types of wireless networks
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 10
Ad hoc Sensor Networks Multiple channel access
Two types of wireless networks (contrsquo)
bull Mesh networks serve as access networks that employ multi-hop wireless forwarding bynon-mobile nodes to relay traffic to and from the wired Internet In such an environmenthybrid wireless network technologies andor hierarchical network organization can beused for ad hoc and infrastructure wireless links
bull Wireless Sensor Networks are a special category of ad hoc networks that are used toprovide a wireless communication infrastructure among the sensors deployed in a specificapplication domain [rarr see Chapter 9 - 11]
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 11
Ad hoc Sensor Networks Multiple channel access
Infrastructured wireless networks Ad hoc wireless networksFixed infrastructure-based Infrastructureless
Guaranteed bandwidth Shared radio channel
Seamless connectivity (low call drops during handoffs)
Frequent path breaks due to mobility
High cost and time of deployment Quick and cost-effective deployment
Reuse of frequency spectrum (cellular principle) Dynamic frequency reuse based on carrier sense mechanism
Application domains include mainly civilian and commercial sectors
Application domains include battlefields emergency search and rescue operations
High cost of network maintenance Self-organization and maintenance properties are built into the network
Widely deployed and currently in the third generation of evolution
Several issues are to be addressed for successful commercial deployment even though widespread use exists in defense
Two types of wireless networks (contrsquo)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 12
Ad hoc Sensor Networks Multiple channel access
Duplexing
bull If the communication between two parties is one way then it is called simplexcommunication Simplex communication is achieved by default by using a singlewireless channel (frequency band) to transmit from sender to receiver
bull If the communication between two parties is two- way then it is called duplexcommunication Duplex communication achieved by
ndash Time Division (TDD) (famous in cellular systems)ndash Frequency Division (FDD) (famous in cellular systems)ndash Some other method like a random access method
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 13
Ad hoc Sensor Networks Multiple channel access
FDDbull A duplex channel consists of two
simplex channels with different carrierfrequencies
ndash Downlink band carries traffic frombase to mobile
ndash Uplink band carries traffic frommobile to base
Duplexing(contrsquo)TDD
bull channel (carrier frequency) is shared intime in a deterministic manner
ndash The time is slotted with fixed slotlength (sec)
ndash Some slots are used for downlinkchannel
ndash Some slots are used for uplink channel
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 14
Ad hoc Sensor Networks Multiple channel access
FDDbull FDD is used in radio systems that can
allocate individual radio frequenciesfor each user
bull For example analog systems AMPSbull In FDD channels are allocated by a
base stationbull A channel for a mobile is allocated
dynamicallybull All channels that a base station will use
are allocated usually staticallybull More suitable for wide-area cellular
networks GSM AMPS all use FDD
Duplexing(contrsquo)TDD
bull Can only be used in digital wirelesssystems (digital modulation)
bull Requires rigid timing andsynchronization
bull Mostly used in short-range and fixedwireless systems so that propagationdelay between base and mobile do notchange much with respect to locationof the mobile
bull Such as cordless phoneshellip
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 15
Ad hoc Sensor Networks Multiple channel access
Multiple Access narrow- vs wideband systemsbull Narrowband System
ndash The bandwidth is small compared to the coherence bandwidth of the channel(BltBcoh)
bull Wideband Systemndash The system bandwidth is much larger that the coherence bandwidth of the
multipath channel (BgtBcoh) A large number of users can access the same channel(frequency band) at the same time
bull Four major multiple access (MA) schemesndash Time Division Multiple Access (TDMA) it could be used in narrowband or
wideband systemndash Frequency Division Multiple Access (FDMA) it is usually used in narrowband
systemndash Code Division Multiple Access (CDMA) it is used in wideband systemndash Space Division Multiple Access (SDMA) for wireless systems with multiple
antennas it can be combined with all of the other MA methods
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 16
Ad hoc Sensor Networks Multiple channel access
Multiple Access cellular standards
Cellular System Multiple Access TechniqueAMPS (2G) FDMAFDD
CT2 Cordless Phone FDMATDD
DECT Cordless Phone FDMATDD
GSM (2G) TDMAFDD
USDC (IS-54 and IS-136) (2G) TDMAFDD
Personal Digital Cellular (2G) TDMAFDD
US IS-95 (2G) CDMAFDD
W-CDMA (3G) CDMAFDD CDMATDD
cdma2000 (3G) CDMAFDD CDMATDD
Narrowband System
Wideband System
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 17
Ad hoc Sensor Networks Multiple channel access
bull Individual radio channels are assigned toindividual users Each user is allocated a Wfrequency band by BS
bull If the channel allocated to a user is idle then itis not used by someone else waste of resource
bull Mobile and base can transmit and receivesimultaneously (FDD)
bull The W bandwidth of FDMA channels arerelatively low Symbol time is usually larger(low data rate) than the delay spread of themultipath channel (implies that inter-symbolinterference is low)
bull Lower complexity systems than TDMAsystems
Multiple Access FDMA
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 18
Ad hoc Sensor Networks Multiple channel access
Multiple Access FDMA (contrsquo)bull Capacity of FDMA systems
Nc = (B-2Bguard)Wwhere B is the total spectrum allocation Bguard is the guard band allocated atthe edge of the spectrum band W is the bandwidth of an individual channel
bull Erlang B and Erlang C system assumesndashinfinite population of sources which jointly offer traffic to Nc channelndashcall attempts arrive following a Poisson process so call arrivals are independentThe average arrival rate is λndashmessage length (holding times) are exponentially distributed (Markovian system)The average call length is hndashthe total amount of traffic offered in erlangs L= λhndashIf a user is rejected his next call attempt is made statistically independent of theprevious attempt (Erlang B)ndashIf all the channels are busy when a request arrives from a user the request isqueued An unlimited number of requests may be held in the queue (Erlang C)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 19
Ad hoc Sensor Networks Multiple channel access
bull Statictical measures of offered load Ncndash In Erlang B system the probability of call
blocking
ndash In Erlang C system the probabilty of waiting
ndash And the average wait time
c
c
cBLOCK
0
Pr
N
Nk
k
LN
Lk
=
=
sum
c
cc
wait 1
c0c
Pr 1
N
NkN
k
LL LL N kN
minus
=
=⎛ ⎞
+ minus⎜ ⎟⎝ ⎠
sum
wait waitc
Pr htN L
=minus
Multiple Access FDMA (contrsquo)Recall from
Queuing theory
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 20
Ad hoc Sensor Networks Multiple channel access
Multiple Access example of Erlang B systembull In an Erlang B system 40 channels are available A blocking probabilty of
less than 1 is required What is the traffic that can be serve if there is oneoperator or three operators
bull (Solution L(1 operator)gtL(3 operators))bull (Note The number of users increases faster than linearly with the number of
available channels The difference between actual increase and linearincrease is called the trunking gain It is preferable to have a large pool ofavailable channels that serves all users (a single operator) The reasons fornot choosing this approach are political not technical)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 21
Ad hoc Sensor Networks Multiple channel access
Multiple Access TDMAbull The allocated radio spectrum for the system is divided into time slots
ndash In each slot a user can transmit or receivendash A user occupies a cyclically repeating slotsndash A channel is logically defined as a particular time slot that repeats with some
periodbull TDMA systems buffer the data until its turn (time slot) comes to transmitbull In TDMATDD half of the slots in the frame is used for uplink channels the
other is used for downlink channelsbull In TDMAFDD a different carrier frequency is used for uplink or downlink
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 22
Ad hoc Sensor Networks Multiple channel access
Multiple Access TDMA (contrsquo)bull Preamble contains address and synchronization info to identify base
station and mobiles to each otherbull Guard times are used to allow synchronization of the receivers between
different slots and framesndash Different mobiles may have different propagation delays to a base station
because of different distances
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 23
Multiple Access TDMA (contrsquo)bull Each frame contains overhead bits and data bits Efficiency of frame is
defined as the percentage of data (information) bits to the total frame size inbits
η =(1 ndash boverheadbtotal) 100 where btotal= Tf R is the total number of bits in a frame Tf is the frameduration in sec boverhead is the number of overhead bits (GSM efficiency 73)
bull TDMA is usually combined with FDMAndash Neighboring cells are allocated and using different carrier frequencies (FDMA)
Inside a cell TDMA can be used Cells may be re-using the same frequency if theyare far from each-other
ndash There may be more than one channel allocated and used inside each cell Eachcarrier frequency (radio channel) may be using TDMA to further multiplex moreuser (ie having TDMA logical channels inside radio channels) For example inGSM each radio channel has 200KHz bandwidth and has 8 time slots (8 logicalchannels) Hence GSM is using FDMA combined with TDMA
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 24
Multiple Access TDMA (contrsquo)
Ad hoc Sensor Networks Multiple channel access
GSM (Europa) IS-54 (USA) PDC (Japan) DECT
Bit Rate 2708 Kbps 486 Kbps 42 Kbps 1152 Mbps
Bandwidth 200 KHz 30 KHz 25 KHz 1728 MHz
Time Slot 0577 ms 67 ms 67 ms 0417 ms
Upstream slots per frame 8 3 3 12
Duplexing FDD FDD FDD TDD
Efficiency 73 80 80 67
Modulation GMSK π4 DQPSK π4 DQPSK GMSK
Adaptive equalized Mandatory Mandatory Optional None
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 25
Multiple Access TDMA (contrsquo)bull Enables the sharing of a single radio channel among N usersbull Requires high data-rate per radio channel to support N users
simultaneously which requires adaptive equalizers to be usedin multipath environments
bull Transmission occurs in bursts (not continually) It enablespower saving by going to sleep modes in unrelated slots Itdiscontinues transmission and also enables mobile assistedhandoff
bull It equires synchronization of the receivers Need guard bits andsync bits which occurs large overhead per slot
bull Allocation of slots to mobile users should not be uniform Itmay depend on the traffic requirement of mobiles This bringsextra flexibility and efficiency compared to FDMA systems
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2620111015 TAacuteMOP ndash 412-082AKMR-2009-0006 26
Ad hoc Sensor Networks Multiple channel access
Multiple Access spread spectrum multiple accessbull SSMA uses signals that have transmission bandwidth that is
several orders of magnitude larger than minimum requiredRF bandwidth
bull It providesndash Immunity to multipath interferencendash Robust multiple access
bull Two techniquesndash Frequency Hopped Multiple Access (FHMA)ndash Direct Sequence Multiple Access (DSMA) or Code Division
Multiple Access (CDMA)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2720111015 TAacuteMOP ndash 412-082AKMR-2009-0006 27
Ad hoc Sensor Networks Multiple channel access
Multiple Access FHMAbull The carrier frequency of users are varied in a pseudo-random
fashionndash Each user is using a narrowband channel (spectrum) at a specific
instance of timendash The random change in frequency make the change of using the same
narrowband channel very lowbull The sender receiver change frequency (calling hopping)
using the same pseudo-random sequence hence they aresynchronized
bull Rate of hopping versus Symbol ratendash If hopping rate is greather Fast Frequency Hoppingndash If symbol rate is greater Slow Frequency Hopping
bull GSM and Bluetooth are example systems
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2820111015 TAacuteMOP ndash 412-082AKMR-2009-0006 28
Ad hoc Sensor Networks Multiple channel access
Multiple Access FHMA (contrsquo)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2920111015 TAacuteMOP ndash 412-082AKMR-2009-0006 29
Ad hoc Sensor Networks Multiple channel access
Multiple Access CDMAbull The narrowband message signal is multiplied by a very large bandwidth
signal called spreading signal (code) before modulation and transmissionover the air This is called spreading
bull Spreading signal use a pseudo-noise (PN) sequence (a pseudo-randomsequence) called codeword which are orthogonal (low autocorrelation)
bull The receiver correlator distinguishes the senders signal by examining thewideband signal with the same time-synchronized spreading code(despreading)
bull Advantagesndash Low power spectral densityndash Interference limited operationndash Privacyndash Reduction of multipath affects by using a larger spectrumndash Users can start their transmission at any time (random access possible)ndash Cell capacity is soft and higher than TDMA and FDMAndash No frequency management and no equalizers and quard time needed
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3020111015 TAacuteMOP ndash 412-082AKMR-2009-0006 30
Ad hoc Sensor Networks Multiple channel access
Multiple Access near far problem and power controlbull At a receiver the signals may come from various sources (multiuser) The
strongest signal usually captures the modulator The other signals areconsidered as noise Each source may have different distances to the basestation
bull In CDMA we want a BS to receive CDMA coded signals from variousmobile users at the same time
ndash Therefore the receiver power at the BS for all mobile users should be close toeach other This requires power control at the mobiles
bull Power Control BS monitors the RSSI (Received Signal Strength Indicator)values from different mobiles and then sends power change commands to themobiles over a forward channel The mobiles then adjust their transmitpower
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3120111015 TAacuteMOP ndash 412-082AKMR-2009-0006 31
Ad hoc Sensor Networks Multiple channel access
Multiple Access hybrid spread spectrum techniquesbull FDMACDMA
ndash Available wideband spectrum is divided into a number narrowband radio channels CDMAis employed inside each channel
bull DSFHMAndash The signals are spread using spreading codes (direct sequence signals are obtained) but
these signal are not transmitted over a constant carrier frequency they are transmitted over afrequency hopping carrier frequency
bull Time Division CDMA (TCDMA)ndash Each cell is using a different spreading code (CDMA employed between cells) that is
conveyed to the mobiles in its rangendash Inside each cell (inside a CDMA channel) TDMA is employed to multiplex multiple users
bull Time Division Frequency Hoppingndash At each time slot the user is hopped to a new frequency according to a pseudo-random
hopping sequencendash Employed in severe co-interference and multi-path environmentsndash Bluetooth and GSM are using this technique
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3220111015 TAacuteMOP ndash 412-082AKMR-2009-0006 32
Ad hoc Sensor Networks Multiple channel access
Multiple Access comparison TDMAFDMACDMA
Techniques TDMA FDMA CDMA SDMA
Idea
segment sending time into disjoint time-slots
Segment the frequency band into disjoint sub-bands
Spread the spectrum using orthogonal codes
Segment space into sectors
Signal separation
Synchronization in the time domain
Filtering in the frequency domain
Code Directed antennas
Advantage
Established flexible
Simple established robust
Flexible less frequency planning needed soft handover
Simple increases capacity
DisadvantageQuard space synchronization difficult
Inflexible scarce source
Complex receiver power control
inflexible
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3320111015 TAacuteMOP ndash 412-082AKMR-2009-0006 33
Ad hoc Sensor Networks Multiple channel access
User capacity per cell
The user capacity C is defined as the total number of active users percell that the system can support while meeting a common BERconstrain For orthogonal multiple access (as FDMA and TDMA)
C = Ncellwhere Ncell is the number of channels assigned to any given cellThe total number of orthogonal channels of bandwidth Bs that can becreated from a total system bandwidth of B is B Bs The reuse factorsatisfies N = (B Bs) Ncell this implies
C = (B Bs) N
Recall from Chapter 1
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3420111015 TAacuteMOP ndash 412-082AKMR-2009-0006 34
Ad hoc Sensor Networks Multiple channel access
10 methods for increasing capacity1 Increasing the amount of spectrum used very expensive2 More efficient modulation format and coding3 Better source coding4 Adaptive modulation and coding5 Discontinuous transmission6 Multiuser detection CDMA systems7 Reduction of cell radius effective but very expensive smaller
cells require more handovers8 Use of sector cells tripled cells have tripled BS antennas9 Use of an overlay structure10 Multiple antennas diversity MIMO systems SDMA
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3520111015 TAacuteMOP ndash 412-082AKMR-2009-0006 35
Ad hoc Sensor Networks Multiple channel access
bull Distributed operation is requiredbull Synchronization is required in TDMA-based systemsbull Hidden terminals are nodes hidden from a senderbull Exposed terminals are exposed nodes preventing a sender
from sendingbull Throughput needs to be maximizedbull Access delay should be minimizedbull Fairness refers to provide an equal share to all competing
nodesbull Real-time traffic support is required for voice video and
real-time databull Resource reservation is required for QoSbull Ability to measure resource availability handles the
resourcesbull Capability for power control reduces the energy
consumption
Random access in adhoc networks
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3620111015 TAacuteMOP ndash 412-082AKMR-2009-0006 36
Ad hoc Sensor Networks Multiple channel access
Random accessbull All random access techniques are based on packetized data or packet radio where user
date is collected into packets of M bitsbull Collision if packets from different users overlap in timebull The transmission time of a packet is τ = M R where R [bps] the data ratebull Analysis of random access techniques assumes that the users generate packets according
to memoryless Poisson process at a rate λ [packets per unit time] the probability that thenumber of packet arrivals in a time period [0t] denoted by X(t) is equal to integer k isgiven by
Pr(X(t)=k) = (λt)k k e-λt
bull The traffic load is defined as L = λ τ If L gt 1 means on avarage more packets arrive inthe system over a given time period than can be transmitted in that period so systemswith Lgt1 are unstable
bull The throughput is defined as the ratio of the average number of packets successfullytransmitted in any given time interval divided by the number of attempted transmissionsin that interval
T = L Pr(succesful packet transmission)bull The effective data rate of the system is RT
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3720111015 TAacuteMOP ndash 412-082AKMR-2009-0006 37
Ad hoc Sensor Networks Multiple channel access
Random access (contrsquo)bull We will examine the following attributes
ndash Channel utilization (Throughput)ndash Latencyndash Collision avoidance (Hidden and exposed terminals problem)ndash Reliability (ACK)ndash Energy efficiency (power control)ndash Fairnessndash Throughput needs to be maximized
bull We will discuss the following techniquesndash Aloha and slotted Alohandash CSMA Protocols (1-persistent non-persistent p-persistent CSMA CSMACD)ndash MACAndash MACAWndash Energy efficient MAC in wireless sensor networks (eg SMAC)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 38
Random Access ALOHA (1970)bull ALOHA was developed for a wireless system at the University of Hawaii
(Abramson et al)ndash Multiple remote stations plus one base stationndash Frame transmissions are made at one frequency from a remote station to the basestation the base station re-broadcasts frames on another frequency Contention is foraccess to the base station
bull Frames can be sent at any time to the base stationndash If no acknowledgment (ACK) is received assume that the frame is lost andretransmit after waiting a random amount of timendash If more than one station broadcasts on the base station access frequency at the sametime ndash a collision ndash interference will destroy the frames
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 39
Random Access ALOHA (contrsquo)
bull The probability that no packets generated during the tima [-τ τ] is given by (nocollision)
Pr(X(t)=0) = (λ2τ)0 0 e-2λτ = e-2Lwith corresponding throughput
T = L e-2L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 05 05e 018
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 40
Random Access slotted ALOHA (1971)bull Improvement Time is slotted and a packet can only be transmitted at the beginning of
one slot Thus it can reduce the collision duration
bull The probability that no packets generated during the time [0 τ] is given by (nocollision)
Pr(X(t)=0) = (λτ)0 0 e-λτ = e-Lwith corresponding throughput T = L e-L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 1 e 037
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 41
bull Slotted Aloha has double the maximumthroughput as pure Aloha and achievesthis maximum at a higher offered loadThe effective data rate is still less than40 of the raw transmission rate
bull Aloha does not listen to the carrier beforetransmission more sophisticatedtechniques are needed to increaseefficiency
bull Further problem the delayndashDelay
ndashThe mean packet delay
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
( ) ( )delayPr 1 e ekL Lkτ minus minus= = minus
( )delay1
1 e e ekL L L
kMPD E kτ
infinminus minus
=
= = minus =sum
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 42
Random Access Carrier Sense Multiple Accessbull CSMA (Carrier Sense Multiple Access) listen to the carrier before transmission
and transmits if channel is idlebull Detection delay and propagation delay are two important parameters
ndash Detection delay time required to sense the carrier and decide if it is idle or busyndash Propagation delay distancespeed_of_ligth The time required for bit to travel
from transmitter to the receiver
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 43
Random Access CSMA variations
Ad hoc Sensor Networks Multiple channel access
1-persistent CSMA Non-persistent CSMA
There will always be a collision iftwo nodes want to retransmit(usually you stop transmissionattempts after few tries)
Random backoff reduces probabilityof collisions Waste idle time if thebackoff time is too long
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 44
Random Access CSMA variations (contrsquo)
Ad hoc Sensor Networks Multiple channel access
p-persistent CSMA CSMA CD (Collision Detection)
A good tradeoff between non-persistent and 1-persistent CSMA
Same with CSMA however a stationalso listen to the carrier whiletransmitting to see if the transmissioncollides with someone elsetransmission It can be used in listen-while-talk capable channels (fullduplex) In single radio channels thetransmission need to be interrupted inorder to sense the channel
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 45
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 46
Random Access CSMA (contrsquo)bull How to select probability p
ndash Assume that M nodes have a packet to send and the medium is busy then Mp is theexpected number of nodes that will attempt to transmit once the medium becomesidle If Mp gt 1 then a collision is expected to occur Therefore the network mustmake sure that Mp lt 1 to avoid collision where M is the maximum number of nodesthat can be active at a time
bull Problems1 The mean packet delay increases exponently with increasing offered load2 CSMA protocols sense the carrier but sensing the carrier does not always releasestrue information about the status of the wireless channel There are two problems thatare unique to wireless channels (different than wireline channels)ndash Hidden terminal problemndash Exposed terminal problem
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 47
Hidden terminal problembull A sends to B C cannot receive Abull C wants to send to B C senses a
ldquofreerdquo medium (CS fails)bull collision at B A cannot receive the
collision (CD fails)bull A is ldquohiddenrdquo for C
Ad hoc Sensor Networks Multiple channel access
Random Access hidden and exposed terminal problemExposed terminal problem
bull B sends to A C wants to send to Dbull C has to wait CS signals a medium
in usebull since A is outside the radio range of
C waiting is not necessarybull C is ldquoexposedrdquo to B
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 48
Random Access MACAWhen a station wants to transmit databull It sends an RTS (Ready-to-Send) packet to the intended receiver
ndash The RTS packet contains the length of the data that needs to be transmittedndash Any station other than the intended recipient hearing RTS defers transmission for atime duration equal to the end of the corresponding CTS reception
bull The receiver sends back CTS (Clear-to-Send) packet back to sender if it isavailable to receivendash The CTS packet contains the length of the data that original sender wants totransmitndash Any station other than the original RTS sender hearing CTS defers transmissionuntil the data is sent
bull The original sender upon reception of the CTS starts transmitting
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 49
Hidden terminal problem
Ad hoc Sensor Networks Multiple channel access
Random Access MACA (contrsquo)Exposed terminal problem
Waiting time of node X is muchsmaller than waiting time of node C
bull C defers transmission upon hearing BrsquosRTS until B could get CTS from Abull After that C can start transmission to DFor that it first sends an RTSbull C is not longer exposed to the datatransmission of B
Problem It does not address the collision of RTS packet
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 50
Random Access reliabilitybull Wireless links are prone to have errors High packet loss rate detrimental to
transport-layer performance Mechanisms needed to reduce packet loss rateexperienced by upper layers
bull When node B receives a data packet from node A node B sends anAcknowledgement (Ack) This approach adopted in many protocols
bull If node A fails to receive an Ack it will retransmit the packet
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 51
Random Access MACAW (CSMACA) bull Physical carrier sense and virtual carrier sense using Network Allocation
Vector (NAV)ndash NAV is updated based on overheard RTSCTSDATAACK packets each of whichspecified duration of a pending transmissionndash Nodes stay silent when carrier sensed (physicalvirtual)
bull Backoff intervals used to reduce collision probabilityndash When transmitting a packet choose a backoff interval in the range [0w] where wis contention windowndash Count down the backoff interval when channel is idlendash Count down is suspended if channel becomes busyndash When backoff interval reaches 0 transmit RTS
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 52
Random Access CSMACA (contrsquo) bull The time spent counting down backoff intervals is a part of MAC overhead
ndash Choosing a large w leads to large backoff intervals and can result in larger overheadndash Choosing a small w leads to a larger number of collisions (when two nodes countdown to 0 simultaneously)ndash Since the number of nodes attempting to transmit simultaneously may change withtime some mechanism to manage contention is needed
bull IEEE 80211 DCF contention window w is chosen dynamically depending oncollision occurrence (binary exponential backoff)ndash When a node fails to receive CTS in response to its RTS it increases the contentionwindowndash w is doubled (up to an upper bound)ndash When a node successfully completes a data transfer it restores w to wmin
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 53
Random Access fairnessbull Simplest definition of fairness all nodes should receive equal bandwidthbull An example of unfairness
ndash Assume that initially A and B both choose a backoff interval in range [031] buttheir RTSs collidendash Nodes A and B then choose from range [063]ndash Node A chooses 4 slots and B choose 60 slotsndash After A transmits a packet it next chooses from range [031]ndash It is possible that A may transmit several packets before B transmits its first packet
Ad hoc Sensor Networks Multiple channel access
Unfairness occurs when one node has backed off much more than some other node
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 54
Random Access fairness (contrsquo) MACAW solution for fairnessbull When a node transmits a packet it appends value w to the packet all nodes
hearing value w use it for their future transmission attemptsbull Since w is an indication of the level of congestion in the vicinity of a specific
receiver node MACAW proposes maintaining w independently for eachreceiver
bull Using per-receiver wi is particularly useful in multi-hop environments sincecongestion level at different receivers can be very different
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 55
Random Access energy efficiency bull Since many mobile hosts are operated by batteries MAC
protocols which conserve energy are of interestbull Two approaches to reduce energy consumption
ndash Power save turn off wireless interface when desirablendash Power control reduce transmit power
bull Power control has some more potential benefitsndash Reduced interference (in ad hoc networks) eg it improves ALOHAefficiency where user with high power can capture a packet even if there is acollisionndash Increased spatial reuse (in infrastructured wireless networks) eg it is usedin CDMA to maintain target SIR of voice and data users or generally it canbe used to maintain target SIR for different user classes (admission control)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 56
Random Access power control bull When C transmits to D at a high power level B cannot receive Arsquos
transmission due to interference from Cbull If C reduces transmit power it can still communicate with D
ndash Reduces energy consumption at node Cndash Allows B to receive Arsquos transmission (spatial reuse)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 57
bull A special wireless ad hoc networkndash Large number of nodesndash Battery poweredndash Topology and density changendash Nodes for a common taskndash In-network data processing
bull Sensor-net applicationsndash Sensor-triggered bursty trafficndash Can often tolerate some delay
Ad hoc Sensor Networks Multiple channel access
Characteristics of Sensor Network
Scalabilty and self configurationEnergy efficiencyAdaptivityFairness not importantLatency
AdaptivityEnergy-delay tradoff
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 58
Ad hoc Sensor Networks Multiple channel access
Wireless sensor node and sensor network
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 59
MAC for WSN attributes bull Attributes
ndash Collision avoidance (Basic task)ndash Energy efficiency (most primary in WSN)ndash Scalability and adaptivityndash Channel utilizationndash Latencyndash Throughputndash Fairness
bull What causes energy wastendash Collisionsndash Control packet overheadndash Overhearing unnecessary trafficndash Long idle time (it consumes 50-100 of the power)
Ad hoc Sensor Networks Multiple channel access
Primary
Secondary
Dominant factor
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 60
MAC for WSN
bull Contention-based protocols need to work hard in all directions for energysavingsndash Reduce idle listening ndash support low duty cyclendash Better collision avoidancendash Reduce control overheadndash Avoid unnecessary overhearing
Ad hoc Sensor Networks Multiple channel access
Scheduled Protocols Contention ProtocolsCollisions No YesEnergy efficiency Good BadScalability and adaptivity Bad GoodMulti-hop communication Difficult EasyTime synchronization Strict Loose or not required
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 61
MAC for WSN classification
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 62
MAC for WSN SMAC (2002)bull It has been developed by Ye Heidemann and Estrinbull Tradeoffs between (latency fairness) and energybull Major components in S-MAC
ndash Periodic listen and sleep turn off radio when sleeping the reduced dutycycle is ~ 10 (eg 120ms on 12s off)ndash Collision avoidance is based on contention similarly to IEEE 80211 adhoc modendash Overhearing avoidance the node is sleeping when neighbors talkndash Massage passing long message is fragmented amp sent in burst operationwith extended Tx time and re-transmit occurs immediately
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 63
Summarybull The algorithms and protocols that enable sharing by multiple users and
controls the access to the wireless channel from different users are calledMAC
bull There are two types of wireless networks Infrastructured wireless networksand ad hoc wireless networks
bull We also want our wireless system to have high user capacity and we canidentify 10 methods for increasing it
bull There are several MACs with no absolute advantage it should be tailoredto the system requirements
bull There are primary concerns for designing a WSN MAC (collisionavoidance energy efficiency scalability and adaptivity) while thesecondary concerns are as follows (channel utilization latency throughputfairness)
bull Next lecture Routing protocols
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2
Peter Pazmany Catholic University
Faculty of Information Technology
Ad hoc Sensor Networks
Multiple channel access
wwwitkppkehu
Eacuterzeacutekelő mobilhaacuteloacutezatok
Toumlbbszoumlroumls hozzaacutefeacutereacutes
Dr Olaacuteh Andraacutes
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3
Ad hoc Sensor Networks Multiple channel access
Lecture 5 reviewbull Signal space representationbull Optimal detection of signal in AWGN (Bayesian decision)bull Probability of error (BER and SER)bull Demodulation and detection for modulation schemesbull BER in fading channelbull Channel equalization
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 4
Outlinebull The goal of medium access controlbull Types of wireless networksbull Duplexing techniquesbull Multiple Accessbull Random Accessbull MAC for Wireless Sensor Networks
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 5
Ad hoc Sensor Networks Multiple channel access
Structure of a wireless communications link
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 6
Ad hoc Sensor Networks Multiple channel access
Layered Communication Approaches
2 Data Link layer it handles access to the unerlyingchannel and defines the data format It is split into twosublayersndash Logical Link Control it manages frames to upper and
lower layer (encapsulation decapsulation) and it enforceserrror control (checksum and parity bits)
ndash Medium Access Control (MAC) it coordinatestransmission between users sharing the spectrum Inwireless systems it must address the hidden terminalproblem and must exercise power control Goals preventcollisions while maximizing throughput and minimizingdelay
Physical
OSI reference model
Data link
Network
Transport
Middleware
Application
Recall from Chapter 1
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 7
Ad hoc Sensor Networks Multiple channel access
Medium Access Control
bull Thus the wireless spectrum (frequency band) is a very precious and limited resourcewe need to use this resource very efficiently
bull We also want our wireless system to have high user capacity with QoS constraintsbull The algorithms and protocols that enables this sharing by multiple users and
controlscoordinates the access to the wireless channel (medium) from differentusers are called MEDIUM ACCESS or MEDIA ACCESS or MULTIPLE ACCESSprotocols techniques schemes etchellip)
Designbull Goal Function Maximum Utilization
(eg throughput user capacity hellip)bull Constraints QoS
(eg delaylt10ms BERlt10-5hellip)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 8
Ad hoc Sensor Networks Multiple channel access
Medium Access Control (contrsquo)bull Multiple Access (Channel Partitioning or Coordinated Schemes)
ndash Techniques TDMA FDMA CDMA SDMAndash Examples GSM 3Ghellip
bull Random Access (or Random Schemes)ndash Techniques MACA MACAW Aloha 80211 MAChellipndash Examples Wifi WSNhellip
bull Polling based schemesndash Access is coordinated by a central nodendash Examples Bluetooth BlueSkyhellip
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 9
Ad hoc Sensor Networks Multiple channel access
bull Infrastructured networksndash base stations are the bridgesndash a mobile host will communicate with the
nearest base station (or access point)ndash handoff is taken when a host roams from one
base to anotherndash Medium access control assisted by BS
bull Ad hoc networksndash infrastructureless no fixed base stationsndash without the assistance of base stations for
communicationndash Due to transmission range constraint two
wireless nodes need multi-hop routing forcommunication
ndash quickly and unpredictably changing topology
Two types of wireless networks
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 10
Ad hoc Sensor Networks Multiple channel access
Two types of wireless networks (contrsquo)
bull Mesh networks serve as access networks that employ multi-hop wireless forwarding bynon-mobile nodes to relay traffic to and from the wired Internet In such an environmenthybrid wireless network technologies andor hierarchical network organization can beused for ad hoc and infrastructure wireless links
bull Wireless Sensor Networks are a special category of ad hoc networks that are used toprovide a wireless communication infrastructure among the sensors deployed in a specificapplication domain [rarr see Chapter 9 - 11]
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 11
Ad hoc Sensor Networks Multiple channel access
Infrastructured wireless networks Ad hoc wireless networksFixed infrastructure-based Infrastructureless
Guaranteed bandwidth Shared radio channel
Seamless connectivity (low call drops during handoffs)
Frequent path breaks due to mobility
High cost and time of deployment Quick and cost-effective deployment
Reuse of frequency spectrum (cellular principle) Dynamic frequency reuse based on carrier sense mechanism
Application domains include mainly civilian and commercial sectors
Application domains include battlefields emergency search and rescue operations
High cost of network maintenance Self-organization and maintenance properties are built into the network
Widely deployed and currently in the third generation of evolution
Several issues are to be addressed for successful commercial deployment even though widespread use exists in defense
Two types of wireless networks (contrsquo)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 12
Ad hoc Sensor Networks Multiple channel access
Duplexing
bull If the communication between two parties is one way then it is called simplexcommunication Simplex communication is achieved by default by using a singlewireless channel (frequency band) to transmit from sender to receiver
bull If the communication between two parties is two- way then it is called duplexcommunication Duplex communication achieved by
ndash Time Division (TDD) (famous in cellular systems)ndash Frequency Division (FDD) (famous in cellular systems)ndash Some other method like a random access method
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 13
Ad hoc Sensor Networks Multiple channel access
FDDbull A duplex channel consists of two
simplex channels with different carrierfrequencies
ndash Downlink band carries traffic frombase to mobile
ndash Uplink band carries traffic frommobile to base
Duplexing(contrsquo)TDD
bull channel (carrier frequency) is shared intime in a deterministic manner
ndash The time is slotted with fixed slotlength (sec)
ndash Some slots are used for downlinkchannel
ndash Some slots are used for uplink channel
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 14
Ad hoc Sensor Networks Multiple channel access
FDDbull FDD is used in radio systems that can
allocate individual radio frequenciesfor each user
bull For example analog systems AMPSbull In FDD channels are allocated by a
base stationbull A channel for a mobile is allocated
dynamicallybull All channels that a base station will use
are allocated usually staticallybull More suitable for wide-area cellular
networks GSM AMPS all use FDD
Duplexing(contrsquo)TDD
bull Can only be used in digital wirelesssystems (digital modulation)
bull Requires rigid timing andsynchronization
bull Mostly used in short-range and fixedwireless systems so that propagationdelay between base and mobile do notchange much with respect to locationof the mobile
bull Such as cordless phoneshellip
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 15
Ad hoc Sensor Networks Multiple channel access
Multiple Access narrow- vs wideband systemsbull Narrowband System
ndash The bandwidth is small compared to the coherence bandwidth of the channel(BltBcoh)
bull Wideband Systemndash The system bandwidth is much larger that the coherence bandwidth of the
multipath channel (BgtBcoh) A large number of users can access the same channel(frequency band) at the same time
bull Four major multiple access (MA) schemesndash Time Division Multiple Access (TDMA) it could be used in narrowband or
wideband systemndash Frequency Division Multiple Access (FDMA) it is usually used in narrowband
systemndash Code Division Multiple Access (CDMA) it is used in wideband systemndash Space Division Multiple Access (SDMA) for wireless systems with multiple
antennas it can be combined with all of the other MA methods
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 16
Ad hoc Sensor Networks Multiple channel access
Multiple Access cellular standards
Cellular System Multiple Access TechniqueAMPS (2G) FDMAFDD
CT2 Cordless Phone FDMATDD
DECT Cordless Phone FDMATDD
GSM (2G) TDMAFDD
USDC (IS-54 and IS-136) (2G) TDMAFDD
Personal Digital Cellular (2G) TDMAFDD
US IS-95 (2G) CDMAFDD
W-CDMA (3G) CDMAFDD CDMATDD
cdma2000 (3G) CDMAFDD CDMATDD
Narrowband System
Wideband System
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 17
Ad hoc Sensor Networks Multiple channel access
bull Individual radio channels are assigned toindividual users Each user is allocated a Wfrequency band by BS
bull If the channel allocated to a user is idle then itis not used by someone else waste of resource
bull Mobile and base can transmit and receivesimultaneously (FDD)
bull The W bandwidth of FDMA channels arerelatively low Symbol time is usually larger(low data rate) than the delay spread of themultipath channel (implies that inter-symbolinterference is low)
bull Lower complexity systems than TDMAsystems
Multiple Access FDMA
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 18
Ad hoc Sensor Networks Multiple channel access
Multiple Access FDMA (contrsquo)bull Capacity of FDMA systems
Nc = (B-2Bguard)Wwhere B is the total spectrum allocation Bguard is the guard band allocated atthe edge of the spectrum band W is the bandwidth of an individual channel
bull Erlang B and Erlang C system assumesndashinfinite population of sources which jointly offer traffic to Nc channelndashcall attempts arrive following a Poisson process so call arrivals are independentThe average arrival rate is λndashmessage length (holding times) are exponentially distributed (Markovian system)The average call length is hndashthe total amount of traffic offered in erlangs L= λhndashIf a user is rejected his next call attempt is made statistically independent of theprevious attempt (Erlang B)ndashIf all the channels are busy when a request arrives from a user the request isqueued An unlimited number of requests may be held in the queue (Erlang C)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 19
Ad hoc Sensor Networks Multiple channel access
bull Statictical measures of offered load Ncndash In Erlang B system the probability of call
blocking
ndash In Erlang C system the probabilty of waiting
ndash And the average wait time
c
c
cBLOCK
0
Pr
N
Nk
k
LN
Lk
=
=
sum
c
cc
wait 1
c0c
Pr 1
N
NkN
k
LL LL N kN
minus
=
=⎛ ⎞
+ minus⎜ ⎟⎝ ⎠
sum
wait waitc
Pr htN L
=minus
Multiple Access FDMA (contrsquo)Recall from
Queuing theory
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 20
Ad hoc Sensor Networks Multiple channel access
Multiple Access example of Erlang B systembull In an Erlang B system 40 channels are available A blocking probabilty of
less than 1 is required What is the traffic that can be serve if there is oneoperator or three operators
bull (Solution L(1 operator)gtL(3 operators))bull (Note The number of users increases faster than linearly with the number of
available channels The difference between actual increase and linearincrease is called the trunking gain It is preferable to have a large pool ofavailable channels that serves all users (a single operator) The reasons fornot choosing this approach are political not technical)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 21
Ad hoc Sensor Networks Multiple channel access
Multiple Access TDMAbull The allocated radio spectrum for the system is divided into time slots
ndash In each slot a user can transmit or receivendash A user occupies a cyclically repeating slotsndash A channel is logically defined as a particular time slot that repeats with some
periodbull TDMA systems buffer the data until its turn (time slot) comes to transmitbull In TDMATDD half of the slots in the frame is used for uplink channels the
other is used for downlink channelsbull In TDMAFDD a different carrier frequency is used for uplink or downlink
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 22
Ad hoc Sensor Networks Multiple channel access
Multiple Access TDMA (contrsquo)bull Preamble contains address and synchronization info to identify base
station and mobiles to each otherbull Guard times are used to allow synchronization of the receivers between
different slots and framesndash Different mobiles may have different propagation delays to a base station
because of different distances
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 23
Multiple Access TDMA (contrsquo)bull Each frame contains overhead bits and data bits Efficiency of frame is
defined as the percentage of data (information) bits to the total frame size inbits
η =(1 ndash boverheadbtotal) 100 where btotal= Tf R is the total number of bits in a frame Tf is the frameduration in sec boverhead is the number of overhead bits (GSM efficiency 73)
bull TDMA is usually combined with FDMAndash Neighboring cells are allocated and using different carrier frequencies (FDMA)
Inside a cell TDMA can be used Cells may be re-using the same frequency if theyare far from each-other
ndash There may be more than one channel allocated and used inside each cell Eachcarrier frequency (radio channel) may be using TDMA to further multiplex moreuser (ie having TDMA logical channels inside radio channels) For example inGSM each radio channel has 200KHz bandwidth and has 8 time slots (8 logicalchannels) Hence GSM is using FDMA combined with TDMA
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 24
Multiple Access TDMA (contrsquo)
Ad hoc Sensor Networks Multiple channel access
GSM (Europa) IS-54 (USA) PDC (Japan) DECT
Bit Rate 2708 Kbps 486 Kbps 42 Kbps 1152 Mbps
Bandwidth 200 KHz 30 KHz 25 KHz 1728 MHz
Time Slot 0577 ms 67 ms 67 ms 0417 ms
Upstream slots per frame 8 3 3 12
Duplexing FDD FDD FDD TDD
Efficiency 73 80 80 67
Modulation GMSK π4 DQPSK π4 DQPSK GMSK
Adaptive equalized Mandatory Mandatory Optional None
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 25
Multiple Access TDMA (contrsquo)bull Enables the sharing of a single radio channel among N usersbull Requires high data-rate per radio channel to support N users
simultaneously which requires adaptive equalizers to be usedin multipath environments
bull Transmission occurs in bursts (not continually) It enablespower saving by going to sleep modes in unrelated slots Itdiscontinues transmission and also enables mobile assistedhandoff
bull It equires synchronization of the receivers Need guard bits andsync bits which occurs large overhead per slot
bull Allocation of slots to mobile users should not be uniform Itmay depend on the traffic requirement of mobiles This bringsextra flexibility and efficiency compared to FDMA systems
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2620111015 TAacuteMOP ndash 412-082AKMR-2009-0006 26
Ad hoc Sensor Networks Multiple channel access
Multiple Access spread spectrum multiple accessbull SSMA uses signals that have transmission bandwidth that is
several orders of magnitude larger than minimum requiredRF bandwidth
bull It providesndash Immunity to multipath interferencendash Robust multiple access
bull Two techniquesndash Frequency Hopped Multiple Access (FHMA)ndash Direct Sequence Multiple Access (DSMA) or Code Division
Multiple Access (CDMA)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2720111015 TAacuteMOP ndash 412-082AKMR-2009-0006 27
Ad hoc Sensor Networks Multiple channel access
Multiple Access FHMAbull The carrier frequency of users are varied in a pseudo-random
fashionndash Each user is using a narrowband channel (spectrum) at a specific
instance of timendash The random change in frequency make the change of using the same
narrowband channel very lowbull The sender receiver change frequency (calling hopping)
using the same pseudo-random sequence hence they aresynchronized
bull Rate of hopping versus Symbol ratendash If hopping rate is greather Fast Frequency Hoppingndash If symbol rate is greater Slow Frequency Hopping
bull GSM and Bluetooth are example systems
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2820111015 TAacuteMOP ndash 412-082AKMR-2009-0006 28
Ad hoc Sensor Networks Multiple channel access
Multiple Access FHMA (contrsquo)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2920111015 TAacuteMOP ndash 412-082AKMR-2009-0006 29
Ad hoc Sensor Networks Multiple channel access
Multiple Access CDMAbull The narrowband message signal is multiplied by a very large bandwidth
signal called spreading signal (code) before modulation and transmissionover the air This is called spreading
bull Spreading signal use a pseudo-noise (PN) sequence (a pseudo-randomsequence) called codeword which are orthogonal (low autocorrelation)
bull The receiver correlator distinguishes the senders signal by examining thewideband signal with the same time-synchronized spreading code(despreading)
bull Advantagesndash Low power spectral densityndash Interference limited operationndash Privacyndash Reduction of multipath affects by using a larger spectrumndash Users can start their transmission at any time (random access possible)ndash Cell capacity is soft and higher than TDMA and FDMAndash No frequency management and no equalizers and quard time needed
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3020111015 TAacuteMOP ndash 412-082AKMR-2009-0006 30
Ad hoc Sensor Networks Multiple channel access
Multiple Access near far problem and power controlbull At a receiver the signals may come from various sources (multiuser) The
strongest signal usually captures the modulator The other signals areconsidered as noise Each source may have different distances to the basestation
bull In CDMA we want a BS to receive CDMA coded signals from variousmobile users at the same time
ndash Therefore the receiver power at the BS for all mobile users should be close toeach other This requires power control at the mobiles
bull Power Control BS monitors the RSSI (Received Signal Strength Indicator)values from different mobiles and then sends power change commands to themobiles over a forward channel The mobiles then adjust their transmitpower
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3120111015 TAacuteMOP ndash 412-082AKMR-2009-0006 31
Ad hoc Sensor Networks Multiple channel access
Multiple Access hybrid spread spectrum techniquesbull FDMACDMA
ndash Available wideband spectrum is divided into a number narrowband radio channels CDMAis employed inside each channel
bull DSFHMAndash The signals are spread using spreading codes (direct sequence signals are obtained) but
these signal are not transmitted over a constant carrier frequency they are transmitted over afrequency hopping carrier frequency
bull Time Division CDMA (TCDMA)ndash Each cell is using a different spreading code (CDMA employed between cells) that is
conveyed to the mobiles in its rangendash Inside each cell (inside a CDMA channel) TDMA is employed to multiplex multiple users
bull Time Division Frequency Hoppingndash At each time slot the user is hopped to a new frequency according to a pseudo-random
hopping sequencendash Employed in severe co-interference and multi-path environmentsndash Bluetooth and GSM are using this technique
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3220111015 TAacuteMOP ndash 412-082AKMR-2009-0006 32
Ad hoc Sensor Networks Multiple channel access
Multiple Access comparison TDMAFDMACDMA
Techniques TDMA FDMA CDMA SDMA
Idea
segment sending time into disjoint time-slots
Segment the frequency band into disjoint sub-bands
Spread the spectrum using orthogonal codes
Segment space into sectors
Signal separation
Synchronization in the time domain
Filtering in the frequency domain
Code Directed antennas
Advantage
Established flexible
Simple established robust
Flexible less frequency planning needed soft handover
Simple increases capacity
DisadvantageQuard space synchronization difficult
Inflexible scarce source
Complex receiver power control
inflexible
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3320111015 TAacuteMOP ndash 412-082AKMR-2009-0006 33
Ad hoc Sensor Networks Multiple channel access
User capacity per cell
The user capacity C is defined as the total number of active users percell that the system can support while meeting a common BERconstrain For orthogonal multiple access (as FDMA and TDMA)
C = Ncellwhere Ncell is the number of channels assigned to any given cellThe total number of orthogonal channels of bandwidth Bs that can becreated from a total system bandwidth of B is B Bs The reuse factorsatisfies N = (B Bs) Ncell this implies
C = (B Bs) N
Recall from Chapter 1
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3420111015 TAacuteMOP ndash 412-082AKMR-2009-0006 34
Ad hoc Sensor Networks Multiple channel access
10 methods for increasing capacity1 Increasing the amount of spectrum used very expensive2 More efficient modulation format and coding3 Better source coding4 Adaptive modulation and coding5 Discontinuous transmission6 Multiuser detection CDMA systems7 Reduction of cell radius effective but very expensive smaller
cells require more handovers8 Use of sector cells tripled cells have tripled BS antennas9 Use of an overlay structure10 Multiple antennas diversity MIMO systems SDMA
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3520111015 TAacuteMOP ndash 412-082AKMR-2009-0006 35
Ad hoc Sensor Networks Multiple channel access
bull Distributed operation is requiredbull Synchronization is required in TDMA-based systemsbull Hidden terminals are nodes hidden from a senderbull Exposed terminals are exposed nodes preventing a sender
from sendingbull Throughput needs to be maximizedbull Access delay should be minimizedbull Fairness refers to provide an equal share to all competing
nodesbull Real-time traffic support is required for voice video and
real-time databull Resource reservation is required for QoSbull Ability to measure resource availability handles the
resourcesbull Capability for power control reduces the energy
consumption
Random access in adhoc networks
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3620111015 TAacuteMOP ndash 412-082AKMR-2009-0006 36
Ad hoc Sensor Networks Multiple channel access
Random accessbull All random access techniques are based on packetized data or packet radio where user
date is collected into packets of M bitsbull Collision if packets from different users overlap in timebull The transmission time of a packet is τ = M R where R [bps] the data ratebull Analysis of random access techniques assumes that the users generate packets according
to memoryless Poisson process at a rate λ [packets per unit time] the probability that thenumber of packet arrivals in a time period [0t] denoted by X(t) is equal to integer k isgiven by
Pr(X(t)=k) = (λt)k k e-λt
bull The traffic load is defined as L = λ τ If L gt 1 means on avarage more packets arrive inthe system over a given time period than can be transmitted in that period so systemswith Lgt1 are unstable
bull The throughput is defined as the ratio of the average number of packets successfullytransmitted in any given time interval divided by the number of attempted transmissionsin that interval
T = L Pr(succesful packet transmission)bull The effective data rate of the system is RT
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3720111015 TAacuteMOP ndash 412-082AKMR-2009-0006 37
Ad hoc Sensor Networks Multiple channel access
Random access (contrsquo)bull We will examine the following attributes
ndash Channel utilization (Throughput)ndash Latencyndash Collision avoidance (Hidden and exposed terminals problem)ndash Reliability (ACK)ndash Energy efficiency (power control)ndash Fairnessndash Throughput needs to be maximized
bull We will discuss the following techniquesndash Aloha and slotted Alohandash CSMA Protocols (1-persistent non-persistent p-persistent CSMA CSMACD)ndash MACAndash MACAWndash Energy efficient MAC in wireless sensor networks (eg SMAC)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 38
Random Access ALOHA (1970)bull ALOHA was developed for a wireless system at the University of Hawaii
(Abramson et al)ndash Multiple remote stations plus one base stationndash Frame transmissions are made at one frequency from a remote station to the basestation the base station re-broadcasts frames on another frequency Contention is foraccess to the base station
bull Frames can be sent at any time to the base stationndash If no acknowledgment (ACK) is received assume that the frame is lost andretransmit after waiting a random amount of timendash If more than one station broadcasts on the base station access frequency at the sametime ndash a collision ndash interference will destroy the frames
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 39
Random Access ALOHA (contrsquo)
bull The probability that no packets generated during the tima [-τ τ] is given by (nocollision)
Pr(X(t)=0) = (λ2τ)0 0 e-2λτ = e-2Lwith corresponding throughput
T = L e-2L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 05 05e 018
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 40
Random Access slotted ALOHA (1971)bull Improvement Time is slotted and a packet can only be transmitted at the beginning of
one slot Thus it can reduce the collision duration
bull The probability that no packets generated during the time [0 τ] is given by (nocollision)
Pr(X(t)=0) = (λτ)0 0 e-λτ = e-Lwith corresponding throughput T = L e-L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 1 e 037
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 41
bull Slotted Aloha has double the maximumthroughput as pure Aloha and achievesthis maximum at a higher offered loadThe effective data rate is still less than40 of the raw transmission rate
bull Aloha does not listen to the carrier beforetransmission more sophisticatedtechniques are needed to increaseefficiency
bull Further problem the delayndashDelay
ndashThe mean packet delay
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
( ) ( )delayPr 1 e ekL Lkτ minus minus= = minus
( )delay1
1 e e ekL L L
kMPD E kτ
infinminus minus
=
= = minus =sum
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 42
Random Access Carrier Sense Multiple Accessbull CSMA (Carrier Sense Multiple Access) listen to the carrier before transmission
and transmits if channel is idlebull Detection delay and propagation delay are two important parameters
ndash Detection delay time required to sense the carrier and decide if it is idle or busyndash Propagation delay distancespeed_of_ligth The time required for bit to travel
from transmitter to the receiver
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 43
Random Access CSMA variations
Ad hoc Sensor Networks Multiple channel access
1-persistent CSMA Non-persistent CSMA
There will always be a collision iftwo nodes want to retransmit(usually you stop transmissionattempts after few tries)
Random backoff reduces probabilityof collisions Waste idle time if thebackoff time is too long
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 44
Random Access CSMA variations (contrsquo)
Ad hoc Sensor Networks Multiple channel access
p-persistent CSMA CSMA CD (Collision Detection)
A good tradeoff between non-persistent and 1-persistent CSMA
Same with CSMA however a stationalso listen to the carrier whiletransmitting to see if the transmissioncollides with someone elsetransmission It can be used in listen-while-talk capable channels (fullduplex) In single radio channels thetransmission need to be interrupted inorder to sense the channel
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 45
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 46
Random Access CSMA (contrsquo)bull How to select probability p
ndash Assume that M nodes have a packet to send and the medium is busy then Mp is theexpected number of nodes that will attempt to transmit once the medium becomesidle If Mp gt 1 then a collision is expected to occur Therefore the network mustmake sure that Mp lt 1 to avoid collision where M is the maximum number of nodesthat can be active at a time
bull Problems1 The mean packet delay increases exponently with increasing offered load2 CSMA protocols sense the carrier but sensing the carrier does not always releasestrue information about the status of the wireless channel There are two problems thatare unique to wireless channels (different than wireline channels)ndash Hidden terminal problemndash Exposed terminal problem
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 47
Hidden terminal problembull A sends to B C cannot receive Abull C wants to send to B C senses a
ldquofreerdquo medium (CS fails)bull collision at B A cannot receive the
collision (CD fails)bull A is ldquohiddenrdquo for C
Ad hoc Sensor Networks Multiple channel access
Random Access hidden and exposed terminal problemExposed terminal problem
bull B sends to A C wants to send to Dbull C has to wait CS signals a medium
in usebull since A is outside the radio range of
C waiting is not necessarybull C is ldquoexposedrdquo to B
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 48
Random Access MACAWhen a station wants to transmit databull It sends an RTS (Ready-to-Send) packet to the intended receiver
ndash The RTS packet contains the length of the data that needs to be transmittedndash Any station other than the intended recipient hearing RTS defers transmission for atime duration equal to the end of the corresponding CTS reception
bull The receiver sends back CTS (Clear-to-Send) packet back to sender if it isavailable to receivendash The CTS packet contains the length of the data that original sender wants totransmitndash Any station other than the original RTS sender hearing CTS defers transmissionuntil the data is sent
bull The original sender upon reception of the CTS starts transmitting
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 49
Hidden terminal problem
Ad hoc Sensor Networks Multiple channel access
Random Access MACA (contrsquo)Exposed terminal problem
Waiting time of node X is muchsmaller than waiting time of node C
bull C defers transmission upon hearing BrsquosRTS until B could get CTS from Abull After that C can start transmission to DFor that it first sends an RTSbull C is not longer exposed to the datatransmission of B
Problem It does not address the collision of RTS packet
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 50
Random Access reliabilitybull Wireless links are prone to have errors High packet loss rate detrimental to
transport-layer performance Mechanisms needed to reduce packet loss rateexperienced by upper layers
bull When node B receives a data packet from node A node B sends anAcknowledgement (Ack) This approach adopted in many protocols
bull If node A fails to receive an Ack it will retransmit the packet
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 51
Random Access MACAW (CSMACA) bull Physical carrier sense and virtual carrier sense using Network Allocation
Vector (NAV)ndash NAV is updated based on overheard RTSCTSDATAACK packets each of whichspecified duration of a pending transmissionndash Nodes stay silent when carrier sensed (physicalvirtual)
bull Backoff intervals used to reduce collision probabilityndash When transmitting a packet choose a backoff interval in the range [0w] where wis contention windowndash Count down the backoff interval when channel is idlendash Count down is suspended if channel becomes busyndash When backoff interval reaches 0 transmit RTS
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 52
Random Access CSMACA (contrsquo) bull The time spent counting down backoff intervals is a part of MAC overhead
ndash Choosing a large w leads to large backoff intervals and can result in larger overheadndash Choosing a small w leads to a larger number of collisions (when two nodes countdown to 0 simultaneously)ndash Since the number of nodes attempting to transmit simultaneously may change withtime some mechanism to manage contention is needed
bull IEEE 80211 DCF contention window w is chosen dynamically depending oncollision occurrence (binary exponential backoff)ndash When a node fails to receive CTS in response to its RTS it increases the contentionwindowndash w is doubled (up to an upper bound)ndash When a node successfully completes a data transfer it restores w to wmin
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 53
Random Access fairnessbull Simplest definition of fairness all nodes should receive equal bandwidthbull An example of unfairness
ndash Assume that initially A and B both choose a backoff interval in range [031] buttheir RTSs collidendash Nodes A and B then choose from range [063]ndash Node A chooses 4 slots and B choose 60 slotsndash After A transmits a packet it next chooses from range [031]ndash It is possible that A may transmit several packets before B transmits its first packet
Ad hoc Sensor Networks Multiple channel access
Unfairness occurs when one node has backed off much more than some other node
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 54
Random Access fairness (contrsquo) MACAW solution for fairnessbull When a node transmits a packet it appends value w to the packet all nodes
hearing value w use it for their future transmission attemptsbull Since w is an indication of the level of congestion in the vicinity of a specific
receiver node MACAW proposes maintaining w independently for eachreceiver
bull Using per-receiver wi is particularly useful in multi-hop environments sincecongestion level at different receivers can be very different
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 55
Random Access energy efficiency bull Since many mobile hosts are operated by batteries MAC
protocols which conserve energy are of interestbull Two approaches to reduce energy consumption
ndash Power save turn off wireless interface when desirablendash Power control reduce transmit power
bull Power control has some more potential benefitsndash Reduced interference (in ad hoc networks) eg it improves ALOHAefficiency where user with high power can capture a packet even if there is acollisionndash Increased spatial reuse (in infrastructured wireless networks) eg it is usedin CDMA to maintain target SIR of voice and data users or generally it canbe used to maintain target SIR for different user classes (admission control)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 56
Random Access power control bull When C transmits to D at a high power level B cannot receive Arsquos
transmission due to interference from Cbull If C reduces transmit power it can still communicate with D
ndash Reduces energy consumption at node Cndash Allows B to receive Arsquos transmission (spatial reuse)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 57
bull A special wireless ad hoc networkndash Large number of nodesndash Battery poweredndash Topology and density changendash Nodes for a common taskndash In-network data processing
bull Sensor-net applicationsndash Sensor-triggered bursty trafficndash Can often tolerate some delay
Ad hoc Sensor Networks Multiple channel access
Characteristics of Sensor Network
Scalabilty and self configurationEnergy efficiencyAdaptivityFairness not importantLatency
AdaptivityEnergy-delay tradoff
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 58
Ad hoc Sensor Networks Multiple channel access
Wireless sensor node and sensor network
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 59
MAC for WSN attributes bull Attributes
ndash Collision avoidance (Basic task)ndash Energy efficiency (most primary in WSN)ndash Scalability and adaptivityndash Channel utilizationndash Latencyndash Throughputndash Fairness
bull What causes energy wastendash Collisionsndash Control packet overheadndash Overhearing unnecessary trafficndash Long idle time (it consumes 50-100 of the power)
Ad hoc Sensor Networks Multiple channel access
Primary
Secondary
Dominant factor
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 60
MAC for WSN
bull Contention-based protocols need to work hard in all directions for energysavingsndash Reduce idle listening ndash support low duty cyclendash Better collision avoidancendash Reduce control overheadndash Avoid unnecessary overhearing
Ad hoc Sensor Networks Multiple channel access
Scheduled Protocols Contention ProtocolsCollisions No YesEnergy efficiency Good BadScalability and adaptivity Bad GoodMulti-hop communication Difficult EasyTime synchronization Strict Loose or not required
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 61
MAC for WSN classification
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 62
MAC for WSN SMAC (2002)bull It has been developed by Ye Heidemann and Estrinbull Tradeoffs between (latency fairness) and energybull Major components in S-MAC
ndash Periodic listen and sleep turn off radio when sleeping the reduced dutycycle is ~ 10 (eg 120ms on 12s off)ndash Collision avoidance is based on contention similarly to IEEE 80211 adhoc modendash Overhearing avoidance the node is sleeping when neighbors talkndash Massage passing long message is fragmented amp sent in burst operationwith extended Tx time and re-transmit occurs immediately
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 63
Summarybull The algorithms and protocols that enable sharing by multiple users and
controls the access to the wireless channel from different users are calledMAC
bull There are two types of wireless networks Infrastructured wireless networksand ad hoc wireless networks
bull We also want our wireless system to have high user capacity and we canidentify 10 methods for increasing it
bull There are several MACs with no absolute advantage it should be tailoredto the system requirements
bull There are primary concerns for designing a WSN MAC (collisionavoidance energy efficiency scalability and adaptivity) while thesecondary concerns are as follows (channel utilization latency throughputfairness)
bull Next lecture Routing protocols
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3
Ad hoc Sensor Networks Multiple channel access
Lecture 5 reviewbull Signal space representationbull Optimal detection of signal in AWGN (Bayesian decision)bull Probability of error (BER and SER)bull Demodulation and detection for modulation schemesbull BER in fading channelbull Channel equalization
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 4
Outlinebull The goal of medium access controlbull Types of wireless networksbull Duplexing techniquesbull Multiple Accessbull Random Accessbull MAC for Wireless Sensor Networks
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 5
Ad hoc Sensor Networks Multiple channel access
Structure of a wireless communications link
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 6
Ad hoc Sensor Networks Multiple channel access
Layered Communication Approaches
2 Data Link layer it handles access to the unerlyingchannel and defines the data format It is split into twosublayersndash Logical Link Control it manages frames to upper and
lower layer (encapsulation decapsulation) and it enforceserrror control (checksum and parity bits)
ndash Medium Access Control (MAC) it coordinatestransmission between users sharing the spectrum Inwireless systems it must address the hidden terminalproblem and must exercise power control Goals preventcollisions while maximizing throughput and minimizingdelay
Physical
OSI reference model
Data link
Network
Transport
Middleware
Application
Recall from Chapter 1
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 7
Ad hoc Sensor Networks Multiple channel access
Medium Access Control
bull Thus the wireless spectrum (frequency band) is a very precious and limited resourcewe need to use this resource very efficiently
bull We also want our wireless system to have high user capacity with QoS constraintsbull The algorithms and protocols that enables this sharing by multiple users and
controlscoordinates the access to the wireless channel (medium) from differentusers are called MEDIUM ACCESS or MEDIA ACCESS or MULTIPLE ACCESSprotocols techniques schemes etchellip)
Designbull Goal Function Maximum Utilization
(eg throughput user capacity hellip)bull Constraints QoS
(eg delaylt10ms BERlt10-5hellip)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 8
Ad hoc Sensor Networks Multiple channel access
Medium Access Control (contrsquo)bull Multiple Access (Channel Partitioning or Coordinated Schemes)
ndash Techniques TDMA FDMA CDMA SDMAndash Examples GSM 3Ghellip
bull Random Access (or Random Schemes)ndash Techniques MACA MACAW Aloha 80211 MAChellipndash Examples Wifi WSNhellip
bull Polling based schemesndash Access is coordinated by a central nodendash Examples Bluetooth BlueSkyhellip
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 9
Ad hoc Sensor Networks Multiple channel access
bull Infrastructured networksndash base stations are the bridgesndash a mobile host will communicate with the
nearest base station (or access point)ndash handoff is taken when a host roams from one
base to anotherndash Medium access control assisted by BS
bull Ad hoc networksndash infrastructureless no fixed base stationsndash without the assistance of base stations for
communicationndash Due to transmission range constraint two
wireless nodes need multi-hop routing forcommunication
ndash quickly and unpredictably changing topology
Two types of wireless networks
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 10
Ad hoc Sensor Networks Multiple channel access
Two types of wireless networks (contrsquo)
bull Mesh networks serve as access networks that employ multi-hop wireless forwarding bynon-mobile nodes to relay traffic to and from the wired Internet In such an environmenthybrid wireless network technologies andor hierarchical network organization can beused for ad hoc and infrastructure wireless links
bull Wireless Sensor Networks are a special category of ad hoc networks that are used toprovide a wireless communication infrastructure among the sensors deployed in a specificapplication domain [rarr see Chapter 9 - 11]
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 11
Ad hoc Sensor Networks Multiple channel access
Infrastructured wireless networks Ad hoc wireless networksFixed infrastructure-based Infrastructureless
Guaranteed bandwidth Shared radio channel
Seamless connectivity (low call drops during handoffs)
Frequent path breaks due to mobility
High cost and time of deployment Quick and cost-effective deployment
Reuse of frequency spectrum (cellular principle) Dynamic frequency reuse based on carrier sense mechanism
Application domains include mainly civilian and commercial sectors
Application domains include battlefields emergency search and rescue operations
High cost of network maintenance Self-organization and maintenance properties are built into the network
Widely deployed and currently in the third generation of evolution
Several issues are to be addressed for successful commercial deployment even though widespread use exists in defense
Two types of wireless networks (contrsquo)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 12
Ad hoc Sensor Networks Multiple channel access
Duplexing
bull If the communication between two parties is one way then it is called simplexcommunication Simplex communication is achieved by default by using a singlewireless channel (frequency band) to transmit from sender to receiver
bull If the communication between two parties is two- way then it is called duplexcommunication Duplex communication achieved by
ndash Time Division (TDD) (famous in cellular systems)ndash Frequency Division (FDD) (famous in cellular systems)ndash Some other method like a random access method
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 13
Ad hoc Sensor Networks Multiple channel access
FDDbull A duplex channel consists of two
simplex channels with different carrierfrequencies
ndash Downlink band carries traffic frombase to mobile
ndash Uplink band carries traffic frommobile to base
Duplexing(contrsquo)TDD
bull channel (carrier frequency) is shared intime in a deterministic manner
ndash The time is slotted with fixed slotlength (sec)
ndash Some slots are used for downlinkchannel
ndash Some slots are used for uplink channel
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 14
Ad hoc Sensor Networks Multiple channel access
FDDbull FDD is used in radio systems that can
allocate individual radio frequenciesfor each user
bull For example analog systems AMPSbull In FDD channels are allocated by a
base stationbull A channel for a mobile is allocated
dynamicallybull All channels that a base station will use
are allocated usually staticallybull More suitable for wide-area cellular
networks GSM AMPS all use FDD
Duplexing(contrsquo)TDD
bull Can only be used in digital wirelesssystems (digital modulation)
bull Requires rigid timing andsynchronization
bull Mostly used in short-range and fixedwireless systems so that propagationdelay between base and mobile do notchange much with respect to locationof the mobile
bull Such as cordless phoneshellip
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 15
Ad hoc Sensor Networks Multiple channel access
Multiple Access narrow- vs wideband systemsbull Narrowband System
ndash The bandwidth is small compared to the coherence bandwidth of the channel(BltBcoh)
bull Wideband Systemndash The system bandwidth is much larger that the coherence bandwidth of the
multipath channel (BgtBcoh) A large number of users can access the same channel(frequency band) at the same time
bull Four major multiple access (MA) schemesndash Time Division Multiple Access (TDMA) it could be used in narrowband or
wideband systemndash Frequency Division Multiple Access (FDMA) it is usually used in narrowband
systemndash Code Division Multiple Access (CDMA) it is used in wideband systemndash Space Division Multiple Access (SDMA) for wireless systems with multiple
antennas it can be combined with all of the other MA methods
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 16
Ad hoc Sensor Networks Multiple channel access
Multiple Access cellular standards
Cellular System Multiple Access TechniqueAMPS (2G) FDMAFDD
CT2 Cordless Phone FDMATDD
DECT Cordless Phone FDMATDD
GSM (2G) TDMAFDD
USDC (IS-54 and IS-136) (2G) TDMAFDD
Personal Digital Cellular (2G) TDMAFDD
US IS-95 (2G) CDMAFDD
W-CDMA (3G) CDMAFDD CDMATDD
cdma2000 (3G) CDMAFDD CDMATDD
Narrowband System
Wideband System
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 17
Ad hoc Sensor Networks Multiple channel access
bull Individual radio channels are assigned toindividual users Each user is allocated a Wfrequency band by BS
bull If the channel allocated to a user is idle then itis not used by someone else waste of resource
bull Mobile and base can transmit and receivesimultaneously (FDD)
bull The W bandwidth of FDMA channels arerelatively low Symbol time is usually larger(low data rate) than the delay spread of themultipath channel (implies that inter-symbolinterference is low)
bull Lower complexity systems than TDMAsystems
Multiple Access FDMA
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 18
Ad hoc Sensor Networks Multiple channel access
Multiple Access FDMA (contrsquo)bull Capacity of FDMA systems
Nc = (B-2Bguard)Wwhere B is the total spectrum allocation Bguard is the guard band allocated atthe edge of the spectrum band W is the bandwidth of an individual channel
bull Erlang B and Erlang C system assumesndashinfinite population of sources which jointly offer traffic to Nc channelndashcall attempts arrive following a Poisson process so call arrivals are independentThe average arrival rate is λndashmessage length (holding times) are exponentially distributed (Markovian system)The average call length is hndashthe total amount of traffic offered in erlangs L= λhndashIf a user is rejected his next call attempt is made statistically independent of theprevious attempt (Erlang B)ndashIf all the channels are busy when a request arrives from a user the request isqueued An unlimited number of requests may be held in the queue (Erlang C)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 19
Ad hoc Sensor Networks Multiple channel access
bull Statictical measures of offered load Ncndash In Erlang B system the probability of call
blocking
ndash In Erlang C system the probabilty of waiting
ndash And the average wait time
c
c
cBLOCK
0
Pr
N
Nk
k
LN
Lk
=
=
sum
c
cc
wait 1
c0c
Pr 1
N
NkN
k
LL LL N kN
minus
=
=⎛ ⎞
+ minus⎜ ⎟⎝ ⎠
sum
wait waitc
Pr htN L
=minus
Multiple Access FDMA (contrsquo)Recall from
Queuing theory
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 20
Ad hoc Sensor Networks Multiple channel access
Multiple Access example of Erlang B systembull In an Erlang B system 40 channels are available A blocking probabilty of
less than 1 is required What is the traffic that can be serve if there is oneoperator or three operators
bull (Solution L(1 operator)gtL(3 operators))bull (Note The number of users increases faster than linearly with the number of
available channels The difference between actual increase and linearincrease is called the trunking gain It is preferable to have a large pool ofavailable channels that serves all users (a single operator) The reasons fornot choosing this approach are political not technical)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 21
Ad hoc Sensor Networks Multiple channel access
Multiple Access TDMAbull The allocated radio spectrum for the system is divided into time slots
ndash In each slot a user can transmit or receivendash A user occupies a cyclically repeating slotsndash A channel is logically defined as a particular time slot that repeats with some
periodbull TDMA systems buffer the data until its turn (time slot) comes to transmitbull In TDMATDD half of the slots in the frame is used for uplink channels the
other is used for downlink channelsbull In TDMAFDD a different carrier frequency is used for uplink or downlink
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 22
Ad hoc Sensor Networks Multiple channel access
Multiple Access TDMA (contrsquo)bull Preamble contains address and synchronization info to identify base
station and mobiles to each otherbull Guard times are used to allow synchronization of the receivers between
different slots and framesndash Different mobiles may have different propagation delays to a base station
because of different distances
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 23
Multiple Access TDMA (contrsquo)bull Each frame contains overhead bits and data bits Efficiency of frame is
defined as the percentage of data (information) bits to the total frame size inbits
η =(1 ndash boverheadbtotal) 100 where btotal= Tf R is the total number of bits in a frame Tf is the frameduration in sec boverhead is the number of overhead bits (GSM efficiency 73)
bull TDMA is usually combined with FDMAndash Neighboring cells are allocated and using different carrier frequencies (FDMA)
Inside a cell TDMA can be used Cells may be re-using the same frequency if theyare far from each-other
ndash There may be more than one channel allocated and used inside each cell Eachcarrier frequency (radio channel) may be using TDMA to further multiplex moreuser (ie having TDMA logical channels inside radio channels) For example inGSM each radio channel has 200KHz bandwidth and has 8 time slots (8 logicalchannels) Hence GSM is using FDMA combined with TDMA
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 24
Multiple Access TDMA (contrsquo)
Ad hoc Sensor Networks Multiple channel access
GSM (Europa) IS-54 (USA) PDC (Japan) DECT
Bit Rate 2708 Kbps 486 Kbps 42 Kbps 1152 Mbps
Bandwidth 200 KHz 30 KHz 25 KHz 1728 MHz
Time Slot 0577 ms 67 ms 67 ms 0417 ms
Upstream slots per frame 8 3 3 12
Duplexing FDD FDD FDD TDD
Efficiency 73 80 80 67
Modulation GMSK π4 DQPSK π4 DQPSK GMSK
Adaptive equalized Mandatory Mandatory Optional None
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 25
Multiple Access TDMA (contrsquo)bull Enables the sharing of a single radio channel among N usersbull Requires high data-rate per radio channel to support N users
simultaneously which requires adaptive equalizers to be usedin multipath environments
bull Transmission occurs in bursts (not continually) It enablespower saving by going to sleep modes in unrelated slots Itdiscontinues transmission and also enables mobile assistedhandoff
bull It equires synchronization of the receivers Need guard bits andsync bits which occurs large overhead per slot
bull Allocation of slots to mobile users should not be uniform Itmay depend on the traffic requirement of mobiles This bringsextra flexibility and efficiency compared to FDMA systems
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2620111015 TAacuteMOP ndash 412-082AKMR-2009-0006 26
Ad hoc Sensor Networks Multiple channel access
Multiple Access spread spectrum multiple accessbull SSMA uses signals that have transmission bandwidth that is
several orders of magnitude larger than minimum requiredRF bandwidth
bull It providesndash Immunity to multipath interferencendash Robust multiple access
bull Two techniquesndash Frequency Hopped Multiple Access (FHMA)ndash Direct Sequence Multiple Access (DSMA) or Code Division
Multiple Access (CDMA)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2720111015 TAacuteMOP ndash 412-082AKMR-2009-0006 27
Ad hoc Sensor Networks Multiple channel access
Multiple Access FHMAbull The carrier frequency of users are varied in a pseudo-random
fashionndash Each user is using a narrowband channel (spectrum) at a specific
instance of timendash The random change in frequency make the change of using the same
narrowband channel very lowbull The sender receiver change frequency (calling hopping)
using the same pseudo-random sequence hence they aresynchronized
bull Rate of hopping versus Symbol ratendash If hopping rate is greather Fast Frequency Hoppingndash If symbol rate is greater Slow Frequency Hopping
bull GSM and Bluetooth are example systems
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2820111015 TAacuteMOP ndash 412-082AKMR-2009-0006 28
Ad hoc Sensor Networks Multiple channel access
Multiple Access FHMA (contrsquo)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2920111015 TAacuteMOP ndash 412-082AKMR-2009-0006 29
Ad hoc Sensor Networks Multiple channel access
Multiple Access CDMAbull The narrowband message signal is multiplied by a very large bandwidth
signal called spreading signal (code) before modulation and transmissionover the air This is called spreading
bull Spreading signal use a pseudo-noise (PN) sequence (a pseudo-randomsequence) called codeword which are orthogonal (low autocorrelation)
bull The receiver correlator distinguishes the senders signal by examining thewideband signal with the same time-synchronized spreading code(despreading)
bull Advantagesndash Low power spectral densityndash Interference limited operationndash Privacyndash Reduction of multipath affects by using a larger spectrumndash Users can start their transmission at any time (random access possible)ndash Cell capacity is soft and higher than TDMA and FDMAndash No frequency management and no equalizers and quard time needed
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3020111015 TAacuteMOP ndash 412-082AKMR-2009-0006 30
Ad hoc Sensor Networks Multiple channel access
Multiple Access near far problem and power controlbull At a receiver the signals may come from various sources (multiuser) The
strongest signal usually captures the modulator The other signals areconsidered as noise Each source may have different distances to the basestation
bull In CDMA we want a BS to receive CDMA coded signals from variousmobile users at the same time
ndash Therefore the receiver power at the BS for all mobile users should be close toeach other This requires power control at the mobiles
bull Power Control BS monitors the RSSI (Received Signal Strength Indicator)values from different mobiles and then sends power change commands to themobiles over a forward channel The mobiles then adjust their transmitpower
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3120111015 TAacuteMOP ndash 412-082AKMR-2009-0006 31
Ad hoc Sensor Networks Multiple channel access
Multiple Access hybrid spread spectrum techniquesbull FDMACDMA
ndash Available wideband spectrum is divided into a number narrowband radio channels CDMAis employed inside each channel
bull DSFHMAndash The signals are spread using spreading codes (direct sequence signals are obtained) but
these signal are not transmitted over a constant carrier frequency they are transmitted over afrequency hopping carrier frequency
bull Time Division CDMA (TCDMA)ndash Each cell is using a different spreading code (CDMA employed between cells) that is
conveyed to the mobiles in its rangendash Inside each cell (inside a CDMA channel) TDMA is employed to multiplex multiple users
bull Time Division Frequency Hoppingndash At each time slot the user is hopped to a new frequency according to a pseudo-random
hopping sequencendash Employed in severe co-interference and multi-path environmentsndash Bluetooth and GSM are using this technique
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3220111015 TAacuteMOP ndash 412-082AKMR-2009-0006 32
Ad hoc Sensor Networks Multiple channel access
Multiple Access comparison TDMAFDMACDMA
Techniques TDMA FDMA CDMA SDMA
Idea
segment sending time into disjoint time-slots
Segment the frequency band into disjoint sub-bands
Spread the spectrum using orthogonal codes
Segment space into sectors
Signal separation
Synchronization in the time domain
Filtering in the frequency domain
Code Directed antennas
Advantage
Established flexible
Simple established robust
Flexible less frequency planning needed soft handover
Simple increases capacity
DisadvantageQuard space synchronization difficult
Inflexible scarce source
Complex receiver power control
inflexible
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3320111015 TAacuteMOP ndash 412-082AKMR-2009-0006 33
Ad hoc Sensor Networks Multiple channel access
User capacity per cell
The user capacity C is defined as the total number of active users percell that the system can support while meeting a common BERconstrain For orthogonal multiple access (as FDMA and TDMA)
C = Ncellwhere Ncell is the number of channels assigned to any given cellThe total number of orthogonal channels of bandwidth Bs that can becreated from a total system bandwidth of B is B Bs The reuse factorsatisfies N = (B Bs) Ncell this implies
C = (B Bs) N
Recall from Chapter 1
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3420111015 TAacuteMOP ndash 412-082AKMR-2009-0006 34
Ad hoc Sensor Networks Multiple channel access
10 methods for increasing capacity1 Increasing the amount of spectrum used very expensive2 More efficient modulation format and coding3 Better source coding4 Adaptive modulation and coding5 Discontinuous transmission6 Multiuser detection CDMA systems7 Reduction of cell radius effective but very expensive smaller
cells require more handovers8 Use of sector cells tripled cells have tripled BS antennas9 Use of an overlay structure10 Multiple antennas diversity MIMO systems SDMA
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3520111015 TAacuteMOP ndash 412-082AKMR-2009-0006 35
Ad hoc Sensor Networks Multiple channel access
bull Distributed operation is requiredbull Synchronization is required in TDMA-based systemsbull Hidden terminals are nodes hidden from a senderbull Exposed terminals are exposed nodes preventing a sender
from sendingbull Throughput needs to be maximizedbull Access delay should be minimizedbull Fairness refers to provide an equal share to all competing
nodesbull Real-time traffic support is required for voice video and
real-time databull Resource reservation is required for QoSbull Ability to measure resource availability handles the
resourcesbull Capability for power control reduces the energy
consumption
Random access in adhoc networks
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3620111015 TAacuteMOP ndash 412-082AKMR-2009-0006 36
Ad hoc Sensor Networks Multiple channel access
Random accessbull All random access techniques are based on packetized data or packet radio where user
date is collected into packets of M bitsbull Collision if packets from different users overlap in timebull The transmission time of a packet is τ = M R where R [bps] the data ratebull Analysis of random access techniques assumes that the users generate packets according
to memoryless Poisson process at a rate λ [packets per unit time] the probability that thenumber of packet arrivals in a time period [0t] denoted by X(t) is equal to integer k isgiven by
Pr(X(t)=k) = (λt)k k e-λt
bull The traffic load is defined as L = λ τ If L gt 1 means on avarage more packets arrive inthe system over a given time period than can be transmitted in that period so systemswith Lgt1 are unstable
bull The throughput is defined as the ratio of the average number of packets successfullytransmitted in any given time interval divided by the number of attempted transmissionsin that interval
T = L Pr(succesful packet transmission)bull The effective data rate of the system is RT
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3720111015 TAacuteMOP ndash 412-082AKMR-2009-0006 37
Ad hoc Sensor Networks Multiple channel access
Random access (contrsquo)bull We will examine the following attributes
ndash Channel utilization (Throughput)ndash Latencyndash Collision avoidance (Hidden and exposed terminals problem)ndash Reliability (ACK)ndash Energy efficiency (power control)ndash Fairnessndash Throughput needs to be maximized
bull We will discuss the following techniquesndash Aloha and slotted Alohandash CSMA Protocols (1-persistent non-persistent p-persistent CSMA CSMACD)ndash MACAndash MACAWndash Energy efficient MAC in wireless sensor networks (eg SMAC)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 38
Random Access ALOHA (1970)bull ALOHA was developed for a wireless system at the University of Hawaii
(Abramson et al)ndash Multiple remote stations plus one base stationndash Frame transmissions are made at one frequency from a remote station to the basestation the base station re-broadcasts frames on another frequency Contention is foraccess to the base station
bull Frames can be sent at any time to the base stationndash If no acknowledgment (ACK) is received assume that the frame is lost andretransmit after waiting a random amount of timendash If more than one station broadcasts on the base station access frequency at the sametime ndash a collision ndash interference will destroy the frames
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 39
Random Access ALOHA (contrsquo)
bull The probability that no packets generated during the tima [-τ τ] is given by (nocollision)
Pr(X(t)=0) = (λ2τ)0 0 e-2λτ = e-2Lwith corresponding throughput
T = L e-2L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 05 05e 018
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 40
Random Access slotted ALOHA (1971)bull Improvement Time is slotted and a packet can only be transmitted at the beginning of
one slot Thus it can reduce the collision duration
bull The probability that no packets generated during the time [0 τ] is given by (nocollision)
Pr(X(t)=0) = (λτ)0 0 e-λτ = e-Lwith corresponding throughput T = L e-L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 1 e 037
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 41
bull Slotted Aloha has double the maximumthroughput as pure Aloha and achievesthis maximum at a higher offered loadThe effective data rate is still less than40 of the raw transmission rate
bull Aloha does not listen to the carrier beforetransmission more sophisticatedtechniques are needed to increaseefficiency
bull Further problem the delayndashDelay
ndashThe mean packet delay
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
( ) ( )delayPr 1 e ekL Lkτ minus minus= = minus
( )delay1
1 e e ekL L L
kMPD E kτ
infinminus minus
=
= = minus =sum
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 42
Random Access Carrier Sense Multiple Accessbull CSMA (Carrier Sense Multiple Access) listen to the carrier before transmission
and transmits if channel is idlebull Detection delay and propagation delay are two important parameters
ndash Detection delay time required to sense the carrier and decide if it is idle or busyndash Propagation delay distancespeed_of_ligth The time required for bit to travel
from transmitter to the receiver
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 43
Random Access CSMA variations
Ad hoc Sensor Networks Multiple channel access
1-persistent CSMA Non-persistent CSMA
There will always be a collision iftwo nodes want to retransmit(usually you stop transmissionattempts after few tries)
Random backoff reduces probabilityof collisions Waste idle time if thebackoff time is too long
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 44
Random Access CSMA variations (contrsquo)
Ad hoc Sensor Networks Multiple channel access
p-persistent CSMA CSMA CD (Collision Detection)
A good tradeoff between non-persistent and 1-persistent CSMA
Same with CSMA however a stationalso listen to the carrier whiletransmitting to see if the transmissioncollides with someone elsetransmission It can be used in listen-while-talk capable channels (fullduplex) In single radio channels thetransmission need to be interrupted inorder to sense the channel
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 45
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 46
Random Access CSMA (contrsquo)bull How to select probability p
ndash Assume that M nodes have a packet to send and the medium is busy then Mp is theexpected number of nodes that will attempt to transmit once the medium becomesidle If Mp gt 1 then a collision is expected to occur Therefore the network mustmake sure that Mp lt 1 to avoid collision where M is the maximum number of nodesthat can be active at a time
bull Problems1 The mean packet delay increases exponently with increasing offered load2 CSMA protocols sense the carrier but sensing the carrier does not always releasestrue information about the status of the wireless channel There are two problems thatare unique to wireless channels (different than wireline channels)ndash Hidden terminal problemndash Exposed terminal problem
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 47
Hidden terminal problembull A sends to B C cannot receive Abull C wants to send to B C senses a
ldquofreerdquo medium (CS fails)bull collision at B A cannot receive the
collision (CD fails)bull A is ldquohiddenrdquo for C
Ad hoc Sensor Networks Multiple channel access
Random Access hidden and exposed terminal problemExposed terminal problem
bull B sends to A C wants to send to Dbull C has to wait CS signals a medium
in usebull since A is outside the radio range of
C waiting is not necessarybull C is ldquoexposedrdquo to B
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 48
Random Access MACAWhen a station wants to transmit databull It sends an RTS (Ready-to-Send) packet to the intended receiver
ndash The RTS packet contains the length of the data that needs to be transmittedndash Any station other than the intended recipient hearing RTS defers transmission for atime duration equal to the end of the corresponding CTS reception
bull The receiver sends back CTS (Clear-to-Send) packet back to sender if it isavailable to receivendash The CTS packet contains the length of the data that original sender wants totransmitndash Any station other than the original RTS sender hearing CTS defers transmissionuntil the data is sent
bull The original sender upon reception of the CTS starts transmitting
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 49
Hidden terminal problem
Ad hoc Sensor Networks Multiple channel access
Random Access MACA (contrsquo)Exposed terminal problem
Waiting time of node X is muchsmaller than waiting time of node C
bull C defers transmission upon hearing BrsquosRTS until B could get CTS from Abull After that C can start transmission to DFor that it first sends an RTSbull C is not longer exposed to the datatransmission of B
Problem It does not address the collision of RTS packet
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 50
Random Access reliabilitybull Wireless links are prone to have errors High packet loss rate detrimental to
transport-layer performance Mechanisms needed to reduce packet loss rateexperienced by upper layers
bull When node B receives a data packet from node A node B sends anAcknowledgement (Ack) This approach adopted in many protocols
bull If node A fails to receive an Ack it will retransmit the packet
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 51
Random Access MACAW (CSMACA) bull Physical carrier sense and virtual carrier sense using Network Allocation
Vector (NAV)ndash NAV is updated based on overheard RTSCTSDATAACK packets each of whichspecified duration of a pending transmissionndash Nodes stay silent when carrier sensed (physicalvirtual)
bull Backoff intervals used to reduce collision probabilityndash When transmitting a packet choose a backoff interval in the range [0w] where wis contention windowndash Count down the backoff interval when channel is idlendash Count down is suspended if channel becomes busyndash When backoff interval reaches 0 transmit RTS
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 52
Random Access CSMACA (contrsquo) bull The time spent counting down backoff intervals is a part of MAC overhead
ndash Choosing a large w leads to large backoff intervals and can result in larger overheadndash Choosing a small w leads to a larger number of collisions (when two nodes countdown to 0 simultaneously)ndash Since the number of nodes attempting to transmit simultaneously may change withtime some mechanism to manage contention is needed
bull IEEE 80211 DCF contention window w is chosen dynamically depending oncollision occurrence (binary exponential backoff)ndash When a node fails to receive CTS in response to its RTS it increases the contentionwindowndash w is doubled (up to an upper bound)ndash When a node successfully completes a data transfer it restores w to wmin
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 53
Random Access fairnessbull Simplest definition of fairness all nodes should receive equal bandwidthbull An example of unfairness
ndash Assume that initially A and B both choose a backoff interval in range [031] buttheir RTSs collidendash Nodes A and B then choose from range [063]ndash Node A chooses 4 slots and B choose 60 slotsndash After A transmits a packet it next chooses from range [031]ndash It is possible that A may transmit several packets before B transmits its first packet
Ad hoc Sensor Networks Multiple channel access
Unfairness occurs when one node has backed off much more than some other node
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 54
Random Access fairness (contrsquo) MACAW solution for fairnessbull When a node transmits a packet it appends value w to the packet all nodes
hearing value w use it for their future transmission attemptsbull Since w is an indication of the level of congestion in the vicinity of a specific
receiver node MACAW proposes maintaining w independently for eachreceiver
bull Using per-receiver wi is particularly useful in multi-hop environments sincecongestion level at different receivers can be very different
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 55
Random Access energy efficiency bull Since many mobile hosts are operated by batteries MAC
protocols which conserve energy are of interestbull Two approaches to reduce energy consumption
ndash Power save turn off wireless interface when desirablendash Power control reduce transmit power
bull Power control has some more potential benefitsndash Reduced interference (in ad hoc networks) eg it improves ALOHAefficiency where user with high power can capture a packet even if there is acollisionndash Increased spatial reuse (in infrastructured wireless networks) eg it is usedin CDMA to maintain target SIR of voice and data users or generally it canbe used to maintain target SIR for different user classes (admission control)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 56
Random Access power control bull When C transmits to D at a high power level B cannot receive Arsquos
transmission due to interference from Cbull If C reduces transmit power it can still communicate with D
ndash Reduces energy consumption at node Cndash Allows B to receive Arsquos transmission (spatial reuse)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 57
bull A special wireless ad hoc networkndash Large number of nodesndash Battery poweredndash Topology and density changendash Nodes for a common taskndash In-network data processing
bull Sensor-net applicationsndash Sensor-triggered bursty trafficndash Can often tolerate some delay
Ad hoc Sensor Networks Multiple channel access
Characteristics of Sensor Network
Scalabilty and self configurationEnergy efficiencyAdaptivityFairness not importantLatency
AdaptivityEnergy-delay tradoff
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 58
Ad hoc Sensor Networks Multiple channel access
Wireless sensor node and sensor network
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 59
MAC for WSN attributes bull Attributes
ndash Collision avoidance (Basic task)ndash Energy efficiency (most primary in WSN)ndash Scalability and adaptivityndash Channel utilizationndash Latencyndash Throughputndash Fairness
bull What causes energy wastendash Collisionsndash Control packet overheadndash Overhearing unnecessary trafficndash Long idle time (it consumes 50-100 of the power)
Ad hoc Sensor Networks Multiple channel access
Primary
Secondary
Dominant factor
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 60
MAC for WSN
bull Contention-based protocols need to work hard in all directions for energysavingsndash Reduce idle listening ndash support low duty cyclendash Better collision avoidancendash Reduce control overheadndash Avoid unnecessary overhearing
Ad hoc Sensor Networks Multiple channel access
Scheduled Protocols Contention ProtocolsCollisions No YesEnergy efficiency Good BadScalability and adaptivity Bad GoodMulti-hop communication Difficult EasyTime synchronization Strict Loose or not required
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 61
MAC for WSN classification
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 62
MAC for WSN SMAC (2002)bull It has been developed by Ye Heidemann and Estrinbull Tradeoffs between (latency fairness) and energybull Major components in S-MAC
ndash Periodic listen and sleep turn off radio when sleeping the reduced dutycycle is ~ 10 (eg 120ms on 12s off)ndash Collision avoidance is based on contention similarly to IEEE 80211 adhoc modendash Overhearing avoidance the node is sleeping when neighbors talkndash Massage passing long message is fragmented amp sent in burst operationwith extended Tx time and re-transmit occurs immediately
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 63
Summarybull The algorithms and protocols that enable sharing by multiple users and
controls the access to the wireless channel from different users are calledMAC
bull There are two types of wireless networks Infrastructured wireless networksand ad hoc wireless networks
bull We also want our wireless system to have high user capacity and we canidentify 10 methods for increasing it
bull There are several MACs with no absolute advantage it should be tailoredto the system requirements
bull There are primary concerns for designing a WSN MAC (collisionavoidance energy efficiency scalability and adaptivity) while thesecondary concerns are as follows (channel utilization latency throughputfairness)
bull Next lecture Routing protocols
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 4
Outlinebull The goal of medium access controlbull Types of wireless networksbull Duplexing techniquesbull Multiple Accessbull Random Accessbull MAC for Wireless Sensor Networks
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 5
Ad hoc Sensor Networks Multiple channel access
Structure of a wireless communications link
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 6
Ad hoc Sensor Networks Multiple channel access
Layered Communication Approaches
2 Data Link layer it handles access to the unerlyingchannel and defines the data format It is split into twosublayersndash Logical Link Control it manages frames to upper and
lower layer (encapsulation decapsulation) and it enforceserrror control (checksum and parity bits)
ndash Medium Access Control (MAC) it coordinatestransmission between users sharing the spectrum Inwireless systems it must address the hidden terminalproblem and must exercise power control Goals preventcollisions while maximizing throughput and minimizingdelay
Physical
OSI reference model
Data link
Network
Transport
Middleware
Application
Recall from Chapter 1
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 7
Ad hoc Sensor Networks Multiple channel access
Medium Access Control
bull Thus the wireless spectrum (frequency band) is a very precious and limited resourcewe need to use this resource very efficiently
bull We also want our wireless system to have high user capacity with QoS constraintsbull The algorithms and protocols that enables this sharing by multiple users and
controlscoordinates the access to the wireless channel (medium) from differentusers are called MEDIUM ACCESS or MEDIA ACCESS or MULTIPLE ACCESSprotocols techniques schemes etchellip)
Designbull Goal Function Maximum Utilization
(eg throughput user capacity hellip)bull Constraints QoS
(eg delaylt10ms BERlt10-5hellip)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 8
Ad hoc Sensor Networks Multiple channel access
Medium Access Control (contrsquo)bull Multiple Access (Channel Partitioning or Coordinated Schemes)
ndash Techniques TDMA FDMA CDMA SDMAndash Examples GSM 3Ghellip
bull Random Access (or Random Schemes)ndash Techniques MACA MACAW Aloha 80211 MAChellipndash Examples Wifi WSNhellip
bull Polling based schemesndash Access is coordinated by a central nodendash Examples Bluetooth BlueSkyhellip
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 9
Ad hoc Sensor Networks Multiple channel access
bull Infrastructured networksndash base stations are the bridgesndash a mobile host will communicate with the
nearest base station (or access point)ndash handoff is taken when a host roams from one
base to anotherndash Medium access control assisted by BS
bull Ad hoc networksndash infrastructureless no fixed base stationsndash without the assistance of base stations for
communicationndash Due to transmission range constraint two
wireless nodes need multi-hop routing forcommunication
ndash quickly and unpredictably changing topology
Two types of wireless networks
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 10
Ad hoc Sensor Networks Multiple channel access
Two types of wireless networks (contrsquo)
bull Mesh networks serve as access networks that employ multi-hop wireless forwarding bynon-mobile nodes to relay traffic to and from the wired Internet In such an environmenthybrid wireless network technologies andor hierarchical network organization can beused for ad hoc and infrastructure wireless links
bull Wireless Sensor Networks are a special category of ad hoc networks that are used toprovide a wireless communication infrastructure among the sensors deployed in a specificapplication domain [rarr see Chapter 9 - 11]
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 11
Ad hoc Sensor Networks Multiple channel access
Infrastructured wireless networks Ad hoc wireless networksFixed infrastructure-based Infrastructureless
Guaranteed bandwidth Shared radio channel
Seamless connectivity (low call drops during handoffs)
Frequent path breaks due to mobility
High cost and time of deployment Quick and cost-effective deployment
Reuse of frequency spectrum (cellular principle) Dynamic frequency reuse based on carrier sense mechanism
Application domains include mainly civilian and commercial sectors
Application domains include battlefields emergency search and rescue operations
High cost of network maintenance Self-organization and maintenance properties are built into the network
Widely deployed and currently in the third generation of evolution
Several issues are to be addressed for successful commercial deployment even though widespread use exists in defense
Two types of wireless networks (contrsquo)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 12
Ad hoc Sensor Networks Multiple channel access
Duplexing
bull If the communication between two parties is one way then it is called simplexcommunication Simplex communication is achieved by default by using a singlewireless channel (frequency band) to transmit from sender to receiver
bull If the communication between two parties is two- way then it is called duplexcommunication Duplex communication achieved by
ndash Time Division (TDD) (famous in cellular systems)ndash Frequency Division (FDD) (famous in cellular systems)ndash Some other method like a random access method
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 13
Ad hoc Sensor Networks Multiple channel access
FDDbull A duplex channel consists of two
simplex channels with different carrierfrequencies
ndash Downlink band carries traffic frombase to mobile
ndash Uplink band carries traffic frommobile to base
Duplexing(contrsquo)TDD
bull channel (carrier frequency) is shared intime in a deterministic manner
ndash The time is slotted with fixed slotlength (sec)
ndash Some slots are used for downlinkchannel
ndash Some slots are used for uplink channel
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 14
Ad hoc Sensor Networks Multiple channel access
FDDbull FDD is used in radio systems that can
allocate individual radio frequenciesfor each user
bull For example analog systems AMPSbull In FDD channels are allocated by a
base stationbull A channel for a mobile is allocated
dynamicallybull All channels that a base station will use
are allocated usually staticallybull More suitable for wide-area cellular
networks GSM AMPS all use FDD
Duplexing(contrsquo)TDD
bull Can only be used in digital wirelesssystems (digital modulation)
bull Requires rigid timing andsynchronization
bull Mostly used in short-range and fixedwireless systems so that propagationdelay between base and mobile do notchange much with respect to locationof the mobile
bull Such as cordless phoneshellip
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 15
Ad hoc Sensor Networks Multiple channel access
Multiple Access narrow- vs wideband systemsbull Narrowband System
ndash The bandwidth is small compared to the coherence bandwidth of the channel(BltBcoh)
bull Wideband Systemndash The system bandwidth is much larger that the coherence bandwidth of the
multipath channel (BgtBcoh) A large number of users can access the same channel(frequency band) at the same time
bull Four major multiple access (MA) schemesndash Time Division Multiple Access (TDMA) it could be used in narrowband or
wideband systemndash Frequency Division Multiple Access (FDMA) it is usually used in narrowband
systemndash Code Division Multiple Access (CDMA) it is used in wideband systemndash Space Division Multiple Access (SDMA) for wireless systems with multiple
antennas it can be combined with all of the other MA methods
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 16
Ad hoc Sensor Networks Multiple channel access
Multiple Access cellular standards
Cellular System Multiple Access TechniqueAMPS (2G) FDMAFDD
CT2 Cordless Phone FDMATDD
DECT Cordless Phone FDMATDD
GSM (2G) TDMAFDD
USDC (IS-54 and IS-136) (2G) TDMAFDD
Personal Digital Cellular (2G) TDMAFDD
US IS-95 (2G) CDMAFDD
W-CDMA (3G) CDMAFDD CDMATDD
cdma2000 (3G) CDMAFDD CDMATDD
Narrowband System
Wideband System
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 17
Ad hoc Sensor Networks Multiple channel access
bull Individual radio channels are assigned toindividual users Each user is allocated a Wfrequency band by BS
bull If the channel allocated to a user is idle then itis not used by someone else waste of resource
bull Mobile and base can transmit and receivesimultaneously (FDD)
bull The W bandwidth of FDMA channels arerelatively low Symbol time is usually larger(low data rate) than the delay spread of themultipath channel (implies that inter-symbolinterference is low)
bull Lower complexity systems than TDMAsystems
Multiple Access FDMA
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 18
Ad hoc Sensor Networks Multiple channel access
Multiple Access FDMA (contrsquo)bull Capacity of FDMA systems
Nc = (B-2Bguard)Wwhere B is the total spectrum allocation Bguard is the guard band allocated atthe edge of the spectrum band W is the bandwidth of an individual channel
bull Erlang B and Erlang C system assumesndashinfinite population of sources which jointly offer traffic to Nc channelndashcall attempts arrive following a Poisson process so call arrivals are independentThe average arrival rate is λndashmessage length (holding times) are exponentially distributed (Markovian system)The average call length is hndashthe total amount of traffic offered in erlangs L= λhndashIf a user is rejected his next call attempt is made statistically independent of theprevious attempt (Erlang B)ndashIf all the channels are busy when a request arrives from a user the request isqueued An unlimited number of requests may be held in the queue (Erlang C)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 19
Ad hoc Sensor Networks Multiple channel access
bull Statictical measures of offered load Ncndash In Erlang B system the probability of call
blocking
ndash In Erlang C system the probabilty of waiting
ndash And the average wait time
c
c
cBLOCK
0
Pr
N
Nk
k
LN
Lk
=
=
sum
c
cc
wait 1
c0c
Pr 1
N
NkN
k
LL LL N kN
minus
=
=⎛ ⎞
+ minus⎜ ⎟⎝ ⎠
sum
wait waitc
Pr htN L
=minus
Multiple Access FDMA (contrsquo)Recall from
Queuing theory
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 20
Ad hoc Sensor Networks Multiple channel access
Multiple Access example of Erlang B systembull In an Erlang B system 40 channels are available A blocking probabilty of
less than 1 is required What is the traffic that can be serve if there is oneoperator or three operators
bull (Solution L(1 operator)gtL(3 operators))bull (Note The number of users increases faster than linearly with the number of
available channels The difference between actual increase and linearincrease is called the trunking gain It is preferable to have a large pool ofavailable channels that serves all users (a single operator) The reasons fornot choosing this approach are political not technical)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 21
Ad hoc Sensor Networks Multiple channel access
Multiple Access TDMAbull The allocated radio spectrum for the system is divided into time slots
ndash In each slot a user can transmit or receivendash A user occupies a cyclically repeating slotsndash A channel is logically defined as a particular time slot that repeats with some
periodbull TDMA systems buffer the data until its turn (time slot) comes to transmitbull In TDMATDD half of the slots in the frame is used for uplink channels the
other is used for downlink channelsbull In TDMAFDD a different carrier frequency is used for uplink or downlink
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 22
Ad hoc Sensor Networks Multiple channel access
Multiple Access TDMA (contrsquo)bull Preamble contains address and synchronization info to identify base
station and mobiles to each otherbull Guard times are used to allow synchronization of the receivers between
different slots and framesndash Different mobiles may have different propagation delays to a base station
because of different distances
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 23
Multiple Access TDMA (contrsquo)bull Each frame contains overhead bits and data bits Efficiency of frame is
defined as the percentage of data (information) bits to the total frame size inbits
η =(1 ndash boverheadbtotal) 100 where btotal= Tf R is the total number of bits in a frame Tf is the frameduration in sec boverhead is the number of overhead bits (GSM efficiency 73)
bull TDMA is usually combined with FDMAndash Neighboring cells are allocated and using different carrier frequencies (FDMA)
Inside a cell TDMA can be used Cells may be re-using the same frequency if theyare far from each-other
ndash There may be more than one channel allocated and used inside each cell Eachcarrier frequency (radio channel) may be using TDMA to further multiplex moreuser (ie having TDMA logical channels inside radio channels) For example inGSM each radio channel has 200KHz bandwidth and has 8 time slots (8 logicalchannels) Hence GSM is using FDMA combined with TDMA
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 24
Multiple Access TDMA (contrsquo)
Ad hoc Sensor Networks Multiple channel access
GSM (Europa) IS-54 (USA) PDC (Japan) DECT
Bit Rate 2708 Kbps 486 Kbps 42 Kbps 1152 Mbps
Bandwidth 200 KHz 30 KHz 25 KHz 1728 MHz
Time Slot 0577 ms 67 ms 67 ms 0417 ms
Upstream slots per frame 8 3 3 12
Duplexing FDD FDD FDD TDD
Efficiency 73 80 80 67
Modulation GMSK π4 DQPSK π4 DQPSK GMSK
Adaptive equalized Mandatory Mandatory Optional None
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 25
Multiple Access TDMA (contrsquo)bull Enables the sharing of a single radio channel among N usersbull Requires high data-rate per radio channel to support N users
simultaneously which requires adaptive equalizers to be usedin multipath environments
bull Transmission occurs in bursts (not continually) It enablespower saving by going to sleep modes in unrelated slots Itdiscontinues transmission and also enables mobile assistedhandoff
bull It equires synchronization of the receivers Need guard bits andsync bits which occurs large overhead per slot
bull Allocation of slots to mobile users should not be uniform Itmay depend on the traffic requirement of mobiles This bringsextra flexibility and efficiency compared to FDMA systems
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2620111015 TAacuteMOP ndash 412-082AKMR-2009-0006 26
Ad hoc Sensor Networks Multiple channel access
Multiple Access spread spectrum multiple accessbull SSMA uses signals that have transmission bandwidth that is
several orders of magnitude larger than minimum requiredRF bandwidth
bull It providesndash Immunity to multipath interferencendash Robust multiple access
bull Two techniquesndash Frequency Hopped Multiple Access (FHMA)ndash Direct Sequence Multiple Access (DSMA) or Code Division
Multiple Access (CDMA)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2720111015 TAacuteMOP ndash 412-082AKMR-2009-0006 27
Ad hoc Sensor Networks Multiple channel access
Multiple Access FHMAbull The carrier frequency of users are varied in a pseudo-random
fashionndash Each user is using a narrowband channel (spectrum) at a specific
instance of timendash The random change in frequency make the change of using the same
narrowband channel very lowbull The sender receiver change frequency (calling hopping)
using the same pseudo-random sequence hence they aresynchronized
bull Rate of hopping versus Symbol ratendash If hopping rate is greather Fast Frequency Hoppingndash If symbol rate is greater Slow Frequency Hopping
bull GSM and Bluetooth are example systems
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2820111015 TAacuteMOP ndash 412-082AKMR-2009-0006 28
Ad hoc Sensor Networks Multiple channel access
Multiple Access FHMA (contrsquo)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2920111015 TAacuteMOP ndash 412-082AKMR-2009-0006 29
Ad hoc Sensor Networks Multiple channel access
Multiple Access CDMAbull The narrowband message signal is multiplied by a very large bandwidth
signal called spreading signal (code) before modulation and transmissionover the air This is called spreading
bull Spreading signal use a pseudo-noise (PN) sequence (a pseudo-randomsequence) called codeword which are orthogonal (low autocorrelation)
bull The receiver correlator distinguishes the senders signal by examining thewideband signal with the same time-synchronized spreading code(despreading)
bull Advantagesndash Low power spectral densityndash Interference limited operationndash Privacyndash Reduction of multipath affects by using a larger spectrumndash Users can start their transmission at any time (random access possible)ndash Cell capacity is soft and higher than TDMA and FDMAndash No frequency management and no equalizers and quard time needed
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3020111015 TAacuteMOP ndash 412-082AKMR-2009-0006 30
Ad hoc Sensor Networks Multiple channel access
Multiple Access near far problem and power controlbull At a receiver the signals may come from various sources (multiuser) The
strongest signal usually captures the modulator The other signals areconsidered as noise Each source may have different distances to the basestation
bull In CDMA we want a BS to receive CDMA coded signals from variousmobile users at the same time
ndash Therefore the receiver power at the BS for all mobile users should be close toeach other This requires power control at the mobiles
bull Power Control BS monitors the RSSI (Received Signal Strength Indicator)values from different mobiles and then sends power change commands to themobiles over a forward channel The mobiles then adjust their transmitpower
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3120111015 TAacuteMOP ndash 412-082AKMR-2009-0006 31
Ad hoc Sensor Networks Multiple channel access
Multiple Access hybrid spread spectrum techniquesbull FDMACDMA
ndash Available wideband spectrum is divided into a number narrowband radio channels CDMAis employed inside each channel
bull DSFHMAndash The signals are spread using spreading codes (direct sequence signals are obtained) but
these signal are not transmitted over a constant carrier frequency they are transmitted over afrequency hopping carrier frequency
bull Time Division CDMA (TCDMA)ndash Each cell is using a different spreading code (CDMA employed between cells) that is
conveyed to the mobiles in its rangendash Inside each cell (inside a CDMA channel) TDMA is employed to multiplex multiple users
bull Time Division Frequency Hoppingndash At each time slot the user is hopped to a new frequency according to a pseudo-random
hopping sequencendash Employed in severe co-interference and multi-path environmentsndash Bluetooth and GSM are using this technique
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3220111015 TAacuteMOP ndash 412-082AKMR-2009-0006 32
Ad hoc Sensor Networks Multiple channel access
Multiple Access comparison TDMAFDMACDMA
Techniques TDMA FDMA CDMA SDMA
Idea
segment sending time into disjoint time-slots
Segment the frequency band into disjoint sub-bands
Spread the spectrum using orthogonal codes
Segment space into sectors
Signal separation
Synchronization in the time domain
Filtering in the frequency domain
Code Directed antennas
Advantage
Established flexible
Simple established robust
Flexible less frequency planning needed soft handover
Simple increases capacity
DisadvantageQuard space synchronization difficult
Inflexible scarce source
Complex receiver power control
inflexible
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3320111015 TAacuteMOP ndash 412-082AKMR-2009-0006 33
Ad hoc Sensor Networks Multiple channel access
User capacity per cell
The user capacity C is defined as the total number of active users percell that the system can support while meeting a common BERconstrain For orthogonal multiple access (as FDMA and TDMA)
C = Ncellwhere Ncell is the number of channels assigned to any given cellThe total number of orthogonal channels of bandwidth Bs that can becreated from a total system bandwidth of B is B Bs The reuse factorsatisfies N = (B Bs) Ncell this implies
C = (B Bs) N
Recall from Chapter 1
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3420111015 TAacuteMOP ndash 412-082AKMR-2009-0006 34
Ad hoc Sensor Networks Multiple channel access
10 methods for increasing capacity1 Increasing the amount of spectrum used very expensive2 More efficient modulation format and coding3 Better source coding4 Adaptive modulation and coding5 Discontinuous transmission6 Multiuser detection CDMA systems7 Reduction of cell radius effective but very expensive smaller
cells require more handovers8 Use of sector cells tripled cells have tripled BS antennas9 Use of an overlay structure10 Multiple antennas diversity MIMO systems SDMA
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3520111015 TAacuteMOP ndash 412-082AKMR-2009-0006 35
Ad hoc Sensor Networks Multiple channel access
bull Distributed operation is requiredbull Synchronization is required in TDMA-based systemsbull Hidden terminals are nodes hidden from a senderbull Exposed terminals are exposed nodes preventing a sender
from sendingbull Throughput needs to be maximizedbull Access delay should be minimizedbull Fairness refers to provide an equal share to all competing
nodesbull Real-time traffic support is required for voice video and
real-time databull Resource reservation is required for QoSbull Ability to measure resource availability handles the
resourcesbull Capability for power control reduces the energy
consumption
Random access in adhoc networks
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3620111015 TAacuteMOP ndash 412-082AKMR-2009-0006 36
Ad hoc Sensor Networks Multiple channel access
Random accessbull All random access techniques are based on packetized data or packet radio where user
date is collected into packets of M bitsbull Collision if packets from different users overlap in timebull The transmission time of a packet is τ = M R where R [bps] the data ratebull Analysis of random access techniques assumes that the users generate packets according
to memoryless Poisson process at a rate λ [packets per unit time] the probability that thenumber of packet arrivals in a time period [0t] denoted by X(t) is equal to integer k isgiven by
Pr(X(t)=k) = (λt)k k e-λt
bull The traffic load is defined as L = λ τ If L gt 1 means on avarage more packets arrive inthe system over a given time period than can be transmitted in that period so systemswith Lgt1 are unstable
bull The throughput is defined as the ratio of the average number of packets successfullytransmitted in any given time interval divided by the number of attempted transmissionsin that interval
T = L Pr(succesful packet transmission)bull The effective data rate of the system is RT
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3720111015 TAacuteMOP ndash 412-082AKMR-2009-0006 37
Ad hoc Sensor Networks Multiple channel access
Random access (contrsquo)bull We will examine the following attributes
ndash Channel utilization (Throughput)ndash Latencyndash Collision avoidance (Hidden and exposed terminals problem)ndash Reliability (ACK)ndash Energy efficiency (power control)ndash Fairnessndash Throughput needs to be maximized
bull We will discuss the following techniquesndash Aloha and slotted Alohandash CSMA Protocols (1-persistent non-persistent p-persistent CSMA CSMACD)ndash MACAndash MACAWndash Energy efficient MAC in wireless sensor networks (eg SMAC)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 38
Random Access ALOHA (1970)bull ALOHA was developed for a wireless system at the University of Hawaii
(Abramson et al)ndash Multiple remote stations plus one base stationndash Frame transmissions are made at one frequency from a remote station to the basestation the base station re-broadcasts frames on another frequency Contention is foraccess to the base station
bull Frames can be sent at any time to the base stationndash If no acknowledgment (ACK) is received assume that the frame is lost andretransmit after waiting a random amount of timendash If more than one station broadcasts on the base station access frequency at the sametime ndash a collision ndash interference will destroy the frames
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 39
Random Access ALOHA (contrsquo)
bull The probability that no packets generated during the tima [-τ τ] is given by (nocollision)
Pr(X(t)=0) = (λ2τ)0 0 e-2λτ = e-2Lwith corresponding throughput
T = L e-2L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 05 05e 018
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 40
Random Access slotted ALOHA (1971)bull Improvement Time is slotted and a packet can only be transmitted at the beginning of
one slot Thus it can reduce the collision duration
bull The probability that no packets generated during the time [0 τ] is given by (nocollision)
Pr(X(t)=0) = (λτ)0 0 e-λτ = e-Lwith corresponding throughput T = L e-L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 1 e 037
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 41
bull Slotted Aloha has double the maximumthroughput as pure Aloha and achievesthis maximum at a higher offered loadThe effective data rate is still less than40 of the raw transmission rate
bull Aloha does not listen to the carrier beforetransmission more sophisticatedtechniques are needed to increaseefficiency
bull Further problem the delayndashDelay
ndashThe mean packet delay
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
( ) ( )delayPr 1 e ekL Lkτ minus minus= = minus
( )delay1
1 e e ekL L L
kMPD E kτ
infinminus minus
=
= = minus =sum
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 42
Random Access Carrier Sense Multiple Accessbull CSMA (Carrier Sense Multiple Access) listen to the carrier before transmission
and transmits if channel is idlebull Detection delay and propagation delay are two important parameters
ndash Detection delay time required to sense the carrier and decide if it is idle or busyndash Propagation delay distancespeed_of_ligth The time required for bit to travel
from transmitter to the receiver
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 43
Random Access CSMA variations
Ad hoc Sensor Networks Multiple channel access
1-persistent CSMA Non-persistent CSMA
There will always be a collision iftwo nodes want to retransmit(usually you stop transmissionattempts after few tries)
Random backoff reduces probabilityof collisions Waste idle time if thebackoff time is too long
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 44
Random Access CSMA variations (contrsquo)
Ad hoc Sensor Networks Multiple channel access
p-persistent CSMA CSMA CD (Collision Detection)
A good tradeoff between non-persistent and 1-persistent CSMA
Same with CSMA however a stationalso listen to the carrier whiletransmitting to see if the transmissioncollides with someone elsetransmission It can be used in listen-while-talk capable channels (fullduplex) In single radio channels thetransmission need to be interrupted inorder to sense the channel
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 45
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 46
Random Access CSMA (contrsquo)bull How to select probability p
ndash Assume that M nodes have a packet to send and the medium is busy then Mp is theexpected number of nodes that will attempt to transmit once the medium becomesidle If Mp gt 1 then a collision is expected to occur Therefore the network mustmake sure that Mp lt 1 to avoid collision where M is the maximum number of nodesthat can be active at a time
bull Problems1 The mean packet delay increases exponently with increasing offered load2 CSMA protocols sense the carrier but sensing the carrier does not always releasestrue information about the status of the wireless channel There are two problems thatare unique to wireless channels (different than wireline channels)ndash Hidden terminal problemndash Exposed terminal problem
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 47
Hidden terminal problembull A sends to B C cannot receive Abull C wants to send to B C senses a
ldquofreerdquo medium (CS fails)bull collision at B A cannot receive the
collision (CD fails)bull A is ldquohiddenrdquo for C
Ad hoc Sensor Networks Multiple channel access
Random Access hidden and exposed terminal problemExposed terminal problem
bull B sends to A C wants to send to Dbull C has to wait CS signals a medium
in usebull since A is outside the radio range of
C waiting is not necessarybull C is ldquoexposedrdquo to B
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 48
Random Access MACAWhen a station wants to transmit databull It sends an RTS (Ready-to-Send) packet to the intended receiver
ndash The RTS packet contains the length of the data that needs to be transmittedndash Any station other than the intended recipient hearing RTS defers transmission for atime duration equal to the end of the corresponding CTS reception
bull The receiver sends back CTS (Clear-to-Send) packet back to sender if it isavailable to receivendash The CTS packet contains the length of the data that original sender wants totransmitndash Any station other than the original RTS sender hearing CTS defers transmissionuntil the data is sent
bull The original sender upon reception of the CTS starts transmitting
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 49
Hidden terminal problem
Ad hoc Sensor Networks Multiple channel access
Random Access MACA (contrsquo)Exposed terminal problem
Waiting time of node X is muchsmaller than waiting time of node C
bull C defers transmission upon hearing BrsquosRTS until B could get CTS from Abull After that C can start transmission to DFor that it first sends an RTSbull C is not longer exposed to the datatransmission of B
Problem It does not address the collision of RTS packet
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 50
Random Access reliabilitybull Wireless links are prone to have errors High packet loss rate detrimental to
transport-layer performance Mechanisms needed to reduce packet loss rateexperienced by upper layers
bull When node B receives a data packet from node A node B sends anAcknowledgement (Ack) This approach adopted in many protocols
bull If node A fails to receive an Ack it will retransmit the packet
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 51
Random Access MACAW (CSMACA) bull Physical carrier sense and virtual carrier sense using Network Allocation
Vector (NAV)ndash NAV is updated based on overheard RTSCTSDATAACK packets each of whichspecified duration of a pending transmissionndash Nodes stay silent when carrier sensed (physicalvirtual)
bull Backoff intervals used to reduce collision probabilityndash When transmitting a packet choose a backoff interval in the range [0w] where wis contention windowndash Count down the backoff interval when channel is idlendash Count down is suspended if channel becomes busyndash When backoff interval reaches 0 transmit RTS
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 52
Random Access CSMACA (contrsquo) bull The time spent counting down backoff intervals is a part of MAC overhead
ndash Choosing a large w leads to large backoff intervals and can result in larger overheadndash Choosing a small w leads to a larger number of collisions (when two nodes countdown to 0 simultaneously)ndash Since the number of nodes attempting to transmit simultaneously may change withtime some mechanism to manage contention is needed
bull IEEE 80211 DCF contention window w is chosen dynamically depending oncollision occurrence (binary exponential backoff)ndash When a node fails to receive CTS in response to its RTS it increases the contentionwindowndash w is doubled (up to an upper bound)ndash When a node successfully completes a data transfer it restores w to wmin
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 53
Random Access fairnessbull Simplest definition of fairness all nodes should receive equal bandwidthbull An example of unfairness
ndash Assume that initially A and B both choose a backoff interval in range [031] buttheir RTSs collidendash Nodes A and B then choose from range [063]ndash Node A chooses 4 slots and B choose 60 slotsndash After A transmits a packet it next chooses from range [031]ndash It is possible that A may transmit several packets before B transmits its first packet
Ad hoc Sensor Networks Multiple channel access
Unfairness occurs when one node has backed off much more than some other node
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 54
Random Access fairness (contrsquo) MACAW solution for fairnessbull When a node transmits a packet it appends value w to the packet all nodes
hearing value w use it for their future transmission attemptsbull Since w is an indication of the level of congestion in the vicinity of a specific
receiver node MACAW proposes maintaining w independently for eachreceiver
bull Using per-receiver wi is particularly useful in multi-hop environments sincecongestion level at different receivers can be very different
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 55
Random Access energy efficiency bull Since many mobile hosts are operated by batteries MAC
protocols which conserve energy are of interestbull Two approaches to reduce energy consumption
ndash Power save turn off wireless interface when desirablendash Power control reduce transmit power
bull Power control has some more potential benefitsndash Reduced interference (in ad hoc networks) eg it improves ALOHAefficiency where user with high power can capture a packet even if there is acollisionndash Increased spatial reuse (in infrastructured wireless networks) eg it is usedin CDMA to maintain target SIR of voice and data users or generally it canbe used to maintain target SIR for different user classes (admission control)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 56
Random Access power control bull When C transmits to D at a high power level B cannot receive Arsquos
transmission due to interference from Cbull If C reduces transmit power it can still communicate with D
ndash Reduces energy consumption at node Cndash Allows B to receive Arsquos transmission (spatial reuse)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 57
bull A special wireless ad hoc networkndash Large number of nodesndash Battery poweredndash Topology and density changendash Nodes for a common taskndash In-network data processing
bull Sensor-net applicationsndash Sensor-triggered bursty trafficndash Can often tolerate some delay
Ad hoc Sensor Networks Multiple channel access
Characteristics of Sensor Network
Scalabilty and self configurationEnergy efficiencyAdaptivityFairness not importantLatency
AdaptivityEnergy-delay tradoff
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 58
Ad hoc Sensor Networks Multiple channel access
Wireless sensor node and sensor network
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 59
MAC for WSN attributes bull Attributes
ndash Collision avoidance (Basic task)ndash Energy efficiency (most primary in WSN)ndash Scalability and adaptivityndash Channel utilizationndash Latencyndash Throughputndash Fairness
bull What causes energy wastendash Collisionsndash Control packet overheadndash Overhearing unnecessary trafficndash Long idle time (it consumes 50-100 of the power)
Ad hoc Sensor Networks Multiple channel access
Primary
Secondary
Dominant factor
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 60
MAC for WSN
bull Contention-based protocols need to work hard in all directions for energysavingsndash Reduce idle listening ndash support low duty cyclendash Better collision avoidancendash Reduce control overheadndash Avoid unnecessary overhearing
Ad hoc Sensor Networks Multiple channel access
Scheduled Protocols Contention ProtocolsCollisions No YesEnergy efficiency Good BadScalability and adaptivity Bad GoodMulti-hop communication Difficult EasyTime synchronization Strict Loose or not required
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 61
MAC for WSN classification
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 62
MAC for WSN SMAC (2002)bull It has been developed by Ye Heidemann and Estrinbull Tradeoffs between (latency fairness) and energybull Major components in S-MAC
ndash Periodic listen and sleep turn off radio when sleeping the reduced dutycycle is ~ 10 (eg 120ms on 12s off)ndash Collision avoidance is based on contention similarly to IEEE 80211 adhoc modendash Overhearing avoidance the node is sleeping when neighbors talkndash Massage passing long message is fragmented amp sent in burst operationwith extended Tx time and re-transmit occurs immediately
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 63
Summarybull The algorithms and protocols that enable sharing by multiple users and
controls the access to the wireless channel from different users are calledMAC
bull There are two types of wireless networks Infrastructured wireless networksand ad hoc wireless networks
bull We also want our wireless system to have high user capacity and we canidentify 10 methods for increasing it
bull There are several MACs with no absolute advantage it should be tailoredto the system requirements
bull There are primary concerns for designing a WSN MAC (collisionavoidance energy efficiency scalability and adaptivity) while thesecondary concerns are as follows (channel utilization latency throughputfairness)
bull Next lecture Routing protocols
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 5
Ad hoc Sensor Networks Multiple channel access
Structure of a wireless communications link
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 6
Ad hoc Sensor Networks Multiple channel access
Layered Communication Approaches
2 Data Link layer it handles access to the unerlyingchannel and defines the data format It is split into twosublayersndash Logical Link Control it manages frames to upper and
lower layer (encapsulation decapsulation) and it enforceserrror control (checksum and parity bits)
ndash Medium Access Control (MAC) it coordinatestransmission between users sharing the spectrum Inwireless systems it must address the hidden terminalproblem and must exercise power control Goals preventcollisions while maximizing throughput and minimizingdelay
Physical
OSI reference model
Data link
Network
Transport
Middleware
Application
Recall from Chapter 1
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 7
Ad hoc Sensor Networks Multiple channel access
Medium Access Control
bull Thus the wireless spectrum (frequency band) is a very precious and limited resourcewe need to use this resource very efficiently
bull We also want our wireless system to have high user capacity with QoS constraintsbull The algorithms and protocols that enables this sharing by multiple users and
controlscoordinates the access to the wireless channel (medium) from differentusers are called MEDIUM ACCESS or MEDIA ACCESS or MULTIPLE ACCESSprotocols techniques schemes etchellip)
Designbull Goal Function Maximum Utilization
(eg throughput user capacity hellip)bull Constraints QoS
(eg delaylt10ms BERlt10-5hellip)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 8
Ad hoc Sensor Networks Multiple channel access
Medium Access Control (contrsquo)bull Multiple Access (Channel Partitioning or Coordinated Schemes)
ndash Techniques TDMA FDMA CDMA SDMAndash Examples GSM 3Ghellip
bull Random Access (or Random Schemes)ndash Techniques MACA MACAW Aloha 80211 MAChellipndash Examples Wifi WSNhellip
bull Polling based schemesndash Access is coordinated by a central nodendash Examples Bluetooth BlueSkyhellip
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 9
Ad hoc Sensor Networks Multiple channel access
bull Infrastructured networksndash base stations are the bridgesndash a mobile host will communicate with the
nearest base station (or access point)ndash handoff is taken when a host roams from one
base to anotherndash Medium access control assisted by BS
bull Ad hoc networksndash infrastructureless no fixed base stationsndash without the assistance of base stations for
communicationndash Due to transmission range constraint two
wireless nodes need multi-hop routing forcommunication
ndash quickly and unpredictably changing topology
Two types of wireless networks
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 10
Ad hoc Sensor Networks Multiple channel access
Two types of wireless networks (contrsquo)
bull Mesh networks serve as access networks that employ multi-hop wireless forwarding bynon-mobile nodes to relay traffic to and from the wired Internet In such an environmenthybrid wireless network technologies andor hierarchical network organization can beused for ad hoc and infrastructure wireless links
bull Wireless Sensor Networks are a special category of ad hoc networks that are used toprovide a wireless communication infrastructure among the sensors deployed in a specificapplication domain [rarr see Chapter 9 - 11]
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 11
Ad hoc Sensor Networks Multiple channel access
Infrastructured wireless networks Ad hoc wireless networksFixed infrastructure-based Infrastructureless
Guaranteed bandwidth Shared radio channel
Seamless connectivity (low call drops during handoffs)
Frequent path breaks due to mobility
High cost and time of deployment Quick and cost-effective deployment
Reuse of frequency spectrum (cellular principle) Dynamic frequency reuse based on carrier sense mechanism
Application domains include mainly civilian and commercial sectors
Application domains include battlefields emergency search and rescue operations
High cost of network maintenance Self-organization and maintenance properties are built into the network
Widely deployed and currently in the third generation of evolution
Several issues are to be addressed for successful commercial deployment even though widespread use exists in defense
Two types of wireless networks (contrsquo)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 12
Ad hoc Sensor Networks Multiple channel access
Duplexing
bull If the communication between two parties is one way then it is called simplexcommunication Simplex communication is achieved by default by using a singlewireless channel (frequency band) to transmit from sender to receiver
bull If the communication between two parties is two- way then it is called duplexcommunication Duplex communication achieved by
ndash Time Division (TDD) (famous in cellular systems)ndash Frequency Division (FDD) (famous in cellular systems)ndash Some other method like a random access method
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 13
Ad hoc Sensor Networks Multiple channel access
FDDbull A duplex channel consists of two
simplex channels with different carrierfrequencies
ndash Downlink band carries traffic frombase to mobile
ndash Uplink band carries traffic frommobile to base
Duplexing(contrsquo)TDD
bull channel (carrier frequency) is shared intime in a deterministic manner
ndash The time is slotted with fixed slotlength (sec)
ndash Some slots are used for downlinkchannel
ndash Some slots are used for uplink channel
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 14
Ad hoc Sensor Networks Multiple channel access
FDDbull FDD is used in radio systems that can
allocate individual radio frequenciesfor each user
bull For example analog systems AMPSbull In FDD channels are allocated by a
base stationbull A channel for a mobile is allocated
dynamicallybull All channels that a base station will use
are allocated usually staticallybull More suitable for wide-area cellular
networks GSM AMPS all use FDD
Duplexing(contrsquo)TDD
bull Can only be used in digital wirelesssystems (digital modulation)
bull Requires rigid timing andsynchronization
bull Mostly used in short-range and fixedwireless systems so that propagationdelay between base and mobile do notchange much with respect to locationof the mobile
bull Such as cordless phoneshellip
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 15
Ad hoc Sensor Networks Multiple channel access
Multiple Access narrow- vs wideband systemsbull Narrowband System
ndash The bandwidth is small compared to the coherence bandwidth of the channel(BltBcoh)
bull Wideband Systemndash The system bandwidth is much larger that the coherence bandwidth of the
multipath channel (BgtBcoh) A large number of users can access the same channel(frequency band) at the same time
bull Four major multiple access (MA) schemesndash Time Division Multiple Access (TDMA) it could be used in narrowband or
wideband systemndash Frequency Division Multiple Access (FDMA) it is usually used in narrowband
systemndash Code Division Multiple Access (CDMA) it is used in wideband systemndash Space Division Multiple Access (SDMA) for wireless systems with multiple
antennas it can be combined with all of the other MA methods
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 16
Ad hoc Sensor Networks Multiple channel access
Multiple Access cellular standards
Cellular System Multiple Access TechniqueAMPS (2G) FDMAFDD
CT2 Cordless Phone FDMATDD
DECT Cordless Phone FDMATDD
GSM (2G) TDMAFDD
USDC (IS-54 and IS-136) (2G) TDMAFDD
Personal Digital Cellular (2G) TDMAFDD
US IS-95 (2G) CDMAFDD
W-CDMA (3G) CDMAFDD CDMATDD
cdma2000 (3G) CDMAFDD CDMATDD
Narrowband System
Wideband System
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 17
Ad hoc Sensor Networks Multiple channel access
bull Individual radio channels are assigned toindividual users Each user is allocated a Wfrequency band by BS
bull If the channel allocated to a user is idle then itis not used by someone else waste of resource
bull Mobile and base can transmit and receivesimultaneously (FDD)
bull The W bandwidth of FDMA channels arerelatively low Symbol time is usually larger(low data rate) than the delay spread of themultipath channel (implies that inter-symbolinterference is low)
bull Lower complexity systems than TDMAsystems
Multiple Access FDMA
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 18
Ad hoc Sensor Networks Multiple channel access
Multiple Access FDMA (contrsquo)bull Capacity of FDMA systems
Nc = (B-2Bguard)Wwhere B is the total spectrum allocation Bguard is the guard band allocated atthe edge of the spectrum band W is the bandwidth of an individual channel
bull Erlang B and Erlang C system assumesndashinfinite population of sources which jointly offer traffic to Nc channelndashcall attempts arrive following a Poisson process so call arrivals are independentThe average arrival rate is λndashmessage length (holding times) are exponentially distributed (Markovian system)The average call length is hndashthe total amount of traffic offered in erlangs L= λhndashIf a user is rejected his next call attempt is made statistically independent of theprevious attempt (Erlang B)ndashIf all the channels are busy when a request arrives from a user the request isqueued An unlimited number of requests may be held in the queue (Erlang C)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 19
Ad hoc Sensor Networks Multiple channel access
bull Statictical measures of offered load Ncndash In Erlang B system the probability of call
blocking
ndash In Erlang C system the probabilty of waiting
ndash And the average wait time
c
c
cBLOCK
0
Pr
N
Nk
k
LN
Lk
=
=
sum
c
cc
wait 1
c0c
Pr 1
N
NkN
k
LL LL N kN
minus
=
=⎛ ⎞
+ minus⎜ ⎟⎝ ⎠
sum
wait waitc
Pr htN L
=minus
Multiple Access FDMA (contrsquo)Recall from
Queuing theory
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 20
Ad hoc Sensor Networks Multiple channel access
Multiple Access example of Erlang B systembull In an Erlang B system 40 channels are available A blocking probabilty of
less than 1 is required What is the traffic that can be serve if there is oneoperator or three operators
bull (Solution L(1 operator)gtL(3 operators))bull (Note The number of users increases faster than linearly with the number of
available channels The difference between actual increase and linearincrease is called the trunking gain It is preferable to have a large pool ofavailable channels that serves all users (a single operator) The reasons fornot choosing this approach are political not technical)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 21
Ad hoc Sensor Networks Multiple channel access
Multiple Access TDMAbull The allocated radio spectrum for the system is divided into time slots
ndash In each slot a user can transmit or receivendash A user occupies a cyclically repeating slotsndash A channel is logically defined as a particular time slot that repeats with some
periodbull TDMA systems buffer the data until its turn (time slot) comes to transmitbull In TDMATDD half of the slots in the frame is used for uplink channels the
other is used for downlink channelsbull In TDMAFDD a different carrier frequency is used for uplink or downlink
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 22
Ad hoc Sensor Networks Multiple channel access
Multiple Access TDMA (contrsquo)bull Preamble contains address and synchronization info to identify base
station and mobiles to each otherbull Guard times are used to allow synchronization of the receivers between
different slots and framesndash Different mobiles may have different propagation delays to a base station
because of different distances
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 23
Multiple Access TDMA (contrsquo)bull Each frame contains overhead bits and data bits Efficiency of frame is
defined as the percentage of data (information) bits to the total frame size inbits
η =(1 ndash boverheadbtotal) 100 where btotal= Tf R is the total number of bits in a frame Tf is the frameduration in sec boverhead is the number of overhead bits (GSM efficiency 73)
bull TDMA is usually combined with FDMAndash Neighboring cells are allocated and using different carrier frequencies (FDMA)
Inside a cell TDMA can be used Cells may be re-using the same frequency if theyare far from each-other
ndash There may be more than one channel allocated and used inside each cell Eachcarrier frequency (radio channel) may be using TDMA to further multiplex moreuser (ie having TDMA logical channels inside radio channels) For example inGSM each radio channel has 200KHz bandwidth and has 8 time slots (8 logicalchannels) Hence GSM is using FDMA combined with TDMA
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 24
Multiple Access TDMA (contrsquo)
Ad hoc Sensor Networks Multiple channel access
GSM (Europa) IS-54 (USA) PDC (Japan) DECT
Bit Rate 2708 Kbps 486 Kbps 42 Kbps 1152 Mbps
Bandwidth 200 KHz 30 KHz 25 KHz 1728 MHz
Time Slot 0577 ms 67 ms 67 ms 0417 ms
Upstream slots per frame 8 3 3 12
Duplexing FDD FDD FDD TDD
Efficiency 73 80 80 67
Modulation GMSK π4 DQPSK π4 DQPSK GMSK
Adaptive equalized Mandatory Mandatory Optional None
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 25
Multiple Access TDMA (contrsquo)bull Enables the sharing of a single radio channel among N usersbull Requires high data-rate per radio channel to support N users
simultaneously which requires adaptive equalizers to be usedin multipath environments
bull Transmission occurs in bursts (not continually) It enablespower saving by going to sleep modes in unrelated slots Itdiscontinues transmission and also enables mobile assistedhandoff
bull It equires synchronization of the receivers Need guard bits andsync bits which occurs large overhead per slot
bull Allocation of slots to mobile users should not be uniform Itmay depend on the traffic requirement of mobiles This bringsextra flexibility and efficiency compared to FDMA systems
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2620111015 TAacuteMOP ndash 412-082AKMR-2009-0006 26
Ad hoc Sensor Networks Multiple channel access
Multiple Access spread spectrum multiple accessbull SSMA uses signals that have transmission bandwidth that is
several orders of magnitude larger than minimum requiredRF bandwidth
bull It providesndash Immunity to multipath interferencendash Robust multiple access
bull Two techniquesndash Frequency Hopped Multiple Access (FHMA)ndash Direct Sequence Multiple Access (DSMA) or Code Division
Multiple Access (CDMA)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2720111015 TAacuteMOP ndash 412-082AKMR-2009-0006 27
Ad hoc Sensor Networks Multiple channel access
Multiple Access FHMAbull The carrier frequency of users are varied in a pseudo-random
fashionndash Each user is using a narrowband channel (spectrum) at a specific
instance of timendash The random change in frequency make the change of using the same
narrowband channel very lowbull The sender receiver change frequency (calling hopping)
using the same pseudo-random sequence hence they aresynchronized
bull Rate of hopping versus Symbol ratendash If hopping rate is greather Fast Frequency Hoppingndash If symbol rate is greater Slow Frequency Hopping
bull GSM and Bluetooth are example systems
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2820111015 TAacuteMOP ndash 412-082AKMR-2009-0006 28
Ad hoc Sensor Networks Multiple channel access
Multiple Access FHMA (contrsquo)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2920111015 TAacuteMOP ndash 412-082AKMR-2009-0006 29
Ad hoc Sensor Networks Multiple channel access
Multiple Access CDMAbull The narrowband message signal is multiplied by a very large bandwidth
signal called spreading signal (code) before modulation and transmissionover the air This is called spreading
bull Spreading signal use a pseudo-noise (PN) sequence (a pseudo-randomsequence) called codeword which are orthogonal (low autocorrelation)
bull The receiver correlator distinguishes the senders signal by examining thewideband signal with the same time-synchronized spreading code(despreading)
bull Advantagesndash Low power spectral densityndash Interference limited operationndash Privacyndash Reduction of multipath affects by using a larger spectrumndash Users can start their transmission at any time (random access possible)ndash Cell capacity is soft and higher than TDMA and FDMAndash No frequency management and no equalizers and quard time needed
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3020111015 TAacuteMOP ndash 412-082AKMR-2009-0006 30
Ad hoc Sensor Networks Multiple channel access
Multiple Access near far problem and power controlbull At a receiver the signals may come from various sources (multiuser) The
strongest signal usually captures the modulator The other signals areconsidered as noise Each source may have different distances to the basestation
bull In CDMA we want a BS to receive CDMA coded signals from variousmobile users at the same time
ndash Therefore the receiver power at the BS for all mobile users should be close toeach other This requires power control at the mobiles
bull Power Control BS monitors the RSSI (Received Signal Strength Indicator)values from different mobiles and then sends power change commands to themobiles over a forward channel The mobiles then adjust their transmitpower
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3120111015 TAacuteMOP ndash 412-082AKMR-2009-0006 31
Ad hoc Sensor Networks Multiple channel access
Multiple Access hybrid spread spectrum techniquesbull FDMACDMA
ndash Available wideband spectrum is divided into a number narrowband radio channels CDMAis employed inside each channel
bull DSFHMAndash The signals are spread using spreading codes (direct sequence signals are obtained) but
these signal are not transmitted over a constant carrier frequency they are transmitted over afrequency hopping carrier frequency
bull Time Division CDMA (TCDMA)ndash Each cell is using a different spreading code (CDMA employed between cells) that is
conveyed to the mobiles in its rangendash Inside each cell (inside a CDMA channel) TDMA is employed to multiplex multiple users
bull Time Division Frequency Hoppingndash At each time slot the user is hopped to a new frequency according to a pseudo-random
hopping sequencendash Employed in severe co-interference and multi-path environmentsndash Bluetooth and GSM are using this technique
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3220111015 TAacuteMOP ndash 412-082AKMR-2009-0006 32
Ad hoc Sensor Networks Multiple channel access
Multiple Access comparison TDMAFDMACDMA
Techniques TDMA FDMA CDMA SDMA
Idea
segment sending time into disjoint time-slots
Segment the frequency band into disjoint sub-bands
Spread the spectrum using orthogonal codes
Segment space into sectors
Signal separation
Synchronization in the time domain
Filtering in the frequency domain
Code Directed antennas
Advantage
Established flexible
Simple established robust
Flexible less frequency planning needed soft handover
Simple increases capacity
DisadvantageQuard space synchronization difficult
Inflexible scarce source
Complex receiver power control
inflexible
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3320111015 TAacuteMOP ndash 412-082AKMR-2009-0006 33
Ad hoc Sensor Networks Multiple channel access
User capacity per cell
The user capacity C is defined as the total number of active users percell that the system can support while meeting a common BERconstrain For orthogonal multiple access (as FDMA and TDMA)
C = Ncellwhere Ncell is the number of channels assigned to any given cellThe total number of orthogonal channels of bandwidth Bs that can becreated from a total system bandwidth of B is B Bs The reuse factorsatisfies N = (B Bs) Ncell this implies
C = (B Bs) N
Recall from Chapter 1
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3420111015 TAacuteMOP ndash 412-082AKMR-2009-0006 34
Ad hoc Sensor Networks Multiple channel access
10 methods for increasing capacity1 Increasing the amount of spectrum used very expensive2 More efficient modulation format and coding3 Better source coding4 Adaptive modulation and coding5 Discontinuous transmission6 Multiuser detection CDMA systems7 Reduction of cell radius effective but very expensive smaller
cells require more handovers8 Use of sector cells tripled cells have tripled BS antennas9 Use of an overlay structure10 Multiple antennas diversity MIMO systems SDMA
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3520111015 TAacuteMOP ndash 412-082AKMR-2009-0006 35
Ad hoc Sensor Networks Multiple channel access
bull Distributed operation is requiredbull Synchronization is required in TDMA-based systemsbull Hidden terminals are nodes hidden from a senderbull Exposed terminals are exposed nodes preventing a sender
from sendingbull Throughput needs to be maximizedbull Access delay should be minimizedbull Fairness refers to provide an equal share to all competing
nodesbull Real-time traffic support is required for voice video and
real-time databull Resource reservation is required for QoSbull Ability to measure resource availability handles the
resourcesbull Capability for power control reduces the energy
consumption
Random access in adhoc networks
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3620111015 TAacuteMOP ndash 412-082AKMR-2009-0006 36
Ad hoc Sensor Networks Multiple channel access
Random accessbull All random access techniques are based on packetized data or packet radio where user
date is collected into packets of M bitsbull Collision if packets from different users overlap in timebull The transmission time of a packet is τ = M R where R [bps] the data ratebull Analysis of random access techniques assumes that the users generate packets according
to memoryless Poisson process at a rate λ [packets per unit time] the probability that thenumber of packet arrivals in a time period [0t] denoted by X(t) is equal to integer k isgiven by
Pr(X(t)=k) = (λt)k k e-λt
bull The traffic load is defined as L = λ τ If L gt 1 means on avarage more packets arrive inthe system over a given time period than can be transmitted in that period so systemswith Lgt1 are unstable
bull The throughput is defined as the ratio of the average number of packets successfullytransmitted in any given time interval divided by the number of attempted transmissionsin that interval
T = L Pr(succesful packet transmission)bull The effective data rate of the system is RT
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3720111015 TAacuteMOP ndash 412-082AKMR-2009-0006 37
Ad hoc Sensor Networks Multiple channel access
Random access (contrsquo)bull We will examine the following attributes
ndash Channel utilization (Throughput)ndash Latencyndash Collision avoidance (Hidden and exposed terminals problem)ndash Reliability (ACK)ndash Energy efficiency (power control)ndash Fairnessndash Throughput needs to be maximized
bull We will discuss the following techniquesndash Aloha and slotted Alohandash CSMA Protocols (1-persistent non-persistent p-persistent CSMA CSMACD)ndash MACAndash MACAWndash Energy efficient MAC in wireless sensor networks (eg SMAC)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 38
Random Access ALOHA (1970)bull ALOHA was developed for a wireless system at the University of Hawaii
(Abramson et al)ndash Multiple remote stations plus one base stationndash Frame transmissions are made at one frequency from a remote station to the basestation the base station re-broadcasts frames on another frequency Contention is foraccess to the base station
bull Frames can be sent at any time to the base stationndash If no acknowledgment (ACK) is received assume that the frame is lost andretransmit after waiting a random amount of timendash If more than one station broadcasts on the base station access frequency at the sametime ndash a collision ndash interference will destroy the frames
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 39
Random Access ALOHA (contrsquo)
bull The probability that no packets generated during the tima [-τ τ] is given by (nocollision)
Pr(X(t)=0) = (λ2τ)0 0 e-2λτ = e-2Lwith corresponding throughput
T = L e-2L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 05 05e 018
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 40
Random Access slotted ALOHA (1971)bull Improvement Time is slotted and a packet can only be transmitted at the beginning of
one slot Thus it can reduce the collision duration
bull The probability that no packets generated during the time [0 τ] is given by (nocollision)
Pr(X(t)=0) = (λτ)0 0 e-λτ = e-Lwith corresponding throughput T = L e-L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 1 e 037
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 41
bull Slotted Aloha has double the maximumthroughput as pure Aloha and achievesthis maximum at a higher offered loadThe effective data rate is still less than40 of the raw transmission rate
bull Aloha does not listen to the carrier beforetransmission more sophisticatedtechniques are needed to increaseefficiency
bull Further problem the delayndashDelay
ndashThe mean packet delay
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
( ) ( )delayPr 1 e ekL Lkτ minus minus= = minus
( )delay1
1 e e ekL L L
kMPD E kτ
infinminus minus
=
= = minus =sum
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 42
Random Access Carrier Sense Multiple Accessbull CSMA (Carrier Sense Multiple Access) listen to the carrier before transmission
and transmits if channel is idlebull Detection delay and propagation delay are two important parameters
ndash Detection delay time required to sense the carrier and decide if it is idle or busyndash Propagation delay distancespeed_of_ligth The time required for bit to travel
from transmitter to the receiver
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 43
Random Access CSMA variations
Ad hoc Sensor Networks Multiple channel access
1-persistent CSMA Non-persistent CSMA
There will always be a collision iftwo nodes want to retransmit(usually you stop transmissionattempts after few tries)
Random backoff reduces probabilityof collisions Waste idle time if thebackoff time is too long
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 44
Random Access CSMA variations (contrsquo)
Ad hoc Sensor Networks Multiple channel access
p-persistent CSMA CSMA CD (Collision Detection)
A good tradeoff between non-persistent and 1-persistent CSMA
Same with CSMA however a stationalso listen to the carrier whiletransmitting to see if the transmissioncollides with someone elsetransmission It can be used in listen-while-talk capable channels (fullduplex) In single radio channels thetransmission need to be interrupted inorder to sense the channel
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 45
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 46
Random Access CSMA (contrsquo)bull How to select probability p
ndash Assume that M nodes have a packet to send and the medium is busy then Mp is theexpected number of nodes that will attempt to transmit once the medium becomesidle If Mp gt 1 then a collision is expected to occur Therefore the network mustmake sure that Mp lt 1 to avoid collision where M is the maximum number of nodesthat can be active at a time
bull Problems1 The mean packet delay increases exponently with increasing offered load2 CSMA protocols sense the carrier but sensing the carrier does not always releasestrue information about the status of the wireless channel There are two problems thatare unique to wireless channels (different than wireline channels)ndash Hidden terminal problemndash Exposed terminal problem
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 47
Hidden terminal problembull A sends to B C cannot receive Abull C wants to send to B C senses a
ldquofreerdquo medium (CS fails)bull collision at B A cannot receive the
collision (CD fails)bull A is ldquohiddenrdquo for C
Ad hoc Sensor Networks Multiple channel access
Random Access hidden and exposed terminal problemExposed terminal problem
bull B sends to A C wants to send to Dbull C has to wait CS signals a medium
in usebull since A is outside the radio range of
C waiting is not necessarybull C is ldquoexposedrdquo to B
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 48
Random Access MACAWhen a station wants to transmit databull It sends an RTS (Ready-to-Send) packet to the intended receiver
ndash The RTS packet contains the length of the data that needs to be transmittedndash Any station other than the intended recipient hearing RTS defers transmission for atime duration equal to the end of the corresponding CTS reception
bull The receiver sends back CTS (Clear-to-Send) packet back to sender if it isavailable to receivendash The CTS packet contains the length of the data that original sender wants totransmitndash Any station other than the original RTS sender hearing CTS defers transmissionuntil the data is sent
bull The original sender upon reception of the CTS starts transmitting
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 49
Hidden terminal problem
Ad hoc Sensor Networks Multiple channel access
Random Access MACA (contrsquo)Exposed terminal problem
Waiting time of node X is muchsmaller than waiting time of node C
bull C defers transmission upon hearing BrsquosRTS until B could get CTS from Abull After that C can start transmission to DFor that it first sends an RTSbull C is not longer exposed to the datatransmission of B
Problem It does not address the collision of RTS packet
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 50
Random Access reliabilitybull Wireless links are prone to have errors High packet loss rate detrimental to
transport-layer performance Mechanisms needed to reduce packet loss rateexperienced by upper layers
bull When node B receives a data packet from node A node B sends anAcknowledgement (Ack) This approach adopted in many protocols
bull If node A fails to receive an Ack it will retransmit the packet
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 51
Random Access MACAW (CSMACA) bull Physical carrier sense and virtual carrier sense using Network Allocation
Vector (NAV)ndash NAV is updated based on overheard RTSCTSDATAACK packets each of whichspecified duration of a pending transmissionndash Nodes stay silent when carrier sensed (physicalvirtual)
bull Backoff intervals used to reduce collision probabilityndash When transmitting a packet choose a backoff interval in the range [0w] where wis contention windowndash Count down the backoff interval when channel is idlendash Count down is suspended if channel becomes busyndash When backoff interval reaches 0 transmit RTS
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 52
Random Access CSMACA (contrsquo) bull The time spent counting down backoff intervals is a part of MAC overhead
ndash Choosing a large w leads to large backoff intervals and can result in larger overheadndash Choosing a small w leads to a larger number of collisions (when two nodes countdown to 0 simultaneously)ndash Since the number of nodes attempting to transmit simultaneously may change withtime some mechanism to manage contention is needed
bull IEEE 80211 DCF contention window w is chosen dynamically depending oncollision occurrence (binary exponential backoff)ndash When a node fails to receive CTS in response to its RTS it increases the contentionwindowndash w is doubled (up to an upper bound)ndash When a node successfully completes a data transfer it restores w to wmin
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 53
Random Access fairnessbull Simplest definition of fairness all nodes should receive equal bandwidthbull An example of unfairness
ndash Assume that initially A and B both choose a backoff interval in range [031] buttheir RTSs collidendash Nodes A and B then choose from range [063]ndash Node A chooses 4 slots and B choose 60 slotsndash After A transmits a packet it next chooses from range [031]ndash It is possible that A may transmit several packets before B transmits its first packet
Ad hoc Sensor Networks Multiple channel access
Unfairness occurs when one node has backed off much more than some other node
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 54
Random Access fairness (contrsquo) MACAW solution for fairnessbull When a node transmits a packet it appends value w to the packet all nodes
hearing value w use it for their future transmission attemptsbull Since w is an indication of the level of congestion in the vicinity of a specific
receiver node MACAW proposes maintaining w independently for eachreceiver
bull Using per-receiver wi is particularly useful in multi-hop environments sincecongestion level at different receivers can be very different
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 55
Random Access energy efficiency bull Since many mobile hosts are operated by batteries MAC
protocols which conserve energy are of interestbull Two approaches to reduce energy consumption
ndash Power save turn off wireless interface when desirablendash Power control reduce transmit power
bull Power control has some more potential benefitsndash Reduced interference (in ad hoc networks) eg it improves ALOHAefficiency where user with high power can capture a packet even if there is acollisionndash Increased spatial reuse (in infrastructured wireless networks) eg it is usedin CDMA to maintain target SIR of voice and data users or generally it canbe used to maintain target SIR for different user classes (admission control)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 56
Random Access power control bull When C transmits to D at a high power level B cannot receive Arsquos
transmission due to interference from Cbull If C reduces transmit power it can still communicate with D
ndash Reduces energy consumption at node Cndash Allows B to receive Arsquos transmission (spatial reuse)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 57
bull A special wireless ad hoc networkndash Large number of nodesndash Battery poweredndash Topology and density changendash Nodes for a common taskndash In-network data processing
bull Sensor-net applicationsndash Sensor-triggered bursty trafficndash Can often tolerate some delay
Ad hoc Sensor Networks Multiple channel access
Characteristics of Sensor Network
Scalabilty and self configurationEnergy efficiencyAdaptivityFairness not importantLatency
AdaptivityEnergy-delay tradoff
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 58
Ad hoc Sensor Networks Multiple channel access
Wireless sensor node and sensor network
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 59
MAC for WSN attributes bull Attributes
ndash Collision avoidance (Basic task)ndash Energy efficiency (most primary in WSN)ndash Scalability and adaptivityndash Channel utilizationndash Latencyndash Throughputndash Fairness
bull What causes energy wastendash Collisionsndash Control packet overheadndash Overhearing unnecessary trafficndash Long idle time (it consumes 50-100 of the power)
Ad hoc Sensor Networks Multiple channel access
Primary
Secondary
Dominant factor
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 60
MAC for WSN
bull Contention-based protocols need to work hard in all directions for energysavingsndash Reduce idle listening ndash support low duty cyclendash Better collision avoidancendash Reduce control overheadndash Avoid unnecessary overhearing
Ad hoc Sensor Networks Multiple channel access
Scheduled Protocols Contention ProtocolsCollisions No YesEnergy efficiency Good BadScalability and adaptivity Bad GoodMulti-hop communication Difficult EasyTime synchronization Strict Loose or not required
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 61
MAC for WSN classification
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 62
MAC for WSN SMAC (2002)bull It has been developed by Ye Heidemann and Estrinbull Tradeoffs between (latency fairness) and energybull Major components in S-MAC
ndash Periodic listen and sleep turn off radio when sleeping the reduced dutycycle is ~ 10 (eg 120ms on 12s off)ndash Collision avoidance is based on contention similarly to IEEE 80211 adhoc modendash Overhearing avoidance the node is sleeping when neighbors talkndash Massage passing long message is fragmented amp sent in burst operationwith extended Tx time and re-transmit occurs immediately
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 63
Summarybull The algorithms and protocols that enable sharing by multiple users and
controls the access to the wireless channel from different users are calledMAC
bull There are two types of wireless networks Infrastructured wireless networksand ad hoc wireless networks
bull We also want our wireless system to have high user capacity and we canidentify 10 methods for increasing it
bull There are several MACs with no absolute advantage it should be tailoredto the system requirements
bull There are primary concerns for designing a WSN MAC (collisionavoidance energy efficiency scalability and adaptivity) while thesecondary concerns are as follows (channel utilization latency throughputfairness)
bull Next lecture Routing protocols
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 6
Ad hoc Sensor Networks Multiple channel access
Layered Communication Approaches
2 Data Link layer it handles access to the unerlyingchannel and defines the data format It is split into twosublayersndash Logical Link Control it manages frames to upper and
lower layer (encapsulation decapsulation) and it enforceserrror control (checksum and parity bits)
ndash Medium Access Control (MAC) it coordinatestransmission between users sharing the spectrum Inwireless systems it must address the hidden terminalproblem and must exercise power control Goals preventcollisions while maximizing throughput and minimizingdelay
Physical
OSI reference model
Data link
Network
Transport
Middleware
Application
Recall from Chapter 1
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 7
Ad hoc Sensor Networks Multiple channel access
Medium Access Control
bull Thus the wireless spectrum (frequency band) is a very precious and limited resourcewe need to use this resource very efficiently
bull We also want our wireless system to have high user capacity with QoS constraintsbull The algorithms and protocols that enables this sharing by multiple users and
controlscoordinates the access to the wireless channel (medium) from differentusers are called MEDIUM ACCESS or MEDIA ACCESS or MULTIPLE ACCESSprotocols techniques schemes etchellip)
Designbull Goal Function Maximum Utilization
(eg throughput user capacity hellip)bull Constraints QoS
(eg delaylt10ms BERlt10-5hellip)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 8
Ad hoc Sensor Networks Multiple channel access
Medium Access Control (contrsquo)bull Multiple Access (Channel Partitioning or Coordinated Schemes)
ndash Techniques TDMA FDMA CDMA SDMAndash Examples GSM 3Ghellip
bull Random Access (or Random Schemes)ndash Techniques MACA MACAW Aloha 80211 MAChellipndash Examples Wifi WSNhellip
bull Polling based schemesndash Access is coordinated by a central nodendash Examples Bluetooth BlueSkyhellip
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 9
Ad hoc Sensor Networks Multiple channel access
bull Infrastructured networksndash base stations are the bridgesndash a mobile host will communicate with the
nearest base station (or access point)ndash handoff is taken when a host roams from one
base to anotherndash Medium access control assisted by BS
bull Ad hoc networksndash infrastructureless no fixed base stationsndash without the assistance of base stations for
communicationndash Due to transmission range constraint two
wireless nodes need multi-hop routing forcommunication
ndash quickly and unpredictably changing topology
Two types of wireless networks
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 10
Ad hoc Sensor Networks Multiple channel access
Two types of wireless networks (contrsquo)
bull Mesh networks serve as access networks that employ multi-hop wireless forwarding bynon-mobile nodes to relay traffic to and from the wired Internet In such an environmenthybrid wireless network technologies andor hierarchical network organization can beused for ad hoc and infrastructure wireless links
bull Wireless Sensor Networks are a special category of ad hoc networks that are used toprovide a wireless communication infrastructure among the sensors deployed in a specificapplication domain [rarr see Chapter 9 - 11]
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 11
Ad hoc Sensor Networks Multiple channel access
Infrastructured wireless networks Ad hoc wireless networksFixed infrastructure-based Infrastructureless
Guaranteed bandwidth Shared radio channel
Seamless connectivity (low call drops during handoffs)
Frequent path breaks due to mobility
High cost and time of deployment Quick and cost-effective deployment
Reuse of frequency spectrum (cellular principle) Dynamic frequency reuse based on carrier sense mechanism
Application domains include mainly civilian and commercial sectors
Application domains include battlefields emergency search and rescue operations
High cost of network maintenance Self-organization and maintenance properties are built into the network
Widely deployed and currently in the third generation of evolution
Several issues are to be addressed for successful commercial deployment even though widespread use exists in defense
Two types of wireless networks (contrsquo)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 12
Ad hoc Sensor Networks Multiple channel access
Duplexing
bull If the communication between two parties is one way then it is called simplexcommunication Simplex communication is achieved by default by using a singlewireless channel (frequency band) to transmit from sender to receiver
bull If the communication between two parties is two- way then it is called duplexcommunication Duplex communication achieved by
ndash Time Division (TDD) (famous in cellular systems)ndash Frequency Division (FDD) (famous in cellular systems)ndash Some other method like a random access method
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 13
Ad hoc Sensor Networks Multiple channel access
FDDbull A duplex channel consists of two
simplex channels with different carrierfrequencies
ndash Downlink band carries traffic frombase to mobile
ndash Uplink band carries traffic frommobile to base
Duplexing(contrsquo)TDD
bull channel (carrier frequency) is shared intime in a deterministic manner
ndash The time is slotted with fixed slotlength (sec)
ndash Some slots are used for downlinkchannel
ndash Some slots are used for uplink channel
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 14
Ad hoc Sensor Networks Multiple channel access
FDDbull FDD is used in radio systems that can
allocate individual radio frequenciesfor each user
bull For example analog systems AMPSbull In FDD channels are allocated by a
base stationbull A channel for a mobile is allocated
dynamicallybull All channels that a base station will use
are allocated usually staticallybull More suitable for wide-area cellular
networks GSM AMPS all use FDD
Duplexing(contrsquo)TDD
bull Can only be used in digital wirelesssystems (digital modulation)
bull Requires rigid timing andsynchronization
bull Mostly used in short-range and fixedwireless systems so that propagationdelay between base and mobile do notchange much with respect to locationof the mobile
bull Such as cordless phoneshellip
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 15
Ad hoc Sensor Networks Multiple channel access
Multiple Access narrow- vs wideband systemsbull Narrowband System
ndash The bandwidth is small compared to the coherence bandwidth of the channel(BltBcoh)
bull Wideband Systemndash The system bandwidth is much larger that the coherence bandwidth of the
multipath channel (BgtBcoh) A large number of users can access the same channel(frequency band) at the same time
bull Four major multiple access (MA) schemesndash Time Division Multiple Access (TDMA) it could be used in narrowband or
wideband systemndash Frequency Division Multiple Access (FDMA) it is usually used in narrowband
systemndash Code Division Multiple Access (CDMA) it is used in wideband systemndash Space Division Multiple Access (SDMA) for wireless systems with multiple
antennas it can be combined with all of the other MA methods
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 16
Ad hoc Sensor Networks Multiple channel access
Multiple Access cellular standards
Cellular System Multiple Access TechniqueAMPS (2G) FDMAFDD
CT2 Cordless Phone FDMATDD
DECT Cordless Phone FDMATDD
GSM (2G) TDMAFDD
USDC (IS-54 and IS-136) (2G) TDMAFDD
Personal Digital Cellular (2G) TDMAFDD
US IS-95 (2G) CDMAFDD
W-CDMA (3G) CDMAFDD CDMATDD
cdma2000 (3G) CDMAFDD CDMATDD
Narrowband System
Wideband System
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 17
Ad hoc Sensor Networks Multiple channel access
bull Individual radio channels are assigned toindividual users Each user is allocated a Wfrequency band by BS
bull If the channel allocated to a user is idle then itis not used by someone else waste of resource
bull Mobile and base can transmit and receivesimultaneously (FDD)
bull The W bandwidth of FDMA channels arerelatively low Symbol time is usually larger(low data rate) than the delay spread of themultipath channel (implies that inter-symbolinterference is low)
bull Lower complexity systems than TDMAsystems
Multiple Access FDMA
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 18
Ad hoc Sensor Networks Multiple channel access
Multiple Access FDMA (contrsquo)bull Capacity of FDMA systems
Nc = (B-2Bguard)Wwhere B is the total spectrum allocation Bguard is the guard band allocated atthe edge of the spectrum band W is the bandwidth of an individual channel
bull Erlang B and Erlang C system assumesndashinfinite population of sources which jointly offer traffic to Nc channelndashcall attempts arrive following a Poisson process so call arrivals are independentThe average arrival rate is λndashmessage length (holding times) are exponentially distributed (Markovian system)The average call length is hndashthe total amount of traffic offered in erlangs L= λhndashIf a user is rejected his next call attempt is made statistically independent of theprevious attempt (Erlang B)ndashIf all the channels are busy when a request arrives from a user the request isqueued An unlimited number of requests may be held in the queue (Erlang C)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 19
Ad hoc Sensor Networks Multiple channel access
bull Statictical measures of offered load Ncndash In Erlang B system the probability of call
blocking
ndash In Erlang C system the probabilty of waiting
ndash And the average wait time
c
c
cBLOCK
0
Pr
N
Nk
k
LN
Lk
=
=
sum
c
cc
wait 1
c0c
Pr 1
N
NkN
k
LL LL N kN
minus
=
=⎛ ⎞
+ minus⎜ ⎟⎝ ⎠
sum
wait waitc
Pr htN L
=minus
Multiple Access FDMA (contrsquo)Recall from
Queuing theory
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 20
Ad hoc Sensor Networks Multiple channel access
Multiple Access example of Erlang B systembull In an Erlang B system 40 channels are available A blocking probabilty of
less than 1 is required What is the traffic that can be serve if there is oneoperator or three operators
bull (Solution L(1 operator)gtL(3 operators))bull (Note The number of users increases faster than linearly with the number of
available channels The difference between actual increase and linearincrease is called the trunking gain It is preferable to have a large pool ofavailable channels that serves all users (a single operator) The reasons fornot choosing this approach are political not technical)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 21
Ad hoc Sensor Networks Multiple channel access
Multiple Access TDMAbull The allocated radio spectrum for the system is divided into time slots
ndash In each slot a user can transmit or receivendash A user occupies a cyclically repeating slotsndash A channel is logically defined as a particular time slot that repeats with some
periodbull TDMA systems buffer the data until its turn (time slot) comes to transmitbull In TDMATDD half of the slots in the frame is used for uplink channels the
other is used for downlink channelsbull In TDMAFDD a different carrier frequency is used for uplink or downlink
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 22
Ad hoc Sensor Networks Multiple channel access
Multiple Access TDMA (contrsquo)bull Preamble contains address and synchronization info to identify base
station and mobiles to each otherbull Guard times are used to allow synchronization of the receivers between
different slots and framesndash Different mobiles may have different propagation delays to a base station
because of different distances
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 23
Multiple Access TDMA (contrsquo)bull Each frame contains overhead bits and data bits Efficiency of frame is
defined as the percentage of data (information) bits to the total frame size inbits
η =(1 ndash boverheadbtotal) 100 where btotal= Tf R is the total number of bits in a frame Tf is the frameduration in sec boverhead is the number of overhead bits (GSM efficiency 73)
bull TDMA is usually combined with FDMAndash Neighboring cells are allocated and using different carrier frequencies (FDMA)
Inside a cell TDMA can be used Cells may be re-using the same frequency if theyare far from each-other
ndash There may be more than one channel allocated and used inside each cell Eachcarrier frequency (radio channel) may be using TDMA to further multiplex moreuser (ie having TDMA logical channels inside radio channels) For example inGSM each radio channel has 200KHz bandwidth and has 8 time slots (8 logicalchannels) Hence GSM is using FDMA combined with TDMA
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 24
Multiple Access TDMA (contrsquo)
Ad hoc Sensor Networks Multiple channel access
GSM (Europa) IS-54 (USA) PDC (Japan) DECT
Bit Rate 2708 Kbps 486 Kbps 42 Kbps 1152 Mbps
Bandwidth 200 KHz 30 KHz 25 KHz 1728 MHz
Time Slot 0577 ms 67 ms 67 ms 0417 ms
Upstream slots per frame 8 3 3 12
Duplexing FDD FDD FDD TDD
Efficiency 73 80 80 67
Modulation GMSK π4 DQPSK π4 DQPSK GMSK
Adaptive equalized Mandatory Mandatory Optional None
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 25
Multiple Access TDMA (contrsquo)bull Enables the sharing of a single radio channel among N usersbull Requires high data-rate per radio channel to support N users
simultaneously which requires adaptive equalizers to be usedin multipath environments
bull Transmission occurs in bursts (not continually) It enablespower saving by going to sleep modes in unrelated slots Itdiscontinues transmission and also enables mobile assistedhandoff
bull It equires synchronization of the receivers Need guard bits andsync bits which occurs large overhead per slot
bull Allocation of slots to mobile users should not be uniform Itmay depend on the traffic requirement of mobiles This bringsextra flexibility and efficiency compared to FDMA systems
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2620111015 TAacuteMOP ndash 412-082AKMR-2009-0006 26
Ad hoc Sensor Networks Multiple channel access
Multiple Access spread spectrum multiple accessbull SSMA uses signals that have transmission bandwidth that is
several orders of magnitude larger than minimum requiredRF bandwidth
bull It providesndash Immunity to multipath interferencendash Robust multiple access
bull Two techniquesndash Frequency Hopped Multiple Access (FHMA)ndash Direct Sequence Multiple Access (DSMA) or Code Division
Multiple Access (CDMA)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2720111015 TAacuteMOP ndash 412-082AKMR-2009-0006 27
Ad hoc Sensor Networks Multiple channel access
Multiple Access FHMAbull The carrier frequency of users are varied in a pseudo-random
fashionndash Each user is using a narrowband channel (spectrum) at a specific
instance of timendash The random change in frequency make the change of using the same
narrowband channel very lowbull The sender receiver change frequency (calling hopping)
using the same pseudo-random sequence hence they aresynchronized
bull Rate of hopping versus Symbol ratendash If hopping rate is greather Fast Frequency Hoppingndash If symbol rate is greater Slow Frequency Hopping
bull GSM and Bluetooth are example systems
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2820111015 TAacuteMOP ndash 412-082AKMR-2009-0006 28
Ad hoc Sensor Networks Multiple channel access
Multiple Access FHMA (contrsquo)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2920111015 TAacuteMOP ndash 412-082AKMR-2009-0006 29
Ad hoc Sensor Networks Multiple channel access
Multiple Access CDMAbull The narrowband message signal is multiplied by a very large bandwidth
signal called spreading signal (code) before modulation and transmissionover the air This is called spreading
bull Spreading signal use a pseudo-noise (PN) sequence (a pseudo-randomsequence) called codeword which are orthogonal (low autocorrelation)
bull The receiver correlator distinguishes the senders signal by examining thewideband signal with the same time-synchronized spreading code(despreading)
bull Advantagesndash Low power spectral densityndash Interference limited operationndash Privacyndash Reduction of multipath affects by using a larger spectrumndash Users can start their transmission at any time (random access possible)ndash Cell capacity is soft and higher than TDMA and FDMAndash No frequency management and no equalizers and quard time needed
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3020111015 TAacuteMOP ndash 412-082AKMR-2009-0006 30
Ad hoc Sensor Networks Multiple channel access
Multiple Access near far problem and power controlbull At a receiver the signals may come from various sources (multiuser) The
strongest signal usually captures the modulator The other signals areconsidered as noise Each source may have different distances to the basestation
bull In CDMA we want a BS to receive CDMA coded signals from variousmobile users at the same time
ndash Therefore the receiver power at the BS for all mobile users should be close toeach other This requires power control at the mobiles
bull Power Control BS monitors the RSSI (Received Signal Strength Indicator)values from different mobiles and then sends power change commands to themobiles over a forward channel The mobiles then adjust their transmitpower
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3120111015 TAacuteMOP ndash 412-082AKMR-2009-0006 31
Ad hoc Sensor Networks Multiple channel access
Multiple Access hybrid spread spectrum techniquesbull FDMACDMA
ndash Available wideband spectrum is divided into a number narrowband radio channels CDMAis employed inside each channel
bull DSFHMAndash The signals are spread using spreading codes (direct sequence signals are obtained) but
these signal are not transmitted over a constant carrier frequency they are transmitted over afrequency hopping carrier frequency
bull Time Division CDMA (TCDMA)ndash Each cell is using a different spreading code (CDMA employed between cells) that is
conveyed to the mobiles in its rangendash Inside each cell (inside a CDMA channel) TDMA is employed to multiplex multiple users
bull Time Division Frequency Hoppingndash At each time slot the user is hopped to a new frequency according to a pseudo-random
hopping sequencendash Employed in severe co-interference and multi-path environmentsndash Bluetooth and GSM are using this technique
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3220111015 TAacuteMOP ndash 412-082AKMR-2009-0006 32
Ad hoc Sensor Networks Multiple channel access
Multiple Access comparison TDMAFDMACDMA
Techniques TDMA FDMA CDMA SDMA
Idea
segment sending time into disjoint time-slots
Segment the frequency band into disjoint sub-bands
Spread the spectrum using orthogonal codes
Segment space into sectors
Signal separation
Synchronization in the time domain
Filtering in the frequency domain
Code Directed antennas
Advantage
Established flexible
Simple established robust
Flexible less frequency planning needed soft handover
Simple increases capacity
DisadvantageQuard space synchronization difficult
Inflexible scarce source
Complex receiver power control
inflexible
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3320111015 TAacuteMOP ndash 412-082AKMR-2009-0006 33
Ad hoc Sensor Networks Multiple channel access
User capacity per cell
The user capacity C is defined as the total number of active users percell that the system can support while meeting a common BERconstrain For orthogonal multiple access (as FDMA and TDMA)
C = Ncellwhere Ncell is the number of channels assigned to any given cellThe total number of orthogonal channels of bandwidth Bs that can becreated from a total system bandwidth of B is B Bs The reuse factorsatisfies N = (B Bs) Ncell this implies
C = (B Bs) N
Recall from Chapter 1
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3420111015 TAacuteMOP ndash 412-082AKMR-2009-0006 34
Ad hoc Sensor Networks Multiple channel access
10 methods for increasing capacity1 Increasing the amount of spectrum used very expensive2 More efficient modulation format and coding3 Better source coding4 Adaptive modulation and coding5 Discontinuous transmission6 Multiuser detection CDMA systems7 Reduction of cell radius effective but very expensive smaller
cells require more handovers8 Use of sector cells tripled cells have tripled BS antennas9 Use of an overlay structure10 Multiple antennas diversity MIMO systems SDMA
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3520111015 TAacuteMOP ndash 412-082AKMR-2009-0006 35
Ad hoc Sensor Networks Multiple channel access
bull Distributed operation is requiredbull Synchronization is required in TDMA-based systemsbull Hidden terminals are nodes hidden from a senderbull Exposed terminals are exposed nodes preventing a sender
from sendingbull Throughput needs to be maximizedbull Access delay should be minimizedbull Fairness refers to provide an equal share to all competing
nodesbull Real-time traffic support is required for voice video and
real-time databull Resource reservation is required for QoSbull Ability to measure resource availability handles the
resourcesbull Capability for power control reduces the energy
consumption
Random access in adhoc networks
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3620111015 TAacuteMOP ndash 412-082AKMR-2009-0006 36
Ad hoc Sensor Networks Multiple channel access
Random accessbull All random access techniques are based on packetized data or packet radio where user
date is collected into packets of M bitsbull Collision if packets from different users overlap in timebull The transmission time of a packet is τ = M R where R [bps] the data ratebull Analysis of random access techniques assumes that the users generate packets according
to memoryless Poisson process at a rate λ [packets per unit time] the probability that thenumber of packet arrivals in a time period [0t] denoted by X(t) is equal to integer k isgiven by
Pr(X(t)=k) = (λt)k k e-λt
bull The traffic load is defined as L = λ τ If L gt 1 means on avarage more packets arrive inthe system over a given time period than can be transmitted in that period so systemswith Lgt1 are unstable
bull The throughput is defined as the ratio of the average number of packets successfullytransmitted in any given time interval divided by the number of attempted transmissionsin that interval
T = L Pr(succesful packet transmission)bull The effective data rate of the system is RT
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3720111015 TAacuteMOP ndash 412-082AKMR-2009-0006 37
Ad hoc Sensor Networks Multiple channel access
Random access (contrsquo)bull We will examine the following attributes
ndash Channel utilization (Throughput)ndash Latencyndash Collision avoidance (Hidden and exposed terminals problem)ndash Reliability (ACK)ndash Energy efficiency (power control)ndash Fairnessndash Throughput needs to be maximized
bull We will discuss the following techniquesndash Aloha and slotted Alohandash CSMA Protocols (1-persistent non-persistent p-persistent CSMA CSMACD)ndash MACAndash MACAWndash Energy efficient MAC in wireless sensor networks (eg SMAC)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 38
Random Access ALOHA (1970)bull ALOHA was developed for a wireless system at the University of Hawaii
(Abramson et al)ndash Multiple remote stations plus one base stationndash Frame transmissions are made at one frequency from a remote station to the basestation the base station re-broadcasts frames on another frequency Contention is foraccess to the base station
bull Frames can be sent at any time to the base stationndash If no acknowledgment (ACK) is received assume that the frame is lost andretransmit after waiting a random amount of timendash If more than one station broadcasts on the base station access frequency at the sametime ndash a collision ndash interference will destroy the frames
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 39
Random Access ALOHA (contrsquo)
bull The probability that no packets generated during the tima [-τ τ] is given by (nocollision)
Pr(X(t)=0) = (λ2τ)0 0 e-2λτ = e-2Lwith corresponding throughput
T = L e-2L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 05 05e 018
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 40
Random Access slotted ALOHA (1971)bull Improvement Time is slotted and a packet can only be transmitted at the beginning of
one slot Thus it can reduce the collision duration
bull The probability that no packets generated during the time [0 τ] is given by (nocollision)
Pr(X(t)=0) = (λτ)0 0 e-λτ = e-Lwith corresponding throughput T = L e-L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 1 e 037
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 41
bull Slotted Aloha has double the maximumthroughput as pure Aloha and achievesthis maximum at a higher offered loadThe effective data rate is still less than40 of the raw transmission rate
bull Aloha does not listen to the carrier beforetransmission more sophisticatedtechniques are needed to increaseefficiency
bull Further problem the delayndashDelay
ndashThe mean packet delay
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
( ) ( )delayPr 1 e ekL Lkτ minus minus= = minus
( )delay1
1 e e ekL L L
kMPD E kτ
infinminus minus
=
= = minus =sum
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 42
Random Access Carrier Sense Multiple Accessbull CSMA (Carrier Sense Multiple Access) listen to the carrier before transmission
and transmits if channel is idlebull Detection delay and propagation delay are two important parameters
ndash Detection delay time required to sense the carrier and decide if it is idle or busyndash Propagation delay distancespeed_of_ligth The time required for bit to travel
from transmitter to the receiver
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 43
Random Access CSMA variations
Ad hoc Sensor Networks Multiple channel access
1-persistent CSMA Non-persistent CSMA
There will always be a collision iftwo nodes want to retransmit(usually you stop transmissionattempts after few tries)
Random backoff reduces probabilityof collisions Waste idle time if thebackoff time is too long
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 44
Random Access CSMA variations (contrsquo)
Ad hoc Sensor Networks Multiple channel access
p-persistent CSMA CSMA CD (Collision Detection)
A good tradeoff between non-persistent and 1-persistent CSMA
Same with CSMA however a stationalso listen to the carrier whiletransmitting to see if the transmissioncollides with someone elsetransmission It can be used in listen-while-talk capable channels (fullduplex) In single radio channels thetransmission need to be interrupted inorder to sense the channel
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 45
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 46
Random Access CSMA (contrsquo)bull How to select probability p
ndash Assume that M nodes have a packet to send and the medium is busy then Mp is theexpected number of nodes that will attempt to transmit once the medium becomesidle If Mp gt 1 then a collision is expected to occur Therefore the network mustmake sure that Mp lt 1 to avoid collision where M is the maximum number of nodesthat can be active at a time
bull Problems1 The mean packet delay increases exponently with increasing offered load2 CSMA protocols sense the carrier but sensing the carrier does not always releasestrue information about the status of the wireless channel There are two problems thatare unique to wireless channels (different than wireline channels)ndash Hidden terminal problemndash Exposed terminal problem
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 47
Hidden terminal problembull A sends to B C cannot receive Abull C wants to send to B C senses a
ldquofreerdquo medium (CS fails)bull collision at B A cannot receive the
collision (CD fails)bull A is ldquohiddenrdquo for C
Ad hoc Sensor Networks Multiple channel access
Random Access hidden and exposed terminal problemExposed terminal problem
bull B sends to A C wants to send to Dbull C has to wait CS signals a medium
in usebull since A is outside the radio range of
C waiting is not necessarybull C is ldquoexposedrdquo to B
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 48
Random Access MACAWhen a station wants to transmit databull It sends an RTS (Ready-to-Send) packet to the intended receiver
ndash The RTS packet contains the length of the data that needs to be transmittedndash Any station other than the intended recipient hearing RTS defers transmission for atime duration equal to the end of the corresponding CTS reception
bull The receiver sends back CTS (Clear-to-Send) packet back to sender if it isavailable to receivendash The CTS packet contains the length of the data that original sender wants totransmitndash Any station other than the original RTS sender hearing CTS defers transmissionuntil the data is sent
bull The original sender upon reception of the CTS starts transmitting
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 49
Hidden terminal problem
Ad hoc Sensor Networks Multiple channel access
Random Access MACA (contrsquo)Exposed terminal problem
Waiting time of node X is muchsmaller than waiting time of node C
bull C defers transmission upon hearing BrsquosRTS until B could get CTS from Abull After that C can start transmission to DFor that it first sends an RTSbull C is not longer exposed to the datatransmission of B
Problem It does not address the collision of RTS packet
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 50
Random Access reliabilitybull Wireless links are prone to have errors High packet loss rate detrimental to
transport-layer performance Mechanisms needed to reduce packet loss rateexperienced by upper layers
bull When node B receives a data packet from node A node B sends anAcknowledgement (Ack) This approach adopted in many protocols
bull If node A fails to receive an Ack it will retransmit the packet
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 51
Random Access MACAW (CSMACA) bull Physical carrier sense and virtual carrier sense using Network Allocation
Vector (NAV)ndash NAV is updated based on overheard RTSCTSDATAACK packets each of whichspecified duration of a pending transmissionndash Nodes stay silent when carrier sensed (physicalvirtual)
bull Backoff intervals used to reduce collision probabilityndash When transmitting a packet choose a backoff interval in the range [0w] where wis contention windowndash Count down the backoff interval when channel is idlendash Count down is suspended if channel becomes busyndash When backoff interval reaches 0 transmit RTS
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 52
Random Access CSMACA (contrsquo) bull The time spent counting down backoff intervals is a part of MAC overhead
ndash Choosing a large w leads to large backoff intervals and can result in larger overheadndash Choosing a small w leads to a larger number of collisions (when two nodes countdown to 0 simultaneously)ndash Since the number of nodes attempting to transmit simultaneously may change withtime some mechanism to manage contention is needed
bull IEEE 80211 DCF contention window w is chosen dynamically depending oncollision occurrence (binary exponential backoff)ndash When a node fails to receive CTS in response to its RTS it increases the contentionwindowndash w is doubled (up to an upper bound)ndash When a node successfully completes a data transfer it restores w to wmin
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 53
Random Access fairnessbull Simplest definition of fairness all nodes should receive equal bandwidthbull An example of unfairness
ndash Assume that initially A and B both choose a backoff interval in range [031] buttheir RTSs collidendash Nodes A and B then choose from range [063]ndash Node A chooses 4 slots and B choose 60 slotsndash After A transmits a packet it next chooses from range [031]ndash It is possible that A may transmit several packets before B transmits its first packet
Ad hoc Sensor Networks Multiple channel access
Unfairness occurs when one node has backed off much more than some other node
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 54
Random Access fairness (contrsquo) MACAW solution for fairnessbull When a node transmits a packet it appends value w to the packet all nodes
hearing value w use it for their future transmission attemptsbull Since w is an indication of the level of congestion in the vicinity of a specific
receiver node MACAW proposes maintaining w independently for eachreceiver
bull Using per-receiver wi is particularly useful in multi-hop environments sincecongestion level at different receivers can be very different
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 55
Random Access energy efficiency bull Since many mobile hosts are operated by batteries MAC
protocols which conserve energy are of interestbull Two approaches to reduce energy consumption
ndash Power save turn off wireless interface when desirablendash Power control reduce transmit power
bull Power control has some more potential benefitsndash Reduced interference (in ad hoc networks) eg it improves ALOHAefficiency where user with high power can capture a packet even if there is acollisionndash Increased spatial reuse (in infrastructured wireless networks) eg it is usedin CDMA to maintain target SIR of voice and data users or generally it canbe used to maintain target SIR for different user classes (admission control)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 56
Random Access power control bull When C transmits to D at a high power level B cannot receive Arsquos
transmission due to interference from Cbull If C reduces transmit power it can still communicate with D
ndash Reduces energy consumption at node Cndash Allows B to receive Arsquos transmission (spatial reuse)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 57
bull A special wireless ad hoc networkndash Large number of nodesndash Battery poweredndash Topology and density changendash Nodes for a common taskndash In-network data processing
bull Sensor-net applicationsndash Sensor-triggered bursty trafficndash Can often tolerate some delay
Ad hoc Sensor Networks Multiple channel access
Characteristics of Sensor Network
Scalabilty and self configurationEnergy efficiencyAdaptivityFairness not importantLatency
AdaptivityEnergy-delay tradoff
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 58
Ad hoc Sensor Networks Multiple channel access
Wireless sensor node and sensor network
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 59
MAC for WSN attributes bull Attributes
ndash Collision avoidance (Basic task)ndash Energy efficiency (most primary in WSN)ndash Scalability and adaptivityndash Channel utilizationndash Latencyndash Throughputndash Fairness
bull What causes energy wastendash Collisionsndash Control packet overheadndash Overhearing unnecessary trafficndash Long idle time (it consumes 50-100 of the power)
Ad hoc Sensor Networks Multiple channel access
Primary
Secondary
Dominant factor
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 60
MAC for WSN
bull Contention-based protocols need to work hard in all directions for energysavingsndash Reduce idle listening ndash support low duty cyclendash Better collision avoidancendash Reduce control overheadndash Avoid unnecessary overhearing
Ad hoc Sensor Networks Multiple channel access
Scheduled Protocols Contention ProtocolsCollisions No YesEnergy efficiency Good BadScalability and adaptivity Bad GoodMulti-hop communication Difficult EasyTime synchronization Strict Loose or not required
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 61
MAC for WSN classification
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 62
MAC for WSN SMAC (2002)bull It has been developed by Ye Heidemann and Estrinbull Tradeoffs between (latency fairness) and energybull Major components in S-MAC
ndash Periodic listen and sleep turn off radio when sleeping the reduced dutycycle is ~ 10 (eg 120ms on 12s off)ndash Collision avoidance is based on contention similarly to IEEE 80211 adhoc modendash Overhearing avoidance the node is sleeping when neighbors talkndash Massage passing long message is fragmented amp sent in burst operationwith extended Tx time and re-transmit occurs immediately
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 63
Summarybull The algorithms and protocols that enable sharing by multiple users and
controls the access to the wireless channel from different users are calledMAC
bull There are two types of wireless networks Infrastructured wireless networksand ad hoc wireless networks
bull We also want our wireless system to have high user capacity and we canidentify 10 methods for increasing it
bull There are several MACs with no absolute advantage it should be tailoredto the system requirements
bull There are primary concerns for designing a WSN MAC (collisionavoidance energy efficiency scalability and adaptivity) while thesecondary concerns are as follows (channel utilization latency throughputfairness)
bull Next lecture Routing protocols
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 7
Ad hoc Sensor Networks Multiple channel access
Medium Access Control
bull Thus the wireless spectrum (frequency band) is a very precious and limited resourcewe need to use this resource very efficiently
bull We also want our wireless system to have high user capacity with QoS constraintsbull The algorithms and protocols that enables this sharing by multiple users and
controlscoordinates the access to the wireless channel (medium) from differentusers are called MEDIUM ACCESS or MEDIA ACCESS or MULTIPLE ACCESSprotocols techniques schemes etchellip)
Designbull Goal Function Maximum Utilization
(eg throughput user capacity hellip)bull Constraints QoS
(eg delaylt10ms BERlt10-5hellip)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 8
Ad hoc Sensor Networks Multiple channel access
Medium Access Control (contrsquo)bull Multiple Access (Channel Partitioning or Coordinated Schemes)
ndash Techniques TDMA FDMA CDMA SDMAndash Examples GSM 3Ghellip
bull Random Access (or Random Schemes)ndash Techniques MACA MACAW Aloha 80211 MAChellipndash Examples Wifi WSNhellip
bull Polling based schemesndash Access is coordinated by a central nodendash Examples Bluetooth BlueSkyhellip
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 9
Ad hoc Sensor Networks Multiple channel access
bull Infrastructured networksndash base stations are the bridgesndash a mobile host will communicate with the
nearest base station (or access point)ndash handoff is taken when a host roams from one
base to anotherndash Medium access control assisted by BS
bull Ad hoc networksndash infrastructureless no fixed base stationsndash without the assistance of base stations for
communicationndash Due to transmission range constraint two
wireless nodes need multi-hop routing forcommunication
ndash quickly and unpredictably changing topology
Two types of wireless networks
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 10
Ad hoc Sensor Networks Multiple channel access
Two types of wireless networks (contrsquo)
bull Mesh networks serve as access networks that employ multi-hop wireless forwarding bynon-mobile nodes to relay traffic to and from the wired Internet In such an environmenthybrid wireless network technologies andor hierarchical network organization can beused for ad hoc and infrastructure wireless links
bull Wireless Sensor Networks are a special category of ad hoc networks that are used toprovide a wireless communication infrastructure among the sensors deployed in a specificapplication domain [rarr see Chapter 9 - 11]
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 11
Ad hoc Sensor Networks Multiple channel access
Infrastructured wireless networks Ad hoc wireless networksFixed infrastructure-based Infrastructureless
Guaranteed bandwidth Shared radio channel
Seamless connectivity (low call drops during handoffs)
Frequent path breaks due to mobility
High cost and time of deployment Quick and cost-effective deployment
Reuse of frequency spectrum (cellular principle) Dynamic frequency reuse based on carrier sense mechanism
Application domains include mainly civilian and commercial sectors
Application domains include battlefields emergency search and rescue operations
High cost of network maintenance Self-organization and maintenance properties are built into the network
Widely deployed and currently in the third generation of evolution
Several issues are to be addressed for successful commercial deployment even though widespread use exists in defense
Two types of wireless networks (contrsquo)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 12
Ad hoc Sensor Networks Multiple channel access
Duplexing
bull If the communication between two parties is one way then it is called simplexcommunication Simplex communication is achieved by default by using a singlewireless channel (frequency band) to transmit from sender to receiver
bull If the communication between two parties is two- way then it is called duplexcommunication Duplex communication achieved by
ndash Time Division (TDD) (famous in cellular systems)ndash Frequency Division (FDD) (famous in cellular systems)ndash Some other method like a random access method
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 13
Ad hoc Sensor Networks Multiple channel access
FDDbull A duplex channel consists of two
simplex channels with different carrierfrequencies
ndash Downlink band carries traffic frombase to mobile
ndash Uplink band carries traffic frommobile to base
Duplexing(contrsquo)TDD
bull channel (carrier frequency) is shared intime in a deterministic manner
ndash The time is slotted with fixed slotlength (sec)
ndash Some slots are used for downlinkchannel
ndash Some slots are used for uplink channel
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 14
Ad hoc Sensor Networks Multiple channel access
FDDbull FDD is used in radio systems that can
allocate individual radio frequenciesfor each user
bull For example analog systems AMPSbull In FDD channels are allocated by a
base stationbull A channel for a mobile is allocated
dynamicallybull All channels that a base station will use
are allocated usually staticallybull More suitable for wide-area cellular
networks GSM AMPS all use FDD
Duplexing(contrsquo)TDD
bull Can only be used in digital wirelesssystems (digital modulation)
bull Requires rigid timing andsynchronization
bull Mostly used in short-range and fixedwireless systems so that propagationdelay between base and mobile do notchange much with respect to locationof the mobile
bull Such as cordless phoneshellip
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 15
Ad hoc Sensor Networks Multiple channel access
Multiple Access narrow- vs wideband systemsbull Narrowband System
ndash The bandwidth is small compared to the coherence bandwidth of the channel(BltBcoh)
bull Wideband Systemndash The system bandwidth is much larger that the coherence bandwidth of the
multipath channel (BgtBcoh) A large number of users can access the same channel(frequency band) at the same time
bull Four major multiple access (MA) schemesndash Time Division Multiple Access (TDMA) it could be used in narrowband or
wideband systemndash Frequency Division Multiple Access (FDMA) it is usually used in narrowband
systemndash Code Division Multiple Access (CDMA) it is used in wideband systemndash Space Division Multiple Access (SDMA) for wireless systems with multiple
antennas it can be combined with all of the other MA methods
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 16
Ad hoc Sensor Networks Multiple channel access
Multiple Access cellular standards
Cellular System Multiple Access TechniqueAMPS (2G) FDMAFDD
CT2 Cordless Phone FDMATDD
DECT Cordless Phone FDMATDD
GSM (2G) TDMAFDD
USDC (IS-54 and IS-136) (2G) TDMAFDD
Personal Digital Cellular (2G) TDMAFDD
US IS-95 (2G) CDMAFDD
W-CDMA (3G) CDMAFDD CDMATDD
cdma2000 (3G) CDMAFDD CDMATDD
Narrowband System
Wideband System
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 17
Ad hoc Sensor Networks Multiple channel access
bull Individual radio channels are assigned toindividual users Each user is allocated a Wfrequency band by BS
bull If the channel allocated to a user is idle then itis not used by someone else waste of resource
bull Mobile and base can transmit and receivesimultaneously (FDD)
bull The W bandwidth of FDMA channels arerelatively low Symbol time is usually larger(low data rate) than the delay spread of themultipath channel (implies that inter-symbolinterference is low)
bull Lower complexity systems than TDMAsystems
Multiple Access FDMA
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 18
Ad hoc Sensor Networks Multiple channel access
Multiple Access FDMA (contrsquo)bull Capacity of FDMA systems
Nc = (B-2Bguard)Wwhere B is the total spectrum allocation Bguard is the guard band allocated atthe edge of the spectrum band W is the bandwidth of an individual channel
bull Erlang B and Erlang C system assumesndashinfinite population of sources which jointly offer traffic to Nc channelndashcall attempts arrive following a Poisson process so call arrivals are independentThe average arrival rate is λndashmessage length (holding times) are exponentially distributed (Markovian system)The average call length is hndashthe total amount of traffic offered in erlangs L= λhndashIf a user is rejected his next call attempt is made statistically independent of theprevious attempt (Erlang B)ndashIf all the channels are busy when a request arrives from a user the request isqueued An unlimited number of requests may be held in the queue (Erlang C)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 19
Ad hoc Sensor Networks Multiple channel access
bull Statictical measures of offered load Ncndash In Erlang B system the probability of call
blocking
ndash In Erlang C system the probabilty of waiting
ndash And the average wait time
c
c
cBLOCK
0
Pr
N
Nk
k
LN
Lk
=
=
sum
c
cc
wait 1
c0c
Pr 1
N
NkN
k
LL LL N kN
minus
=
=⎛ ⎞
+ minus⎜ ⎟⎝ ⎠
sum
wait waitc
Pr htN L
=minus
Multiple Access FDMA (contrsquo)Recall from
Queuing theory
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 20
Ad hoc Sensor Networks Multiple channel access
Multiple Access example of Erlang B systembull In an Erlang B system 40 channels are available A blocking probabilty of
less than 1 is required What is the traffic that can be serve if there is oneoperator or three operators
bull (Solution L(1 operator)gtL(3 operators))bull (Note The number of users increases faster than linearly with the number of
available channels The difference between actual increase and linearincrease is called the trunking gain It is preferable to have a large pool ofavailable channels that serves all users (a single operator) The reasons fornot choosing this approach are political not technical)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 21
Ad hoc Sensor Networks Multiple channel access
Multiple Access TDMAbull The allocated radio spectrum for the system is divided into time slots
ndash In each slot a user can transmit or receivendash A user occupies a cyclically repeating slotsndash A channel is logically defined as a particular time slot that repeats with some
periodbull TDMA systems buffer the data until its turn (time slot) comes to transmitbull In TDMATDD half of the slots in the frame is used for uplink channels the
other is used for downlink channelsbull In TDMAFDD a different carrier frequency is used for uplink or downlink
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 22
Ad hoc Sensor Networks Multiple channel access
Multiple Access TDMA (contrsquo)bull Preamble contains address and synchronization info to identify base
station and mobiles to each otherbull Guard times are used to allow synchronization of the receivers between
different slots and framesndash Different mobiles may have different propagation delays to a base station
because of different distances
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 23
Multiple Access TDMA (contrsquo)bull Each frame contains overhead bits and data bits Efficiency of frame is
defined as the percentage of data (information) bits to the total frame size inbits
η =(1 ndash boverheadbtotal) 100 where btotal= Tf R is the total number of bits in a frame Tf is the frameduration in sec boverhead is the number of overhead bits (GSM efficiency 73)
bull TDMA is usually combined with FDMAndash Neighboring cells are allocated and using different carrier frequencies (FDMA)
Inside a cell TDMA can be used Cells may be re-using the same frequency if theyare far from each-other
ndash There may be more than one channel allocated and used inside each cell Eachcarrier frequency (radio channel) may be using TDMA to further multiplex moreuser (ie having TDMA logical channels inside radio channels) For example inGSM each radio channel has 200KHz bandwidth and has 8 time slots (8 logicalchannels) Hence GSM is using FDMA combined with TDMA
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 24
Multiple Access TDMA (contrsquo)
Ad hoc Sensor Networks Multiple channel access
GSM (Europa) IS-54 (USA) PDC (Japan) DECT
Bit Rate 2708 Kbps 486 Kbps 42 Kbps 1152 Mbps
Bandwidth 200 KHz 30 KHz 25 KHz 1728 MHz
Time Slot 0577 ms 67 ms 67 ms 0417 ms
Upstream slots per frame 8 3 3 12
Duplexing FDD FDD FDD TDD
Efficiency 73 80 80 67
Modulation GMSK π4 DQPSK π4 DQPSK GMSK
Adaptive equalized Mandatory Mandatory Optional None
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 25
Multiple Access TDMA (contrsquo)bull Enables the sharing of a single radio channel among N usersbull Requires high data-rate per radio channel to support N users
simultaneously which requires adaptive equalizers to be usedin multipath environments
bull Transmission occurs in bursts (not continually) It enablespower saving by going to sleep modes in unrelated slots Itdiscontinues transmission and also enables mobile assistedhandoff
bull It equires synchronization of the receivers Need guard bits andsync bits which occurs large overhead per slot
bull Allocation of slots to mobile users should not be uniform Itmay depend on the traffic requirement of mobiles This bringsextra flexibility and efficiency compared to FDMA systems
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2620111015 TAacuteMOP ndash 412-082AKMR-2009-0006 26
Ad hoc Sensor Networks Multiple channel access
Multiple Access spread spectrum multiple accessbull SSMA uses signals that have transmission bandwidth that is
several orders of magnitude larger than minimum requiredRF bandwidth
bull It providesndash Immunity to multipath interferencendash Robust multiple access
bull Two techniquesndash Frequency Hopped Multiple Access (FHMA)ndash Direct Sequence Multiple Access (DSMA) or Code Division
Multiple Access (CDMA)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2720111015 TAacuteMOP ndash 412-082AKMR-2009-0006 27
Ad hoc Sensor Networks Multiple channel access
Multiple Access FHMAbull The carrier frequency of users are varied in a pseudo-random
fashionndash Each user is using a narrowband channel (spectrum) at a specific
instance of timendash The random change in frequency make the change of using the same
narrowband channel very lowbull The sender receiver change frequency (calling hopping)
using the same pseudo-random sequence hence they aresynchronized
bull Rate of hopping versus Symbol ratendash If hopping rate is greather Fast Frequency Hoppingndash If symbol rate is greater Slow Frequency Hopping
bull GSM and Bluetooth are example systems
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2820111015 TAacuteMOP ndash 412-082AKMR-2009-0006 28
Ad hoc Sensor Networks Multiple channel access
Multiple Access FHMA (contrsquo)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2920111015 TAacuteMOP ndash 412-082AKMR-2009-0006 29
Ad hoc Sensor Networks Multiple channel access
Multiple Access CDMAbull The narrowband message signal is multiplied by a very large bandwidth
signal called spreading signal (code) before modulation and transmissionover the air This is called spreading
bull Spreading signal use a pseudo-noise (PN) sequence (a pseudo-randomsequence) called codeword which are orthogonal (low autocorrelation)
bull The receiver correlator distinguishes the senders signal by examining thewideband signal with the same time-synchronized spreading code(despreading)
bull Advantagesndash Low power spectral densityndash Interference limited operationndash Privacyndash Reduction of multipath affects by using a larger spectrumndash Users can start their transmission at any time (random access possible)ndash Cell capacity is soft and higher than TDMA and FDMAndash No frequency management and no equalizers and quard time needed
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3020111015 TAacuteMOP ndash 412-082AKMR-2009-0006 30
Ad hoc Sensor Networks Multiple channel access
Multiple Access near far problem and power controlbull At a receiver the signals may come from various sources (multiuser) The
strongest signal usually captures the modulator The other signals areconsidered as noise Each source may have different distances to the basestation
bull In CDMA we want a BS to receive CDMA coded signals from variousmobile users at the same time
ndash Therefore the receiver power at the BS for all mobile users should be close toeach other This requires power control at the mobiles
bull Power Control BS monitors the RSSI (Received Signal Strength Indicator)values from different mobiles and then sends power change commands to themobiles over a forward channel The mobiles then adjust their transmitpower
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3120111015 TAacuteMOP ndash 412-082AKMR-2009-0006 31
Ad hoc Sensor Networks Multiple channel access
Multiple Access hybrid spread spectrum techniquesbull FDMACDMA
ndash Available wideband spectrum is divided into a number narrowband radio channels CDMAis employed inside each channel
bull DSFHMAndash The signals are spread using spreading codes (direct sequence signals are obtained) but
these signal are not transmitted over a constant carrier frequency they are transmitted over afrequency hopping carrier frequency
bull Time Division CDMA (TCDMA)ndash Each cell is using a different spreading code (CDMA employed between cells) that is
conveyed to the mobiles in its rangendash Inside each cell (inside a CDMA channel) TDMA is employed to multiplex multiple users
bull Time Division Frequency Hoppingndash At each time slot the user is hopped to a new frequency according to a pseudo-random
hopping sequencendash Employed in severe co-interference and multi-path environmentsndash Bluetooth and GSM are using this technique
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3220111015 TAacuteMOP ndash 412-082AKMR-2009-0006 32
Ad hoc Sensor Networks Multiple channel access
Multiple Access comparison TDMAFDMACDMA
Techniques TDMA FDMA CDMA SDMA
Idea
segment sending time into disjoint time-slots
Segment the frequency band into disjoint sub-bands
Spread the spectrum using orthogonal codes
Segment space into sectors
Signal separation
Synchronization in the time domain
Filtering in the frequency domain
Code Directed antennas
Advantage
Established flexible
Simple established robust
Flexible less frequency planning needed soft handover
Simple increases capacity
DisadvantageQuard space synchronization difficult
Inflexible scarce source
Complex receiver power control
inflexible
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3320111015 TAacuteMOP ndash 412-082AKMR-2009-0006 33
Ad hoc Sensor Networks Multiple channel access
User capacity per cell
The user capacity C is defined as the total number of active users percell that the system can support while meeting a common BERconstrain For orthogonal multiple access (as FDMA and TDMA)
C = Ncellwhere Ncell is the number of channels assigned to any given cellThe total number of orthogonal channels of bandwidth Bs that can becreated from a total system bandwidth of B is B Bs The reuse factorsatisfies N = (B Bs) Ncell this implies
C = (B Bs) N
Recall from Chapter 1
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3420111015 TAacuteMOP ndash 412-082AKMR-2009-0006 34
Ad hoc Sensor Networks Multiple channel access
10 methods for increasing capacity1 Increasing the amount of spectrum used very expensive2 More efficient modulation format and coding3 Better source coding4 Adaptive modulation and coding5 Discontinuous transmission6 Multiuser detection CDMA systems7 Reduction of cell radius effective but very expensive smaller
cells require more handovers8 Use of sector cells tripled cells have tripled BS antennas9 Use of an overlay structure10 Multiple antennas diversity MIMO systems SDMA
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3520111015 TAacuteMOP ndash 412-082AKMR-2009-0006 35
Ad hoc Sensor Networks Multiple channel access
bull Distributed operation is requiredbull Synchronization is required in TDMA-based systemsbull Hidden terminals are nodes hidden from a senderbull Exposed terminals are exposed nodes preventing a sender
from sendingbull Throughput needs to be maximizedbull Access delay should be minimizedbull Fairness refers to provide an equal share to all competing
nodesbull Real-time traffic support is required for voice video and
real-time databull Resource reservation is required for QoSbull Ability to measure resource availability handles the
resourcesbull Capability for power control reduces the energy
consumption
Random access in adhoc networks
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3620111015 TAacuteMOP ndash 412-082AKMR-2009-0006 36
Ad hoc Sensor Networks Multiple channel access
Random accessbull All random access techniques are based on packetized data or packet radio where user
date is collected into packets of M bitsbull Collision if packets from different users overlap in timebull The transmission time of a packet is τ = M R where R [bps] the data ratebull Analysis of random access techniques assumes that the users generate packets according
to memoryless Poisson process at a rate λ [packets per unit time] the probability that thenumber of packet arrivals in a time period [0t] denoted by X(t) is equal to integer k isgiven by
Pr(X(t)=k) = (λt)k k e-λt
bull The traffic load is defined as L = λ τ If L gt 1 means on avarage more packets arrive inthe system over a given time period than can be transmitted in that period so systemswith Lgt1 are unstable
bull The throughput is defined as the ratio of the average number of packets successfullytransmitted in any given time interval divided by the number of attempted transmissionsin that interval
T = L Pr(succesful packet transmission)bull The effective data rate of the system is RT
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3720111015 TAacuteMOP ndash 412-082AKMR-2009-0006 37
Ad hoc Sensor Networks Multiple channel access
Random access (contrsquo)bull We will examine the following attributes
ndash Channel utilization (Throughput)ndash Latencyndash Collision avoidance (Hidden and exposed terminals problem)ndash Reliability (ACK)ndash Energy efficiency (power control)ndash Fairnessndash Throughput needs to be maximized
bull We will discuss the following techniquesndash Aloha and slotted Alohandash CSMA Protocols (1-persistent non-persistent p-persistent CSMA CSMACD)ndash MACAndash MACAWndash Energy efficient MAC in wireless sensor networks (eg SMAC)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 38
Random Access ALOHA (1970)bull ALOHA was developed for a wireless system at the University of Hawaii
(Abramson et al)ndash Multiple remote stations plus one base stationndash Frame transmissions are made at one frequency from a remote station to the basestation the base station re-broadcasts frames on another frequency Contention is foraccess to the base station
bull Frames can be sent at any time to the base stationndash If no acknowledgment (ACK) is received assume that the frame is lost andretransmit after waiting a random amount of timendash If more than one station broadcasts on the base station access frequency at the sametime ndash a collision ndash interference will destroy the frames
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 39
Random Access ALOHA (contrsquo)
bull The probability that no packets generated during the tima [-τ τ] is given by (nocollision)
Pr(X(t)=0) = (λ2τ)0 0 e-2λτ = e-2Lwith corresponding throughput
T = L e-2L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 05 05e 018
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 40
Random Access slotted ALOHA (1971)bull Improvement Time is slotted and a packet can only be transmitted at the beginning of
one slot Thus it can reduce the collision duration
bull The probability that no packets generated during the time [0 τ] is given by (nocollision)
Pr(X(t)=0) = (λτ)0 0 e-λτ = e-Lwith corresponding throughput T = L e-L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 1 e 037
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 41
bull Slotted Aloha has double the maximumthroughput as pure Aloha and achievesthis maximum at a higher offered loadThe effective data rate is still less than40 of the raw transmission rate
bull Aloha does not listen to the carrier beforetransmission more sophisticatedtechniques are needed to increaseefficiency
bull Further problem the delayndashDelay
ndashThe mean packet delay
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
( ) ( )delayPr 1 e ekL Lkτ minus minus= = minus
( )delay1
1 e e ekL L L
kMPD E kτ
infinminus minus
=
= = minus =sum
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 42
Random Access Carrier Sense Multiple Accessbull CSMA (Carrier Sense Multiple Access) listen to the carrier before transmission
and transmits if channel is idlebull Detection delay and propagation delay are two important parameters
ndash Detection delay time required to sense the carrier and decide if it is idle or busyndash Propagation delay distancespeed_of_ligth The time required for bit to travel
from transmitter to the receiver
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 43
Random Access CSMA variations
Ad hoc Sensor Networks Multiple channel access
1-persistent CSMA Non-persistent CSMA
There will always be a collision iftwo nodes want to retransmit(usually you stop transmissionattempts after few tries)
Random backoff reduces probabilityof collisions Waste idle time if thebackoff time is too long
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 44
Random Access CSMA variations (contrsquo)
Ad hoc Sensor Networks Multiple channel access
p-persistent CSMA CSMA CD (Collision Detection)
A good tradeoff between non-persistent and 1-persistent CSMA
Same with CSMA however a stationalso listen to the carrier whiletransmitting to see if the transmissioncollides with someone elsetransmission It can be used in listen-while-talk capable channels (fullduplex) In single radio channels thetransmission need to be interrupted inorder to sense the channel
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 45
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 46
Random Access CSMA (contrsquo)bull How to select probability p
ndash Assume that M nodes have a packet to send and the medium is busy then Mp is theexpected number of nodes that will attempt to transmit once the medium becomesidle If Mp gt 1 then a collision is expected to occur Therefore the network mustmake sure that Mp lt 1 to avoid collision where M is the maximum number of nodesthat can be active at a time
bull Problems1 The mean packet delay increases exponently with increasing offered load2 CSMA protocols sense the carrier but sensing the carrier does not always releasestrue information about the status of the wireless channel There are two problems thatare unique to wireless channels (different than wireline channels)ndash Hidden terminal problemndash Exposed terminal problem
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 47
Hidden terminal problembull A sends to B C cannot receive Abull C wants to send to B C senses a
ldquofreerdquo medium (CS fails)bull collision at B A cannot receive the
collision (CD fails)bull A is ldquohiddenrdquo for C
Ad hoc Sensor Networks Multiple channel access
Random Access hidden and exposed terminal problemExposed terminal problem
bull B sends to A C wants to send to Dbull C has to wait CS signals a medium
in usebull since A is outside the radio range of
C waiting is not necessarybull C is ldquoexposedrdquo to B
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 48
Random Access MACAWhen a station wants to transmit databull It sends an RTS (Ready-to-Send) packet to the intended receiver
ndash The RTS packet contains the length of the data that needs to be transmittedndash Any station other than the intended recipient hearing RTS defers transmission for atime duration equal to the end of the corresponding CTS reception
bull The receiver sends back CTS (Clear-to-Send) packet back to sender if it isavailable to receivendash The CTS packet contains the length of the data that original sender wants totransmitndash Any station other than the original RTS sender hearing CTS defers transmissionuntil the data is sent
bull The original sender upon reception of the CTS starts transmitting
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 49
Hidden terminal problem
Ad hoc Sensor Networks Multiple channel access
Random Access MACA (contrsquo)Exposed terminal problem
Waiting time of node X is muchsmaller than waiting time of node C
bull C defers transmission upon hearing BrsquosRTS until B could get CTS from Abull After that C can start transmission to DFor that it first sends an RTSbull C is not longer exposed to the datatransmission of B
Problem It does not address the collision of RTS packet
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 50
Random Access reliabilitybull Wireless links are prone to have errors High packet loss rate detrimental to
transport-layer performance Mechanisms needed to reduce packet loss rateexperienced by upper layers
bull When node B receives a data packet from node A node B sends anAcknowledgement (Ack) This approach adopted in many protocols
bull If node A fails to receive an Ack it will retransmit the packet
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 51
Random Access MACAW (CSMACA) bull Physical carrier sense and virtual carrier sense using Network Allocation
Vector (NAV)ndash NAV is updated based on overheard RTSCTSDATAACK packets each of whichspecified duration of a pending transmissionndash Nodes stay silent when carrier sensed (physicalvirtual)
bull Backoff intervals used to reduce collision probabilityndash When transmitting a packet choose a backoff interval in the range [0w] where wis contention windowndash Count down the backoff interval when channel is idlendash Count down is suspended if channel becomes busyndash When backoff interval reaches 0 transmit RTS
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 52
Random Access CSMACA (contrsquo) bull The time spent counting down backoff intervals is a part of MAC overhead
ndash Choosing a large w leads to large backoff intervals and can result in larger overheadndash Choosing a small w leads to a larger number of collisions (when two nodes countdown to 0 simultaneously)ndash Since the number of nodes attempting to transmit simultaneously may change withtime some mechanism to manage contention is needed
bull IEEE 80211 DCF contention window w is chosen dynamically depending oncollision occurrence (binary exponential backoff)ndash When a node fails to receive CTS in response to its RTS it increases the contentionwindowndash w is doubled (up to an upper bound)ndash When a node successfully completes a data transfer it restores w to wmin
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 53
Random Access fairnessbull Simplest definition of fairness all nodes should receive equal bandwidthbull An example of unfairness
ndash Assume that initially A and B both choose a backoff interval in range [031] buttheir RTSs collidendash Nodes A and B then choose from range [063]ndash Node A chooses 4 slots and B choose 60 slotsndash After A transmits a packet it next chooses from range [031]ndash It is possible that A may transmit several packets before B transmits its first packet
Ad hoc Sensor Networks Multiple channel access
Unfairness occurs when one node has backed off much more than some other node
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 54
Random Access fairness (contrsquo) MACAW solution for fairnessbull When a node transmits a packet it appends value w to the packet all nodes
hearing value w use it for their future transmission attemptsbull Since w is an indication of the level of congestion in the vicinity of a specific
receiver node MACAW proposes maintaining w independently for eachreceiver
bull Using per-receiver wi is particularly useful in multi-hop environments sincecongestion level at different receivers can be very different
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 55
Random Access energy efficiency bull Since many mobile hosts are operated by batteries MAC
protocols which conserve energy are of interestbull Two approaches to reduce energy consumption
ndash Power save turn off wireless interface when desirablendash Power control reduce transmit power
bull Power control has some more potential benefitsndash Reduced interference (in ad hoc networks) eg it improves ALOHAefficiency where user with high power can capture a packet even if there is acollisionndash Increased spatial reuse (in infrastructured wireless networks) eg it is usedin CDMA to maintain target SIR of voice and data users or generally it canbe used to maintain target SIR for different user classes (admission control)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 56
Random Access power control bull When C transmits to D at a high power level B cannot receive Arsquos
transmission due to interference from Cbull If C reduces transmit power it can still communicate with D
ndash Reduces energy consumption at node Cndash Allows B to receive Arsquos transmission (spatial reuse)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 57
bull A special wireless ad hoc networkndash Large number of nodesndash Battery poweredndash Topology and density changendash Nodes for a common taskndash In-network data processing
bull Sensor-net applicationsndash Sensor-triggered bursty trafficndash Can often tolerate some delay
Ad hoc Sensor Networks Multiple channel access
Characteristics of Sensor Network
Scalabilty and self configurationEnergy efficiencyAdaptivityFairness not importantLatency
AdaptivityEnergy-delay tradoff
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 58
Ad hoc Sensor Networks Multiple channel access
Wireless sensor node and sensor network
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 59
MAC for WSN attributes bull Attributes
ndash Collision avoidance (Basic task)ndash Energy efficiency (most primary in WSN)ndash Scalability and adaptivityndash Channel utilizationndash Latencyndash Throughputndash Fairness
bull What causes energy wastendash Collisionsndash Control packet overheadndash Overhearing unnecessary trafficndash Long idle time (it consumes 50-100 of the power)
Ad hoc Sensor Networks Multiple channel access
Primary
Secondary
Dominant factor
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 60
MAC for WSN
bull Contention-based protocols need to work hard in all directions for energysavingsndash Reduce idle listening ndash support low duty cyclendash Better collision avoidancendash Reduce control overheadndash Avoid unnecessary overhearing
Ad hoc Sensor Networks Multiple channel access
Scheduled Protocols Contention ProtocolsCollisions No YesEnergy efficiency Good BadScalability and adaptivity Bad GoodMulti-hop communication Difficult EasyTime synchronization Strict Loose or not required
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 61
MAC for WSN classification
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 62
MAC for WSN SMAC (2002)bull It has been developed by Ye Heidemann and Estrinbull Tradeoffs between (latency fairness) and energybull Major components in S-MAC
ndash Periodic listen and sleep turn off radio when sleeping the reduced dutycycle is ~ 10 (eg 120ms on 12s off)ndash Collision avoidance is based on contention similarly to IEEE 80211 adhoc modendash Overhearing avoidance the node is sleeping when neighbors talkndash Massage passing long message is fragmented amp sent in burst operationwith extended Tx time and re-transmit occurs immediately
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 63
Summarybull The algorithms and protocols that enable sharing by multiple users and
controls the access to the wireless channel from different users are calledMAC
bull There are two types of wireless networks Infrastructured wireless networksand ad hoc wireless networks
bull We also want our wireless system to have high user capacity and we canidentify 10 methods for increasing it
bull There are several MACs with no absolute advantage it should be tailoredto the system requirements
bull There are primary concerns for designing a WSN MAC (collisionavoidance energy efficiency scalability and adaptivity) while thesecondary concerns are as follows (channel utilization latency throughputfairness)
bull Next lecture Routing protocols
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 8
Ad hoc Sensor Networks Multiple channel access
Medium Access Control (contrsquo)bull Multiple Access (Channel Partitioning or Coordinated Schemes)
ndash Techniques TDMA FDMA CDMA SDMAndash Examples GSM 3Ghellip
bull Random Access (or Random Schemes)ndash Techniques MACA MACAW Aloha 80211 MAChellipndash Examples Wifi WSNhellip
bull Polling based schemesndash Access is coordinated by a central nodendash Examples Bluetooth BlueSkyhellip
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 9
Ad hoc Sensor Networks Multiple channel access
bull Infrastructured networksndash base stations are the bridgesndash a mobile host will communicate with the
nearest base station (or access point)ndash handoff is taken when a host roams from one
base to anotherndash Medium access control assisted by BS
bull Ad hoc networksndash infrastructureless no fixed base stationsndash without the assistance of base stations for
communicationndash Due to transmission range constraint two
wireless nodes need multi-hop routing forcommunication
ndash quickly and unpredictably changing topology
Two types of wireless networks
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 10
Ad hoc Sensor Networks Multiple channel access
Two types of wireless networks (contrsquo)
bull Mesh networks serve as access networks that employ multi-hop wireless forwarding bynon-mobile nodes to relay traffic to and from the wired Internet In such an environmenthybrid wireless network technologies andor hierarchical network organization can beused for ad hoc and infrastructure wireless links
bull Wireless Sensor Networks are a special category of ad hoc networks that are used toprovide a wireless communication infrastructure among the sensors deployed in a specificapplication domain [rarr see Chapter 9 - 11]
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 11
Ad hoc Sensor Networks Multiple channel access
Infrastructured wireless networks Ad hoc wireless networksFixed infrastructure-based Infrastructureless
Guaranteed bandwidth Shared radio channel
Seamless connectivity (low call drops during handoffs)
Frequent path breaks due to mobility
High cost and time of deployment Quick and cost-effective deployment
Reuse of frequency spectrum (cellular principle) Dynamic frequency reuse based on carrier sense mechanism
Application domains include mainly civilian and commercial sectors
Application domains include battlefields emergency search and rescue operations
High cost of network maintenance Self-organization and maintenance properties are built into the network
Widely deployed and currently in the third generation of evolution
Several issues are to be addressed for successful commercial deployment even though widespread use exists in defense
Two types of wireless networks (contrsquo)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 12
Ad hoc Sensor Networks Multiple channel access
Duplexing
bull If the communication between two parties is one way then it is called simplexcommunication Simplex communication is achieved by default by using a singlewireless channel (frequency band) to transmit from sender to receiver
bull If the communication between two parties is two- way then it is called duplexcommunication Duplex communication achieved by
ndash Time Division (TDD) (famous in cellular systems)ndash Frequency Division (FDD) (famous in cellular systems)ndash Some other method like a random access method
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 13
Ad hoc Sensor Networks Multiple channel access
FDDbull A duplex channel consists of two
simplex channels with different carrierfrequencies
ndash Downlink band carries traffic frombase to mobile
ndash Uplink band carries traffic frommobile to base
Duplexing(contrsquo)TDD
bull channel (carrier frequency) is shared intime in a deterministic manner
ndash The time is slotted with fixed slotlength (sec)
ndash Some slots are used for downlinkchannel
ndash Some slots are used for uplink channel
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 14
Ad hoc Sensor Networks Multiple channel access
FDDbull FDD is used in radio systems that can
allocate individual radio frequenciesfor each user
bull For example analog systems AMPSbull In FDD channels are allocated by a
base stationbull A channel for a mobile is allocated
dynamicallybull All channels that a base station will use
are allocated usually staticallybull More suitable for wide-area cellular
networks GSM AMPS all use FDD
Duplexing(contrsquo)TDD
bull Can only be used in digital wirelesssystems (digital modulation)
bull Requires rigid timing andsynchronization
bull Mostly used in short-range and fixedwireless systems so that propagationdelay between base and mobile do notchange much with respect to locationof the mobile
bull Such as cordless phoneshellip
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 15
Ad hoc Sensor Networks Multiple channel access
Multiple Access narrow- vs wideband systemsbull Narrowband System
ndash The bandwidth is small compared to the coherence bandwidth of the channel(BltBcoh)
bull Wideband Systemndash The system bandwidth is much larger that the coherence bandwidth of the
multipath channel (BgtBcoh) A large number of users can access the same channel(frequency band) at the same time
bull Four major multiple access (MA) schemesndash Time Division Multiple Access (TDMA) it could be used in narrowband or
wideband systemndash Frequency Division Multiple Access (FDMA) it is usually used in narrowband
systemndash Code Division Multiple Access (CDMA) it is used in wideband systemndash Space Division Multiple Access (SDMA) for wireless systems with multiple
antennas it can be combined with all of the other MA methods
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 16
Ad hoc Sensor Networks Multiple channel access
Multiple Access cellular standards
Cellular System Multiple Access TechniqueAMPS (2G) FDMAFDD
CT2 Cordless Phone FDMATDD
DECT Cordless Phone FDMATDD
GSM (2G) TDMAFDD
USDC (IS-54 and IS-136) (2G) TDMAFDD
Personal Digital Cellular (2G) TDMAFDD
US IS-95 (2G) CDMAFDD
W-CDMA (3G) CDMAFDD CDMATDD
cdma2000 (3G) CDMAFDD CDMATDD
Narrowband System
Wideband System
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 17
Ad hoc Sensor Networks Multiple channel access
bull Individual radio channels are assigned toindividual users Each user is allocated a Wfrequency band by BS
bull If the channel allocated to a user is idle then itis not used by someone else waste of resource
bull Mobile and base can transmit and receivesimultaneously (FDD)
bull The W bandwidth of FDMA channels arerelatively low Symbol time is usually larger(low data rate) than the delay spread of themultipath channel (implies that inter-symbolinterference is low)
bull Lower complexity systems than TDMAsystems
Multiple Access FDMA
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 18
Ad hoc Sensor Networks Multiple channel access
Multiple Access FDMA (contrsquo)bull Capacity of FDMA systems
Nc = (B-2Bguard)Wwhere B is the total spectrum allocation Bguard is the guard band allocated atthe edge of the spectrum band W is the bandwidth of an individual channel
bull Erlang B and Erlang C system assumesndashinfinite population of sources which jointly offer traffic to Nc channelndashcall attempts arrive following a Poisson process so call arrivals are independentThe average arrival rate is λndashmessage length (holding times) are exponentially distributed (Markovian system)The average call length is hndashthe total amount of traffic offered in erlangs L= λhndashIf a user is rejected his next call attempt is made statistically independent of theprevious attempt (Erlang B)ndashIf all the channels are busy when a request arrives from a user the request isqueued An unlimited number of requests may be held in the queue (Erlang C)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 19
Ad hoc Sensor Networks Multiple channel access
bull Statictical measures of offered load Ncndash In Erlang B system the probability of call
blocking
ndash In Erlang C system the probabilty of waiting
ndash And the average wait time
c
c
cBLOCK
0
Pr
N
Nk
k
LN
Lk
=
=
sum
c
cc
wait 1
c0c
Pr 1
N
NkN
k
LL LL N kN
minus
=
=⎛ ⎞
+ minus⎜ ⎟⎝ ⎠
sum
wait waitc
Pr htN L
=minus
Multiple Access FDMA (contrsquo)Recall from
Queuing theory
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 20
Ad hoc Sensor Networks Multiple channel access
Multiple Access example of Erlang B systembull In an Erlang B system 40 channels are available A blocking probabilty of
less than 1 is required What is the traffic that can be serve if there is oneoperator or three operators
bull (Solution L(1 operator)gtL(3 operators))bull (Note The number of users increases faster than linearly with the number of
available channels The difference between actual increase and linearincrease is called the trunking gain It is preferable to have a large pool ofavailable channels that serves all users (a single operator) The reasons fornot choosing this approach are political not technical)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 21
Ad hoc Sensor Networks Multiple channel access
Multiple Access TDMAbull The allocated radio spectrum for the system is divided into time slots
ndash In each slot a user can transmit or receivendash A user occupies a cyclically repeating slotsndash A channel is logically defined as a particular time slot that repeats with some
periodbull TDMA systems buffer the data until its turn (time slot) comes to transmitbull In TDMATDD half of the slots in the frame is used for uplink channels the
other is used for downlink channelsbull In TDMAFDD a different carrier frequency is used for uplink or downlink
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 22
Ad hoc Sensor Networks Multiple channel access
Multiple Access TDMA (contrsquo)bull Preamble contains address and synchronization info to identify base
station and mobiles to each otherbull Guard times are used to allow synchronization of the receivers between
different slots and framesndash Different mobiles may have different propagation delays to a base station
because of different distances
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 23
Multiple Access TDMA (contrsquo)bull Each frame contains overhead bits and data bits Efficiency of frame is
defined as the percentage of data (information) bits to the total frame size inbits
η =(1 ndash boverheadbtotal) 100 where btotal= Tf R is the total number of bits in a frame Tf is the frameduration in sec boverhead is the number of overhead bits (GSM efficiency 73)
bull TDMA is usually combined with FDMAndash Neighboring cells are allocated and using different carrier frequencies (FDMA)
Inside a cell TDMA can be used Cells may be re-using the same frequency if theyare far from each-other
ndash There may be more than one channel allocated and used inside each cell Eachcarrier frequency (radio channel) may be using TDMA to further multiplex moreuser (ie having TDMA logical channels inside radio channels) For example inGSM each radio channel has 200KHz bandwidth and has 8 time slots (8 logicalchannels) Hence GSM is using FDMA combined with TDMA
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 24
Multiple Access TDMA (contrsquo)
Ad hoc Sensor Networks Multiple channel access
GSM (Europa) IS-54 (USA) PDC (Japan) DECT
Bit Rate 2708 Kbps 486 Kbps 42 Kbps 1152 Mbps
Bandwidth 200 KHz 30 KHz 25 KHz 1728 MHz
Time Slot 0577 ms 67 ms 67 ms 0417 ms
Upstream slots per frame 8 3 3 12
Duplexing FDD FDD FDD TDD
Efficiency 73 80 80 67
Modulation GMSK π4 DQPSK π4 DQPSK GMSK
Adaptive equalized Mandatory Mandatory Optional None
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 25
Multiple Access TDMA (contrsquo)bull Enables the sharing of a single radio channel among N usersbull Requires high data-rate per radio channel to support N users
simultaneously which requires adaptive equalizers to be usedin multipath environments
bull Transmission occurs in bursts (not continually) It enablespower saving by going to sleep modes in unrelated slots Itdiscontinues transmission and also enables mobile assistedhandoff
bull It equires synchronization of the receivers Need guard bits andsync bits which occurs large overhead per slot
bull Allocation of slots to mobile users should not be uniform Itmay depend on the traffic requirement of mobiles This bringsextra flexibility and efficiency compared to FDMA systems
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2620111015 TAacuteMOP ndash 412-082AKMR-2009-0006 26
Ad hoc Sensor Networks Multiple channel access
Multiple Access spread spectrum multiple accessbull SSMA uses signals that have transmission bandwidth that is
several orders of magnitude larger than minimum requiredRF bandwidth
bull It providesndash Immunity to multipath interferencendash Robust multiple access
bull Two techniquesndash Frequency Hopped Multiple Access (FHMA)ndash Direct Sequence Multiple Access (DSMA) or Code Division
Multiple Access (CDMA)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2720111015 TAacuteMOP ndash 412-082AKMR-2009-0006 27
Ad hoc Sensor Networks Multiple channel access
Multiple Access FHMAbull The carrier frequency of users are varied in a pseudo-random
fashionndash Each user is using a narrowband channel (spectrum) at a specific
instance of timendash The random change in frequency make the change of using the same
narrowband channel very lowbull The sender receiver change frequency (calling hopping)
using the same pseudo-random sequence hence they aresynchronized
bull Rate of hopping versus Symbol ratendash If hopping rate is greather Fast Frequency Hoppingndash If symbol rate is greater Slow Frequency Hopping
bull GSM and Bluetooth are example systems
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2820111015 TAacuteMOP ndash 412-082AKMR-2009-0006 28
Ad hoc Sensor Networks Multiple channel access
Multiple Access FHMA (contrsquo)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2920111015 TAacuteMOP ndash 412-082AKMR-2009-0006 29
Ad hoc Sensor Networks Multiple channel access
Multiple Access CDMAbull The narrowband message signal is multiplied by a very large bandwidth
signal called spreading signal (code) before modulation and transmissionover the air This is called spreading
bull Spreading signal use a pseudo-noise (PN) sequence (a pseudo-randomsequence) called codeword which are orthogonal (low autocorrelation)
bull The receiver correlator distinguishes the senders signal by examining thewideband signal with the same time-synchronized spreading code(despreading)
bull Advantagesndash Low power spectral densityndash Interference limited operationndash Privacyndash Reduction of multipath affects by using a larger spectrumndash Users can start their transmission at any time (random access possible)ndash Cell capacity is soft and higher than TDMA and FDMAndash No frequency management and no equalizers and quard time needed
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3020111015 TAacuteMOP ndash 412-082AKMR-2009-0006 30
Ad hoc Sensor Networks Multiple channel access
Multiple Access near far problem and power controlbull At a receiver the signals may come from various sources (multiuser) The
strongest signal usually captures the modulator The other signals areconsidered as noise Each source may have different distances to the basestation
bull In CDMA we want a BS to receive CDMA coded signals from variousmobile users at the same time
ndash Therefore the receiver power at the BS for all mobile users should be close toeach other This requires power control at the mobiles
bull Power Control BS monitors the RSSI (Received Signal Strength Indicator)values from different mobiles and then sends power change commands to themobiles over a forward channel The mobiles then adjust their transmitpower
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3120111015 TAacuteMOP ndash 412-082AKMR-2009-0006 31
Ad hoc Sensor Networks Multiple channel access
Multiple Access hybrid spread spectrum techniquesbull FDMACDMA
ndash Available wideband spectrum is divided into a number narrowband radio channels CDMAis employed inside each channel
bull DSFHMAndash The signals are spread using spreading codes (direct sequence signals are obtained) but
these signal are not transmitted over a constant carrier frequency they are transmitted over afrequency hopping carrier frequency
bull Time Division CDMA (TCDMA)ndash Each cell is using a different spreading code (CDMA employed between cells) that is
conveyed to the mobiles in its rangendash Inside each cell (inside a CDMA channel) TDMA is employed to multiplex multiple users
bull Time Division Frequency Hoppingndash At each time slot the user is hopped to a new frequency according to a pseudo-random
hopping sequencendash Employed in severe co-interference and multi-path environmentsndash Bluetooth and GSM are using this technique
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3220111015 TAacuteMOP ndash 412-082AKMR-2009-0006 32
Ad hoc Sensor Networks Multiple channel access
Multiple Access comparison TDMAFDMACDMA
Techniques TDMA FDMA CDMA SDMA
Idea
segment sending time into disjoint time-slots
Segment the frequency band into disjoint sub-bands
Spread the spectrum using orthogonal codes
Segment space into sectors
Signal separation
Synchronization in the time domain
Filtering in the frequency domain
Code Directed antennas
Advantage
Established flexible
Simple established robust
Flexible less frequency planning needed soft handover
Simple increases capacity
DisadvantageQuard space synchronization difficult
Inflexible scarce source
Complex receiver power control
inflexible
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3320111015 TAacuteMOP ndash 412-082AKMR-2009-0006 33
Ad hoc Sensor Networks Multiple channel access
User capacity per cell
The user capacity C is defined as the total number of active users percell that the system can support while meeting a common BERconstrain For orthogonal multiple access (as FDMA and TDMA)
C = Ncellwhere Ncell is the number of channels assigned to any given cellThe total number of orthogonal channels of bandwidth Bs that can becreated from a total system bandwidth of B is B Bs The reuse factorsatisfies N = (B Bs) Ncell this implies
C = (B Bs) N
Recall from Chapter 1
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3420111015 TAacuteMOP ndash 412-082AKMR-2009-0006 34
Ad hoc Sensor Networks Multiple channel access
10 methods for increasing capacity1 Increasing the amount of spectrum used very expensive2 More efficient modulation format and coding3 Better source coding4 Adaptive modulation and coding5 Discontinuous transmission6 Multiuser detection CDMA systems7 Reduction of cell radius effective but very expensive smaller
cells require more handovers8 Use of sector cells tripled cells have tripled BS antennas9 Use of an overlay structure10 Multiple antennas diversity MIMO systems SDMA
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3520111015 TAacuteMOP ndash 412-082AKMR-2009-0006 35
Ad hoc Sensor Networks Multiple channel access
bull Distributed operation is requiredbull Synchronization is required in TDMA-based systemsbull Hidden terminals are nodes hidden from a senderbull Exposed terminals are exposed nodes preventing a sender
from sendingbull Throughput needs to be maximizedbull Access delay should be minimizedbull Fairness refers to provide an equal share to all competing
nodesbull Real-time traffic support is required for voice video and
real-time databull Resource reservation is required for QoSbull Ability to measure resource availability handles the
resourcesbull Capability for power control reduces the energy
consumption
Random access in adhoc networks
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3620111015 TAacuteMOP ndash 412-082AKMR-2009-0006 36
Ad hoc Sensor Networks Multiple channel access
Random accessbull All random access techniques are based on packetized data or packet radio where user
date is collected into packets of M bitsbull Collision if packets from different users overlap in timebull The transmission time of a packet is τ = M R where R [bps] the data ratebull Analysis of random access techniques assumes that the users generate packets according
to memoryless Poisson process at a rate λ [packets per unit time] the probability that thenumber of packet arrivals in a time period [0t] denoted by X(t) is equal to integer k isgiven by
Pr(X(t)=k) = (λt)k k e-λt
bull The traffic load is defined as L = λ τ If L gt 1 means on avarage more packets arrive inthe system over a given time period than can be transmitted in that period so systemswith Lgt1 are unstable
bull The throughput is defined as the ratio of the average number of packets successfullytransmitted in any given time interval divided by the number of attempted transmissionsin that interval
T = L Pr(succesful packet transmission)bull The effective data rate of the system is RT
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3720111015 TAacuteMOP ndash 412-082AKMR-2009-0006 37
Ad hoc Sensor Networks Multiple channel access
Random access (contrsquo)bull We will examine the following attributes
ndash Channel utilization (Throughput)ndash Latencyndash Collision avoidance (Hidden and exposed terminals problem)ndash Reliability (ACK)ndash Energy efficiency (power control)ndash Fairnessndash Throughput needs to be maximized
bull We will discuss the following techniquesndash Aloha and slotted Alohandash CSMA Protocols (1-persistent non-persistent p-persistent CSMA CSMACD)ndash MACAndash MACAWndash Energy efficient MAC in wireless sensor networks (eg SMAC)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 38
Random Access ALOHA (1970)bull ALOHA was developed for a wireless system at the University of Hawaii
(Abramson et al)ndash Multiple remote stations plus one base stationndash Frame transmissions are made at one frequency from a remote station to the basestation the base station re-broadcasts frames on another frequency Contention is foraccess to the base station
bull Frames can be sent at any time to the base stationndash If no acknowledgment (ACK) is received assume that the frame is lost andretransmit after waiting a random amount of timendash If more than one station broadcasts on the base station access frequency at the sametime ndash a collision ndash interference will destroy the frames
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 39
Random Access ALOHA (contrsquo)
bull The probability that no packets generated during the tima [-τ τ] is given by (nocollision)
Pr(X(t)=0) = (λ2τ)0 0 e-2λτ = e-2Lwith corresponding throughput
T = L e-2L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 05 05e 018
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 40
Random Access slotted ALOHA (1971)bull Improvement Time is slotted and a packet can only be transmitted at the beginning of
one slot Thus it can reduce the collision duration
bull The probability that no packets generated during the time [0 τ] is given by (nocollision)
Pr(X(t)=0) = (λτ)0 0 e-λτ = e-Lwith corresponding throughput T = L e-L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 1 e 037
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 41
bull Slotted Aloha has double the maximumthroughput as pure Aloha and achievesthis maximum at a higher offered loadThe effective data rate is still less than40 of the raw transmission rate
bull Aloha does not listen to the carrier beforetransmission more sophisticatedtechniques are needed to increaseefficiency
bull Further problem the delayndashDelay
ndashThe mean packet delay
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
( ) ( )delayPr 1 e ekL Lkτ minus minus= = minus
( )delay1
1 e e ekL L L
kMPD E kτ
infinminus minus
=
= = minus =sum
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 42
Random Access Carrier Sense Multiple Accessbull CSMA (Carrier Sense Multiple Access) listen to the carrier before transmission
and transmits if channel is idlebull Detection delay and propagation delay are two important parameters
ndash Detection delay time required to sense the carrier and decide if it is idle or busyndash Propagation delay distancespeed_of_ligth The time required for bit to travel
from transmitter to the receiver
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 43
Random Access CSMA variations
Ad hoc Sensor Networks Multiple channel access
1-persistent CSMA Non-persistent CSMA
There will always be a collision iftwo nodes want to retransmit(usually you stop transmissionattempts after few tries)
Random backoff reduces probabilityof collisions Waste idle time if thebackoff time is too long
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 44
Random Access CSMA variations (contrsquo)
Ad hoc Sensor Networks Multiple channel access
p-persistent CSMA CSMA CD (Collision Detection)
A good tradeoff between non-persistent and 1-persistent CSMA
Same with CSMA however a stationalso listen to the carrier whiletransmitting to see if the transmissioncollides with someone elsetransmission It can be used in listen-while-talk capable channels (fullduplex) In single radio channels thetransmission need to be interrupted inorder to sense the channel
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 45
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 46
Random Access CSMA (contrsquo)bull How to select probability p
ndash Assume that M nodes have a packet to send and the medium is busy then Mp is theexpected number of nodes that will attempt to transmit once the medium becomesidle If Mp gt 1 then a collision is expected to occur Therefore the network mustmake sure that Mp lt 1 to avoid collision where M is the maximum number of nodesthat can be active at a time
bull Problems1 The mean packet delay increases exponently with increasing offered load2 CSMA protocols sense the carrier but sensing the carrier does not always releasestrue information about the status of the wireless channel There are two problems thatare unique to wireless channels (different than wireline channels)ndash Hidden terminal problemndash Exposed terminal problem
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 47
Hidden terminal problembull A sends to B C cannot receive Abull C wants to send to B C senses a
ldquofreerdquo medium (CS fails)bull collision at B A cannot receive the
collision (CD fails)bull A is ldquohiddenrdquo for C
Ad hoc Sensor Networks Multiple channel access
Random Access hidden and exposed terminal problemExposed terminal problem
bull B sends to A C wants to send to Dbull C has to wait CS signals a medium
in usebull since A is outside the radio range of
C waiting is not necessarybull C is ldquoexposedrdquo to B
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 48
Random Access MACAWhen a station wants to transmit databull It sends an RTS (Ready-to-Send) packet to the intended receiver
ndash The RTS packet contains the length of the data that needs to be transmittedndash Any station other than the intended recipient hearing RTS defers transmission for atime duration equal to the end of the corresponding CTS reception
bull The receiver sends back CTS (Clear-to-Send) packet back to sender if it isavailable to receivendash The CTS packet contains the length of the data that original sender wants totransmitndash Any station other than the original RTS sender hearing CTS defers transmissionuntil the data is sent
bull The original sender upon reception of the CTS starts transmitting
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 49
Hidden terminal problem
Ad hoc Sensor Networks Multiple channel access
Random Access MACA (contrsquo)Exposed terminal problem
Waiting time of node X is muchsmaller than waiting time of node C
bull C defers transmission upon hearing BrsquosRTS until B could get CTS from Abull After that C can start transmission to DFor that it first sends an RTSbull C is not longer exposed to the datatransmission of B
Problem It does not address the collision of RTS packet
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 50
Random Access reliabilitybull Wireless links are prone to have errors High packet loss rate detrimental to
transport-layer performance Mechanisms needed to reduce packet loss rateexperienced by upper layers
bull When node B receives a data packet from node A node B sends anAcknowledgement (Ack) This approach adopted in many protocols
bull If node A fails to receive an Ack it will retransmit the packet
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 51
Random Access MACAW (CSMACA) bull Physical carrier sense and virtual carrier sense using Network Allocation
Vector (NAV)ndash NAV is updated based on overheard RTSCTSDATAACK packets each of whichspecified duration of a pending transmissionndash Nodes stay silent when carrier sensed (physicalvirtual)
bull Backoff intervals used to reduce collision probabilityndash When transmitting a packet choose a backoff interval in the range [0w] where wis contention windowndash Count down the backoff interval when channel is idlendash Count down is suspended if channel becomes busyndash When backoff interval reaches 0 transmit RTS
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 52
Random Access CSMACA (contrsquo) bull The time spent counting down backoff intervals is a part of MAC overhead
ndash Choosing a large w leads to large backoff intervals and can result in larger overheadndash Choosing a small w leads to a larger number of collisions (when two nodes countdown to 0 simultaneously)ndash Since the number of nodes attempting to transmit simultaneously may change withtime some mechanism to manage contention is needed
bull IEEE 80211 DCF contention window w is chosen dynamically depending oncollision occurrence (binary exponential backoff)ndash When a node fails to receive CTS in response to its RTS it increases the contentionwindowndash w is doubled (up to an upper bound)ndash When a node successfully completes a data transfer it restores w to wmin
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 53
Random Access fairnessbull Simplest definition of fairness all nodes should receive equal bandwidthbull An example of unfairness
ndash Assume that initially A and B both choose a backoff interval in range [031] buttheir RTSs collidendash Nodes A and B then choose from range [063]ndash Node A chooses 4 slots and B choose 60 slotsndash After A transmits a packet it next chooses from range [031]ndash It is possible that A may transmit several packets before B transmits its first packet
Ad hoc Sensor Networks Multiple channel access
Unfairness occurs when one node has backed off much more than some other node
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 54
Random Access fairness (contrsquo) MACAW solution for fairnessbull When a node transmits a packet it appends value w to the packet all nodes
hearing value w use it for their future transmission attemptsbull Since w is an indication of the level of congestion in the vicinity of a specific
receiver node MACAW proposes maintaining w independently for eachreceiver
bull Using per-receiver wi is particularly useful in multi-hop environments sincecongestion level at different receivers can be very different
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 55
Random Access energy efficiency bull Since many mobile hosts are operated by batteries MAC
protocols which conserve energy are of interestbull Two approaches to reduce energy consumption
ndash Power save turn off wireless interface when desirablendash Power control reduce transmit power
bull Power control has some more potential benefitsndash Reduced interference (in ad hoc networks) eg it improves ALOHAefficiency where user with high power can capture a packet even if there is acollisionndash Increased spatial reuse (in infrastructured wireless networks) eg it is usedin CDMA to maintain target SIR of voice and data users or generally it canbe used to maintain target SIR for different user classes (admission control)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 56
Random Access power control bull When C transmits to D at a high power level B cannot receive Arsquos
transmission due to interference from Cbull If C reduces transmit power it can still communicate with D
ndash Reduces energy consumption at node Cndash Allows B to receive Arsquos transmission (spatial reuse)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 57
bull A special wireless ad hoc networkndash Large number of nodesndash Battery poweredndash Topology and density changendash Nodes for a common taskndash In-network data processing
bull Sensor-net applicationsndash Sensor-triggered bursty trafficndash Can often tolerate some delay
Ad hoc Sensor Networks Multiple channel access
Characteristics of Sensor Network
Scalabilty and self configurationEnergy efficiencyAdaptivityFairness not importantLatency
AdaptivityEnergy-delay tradoff
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 58
Ad hoc Sensor Networks Multiple channel access
Wireless sensor node and sensor network
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 59
MAC for WSN attributes bull Attributes
ndash Collision avoidance (Basic task)ndash Energy efficiency (most primary in WSN)ndash Scalability and adaptivityndash Channel utilizationndash Latencyndash Throughputndash Fairness
bull What causes energy wastendash Collisionsndash Control packet overheadndash Overhearing unnecessary trafficndash Long idle time (it consumes 50-100 of the power)
Ad hoc Sensor Networks Multiple channel access
Primary
Secondary
Dominant factor
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 60
MAC for WSN
bull Contention-based protocols need to work hard in all directions for energysavingsndash Reduce idle listening ndash support low duty cyclendash Better collision avoidancendash Reduce control overheadndash Avoid unnecessary overhearing
Ad hoc Sensor Networks Multiple channel access
Scheduled Protocols Contention ProtocolsCollisions No YesEnergy efficiency Good BadScalability and adaptivity Bad GoodMulti-hop communication Difficult EasyTime synchronization Strict Loose or not required
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 61
MAC for WSN classification
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 62
MAC for WSN SMAC (2002)bull It has been developed by Ye Heidemann and Estrinbull Tradeoffs between (latency fairness) and energybull Major components in S-MAC
ndash Periodic listen and sleep turn off radio when sleeping the reduced dutycycle is ~ 10 (eg 120ms on 12s off)ndash Collision avoidance is based on contention similarly to IEEE 80211 adhoc modendash Overhearing avoidance the node is sleeping when neighbors talkndash Massage passing long message is fragmented amp sent in burst operationwith extended Tx time and re-transmit occurs immediately
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 63
Summarybull The algorithms and protocols that enable sharing by multiple users and
controls the access to the wireless channel from different users are calledMAC
bull There are two types of wireless networks Infrastructured wireless networksand ad hoc wireless networks
bull We also want our wireless system to have high user capacity and we canidentify 10 methods for increasing it
bull There are several MACs with no absolute advantage it should be tailoredto the system requirements
bull There are primary concerns for designing a WSN MAC (collisionavoidance energy efficiency scalability and adaptivity) while thesecondary concerns are as follows (channel utilization latency throughputfairness)
bull Next lecture Routing protocols
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 9
Ad hoc Sensor Networks Multiple channel access
bull Infrastructured networksndash base stations are the bridgesndash a mobile host will communicate with the
nearest base station (or access point)ndash handoff is taken when a host roams from one
base to anotherndash Medium access control assisted by BS
bull Ad hoc networksndash infrastructureless no fixed base stationsndash without the assistance of base stations for
communicationndash Due to transmission range constraint two
wireless nodes need multi-hop routing forcommunication
ndash quickly and unpredictably changing topology
Two types of wireless networks
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 10
Ad hoc Sensor Networks Multiple channel access
Two types of wireless networks (contrsquo)
bull Mesh networks serve as access networks that employ multi-hop wireless forwarding bynon-mobile nodes to relay traffic to and from the wired Internet In such an environmenthybrid wireless network technologies andor hierarchical network organization can beused for ad hoc and infrastructure wireless links
bull Wireless Sensor Networks are a special category of ad hoc networks that are used toprovide a wireless communication infrastructure among the sensors deployed in a specificapplication domain [rarr see Chapter 9 - 11]
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 11
Ad hoc Sensor Networks Multiple channel access
Infrastructured wireless networks Ad hoc wireless networksFixed infrastructure-based Infrastructureless
Guaranteed bandwidth Shared radio channel
Seamless connectivity (low call drops during handoffs)
Frequent path breaks due to mobility
High cost and time of deployment Quick and cost-effective deployment
Reuse of frequency spectrum (cellular principle) Dynamic frequency reuse based on carrier sense mechanism
Application domains include mainly civilian and commercial sectors
Application domains include battlefields emergency search and rescue operations
High cost of network maintenance Self-organization and maintenance properties are built into the network
Widely deployed and currently in the third generation of evolution
Several issues are to be addressed for successful commercial deployment even though widespread use exists in defense
Two types of wireless networks (contrsquo)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 12
Ad hoc Sensor Networks Multiple channel access
Duplexing
bull If the communication between two parties is one way then it is called simplexcommunication Simplex communication is achieved by default by using a singlewireless channel (frequency band) to transmit from sender to receiver
bull If the communication between two parties is two- way then it is called duplexcommunication Duplex communication achieved by
ndash Time Division (TDD) (famous in cellular systems)ndash Frequency Division (FDD) (famous in cellular systems)ndash Some other method like a random access method
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 13
Ad hoc Sensor Networks Multiple channel access
FDDbull A duplex channel consists of two
simplex channels with different carrierfrequencies
ndash Downlink band carries traffic frombase to mobile
ndash Uplink band carries traffic frommobile to base
Duplexing(contrsquo)TDD
bull channel (carrier frequency) is shared intime in a deterministic manner
ndash The time is slotted with fixed slotlength (sec)
ndash Some slots are used for downlinkchannel
ndash Some slots are used for uplink channel
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 14
Ad hoc Sensor Networks Multiple channel access
FDDbull FDD is used in radio systems that can
allocate individual radio frequenciesfor each user
bull For example analog systems AMPSbull In FDD channels are allocated by a
base stationbull A channel for a mobile is allocated
dynamicallybull All channels that a base station will use
are allocated usually staticallybull More suitable for wide-area cellular
networks GSM AMPS all use FDD
Duplexing(contrsquo)TDD
bull Can only be used in digital wirelesssystems (digital modulation)
bull Requires rigid timing andsynchronization
bull Mostly used in short-range and fixedwireless systems so that propagationdelay between base and mobile do notchange much with respect to locationof the mobile
bull Such as cordless phoneshellip
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 15
Ad hoc Sensor Networks Multiple channel access
Multiple Access narrow- vs wideband systemsbull Narrowband System
ndash The bandwidth is small compared to the coherence bandwidth of the channel(BltBcoh)
bull Wideband Systemndash The system bandwidth is much larger that the coherence bandwidth of the
multipath channel (BgtBcoh) A large number of users can access the same channel(frequency band) at the same time
bull Four major multiple access (MA) schemesndash Time Division Multiple Access (TDMA) it could be used in narrowband or
wideband systemndash Frequency Division Multiple Access (FDMA) it is usually used in narrowband
systemndash Code Division Multiple Access (CDMA) it is used in wideband systemndash Space Division Multiple Access (SDMA) for wireless systems with multiple
antennas it can be combined with all of the other MA methods
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 16
Ad hoc Sensor Networks Multiple channel access
Multiple Access cellular standards
Cellular System Multiple Access TechniqueAMPS (2G) FDMAFDD
CT2 Cordless Phone FDMATDD
DECT Cordless Phone FDMATDD
GSM (2G) TDMAFDD
USDC (IS-54 and IS-136) (2G) TDMAFDD
Personal Digital Cellular (2G) TDMAFDD
US IS-95 (2G) CDMAFDD
W-CDMA (3G) CDMAFDD CDMATDD
cdma2000 (3G) CDMAFDD CDMATDD
Narrowband System
Wideband System
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 17
Ad hoc Sensor Networks Multiple channel access
bull Individual radio channels are assigned toindividual users Each user is allocated a Wfrequency band by BS
bull If the channel allocated to a user is idle then itis not used by someone else waste of resource
bull Mobile and base can transmit and receivesimultaneously (FDD)
bull The W bandwidth of FDMA channels arerelatively low Symbol time is usually larger(low data rate) than the delay spread of themultipath channel (implies that inter-symbolinterference is low)
bull Lower complexity systems than TDMAsystems
Multiple Access FDMA
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 18
Ad hoc Sensor Networks Multiple channel access
Multiple Access FDMA (contrsquo)bull Capacity of FDMA systems
Nc = (B-2Bguard)Wwhere B is the total spectrum allocation Bguard is the guard band allocated atthe edge of the spectrum band W is the bandwidth of an individual channel
bull Erlang B and Erlang C system assumesndashinfinite population of sources which jointly offer traffic to Nc channelndashcall attempts arrive following a Poisson process so call arrivals are independentThe average arrival rate is λndashmessage length (holding times) are exponentially distributed (Markovian system)The average call length is hndashthe total amount of traffic offered in erlangs L= λhndashIf a user is rejected his next call attempt is made statistically independent of theprevious attempt (Erlang B)ndashIf all the channels are busy when a request arrives from a user the request isqueued An unlimited number of requests may be held in the queue (Erlang C)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 19
Ad hoc Sensor Networks Multiple channel access
bull Statictical measures of offered load Ncndash In Erlang B system the probability of call
blocking
ndash In Erlang C system the probabilty of waiting
ndash And the average wait time
c
c
cBLOCK
0
Pr
N
Nk
k
LN
Lk
=
=
sum
c
cc
wait 1
c0c
Pr 1
N
NkN
k
LL LL N kN
minus
=
=⎛ ⎞
+ minus⎜ ⎟⎝ ⎠
sum
wait waitc
Pr htN L
=minus
Multiple Access FDMA (contrsquo)Recall from
Queuing theory
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 20
Ad hoc Sensor Networks Multiple channel access
Multiple Access example of Erlang B systembull In an Erlang B system 40 channels are available A blocking probabilty of
less than 1 is required What is the traffic that can be serve if there is oneoperator or three operators
bull (Solution L(1 operator)gtL(3 operators))bull (Note The number of users increases faster than linearly with the number of
available channels The difference between actual increase and linearincrease is called the trunking gain It is preferable to have a large pool ofavailable channels that serves all users (a single operator) The reasons fornot choosing this approach are political not technical)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 21
Ad hoc Sensor Networks Multiple channel access
Multiple Access TDMAbull The allocated radio spectrum for the system is divided into time slots
ndash In each slot a user can transmit or receivendash A user occupies a cyclically repeating slotsndash A channel is logically defined as a particular time slot that repeats with some
periodbull TDMA systems buffer the data until its turn (time slot) comes to transmitbull In TDMATDD half of the slots in the frame is used for uplink channels the
other is used for downlink channelsbull In TDMAFDD a different carrier frequency is used for uplink or downlink
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 22
Ad hoc Sensor Networks Multiple channel access
Multiple Access TDMA (contrsquo)bull Preamble contains address and synchronization info to identify base
station and mobiles to each otherbull Guard times are used to allow synchronization of the receivers between
different slots and framesndash Different mobiles may have different propagation delays to a base station
because of different distances
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 23
Multiple Access TDMA (contrsquo)bull Each frame contains overhead bits and data bits Efficiency of frame is
defined as the percentage of data (information) bits to the total frame size inbits
η =(1 ndash boverheadbtotal) 100 where btotal= Tf R is the total number of bits in a frame Tf is the frameduration in sec boverhead is the number of overhead bits (GSM efficiency 73)
bull TDMA is usually combined with FDMAndash Neighboring cells are allocated and using different carrier frequencies (FDMA)
Inside a cell TDMA can be used Cells may be re-using the same frequency if theyare far from each-other
ndash There may be more than one channel allocated and used inside each cell Eachcarrier frequency (radio channel) may be using TDMA to further multiplex moreuser (ie having TDMA logical channels inside radio channels) For example inGSM each radio channel has 200KHz bandwidth and has 8 time slots (8 logicalchannels) Hence GSM is using FDMA combined with TDMA
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 24
Multiple Access TDMA (contrsquo)
Ad hoc Sensor Networks Multiple channel access
GSM (Europa) IS-54 (USA) PDC (Japan) DECT
Bit Rate 2708 Kbps 486 Kbps 42 Kbps 1152 Mbps
Bandwidth 200 KHz 30 KHz 25 KHz 1728 MHz
Time Slot 0577 ms 67 ms 67 ms 0417 ms
Upstream slots per frame 8 3 3 12
Duplexing FDD FDD FDD TDD
Efficiency 73 80 80 67
Modulation GMSK π4 DQPSK π4 DQPSK GMSK
Adaptive equalized Mandatory Mandatory Optional None
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 25
Multiple Access TDMA (contrsquo)bull Enables the sharing of a single radio channel among N usersbull Requires high data-rate per radio channel to support N users
simultaneously which requires adaptive equalizers to be usedin multipath environments
bull Transmission occurs in bursts (not continually) It enablespower saving by going to sleep modes in unrelated slots Itdiscontinues transmission and also enables mobile assistedhandoff
bull It equires synchronization of the receivers Need guard bits andsync bits which occurs large overhead per slot
bull Allocation of slots to mobile users should not be uniform Itmay depend on the traffic requirement of mobiles This bringsextra flexibility and efficiency compared to FDMA systems
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2620111015 TAacuteMOP ndash 412-082AKMR-2009-0006 26
Ad hoc Sensor Networks Multiple channel access
Multiple Access spread spectrum multiple accessbull SSMA uses signals that have transmission bandwidth that is
several orders of magnitude larger than minimum requiredRF bandwidth
bull It providesndash Immunity to multipath interferencendash Robust multiple access
bull Two techniquesndash Frequency Hopped Multiple Access (FHMA)ndash Direct Sequence Multiple Access (DSMA) or Code Division
Multiple Access (CDMA)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2720111015 TAacuteMOP ndash 412-082AKMR-2009-0006 27
Ad hoc Sensor Networks Multiple channel access
Multiple Access FHMAbull The carrier frequency of users are varied in a pseudo-random
fashionndash Each user is using a narrowband channel (spectrum) at a specific
instance of timendash The random change in frequency make the change of using the same
narrowband channel very lowbull The sender receiver change frequency (calling hopping)
using the same pseudo-random sequence hence they aresynchronized
bull Rate of hopping versus Symbol ratendash If hopping rate is greather Fast Frequency Hoppingndash If symbol rate is greater Slow Frequency Hopping
bull GSM and Bluetooth are example systems
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2820111015 TAacuteMOP ndash 412-082AKMR-2009-0006 28
Ad hoc Sensor Networks Multiple channel access
Multiple Access FHMA (contrsquo)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2920111015 TAacuteMOP ndash 412-082AKMR-2009-0006 29
Ad hoc Sensor Networks Multiple channel access
Multiple Access CDMAbull The narrowband message signal is multiplied by a very large bandwidth
signal called spreading signal (code) before modulation and transmissionover the air This is called spreading
bull Spreading signal use a pseudo-noise (PN) sequence (a pseudo-randomsequence) called codeword which are orthogonal (low autocorrelation)
bull The receiver correlator distinguishes the senders signal by examining thewideband signal with the same time-synchronized spreading code(despreading)
bull Advantagesndash Low power spectral densityndash Interference limited operationndash Privacyndash Reduction of multipath affects by using a larger spectrumndash Users can start their transmission at any time (random access possible)ndash Cell capacity is soft and higher than TDMA and FDMAndash No frequency management and no equalizers and quard time needed
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3020111015 TAacuteMOP ndash 412-082AKMR-2009-0006 30
Ad hoc Sensor Networks Multiple channel access
Multiple Access near far problem and power controlbull At a receiver the signals may come from various sources (multiuser) The
strongest signal usually captures the modulator The other signals areconsidered as noise Each source may have different distances to the basestation
bull In CDMA we want a BS to receive CDMA coded signals from variousmobile users at the same time
ndash Therefore the receiver power at the BS for all mobile users should be close toeach other This requires power control at the mobiles
bull Power Control BS monitors the RSSI (Received Signal Strength Indicator)values from different mobiles and then sends power change commands to themobiles over a forward channel The mobiles then adjust their transmitpower
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3120111015 TAacuteMOP ndash 412-082AKMR-2009-0006 31
Ad hoc Sensor Networks Multiple channel access
Multiple Access hybrid spread spectrum techniquesbull FDMACDMA
ndash Available wideband spectrum is divided into a number narrowband radio channels CDMAis employed inside each channel
bull DSFHMAndash The signals are spread using spreading codes (direct sequence signals are obtained) but
these signal are not transmitted over a constant carrier frequency they are transmitted over afrequency hopping carrier frequency
bull Time Division CDMA (TCDMA)ndash Each cell is using a different spreading code (CDMA employed between cells) that is
conveyed to the mobiles in its rangendash Inside each cell (inside a CDMA channel) TDMA is employed to multiplex multiple users
bull Time Division Frequency Hoppingndash At each time slot the user is hopped to a new frequency according to a pseudo-random
hopping sequencendash Employed in severe co-interference and multi-path environmentsndash Bluetooth and GSM are using this technique
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3220111015 TAacuteMOP ndash 412-082AKMR-2009-0006 32
Ad hoc Sensor Networks Multiple channel access
Multiple Access comparison TDMAFDMACDMA
Techniques TDMA FDMA CDMA SDMA
Idea
segment sending time into disjoint time-slots
Segment the frequency band into disjoint sub-bands
Spread the spectrum using orthogonal codes
Segment space into sectors
Signal separation
Synchronization in the time domain
Filtering in the frequency domain
Code Directed antennas
Advantage
Established flexible
Simple established robust
Flexible less frequency planning needed soft handover
Simple increases capacity
DisadvantageQuard space synchronization difficult
Inflexible scarce source
Complex receiver power control
inflexible
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3320111015 TAacuteMOP ndash 412-082AKMR-2009-0006 33
Ad hoc Sensor Networks Multiple channel access
User capacity per cell
The user capacity C is defined as the total number of active users percell that the system can support while meeting a common BERconstrain For orthogonal multiple access (as FDMA and TDMA)
C = Ncellwhere Ncell is the number of channels assigned to any given cellThe total number of orthogonal channels of bandwidth Bs that can becreated from a total system bandwidth of B is B Bs The reuse factorsatisfies N = (B Bs) Ncell this implies
C = (B Bs) N
Recall from Chapter 1
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3420111015 TAacuteMOP ndash 412-082AKMR-2009-0006 34
Ad hoc Sensor Networks Multiple channel access
10 methods for increasing capacity1 Increasing the amount of spectrum used very expensive2 More efficient modulation format and coding3 Better source coding4 Adaptive modulation and coding5 Discontinuous transmission6 Multiuser detection CDMA systems7 Reduction of cell radius effective but very expensive smaller
cells require more handovers8 Use of sector cells tripled cells have tripled BS antennas9 Use of an overlay structure10 Multiple antennas diversity MIMO systems SDMA
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3520111015 TAacuteMOP ndash 412-082AKMR-2009-0006 35
Ad hoc Sensor Networks Multiple channel access
bull Distributed operation is requiredbull Synchronization is required in TDMA-based systemsbull Hidden terminals are nodes hidden from a senderbull Exposed terminals are exposed nodes preventing a sender
from sendingbull Throughput needs to be maximizedbull Access delay should be minimizedbull Fairness refers to provide an equal share to all competing
nodesbull Real-time traffic support is required for voice video and
real-time databull Resource reservation is required for QoSbull Ability to measure resource availability handles the
resourcesbull Capability for power control reduces the energy
consumption
Random access in adhoc networks
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3620111015 TAacuteMOP ndash 412-082AKMR-2009-0006 36
Ad hoc Sensor Networks Multiple channel access
Random accessbull All random access techniques are based on packetized data or packet radio where user
date is collected into packets of M bitsbull Collision if packets from different users overlap in timebull The transmission time of a packet is τ = M R where R [bps] the data ratebull Analysis of random access techniques assumes that the users generate packets according
to memoryless Poisson process at a rate λ [packets per unit time] the probability that thenumber of packet arrivals in a time period [0t] denoted by X(t) is equal to integer k isgiven by
Pr(X(t)=k) = (λt)k k e-λt
bull The traffic load is defined as L = λ τ If L gt 1 means on avarage more packets arrive inthe system over a given time period than can be transmitted in that period so systemswith Lgt1 are unstable
bull The throughput is defined as the ratio of the average number of packets successfullytransmitted in any given time interval divided by the number of attempted transmissionsin that interval
T = L Pr(succesful packet transmission)bull The effective data rate of the system is RT
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3720111015 TAacuteMOP ndash 412-082AKMR-2009-0006 37
Ad hoc Sensor Networks Multiple channel access
Random access (contrsquo)bull We will examine the following attributes
ndash Channel utilization (Throughput)ndash Latencyndash Collision avoidance (Hidden and exposed terminals problem)ndash Reliability (ACK)ndash Energy efficiency (power control)ndash Fairnessndash Throughput needs to be maximized
bull We will discuss the following techniquesndash Aloha and slotted Alohandash CSMA Protocols (1-persistent non-persistent p-persistent CSMA CSMACD)ndash MACAndash MACAWndash Energy efficient MAC in wireless sensor networks (eg SMAC)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 38
Random Access ALOHA (1970)bull ALOHA was developed for a wireless system at the University of Hawaii
(Abramson et al)ndash Multiple remote stations plus one base stationndash Frame transmissions are made at one frequency from a remote station to the basestation the base station re-broadcasts frames on another frequency Contention is foraccess to the base station
bull Frames can be sent at any time to the base stationndash If no acknowledgment (ACK) is received assume that the frame is lost andretransmit after waiting a random amount of timendash If more than one station broadcasts on the base station access frequency at the sametime ndash a collision ndash interference will destroy the frames
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 39
Random Access ALOHA (contrsquo)
bull The probability that no packets generated during the tima [-τ τ] is given by (nocollision)
Pr(X(t)=0) = (λ2τ)0 0 e-2λτ = e-2Lwith corresponding throughput
T = L e-2L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 05 05e 018
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 40
Random Access slotted ALOHA (1971)bull Improvement Time is slotted and a packet can only be transmitted at the beginning of
one slot Thus it can reduce the collision duration
bull The probability that no packets generated during the time [0 τ] is given by (nocollision)
Pr(X(t)=0) = (λτ)0 0 e-λτ = e-Lwith corresponding throughput T = L e-L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 1 e 037
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 41
bull Slotted Aloha has double the maximumthroughput as pure Aloha and achievesthis maximum at a higher offered loadThe effective data rate is still less than40 of the raw transmission rate
bull Aloha does not listen to the carrier beforetransmission more sophisticatedtechniques are needed to increaseefficiency
bull Further problem the delayndashDelay
ndashThe mean packet delay
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
( ) ( )delayPr 1 e ekL Lkτ minus minus= = minus
( )delay1
1 e e ekL L L
kMPD E kτ
infinminus minus
=
= = minus =sum
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 42
Random Access Carrier Sense Multiple Accessbull CSMA (Carrier Sense Multiple Access) listen to the carrier before transmission
and transmits if channel is idlebull Detection delay and propagation delay are two important parameters
ndash Detection delay time required to sense the carrier and decide if it is idle or busyndash Propagation delay distancespeed_of_ligth The time required for bit to travel
from transmitter to the receiver
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 43
Random Access CSMA variations
Ad hoc Sensor Networks Multiple channel access
1-persistent CSMA Non-persistent CSMA
There will always be a collision iftwo nodes want to retransmit(usually you stop transmissionattempts after few tries)
Random backoff reduces probabilityof collisions Waste idle time if thebackoff time is too long
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 44
Random Access CSMA variations (contrsquo)
Ad hoc Sensor Networks Multiple channel access
p-persistent CSMA CSMA CD (Collision Detection)
A good tradeoff between non-persistent and 1-persistent CSMA
Same with CSMA however a stationalso listen to the carrier whiletransmitting to see if the transmissioncollides with someone elsetransmission It can be used in listen-while-talk capable channels (fullduplex) In single radio channels thetransmission need to be interrupted inorder to sense the channel
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 45
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 46
Random Access CSMA (contrsquo)bull How to select probability p
ndash Assume that M nodes have a packet to send and the medium is busy then Mp is theexpected number of nodes that will attempt to transmit once the medium becomesidle If Mp gt 1 then a collision is expected to occur Therefore the network mustmake sure that Mp lt 1 to avoid collision where M is the maximum number of nodesthat can be active at a time
bull Problems1 The mean packet delay increases exponently with increasing offered load2 CSMA protocols sense the carrier but sensing the carrier does not always releasestrue information about the status of the wireless channel There are two problems thatare unique to wireless channels (different than wireline channels)ndash Hidden terminal problemndash Exposed terminal problem
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 47
Hidden terminal problembull A sends to B C cannot receive Abull C wants to send to B C senses a
ldquofreerdquo medium (CS fails)bull collision at B A cannot receive the
collision (CD fails)bull A is ldquohiddenrdquo for C
Ad hoc Sensor Networks Multiple channel access
Random Access hidden and exposed terminal problemExposed terminal problem
bull B sends to A C wants to send to Dbull C has to wait CS signals a medium
in usebull since A is outside the radio range of
C waiting is not necessarybull C is ldquoexposedrdquo to B
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 48
Random Access MACAWhen a station wants to transmit databull It sends an RTS (Ready-to-Send) packet to the intended receiver
ndash The RTS packet contains the length of the data that needs to be transmittedndash Any station other than the intended recipient hearing RTS defers transmission for atime duration equal to the end of the corresponding CTS reception
bull The receiver sends back CTS (Clear-to-Send) packet back to sender if it isavailable to receivendash The CTS packet contains the length of the data that original sender wants totransmitndash Any station other than the original RTS sender hearing CTS defers transmissionuntil the data is sent
bull The original sender upon reception of the CTS starts transmitting
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 49
Hidden terminal problem
Ad hoc Sensor Networks Multiple channel access
Random Access MACA (contrsquo)Exposed terminal problem
Waiting time of node X is muchsmaller than waiting time of node C
bull C defers transmission upon hearing BrsquosRTS until B could get CTS from Abull After that C can start transmission to DFor that it first sends an RTSbull C is not longer exposed to the datatransmission of B
Problem It does not address the collision of RTS packet
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 50
Random Access reliabilitybull Wireless links are prone to have errors High packet loss rate detrimental to
transport-layer performance Mechanisms needed to reduce packet loss rateexperienced by upper layers
bull When node B receives a data packet from node A node B sends anAcknowledgement (Ack) This approach adopted in many protocols
bull If node A fails to receive an Ack it will retransmit the packet
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 51
Random Access MACAW (CSMACA) bull Physical carrier sense and virtual carrier sense using Network Allocation
Vector (NAV)ndash NAV is updated based on overheard RTSCTSDATAACK packets each of whichspecified duration of a pending transmissionndash Nodes stay silent when carrier sensed (physicalvirtual)
bull Backoff intervals used to reduce collision probabilityndash When transmitting a packet choose a backoff interval in the range [0w] where wis contention windowndash Count down the backoff interval when channel is idlendash Count down is suspended if channel becomes busyndash When backoff interval reaches 0 transmit RTS
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 52
Random Access CSMACA (contrsquo) bull The time spent counting down backoff intervals is a part of MAC overhead
ndash Choosing a large w leads to large backoff intervals and can result in larger overheadndash Choosing a small w leads to a larger number of collisions (when two nodes countdown to 0 simultaneously)ndash Since the number of nodes attempting to transmit simultaneously may change withtime some mechanism to manage contention is needed
bull IEEE 80211 DCF contention window w is chosen dynamically depending oncollision occurrence (binary exponential backoff)ndash When a node fails to receive CTS in response to its RTS it increases the contentionwindowndash w is doubled (up to an upper bound)ndash When a node successfully completes a data transfer it restores w to wmin
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 53
Random Access fairnessbull Simplest definition of fairness all nodes should receive equal bandwidthbull An example of unfairness
ndash Assume that initially A and B both choose a backoff interval in range [031] buttheir RTSs collidendash Nodes A and B then choose from range [063]ndash Node A chooses 4 slots and B choose 60 slotsndash After A transmits a packet it next chooses from range [031]ndash It is possible that A may transmit several packets before B transmits its first packet
Ad hoc Sensor Networks Multiple channel access
Unfairness occurs when one node has backed off much more than some other node
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 54
Random Access fairness (contrsquo) MACAW solution for fairnessbull When a node transmits a packet it appends value w to the packet all nodes
hearing value w use it for their future transmission attemptsbull Since w is an indication of the level of congestion in the vicinity of a specific
receiver node MACAW proposes maintaining w independently for eachreceiver
bull Using per-receiver wi is particularly useful in multi-hop environments sincecongestion level at different receivers can be very different
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 55
Random Access energy efficiency bull Since many mobile hosts are operated by batteries MAC
protocols which conserve energy are of interestbull Two approaches to reduce energy consumption
ndash Power save turn off wireless interface when desirablendash Power control reduce transmit power
bull Power control has some more potential benefitsndash Reduced interference (in ad hoc networks) eg it improves ALOHAefficiency where user with high power can capture a packet even if there is acollisionndash Increased spatial reuse (in infrastructured wireless networks) eg it is usedin CDMA to maintain target SIR of voice and data users or generally it canbe used to maintain target SIR for different user classes (admission control)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 56
Random Access power control bull When C transmits to D at a high power level B cannot receive Arsquos
transmission due to interference from Cbull If C reduces transmit power it can still communicate with D
ndash Reduces energy consumption at node Cndash Allows B to receive Arsquos transmission (spatial reuse)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 57
bull A special wireless ad hoc networkndash Large number of nodesndash Battery poweredndash Topology and density changendash Nodes for a common taskndash In-network data processing
bull Sensor-net applicationsndash Sensor-triggered bursty trafficndash Can often tolerate some delay
Ad hoc Sensor Networks Multiple channel access
Characteristics of Sensor Network
Scalabilty and self configurationEnergy efficiencyAdaptivityFairness not importantLatency
AdaptivityEnergy-delay tradoff
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 58
Ad hoc Sensor Networks Multiple channel access
Wireless sensor node and sensor network
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 59
MAC for WSN attributes bull Attributes
ndash Collision avoidance (Basic task)ndash Energy efficiency (most primary in WSN)ndash Scalability and adaptivityndash Channel utilizationndash Latencyndash Throughputndash Fairness
bull What causes energy wastendash Collisionsndash Control packet overheadndash Overhearing unnecessary trafficndash Long idle time (it consumes 50-100 of the power)
Ad hoc Sensor Networks Multiple channel access
Primary
Secondary
Dominant factor
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 60
MAC for WSN
bull Contention-based protocols need to work hard in all directions for energysavingsndash Reduce idle listening ndash support low duty cyclendash Better collision avoidancendash Reduce control overheadndash Avoid unnecessary overhearing
Ad hoc Sensor Networks Multiple channel access
Scheduled Protocols Contention ProtocolsCollisions No YesEnergy efficiency Good BadScalability and adaptivity Bad GoodMulti-hop communication Difficult EasyTime synchronization Strict Loose or not required
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 61
MAC for WSN classification
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 62
MAC for WSN SMAC (2002)bull It has been developed by Ye Heidemann and Estrinbull Tradeoffs between (latency fairness) and energybull Major components in S-MAC
ndash Periodic listen and sleep turn off radio when sleeping the reduced dutycycle is ~ 10 (eg 120ms on 12s off)ndash Collision avoidance is based on contention similarly to IEEE 80211 adhoc modendash Overhearing avoidance the node is sleeping when neighbors talkndash Massage passing long message is fragmented amp sent in burst operationwith extended Tx time and re-transmit occurs immediately
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 63
Summarybull The algorithms and protocols that enable sharing by multiple users and
controls the access to the wireless channel from different users are calledMAC
bull There are two types of wireless networks Infrastructured wireless networksand ad hoc wireless networks
bull We also want our wireless system to have high user capacity and we canidentify 10 methods for increasing it
bull There are several MACs with no absolute advantage it should be tailoredto the system requirements
bull There are primary concerns for designing a WSN MAC (collisionavoidance energy efficiency scalability and adaptivity) while thesecondary concerns are as follows (channel utilization latency throughputfairness)
bull Next lecture Routing protocols
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 10
Ad hoc Sensor Networks Multiple channel access
Two types of wireless networks (contrsquo)
bull Mesh networks serve as access networks that employ multi-hop wireless forwarding bynon-mobile nodes to relay traffic to and from the wired Internet In such an environmenthybrid wireless network technologies andor hierarchical network organization can beused for ad hoc and infrastructure wireless links
bull Wireless Sensor Networks are a special category of ad hoc networks that are used toprovide a wireless communication infrastructure among the sensors deployed in a specificapplication domain [rarr see Chapter 9 - 11]
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 11
Ad hoc Sensor Networks Multiple channel access
Infrastructured wireless networks Ad hoc wireless networksFixed infrastructure-based Infrastructureless
Guaranteed bandwidth Shared radio channel
Seamless connectivity (low call drops during handoffs)
Frequent path breaks due to mobility
High cost and time of deployment Quick and cost-effective deployment
Reuse of frequency spectrum (cellular principle) Dynamic frequency reuse based on carrier sense mechanism
Application domains include mainly civilian and commercial sectors
Application domains include battlefields emergency search and rescue operations
High cost of network maintenance Self-organization and maintenance properties are built into the network
Widely deployed and currently in the third generation of evolution
Several issues are to be addressed for successful commercial deployment even though widespread use exists in defense
Two types of wireless networks (contrsquo)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 12
Ad hoc Sensor Networks Multiple channel access
Duplexing
bull If the communication between two parties is one way then it is called simplexcommunication Simplex communication is achieved by default by using a singlewireless channel (frequency band) to transmit from sender to receiver
bull If the communication between two parties is two- way then it is called duplexcommunication Duplex communication achieved by
ndash Time Division (TDD) (famous in cellular systems)ndash Frequency Division (FDD) (famous in cellular systems)ndash Some other method like a random access method
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 13
Ad hoc Sensor Networks Multiple channel access
FDDbull A duplex channel consists of two
simplex channels with different carrierfrequencies
ndash Downlink band carries traffic frombase to mobile
ndash Uplink band carries traffic frommobile to base
Duplexing(contrsquo)TDD
bull channel (carrier frequency) is shared intime in a deterministic manner
ndash The time is slotted with fixed slotlength (sec)
ndash Some slots are used for downlinkchannel
ndash Some slots are used for uplink channel
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 14
Ad hoc Sensor Networks Multiple channel access
FDDbull FDD is used in radio systems that can
allocate individual radio frequenciesfor each user
bull For example analog systems AMPSbull In FDD channels are allocated by a
base stationbull A channel for a mobile is allocated
dynamicallybull All channels that a base station will use
are allocated usually staticallybull More suitable for wide-area cellular
networks GSM AMPS all use FDD
Duplexing(contrsquo)TDD
bull Can only be used in digital wirelesssystems (digital modulation)
bull Requires rigid timing andsynchronization
bull Mostly used in short-range and fixedwireless systems so that propagationdelay between base and mobile do notchange much with respect to locationof the mobile
bull Such as cordless phoneshellip
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 15
Ad hoc Sensor Networks Multiple channel access
Multiple Access narrow- vs wideband systemsbull Narrowband System
ndash The bandwidth is small compared to the coherence bandwidth of the channel(BltBcoh)
bull Wideband Systemndash The system bandwidth is much larger that the coherence bandwidth of the
multipath channel (BgtBcoh) A large number of users can access the same channel(frequency band) at the same time
bull Four major multiple access (MA) schemesndash Time Division Multiple Access (TDMA) it could be used in narrowband or
wideband systemndash Frequency Division Multiple Access (FDMA) it is usually used in narrowband
systemndash Code Division Multiple Access (CDMA) it is used in wideband systemndash Space Division Multiple Access (SDMA) for wireless systems with multiple
antennas it can be combined with all of the other MA methods
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 16
Ad hoc Sensor Networks Multiple channel access
Multiple Access cellular standards
Cellular System Multiple Access TechniqueAMPS (2G) FDMAFDD
CT2 Cordless Phone FDMATDD
DECT Cordless Phone FDMATDD
GSM (2G) TDMAFDD
USDC (IS-54 and IS-136) (2G) TDMAFDD
Personal Digital Cellular (2G) TDMAFDD
US IS-95 (2G) CDMAFDD
W-CDMA (3G) CDMAFDD CDMATDD
cdma2000 (3G) CDMAFDD CDMATDD
Narrowband System
Wideband System
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 17
Ad hoc Sensor Networks Multiple channel access
bull Individual radio channels are assigned toindividual users Each user is allocated a Wfrequency band by BS
bull If the channel allocated to a user is idle then itis not used by someone else waste of resource
bull Mobile and base can transmit and receivesimultaneously (FDD)
bull The W bandwidth of FDMA channels arerelatively low Symbol time is usually larger(low data rate) than the delay spread of themultipath channel (implies that inter-symbolinterference is low)
bull Lower complexity systems than TDMAsystems
Multiple Access FDMA
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 18
Ad hoc Sensor Networks Multiple channel access
Multiple Access FDMA (contrsquo)bull Capacity of FDMA systems
Nc = (B-2Bguard)Wwhere B is the total spectrum allocation Bguard is the guard band allocated atthe edge of the spectrum band W is the bandwidth of an individual channel
bull Erlang B and Erlang C system assumesndashinfinite population of sources which jointly offer traffic to Nc channelndashcall attempts arrive following a Poisson process so call arrivals are independentThe average arrival rate is λndashmessage length (holding times) are exponentially distributed (Markovian system)The average call length is hndashthe total amount of traffic offered in erlangs L= λhndashIf a user is rejected his next call attempt is made statistically independent of theprevious attempt (Erlang B)ndashIf all the channels are busy when a request arrives from a user the request isqueued An unlimited number of requests may be held in the queue (Erlang C)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 19
Ad hoc Sensor Networks Multiple channel access
bull Statictical measures of offered load Ncndash In Erlang B system the probability of call
blocking
ndash In Erlang C system the probabilty of waiting
ndash And the average wait time
c
c
cBLOCK
0
Pr
N
Nk
k
LN
Lk
=
=
sum
c
cc
wait 1
c0c
Pr 1
N
NkN
k
LL LL N kN
minus
=
=⎛ ⎞
+ minus⎜ ⎟⎝ ⎠
sum
wait waitc
Pr htN L
=minus
Multiple Access FDMA (contrsquo)Recall from
Queuing theory
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 20
Ad hoc Sensor Networks Multiple channel access
Multiple Access example of Erlang B systembull In an Erlang B system 40 channels are available A blocking probabilty of
less than 1 is required What is the traffic that can be serve if there is oneoperator or three operators
bull (Solution L(1 operator)gtL(3 operators))bull (Note The number of users increases faster than linearly with the number of
available channels The difference between actual increase and linearincrease is called the trunking gain It is preferable to have a large pool ofavailable channels that serves all users (a single operator) The reasons fornot choosing this approach are political not technical)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 21
Ad hoc Sensor Networks Multiple channel access
Multiple Access TDMAbull The allocated radio spectrum for the system is divided into time slots
ndash In each slot a user can transmit or receivendash A user occupies a cyclically repeating slotsndash A channel is logically defined as a particular time slot that repeats with some
periodbull TDMA systems buffer the data until its turn (time slot) comes to transmitbull In TDMATDD half of the slots in the frame is used for uplink channels the
other is used for downlink channelsbull In TDMAFDD a different carrier frequency is used for uplink or downlink
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 22
Ad hoc Sensor Networks Multiple channel access
Multiple Access TDMA (contrsquo)bull Preamble contains address and synchronization info to identify base
station and mobiles to each otherbull Guard times are used to allow synchronization of the receivers between
different slots and framesndash Different mobiles may have different propagation delays to a base station
because of different distances
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 23
Multiple Access TDMA (contrsquo)bull Each frame contains overhead bits and data bits Efficiency of frame is
defined as the percentage of data (information) bits to the total frame size inbits
η =(1 ndash boverheadbtotal) 100 where btotal= Tf R is the total number of bits in a frame Tf is the frameduration in sec boverhead is the number of overhead bits (GSM efficiency 73)
bull TDMA is usually combined with FDMAndash Neighboring cells are allocated and using different carrier frequencies (FDMA)
Inside a cell TDMA can be used Cells may be re-using the same frequency if theyare far from each-other
ndash There may be more than one channel allocated and used inside each cell Eachcarrier frequency (radio channel) may be using TDMA to further multiplex moreuser (ie having TDMA logical channels inside radio channels) For example inGSM each radio channel has 200KHz bandwidth and has 8 time slots (8 logicalchannels) Hence GSM is using FDMA combined with TDMA
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 24
Multiple Access TDMA (contrsquo)
Ad hoc Sensor Networks Multiple channel access
GSM (Europa) IS-54 (USA) PDC (Japan) DECT
Bit Rate 2708 Kbps 486 Kbps 42 Kbps 1152 Mbps
Bandwidth 200 KHz 30 KHz 25 KHz 1728 MHz
Time Slot 0577 ms 67 ms 67 ms 0417 ms
Upstream slots per frame 8 3 3 12
Duplexing FDD FDD FDD TDD
Efficiency 73 80 80 67
Modulation GMSK π4 DQPSK π4 DQPSK GMSK
Adaptive equalized Mandatory Mandatory Optional None
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 25
Multiple Access TDMA (contrsquo)bull Enables the sharing of a single radio channel among N usersbull Requires high data-rate per radio channel to support N users
simultaneously which requires adaptive equalizers to be usedin multipath environments
bull Transmission occurs in bursts (not continually) It enablespower saving by going to sleep modes in unrelated slots Itdiscontinues transmission and also enables mobile assistedhandoff
bull It equires synchronization of the receivers Need guard bits andsync bits which occurs large overhead per slot
bull Allocation of slots to mobile users should not be uniform Itmay depend on the traffic requirement of mobiles This bringsextra flexibility and efficiency compared to FDMA systems
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2620111015 TAacuteMOP ndash 412-082AKMR-2009-0006 26
Ad hoc Sensor Networks Multiple channel access
Multiple Access spread spectrum multiple accessbull SSMA uses signals that have transmission bandwidth that is
several orders of magnitude larger than minimum requiredRF bandwidth
bull It providesndash Immunity to multipath interferencendash Robust multiple access
bull Two techniquesndash Frequency Hopped Multiple Access (FHMA)ndash Direct Sequence Multiple Access (DSMA) or Code Division
Multiple Access (CDMA)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2720111015 TAacuteMOP ndash 412-082AKMR-2009-0006 27
Ad hoc Sensor Networks Multiple channel access
Multiple Access FHMAbull The carrier frequency of users are varied in a pseudo-random
fashionndash Each user is using a narrowband channel (spectrum) at a specific
instance of timendash The random change in frequency make the change of using the same
narrowband channel very lowbull The sender receiver change frequency (calling hopping)
using the same pseudo-random sequence hence they aresynchronized
bull Rate of hopping versus Symbol ratendash If hopping rate is greather Fast Frequency Hoppingndash If symbol rate is greater Slow Frequency Hopping
bull GSM and Bluetooth are example systems
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2820111015 TAacuteMOP ndash 412-082AKMR-2009-0006 28
Ad hoc Sensor Networks Multiple channel access
Multiple Access FHMA (contrsquo)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2920111015 TAacuteMOP ndash 412-082AKMR-2009-0006 29
Ad hoc Sensor Networks Multiple channel access
Multiple Access CDMAbull The narrowband message signal is multiplied by a very large bandwidth
signal called spreading signal (code) before modulation and transmissionover the air This is called spreading
bull Spreading signal use a pseudo-noise (PN) sequence (a pseudo-randomsequence) called codeword which are orthogonal (low autocorrelation)
bull The receiver correlator distinguishes the senders signal by examining thewideband signal with the same time-synchronized spreading code(despreading)
bull Advantagesndash Low power spectral densityndash Interference limited operationndash Privacyndash Reduction of multipath affects by using a larger spectrumndash Users can start their transmission at any time (random access possible)ndash Cell capacity is soft and higher than TDMA and FDMAndash No frequency management and no equalizers and quard time needed
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3020111015 TAacuteMOP ndash 412-082AKMR-2009-0006 30
Ad hoc Sensor Networks Multiple channel access
Multiple Access near far problem and power controlbull At a receiver the signals may come from various sources (multiuser) The
strongest signal usually captures the modulator The other signals areconsidered as noise Each source may have different distances to the basestation
bull In CDMA we want a BS to receive CDMA coded signals from variousmobile users at the same time
ndash Therefore the receiver power at the BS for all mobile users should be close toeach other This requires power control at the mobiles
bull Power Control BS monitors the RSSI (Received Signal Strength Indicator)values from different mobiles and then sends power change commands to themobiles over a forward channel The mobiles then adjust their transmitpower
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3120111015 TAacuteMOP ndash 412-082AKMR-2009-0006 31
Ad hoc Sensor Networks Multiple channel access
Multiple Access hybrid spread spectrum techniquesbull FDMACDMA
ndash Available wideband spectrum is divided into a number narrowband radio channels CDMAis employed inside each channel
bull DSFHMAndash The signals are spread using spreading codes (direct sequence signals are obtained) but
these signal are not transmitted over a constant carrier frequency they are transmitted over afrequency hopping carrier frequency
bull Time Division CDMA (TCDMA)ndash Each cell is using a different spreading code (CDMA employed between cells) that is
conveyed to the mobiles in its rangendash Inside each cell (inside a CDMA channel) TDMA is employed to multiplex multiple users
bull Time Division Frequency Hoppingndash At each time slot the user is hopped to a new frequency according to a pseudo-random
hopping sequencendash Employed in severe co-interference and multi-path environmentsndash Bluetooth and GSM are using this technique
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3220111015 TAacuteMOP ndash 412-082AKMR-2009-0006 32
Ad hoc Sensor Networks Multiple channel access
Multiple Access comparison TDMAFDMACDMA
Techniques TDMA FDMA CDMA SDMA
Idea
segment sending time into disjoint time-slots
Segment the frequency band into disjoint sub-bands
Spread the spectrum using orthogonal codes
Segment space into sectors
Signal separation
Synchronization in the time domain
Filtering in the frequency domain
Code Directed antennas
Advantage
Established flexible
Simple established robust
Flexible less frequency planning needed soft handover
Simple increases capacity
DisadvantageQuard space synchronization difficult
Inflexible scarce source
Complex receiver power control
inflexible
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3320111015 TAacuteMOP ndash 412-082AKMR-2009-0006 33
Ad hoc Sensor Networks Multiple channel access
User capacity per cell
The user capacity C is defined as the total number of active users percell that the system can support while meeting a common BERconstrain For orthogonal multiple access (as FDMA and TDMA)
C = Ncellwhere Ncell is the number of channels assigned to any given cellThe total number of orthogonal channels of bandwidth Bs that can becreated from a total system bandwidth of B is B Bs The reuse factorsatisfies N = (B Bs) Ncell this implies
C = (B Bs) N
Recall from Chapter 1
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3420111015 TAacuteMOP ndash 412-082AKMR-2009-0006 34
Ad hoc Sensor Networks Multiple channel access
10 methods for increasing capacity1 Increasing the amount of spectrum used very expensive2 More efficient modulation format and coding3 Better source coding4 Adaptive modulation and coding5 Discontinuous transmission6 Multiuser detection CDMA systems7 Reduction of cell radius effective but very expensive smaller
cells require more handovers8 Use of sector cells tripled cells have tripled BS antennas9 Use of an overlay structure10 Multiple antennas diversity MIMO systems SDMA
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3520111015 TAacuteMOP ndash 412-082AKMR-2009-0006 35
Ad hoc Sensor Networks Multiple channel access
bull Distributed operation is requiredbull Synchronization is required in TDMA-based systemsbull Hidden terminals are nodes hidden from a senderbull Exposed terminals are exposed nodes preventing a sender
from sendingbull Throughput needs to be maximizedbull Access delay should be minimizedbull Fairness refers to provide an equal share to all competing
nodesbull Real-time traffic support is required for voice video and
real-time databull Resource reservation is required for QoSbull Ability to measure resource availability handles the
resourcesbull Capability for power control reduces the energy
consumption
Random access in adhoc networks
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3620111015 TAacuteMOP ndash 412-082AKMR-2009-0006 36
Ad hoc Sensor Networks Multiple channel access
Random accessbull All random access techniques are based on packetized data or packet radio where user
date is collected into packets of M bitsbull Collision if packets from different users overlap in timebull The transmission time of a packet is τ = M R where R [bps] the data ratebull Analysis of random access techniques assumes that the users generate packets according
to memoryless Poisson process at a rate λ [packets per unit time] the probability that thenumber of packet arrivals in a time period [0t] denoted by X(t) is equal to integer k isgiven by
Pr(X(t)=k) = (λt)k k e-λt
bull The traffic load is defined as L = λ τ If L gt 1 means on avarage more packets arrive inthe system over a given time period than can be transmitted in that period so systemswith Lgt1 are unstable
bull The throughput is defined as the ratio of the average number of packets successfullytransmitted in any given time interval divided by the number of attempted transmissionsin that interval
T = L Pr(succesful packet transmission)bull The effective data rate of the system is RT
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3720111015 TAacuteMOP ndash 412-082AKMR-2009-0006 37
Ad hoc Sensor Networks Multiple channel access
Random access (contrsquo)bull We will examine the following attributes
ndash Channel utilization (Throughput)ndash Latencyndash Collision avoidance (Hidden and exposed terminals problem)ndash Reliability (ACK)ndash Energy efficiency (power control)ndash Fairnessndash Throughput needs to be maximized
bull We will discuss the following techniquesndash Aloha and slotted Alohandash CSMA Protocols (1-persistent non-persistent p-persistent CSMA CSMACD)ndash MACAndash MACAWndash Energy efficient MAC in wireless sensor networks (eg SMAC)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 38
Random Access ALOHA (1970)bull ALOHA was developed for a wireless system at the University of Hawaii
(Abramson et al)ndash Multiple remote stations plus one base stationndash Frame transmissions are made at one frequency from a remote station to the basestation the base station re-broadcasts frames on another frequency Contention is foraccess to the base station
bull Frames can be sent at any time to the base stationndash If no acknowledgment (ACK) is received assume that the frame is lost andretransmit after waiting a random amount of timendash If more than one station broadcasts on the base station access frequency at the sametime ndash a collision ndash interference will destroy the frames
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 39
Random Access ALOHA (contrsquo)
bull The probability that no packets generated during the tima [-τ τ] is given by (nocollision)
Pr(X(t)=0) = (λ2τ)0 0 e-2λτ = e-2Lwith corresponding throughput
T = L e-2L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 05 05e 018
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 40
Random Access slotted ALOHA (1971)bull Improvement Time is slotted and a packet can only be transmitted at the beginning of
one slot Thus it can reduce the collision duration
bull The probability that no packets generated during the time [0 τ] is given by (nocollision)
Pr(X(t)=0) = (λτ)0 0 e-λτ = e-Lwith corresponding throughput T = L e-L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 1 e 037
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 41
bull Slotted Aloha has double the maximumthroughput as pure Aloha and achievesthis maximum at a higher offered loadThe effective data rate is still less than40 of the raw transmission rate
bull Aloha does not listen to the carrier beforetransmission more sophisticatedtechniques are needed to increaseefficiency
bull Further problem the delayndashDelay
ndashThe mean packet delay
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
( ) ( )delayPr 1 e ekL Lkτ minus minus= = minus
( )delay1
1 e e ekL L L
kMPD E kτ
infinminus minus
=
= = minus =sum
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 42
Random Access Carrier Sense Multiple Accessbull CSMA (Carrier Sense Multiple Access) listen to the carrier before transmission
and transmits if channel is idlebull Detection delay and propagation delay are two important parameters
ndash Detection delay time required to sense the carrier and decide if it is idle or busyndash Propagation delay distancespeed_of_ligth The time required for bit to travel
from transmitter to the receiver
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 43
Random Access CSMA variations
Ad hoc Sensor Networks Multiple channel access
1-persistent CSMA Non-persistent CSMA
There will always be a collision iftwo nodes want to retransmit(usually you stop transmissionattempts after few tries)
Random backoff reduces probabilityof collisions Waste idle time if thebackoff time is too long
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 44
Random Access CSMA variations (contrsquo)
Ad hoc Sensor Networks Multiple channel access
p-persistent CSMA CSMA CD (Collision Detection)
A good tradeoff between non-persistent and 1-persistent CSMA
Same with CSMA however a stationalso listen to the carrier whiletransmitting to see if the transmissioncollides with someone elsetransmission It can be used in listen-while-talk capable channels (fullduplex) In single radio channels thetransmission need to be interrupted inorder to sense the channel
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 45
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 46
Random Access CSMA (contrsquo)bull How to select probability p
ndash Assume that M nodes have a packet to send and the medium is busy then Mp is theexpected number of nodes that will attempt to transmit once the medium becomesidle If Mp gt 1 then a collision is expected to occur Therefore the network mustmake sure that Mp lt 1 to avoid collision where M is the maximum number of nodesthat can be active at a time
bull Problems1 The mean packet delay increases exponently with increasing offered load2 CSMA protocols sense the carrier but sensing the carrier does not always releasestrue information about the status of the wireless channel There are two problems thatare unique to wireless channels (different than wireline channels)ndash Hidden terminal problemndash Exposed terminal problem
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 47
Hidden terminal problembull A sends to B C cannot receive Abull C wants to send to B C senses a
ldquofreerdquo medium (CS fails)bull collision at B A cannot receive the
collision (CD fails)bull A is ldquohiddenrdquo for C
Ad hoc Sensor Networks Multiple channel access
Random Access hidden and exposed terminal problemExposed terminal problem
bull B sends to A C wants to send to Dbull C has to wait CS signals a medium
in usebull since A is outside the radio range of
C waiting is not necessarybull C is ldquoexposedrdquo to B
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 48
Random Access MACAWhen a station wants to transmit databull It sends an RTS (Ready-to-Send) packet to the intended receiver
ndash The RTS packet contains the length of the data that needs to be transmittedndash Any station other than the intended recipient hearing RTS defers transmission for atime duration equal to the end of the corresponding CTS reception
bull The receiver sends back CTS (Clear-to-Send) packet back to sender if it isavailable to receivendash The CTS packet contains the length of the data that original sender wants totransmitndash Any station other than the original RTS sender hearing CTS defers transmissionuntil the data is sent
bull The original sender upon reception of the CTS starts transmitting
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 49
Hidden terminal problem
Ad hoc Sensor Networks Multiple channel access
Random Access MACA (contrsquo)Exposed terminal problem
Waiting time of node X is muchsmaller than waiting time of node C
bull C defers transmission upon hearing BrsquosRTS until B could get CTS from Abull After that C can start transmission to DFor that it first sends an RTSbull C is not longer exposed to the datatransmission of B
Problem It does not address the collision of RTS packet
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 50
Random Access reliabilitybull Wireless links are prone to have errors High packet loss rate detrimental to
transport-layer performance Mechanisms needed to reduce packet loss rateexperienced by upper layers
bull When node B receives a data packet from node A node B sends anAcknowledgement (Ack) This approach adopted in many protocols
bull If node A fails to receive an Ack it will retransmit the packet
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 51
Random Access MACAW (CSMACA) bull Physical carrier sense and virtual carrier sense using Network Allocation
Vector (NAV)ndash NAV is updated based on overheard RTSCTSDATAACK packets each of whichspecified duration of a pending transmissionndash Nodes stay silent when carrier sensed (physicalvirtual)
bull Backoff intervals used to reduce collision probabilityndash When transmitting a packet choose a backoff interval in the range [0w] where wis contention windowndash Count down the backoff interval when channel is idlendash Count down is suspended if channel becomes busyndash When backoff interval reaches 0 transmit RTS
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 52
Random Access CSMACA (contrsquo) bull The time spent counting down backoff intervals is a part of MAC overhead
ndash Choosing a large w leads to large backoff intervals and can result in larger overheadndash Choosing a small w leads to a larger number of collisions (when two nodes countdown to 0 simultaneously)ndash Since the number of nodes attempting to transmit simultaneously may change withtime some mechanism to manage contention is needed
bull IEEE 80211 DCF contention window w is chosen dynamically depending oncollision occurrence (binary exponential backoff)ndash When a node fails to receive CTS in response to its RTS it increases the contentionwindowndash w is doubled (up to an upper bound)ndash When a node successfully completes a data transfer it restores w to wmin
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 53
Random Access fairnessbull Simplest definition of fairness all nodes should receive equal bandwidthbull An example of unfairness
ndash Assume that initially A and B both choose a backoff interval in range [031] buttheir RTSs collidendash Nodes A and B then choose from range [063]ndash Node A chooses 4 slots and B choose 60 slotsndash After A transmits a packet it next chooses from range [031]ndash It is possible that A may transmit several packets before B transmits its first packet
Ad hoc Sensor Networks Multiple channel access
Unfairness occurs when one node has backed off much more than some other node
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 54
Random Access fairness (contrsquo) MACAW solution for fairnessbull When a node transmits a packet it appends value w to the packet all nodes
hearing value w use it for their future transmission attemptsbull Since w is an indication of the level of congestion in the vicinity of a specific
receiver node MACAW proposes maintaining w independently for eachreceiver
bull Using per-receiver wi is particularly useful in multi-hop environments sincecongestion level at different receivers can be very different
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 55
Random Access energy efficiency bull Since many mobile hosts are operated by batteries MAC
protocols which conserve energy are of interestbull Two approaches to reduce energy consumption
ndash Power save turn off wireless interface when desirablendash Power control reduce transmit power
bull Power control has some more potential benefitsndash Reduced interference (in ad hoc networks) eg it improves ALOHAefficiency where user with high power can capture a packet even if there is acollisionndash Increased spatial reuse (in infrastructured wireless networks) eg it is usedin CDMA to maintain target SIR of voice and data users or generally it canbe used to maintain target SIR for different user classes (admission control)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 56
Random Access power control bull When C transmits to D at a high power level B cannot receive Arsquos
transmission due to interference from Cbull If C reduces transmit power it can still communicate with D
ndash Reduces energy consumption at node Cndash Allows B to receive Arsquos transmission (spatial reuse)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 57
bull A special wireless ad hoc networkndash Large number of nodesndash Battery poweredndash Topology and density changendash Nodes for a common taskndash In-network data processing
bull Sensor-net applicationsndash Sensor-triggered bursty trafficndash Can often tolerate some delay
Ad hoc Sensor Networks Multiple channel access
Characteristics of Sensor Network
Scalabilty and self configurationEnergy efficiencyAdaptivityFairness not importantLatency
AdaptivityEnergy-delay tradoff
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 58
Ad hoc Sensor Networks Multiple channel access
Wireless sensor node and sensor network
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 59
MAC for WSN attributes bull Attributes
ndash Collision avoidance (Basic task)ndash Energy efficiency (most primary in WSN)ndash Scalability and adaptivityndash Channel utilizationndash Latencyndash Throughputndash Fairness
bull What causes energy wastendash Collisionsndash Control packet overheadndash Overhearing unnecessary trafficndash Long idle time (it consumes 50-100 of the power)
Ad hoc Sensor Networks Multiple channel access
Primary
Secondary
Dominant factor
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 60
MAC for WSN
bull Contention-based protocols need to work hard in all directions for energysavingsndash Reduce idle listening ndash support low duty cyclendash Better collision avoidancendash Reduce control overheadndash Avoid unnecessary overhearing
Ad hoc Sensor Networks Multiple channel access
Scheduled Protocols Contention ProtocolsCollisions No YesEnergy efficiency Good BadScalability and adaptivity Bad GoodMulti-hop communication Difficult EasyTime synchronization Strict Loose or not required
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 61
MAC for WSN classification
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 62
MAC for WSN SMAC (2002)bull It has been developed by Ye Heidemann and Estrinbull Tradeoffs between (latency fairness) and energybull Major components in S-MAC
ndash Periodic listen and sleep turn off radio when sleeping the reduced dutycycle is ~ 10 (eg 120ms on 12s off)ndash Collision avoidance is based on contention similarly to IEEE 80211 adhoc modendash Overhearing avoidance the node is sleeping when neighbors talkndash Massage passing long message is fragmented amp sent in burst operationwith extended Tx time and re-transmit occurs immediately
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 63
Summarybull The algorithms and protocols that enable sharing by multiple users and
controls the access to the wireless channel from different users are calledMAC
bull There are two types of wireless networks Infrastructured wireless networksand ad hoc wireless networks
bull We also want our wireless system to have high user capacity and we canidentify 10 methods for increasing it
bull There are several MACs with no absolute advantage it should be tailoredto the system requirements
bull There are primary concerns for designing a WSN MAC (collisionavoidance energy efficiency scalability and adaptivity) while thesecondary concerns are as follows (channel utilization latency throughputfairness)
bull Next lecture Routing protocols
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 11
Ad hoc Sensor Networks Multiple channel access
Infrastructured wireless networks Ad hoc wireless networksFixed infrastructure-based Infrastructureless
Guaranteed bandwidth Shared radio channel
Seamless connectivity (low call drops during handoffs)
Frequent path breaks due to mobility
High cost and time of deployment Quick and cost-effective deployment
Reuse of frequency spectrum (cellular principle) Dynamic frequency reuse based on carrier sense mechanism
Application domains include mainly civilian and commercial sectors
Application domains include battlefields emergency search and rescue operations
High cost of network maintenance Self-organization and maintenance properties are built into the network
Widely deployed and currently in the third generation of evolution
Several issues are to be addressed for successful commercial deployment even though widespread use exists in defense
Two types of wireless networks (contrsquo)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 12
Ad hoc Sensor Networks Multiple channel access
Duplexing
bull If the communication between two parties is one way then it is called simplexcommunication Simplex communication is achieved by default by using a singlewireless channel (frequency band) to transmit from sender to receiver
bull If the communication between two parties is two- way then it is called duplexcommunication Duplex communication achieved by
ndash Time Division (TDD) (famous in cellular systems)ndash Frequency Division (FDD) (famous in cellular systems)ndash Some other method like a random access method
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 13
Ad hoc Sensor Networks Multiple channel access
FDDbull A duplex channel consists of two
simplex channels with different carrierfrequencies
ndash Downlink band carries traffic frombase to mobile
ndash Uplink band carries traffic frommobile to base
Duplexing(contrsquo)TDD
bull channel (carrier frequency) is shared intime in a deterministic manner
ndash The time is slotted with fixed slotlength (sec)
ndash Some slots are used for downlinkchannel
ndash Some slots are used for uplink channel
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 14
Ad hoc Sensor Networks Multiple channel access
FDDbull FDD is used in radio systems that can
allocate individual radio frequenciesfor each user
bull For example analog systems AMPSbull In FDD channels are allocated by a
base stationbull A channel for a mobile is allocated
dynamicallybull All channels that a base station will use
are allocated usually staticallybull More suitable for wide-area cellular
networks GSM AMPS all use FDD
Duplexing(contrsquo)TDD
bull Can only be used in digital wirelesssystems (digital modulation)
bull Requires rigid timing andsynchronization
bull Mostly used in short-range and fixedwireless systems so that propagationdelay between base and mobile do notchange much with respect to locationof the mobile
bull Such as cordless phoneshellip
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 15
Ad hoc Sensor Networks Multiple channel access
Multiple Access narrow- vs wideband systemsbull Narrowband System
ndash The bandwidth is small compared to the coherence bandwidth of the channel(BltBcoh)
bull Wideband Systemndash The system bandwidth is much larger that the coherence bandwidth of the
multipath channel (BgtBcoh) A large number of users can access the same channel(frequency band) at the same time
bull Four major multiple access (MA) schemesndash Time Division Multiple Access (TDMA) it could be used in narrowband or
wideband systemndash Frequency Division Multiple Access (FDMA) it is usually used in narrowband
systemndash Code Division Multiple Access (CDMA) it is used in wideband systemndash Space Division Multiple Access (SDMA) for wireless systems with multiple
antennas it can be combined with all of the other MA methods
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 16
Ad hoc Sensor Networks Multiple channel access
Multiple Access cellular standards
Cellular System Multiple Access TechniqueAMPS (2G) FDMAFDD
CT2 Cordless Phone FDMATDD
DECT Cordless Phone FDMATDD
GSM (2G) TDMAFDD
USDC (IS-54 and IS-136) (2G) TDMAFDD
Personal Digital Cellular (2G) TDMAFDD
US IS-95 (2G) CDMAFDD
W-CDMA (3G) CDMAFDD CDMATDD
cdma2000 (3G) CDMAFDD CDMATDD
Narrowband System
Wideband System
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 17
Ad hoc Sensor Networks Multiple channel access
bull Individual radio channels are assigned toindividual users Each user is allocated a Wfrequency band by BS
bull If the channel allocated to a user is idle then itis not used by someone else waste of resource
bull Mobile and base can transmit and receivesimultaneously (FDD)
bull The W bandwidth of FDMA channels arerelatively low Symbol time is usually larger(low data rate) than the delay spread of themultipath channel (implies that inter-symbolinterference is low)
bull Lower complexity systems than TDMAsystems
Multiple Access FDMA
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 18
Ad hoc Sensor Networks Multiple channel access
Multiple Access FDMA (contrsquo)bull Capacity of FDMA systems
Nc = (B-2Bguard)Wwhere B is the total spectrum allocation Bguard is the guard band allocated atthe edge of the spectrum band W is the bandwidth of an individual channel
bull Erlang B and Erlang C system assumesndashinfinite population of sources which jointly offer traffic to Nc channelndashcall attempts arrive following a Poisson process so call arrivals are independentThe average arrival rate is λndashmessage length (holding times) are exponentially distributed (Markovian system)The average call length is hndashthe total amount of traffic offered in erlangs L= λhndashIf a user is rejected his next call attempt is made statistically independent of theprevious attempt (Erlang B)ndashIf all the channels are busy when a request arrives from a user the request isqueued An unlimited number of requests may be held in the queue (Erlang C)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 19
Ad hoc Sensor Networks Multiple channel access
bull Statictical measures of offered load Ncndash In Erlang B system the probability of call
blocking
ndash In Erlang C system the probabilty of waiting
ndash And the average wait time
c
c
cBLOCK
0
Pr
N
Nk
k
LN
Lk
=
=
sum
c
cc
wait 1
c0c
Pr 1
N
NkN
k
LL LL N kN
minus
=
=⎛ ⎞
+ minus⎜ ⎟⎝ ⎠
sum
wait waitc
Pr htN L
=minus
Multiple Access FDMA (contrsquo)Recall from
Queuing theory
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 20
Ad hoc Sensor Networks Multiple channel access
Multiple Access example of Erlang B systembull In an Erlang B system 40 channels are available A blocking probabilty of
less than 1 is required What is the traffic that can be serve if there is oneoperator or three operators
bull (Solution L(1 operator)gtL(3 operators))bull (Note The number of users increases faster than linearly with the number of
available channels The difference between actual increase and linearincrease is called the trunking gain It is preferable to have a large pool ofavailable channels that serves all users (a single operator) The reasons fornot choosing this approach are political not technical)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 21
Ad hoc Sensor Networks Multiple channel access
Multiple Access TDMAbull The allocated radio spectrum for the system is divided into time slots
ndash In each slot a user can transmit or receivendash A user occupies a cyclically repeating slotsndash A channel is logically defined as a particular time slot that repeats with some
periodbull TDMA systems buffer the data until its turn (time slot) comes to transmitbull In TDMATDD half of the slots in the frame is used for uplink channels the
other is used for downlink channelsbull In TDMAFDD a different carrier frequency is used for uplink or downlink
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 22
Ad hoc Sensor Networks Multiple channel access
Multiple Access TDMA (contrsquo)bull Preamble contains address and synchronization info to identify base
station and mobiles to each otherbull Guard times are used to allow synchronization of the receivers between
different slots and framesndash Different mobiles may have different propagation delays to a base station
because of different distances
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 23
Multiple Access TDMA (contrsquo)bull Each frame contains overhead bits and data bits Efficiency of frame is
defined as the percentage of data (information) bits to the total frame size inbits
η =(1 ndash boverheadbtotal) 100 where btotal= Tf R is the total number of bits in a frame Tf is the frameduration in sec boverhead is the number of overhead bits (GSM efficiency 73)
bull TDMA is usually combined with FDMAndash Neighboring cells are allocated and using different carrier frequencies (FDMA)
Inside a cell TDMA can be used Cells may be re-using the same frequency if theyare far from each-other
ndash There may be more than one channel allocated and used inside each cell Eachcarrier frequency (radio channel) may be using TDMA to further multiplex moreuser (ie having TDMA logical channels inside radio channels) For example inGSM each radio channel has 200KHz bandwidth and has 8 time slots (8 logicalchannels) Hence GSM is using FDMA combined with TDMA
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 24
Multiple Access TDMA (contrsquo)
Ad hoc Sensor Networks Multiple channel access
GSM (Europa) IS-54 (USA) PDC (Japan) DECT
Bit Rate 2708 Kbps 486 Kbps 42 Kbps 1152 Mbps
Bandwidth 200 KHz 30 KHz 25 KHz 1728 MHz
Time Slot 0577 ms 67 ms 67 ms 0417 ms
Upstream slots per frame 8 3 3 12
Duplexing FDD FDD FDD TDD
Efficiency 73 80 80 67
Modulation GMSK π4 DQPSK π4 DQPSK GMSK
Adaptive equalized Mandatory Mandatory Optional None
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 25
Multiple Access TDMA (contrsquo)bull Enables the sharing of a single radio channel among N usersbull Requires high data-rate per radio channel to support N users
simultaneously which requires adaptive equalizers to be usedin multipath environments
bull Transmission occurs in bursts (not continually) It enablespower saving by going to sleep modes in unrelated slots Itdiscontinues transmission and also enables mobile assistedhandoff
bull It equires synchronization of the receivers Need guard bits andsync bits which occurs large overhead per slot
bull Allocation of slots to mobile users should not be uniform Itmay depend on the traffic requirement of mobiles This bringsextra flexibility and efficiency compared to FDMA systems
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2620111015 TAacuteMOP ndash 412-082AKMR-2009-0006 26
Ad hoc Sensor Networks Multiple channel access
Multiple Access spread spectrum multiple accessbull SSMA uses signals that have transmission bandwidth that is
several orders of magnitude larger than minimum requiredRF bandwidth
bull It providesndash Immunity to multipath interferencendash Robust multiple access
bull Two techniquesndash Frequency Hopped Multiple Access (FHMA)ndash Direct Sequence Multiple Access (DSMA) or Code Division
Multiple Access (CDMA)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2720111015 TAacuteMOP ndash 412-082AKMR-2009-0006 27
Ad hoc Sensor Networks Multiple channel access
Multiple Access FHMAbull The carrier frequency of users are varied in a pseudo-random
fashionndash Each user is using a narrowband channel (spectrum) at a specific
instance of timendash The random change in frequency make the change of using the same
narrowband channel very lowbull The sender receiver change frequency (calling hopping)
using the same pseudo-random sequence hence they aresynchronized
bull Rate of hopping versus Symbol ratendash If hopping rate is greather Fast Frequency Hoppingndash If symbol rate is greater Slow Frequency Hopping
bull GSM and Bluetooth are example systems
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2820111015 TAacuteMOP ndash 412-082AKMR-2009-0006 28
Ad hoc Sensor Networks Multiple channel access
Multiple Access FHMA (contrsquo)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2920111015 TAacuteMOP ndash 412-082AKMR-2009-0006 29
Ad hoc Sensor Networks Multiple channel access
Multiple Access CDMAbull The narrowband message signal is multiplied by a very large bandwidth
signal called spreading signal (code) before modulation and transmissionover the air This is called spreading
bull Spreading signal use a pseudo-noise (PN) sequence (a pseudo-randomsequence) called codeword which are orthogonal (low autocorrelation)
bull The receiver correlator distinguishes the senders signal by examining thewideband signal with the same time-synchronized spreading code(despreading)
bull Advantagesndash Low power spectral densityndash Interference limited operationndash Privacyndash Reduction of multipath affects by using a larger spectrumndash Users can start their transmission at any time (random access possible)ndash Cell capacity is soft and higher than TDMA and FDMAndash No frequency management and no equalizers and quard time needed
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3020111015 TAacuteMOP ndash 412-082AKMR-2009-0006 30
Ad hoc Sensor Networks Multiple channel access
Multiple Access near far problem and power controlbull At a receiver the signals may come from various sources (multiuser) The
strongest signal usually captures the modulator The other signals areconsidered as noise Each source may have different distances to the basestation
bull In CDMA we want a BS to receive CDMA coded signals from variousmobile users at the same time
ndash Therefore the receiver power at the BS for all mobile users should be close toeach other This requires power control at the mobiles
bull Power Control BS monitors the RSSI (Received Signal Strength Indicator)values from different mobiles and then sends power change commands to themobiles over a forward channel The mobiles then adjust their transmitpower
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3120111015 TAacuteMOP ndash 412-082AKMR-2009-0006 31
Ad hoc Sensor Networks Multiple channel access
Multiple Access hybrid spread spectrum techniquesbull FDMACDMA
ndash Available wideband spectrum is divided into a number narrowband radio channels CDMAis employed inside each channel
bull DSFHMAndash The signals are spread using spreading codes (direct sequence signals are obtained) but
these signal are not transmitted over a constant carrier frequency they are transmitted over afrequency hopping carrier frequency
bull Time Division CDMA (TCDMA)ndash Each cell is using a different spreading code (CDMA employed between cells) that is
conveyed to the mobiles in its rangendash Inside each cell (inside a CDMA channel) TDMA is employed to multiplex multiple users
bull Time Division Frequency Hoppingndash At each time slot the user is hopped to a new frequency according to a pseudo-random
hopping sequencendash Employed in severe co-interference and multi-path environmentsndash Bluetooth and GSM are using this technique
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3220111015 TAacuteMOP ndash 412-082AKMR-2009-0006 32
Ad hoc Sensor Networks Multiple channel access
Multiple Access comparison TDMAFDMACDMA
Techniques TDMA FDMA CDMA SDMA
Idea
segment sending time into disjoint time-slots
Segment the frequency band into disjoint sub-bands
Spread the spectrum using orthogonal codes
Segment space into sectors
Signal separation
Synchronization in the time domain
Filtering in the frequency domain
Code Directed antennas
Advantage
Established flexible
Simple established robust
Flexible less frequency planning needed soft handover
Simple increases capacity
DisadvantageQuard space synchronization difficult
Inflexible scarce source
Complex receiver power control
inflexible
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3320111015 TAacuteMOP ndash 412-082AKMR-2009-0006 33
Ad hoc Sensor Networks Multiple channel access
User capacity per cell
The user capacity C is defined as the total number of active users percell that the system can support while meeting a common BERconstrain For orthogonal multiple access (as FDMA and TDMA)
C = Ncellwhere Ncell is the number of channels assigned to any given cellThe total number of orthogonal channels of bandwidth Bs that can becreated from a total system bandwidth of B is B Bs The reuse factorsatisfies N = (B Bs) Ncell this implies
C = (B Bs) N
Recall from Chapter 1
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3420111015 TAacuteMOP ndash 412-082AKMR-2009-0006 34
Ad hoc Sensor Networks Multiple channel access
10 methods for increasing capacity1 Increasing the amount of spectrum used very expensive2 More efficient modulation format and coding3 Better source coding4 Adaptive modulation and coding5 Discontinuous transmission6 Multiuser detection CDMA systems7 Reduction of cell radius effective but very expensive smaller
cells require more handovers8 Use of sector cells tripled cells have tripled BS antennas9 Use of an overlay structure10 Multiple antennas diversity MIMO systems SDMA
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3520111015 TAacuteMOP ndash 412-082AKMR-2009-0006 35
Ad hoc Sensor Networks Multiple channel access
bull Distributed operation is requiredbull Synchronization is required in TDMA-based systemsbull Hidden terminals are nodes hidden from a senderbull Exposed terminals are exposed nodes preventing a sender
from sendingbull Throughput needs to be maximizedbull Access delay should be minimizedbull Fairness refers to provide an equal share to all competing
nodesbull Real-time traffic support is required for voice video and
real-time databull Resource reservation is required for QoSbull Ability to measure resource availability handles the
resourcesbull Capability for power control reduces the energy
consumption
Random access in adhoc networks
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3620111015 TAacuteMOP ndash 412-082AKMR-2009-0006 36
Ad hoc Sensor Networks Multiple channel access
Random accessbull All random access techniques are based on packetized data or packet radio where user
date is collected into packets of M bitsbull Collision if packets from different users overlap in timebull The transmission time of a packet is τ = M R where R [bps] the data ratebull Analysis of random access techniques assumes that the users generate packets according
to memoryless Poisson process at a rate λ [packets per unit time] the probability that thenumber of packet arrivals in a time period [0t] denoted by X(t) is equal to integer k isgiven by
Pr(X(t)=k) = (λt)k k e-λt
bull The traffic load is defined as L = λ τ If L gt 1 means on avarage more packets arrive inthe system over a given time period than can be transmitted in that period so systemswith Lgt1 are unstable
bull The throughput is defined as the ratio of the average number of packets successfullytransmitted in any given time interval divided by the number of attempted transmissionsin that interval
T = L Pr(succesful packet transmission)bull The effective data rate of the system is RT
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3720111015 TAacuteMOP ndash 412-082AKMR-2009-0006 37
Ad hoc Sensor Networks Multiple channel access
Random access (contrsquo)bull We will examine the following attributes
ndash Channel utilization (Throughput)ndash Latencyndash Collision avoidance (Hidden and exposed terminals problem)ndash Reliability (ACK)ndash Energy efficiency (power control)ndash Fairnessndash Throughput needs to be maximized
bull We will discuss the following techniquesndash Aloha and slotted Alohandash CSMA Protocols (1-persistent non-persistent p-persistent CSMA CSMACD)ndash MACAndash MACAWndash Energy efficient MAC in wireless sensor networks (eg SMAC)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 38
Random Access ALOHA (1970)bull ALOHA was developed for a wireless system at the University of Hawaii
(Abramson et al)ndash Multiple remote stations plus one base stationndash Frame transmissions are made at one frequency from a remote station to the basestation the base station re-broadcasts frames on another frequency Contention is foraccess to the base station
bull Frames can be sent at any time to the base stationndash If no acknowledgment (ACK) is received assume that the frame is lost andretransmit after waiting a random amount of timendash If more than one station broadcasts on the base station access frequency at the sametime ndash a collision ndash interference will destroy the frames
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 39
Random Access ALOHA (contrsquo)
bull The probability that no packets generated during the tima [-τ τ] is given by (nocollision)
Pr(X(t)=0) = (λ2τ)0 0 e-2λτ = e-2Lwith corresponding throughput
T = L e-2L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 05 05e 018
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 40
Random Access slotted ALOHA (1971)bull Improvement Time is slotted and a packet can only be transmitted at the beginning of
one slot Thus it can reduce the collision duration
bull The probability that no packets generated during the time [0 τ] is given by (nocollision)
Pr(X(t)=0) = (λτ)0 0 e-λτ = e-Lwith corresponding throughput T = L e-L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 1 e 037
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 41
bull Slotted Aloha has double the maximumthroughput as pure Aloha and achievesthis maximum at a higher offered loadThe effective data rate is still less than40 of the raw transmission rate
bull Aloha does not listen to the carrier beforetransmission more sophisticatedtechniques are needed to increaseefficiency
bull Further problem the delayndashDelay
ndashThe mean packet delay
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
( ) ( )delayPr 1 e ekL Lkτ minus minus= = minus
( )delay1
1 e e ekL L L
kMPD E kτ
infinminus minus
=
= = minus =sum
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 42
Random Access Carrier Sense Multiple Accessbull CSMA (Carrier Sense Multiple Access) listen to the carrier before transmission
and transmits if channel is idlebull Detection delay and propagation delay are two important parameters
ndash Detection delay time required to sense the carrier and decide if it is idle or busyndash Propagation delay distancespeed_of_ligth The time required for bit to travel
from transmitter to the receiver
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 43
Random Access CSMA variations
Ad hoc Sensor Networks Multiple channel access
1-persistent CSMA Non-persistent CSMA
There will always be a collision iftwo nodes want to retransmit(usually you stop transmissionattempts after few tries)
Random backoff reduces probabilityof collisions Waste idle time if thebackoff time is too long
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 44
Random Access CSMA variations (contrsquo)
Ad hoc Sensor Networks Multiple channel access
p-persistent CSMA CSMA CD (Collision Detection)
A good tradeoff between non-persistent and 1-persistent CSMA
Same with CSMA however a stationalso listen to the carrier whiletransmitting to see if the transmissioncollides with someone elsetransmission It can be used in listen-while-talk capable channels (fullduplex) In single radio channels thetransmission need to be interrupted inorder to sense the channel
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 45
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 46
Random Access CSMA (contrsquo)bull How to select probability p
ndash Assume that M nodes have a packet to send and the medium is busy then Mp is theexpected number of nodes that will attempt to transmit once the medium becomesidle If Mp gt 1 then a collision is expected to occur Therefore the network mustmake sure that Mp lt 1 to avoid collision where M is the maximum number of nodesthat can be active at a time
bull Problems1 The mean packet delay increases exponently with increasing offered load2 CSMA protocols sense the carrier but sensing the carrier does not always releasestrue information about the status of the wireless channel There are two problems thatare unique to wireless channels (different than wireline channels)ndash Hidden terminal problemndash Exposed terminal problem
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 47
Hidden terminal problembull A sends to B C cannot receive Abull C wants to send to B C senses a
ldquofreerdquo medium (CS fails)bull collision at B A cannot receive the
collision (CD fails)bull A is ldquohiddenrdquo for C
Ad hoc Sensor Networks Multiple channel access
Random Access hidden and exposed terminal problemExposed terminal problem
bull B sends to A C wants to send to Dbull C has to wait CS signals a medium
in usebull since A is outside the radio range of
C waiting is not necessarybull C is ldquoexposedrdquo to B
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 48
Random Access MACAWhen a station wants to transmit databull It sends an RTS (Ready-to-Send) packet to the intended receiver
ndash The RTS packet contains the length of the data that needs to be transmittedndash Any station other than the intended recipient hearing RTS defers transmission for atime duration equal to the end of the corresponding CTS reception
bull The receiver sends back CTS (Clear-to-Send) packet back to sender if it isavailable to receivendash The CTS packet contains the length of the data that original sender wants totransmitndash Any station other than the original RTS sender hearing CTS defers transmissionuntil the data is sent
bull The original sender upon reception of the CTS starts transmitting
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 49
Hidden terminal problem
Ad hoc Sensor Networks Multiple channel access
Random Access MACA (contrsquo)Exposed terminal problem
Waiting time of node X is muchsmaller than waiting time of node C
bull C defers transmission upon hearing BrsquosRTS until B could get CTS from Abull After that C can start transmission to DFor that it first sends an RTSbull C is not longer exposed to the datatransmission of B
Problem It does not address the collision of RTS packet
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 50
Random Access reliabilitybull Wireless links are prone to have errors High packet loss rate detrimental to
transport-layer performance Mechanisms needed to reduce packet loss rateexperienced by upper layers
bull When node B receives a data packet from node A node B sends anAcknowledgement (Ack) This approach adopted in many protocols
bull If node A fails to receive an Ack it will retransmit the packet
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 51
Random Access MACAW (CSMACA) bull Physical carrier sense and virtual carrier sense using Network Allocation
Vector (NAV)ndash NAV is updated based on overheard RTSCTSDATAACK packets each of whichspecified duration of a pending transmissionndash Nodes stay silent when carrier sensed (physicalvirtual)
bull Backoff intervals used to reduce collision probabilityndash When transmitting a packet choose a backoff interval in the range [0w] where wis contention windowndash Count down the backoff interval when channel is idlendash Count down is suspended if channel becomes busyndash When backoff interval reaches 0 transmit RTS
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 52
Random Access CSMACA (contrsquo) bull The time spent counting down backoff intervals is a part of MAC overhead
ndash Choosing a large w leads to large backoff intervals and can result in larger overheadndash Choosing a small w leads to a larger number of collisions (when two nodes countdown to 0 simultaneously)ndash Since the number of nodes attempting to transmit simultaneously may change withtime some mechanism to manage contention is needed
bull IEEE 80211 DCF contention window w is chosen dynamically depending oncollision occurrence (binary exponential backoff)ndash When a node fails to receive CTS in response to its RTS it increases the contentionwindowndash w is doubled (up to an upper bound)ndash When a node successfully completes a data transfer it restores w to wmin
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 53
Random Access fairnessbull Simplest definition of fairness all nodes should receive equal bandwidthbull An example of unfairness
ndash Assume that initially A and B both choose a backoff interval in range [031] buttheir RTSs collidendash Nodes A and B then choose from range [063]ndash Node A chooses 4 slots and B choose 60 slotsndash After A transmits a packet it next chooses from range [031]ndash It is possible that A may transmit several packets before B transmits its first packet
Ad hoc Sensor Networks Multiple channel access
Unfairness occurs when one node has backed off much more than some other node
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 54
Random Access fairness (contrsquo) MACAW solution for fairnessbull When a node transmits a packet it appends value w to the packet all nodes
hearing value w use it for their future transmission attemptsbull Since w is an indication of the level of congestion in the vicinity of a specific
receiver node MACAW proposes maintaining w independently for eachreceiver
bull Using per-receiver wi is particularly useful in multi-hop environments sincecongestion level at different receivers can be very different
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 55
Random Access energy efficiency bull Since many mobile hosts are operated by batteries MAC
protocols which conserve energy are of interestbull Two approaches to reduce energy consumption
ndash Power save turn off wireless interface when desirablendash Power control reduce transmit power
bull Power control has some more potential benefitsndash Reduced interference (in ad hoc networks) eg it improves ALOHAefficiency where user with high power can capture a packet even if there is acollisionndash Increased spatial reuse (in infrastructured wireless networks) eg it is usedin CDMA to maintain target SIR of voice and data users or generally it canbe used to maintain target SIR for different user classes (admission control)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 56
Random Access power control bull When C transmits to D at a high power level B cannot receive Arsquos
transmission due to interference from Cbull If C reduces transmit power it can still communicate with D
ndash Reduces energy consumption at node Cndash Allows B to receive Arsquos transmission (spatial reuse)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 57
bull A special wireless ad hoc networkndash Large number of nodesndash Battery poweredndash Topology and density changendash Nodes for a common taskndash In-network data processing
bull Sensor-net applicationsndash Sensor-triggered bursty trafficndash Can often tolerate some delay
Ad hoc Sensor Networks Multiple channel access
Characteristics of Sensor Network
Scalabilty and self configurationEnergy efficiencyAdaptivityFairness not importantLatency
AdaptivityEnergy-delay tradoff
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 58
Ad hoc Sensor Networks Multiple channel access
Wireless sensor node and sensor network
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 59
MAC for WSN attributes bull Attributes
ndash Collision avoidance (Basic task)ndash Energy efficiency (most primary in WSN)ndash Scalability and adaptivityndash Channel utilizationndash Latencyndash Throughputndash Fairness
bull What causes energy wastendash Collisionsndash Control packet overheadndash Overhearing unnecessary trafficndash Long idle time (it consumes 50-100 of the power)
Ad hoc Sensor Networks Multiple channel access
Primary
Secondary
Dominant factor
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 60
MAC for WSN
bull Contention-based protocols need to work hard in all directions for energysavingsndash Reduce idle listening ndash support low duty cyclendash Better collision avoidancendash Reduce control overheadndash Avoid unnecessary overhearing
Ad hoc Sensor Networks Multiple channel access
Scheduled Protocols Contention ProtocolsCollisions No YesEnergy efficiency Good BadScalability and adaptivity Bad GoodMulti-hop communication Difficult EasyTime synchronization Strict Loose or not required
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 61
MAC for WSN classification
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 62
MAC for WSN SMAC (2002)bull It has been developed by Ye Heidemann and Estrinbull Tradeoffs between (latency fairness) and energybull Major components in S-MAC
ndash Periodic listen and sleep turn off radio when sleeping the reduced dutycycle is ~ 10 (eg 120ms on 12s off)ndash Collision avoidance is based on contention similarly to IEEE 80211 adhoc modendash Overhearing avoidance the node is sleeping when neighbors talkndash Massage passing long message is fragmented amp sent in burst operationwith extended Tx time and re-transmit occurs immediately
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 63
Summarybull The algorithms and protocols that enable sharing by multiple users and
controls the access to the wireless channel from different users are calledMAC
bull There are two types of wireless networks Infrastructured wireless networksand ad hoc wireless networks
bull We also want our wireless system to have high user capacity and we canidentify 10 methods for increasing it
bull There are several MACs with no absolute advantage it should be tailoredto the system requirements
bull There are primary concerns for designing a WSN MAC (collisionavoidance energy efficiency scalability and adaptivity) while thesecondary concerns are as follows (channel utilization latency throughputfairness)
bull Next lecture Routing protocols
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 12
Ad hoc Sensor Networks Multiple channel access
Duplexing
bull If the communication between two parties is one way then it is called simplexcommunication Simplex communication is achieved by default by using a singlewireless channel (frequency band) to transmit from sender to receiver
bull If the communication between two parties is two- way then it is called duplexcommunication Duplex communication achieved by
ndash Time Division (TDD) (famous in cellular systems)ndash Frequency Division (FDD) (famous in cellular systems)ndash Some other method like a random access method
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 13
Ad hoc Sensor Networks Multiple channel access
FDDbull A duplex channel consists of two
simplex channels with different carrierfrequencies
ndash Downlink band carries traffic frombase to mobile
ndash Uplink band carries traffic frommobile to base
Duplexing(contrsquo)TDD
bull channel (carrier frequency) is shared intime in a deterministic manner
ndash The time is slotted with fixed slotlength (sec)
ndash Some slots are used for downlinkchannel
ndash Some slots are used for uplink channel
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 14
Ad hoc Sensor Networks Multiple channel access
FDDbull FDD is used in radio systems that can
allocate individual radio frequenciesfor each user
bull For example analog systems AMPSbull In FDD channels are allocated by a
base stationbull A channel for a mobile is allocated
dynamicallybull All channels that a base station will use
are allocated usually staticallybull More suitable for wide-area cellular
networks GSM AMPS all use FDD
Duplexing(contrsquo)TDD
bull Can only be used in digital wirelesssystems (digital modulation)
bull Requires rigid timing andsynchronization
bull Mostly used in short-range and fixedwireless systems so that propagationdelay between base and mobile do notchange much with respect to locationof the mobile
bull Such as cordless phoneshellip
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 15
Ad hoc Sensor Networks Multiple channel access
Multiple Access narrow- vs wideband systemsbull Narrowband System
ndash The bandwidth is small compared to the coherence bandwidth of the channel(BltBcoh)
bull Wideband Systemndash The system bandwidth is much larger that the coherence bandwidth of the
multipath channel (BgtBcoh) A large number of users can access the same channel(frequency band) at the same time
bull Four major multiple access (MA) schemesndash Time Division Multiple Access (TDMA) it could be used in narrowband or
wideband systemndash Frequency Division Multiple Access (FDMA) it is usually used in narrowband
systemndash Code Division Multiple Access (CDMA) it is used in wideband systemndash Space Division Multiple Access (SDMA) for wireless systems with multiple
antennas it can be combined with all of the other MA methods
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 16
Ad hoc Sensor Networks Multiple channel access
Multiple Access cellular standards
Cellular System Multiple Access TechniqueAMPS (2G) FDMAFDD
CT2 Cordless Phone FDMATDD
DECT Cordless Phone FDMATDD
GSM (2G) TDMAFDD
USDC (IS-54 and IS-136) (2G) TDMAFDD
Personal Digital Cellular (2G) TDMAFDD
US IS-95 (2G) CDMAFDD
W-CDMA (3G) CDMAFDD CDMATDD
cdma2000 (3G) CDMAFDD CDMATDD
Narrowband System
Wideband System
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 17
Ad hoc Sensor Networks Multiple channel access
bull Individual radio channels are assigned toindividual users Each user is allocated a Wfrequency band by BS
bull If the channel allocated to a user is idle then itis not used by someone else waste of resource
bull Mobile and base can transmit and receivesimultaneously (FDD)
bull The W bandwidth of FDMA channels arerelatively low Symbol time is usually larger(low data rate) than the delay spread of themultipath channel (implies that inter-symbolinterference is low)
bull Lower complexity systems than TDMAsystems
Multiple Access FDMA
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 18
Ad hoc Sensor Networks Multiple channel access
Multiple Access FDMA (contrsquo)bull Capacity of FDMA systems
Nc = (B-2Bguard)Wwhere B is the total spectrum allocation Bguard is the guard band allocated atthe edge of the spectrum band W is the bandwidth of an individual channel
bull Erlang B and Erlang C system assumesndashinfinite population of sources which jointly offer traffic to Nc channelndashcall attempts arrive following a Poisson process so call arrivals are independentThe average arrival rate is λndashmessage length (holding times) are exponentially distributed (Markovian system)The average call length is hndashthe total amount of traffic offered in erlangs L= λhndashIf a user is rejected his next call attempt is made statistically independent of theprevious attempt (Erlang B)ndashIf all the channels are busy when a request arrives from a user the request isqueued An unlimited number of requests may be held in the queue (Erlang C)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 19
Ad hoc Sensor Networks Multiple channel access
bull Statictical measures of offered load Ncndash In Erlang B system the probability of call
blocking
ndash In Erlang C system the probabilty of waiting
ndash And the average wait time
c
c
cBLOCK
0
Pr
N
Nk
k
LN
Lk
=
=
sum
c
cc
wait 1
c0c
Pr 1
N
NkN
k
LL LL N kN
minus
=
=⎛ ⎞
+ minus⎜ ⎟⎝ ⎠
sum
wait waitc
Pr htN L
=minus
Multiple Access FDMA (contrsquo)Recall from
Queuing theory
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 20
Ad hoc Sensor Networks Multiple channel access
Multiple Access example of Erlang B systembull In an Erlang B system 40 channels are available A blocking probabilty of
less than 1 is required What is the traffic that can be serve if there is oneoperator or three operators
bull (Solution L(1 operator)gtL(3 operators))bull (Note The number of users increases faster than linearly with the number of
available channels The difference between actual increase and linearincrease is called the trunking gain It is preferable to have a large pool ofavailable channels that serves all users (a single operator) The reasons fornot choosing this approach are political not technical)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 21
Ad hoc Sensor Networks Multiple channel access
Multiple Access TDMAbull The allocated radio spectrum for the system is divided into time slots
ndash In each slot a user can transmit or receivendash A user occupies a cyclically repeating slotsndash A channel is logically defined as a particular time slot that repeats with some
periodbull TDMA systems buffer the data until its turn (time slot) comes to transmitbull In TDMATDD half of the slots in the frame is used for uplink channels the
other is used for downlink channelsbull In TDMAFDD a different carrier frequency is used for uplink or downlink
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 22
Ad hoc Sensor Networks Multiple channel access
Multiple Access TDMA (contrsquo)bull Preamble contains address and synchronization info to identify base
station and mobiles to each otherbull Guard times are used to allow synchronization of the receivers between
different slots and framesndash Different mobiles may have different propagation delays to a base station
because of different distances
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 23
Multiple Access TDMA (contrsquo)bull Each frame contains overhead bits and data bits Efficiency of frame is
defined as the percentage of data (information) bits to the total frame size inbits
η =(1 ndash boverheadbtotal) 100 where btotal= Tf R is the total number of bits in a frame Tf is the frameduration in sec boverhead is the number of overhead bits (GSM efficiency 73)
bull TDMA is usually combined with FDMAndash Neighboring cells are allocated and using different carrier frequencies (FDMA)
Inside a cell TDMA can be used Cells may be re-using the same frequency if theyare far from each-other
ndash There may be more than one channel allocated and used inside each cell Eachcarrier frequency (radio channel) may be using TDMA to further multiplex moreuser (ie having TDMA logical channels inside radio channels) For example inGSM each radio channel has 200KHz bandwidth and has 8 time slots (8 logicalchannels) Hence GSM is using FDMA combined with TDMA
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 24
Multiple Access TDMA (contrsquo)
Ad hoc Sensor Networks Multiple channel access
GSM (Europa) IS-54 (USA) PDC (Japan) DECT
Bit Rate 2708 Kbps 486 Kbps 42 Kbps 1152 Mbps
Bandwidth 200 KHz 30 KHz 25 KHz 1728 MHz
Time Slot 0577 ms 67 ms 67 ms 0417 ms
Upstream slots per frame 8 3 3 12
Duplexing FDD FDD FDD TDD
Efficiency 73 80 80 67
Modulation GMSK π4 DQPSK π4 DQPSK GMSK
Adaptive equalized Mandatory Mandatory Optional None
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 25
Multiple Access TDMA (contrsquo)bull Enables the sharing of a single radio channel among N usersbull Requires high data-rate per radio channel to support N users
simultaneously which requires adaptive equalizers to be usedin multipath environments
bull Transmission occurs in bursts (not continually) It enablespower saving by going to sleep modes in unrelated slots Itdiscontinues transmission and also enables mobile assistedhandoff
bull It equires synchronization of the receivers Need guard bits andsync bits which occurs large overhead per slot
bull Allocation of slots to mobile users should not be uniform Itmay depend on the traffic requirement of mobiles This bringsextra flexibility and efficiency compared to FDMA systems
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2620111015 TAacuteMOP ndash 412-082AKMR-2009-0006 26
Ad hoc Sensor Networks Multiple channel access
Multiple Access spread spectrum multiple accessbull SSMA uses signals that have transmission bandwidth that is
several orders of magnitude larger than minimum requiredRF bandwidth
bull It providesndash Immunity to multipath interferencendash Robust multiple access
bull Two techniquesndash Frequency Hopped Multiple Access (FHMA)ndash Direct Sequence Multiple Access (DSMA) or Code Division
Multiple Access (CDMA)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2720111015 TAacuteMOP ndash 412-082AKMR-2009-0006 27
Ad hoc Sensor Networks Multiple channel access
Multiple Access FHMAbull The carrier frequency of users are varied in a pseudo-random
fashionndash Each user is using a narrowband channel (spectrum) at a specific
instance of timendash The random change in frequency make the change of using the same
narrowband channel very lowbull The sender receiver change frequency (calling hopping)
using the same pseudo-random sequence hence they aresynchronized
bull Rate of hopping versus Symbol ratendash If hopping rate is greather Fast Frequency Hoppingndash If symbol rate is greater Slow Frequency Hopping
bull GSM and Bluetooth are example systems
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2820111015 TAacuteMOP ndash 412-082AKMR-2009-0006 28
Ad hoc Sensor Networks Multiple channel access
Multiple Access FHMA (contrsquo)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2920111015 TAacuteMOP ndash 412-082AKMR-2009-0006 29
Ad hoc Sensor Networks Multiple channel access
Multiple Access CDMAbull The narrowband message signal is multiplied by a very large bandwidth
signal called spreading signal (code) before modulation and transmissionover the air This is called spreading
bull Spreading signal use a pseudo-noise (PN) sequence (a pseudo-randomsequence) called codeword which are orthogonal (low autocorrelation)
bull The receiver correlator distinguishes the senders signal by examining thewideband signal with the same time-synchronized spreading code(despreading)
bull Advantagesndash Low power spectral densityndash Interference limited operationndash Privacyndash Reduction of multipath affects by using a larger spectrumndash Users can start their transmission at any time (random access possible)ndash Cell capacity is soft and higher than TDMA and FDMAndash No frequency management and no equalizers and quard time needed
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3020111015 TAacuteMOP ndash 412-082AKMR-2009-0006 30
Ad hoc Sensor Networks Multiple channel access
Multiple Access near far problem and power controlbull At a receiver the signals may come from various sources (multiuser) The
strongest signal usually captures the modulator The other signals areconsidered as noise Each source may have different distances to the basestation
bull In CDMA we want a BS to receive CDMA coded signals from variousmobile users at the same time
ndash Therefore the receiver power at the BS for all mobile users should be close toeach other This requires power control at the mobiles
bull Power Control BS monitors the RSSI (Received Signal Strength Indicator)values from different mobiles and then sends power change commands to themobiles over a forward channel The mobiles then adjust their transmitpower
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3120111015 TAacuteMOP ndash 412-082AKMR-2009-0006 31
Ad hoc Sensor Networks Multiple channel access
Multiple Access hybrid spread spectrum techniquesbull FDMACDMA
ndash Available wideband spectrum is divided into a number narrowband radio channels CDMAis employed inside each channel
bull DSFHMAndash The signals are spread using spreading codes (direct sequence signals are obtained) but
these signal are not transmitted over a constant carrier frequency they are transmitted over afrequency hopping carrier frequency
bull Time Division CDMA (TCDMA)ndash Each cell is using a different spreading code (CDMA employed between cells) that is
conveyed to the mobiles in its rangendash Inside each cell (inside a CDMA channel) TDMA is employed to multiplex multiple users
bull Time Division Frequency Hoppingndash At each time slot the user is hopped to a new frequency according to a pseudo-random
hopping sequencendash Employed in severe co-interference and multi-path environmentsndash Bluetooth and GSM are using this technique
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3220111015 TAacuteMOP ndash 412-082AKMR-2009-0006 32
Ad hoc Sensor Networks Multiple channel access
Multiple Access comparison TDMAFDMACDMA
Techniques TDMA FDMA CDMA SDMA
Idea
segment sending time into disjoint time-slots
Segment the frequency band into disjoint sub-bands
Spread the spectrum using orthogonal codes
Segment space into sectors
Signal separation
Synchronization in the time domain
Filtering in the frequency domain
Code Directed antennas
Advantage
Established flexible
Simple established robust
Flexible less frequency planning needed soft handover
Simple increases capacity
DisadvantageQuard space synchronization difficult
Inflexible scarce source
Complex receiver power control
inflexible
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3320111015 TAacuteMOP ndash 412-082AKMR-2009-0006 33
Ad hoc Sensor Networks Multiple channel access
User capacity per cell
The user capacity C is defined as the total number of active users percell that the system can support while meeting a common BERconstrain For orthogonal multiple access (as FDMA and TDMA)
C = Ncellwhere Ncell is the number of channels assigned to any given cellThe total number of orthogonal channels of bandwidth Bs that can becreated from a total system bandwidth of B is B Bs The reuse factorsatisfies N = (B Bs) Ncell this implies
C = (B Bs) N
Recall from Chapter 1
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3420111015 TAacuteMOP ndash 412-082AKMR-2009-0006 34
Ad hoc Sensor Networks Multiple channel access
10 methods for increasing capacity1 Increasing the amount of spectrum used very expensive2 More efficient modulation format and coding3 Better source coding4 Adaptive modulation and coding5 Discontinuous transmission6 Multiuser detection CDMA systems7 Reduction of cell radius effective but very expensive smaller
cells require more handovers8 Use of sector cells tripled cells have tripled BS antennas9 Use of an overlay structure10 Multiple antennas diversity MIMO systems SDMA
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3520111015 TAacuteMOP ndash 412-082AKMR-2009-0006 35
Ad hoc Sensor Networks Multiple channel access
bull Distributed operation is requiredbull Synchronization is required in TDMA-based systemsbull Hidden terminals are nodes hidden from a senderbull Exposed terminals are exposed nodes preventing a sender
from sendingbull Throughput needs to be maximizedbull Access delay should be minimizedbull Fairness refers to provide an equal share to all competing
nodesbull Real-time traffic support is required for voice video and
real-time databull Resource reservation is required for QoSbull Ability to measure resource availability handles the
resourcesbull Capability for power control reduces the energy
consumption
Random access in adhoc networks
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3620111015 TAacuteMOP ndash 412-082AKMR-2009-0006 36
Ad hoc Sensor Networks Multiple channel access
Random accessbull All random access techniques are based on packetized data or packet radio where user
date is collected into packets of M bitsbull Collision if packets from different users overlap in timebull The transmission time of a packet is τ = M R where R [bps] the data ratebull Analysis of random access techniques assumes that the users generate packets according
to memoryless Poisson process at a rate λ [packets per unit time] the probability that thenumber of packet arrivals in a time period [0t] denoted by X(t) is equal to integer k isgiven by
Pr(X(t)=k) = (λt)k k e-λt
bull The traffic load is defined as L = λ τ If L gt 1 means on avarage more packets arrive inthe system over a given time period than can be transmitted in that period so systemswith Lgt1 are unstable
bull The throughput is defined as the ratio of the average number of packets successfullytransmitted in any given time interval divided by the number of attempted transmissionsin that interval
T = L Pr(succesful packet transmission)bull The effective data rate of the system is RT
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3720111015 TAacuteMOP ndash 412-082AKMR-2009-0006 37
Ad hoc Sensor Networks Multiple channel access
Random access (contrsquo)bull We will examine the following attributes
ndash Channel utilization (Throughput)ndash Latencyndash Collision avoidance (Hidden and exposed terminals problem)ndash Reliability (ACK)ndash Energy efficiency (power control)ndash Fairnessndash Throughput needs to be maximized
bull We will discuss the following techniquesndash Aloha and slotted Alohandash CSMA Protocols (1-persistent non-persistent p-persistent CSMA CSMACD)ndash MACAndash MACAWndash Energy efficient MAC in wireless sensor networks (eg SMAC)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 38
Random Access ALOHA (1970)bull ALOHA was developed for a wireless system at the University of Hawaii
(Abramson et al)ndash Multiple remote stations plus one base stationndash Frame transmissions are made at one frequency from a remote station to the basestation the base station re-broadcasts frames on another frequency Contention is foraccess to the base station
bull Frames can be sent at any time to the base stationndash If no acknowledgment (ACK) is received assume that the frame is lost andretransmit after waiting a random amount of timendash If more than one station broadcasts on the base station access frequency at the sametime ndash a collision ndash interference will destroy the frames
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 39
Random Access ALOHA (contrsquo)
bull The probability that no packets generated during the tima [-τ τ] is given by (nocollision)
Pr(X(t)=0) = (λ2τ)0 0 e-2λτ = e-2Lwith corresponding throughput
T = L e-2L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 05 05e 018
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 40
Random Access slotted ALOHA (1971)bull Improvement Time is slotted and a packet can only be transmitted at the beginning of
one slot Thus it can reduce the collision duration
bull The probability that no packets generated during the time [0 τ] is given by (nocollision)
Pr(X(t)=0) = (λτ)0 0 e-λτ = e-Lwith corresponding throughput T = L e-L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 1 e 037
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 41
bull Slotted Aloha has double the maximumthroughput as pure Aloha and achievesthis maximum at a higher offered loadThe effective data rate is still less than40 of the raw transmission rate
bull Aloha does not listen to the carrier beforetransmission more sophisticatedtechniques are needed to increaseefficiency
bull Further problem the delayndashDelay
ndashThe mean packet delay
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
( ) ( )delayPr 1 e ekL Lkτ minus minus= = minus
( )delay1
1 e e ekL L L
kMPD E kτ
infinminus minus
=
= = minus =sum
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 42
Random Access Carrier Sense Multiple Accessbull CSMA (Carrier Sense Multiple Access) listen to the carrier before transmission
and transmits if channel is idlebull Detection delay and propagation delay are two important parameters
ndash Detection delay time required to sense the carrier and decide if it is idle or busyndash Propagation delay distancespeed_of_ligth The time required for bit to travel
from transmitter to the receiver
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 43
Random Access CSMA variations
Ad hoc Sensor Networks Multiple channel access
1-persistent CSMA Non-persistent CSMA
There will always be a collision iftwo nodes want to retransmit(usually you stop transmissionattempts after few tries)
Random backoff reduces probabilityof collisions Waste idle time if thebackoff time is too long
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 44
Random Access CSMA variations (contrsquo)
Ad hoc Sensor Networks Multiple channel access
p-persistent CSMA CSMA CD (Collision Detection)
A good tradeoff between non-persistent and 1-persistent CSMA
Same with CSMA however a stationalso listen to the carrier whiletransmitting to see if the transmissioncollides with someone elsetransmission It can be used in listen-while-talk capable channels (fullduplex) In single radio channels thetransmission need to be interrupted inorder to sense the channel
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 45
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 46
Random Access CSMA (contrsquo)bull How to select probability p
ndash Assume that M nodes have a packet to send and the medium is busy then Mp is theexpected number of nodes that will attempt to transmit once the medium becomesidle If Mp gt 1 then a collision is expected to occur Therefore the network mustmake sure that Mp lt 1 to avoid collision where M is the maximum number of nodesthat can be active at a time
bull Problems1 The mean packet delay increases exponently with increasing offered load2 CSMA protocols sense the carrier but sensing the carrier does not always releasestrue information about the status of the wireless channel There are two problems thatare unique to wireless channels (different than wireline channels)ndash Hidden terminal problemndash Exposed terminal problem
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 47
Hidden terminal problembull A sends to B C cannot receive Abull C wants to send to B C senses a
ldquofreerdquo medium (CS fails)bull collision at B A cannot receive the
collision (CD fails)bull A is ldquohiddenrdquo for C
Ad hoc Sensor Networks Multiple channel access
Random Access hidden and exposed terminal problemExposed terminal problem
bull B sends to A C wants to send to Dbull C has to wait CS signals a medium
in usebull since A is outside the radio range of
C waiting is not necessarybull C is ldquoexposedrdquo to B
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 48
Random Access MACAWhen a station wants to transmit databull It sends an RTS (Ready-to-Send) packet to the intended receiver
ndash The RTS packet contains the length of the data that needs to be transmittedndash Any station other than the intended recipient hearing RTS defers transmission for atime duration equal to the end of the corresponding CTS reception
bull The receiver sends back CTS (Clear-to-Send) packet back to sender if it isavailable to receivendash The CTS packet contains the length of the data that original sender wants totransmitndash Any station other than the original RTS sender hearing CTS defers transmissionuntil the data is sent
bull The original sender upon reception of the CTS starts transmitting
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 49
Hidden terminal problem
Ad hoc Sensor Networks Multiple channel access
Random Access MACA (contrsquo)Exposed terminal problem
Waiting time of node X is muchsmaller than waiting time of node C
bull C defers transmission upon hearing BrsquosRTS until B could get CTS from Abull After that C can start transmission to DFor that it first sends an RTSbull C is not longer exposed to the datatransmission of B
Problem It does not address the collision of RTS packet
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 50
Random Access reliabilitybull Wireless links are prone to have errors High packet loss rate detrimental to
transport-layer performance Mechanisms needed to reduce packet loss rateexperienced by upper layers
bull When node B receives a data packet from node A node B sends anAcknowledgement (Ack) This approach adopted in many protocols
bull If node A fails to receive an Ack it will retransmit the packet
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 51
Random Access MACAW (CSMACA) bull Physical carrier sense and virtual carrier sense using Network Allocation
Vector (NAV)ndash NAV is updated based on overheard RTSCTSDATAACK packets each of whichspecified duration of a pending transmissionndash Nodes stay silent when carrier sensed (physicalvirtual)
bull Backoff intervals used to reduce collision probabilityndash When transmitting a packet choose a backoff interval in the range [0w] where wis contention windowndash Count down the backoff interval when channel is idlendash Count down is suspended if channel becomes busyndash When backoff interval reaches 0 transmit RTS
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 52
Random Access CSMACA (contrsquo) bull The time spent counting down backoff intervals is a part of MAC overhead
ndash Choosing a large w leads to large backoff intervals and can result in larger overheadndash Choosing a small w leads to a larger number of collisions (when two nodes countdown to 0 simultaneously)ndash Since the number of nodes attempting to transmit simultaneously may change withtime some mechanism to manage contention is needed
bull IEEE 80211 DCF contention window w is chosen dynamically depending oncollision occurrence (binary exponential backoff)ndash When a node fails to receive CTS in response to its RTS it increases the contentionwindowndash w is doubled (up to an upper bound)ndash When a node successfully completes a data transfer it restores w to wmin
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 53
Random Access fairnessbull Simplest definition of fairness all nodes should receive equal bandwidthbull An example of unfairness
ndash Assume that initially A and B both choose a backoff interval in range [031] buttheir RTSs collidendash Nodes A and B then choose from range [063]ndash Node A chooses 4 slots and B choose 60 slotsndash After A transmits a packet it next chooses from range [031]ndash It is possible that A may transmit several packets before B transmits its first packet
Ad hoc Sensor Networks Multiple channel access
Unfairness occurs when one node has backed off much more than some other node
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 54
Random Access fairness (contrsquo) MACAW solution for fairnessbull When a node transmits a packet it appends value w to the packet all nodes
hearing value w use it for their future transmission attemptsbull Since w is an indication of the level of congestion in the vicinity of a specific
receiver node MACAW proposes maintaining w independently for eachreceiver
bull Using per-receiver wi is particularly useful in multi-hop environments sincecongestion level at different receivers can be very different
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 55
Random Access energy efficiency bull Since many mobile hosts are operated by batteries MAC
protocols which conserve energy are of interestbull Two approaches to reduce energy consumption
ndash Power save turn off wireless interface when desirablendash Power control reduce transmit power
bull Power control has some more potential benefitsndash Reduced interference (in ad hoc networks) eg it improves ALOHAefficiency where user with high power can capture a packet even if there is acollisionndash Increased spatial reuse (in infrastructured wireless networks) eg it is usedin CDMA to maintain target SIR of voice and data users or generally it canbe used to maintain target SIR for different user classes (admission control)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 56
Random Access power control bull When C transmits to D at a high power level B cannot receive Arsquos
transmission due to interference from Cbull If C reduces transmit power it can still communicate with D
ndash Reduces energy consumption at node Cndash Allows B to receive Arsquos transmission (spatial reuse)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 57
bull A special wireless ad hoc networkndash Large number of nodesndash Battery poweredndash Topology and density changendash Nodes for a common taskndash In-network data processing
bull Sensor-net applicationsndash Sensor-triggered bursty trafficndash Can often tolerate some delay
Ad hoc Sensor Networks Multiple channel access
Characteristics of Sensor Network
Scalabilty and self configurationEnergy efficiencyAdaptivityFairness not importantLatency
AdaptivityEnergy-delay tradoff
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 58
Ad hoc Sensor Networks Multiple channel access
Wireless sensor node and sensor network
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 59
MAC for WSN attributes bull Attributes
ndash Collision avoidance (Basic task)ndash Energy efficiency (most primary in WSN)ndash Scalability and adaptivityndash Channel utilizationndash Latencyndash Throughputndash Fairness
bull What causes energy wastendash Collisionsndash Control packet overheadndash Overhearing unnecessary trafficndash Long idle time (it consumes 50-100 of the power)
Ad hoc Sensor Networks Multiple channel access
Primary
Secondary
Dominant factor
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 60
MAC for WSN
bull Contention-based protocols need to work hard in all directions for energysavingsndash Reduce idle listening ndash support low duty cyclendash Better collision avoidancendash Reduce control overheadndash Avoid unnecessary overhearing
Ad hoc Sensor Networks Multiple channel access
Scheduled Protocols Contention ProtocolsCollisions No YesEnergy efficiency Good BadScalability and adaptivity Bad GoodMulti-hop communication Difficult EasyTime synchronization Strict Loose or not required
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 61
MAC for WSN classification
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 62
MAC for WSN SMAC (2002)bull It has been developed by Ye Heidemann and Estrinbull Tradeoffs between (latency fairness) and energybull Major components in S-MAC
ndash Periodic listen and sleep turn off radio when sleeping the reduced dutycycle is ~ 10 (eg 120ms on 12s off)ndash Collision avoidance is based on contention similarly to IEEE 80211 adhoc modendash Overhearing avoidance the node is sleeping when neighbors talkndash Massage passing long message is fragmented amp sent in burst operationwith extended Tx time and re-transmit occurs immediately
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 63
Summarybull The algorithms and protocols that enable sharing by multiple users and
controls the access to the wireless channel from different users are calledMAC
bull There are two types of wireless networks Infrastructured wireless networksand ad hoc wireless networks
bull We also want our wireless system to have high user capacity and we canidentify 10 methods for increasing it
bull There are several MACs with no absolute advantage it should be tailoredto the system requirements
bull There are primary concerns for designing a WSN MAC (collisionavoidance energy efficiency scalability and adaptivity) while thesecondary concerns are as follows (channel utilization latency throughputfairness)
bull Next lecture Routing protocols
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 13
Ad hoc Sensor Networks Multiple channel access
FDDbull A duplex channel consists of two
simplex channels with different carrierfrequencies
ndash Downlink band carries traffic frombase to mobile
ndash Uplink band carries traffic frommobile to base
Duplexing(contrsquo)TDD
bull channel (carrier frequency) is shared intime in a deterministic manner
ndash The time is slotted with fixed slotlength (sec)
ndash Some slots are used for downlinkchannel
ndash Some slots are used for uplink channel
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 14
Ad hoc Sensor Networks Multiple channel access
FDDbull FDD is used in radio systems that can
allocate individual radio frequenciesfor each user
bull For example analog systems AMPSbull In FDD channels are allocated by a
base stationbull A channel for a mobile is allocated
dynamicallybull All channels that a base station will use
are allocated usually staticallybull More suitable for wide-area cellular
networks GSM AMPS all use FDD
Duplexing(contrsquo)TDD
bull Can only be used in digital wirelesssystems (digital modulation)
bull Requires rigid timing andsynchronization
bull Mostly used in short-range and fixedwireless systems so that propagationdelay between base and mobile do notchange much with respect to locationof the mobile
bull Such as cordless phoneshellip
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 15
Ad hoc Sensor Networks Multiple channel access
Multiple Access narrow- vs wideband systemsbull Narrowband System
ndash The bandwidth is small compared to the coherence bandwidth of the channel(BltBcoh)
bull Wideband Systemndash The system bandwidth is much larger that the coherence bandwidth of the
multipath channel (BgtBcoh) A large number of users can access the same channel(frequency band) at the same time
bull Four major multiple access (MA) schemesndash Time Division Multiple Access (TDMA) it could be used in narrowband or
wideband systemndash Frequency Division Multiple Access (FDMA) it is usually used in narrowband
systemndash Code Division Multiple Access (CDMA) it is used in wideband systemndash Space Division Multiple Access (SDMA) for wireless systems with multiple
antennas it can be combined with all of the other MA methods
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 16
Ad hoc Sensor Networks Multiple channel access
Multiple Access cellular standards
Cellular System Multiple Access TechniqueAMPS (2G) FDMAFDD
CT2 Cordless Phone FDMATDD
DECT Cordless Phone FDMATDD
GSM (2G) TDMAFDD
USDC (IS-54 and IS-136) (2G) TDMAFDD
Personal Digital Cellular (2G) TDMAFDD
US IS-95 (2G) CDMAFDD
W-CDMA (3G) CDMAFDD CDMATDD
cdma2000 (3G) CDMAFDD CDMATDD
Narrowband System
Wideband System
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 17
Ad hoc Sensor Networks Multiple channel access
bull Individual radio channels are assigned toindividual users Each user is allocated a Wfrequency band by BS
bull If the channel allocated to a user is idle then itis not used by someone else waste of resource
bull Mobile and base can transmit and receivesimultaneously (FDD)
bull The W bandwidth of FDMA channels arerelatively low Symbol time is usually larger(low data rate) than the delay spread of themultipath channel (implies that inter-symbolinterference is low)
bull Lower complexity systems than TDMAsystems
Multiple Access FDMA
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 18
Ad hoc Sensor Networks Multiple channel access
Multiple Access FDMA (contrsquo)bull Capacity of FDMA systems
Nc = (B-2Bguard)Wwhere B is the total spectrum allocation Bguard is the guard band allocated atthe edge of the spectrum band W is the bandwidth of an individual channel
bull Erlang B and Erlang C system assumesndashinfinite population of sources which jointly offer traffic to Nc channelndashcall attempts arrive following a Poisson process so call arrivals are independentThe average arrival rate is λndashmessage length (holding times) are exponentially distributed (Markovian system)The average call length is hndashthe total amount of traffic offered in erlangs L= λhndashIf a user is rejected his next call attempt is made statistically independent of theprevious attempt (Erlang B)ndashIf all the channels are busy when a request arrives from a user the request isqueued An unlimited number of requests may be held in the queue (Erlang C)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 19
Ad hoc Sensor Networks Multiple channel access
bull Statictical measures of offered load Ncndash In Erlang B system the probability of call
blocking
ndash In Erlang C system the probabilty of waiting
ndash And the average wait time
c
c
cBLOCK
0
Pr
N
Nk
k
LN
Lk
=
=
sum
c
cc
wait 1
c0c
Pr 1
N
NkN
k
LL LL N kN
minus
=
=⎛ ⎞
+ minus⎜ ⎟⎝ ⎠
sum
wait waitc
Pr htN L
=minus
Multiple Access FDMA (contrsquo)Recall from
Queuing theory
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 20
Ad hoc Sensor Networks Multiple channel access
Multiple Access example of Erlang B systembull In an Erlang B system 40 channels are available A blocking probabilty of
less than 1 is required What is the traffic that can be serve if there is oneoperator or three operators
bull (Solution L(1 operator)gtL(3 operators))bull (Note The number of users increases faster than linearly with the number of
available channels The difference between actual increase and linearincrease is called the trunking gain It is preferable to have a large pool ofavailable channels that serves all users (a single operator) The reasons fornot choosing this approach are political not technical)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 21
Ad hoc Sensor Networks Multiple channel access
Multiple Access TDMAbull The allocated radio spectrum for the system is divided into time slots
ndash In each slot a user can transmit or receivendash A user occupies a cyclically repeating slotsndash A channel is logically defined as a particular time slot that repeats with some
periodbull TDMA systems buffer the data until its turn (time slot) comes to transmitbull In TDMATDD half of the slots in the frame is used for uplink channels the
other is used for downlink channelsbull In TDMAFDD a different carrier frequency is used for uplink or downlink
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 22
Ad hoc Sensor Networks Multiple channel access
Multiple Access TDMA (contrsquo)bull Preamble contains address and synchronization info to identify base
station and mobiles to each otherbull Guard times are used to allow synchronization of the receivers between
different slots and framesndash Different mobiles may have different propagation delays to a base station
because of different distances
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 23
Multiple Access TDMA (contrsquo)bull Each frame contains overhead bits and data bits Efficiency of frame is
defined as the percentage of data (information) bits to the total frame size inbits
η =(1 ndash boverheadbtotal) 100 where btotal= Tf R is the total number of bits in a frame Tf is the frameduration in sec boverhead is the number of overhead bits (GSM efficiency 73)
bull TDMA is usually combined with FDMAndash Neighboring cells are allocated and using different carrier frequencies (FDMA)
Inside a cell TDMA can be used Cells may be re-using the same frequency if theyare far from each-other
ndash There may be more than one channel allocated and used inside each cell Eachcarrier frequency (radio channel) may be using TDMA to further multiplex moreuser (ie having TDMA logical channels inside radio channels) For example inGSM each radio channel has 200KHz bandwidth and has 8 time slots (8 logicalchannels) Hence GSM is using FDMA combined with TDMA
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 24
Multiple Access TDMA (contrsquo)
Ad hoc Sensor Networks Multiple channel access
GSM (Europa) IS-54 (USA) PDC (Japan) DECT
Bit Rate 2708 Kbps 486 Kbps 42 Kbps 1152 Mbps
Bandwidth 200 KHz 30 KHz 25 KHz 1728 MHz
Time Slot 0577 ms 67 ms 67 ms 0417 ms
Upstream slots per frame 8 3 3 12
Duplexing FDD FDD FDD TDD
Efficiency 73 80 80 67
Modulation GMSK π4 DQPSK π4 DQPSK GMSK
Adaptive equalized Mandatory Mandatory Optional None
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 25
Multiple Access TDMA (contrsquo)bull Enables the sharing of a single radio channel among N usersbull Requires high data-rate per radio channel to support N users
simultaneously which requires adaptive equalizers to be usedin multipath environments
bull Transmission occurs in bursts (not continually) It enablespower saving by going to sleep modes in unrelated slots Itdiscontinues transmission and also enables mobile assistedhandoff
bull It equires synchronization of the receivers Need guard bits andsync bits which occurs large overhead per slot
bull Allocation of slots to mobile users should not be uniform Itmay depend on the traffic requirement of mobiles This bringsextra flexibility and efficiency compared to FDMA systems
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2620111015 TAacuteMOP ndash 412-082AKMR-2009-0006 26
Ad hoc Sensor Networks Multiple channel access
Multiple Access spread spectrum multiple accessbull SSMA uses signals that have transmission bandwidth that is
several orders of magnitude larger than minimum requiredRF bandwidth
bull It providesndash Immunity to multipath interferencendash Robust multiple access
bull Two techniquesndash Frequency Hopped Multiple Access (FHMA)ndash Direct Sequence Multiple Access (DSMA) or Code Division
Multiple Access (CDMA)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2720111015 TAacuteMOP ndash 412-082AKMR-2009-0006 27
Ad hoc Sensor Networks Multiple channel access
Multiple Access FHMAbull The carrier frequency of users are varied in a pseudo-random
fashionndash Each user is using a narrowband channel (spectrum) at a specific
instance of timendash The random change in frequency make the change of using the same
narrowband channel very lowbull The sender receiver change frequency (calling hopping)
using the same pseudo-random sequence hence they aresynchronized
bull Rate of hopping versus Symbol ratendash If hopping rate is greather Fast Frequency Hoppingndash If symbol rate is greater Slow Frequency Hopping
bull GSM and Bluetooth are example systems
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2820111015 TAacuteMOP ndash 412-082AKMR-2009-0006 28
Ad hoc Sensor Networks Multiple channel access
Multiple Access FHMA (contrsquo)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2920111015 TAacuteMOP ndash 412-082AKMR-2009-0006 29
Ad hoc Sensor Networks Multiple channel access
Multiple Access CDMAbull The narrowband message signal is multiplied by a very large bandwidth
signal called spreading signal (code) before modulation and transmissionover the air This is called spreading
bull Spreading signal use a pseudo-noise (PN) sequence (a pseudo-randomsequence) called codeword which are orthogonal (low autocorrelation)
bull The receiver correlator distinguishes the senders signal by examining thewideband signal with the same time-synchronized spreading code(despreading)
bull Advantagesndash Low power spectral densityndash Interference limited operationndash Privacyndash Reduction of multipath affects by using a larger spectrumndash Users can start their transmission at any time (random access possible)ndash Cell capacity is soft and higher than TDMA and FDMAndash No frequency management and no equalizers and quard time needed
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3020111015 TAacuteMOP ndash 412-082AKMR-2009-0006 30
Ad hoc Sensor Networks Multiple channel access
Multiple Access near far problem and power controlbull At a receiver the signals may come from various sources (multiuser) The
strongest signal usually captures the modulator The other signals areconsidered as noise Each source may have different distances to the basestation
bull In CDMA we want a BS to receive CDMA coded signals from variousmobile users at the same time
ndash Therefore the receiver power at the BS for all mobile users should be close toeach other This requires power control at the mobiles
bull Power Control BS monitors the RSSI (Received Signal Strength Indicator)values from different mobiles and then sends power change commands to themobiles over a forward channel The mobiles then adjust their transmitpower
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3120111015 TAacuteMOP ndash 412-082AKMR-2009-0006 31
Ad hoc Sensor Networks Multiple channel access
Multiple Access hybrid spread spectrum techniquesbull FDMACDMA
ndash Available wideband spectrum is divided into a number narrowband radio channels CDMAis employed inside each channel
bull DSFHMAndash The signals are spread using spreading codes (direct sequence signals are obtained) but
these signal are not transmitted over a constant carrier frequency they are transmitted over afrequency hopping carrier frequency
bull Time Division CDMA (TCDMA)ndash Each cell is using a different spreading code (CDMA employed between cells) that is
conveyed to the mobiles in its rangendash Inside each cell (inside a CDMA channel) TDMA is employed to multiplex multiple users
bull Time Division Frequency Hoppingndash At each time slot the user is hopped to a new frequency according to a pseudo-random
hopping sequencendash Employed in severe co-interference and multi-path environmentsndash Bluetooth and GSM are using this technique
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3220111015 TAacuteMOP ndash 412-082AKMR-2009-0006 32
Ad hoc Sensor Networks Multiple channel access
Multiple Access comparison TDMAFDMACDMA
Techniques TDMA FDMA CDMA SDMA
Idea
segment sending time into disjoint time-slots
Segment the frequency band into disjoint sub-bands
Spread the spectrum using orthogonal codes
Segment space into sectors
Signal separation
Synchronization in the time domain
Filtering in the frequency domain
Code Directed antennas
Advantage
Established flexible
Simple established robust
Flexible less frequency planning needed soft handover
Simple increases capacity
DisadvantageQuard space synchronization difficult
Inflexible scarce source
Complex receiver power control
inflexible
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3320111015 TAacuteMOP ndash 412-082AKMR-2009-0006 33
Ad hoc Sensor Networks Multiple channel access
User capacity per cell
The user capacity C is defined as the total number of active users percell that the system can support while meeting a common BERconstrain For orthogonal multiple access (as FDMA and TDMA)
C = Ncellwhere Ncell is the number of channels assigned to any given cellThe total number of orthogonal channels of bandwidth Bs that can becreated from a total system bandwidth of B is B Bs The reuse factorsatisfies N = (B Bs) Ncell this implies
C = (B Bs) N
Recall from Chapter 1
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3420111015 TAacuteMOP ndash 412-082AKMR-2009-0006 34
Ad hoc Sensor Networks Multiple channel access
10 methods for increasing capacity1 Increasing the amount of spectrum used very expensive2 More efficient modulation format and coding3 Better source coding4 Adaptive modulation and coding5 Discontinuous transmission6 Multiuser detection CDMA systems7 Reduction of cell radius effective but very expensive smaller
cells require more handovers8 Use of sector cells tripled cells have tripled BS antennas9 Use of an overlay structure10 Multiple antennas diversity MIMO systems SDMA
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3520111015 TAacuteMOP ndash 412-082AKMR-2009-0006 35
Ad hoc Sensor Networks Multiple channel access
bull Distributed operation is requiredbull Synchronization is required in TDMA-based systemsbull Hidden terminals are nodes hidden from a senderbull Exposed terminals are exposed nodes preventing a sender
from sendingbull Throughput needs to be maximizedbull Access delay should be minimizedbull Fairness refers to provide an equal share to all competing
nodesbull Real-time traffic support is required for voice video and
real-time databull Resource reservation is required for QoSbull Ability to measure resource availability handles the
resourcesbull Capability for power control reduces the energy
consumption
Random access in adhoc networks
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3620111015 TAacuteMOP ndash 412-082AKMR-2009-0006 36
Ad hoc Sensor Networks Multiple channel access
Random accessbull All random access techniques are based on packetized data or packet radio where user
date is collected into packets of M bitsbull Collision if packets from different users overlap in timebull The transmission time of a packet is τ = M R where R [bps] the data ratebull Analysis of random access techniques assumes that the users generate packets according
to memoryless Poisson process at a rate λ [packets per unit time] the probability that thenumber of packet arrivals in a time period [0t] denoted by X(t) is equal to integer k isgiven by
Pr(X(t)=k) = (λt)k k e-λt
bull The traffic load is defined as L = λ τ If L gt 1 means on avarage more packets arrive inthe system over a given time period than can be transmitted in that period so systemswith Lgt1 are unstable
bull The throughput is defined as the ratio of the average number of packets successfullytransmitted in any given time interval divided by the number of attempted transmissionsin that interval
T = L Pr(succesful packet transmission)bull The effective data rate of the system is RT
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3720111015 TAacuteMOP ndash 412-082AKMR-2009-0006 37
Ad hoc Sensor Networks Multiple channel access
Random access (contrsquo)bull We will examine the following attributes
ndash Channel utilization (Throughput)ndash Latencyndash Collision avoidance (Hidden and exposed terminals problem)ndash Reliability (ACK)ndash Energy efficiency (power control)ndash Fairnessndash Throughput needs to be maximized
bull We will discuss the following techniquesndash Aloha and slotted Alohandash CSMA Protocols (1-persistent non-persistent p-persistent CSMA CSMACD)ndash MACAndash MACAWndash Energy efficient MAC in wireless sensor networks (eg SMAC)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 38
Random Access ALOHA (1970)bull ALOHA was developed for a wireless system at the University of Hawaii
(Abramson et al)ndash Multiple remote stations plus one base stationndash Frame transmissions are made at one frequency from a remote station to the basestation the base station re-broadcasts frames on another frequency Contention is foraccess to the base station
bull Frames can be sent at any time to the base stationndash If no acknowledgment (ACK) is received assume that the frame is lost andretransmit after waiting a random amount of timendash If more than one station broadcasts on the base station access frequency at the sametime ndash a collision ndash interference will destroy the frames
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 39
Random Access ALOHA (contrsquo)
bull The probability that no packets generated during the tima [-τ τ] is given by (nocollision)
Pr(X(t)=0) = (λ2τ)0 0 e-2λτ = e-2Lwith corresponding throughput
T = L e-2L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 05 05e 018
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 40
Random Access slotted ALOHA (1971)bull Improvement Time is slotted and a packet can only be transmitted at the beginning of
one slot Thus it can reduce the collision duration
bull The probability that no packets generated during the time [0 τ] is given by (nocollision)
Pr(X(t)=0) = (λτ)0 0 e-λτ = e-Lwith corresponding throughput T = L e-L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 1 e 037
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 41
bull Slotted Aloha has double the maximumthroughput as pure Aloha and achievesthis maximum at a higher offered loadThe effective data rate is still less than40 of the raw transmission rate
bull Aloha does not listen to the carrier beforetransmission more sophisticatedtechniques are needed to increaseefficiency
bull Further problem the delayndashDelay
ndashThe mean packet delay
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
( ) ( )delayPr 1 e ekL Lkτ minus minus= = minus
( )delay1
1 e e ekL L L
kMPD E kτ
infinminus minus
=
= = minus =sum
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 42
Random Access Carrier Sense Multiple Accessbull CSMA (Carrier Sense Multiple Access) listen to the carrier before transmission
and transmits if channel is idlebull Detection delay and propagation delay are two important parameters
ndash Detection delay time required to sense the carrier and decide if it is idle or busyndash Propagation delay distancespeed_of_ligth The time required for bit to travel
from transmitter to the receiver
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 43
Random Access CSMA variations
Ad hoc Sensor Networks Multiple channel access
1-persistent CSMA Non-persistent CSMA
There will always be a collision iftwo nodes want to retransmit(usually you stop transmissionattempts after few tries)
Random backoff reduces probabilityof collisions Waste idle time if thebackoff time is too long
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 44
Random Access CSMA variations (contrsquo)
Ad hoc Sensor Networks Multiple channel access
p-persistent CSMA CSMA CD (Collision Detection)
A good tradeoff between non-persistent and 1-persistent CSMA
Same with CSMA however a stationalso listen to the carrier whiletransmitting to see if the transmissioncollides with someone elsetransmission It can be used in listen-while-talk capable channels (fullduplex) In single radio channels thetransmission need to be interrupted inorder to sense the channel
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 45
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 46
Random Access CSMA (contrsquo)bull How to select probability p
ndash Assume that M nodes have a packet to send and the medium is busy then Mp is theexpected number of nodes that will attempt to transmit once the medium becomesidle If Mp gt 1 then a collision is expected to occur Therefore the network mustmake sure that Mp lt 1 to avoid collision where M is the maximum number of nodesthat can be active at a time
bull Problems1 The mean packet delay increases exponently with increasing offered load2 CSMA protocols sense the carrier but sensing the carrier does not always releasestrue information about the status of the wireless channel There are two problems thatare unique to wireless channels (different than wireline channels)ndash Hidden terminal problemndash Exposed terminal problem
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 47
Hidden terminal problembull A sends to B C cannot receive Abull C wants to send to B C senses a
ldquofreerdquo medium (CS fails)bull collision at B A cannot receive the
collision (CD fails)bull A is ldquohiddenrdquo for C
Ad hoc Sensor Networks Multiple channel access
Random Access hidden and exposed terminal problemExposed terminal problem
bull B sends to A C wants to send to Dbull C has to wait CS signals a medium
in usebull since A is outside the radio range of
C waiting is not necessarybull C is ldquoexposedrdquo to B
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 48
Random Access MACAWhen a station wants to transmit databull It sends an RTS (Ready-to-Send) packet to the intended receiver
ndash The RTS packet contains the length of the data that needs to be transmittedndash Any station other than the intended recipient hearing RTS defers transmission for atime duration equal to the end of the corresponding CTS reception
bull The receiver sends back CTS (Clear-to-Send) packet back to sender if it isavailable to receivendash The CTS packet contains the length of the data that original sender wants totransmitndash Any station other than the original RTS sender hearing CTS defers transmissionuntil the data is sent
bull The original sender upon reception of the CTS starts transmitting
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 49
Hidden terminal problem
Ad hoc Sensor Networks Multiple channel access
Random Access MACA (contrsquo)Exposed terminal problem
Waiting time of node X is muchsmaller than waiting time of node C
bull C defers transmission upon hearing BrsquosRTS until B could get CTS from Abull After that C can start transmission to DFor that it first sends an RTSbull C is not longer exposed to the datatransmission of B
Problem It does not address the collision of RTS packet
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 50
Random Access reliabilitybull Wireless links are prone to have errors High packet loss rate detrimental to
transport-layer performance Mechanisms needed to reduce packet loss rateexperienced by upper layers
bull When node B receives a data packet from node A node B sends anAcknowledgement (Ack) This approach adopted in many protocols
bull If node A fails to receive an Ack it will retransmit the packet
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 51
Random Access MACAW (CSMACA) bull Physical carrier sense and virtual carrier sense using Network Allocation
Vector (NAV)ndash NAV is updated based on overheard RTSCTSDATAACK packets each of whichspecified duration of a pending transmissionndash Nodes stay silent when carrier sensed (physicalvirtual)
bull Backoff intervals used to reduce collision probabilityndash When transmitting a packet choose a backoff interval in the range [0w] where wis contention windowndash Count down the backoff interval when channel is idlendash Count down is suspended if channel becomes busyndash When backoff interval reaches 0 transmit RTS
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 52
Random Access CSMACA (contrsquo) bull The time spent counting down backoff intervals is a part of MAC overhead
ndash Choosing a large w leads to large backoff intervals and can result in larger overheadndash Choosing a small w leads to a larger number of collisions (when two nodes countdown to 0 simultaneously)ndash Since the number of nodes attempting to transmit simultaneously may change withtime some mechanism to manage contention is needed
bull IEEE 80211 DCF contention window w is chosen dynamically depending oncollision occurrence (binary exponential backoff)ndash When a node fails to receive CTS in response to its RTS it increases the contentionwindowndash w is doubled (up to an upper bound)ndash When a node successfully completes a data transfer it restores w to wmin
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 53
Random Access fairnessbull Simplest definition of fairness all nodes should receive equal bandwidthbull An example of unfairness
ndash Assume that initially A and B both choose a backoff interval in range [031] buttheir RTSs collidendash Nodes A and B then choose from range [063]ndash Node A chooses 4 slots and B choose 60 slotsndash After A transmits a packet it next chooses from range [031]ndash It is possible that A may transmit several packets before B transmits its first packet
Ad hoc Sensor Networks Multiple channel access
Unfairness occurs when one node has backed off much more than some other node
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 54
Random Access fairness (contrsquo) MACAW solution for fairnessbull When a node transmits a packet it appends value w to the packet all nodes
hearing value w use it for their future transmission attemptsbull Since w is an indication of the level of congestion in the vicinity of a specific
receiver node MACAW proposes maintaining w independently for eachreceiver
bull Using per-receiver wi is particularly useful in multi-hop environments sincecongestion level at different receivers can be very different
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 55
Random Access energy efficiency bull Since many mobile hosts are operated by batteries MAC
protocols which conserve energy are of interestbull Two approaches to reduce energy consumption
ndash Power save turn off wireless interface when desirablendash Power control reduce transmit power
bull Power control has some more potential benefitsndash Reduced interference (in ad hoc networks) eg it improves ALOHAefficiency where user with high power can capture a packet even if there is acollisionndash Increased spatial reuse (in infrastructured wireless networks) eg it is usedin CDMA to maintain target SIR of voice and data users or generally it canbe used to maintain target SIR for different user classes (admission control)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 56
Random Access power control bull When C transmits to D at a high power level B cannot receive Arsquos
transmission due to interference from Cbull If C reduces transmit power it can still communicate with D
ndash Reduces energy consumption at node Cndash Allows B to receive Arsquos transmission (spatial reuse)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 57
bull A special wireless ad hoc networkndash Large number of nodesndash Battery poweredndash Topology and density changendash Nodes for a common taskndash In-network data processing
bull Sensor-net applicationsndash Sensor-triggered bursty trafficndash Can often tolerate some delay
Ad hoc Sensor Networks Multiple channel access
Characteristics of Sensor Network
Scalabilty and self configurationEnergy efficiencyAdaptivityFairness not importantLatency
AdaptivityEnergy-delay tradoff
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 58
Ad hoc Sensor Networks Multiple channel access
Wireless sensor node and sensor network
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 59
MAC for WSN attributes bull Attributes
ndash Collision avoidance (Basic task)ndash Energy efficiency (most primary in WSN)ndash Scalability and adaptivityndash Channel utilizationndash Latencyndash Throughputndash Fairness
bull What causes energy wastendash Collisionsndash Control packet overheadndash Overhearing unnecessary trafficndash Long idle time (it consumes 50-100 of the power)
Ad hoc Sensor Networks Multiple channel access
Primary
Secondary
Dominant factor
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 60
MAC for WSN
bull Contention-based protocols need to work hard in all directions for energysavingsndash Reduce idle listening ndash support low duty cyclendash Better collision avoidancendash Reduce control overheadndash Avoid unnecessary overhearing
Ad hoc Sensor Networks Multiple channel access
Scheduled Protocols Contention ProtocolsCollisions No YesEnergy efficiency Good BadScalability and adaptivity Bad GoodMulti-hop communication Difficult EasyTime synchronization Strict Loose or not required
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 61
MAC for WSN classification
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 62
MAC for WSN SMAC (2002)bull It has been developed by Ye Heidemann and Estrinbull Tradeoffs between (latency fairness) and energybull Major components in S-MAC
ndash Periodic listen and sleep turn off radio when sleeping the reduced dutycycle is ~ 10 (eg 120ms on 12s off)ndash Collision avoidance is based on contention similarly to IEEE 80211 adhoc modendash Overhearing avoidance the node is sleeping when neighbors talkndash Massage passing long message is fragmented amp sent in burst operationwith extended Tx time and re-transmit occurs immediately
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 63
Summarybull The algorithms and protocols that enable sharing by multiple users and
controls the access to the wireless channel from different users are calledMAC
bull There are two types of wireless networks Infrastructured wireless networksand ad hoc wireless networks
bull We also want our wireless system to have high user capacity and we canidentify 10 methods for increasing it
bull There are several MACs with no absolute advantage it should be tailoredto the system requirements
bull There are primary concerns for designing a WSN MAC (collisionavoidance energy efficiency scalability and adaptivity) while thesecondary concerns are as follows (channel utilization latency throughputfairness)
bull Next lecture Routing protocols
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 14
Ad hoc Sensor Networks Multiple channel access
FDDbull FDD is used in radio systems that can
allocate individual radio frequenciesfor each user
bull For example analog systems AMPSbull In FDD channels are allocated by a
base stationbull A channel for a mobile is allocated
dynamicallybull All channels that a base station will use
are allocated usually staticallybull More suitable for wide-area cellular
networks GSM AMPS all use FDD
Duplexing(contrsquo)TDD
bull Can only be used in digital wirelesssystems (digital modulation)
bull Requires rigid timing andsynchronization
bull Mostly used in short-range and fixedwireless systems so that propagationdelay between base and mobile do notchange much with respect to locationof the mobile
bull Such as cordless phoneshellip
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 15
Ad hoc Sensor Networks Multiple channel access
Multiple Access narrow- vs wideband systemsbull Narrowband System
ndash The bandwidth is small compared to the coherence bandwidth of the channel(BltBcoh)
bull Wideband Systemndash The system bandwidth is much larger that the coherence bandwidth of the
multipath channel (BgtBcoh) A large number of users can access the same channel(frequency band) at the same time
bull Four major multiple access (MA) schemesndash Time Division Multiple Access (TDMA) it could be used in narrowband or
wideband systemndash Frequency Division Multiple Access (FDMA) it is usually used in narrowband
systemndash Code Division Multiple Access (CDMA) it is used in wideband systemndash Space Division Multiple Access (SDMA) for wireless systems with multiple
antennas it can be combined with all of the other MA methods
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 16
Ad hoc Sensor Networks Multiple channel access
Multiple Access cellular standards
Cellular System Multiple Access TechniqueAMPS (2G) FDMAFDD
CT2 Cordless Phone FDMATDD
DECT Cordless Phone FDMATDD
GSM (2G) TDMAFDD
USDC (IS-54 and IS-136) (2G) TDMAFDD
Personal Digital Cellular (2G) TDMAFDD
US IS-95 (2G) CDMAFDD
W-CDMA (3G) CDMAFDD CDMATDD
cdma2000 (3G) CDMAFDD CDMATDD
Narrowband System
Wideband System
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 17
Ad hoc Sensor Networks Multiple channel access
bull Individual radio channels are assigned toindividual users Each user is allocated a Wfrequency band by BS
bull If the channel allocated to a user is idle then itis not used by someone else waste of resource
bull Mobile and base can transmit and receivesimultaneously (FDD)
bull The W bandwidth of FDMA channels arerelatively low Symbol time is usually larger(low data rate) than the delay spread of themultipath channel (implies that inter-symbolinterference is low)
bull Lower complexity systems than TDMAsystems
Multiple Access FDMA
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 18
Ad hoc Sensor Networks Multiple channel access
Multiple Access FDMA (contrsquo)bull Capacity of FDMA systems
Nc = (B-2Bguard)Wwhere B is the total spectrum allocation Bguard is the guard band allocated atthe edge of the spectrum band W is the bandwidth of an individual channel
bull Erlang B and Erlang C system assumesndashinfinite population of sources which jointly offer traffic to Nc channelndashcall attempts arrive following a Poisson process so call arrivals are independentThe average arrival rate is λndashmessage length (holding times) are exponentially distributed (Markovian system)The average call length is hndashthe total amount of traffic offered in erlangs L= λhndashIf a user is rejected his next call attempt is made statistically independent of theprevious attempt (Erlang B)ndashIf all the channels are busy when a request arrives from a user the request isqueued An unlimited number of requests may be held in the queue (Erlang C)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 19
Ad hoc Sensor Networks Multiple channel access
bull Statictical measures of offered load Ncndash In Erlang B system the probability of call
blocking
ndash In Erlang C system the probabilty of waiting
ndash And the average wait time
c
c
cBLOCK
0
Pr
N
Nk
k
LN
Lk
=
=
sum
c
cc
wait 1
c0c
Pr 1
N
NkN
k
LL LL N kN
minus
=
=⎛ ⎞
+ minus⎜ ⎟⎝ ⎠
sum
wait waitc
Pr htN L
=minus
Multiple Access FDMA (contrsquo)Recall from
Queuing theory
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 20
Ad hoc Sensor Networks Multiple channel access
Multiple Access example of Erlang B systembull In an Erlang B system 40 channels are available A blocking probabilty of
less than 1 is required What is the traffic that can be serve if there is oneoperator or three operators
bull (Solution L(1 operator)gtL(3 operators))bull (Note The number of users increases faster than linearly with the number of
available channels The difference between actual increase and linearincrease is called the trunking gain It is preferable to have a large pool ofavailable channels that serves all users (a single operator) The reasons fornot choosing this approach are political not technical)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 21
Ad hoc Sensor Networks Multiple channel access
Multiple Access TDMAbull The allocated radio spectrum for the system is divided into time slots
ndash In each slot a user can transmit or receivendash A user occupies a cyclically repeating slotsndash A channel is logically defined as a particular time slot that repeats with some
periodbull TDMA systems buffer the data until its turn (time slot) comes to transmitbull In TDMATDD half of the slots in the frame is used for uplink channels the
other is used for downlink channelsbull In TDMAFDD a different carrier frequency is used for uplink or downlink
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 22
Ad hoc Sensor Networks Multiple channel access
Multiple Access TDMA (contrsquo)bull Preamble contains address and synchronization info to identify base
station and mobiles to each otherbull Guard times are used to allow synchronization of the receivers between
different slots and framesndash Different mobiles may have different propagation delays to a base station
because of different distances
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 23
Multiple Access TDMA (contrsquo)bull Each frame contains overhead bits and data bits Efficiency of frame is
defined as the percentage of data (information) bits to the total frame size inbits
η =(1 ndash boverheadbtotal) 100 where btotal= Tf R is the total number of bits in a frame Tf is the frameduration in sec boverhead is the number of overhead bits (GSM efficiency 73)
bull TDMA is usually combined with FDMAndash Neighboring cells are allocated and using different carrier frequencies (FDMA)
Inside a cell TDMA can be used Cells may be re-using the same frequency if theyare far from each-other
ndash There may be more than one channel allocated and used inside each cell Eachcarrier frequency (radio channel) may be using TDMA to further multiplex moreuser (ie having TDMA logical channels inside radio channels) For example inGSM each radio channel has 200KHz bandwidth and has 8 time slots (8 logicalchannels) Hence GSM is using FDMA combined with TDMA
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 24
Multiple Access TDMA (contrsquo)
Ad hoc Sensor Networks Multiple channel access
GSM (Europa) IS-54 (USA) PDC (Japan) DECT
Bit Rate 2708 Kbps 486 Kbps 42 Kbps 1152 Mbps
Bandwidth 200 KHz 30 KHz 25 KHz 1728 MHz
Time Slot 0577 ms 67 ms 67 ms 0417 ms
Upstream slots per frame 8 3 3 12
Duplexing FDD FDD FDD TDD
Efficiency 73 80 80 67
Modulation GMSK π4 DQPSK π4 DQPSK GMSK
Adaptive equalized Mandatory Mandatory Optional None
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 25
Multiple Access TDMA (contrsquo)bull Enables the sharing of a single radio channel among N usersbull Requires high data-rate per radio channel to support N users
simultaneously which requires adaptive equalizers to be usedin multipath environments
bull Transmission occurs in bursts (not continually) It enablespower saving by going to sleep modes in unrelated slots Itdiscontinues transmission and also enables mobile assistedhandoff
bull It equires synchronization of the receivers Need guard bits andsync bits which occurs large overhead per slot
bull Allocation of slots to mobile users should not be uniform Itmay depend on the traffic requirement of mobiles This bringsextra flexibility and efficiency compared to FDMA systems
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2620111015 TAacuteMOP ndash 412-082AKMR-2009-0006 26
Ad hoc Sensor Networks Multiple channel access
Multiple Access spread spectrum multiple accessbull SSMA uses signals that have transmission bandwidth that is
several orders of magnitude larger than minimum requiredRF bandwidth
bull It providesndash Immunity to multipath interferencendash Robust multiple access
bull Two techniquesndash Frequency Hopped Multiple Access (FHMA)ndash Direct Sequence Multiple Access (DSMA) or Code Division
Multiple Access (CDMA)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2720111015 TAacuteMOP ndash 412-082AKMR-2009-0006 27
Ad hoc Sensor Networks Multiple channel access
Multiple Access FHMAbull The carrier frequency of users are varied in a pseudo-random
fashionndash Each user is using a narrowband channel (spectrum) at a specific
instance of timendash The random change in frequency make the change of using the same
narrowband channel very lowbull The sender receiver change frequency (calling hopping)
using the same pseudo-random sequence hence they aresynchronized
bull Rate of hopping versus Symbol ratendash If hopping rate is greather Fast Frequency Hoppingndash If symbol rate is greater Slow Frequency Hopping
bull GSM and Bluetooth are example systems
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2820111015 TAacuteMOP ndash 412-082AKMR-2009-0006 28
Ad hoc Sensor Networks Multiple channel access
Multiple Access FHMA (contrsquo)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2920111015 TAacuteMOP ndash 412-082AKMR-2009-0006 29
Ad hoc Sensor Networks Multiple channel access
Multiple Access CDMAbull The narrowband message signal is multiplied by a very large bandwidth
signal called spreading signal (code) before modulation and transmissionover the air This is called spreading
bull Spreading signal use a pseudo-noise (PN) sequence (a pseudo-randomsequence) called codeword which are orthogonal (low autocorrelation)
bull The receiver correlator distinguishes the senders signal by examining thewideband signal with the same time-synchronized spreading code(despreading)
bull Advantagesndash Low power spectral densityndash Interference limited operationndash Privacyndash Reduction of multipath affects by using a larger spectrumndash Users can start their transmission at any time (random access possible)ndash Cell capacity is soft and higher than TDMA and FDMAndash No frequency management and no equalizers and quard time needed
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3020111015 TAacuteMOP ndash 412-082AKMR-2009-0006 30
Ad hoc Sensor Networks Multiple channel access
Multiple Access near far problem and power controlbull At a receiver the signals may come from various sources (multiuser) The
strongest signal usually captures the modulator The other signals areconsidered as noise Each source may have different distances to the basestation
bull In CDMA we want a BS to receive CDMA coded signals from variousmobile users at the same time
ndash Therefore the receiver power at the BS for all mobile users should be close toeach other This requires power control at the mobiles
bull Power Control BS monitors the RSSI (Received Signal Strength Indicator)values from different mobiles and then sends power change commands to themobiles over a forward channel The mobiles then adjust their transmitpower
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3120111015 TAacuteMOP ndash 412-082AKMR-2009-0006 31
Ad hoc Sensor Networks Multiple channel access
Multiple Access hybrid spread spectrum techniquesbull FDMACDMA
ndash Available wideband spectrum is divided into a number narrowband radio channels CDMAis employed inside each channel
bull DSFHMAndash The signals are spread using spreading codes (direct sequence signals are obtained) but
these signal are not transmitted over a constant carrier frequency they are transmitted over afrequency hopping carrier frequency
bull Time Division CDMA (TCDMA)ndash Each cell is using a different spreading code (CDMA employed between cells) that is
conveyed to the mobiles in its rangendash Inside each cell (inside a CDMA channel) TDMA is employed to multiplex multiple users
bull Time Division Frequency Hoppingndash At each time slot the user is hopped to a new frequency according to a pseudo-random
hopping sequencendash Employed in severe co-interference and multi-path environmentsndash Bluetooth and GSM are using this technique
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3220111015 TAacuteMOP ndash 412-082AKMR-2009-0006 32
Ad hoc Sensor Networks Multiple channel access
Multiple Access comparison TDMAFDMACDMA
Techniques TDMA FDMA CDMA SDMA
Idea
segment sending time into disjoint time-slots
Segment the frequency band into disjoint sub-bands
Spread the spectrum using orthogonal codes
Segment space into sectors
Signal separation
Synchronization in the time domain
Filtering in the frequency domain
Code Directed antennas
Advantage
Established flexible
Simple established robust
Flexible less frequency planning needed soft handover
Simple increases capacity
DisadvantageQuard space synchronization difficult
Inflexible scarce source
Complex receiver power control
inflexible
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3320111015 TAacuteMOP ndash 412-082AKMR-2009-0006 33
Ad hoc Sensor Networks Multiple channel access
User capacity per cell
The user capacity C is defined as the total number of active users percell that the system can support while meeting a common BERconstrain For orthogonal multiple access (as FDMA and TDMA)
C = Ncellwhere Ncell is the number of channels assigned to any given cellThe total number of orthogonal channels of bandwidth Bs that can becreated from a total system bandwidth of B is B Bs The reuse factorsatisfies N = (B Bs) Ncell this implies
C = (B Bs) N
Recall from Chapter 1
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3420111015 TAacuteMOP ndash 412-082AKMR-2009-0006 34
Ad hoc Sensor Networks Multiple channel access
10 methods for increasing capacity1 Increasing the amount of spectrum used very expensive2 More efficient modulation format and coding3 Better source coding4 Adaptive modulation and coding5 Discontinuous transmission6 Multiuser detection CDMA systems7 Reduction of cell radius effective but very expensive smaller
cells require more handovers8 Use of sector cells tripled cells have tripled BS antennas9 Use of an overlay structure10 Multiple antennas diversity MIMO systems SDMA
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3520111015 TAacuteMOP ndash 412-082AKMR-2009-0006 35
Ad hoc Sensor Networks Multiple channel access
bull Distributed operation is requiredbull Synchronization is required in TDMA-based systemsbull Hidden terminals are nodes hidden from a senderbull Exposed terminals are exposed nodes preventing a sender
from sendingbull Throughput needs to be maximizedbull Access delay should be minimizedbull Fairness refers to provide an equal share to all competing
nodesbull Real-time traffic support is required for voice video and
real-time databull Resource reservation is required for QoSbull Ability to measure resource availability handles the
resourcesbull Capability for power control reduces the energy
consumption
Random access in adhoc networks
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3620111015 TAacuteMOP ndash 412-082AKMR-2009-0006 36
Ad hoc Sensor Networks Multiple channel access
Random accessbull All random access techniques are based on packetized data or packet radio where user
date is collected into packets of M bitsbull Collision if packets from different users overlap in timebull The transmission time of a packet is τ = M R where R [bps] the data ratebull Analysis of random access techniques assumes that the users generate packets according
to memoryless Poisson process at a rate λ [packets per unit time] the probability that thenumber of packet arrivals in a time period [0t] denoted by X(t) is equal to integer k isgiven by
Pr(X(t)=k) = (λt)k k e-λt
bull The traffic load is defined as L = λ τ If L gt 1 means on avarage more packets arrive inthe system over a given time period than can be transmitted in that period so systemswith Lgt1 are unstable
bull The throughput is defined as the ratio of the average number of packets successfullytransmitted in any given time interval divided by the number of attempted transmissionsin that interval
T = L Pr(succesful packet transmission)bull The effective data rate of the system is RT
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3720111015 TAacuteMOP ndash 412-082AKMR-2009-0006 37
Ad hoc Sensor Networks Multiple channel access
Random access (contrsquo)bull We will examine the following attributes
ndash Channel utilization (Throughput)ndash Latencyndash Collision avoidance (Hidden and exposed terminals problem)ndash Reliability (ACK)ndash Energy efficiency (power control)ndash Fairnessndash Throughput needs to be maximized
bull We will discuss the following techniquesndash Aloha and slotted Alohandash CSMA Protocols (1-persistent non-persistent p-persistent CSMA CSMACD)ndash MACAndash MACAWndash Energy efficient MAC in wireless sensor networks (eg SMAC)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 38
Random Access ALOHA (1970)bull ALOHA was developed for a wireless system at the University of Hawaii
(Abramson et al)ndash Multiple remote stations plus one base stationndash Frame transmissions are made at one frequency from a remote station to the basestation the base station re-broadcasts frames on another frequency Contention is foraccess to the base station
bull Frames can be sent at any time to the base stationndash If no acknowledgment (ACK) is received assume that the frame is lost andretransmit after waiting a random amount of timendash If more than one station broadcasts on the base station access frequency at the sametime ndash a collision ndash interference will destroy the frames
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 39
Random Access ALOHA (contrsquo)
bull The probability that no packets generated during the tima [-τ τ] is given by (nocollision)
Pr(X(t)=0) = (λ2τ)0 0 e-2λτ = e-2Lwith corresponding throughput
T = L e-2L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 05 05e 018
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 40
Random Access slotted ALOHA (1971)bull Improvement Time is slotted and a packet can only be transmitted at the beginning of
one slot Thus it can reduce the collision duration
bull The probability that no packets generated during the time [0 τ] is given by (nocollision)
Pr(X(t)=0) = (λτ)0 0 e-λτ = e-Lwith corresponding throughput T = L e-L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 1 e 037
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 41
bull Slotted Aloha has double the maximumthroughput as pure Aloha and achievesthis maximum at a higher offered loadThe effective data rate is still less than40 of the raw transmission rate
bull Aloha does not listen to the carrier beforetransmission more sophisticatedtechniques are needed to increaseefficiency
bull Further problem the delayndashDelay
ndashThe mean packet delay
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
( ) ( )delayPr 1 e ekL Lkτ minus minus= = minus
( )delay1
1 e e ekL L L
kMPD E kτ
infinminus minus
=
= = minus =sum
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 42
Random Access Carrier Sense Multiple Accessbull CSMA (Carrier Sense Multiple Access) listen to the carrier before transmission
and transmits if channel is idlebull Detection delay and propagation delay are two important parameters
ndash Detection delay time required to sense the carrier and decide if it is idle or busyndash Propagation delay distancespeed_of_ligth The time required for bit to travel
from transmitter to the receiver
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 43
Random Access CSMA variations
Ad hoc Sensor Networks Multiple channel access
1-persistent CSMA Non-persistent CSMA
There will always be a collision iftwo nodes want to retransmit(usually you stop transmissionattempts after few tries)
Random backoff reduces probabilityof collisions Waste idle time if thebackoff time is too long
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 44
Random Access CSMA variations (contrsquo)
Ad hoc Sensor Networks Multiple channel access
p-persistent CSMA CSMA CD (Collision Detection)
A good tradeoff between non-persistent and 1-persistent CSMA
Same with CSMA however a stationalso listen to the carrier whiletransmitting to see if the transmissioncollides with someone elsetransmission It can be used in listen-while-talk capable channels (fullduplex) In single radio channels thetransmission need to be interrupted inorder to sense the channel
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 45
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 46
Random Access CSMA (contrsquo)bull How to select probability p
ndash Assume that M nodes have a packet to send and the medium is busy then Mp is theexpected number of nodes that will attempt to transmit once the medium becomesidle If Mp gt 1 then a collision is expected to occur Therefore the network mustmake sure that Mp lt 1 to avoid collision where M is the maximum number of nodesthat can be active at a time
bull Problems1 The mean packet delay increases exponently with increasing offered load2 CSMA protocols sense the carrier but sensing the carrier does not always releasestrue information about the status of the wireless channel There are two problems thatare unique to wireless channels (different than wireline channels)ndash Hidden terminal problemndash Exposed terminal problem
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 47
Hidden terminal problembull A sends to B C cannot receive Abull C wants to send to B C senses a
ldquofreerdquo medium (CS fails)bull collision at B A cannot receive the
collision (CD fails)bull A is ldquohiddenrdquo for C
Ad hoc Sensor Networks Multiple channel access
Random Access hidden and exposed terminal problemExposed terminal problem
bull B sends to A C wants to send to Dbull C has to wait CS signals a medium
in usebull since A is outside the radio range of
C waiting is not necessarybull C is ldquoexposedrdquo to B
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 48
Random Access MACAWhen a station wants to transmit databull It sends an RTS (Ready-to-Send) packet to the intended receiver
ndash The RTS packet contains the length of the data that needs to be transmittedndash Any station other than the intended recipient hearing RTS defers transmission for atime duration equal to the end of the corresponding CTS reception
bull The receiver sends back CTS (Clear-to-Send) packet back to sender if it isavailable to receivendash The CTS packet contains the length of the data that original sender wants totransmitndash Any station other than the original RTS sender hearing CTS defers transmissionuntil the data is sent
bull The original sender upon reception of the CTS starts transmitting
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 49
Hidden terminal problem
Ad hoc Sensor Networks Multiple channel access
Random Access MACA (contrsquo)Exposed terminal problem
Waiting time of node X is muchsmaller than waiting time of node C
bull C defers transmission upon hearing BrsquosRTS until B could get CTS from Abull After that C can start transmission to DFor that it first sends an RTSbull C is not longer exposed to the datatransmission of B
Problem It does not address the collision of RTS packet
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 50
Random Access reliabilitybull Wireless links are prone to have errors High packet loss rate detrimental to
transport-layer performance Mechanisms needed to reduce packet loss rateexperienced by upper layers
bull When node B receives a data packet from node A node B sends anAcknowledgement (Ack) This approach adopted in many protocols
bull If node A fails to receive an Ack it will retransmit the packet
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 51
Random Access MACAW (CSMACA) bull Physical carrier sense and virtual carrier sense using Network Allocation
Vector (NAV)ndash NAV is updated based on overheard RTSCTSDATAACK packets each of whichspecified duration of a pending transmissionndash Nodes stay silent when carrier sensed (physicalvirtual)
bull Backoff intervals used to reduce collision probabilityndash When transmitting a packet choose a backoff interval in the range [0w] where wis contention windowndash Count down the backoff interval when channel is idlendash Count down is suspended if channel becomes busyndash When backoff interval reaches 0 transmit RTS
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 52
Random Access CSMACA (contrsquo) bull The time spent counting down backoff intervals is a part of MAC overhead
ndash Choosing a large w leads to large backoff intervals and can result in larger overheadndash Choosing a small w leads to a larger number of collisions (when two nodes countdown to 0 simultaneously)ndash Since the number of nodes attempting to transmit simultaneously may change withtime some mechanism to manage contention is needed
bull IEEE 80211 DCF contention window w is chosen dynamically depending oncollision occurrence (binary exponential backoff)ndash When a node fails to receive CTS in response to its RTS it increases the contentionwindowndash w is doubled (up to an upper bound)ndash When a node successfully completes a data transfer it restores w to wmin
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 53
Random Access fairnessbull Simplest definition of fairness all nodes should receive equal bandwidthbull An example of unfairness
ndash Assume that initially A and B both choose a backoff interval in range [031] buttheir RTSs collidendash Nodes A and B then choose from range [063]ndash Node A chooses 4 slots and B choose 60 slotsndash After A transmits a packet it next chooses from range [031]ndash It is possible that A may transmit several packets before B transmits its first packet
Ad hoc Sensor Networks Multiple channel access
Unfairness occurs when one node has backed off much more than some other node
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 54
Random Access fairness (contrsquo) MACAW solution for fairnessbull When a node transmits a packet it appends value w to the packet all nodes
hearing value w use it for their future transmission attemptsbull Since w is an indication of the level of congestion in the vicinity of a specific
receiver node MACAW proposes maintaining w independently for eachreceiver
bull Using per-receiver wi is particularly useful in multi-hop environments sincecongestion level at different receivers can be very different
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 55
Random Access energy efficiency bull Since many mobile hosts are operated by batteries MAC
protocols which conserve energy are of interestbull Two approaches to reduce energy consumption
ndash Power save turn off wireless interface when desirablendash Power control reduce transmit power
bull Power control has some more potential benefitsndash Reduced interference (in ad hoc networks) eg it improves ALOHAefficiency where user with high power can capture a packet even if there is acollisionndash Increased spatial reuse (in infrastructured wireless networks) eg it is usedin CDMA to maintain target SIR of voice and data users or generally it canbe used to maintain target SIR for different user classes (admission control)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 56
Random Access power control bull When C transmits to D at a high power level B cannot receive Arsquos
transmission due to interference from Cbull If C reduces transmit power it can still communicate with D
ndash Reduces energy consumption at node Cndash Allows B to receive Arsquos transmission (spatial reuse)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 57
bull A special wireless ad hoc networkndash Large number of nodesndash Battery poweredndash Topology and density changendash Nodes for a common taskndash In-network data processing
bull Sensor-net applicationsndash Sensor-triggered bursty trafficndash Can often tolerate some delay
Ad hoc Sensor Networks Multiple channel access
Characteristics of Sensor Network
Scalabilty and self configurationEnergy efficiencyAdaptivityFairness not importantLatency
AdaptivityEnergy-delay tradoff
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 58
Ad hoc Sensor Networks Multiple channel access
Wireless sensor node and sensor network
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 59
MAC for WSN attributes bull Attributes
ndash Collision avoidance (Basic task)ndash Energy efficiency (most primary in WSN)ndash Scalability and adaptivityndash Channel utilizationndash Latencyndash Throughputndash Fairness
bull What causes energy wastendash Collisionsndash Control packet overheadndash Overhearing unnecessary trafficndash Long idle time (it consumes 50-100 of the power)
Ad hoc Sensor Networks Multiple channel access
Primary
Secondary
Dominant factor
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 60
MAC for WSN
bull Contention-based protocols need to work hard in all directions for energysavingsndash Reduce idle listening ndash support low duty cyclendash Better collision avoidancendash Reduce control overheadndash Avoid unnecessary overhearing
Ad hoc Sensor Networks Multiple channel access
Scheduled Protocols Contention ProtocolsCollisions No YesEnergy efficiency Good BadScalability and adaptivity Bad GoodMulti-hop communication Difficult EasyTime synchronization Strict Loose or not required
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 61
MAC for WSN classification
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 62
MAC for WSN SMAC (2002)bull It has been developed by Ye Heidemann and Estrinbull Tradeoffs between (latency fairness) and energybull Major components in S-MAC
ndash Periodic listen and sleep turn off radio when sleeping the reduced dutycycle is ~ 10 (eg 120ms on 12s off)ndash Collision avoidance is based on contention similarly to IEEE 80211 adhoc modendash Overhearing avoidance the node is sleeping when neighbors talkndash Massage passing long message is fragmented amp sent in burst operationwith extended Tx time and re-transmit occurs immediately
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 63
Summarybull The algorithms and protocols that enable sharing by multiple users and
controls the access to the wireless channel from different users are calledMAC
bull There are two types of wireless networks Infrastructured wireless networksand ad hoc wireless networks
bull We also want our wireless system to have high user capacity and we canidentify 10 methods for increasing it
bull There are several MACs with no absolute advantage it should be tailoredto the system requirements
bull There are primary concerns for designing a WSN MAC (collisionavoidance energy efficiency scalability and adaptivity) while thesecondary concerns are as follows (channel utilization latency throughputfairness)
bull Next lecture Routing protocols
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 15
Ad hoc Sensor Networks Multiple channel access
Multiple Access narrow- vs wideband systemsbull Narrowband System
ndash The bandwidth is small compared to the coherence bandwidth of the channel(BltBcoh)
bull Wideband Systemndash The system bandwidth is much larger that the coherence bandwidth of the
multipath channel (BgtBcoh) A large number of users can access the same channel(frequency band) at the same time
bull Four major multiple access (MA) schemesndash Time Division Multiple Access (TDMA) it could be used in narrowband or
wideband systemndash Frequency Division Multiple Access (FDMA) it is usually used in narrowband
systemndash Code Division Multiple Access (CDMA) it is used in wideband systemndash Space Division Multiple Access (SDMA) for wireless systems with multiple
antennas it can be combined with all of the other MA methods
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 16
Ad hoc Sensor Networks Multiple channel access
Multiple Access cellular standards
Cellular System Multiple Access TechniqueAMPS (2G) FDMAFDD
CT2 Cordless Phone FDMATDD
DECT Cordless Phone FDMATDD
GSM (2G) TDMAFDD
USDC (IS-54 and IS-136) (2G) TDMAFDD
Personal Digital Cellular (2G) TDMAFDD
US IS-95 (2G) CDMAFDD
W-CDMA (3G) CDMAFDD CDMATDD
cdma2000 (3G) CDMAFDD CDMATDD
Narrowband System
Wideband System
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 17
Ad hoc Sensor Networks Multiple channel access
bull Individual radio channels are assigned toindividual users Each user is allocated a Wfrequency band by BS
bull If the channel allocated to a user is idle then itis not used by someone else waste of resource
bull Mobile and base can transmit and receivesimultaneously (FDD)
bull The W bandwidth of FDMA channels arerelatively low Symbol time is usually larger(low data rate) than the delay spread of themultipath channel (implies that inter-symbolinterference is low)
bull Lower complexity systems than TDMAsystems
Multiple Access FDMA
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 18
Ad hoc Sensor Networks Multiple channel access
Multiple Access FDMA (contrsquo)bull Capacity of FDMA systems
Nc = (B-2Bguard)Wwhere B is the total spectrum allocation Bguard is the guard band allocated atthe edge of the spectrum band W is the bandwidth of an individual channel
bull Erlang B and Erlang C system assumesndashinfinite population of sources which jointly offer traffic to Nc channelndashcall attempts arrive following a Poisson process so call arrivals are independentThe average arrival rate is λndashmessage length (holding times) are exponentially distributed (Markovian system)The average call length is hndashthe total amount of traffic offered in erlangs L= λhndashIf a user is rejected his next call attempt is made statistically independent of theprevious attempt (Erlang B)ndashIf all the channels are busy when a request arrives from a user the request isqueued An unlimited number of requests may be held in the queue (Erlang C)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 19
Ad hoc Sensor Networks Multiple channel access
bull Statictical measures of offered load Ncndash In Erlang B system the probability of call
blocking
ndash In Erlang C system the probabilty of waiting
ndash And the average wait time
c
c
cBLOCK
0
Pr
N
Nk
k
LN
Lk
=
=
sum
c
cc
wait 1
c0c
Pr 1
N
NkN
k
LL LL N kN
minus
=
=⎛ ⎞
+ minus⎜ ⎟⎝ ⎠
sum
wait waitc
Pr htN L
=minus
Multiple Access FDMA (contrsquo)Recall from
Queuing theory
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 20
Ad hoc Sensor Networks Multiple channel access
Multiple Access example of Erlang B systembull In an Erlang B system 40 channels are available A blocking probabilty of
less than 1 is required What is the traffic that can be serve if there is oneoperator or three operators
bull (Solution L(1 operator)gtL(3 operators))bull (Note The number of users increases faster than linearly with the number of
available channels The difference between actual increase and linearincrease is called the trunking gain It is preferable to have a large pool ofavailable channels that serves all users (a single operator) The reasons fornot choosing this approach are political not technical)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 21
Ad hoc Sensor Networks Multiple channel access
Multiple Access TDMAbull The allocated radio spectrum for the system is divided into time slots
ndash In each slot a user can transmit or receivendash A user occupies a cyclically repeating slotsndash A channel is logically defined as a particular time slot that repeats with some
periodbull TDMA systems buffer the data until its turn (time slot) comes to transmitbull In TDMATDD half of the slots in the frame is used for uplink channels the
other is used for downlink channelsbull In TDMAFDD a different carrier frequency is used for uplink or downlink
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 22
Ad hoc Sensor Networks Multiple channel access
Multiple Access TDMA (contrsquo)bull Preamble contains address and synchronization info to identify base
station and mobiles to each otherbull Guard times are used to allow synchronization of the receivers between
different slots and framesndash Different mobiles may have different propagation delays to a base station
because of different distances
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 23
Multiple Access TDMA (contrsquo)bull Each frame contains overhead bits and data bits Efficiency of frame is
defined as the percentage of data (information) bits to the total frame size inbits
η =(1 ndash boverheadbtotal) 100 where btotal= Tf R is the total number of bits in a frame Tf is the frameduration in sec boverhead is the number of overhead bits (GSM efficiency 73)
bull TDMA is usually combined with FDMAndash Neighboring cells are allocated and using different carrier frequencies (FDMA)
Inside a cell TDMA can be used Cells may be re-using the same frequency if theyare far from each-other
ndash There may be more than one channel allocated and used inside each cell Eachcarrier frequency (radio channel) may be using TDMA to further multiplex moreuser (ie having TDMA logical channels inside radio channels) For example inGSM each radio channel has 200KHz bandwidth and has 8 time slots (8 logicalchannels) Hence GSM is using FDMA combined with TDMA
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 24
Multiple Access TDMA (contrsquo)
Ad hoc Sensor Networks Multiple channel access
GSM (Europa) IS-54 (USA) PDC (Japan) DECT
Bit Rate 2708 Kbps 486 Kbps 42 Kbps 1152 Mbps
Bandwidth 200 KHz 30 KHz 25 KHz 1728 MHz
Time Slot 0577 ms 67 ms 67 ms 0417 ms
Upstream slots per frame 8 3 3 12
Duplexing FDD FDD FDD TDD
Efficiency 73 80 80 67
Modulation GMSK π4 DQPSK π4 DQPSK GMSK
Adaptive equalized Mandatory Mandatory Optional None
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 25
Multiple Access TDMA (contrsquo)bull Enables the sharing of a single radio channel among N usersbull Requires high data-rate per radio channel to support N users
simultaneously which requires adaptive equalizers to be usedin multipath environments
bull Transmission occurs in bursts (not continually) It enablespower saving by going to sleep modes in unrelated slots Itdiscontinues transmission and also enables mobile assistedhandoff
bull It equires synchronization of the receivers Need guard bits andsync bits which occurs large overhead per slot
bull Allocation of slots to mobile users should not be uniform Itmay depend on the traffic requirement of mobiles This bringsextra flexibility and efficiency compared to FDMA systems
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2620111015 TAacuteMOP ndash 412-082AKMR-2009-0006 26
Ad hoc Sensor Networks Multiple channel access
Multiple Access spread spectrum multiple accessbull SSMA uses signals that have transmission bandwidth that is
several orders of magnitude larger than minimum requiredRF bandwidth
bull It providesndash Immunity to multipath interferencendash Robust multiple access
bull Two techniquesndash Frequency Hopped Multiple Access (FHMA)ndash Direct Sequence Multiple Access (DSMA) or Code Division
Multiple Access (CDMA)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2720111015 TAacuteMOP ndash 412-082AKMR-2009-0006 27
Ad hoc Sensor Networks Multiple channel access
Multiple Access FHMAbull The carrier frequency of users are varied in a pseudo-random
fashionndash Each user is using a narrowband channel (spectrum) at a specific
instance of timendash The random change in frequency make the change of using the same
narrowband channel very lowbull The sender receiver change frequency (calling hopping)
using the same pseudo-random sequence hence they aresynchronized
bull Rate of hopping versus Symbol ratendash If hopping rate is greather Fast Frequency Hoppingndash If symbol rate is greater Slow Frequency Hopping
bull GSM and Bluetooth are example systems
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2820111015 TAacuteMOP ndash 412-082AKMR-2009-0006 28
Ad hoc Sensor Networks Multiple channel access
Multiple Access FHMA (contrsquo)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2920111015 TAacuteMOP ndash 412-082AKMR-2009-0006 29
Ad hoc Sensor Networks Multiple channel access
Multiple Access CDMAbull The narrowband message signal is multiplied by a very large bandwidth
signal called spreading signal (code) before modulation and transmissionover the air This is called spreading
bull Spreading signal use a pseudo-noise (PN) sequence (a pseudo-randomsequence) called codeword which are orthogonal (low autocorrelation)
bull The receiver correlator distinguishes the senders signal by examining thewideband signal with the same time-synchronized spreading code(despreading)
bull Advantagesndash Low power spectral densityndash Interference limited operationndash Privacyndash Reduction of multipath affects by using a larger spectrumndash Users can start their transmission at any time (random access possible)ndash Cell capacity is soft and higher than TDMA and FDMAndash No frequency management and no equalizers and quard time needed
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3020111015 TAacuteMOP ndash 412-082AKMR-2009-0006 30
Ad hoc Sensor Networks Multiple channel access
Multiple Access near far problem and power controlbull At a receiver the signals may come from various sources (multiuser) The
strongest signal usually captures the modulator The other signals areconsidered as noise Each source may have different distances to the basestation
bull In CDMA we want a BS to receive CDMA coded signals from variousmobile users at the same time
ndash Therefore the receiver power at the BS for all mobile users should be close toeach other This requires power control at the mobiles
bull Power Control BS monitors the RSSI (Received Signal Strength Indicator)values from different mobiles and then sends power change commands to themobiles over a forward channel The mobiles then adjust their transmitpower
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3120111015 TAacuteMOP ndash 412-082AKMR-2009-0006 31
Ad hoc Sensor Networks Multiple channel access
Multiple Access hybrid spread spectrum techniquesbull FDMACDMA
ndash Available wideband spectrum is divided into a number narrowband radio channels CDMAis employed inside each channel
bull DSFHMAndash The signals are spread using spreading codes (direct sequence signals are obtained) but
these signal are not transmitted over a constant carrier frequency they are transmitted over afrequency hopping carrier frequency
bull Time Division CDMA (TCDMA)ndash Each cell is using a different spreading code (CDMA employed between cells) that is
conveyed to the mobiles in its rangendash Inside each cell (inside a CDMA channel) TDMA is employed to multiplex multiple users
bull Time Division Frequency Hoppingndash At each time slot the user is hopped to a new frequency according to a pseudo-random
hopping sequencendash Employed in severe co-interference and multi-path environmentsndash Bluetooth and GSM are using this technique
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3220111015 TAacuteMOP ndash 412-082AKMR-2009-0006 32
Ad hoc Sensor Networks Multiple channel access
Multiple Access comparison TDMAFDMACDMA
Techniques TDMA FDMA CDMA SDMA
Idea
segment sending time into disjoint time-slots
Segment the frequency band into disjoint sub-bands
Spread the spectrum using orthogonal codes
Segment space into sectors
Signal separation
Synchronization in the time domain
Filtering in the frequency domain
Code Directed antennas
Advantage
Established flexible
Simple established robust
Flexible less frequency planning needed soft handover
Simple increases capacity
DisadvantageQuard space synchronization difficult
Inflexible scarce source
Complex receiver power control
inflexible
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3320111015 TAacuteMOP ndash 412-082AKMR-2009-0006 33
Ad hoc Sensor Networks Multiple channel access
User capacity per cell
The user capacity C is defined as the total number of active users percell that the system can support while meeting a common BERconstrain For orthogonal multiple access (as FDMA and TDMA)
C = Ncellwhere Ncell is the number of channels assigned to any given cellThe total number of orthogonal channels of bandwidth Bs that can becreated from a total system bandwidth of B is B Bs The reuse factorsatisfies N = (B Bs) Ncell this implies
C = (B Bs) N
Recall from Chapter 1
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3420111015 TAacuteMOP ndash 412-082AKMR-2009-0006 34
Ad hoc Sensor Networks Multiple channel access
10 methods for increasing capacity1 Increasing the amount of spectrum used very expensive2 More efficient modulation format and coding3 Better source coding4 Adaptive modulation and coding5 Discontinuous transmission6 Multiuser detection CDMA systems7 Reduction of cell radius effective but very expensive smaller
cells require more handovers8 Use of sector cells tripled cells have tripled BS antennas9 Use of an overlay structure10 Multiple antennas diversity MIMO systems SDMA
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3520111015 TAacuteMOP ndash 412-082AKMR-2009-0006 35
Ad hoc Sensor Networks Multiple channel access
bull Distributed operation is requiredbull Synchronization is required in TDMA-based systemsbull Hidden terminals are nodes hidden from a senderbull Exposed terminals are exposed nodes preventing a sender
from sendingbull Throughput needs to be maximizedbull Access delay should be minimizedbull Fairness refers to provide an equal share to all competing
nodesbull Real-time traffic support is required for voice video and
real-time databull Resource reservation is required for QoSbull Ability to measure resource availability handles the
resourcesbull Capability for power control reduces the energy
consumption
Random access in adhoc networks
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3620111015 TAacuteMOP ndash 412-082AKMR-2009-0006 36
Ad hoc Sensor Networks Multiple channel access
Random accessbull All random access techniques are based on packetized data or packet radio where user
date is collected into packets of M bitsbull Collision if packets from different users overlap in timebull The transmission time of a packet is τ = M R where R [bps] the data ratebull Analysis of random access techniques assumes that the users generate packets according
to memoryless Poisson process at a rate λ [packets per unit time] the probability that thenumber of packet arrivals in a time period [0t] denoted by X(t) is equal to integer k isgiven by
Pr(X(t)=k) = (λt)k k e-λt
bull The traffic load is defined as L = λ τ If L gt 1 means on avarage more packets arrive inthe system over a given time period than can be transmitted in that period so systemswith Lgt1 are unstable
bull The throughput is defined as the ratio of the average number of packets successfullytransmitted in any given time interval divided by the number of attempted transmissionsin that interval
T = L Pr(succesful packet transmission)bull The effective data rate of the system is RT
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3720111015 TAacuteMOP ndash 412-082AKMR-2009-0006 37
Ad hoc Sensor Networks Multiple channel access
Random access (contrsquo)bull We will examine the following attributes
ndash Channel utilization (Throughput)ndash Latencyndash Collision avoidance (Hidden and exposed terminals problem)ndash Reliability (ACK)ndash Energy efficiency (power control)ndash Fairnessndash Throughput needs to be maximized
bull We will discuss the following techniquesndash Aloha and slotted Alohandash CSMA Protocols (1-persistent non-persistent p-persistent CSMA CSMACD)ndash MACAndash MACAWndash Energy efficient MAC in wireless sensor networks (eg SMAC)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 38
Random Access ALOHA (1970)bull ALOHA was developed for a wireless system at the University of Hawaii
(Abramson et al)ndash Multiple remote stations plus one base stationndash Frame transmissions are made at one frequency from a remote station to the basestation the base station re-broadcasts frames on another frequency Contention is foraccess to the base station
bull Frames can be sent at any time to the base stationndash If no acknowledgment (ACK) is received assume that the frame is lost andretransmit after waiting a random amount of timendash If more than one station broadcasts on the base station access frequency at the sametime ndash a collision ndash interference will destroy the frames
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 39
Random Access ALOHA (contrsquo)
bull The probability that no packets generated during the tima [-τ τ] is given by (nocollision)
Pr(X(t)=0) = (λ2τ)0 0 e-2λτ = e-2Lwith corresponding throughput
T = L e-2L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 05 05e 018
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 40
Random Access slotted ALOHA (1971)bull Improvement Time is slotted and a packet can only be transmitted at the beginning of
one slot Thus it can reduce the collision duration
bull The probability that no packets generated during the time [0 τ] is given by (nocollision)
Pr(X(t)=0) = (λτ)0 0 e-λτ = e-Lwith corresponding throughput T = L e-L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 1 e 037
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 41
bull Slotted Aloha has double the maximumthroughput as pure Aloha and achievesthis maximum at a higher offered loadThe effective data rate is still less than40 of the raw transmission rate
bull Aloha does not listen to the carrier beforetransmission more sophisticatedtechniques are needed to increaseefficiency
bull Further problem the delayndashDelay
ndashThe mean packet delay
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
( ) ( )delayPr 1 e ekL Lkτ minus minus= = minus
( )delay1
1 e e ekL L L
kMPD E kτ
infinminus minus
=
= = minus =sum
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 42
Random Access Carrier Sense Multiple Accessbull CSMA (Carrier Sense Multiple Access) listen to the carrier before transmission
and transmits if channel is idlebull Detection delay and propagation delay are two important parameters
ndash Detection delay time required to sense the carrier and decide if it is idle or busyndash Propagation delay distancespeed_of_ligth The time required for bit to travel
from transmitter to the receiver
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 43
Random Access CSMA variations
Ad hoc Sensor Networks Multiple channel access
1-persistent CSMA Non-persistent CSMA
There will always be a collision iftwo nodes want to retransmit(usually you stop transmissionattempts after few tries)
Random backoff reduces probabilityof collisions Waste idle time if thebackoff time is too long
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 44
Random Access CSMA variations (contrsquo)
Ad hoc Sensor Networks Multiple channel access
p-persistent CSMA CSMA CD (Collision Detection)
A good tradeoff between non-persistent and 1-persistent CSMA
Same with CSMA however a stationalso listen to the carrier whiletransmitting to see if the transmissioncollides with someone elsetransmission It can be used in listen-while-talk capable channels (fullduplex) In single radio channels thetransmission need to be interrupted inorder to sense the channel
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 45
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 46
Random Access CSMA (contrsquo)bull How to select probability p
ndash Assume that M nodes have a packet to send and the medium is busy then Mp is theexpected number of nodes that will attempt to transmit once the medium becomesidle If Mp gt 1 then a collision is expected to occur Therefore the network mustmake sure that Mp lt 1 to avoid collision where M is the maximum number of nodesthat can be active at a time
bull Problems1 The mean packet delay increases exponently with increasing offered load2 CSMA protocols sense the carrier but sensing the carrier does not always releasestrue information about the status of the wireless channel There are two problems thatare unique to wireless channels (different than wireline channels)ndash Hidden terminal problemndash Exposed terminal problem
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 47
Hidden terminal problembull A sends to B C cannot receive Abull C wants to send to B C senses a
ldquofreerdquo medium (CS fails)bull collision at B A cannot receive the
collision (CD fails)bull A is ldquohiddenrdquo for C
Ad hoc Sensor Networks Multiple channel access
Random Access hidden and exposed terminal problemExposed terminal problem
bull B sends to A C wants to send to Dbull C has to wait CS signals a medium
in usebull since A is outside the radio range of
C waiting is not necessarybull C is ldquoexposedrdquo to B
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 48
Random Access MACAWhen a station wants to transmit databull It sends an RTS (Ready-to-Send) packet to the intended receiver
ndash The RTS packet contains the length of the data that needs to be transmittedndash Any station other than the intended recipient hearing RTS defers transmission for atime duration equal to the end of the corresponding CTS reception
bull The receiver sends back CTS (Clear-to-Send) packet back to sender if it isavailable to receivendash The CTS packet contains the length of the data that original sender wants totransmitndash Any station other than the original RTS sender hearing CTS defers transmissionuntil the data is sent
bull The original sender upon reception of the CTS starts transmitting
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 49
Hidden terminal problem
Ad hoc Sensor Networks Multiple channel access
Random Access MACA (contrsquo)Exposed terminal problem
Waiting time of node X is muchsmaller than waiting time of node C
bull C defers transmission upon hearing BrsquosRTS until B could get CTS from Abull After that C can start transmission to DFor that it first sends an RTSbull C is not longer exposed to the datatransmission of B
Problem It does not address the collision of RTS packet
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 50
Random Access reliabilitybull Wireless links are prone to have errors High packet loss rate detrimental to
transport-layer performance Mechanisms needed to reduce packet loss rateexperienced by upper layers
bull When node B receives a data packet from node A node B sends anAcknowledgement (Ack) This approach adopted in many protocols
bull If node A fails to receive an Ack it will retransmit the packet
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 51
Random Access MACAW (CSMACA) bull Physical carrier sense and virtual carrier sense using Network Allocation
Vector (NAV)ndash NAV is updated based on overheard RTSCTSDATAACK packets each of whichspecified duration of a pending transmissionndash Nodes stay silent when carrier sensed (physicalvirtual)
bull Backoff intervals used to reduce collision probabilityndash When transmitting a packet choose a backoff interval in the range [0w] where wis contention windowndash Count down the backoff interval when channel is idlendash Count down is suspended if channel becomes busyndash When backoff interval reaches 0 transmit RTS
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 52
Random Access CSMACA (contrsquo) bull The time spent counting down backoff intervals is a part of MAC overhead
ndash Choosing a large w leads to large backoff intervals and can result in larger overheadndash Choosing a small w leads to a larger number of collisions (when two nodes countdown to 0 simultaneously)ndash Since the number of nodes attempting to transmit simultaneously may change withtime some mechanism to manage contention is needed
bull IEEE 80211 DCF contention window w is chosen dynamically depending oncollision occurrence (binary exponential backoff)ndash When a node fails to receive CTS in response to its RTS it increases the contentionwindowndash w is doubled (up to an upper bound)ndash When a node successfully completes a data transfer it restores w to wmin
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 53
Random Access fairnessbull Simplest definition of fairness all nodes should receive equal bandwidthbull An example of unfairness
ndash Assume that initially A and B both choose a backoff interval in range [031] buttheir RTSs collidendash Nodes A and B then choose from range [063]ndash Node A chooses 4 slots and B choose 60 slotsndash After A transmits a packet it next chooses from range [031]ndash It is possible that A may transmit several packets before B transmits its first packet
Ad hoc Sensor Networks Multiple channel access
Unfairness occurs when one node has backed off much more than some other node
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 54
Random Access fairness (contrsquo) MACAW solution for fairnessbull When a node transmits a packet it appends value w to the packet all nodes
hearing value w use it for their future transmission attemptsbull Since w is an indication of the level of congestion in the vicinity of a specific
receiver node MACAW proposes maintaining w independently for eachreceiver
bull Using per-receiver wi is particularly useful in multi-hop environments sincecongestion level at different receivers can be very different
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 55
Random Access energy efficiency bull Since many mobile hosts are operated by batteries MAC
protocols which conserve energy are of interestbull Two approaches to reduce energy consumption
ndash Power save turn off wireless interface when desirablendash Power control reduce transmit power
bull Power control has some more potential benefitsndash Reduced interference (in ad hoc networks) eg it improves ALOHAefficiency where user with high power can capture a packet even if there is acollisionndash Increased spatial reuse (in infrastructured wireless networks) eg it is usedin CDMA to maintain target SIR of voice and data users or generally it canbe used to maintain target SIR for different user classes (admission control)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 56
Random Access power control bull When C transmits to D at a high power level B cannot receive Arsquos
transmission due to interference from Cbull If C reduces transmit power it can still communicate with D
ndash Reduces energy consumption at node Cndash Allows B to receive Arsquos transmission (spatial reuse)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 57
bull A special wireless ad hoc networkndash Large number of nodesndash Battery poweredndash Topology and density changendash Nodes for a common taskndash In-network data processing
bull Sensor-net applicationsndash Sensor-triggered bursty trafficndash Can often tolerate some delay
Ad hoc Sensor Networks Multiple channel access
Characteristics of Sensor Network
Scalabilty and self configurationEnergy efficiencyAdaptivityFairness not importantLatency
AdaptivityEnergy-delay tradoff
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 58
Ad hoc Sensor Networks Multiple channel access
Wireless sensor node and sensor network
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 59
MAC for WSN attributes bull Attributes
ndash Collision avoidance (Basic task)ndash Energy efficiency (most primary in WSN)ndash Scalability and adaptivityndash Channel utilizationndash Latencyndash Throughputndash Fairness
bull What causes energy wastendash Collisionsndash Control packet overheadndash Overhearing unnecessary trafficndash Long idle time (it consumes 50-100 of the power)
Ad hoc Sensor Networks Multiple channel access
Primary
Secondary
Dominant factor
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 60
MAC for WSN
bull Contention-based protocols need to work hard in all directions for energysavingsndash Reduce idle listening ndash support low duty cyclendash Better collision avoidancendash Reduce control overheadndash Avoid unnecessary overhearing
Ad hoc Sensor Networks Multiple channel access
Scheduled Protocols Contention ProtocolsCollisions No YesEnergy efficiency Good BadScalability and adaptivity Bad GoodMulti-hop communication Difficult EasyTime synchronization Strict Loose or not required
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 61
MAC for WSN classification
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 62
MAC for WSN SMAC (2002)bull It has been developed by Ye Heidemann and Estrinbull Tradeoffs between (latency fairness) and energybull Major components in S-MAC
ndash Periodic listen and sleep turn off radio when sleeping the reduced dutycycle is ~ 10 (eg 120ms on 12s off)ndash Collision avoidance is based on contention similarly to IEEE 80211 adhoc modendash Overhearing avoidance the node is sleeping when neighbors talkndash Massage passing long message is fragmented amp sent in burst operationwith extended Tx time and re-transmit occurs immediately
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 63
Summarybull The algorithms and protocols that enable sharing by multiple users and
controls the access to the wireless channel from different users are calledMAC
bull There are two types of wireless networks Infrastructured wireless networksand ad hoc wireless networks
bull We also want our wireless system to have high user capacity and we canidentify 10 methods for increasing it
bull There are several MACs with no absolute advantage it should be tailoredto the system requirements
bull There are primary concerns for designing a WSN MAC (collisionavoidance energy efficiency scalability and adaptivity) while thesecondary concerns are as follows (channel utilization latency throughputfairness)
bull Next lecture Routing protocols
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 16
Ad hoc Sensor Networks Multiple channel access
Multiple Access cellular standards
Cellular System Multiple Access TechniqueAMPS (2G) FDMAFDD
CT2 Cordless Phone FDMATDD
DECT Cordless Phone FDMATDD
GSM (2G) TDMAFDD
USDC (IS-54 and IS-136) (2G) TDMAFDD
Personal Digital Cellular (2G) TDMAFDD
US IS-95 (2G) CDMAFDD
W-CDMA (3G) CDMAFDD CDMATDD
cdma2000 (3G) CDMAFDD CDMATDD
Narrowband System
Wideband System
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 17
Ad hoc Sensor Networks Multiple channel access
bull Individual radio channels are assigned toindividual users Each user is allocated a Wfrequency band by BS
bull If the channel allocated to a user is idle then itis not used by someone else waste of resource
bull Mobile and base can transmit and receivesimultaneously (FDD)
bull The W bandwidth of FDMA channels arerelatively low Symbol time is usually larger(low data rate) than the delay spread of themultipath channel (implies that inter-symbolinterference is low)
bull Lower complexity systems than TDMAsystems
Multiple Access FDMA
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 18
Ad hoc Sensor Networks Multiple channel access
Multiple Access FDMA (contrsquo)bull Capacity of FDMA systems
Nc = (B-2Bguard)Wwhere B is the total spectrum allocation Bguard is the guard band allocated atthe edge of the spectrum band W is the bandwidth of an individual channel
bull Erlang B and Erlang C system assumesndashinfinite population of sources which jointly offer traffic to Nc channelndashcall attempts arrive following a Poisson process so call arrivals are independentThe average arrival rate is λndashmessage length (holding times) are exponentially distributed (Markovian system)The average call length is hndashthe total amount of traffic offered in erlangs L= λhndashIf a user is rejected his next call attempt is made statistically independent of theprevious attempt (Erlang B)ndashIf all the channels are busy when a request arrives from a user the request isqueued An unlimited number of requests may be held in the queue (Erlang C)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 19
Ad hoc Sensor Networks Multiple channel access
bull Statictical measures of offered load Ncndash In Erlang B system the probability of call
blocking
ndash In Erlang C system the probabilty of waiting
ndash And the average wait time
c
c
cBLOCK
0
Pr
N
Nk
k
LN
Lk
=
=
sum
c
cc
wait 1
c0c
Pr 1
N
NkN
k
LL LL N kN
minus
=
=⎛ ⎞
+ minus⎜ ⎟⎝ ⎠
sum
wait waitc
Pr htN L
=minus
Multiple Access FDMA (contrsquo)Recall from
Queuing theory
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 20
Ad hoc Sensor Networks Multiple channel access
Multiple Access example of Erlang B systembull In an Erlang B system 40 channels are available A blocking probabilty of
less than 1 is required What is the traffic that can be serve if there is oneoperator or three operators
bull (Solution L(1 operator)gtL(3 operators))bull (Note The number of users increases faster than linearly with the number of
available channels The difference between actual increase and linearincrease is called the trunking gain It is preferable to have a large pool ofavailable channels that serves all users (a single operator) The reasons fornot choosing this approach are political not technical)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 21
Ad hoc Sensor Networks Multiple channel access
Multiple Access TDMAbull The allocated radio spectrum for the system is divided into time slots
ndash In each slot a user can transmit or receivendash A user occupies a cyclically repeating slotsndash A channel is logically defined as a particular time slot that repeats with some
periodbull TDMA systems buffer the data until its turn (time slot) comes to transmitbull In TDMATDD half of the slots in the frame is used for uplink channels the
other is used for downlink channelsbull In TDMAFDD a different carrier frequency is used for uplink or downlink
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 22
Ad hoc Sensor Networks Multiple channel access
Multiple Access TDMA (contrsquo)bull Preamble contains address and synchronization info to identify base
station and mobiles to each otherbull Guard times are used to allow synchronization of the receivers between
different slots and framesndash Different mobiles may have different propagation delays to a base station
because of different distances
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 23
Multiple Access TDMA (contrsquo)bull Each frame contains overhead bits and data bits Efficiency of frame is
defined as the percentage of data (information) bits to the total frame size inbits
η =(1 ndash boverheadbtotal) 100 where btotal= Tf R is the total number of bits in a frame Tf is the frameduration in sec boverhead is the number of overhead bits (GSM efficiency 73)
bull TDMA is usually combined with FDMAndash Neighboring cells are allocated and using different carrier frequencies (FDMA)
Inside a cell TDMA can be used Cells may be re-using the same frequency if theyare far from each-other
ndash There may be more than one channel allocated and used inside each cell Eachcarrier frequency (radio channel) may be using TDMA to further multiplex moreuser (ie having TDMA logical channels inside radio channels) For example inGSM each radio channel has 200KHz bandwidth and has 8 time slots (8 logicalchannels) Hence GSM is using FDMA combined with TDMA
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 24
Multiple Access TDMA (contrsquo)
Ad hoc Sensor Networks Multiple channel access
GSM (Europa) IS-54 (USA) PDC (Japan) DECT
Bit Rate 2708 Kbps 486 Kbps 42 Kbps 1152 Mbps
Bandwidth 200 KHz 30 KHz 25 KHz 1728 MHz
Time Slot 0577 ms 67 ms 67 ms 0417 ms
Upstream slots per frame 8 3 3 12
Duplexing FDD FDD FDD TDD
Efficiency 73 80 80 67
Modulation GMSK π4 DQPSK π4 DQPSK GMSK
Adaptive equalized Mandatory Mandatory Optional None
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 25
Multiple Access TDMA (contrsquo)bull Enables the sharing of a single radio channel among N usersbull Requires high data-rate per radio channel to support N users
simultaneously which requires adaptive equalizers to be usedin multipath environments
bull Transmission occurs in bursts (not continually) It enablespower saving by going to sleep modes in unrelated slots Itdiscontinues transmission and also enables mobile assistedhandoff
bull It equires synchronization of the receivers Need guard bits andsync bits which occurs large overhead per slot
bull Allocation of slots to mobile users should not be uniform Itmay depend on the traffic requirement of mobiles This bringsextra flexibility and efficiency compared to FDMA systems
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2620111015 TAacuteMOP ndash 412-082AKMR-2009-0006 26
Ad hoc Sensor Networks Multiple channel access
Multiple Access spread spectrum multiple accessbull SSMA uses signals that have transmission bandwidth that is
several orders of magnitude larger than minimum requiredRF bandwidth
bull It providesndash Immunity to multipath interferencendash Robust multiple access
bull Two techniquesndash Frequency Hopped Multiple Access (FHMA)ndash Direct Sequence Multiple Access (DSMA) or Code Division
Multiple Access (CDMA)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2720111015 TAacuteMOP ndash 412-082AKMR-2009-0006 27
Ad hoc Sensor Networks Multiple channel access
Multiple Access FHMAbull The carrier frequency of users are varied in a pseudo-random
fashionndash Each user is using a narrowband channel (spectrum) at a specific
instance of timendash The random change in frequency make the change of using the same
narrowband channel very lowbull The sender receiver change frequency (calling hopping)
using the same pseudo-random sequence hence they aresynchronized
bull Rate of hopping versus Symbol ratendash If hopping rate is greather Fast Frequency Hoppingndash If symbol rate is greater Slow Frequency Hopping
bull GSM and Bluetooth are example systems
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2820111015 TAacuteMOP ndash 412-082AKMR-2009-0006 28
Ad hoc Sensor Networks Multiple channel access
Multiple Access FHMA (contrsquo)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2920111015 TAacuteMOP ndash 412-082AKMR-2009-0006 29
Ad hoc Sensor Networks Multiple channel access
Multiple Access CDMAbull The narrowband message signal is multiplied by a very large bandwidth
signal called spreading signal (code) before modulation and transmissionover the air This is called spreading
bull Spreading signal use a pseudo-noise (PN) sequence (a pseudo-randomsequence) called codeword which are orthogonal (low autocorrelation)
bull The receiver correlator distinguishes the senders signal by examining thewideband signal with the same time-synchronized spreading code(despreading)
bull Advantagesndash Low power spectral densityndash Interference limited operationndash Privacyndash Reduction of multipath affects by using a larger spectrumndash Users can start their transmission at any time (random access possible)ndash Cell capacity is soft and higher than TDMA and FDMAndash No frequency management and no equalizers and quard time needed
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3020111015 TAacuteMOP ndash 412-082AKMR-2009-0006 30
Ad hoc Sensor Networks Multiple channel access
Multiple Access near far problem and power controlbull At a receiver the signals may come from various sources (multiuser) The
strongest signal usually captures the modulator The other signals areconsidered as noise Each source may have different distances to the basestation
bull In CDMA we want a BS to receive CDMA coded signals from variousmobile users at the same time
ndash Therefore the receiver power at the BS for all mobile users should be close toeach other This requires power control at the mobiles
bull Power Control BS monitors the RSSI (Received Signal Strength Indicator)values from different mobiles and then sends power change commands to themobiles over a forward channel The mobiles then adjust their transmitpower
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3120111015 TAacuteMOP ndash 412-082AKMR-2009-0006 31
Ad hoc Sensor Networks Multiple channel access
Multiple Access hybrid spread spectrum techniquesbull FDMACDMA
ndash Available wideband spectrum is divided into a number narrowband radio channels CDMAis employed inside each channel
bull DSFHMAndash The signals are spread using spreading codes (direct sequence signals are obtained) but
these signal are not transmitted over a constant carrier frequency they are transmitted over afrequency hopping carrier frequency
bull Time Division CDMA (TCDMA)ndash Each cell is using a different spreading code (CDMA employed between cells) that is
conveyed to the mobiles in its rangendash Inside each cell (inside a CDMA channel) TDMA is employed to multiplex multiple users
bull Time Division Frequency Hoppingndash At each time slot the user is hopped to a new frequency according to a pseudo-random
hopping sequencendash Employed in severe co-interference and multi-path environmentsndash Bluetooth and GSM are using this technique
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3220111015 TAacuteMOP ndash 412-082AKMR-2009-0006 32
Ad hoc Sensor Networks Multiple channel access
Multiple Access comparison TDMAFDMACDMA
Techniques TDMA FDMA CDMA SDMA
Idea
segment sending time into disjoint time-slots
Segment the frequency band into disjoint sub-bands
Spread the spectrum using orthogonal codes
Segment space into sectors
Signal separation
Synchronization in the time domain
Filtering in the frequency domain
Code Directed antennas
Advantage
Established flexible
Simple established robust
Flexible less frequency planning needed soft handover
Simple increases capacity
DisadvantageQuard space synchronization difficult
Inflexible scarce source
Complex receiver power control
inflexible
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3320111015 TAacuteMOP ndash 412-082AKMR-2009-0006 33
Ad hoc Sensor Networks Multiple channel access
User capacity per cell
The user capacity C is defined as the total number of active users percell that the system can support while meeting a common BERconstrain For orthogonal multiple access (as FDMA and TDMA)
C = Ncellwhere Ncell is the number of channels assigned to any given cellThe total number of orthogonal channels of bandwidth Bs that can becreated from a total system bandwidth of B is B Bs The reuse factorsatisfies N = (B Bs) Ncell this implies
C = (B Bs) N
Recall from Chapter 1
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3420111015 TAacuteMOP ndash 412-082AKMR-2009-0006 34
Ad hoc Sensor Networks Multiple channel access
10 methods for increasing capacity1 Increasing the amount of spectrum used very expensive2 More efficient modulation format and coding3 Better source coding4 Adaptive modulation and coding5 Discontinuous transmission6 Multiuser detection CDMA systems7 Reduction of cell radius effective but very expensive smaller
cells require more handovers8 Use of sector cells tripled cells have tripled BS antennas9 Use of an overlay structure10 Multiple antennas diversity MIMO systems SDMA
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3520111015 TAacuteMOP ndash 412-082AKMR-2009-0006 35
Ad hoc Sensor Networks Multiple channel access
bull Distributed operation is requiredbull Synchronization is required in TDMA-based systemsbull Hidden terminals are nodes hidden from a senderbull Exposed terminals are exposed nodes preventing a sender
from sendingbull Throughput needs to be maximizedbull Access delay should be minimizedbull Fairness refers to provide an equal share to all competing
nodesbull Real-time traffic support is required for voice video and
real-time databull Resource reservation is required for QoSbull Ability to measure resource availability handles the
resourcesbull Capability for power control reduces the energy
consumption
Random access in adhoc networks
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3620111015 TAacuteMOP ndash 412-082AKMR-2009-0006 36
Ad hoc Sensor Networks Multiple channel access
Random accessbull All random access techniques are based on packetized data or packet radio where user
date is collected into packets of M bitsbull Collision if packets from different users overlap in timebull The transmission time of a packet is τ = M R where R [bps] the data ratebull Analysis of random access techniques assumes that the users generate packets according
to memoryless Poisson process at a rate λ [packets per unit time] the probability that thenumber of packet arrivals in a time period [0t] denoted by X(t) is equal to integer k isgiven by
Pr(X(t)=k) = (λt)k k e-λt
bull The traffic load is defined as L = λ τ If L gt 1 means on avarage more packets arrive inthe system over a given time period than can be transmitted in that period so systemswith Lgt1 are unstable
bull The throughput is defined as the ratio of the average number of packets successfullytransmitted in any given time interval divided by the number of attempted transmissionsin that interval
T = L Pr(succesful packet transmission)bull The effective data rate of the system is RT
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3720111015 TAacuteMOP ndash 412-082AKMR-2009-0006 37
Ad hoc Sensor Networks Multiple channel access
Random access (contrsquo)bull We will examine the following attributes
ndash Channel utilization (Throughput)ndash Latencyndash Collision avoidance (Hidden and exposed terminals problem)ndash Reliability (ACK)ndash Energy efficiency (power control)ndash Fairnessndash Throughput needs to be maximized
bull We will discuss the following techniquesndash Aloha and slotted Alohandash CSMA Protocols (1-persistent non-persistent p-persistent CSMA CSMACD)ndash MACAndash MACAWndash Energy efficient MAC in wireless sensor networks (eg SMAC)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 38
Random Access ALOHA (1970)bull ALOHA was developed for a wireless system at the University of Hawaii
(Abramson et al)ndash Multiple remote stations plus one base stationndash Frame transmissions are made at one frequency from a remote station to the basestation the base station re-broadcasts frames on another frequency Contention is foraccess to the base station
bull Frames can be sent at any time to the base stationndash If no acknowledgment (ACK) is received assume that the frame is lost andretransmit after waiting a random amount of timendash If more than one station broadcasts on the base station access frequency at the sametime ndash a collision ndash interference will destroy the frames
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 39
Random Access ALOHA (contrsquo)
bull The probability that no packets generated during the tima [-τ τ] is given by (nocollision)
Pr(X(t)=0) = (λ2τ)0 0 e-2λτ = e-2Lwith corresponding throughput
T = L e-2L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 05 05e 018
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 40
Random Access slotted ALOHA (1971)bull Improvement Time is slotted and a packet can only be transmitted at the beginning of
one slot Thus it can reduce the collision duration
bull The probability that no packets generated during the time [0 τ] is given by (nocollision)
Pr(X(t)=0) = (λτ)0 0 e-λτ = e-Lwith corresponding throughput T = L e-L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 1 e 037
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 41
bull Slotted Aloha has double the maximumthroughput as pure Aloha and achievesthis maximum at a higher offered loadThe effective data rate is still less than40 of the raw transmission rate
bull Aloha does not listen to the carrier beforetransmission more sophisticatedtechniques are needed to increaseefficiency
bull Further problem the delayndashDelay
ndashThe mean packet delay
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
( ) ( )delayPr 1 e ekL Lkτ minus minus= = minus
( )delay1
1 e e ekL L L
kMPD E kτ
infinminus minus
=
= = minus =sum
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 42
Random Access Carrier Sense Multiple Accessbull CSMA (Carrier Sense Multiple Access) listen to the carrier before transmission
and transmits if channel is idlebull Detection delay and propagation delay are two important parameters
ndash Detection delay time required to sense the carrier and decide if it is idle or busyndash Propagation delay distancespeed_of_ligth The time required for bit to travel
from transmitter to the receiver
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 43
Random Access CSMA variations
Ad hoc Sensor Networks Multiple channel access
1-persistent CSMA Non-persistent CSMA
There will always be a collision iftwo nodes want to retransmit(usually you stop transmissionattempts after few tries)
Random backoff reduces probabilityof collisions Waste idle time if thebackoff time is too long
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 44
Random Access CSMA variations (contrsquo)
Ad hoc Sensor Networks Multiple channel access
p-persistent CSMA CSMA CD (Collision Detection)
A good tradeoff between non-persistent and 1-persistent CSMA
Same with CSMA however a stationalso listen to the carrier whiletransmitting to see if the transmissioncollides with someone elsetransmission It can be used in listen-while-talk capable channels (fullduplex) In single radio channels thetransmission need to be interrupted inorder to sense the channel
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 45
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 46
Random Access CSMA (contrsquo)bull How to select probability p
ndash Assume that M nodes have a packet to send and the medium is busy then Mp is theexpected number of nodes that will attempt to transmit once the medium becomesidle If Mp gt 1 then a collision is expected to occur Therefore the network mustmake sure that Mp lt 1 to avoid collision where M is the maximum number of nodesthat can be active at a time
bull Problems1 The mean packet delay increases exponently with increasing offered load2 CSMA protocols sense the carrier but sensing the carrier does not always releasestrue information about the status of the wireless channel There are two problems thatare unique to wireless channels (different than wireline channels)ndash Hidden terminal problemndash Exposed terminal problem
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 47
Hidden terminal problembull A sends to B C cannot receive Abull C wants to send to B C senses a
ldquofreerdquo medium (CS fails)bull collision at B A cannot receive the
collision (CD fails)bull A is ldquohiddenrdquo for C
Ad hoc Sensor Networks Multiple channel access
Random Access hidden and exposed terminal problemExposed terminal problem
bull B sends to A C wants to send to Dbull C has to wait CS signals a medium
in usebull since A is outside the radio range of
C waiting is not necessarybull C is ldquoexposedrdquo to B
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 48
Random Access MACAWhen a station wants to transmit databull It sends an RTS (Ready-to-Send) packet to the intended receiver
ndash The RTS packet contains the length of the data that needs to be transmittedndash Any station other than the intended recipient hearing RTS defers transmission for atime duration equal to the end of the corresponding CTS reception
bull The receiver sends back CTS (Clear-to-Send) packet back to sender if it isavailable to receivendash The CTS packet contains the length of the data that original sender wants totransmitndash Any station other than the original RTS sender hearing CTS defers transmissionuntil the data is sent
bull The original sender upon reception of the CTS starts transmitting
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 49
Hidden terminal problem
Ad hoc Sensor Networks Multiple channel access
Random Access MACA (contrsquo)Exposed terminal problem
Waiting time of node X is muchsmaller than waiting time of node C
bull C defers transmission upon hearing BrsquosRTS until B could get CTS from Abull After that C can start transmission to DFor that it first sends an RTSbull C is not longer exposed to the datatransmission of B
Problem It does not address the collision of RTS packet
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 50
Random Access reliabilitybull Wireless links are prone to have errors High packet loss rate detrimental to
transport-layer performance Mechanisms needed to reduce packet loss rateexperienced by upper layers
bull When node B receives a data packet from node A node B sends anAcknowledgement (Ack) This approach adopted in many protocols
bull If node A fails to receive an Ack it will retransmit the packet
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 51
Random Access MACAW (CSMACA) bull Physical carrier sense and virtual carrier sense using Network Allocation
Vector (NAV)ndash NAV is updated based on overheard RTSCTSDATAACK packets each of whichspecified duration of a pending transmissionndash Nodes stay silent when carrier sensed (physicalvirtual)
bull Backoff intervals used to reduce collision probabilityndash When transmitting a packet choose a backoff interval in the range [0w] where wis contention windowndash Count down the backoff interval when channel is idlendash Count down is suspended if channel becomes busyndash When backoff interval reaches 0 transmit RTS
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 52
Random Access CSMACA (contrsquo) bull The time spent counting down backoff intervals is a part of MAC overhead
ndash Choosing a large w leads to large backoff intervals and can result in larger overheadndash Choosing a small w leads to a larger number of collisions (when two nodes countdown to 0 simultaneously)ndash Since the number of nodes attempting to transmit simultaneously may change withtime some mechanism to manage contention is needed
bull IEEE 80211 DCF contention window w is chosen dynamically depending oncollision occurrence (binary exponential backoff)ndash When a node fails to receive CTS in response to its RTS it increases the contentionwindowndash w is doubled (up to an upper bound)ndash When a node successfully completes a data transfer it restores w to wmin
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 53
Random Access fairnessbull Simplest definition of fairness all nodes should receive equal bandwidthbull An example of unfairness
ndash Assume that initially A and B both choose a backoff interval in range [031] buttheir RTSs collidendash Nodes A and B then choose from range [063]ndash Node A chooses 4 slots and B choose 60 slotsndash After A transmits a packet it next chooses from range [031]ndash It is possible that A may transmit several packets before B transmits its first packet
Ad hoc Sensor Networks Multiple channel access
Unfairness occurs when one node has backed off much more than some other node
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 54
Random Access fairness (contrsquo) MACAW solution for fairnessbull When a node transmits a packet it appends value w to the packet all nodes
hearing value w use it for their future transmission attemptsbull Since w is an indication of the level of congestion in the vicinity of a specific
receiver node MACAW proposes maintaining w independently for eachreceiver
bull Using per-receiver wi is particularly useful in multi-hop environments sincecongestion level at different receivers can be very different
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 55
Random Access energy efficiency bull Since many mobile hosts are operated by batteries MAC
protocols which conserve energy are of interestbull Two approaches to reduce energy consumption
ndash Power save turn off wireless interface when desirablendash Power control reduce transmit power
bull Power control has some more potential benefitsndash Reduced interference (in ad hoc networks) eg it improves ALOHAefficiency where user with high power can capture a packet even if there is acollisionndash Increased spatial reuse (in infrastructured wireless networks) eg it is usedin CDMA to maintain target SIR of voice and data users or generally it canbe used to maintain target SIR for different user classes (admission control)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 56
Random Access power control bull When C transmits to D at a high power level B cannot receive Arsquos
transmission due to interference from Cbull If C reduces transmit power it can still communicate with D
ndash Reduces energy consumption at node Cndash Allows B to receive Arsquos transmission (spatial reuse)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 57
bull A special wireless ad hoc networkndash Large number of nodesndash Battery poweredndash Topology and density changendash Nodes for a common taskndash In-network data processing
bull Sensor-net applicationsndash Sensor-triggered bursty trafficndash Can often tolerate some delay
Ad hoc Sensor Networks Multiple channel access
Characteristics of Sensor Network
Scalabilty and self configurationEnergy efficiencyAdaptivityFairness not importantLatency
AdaptivityEnergy-delay tradoff
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 58
Ad hoc Sensor Networks Multiple channel access
Wireless sensor node and sensor network
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 59
MAC for WSN attributes bull Attributes
ndash Collision avoidance (Basic task)ndash Energy efficiency (most primary in WSN)ndash Scalability and adaptivityndash Channel utilizationndash Latencyndash Throughputndash Fairness
bull What causes energy wastendash Collisionsndash Control packet overheadndash Overhearing unnecessary trafficndash Long idle time (it consumes 50-100 of the power)
Ad hoc Sensor Networks Multiple channel access
Primary
Secondary
Dominant factor
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 60
MAC for WSN
bull Contention-based protocols need to work hard in all directions for energysavingsndash Reduce idle listening ndash support low duty cyclendash Better collision avoidancendash Reduce control overheadndash Avoid unnecessary overhearing
Ad hoc Sensor Networks Multiple channel access
Scheduled Protocols Contention ProtocolsCollisions No YesEnergy efficiency Good BadScalability and adaptivity Bad GoodMulti-hop communication Difficult EasyTime synchronization Strict Loose or not required
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 61
MAC for WSN classification
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 62
MAC for WSN SMAC (2002)bull It has been developed by Ye Heidemann and Estrinbull Tradeoffs between (latency fairness) and energybull Major components in S-MAC
ndash Periodic listen and sleep turn off radio when sleeping the reduced dutycycle is ~ 10 (eg 120ms on 12s off)ndash Collision avoidance is based on contention similarly to IEEE 80211 adhoc modendash Overhearing avoidance the node is sleeping when neighbors talkndash Massage passing long message is fragmented amp sent in burst operationwith extended Tx time and re-transmit occurs immediately
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 63
Summarybull The algorithms and protocols that enable sharing by multiple users and
controls the access to the wireless channel from different users are calledMAC
bull There are two types of wireless networks Infrastructured wireless networksand ad hoc wireless networks
bull We also want our wireless system to have high user capacity and we canidentify 10 methods for increasing it
bull There are several MACs with no absolute advantage it should be tailoredto the system requirements
bull There are primary concerns for designing a WSN MAC (collisionavoidance energy efficiency scalability and adaptivity) while thesecondary concerns are as follows (channel utilization latency throughputfairness)
bull Next lecture Routing protocols
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 17
Ad hoc Sensor Networks Multiple channel access
bull Individual radio channels are assigned toindividual users Each user is allocated a Wfrequency band by BS
bull If the channel allocated to a user is idle then itis not used by someone else waste of resource
bull Mobile and base can transmit and receivesimultaneously (FDD)
bull The W bandwidth of FDMA channels arerelatively low Symbol time is usually larger(low data rate) than the delay spread of themultipath channel (implies that inter-symbolinterference is low)
bull Lower complexity systems than TDMAsystems
Multiple Access FDMA
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 18
Ad hoc Sensor Networks Multiple channel access
Multiple Access FDMA (contrsquo)bull Capacity of FDMA systems
Nc = (B-2Bguard)Wwhere B is the total spectrum allocation Bguard is the guard band allocated atthe edge of the spectrum band W is the bandwidth of an individual channel
bull Erlang B and Erlang C system assumesndashinfinite population of sources which jointly offer traffic to Nc channelndashcall attempts arrive following a Poisson process so call arrivals are independentThe average arrival rate is λndashmessage length (holding times) are exponentially distributed (Markovian system)The average call length is hndashthe total amount of traffic offered in erlangs L= λhndashIf a user is rejected his next call attempt is made statistically independent of theprevious attempt (Erlang B)ndashIf all the channels are busy when a request arrives from a user the request isqueued An unlimited number of requests may be held in the queue (Erlang C)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 19
Ad hoc Sensor Networks Multiple channel access
bull Statictical measures of offered load Ncndash In Erlang B system the probability of call
blocking
ndash In Erlang C system the probabilty of waiting
ndash And the average wait time
c
c
cBLOCK
0
Pr
N
Nk
k
LN
Lk
=
=
sum
c
cc
wait 1
c0c
Pr 1
N
NkN
k
LL LL N kN
minus
=
=⎛ ⎞
+ minus⎜ ⎟⎝ ⎠
sum
wait waitc
Pr htN L
=minus
Multiple Access FDMA (contrsquo)Recall from
Queuing theory
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 20
Ad hoc Sensor Networks Multiple channel access
Multiple Access example of Erlang B systembull In an Erlang B system 40 channels are available A blocking probabilty of
less than 1 is required What is the traffic that can be serve if there is oneoperator or three operators
bull (Solution L(1 operator)gtL(3 operators))bull (Note The number of users increases faster than linearly with the number of
available channels The difference between actual increase and linearincrease is called the trunking gain It is preferable to have a large pool ofavailable channels that serves all users (a single operator) The reasons fornot choosing this approach are political not technical)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 21
Ad hoc Sensor Networks Multiple channel access
Multiple Access TDMAbull The allocated radio spectrum for the system is divided into time slots
ndash In each slot a user can transmit or receivendash A user occupies a cyclically repeating slotsndash A channel is logically defined as a particular time slot that repeats with some
periodbull TDMA systems buffer the data until its turn (time slot) comes to transmitbull In TDMATDD half of the slots in the frame is used for uplink channels the
other is used for downlink channelsbull In TDMAFDD a different carrier frequency is used for uplink or downlink
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 22
Ad hoc Sensor Networks Multiple channel access
Multiple Access TDMA (contrsquo)bull Preamble contains address and synchronization info to identify base
station and mobiles to each otherbull Guard times are used to allow synchronization of the receivers between
different slots and framesndash Different mobiles may have different propagation delays to a base station
because of different distances
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 23
Multiple Access TDMA (contrsquo)bull Each frame contains overhead bits and data bits Efficiency of frame is
defined as the percentage of data (information) bits to the total frame size inbits
η =(1 ndash boverheadbtotal) 100 where btotal= Tf R is the total number of bits in a frame Tf is the frameduration in sec boverhead is the number of overhead bits (GSM efficiency 73)
bull TDMA is usually combined with FDMAndash Neighboring cells are allocated and using different carrier frequencies (FDMA)
Inside a cell TDMA can be used Cells may be re-using the same frequency if theyare far from each-other
ndash There may be more than one channel allocated and used inside each cell Eachcarrier frequency (radio channel) may be using TDMA to further multiplex moreuser (ie having TDMA logical channels inside radio channels) For example inGSM each radio channel has 200KHz bandwidth and has 8 time slots (8 logicalchannels) Hence GSM is using FDMA combined with TDMA
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 24
Multiple Access TDMA (contrsquo)
Ad hoc Sensor Networks Multiple channel access
GSM (Europa) IS-54 (USA) PDC (Japan) DECT
Bit Rate 2708 Kbps 486 Kbps 42 Kbps 1152 Mbps
Bandwidth 200 KHz 30 KHz 25 KHz 1728 MHz
Time Slot 0577 ms 67 ms 67 ms 0417 ms
Upstream slots per frame 8 3 3 12
Duplexing FDD FDD FDD TDD
Efficiency 73 80 80 67
Modulation GMSK π4 DQPSK π4 DQPSK GMSK
Adaptive equalized Mandatory Mandatory Optional None
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 25
Multiple Access TDMA (contrsquo)bull Enables the sharing of a single radio channel among N usersbull Requires high data-rate per radio channel to support N users
simultaneously which requires adaptive equalizers to be usedin multipath environments
bull Transmission occurs in bursts (not continually) It enablespower saving by going to sleep modes in unrelated slots Itdiscontinues transmission and also enables mobile assistedhandoff
bull It equires synchronization of the receivers Need guard bits andsync bits which occurs large overhead per slot
bull Allocation of slots to mobile users should not be uniform Itmay depend on the traffic requirement of mobiles This bringsextra flexibility and efficiency compared to FDMA systems
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2620111015 TAacuteMOP ndash 412-082AKMR-2009-0006 26
Ad hoc Sensor Networks Multiple channel access
Multiple Access spread spectrum multiple accessbull SSMA uses signals that have transmission bandwidth that is
several orders of magnitude larger than minimum requiredRF bandwidth
bull It providesndash Immunity to multipath interferencendash Robust multiple access
bull Two techniquesndash Frequency Hopped Multiple Access (FHMA)ndash Direct Sequence Multiple Access (DSMA) or Code Division
Multiple Access (CDMA)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2720111015 TAacuteMOP ndash 412-082AKMR-2009-0006 27
Ad hoc Sensor Networks Multiple channel access
Multiple Access FHMAbull The carrier frequency of users are varied in a pseudo-random
fashionndash Each user is using a narrowband channel (spectrum) at a specific
instance of timendash The random change in frequency make the change of using the same
narrowband channel very lowbull The sender receiver change frequency (calling hopping)
using the same pseudo-random sequence hence they aresynchronized
bull Rate of hopping versus Symbol ratendash If hopping rate is greather Fast Frequency Hoppingndash If symbol rate is greater Slow Frequency Hopping
bull GSM and Bluetooth are example systems
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2820111015 TAacuteMOP ndash 412-082AKMR-2009-0006 28
Ad hoc Sensor Networks Multiple channel access
Multiple Access FHMA (contrsquo)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2920111015 TAacuteMOP ndash 412-082AKMR-2009-0006 29
Ad hoc Sensor Networks Multiple channel access
Multiple Access CDMAbull The narrowband message signal is multiplied by a very large bandwidth
signal called spreading signal (code) before modulation and transmissionover the air This is called spreading
bull Spreading signal use a pseudo-noise (PN) sequence (a pseudo-randomsequence) called codeword which are orthogonal (low autocorrelation)
bull The receiver correlator distinguishes the senders signal by examining thewideband signal with the same time-synchronized spreading code(despreading)
bull Advantagesndash Low power spectral densityndash Interference limited operationndash Privacyndash Reduction of multipath affects by using a larger spectrumndash Users can start their transmission at any time (random access possible)ndash Cell capacity is soft and higher than TDMA and FDMAndash No frequency management and no equalizers and quard time needed
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3020111015 TAacuteMOP ndash 412-082AKMR-2009-0006 30
Ad hoc Sensor Networks Multiple channel access
Multiple Access near far problem and power controlbull At a receiver the signals may come from various sources (multiuser) The
strongest signal usually captures the modulator The other signals areconsidered as noise Each source may have different distances to the basestation
bull In CDMA we want a BS to receive CDMA coded signals from variousmobile users at the same time
ndash Therefore the receiver power at the BS for all mobile users should be close toeach other This requires power control at the mobiles
bull Power Control BS monitors the RSSI (Received Signal Strength Indicator)values from different mobiles and then sends power change commands to themobiles over a forward channel The mobiles then adjust their transmitpower
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3120111015 TAacuteMOP ndash 412-082AKMR-2009-0006 31
Ad hoc Sensor Networks Multiple channel access
Multiple Access hybrid spread spectrum techniquesbull FDMACDMA
ndash Available wideband spectrum is divided into a number narrowband radio channels CDMAis employed inside each channel
bull DSFHMAndash The signals are spread using spreading codes (direct sequence signals are obtained) but
these signal are not transmitted over a constant carrier frequency they are transmitted over afrequency hopping carrier frequency
bull Time Division CDMA (TCDMA)ndash Each cell is using a different spreading code (CDMA employed between cells) that is
conveyed to the mobiles in its rangendash Inside each cell (inside a CDMA channel) TDMA is employed to multiplex multiple users
bull Time Division Frequency Hoppingndash At each time slot the user is hopped to a new frequency according to a pseudo-random
hopping sequencendash Employed in severe co-interference and multi-path environmentsndash Bluetooth and GSM are using this technique
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3220111015 TAacuteMOP ndash 412-082AKMR-2009-0006 32
Ad hoc Sensor Networks Multiple channel access
Multiple Access comparison TDMAFDMACDMA
Techniques TDMA FDMA CDMA SDMA
Idea
segment sending time into disjoint time-slots
Segment the frequency band into disjoint sub-bands
Spread the spectrum using orthogonal codes
Segment space into sectors
Signal separation
Synchronization in the time domain
Filtering in the frequency domain
Code Directed antennas
Advantage
Established flexible
Simple established robust
Flexible less frequency planning needed soft handover
Simple increases capacity
DisadvantageQuard space synchronization difficult
Inflexible scarce source
Complex receiver power control
inflexible
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3320111015 TAacuteMOP ndash 412-082AKMR-2009-0006 33
Ad hoc Sensor Networks Multiple channel access
User capacity per cell
The user capacity C is defined as the total number of active users percell that the system can support while meeting a common BERconstrain For orthogonal multiple access (as FDMA and TDMA)
C = Ncellwhere Ncell is the number of channels assigned to any given cellThe total number of orthogonal channels of bandwidth Bs that can becreated from a total system bandwidth of B is B Bs The reuse factorsatisfies N = (B Bs) Ncell this implies
C = (B Bs) N
Recall from Chapter 1
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3420111015 TAacuteMOP ndash 412-082AKMR-2009-0006 34
Ad hoc Sensor Networks Multiple channel access
10 methods for increasing capacity1 Increasing the amount of spectrum used very expensive2 More efficient modulation format and coding3 Better source coding4 Adaptive modulation and coding5 Discontinuous transmission6 Multiuser detection CDMA systems7 Reduction of cell radius effective but very expensive smaller
cells require more handovers8 Use of sector cells tripled cells have tripled BS antennas9 Use of an overlay structure10 Multiple antennas diversity MIMO systems SDMA
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3520111015 TAacuteMOP ndash 412-082AKMR-2009-0006 35
Ad hoc Sensor Networks Multiple channel access
bull Distributed operation is requiredbull Synchronization is required in TDMA-based systemsbull Hidden terminals are nodes hidden from a senderbull Exposed terminals are exposed nodes preventing a sender
from sendingbull Throughput needs to be maximizedbull Access delay should be minimizedbull Fairness refers to provide an equal share to all competing
nodesbull Real-time traffic support is required for voice video and
real-time databull Resource reservation is required for QoSbull Ability to measure resource availability handles the
resourcesbull Capability for power control reduces the energy
consumption
Random access in adhoc networks
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3620111015 TAacuteMOP ndash 412-082AKMR-2009-0006 36
Ad hoc Sensor Networks Multiple channel access
Random accessbull All random access techniques are based on packetized data or packet radio where user
date is collected into packets of M bitsbull Collision if packets from different users overlap in timebull The transmission time of a packet is τ = M R where R [bps] the data ratebull Analysis of random access techniques assumes that the users generate packets according
to memoryless Poisson process at a rate λ [packets per unit time] the probability that thenumber of packet arrivals in a time period [0t] denoted by X(t) is equal to integer k isgiven by
Pr(X(t)=k) = (λt)k k e-λt
bull The traffic load is defined as L = λ τ If L gt 1 means on avarage more packets arrive inthe system over a given time period than can be transmitted in that period so systemswith Lgt1 are unstable
bull The throughput is defined as the ratio of the average number of packets successfullytransmitted in any given time interval divided by the number of attempted transmissionsin that interval
T = L Pr(succesful packet transmission)bull The effective data rate of the system is RT
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3720111015 TAacuteMOP ndash 412-082AKMR-2009-0006 37
Ad hoc Sensor Networks Multiple channel access
Random access (contrsquo)bull We will examine the following attributes
ndash Channel utilization (Throughput)ndash Latencyndash Collision avoidance (Hidden and exposed terminals problem)ndash Reliability (ACK)ndash Energy efficiency (power control)ndash Fairnessndash Throughput needs to be maximized
bull We will discuss the following techniquesndash Aloha and slotted Alohandash CSMA Protocols (1-persistent non-persistent p-persistent CSMA CSMACD)ndash MACAndash MACAWndash Energy efficient MAC in wireless sensor networks (eg SMAC)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 38
Random Access ALOHA (1970)bull ALOHA was developed for a wireless system at the University of Hawaii
(Abramson et al)ndash Multiple remote stations plus one base stationndash Frame transmissions are made at one frequency from a remote station to the basestation the base station re-broadcasts frames on another frequency Contention is foraccess to the base station
bull Frames can be sent at any time to the base stationndash If no acknowledgment (ACK) is received assume that the frame is lost andretransmit after waiting a random amount of timendash If more than one station broadcasts on the base station access frequency at the sametime ndash a collision ndash interference will destroy the frames
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 39
Random Access ALOHA (contrsquo)
bull The probability that no packets generated during the tima [-τ τ] is given by (nocollision)
Pr(X(t)=0) = (λ2τ)0 0 e-2λτ = e-2Lwith corresponding throughput
T = L e-2L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 05 05e 018
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 40
Random Access slotted ALOHA (1971)bull Improvement Time is slotted and a packet can only be transmitted at the beginning of
one slot Thus it can reduce the collision duration
bull The probability that no packets generated during the time [0 τ] is given by (nocollision)
Pr(X(t)=0) = (λτ)0 0 e-λτ = e-Lwith corresponding throughput T = L e-L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 1 e 037
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 41
bull Slotted Aloha has double the maximumthroughput as pure Aloha and achievesthis maximum at a higher offered loadThe effective data rate is still less than40 of the raw transmission rate
bull Aloha does not listen to the carrier beforetransmission more sophisticatedtechniques are needed to increaseefficiency
bull Further problem the delayndashDelay
ndashThe mean packet delay
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
( ) ( )delayPr 1 e ekL Lkτ minus minus= = minus
( )delay1
1 e e ekL L L
kMPD E kτ
infinminus minus
=
= = minus =sum
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 42
Random Access Carrier Sense Multiple Accessbull CSMA (Carrier Sense Multiple Access) listen to the carrier before transmission
and transmits if channel is idlebull Detection delay and propagation delay are two important parameters
ndash Detection delay time required to sense the carrier and decide if it is idle or busyndash Propagation delay distancespeed_of_ligth The time required for bit to travel
from transmitter to the receiver
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 43
Random Access CSMA variations
Ad hoc Sensor Networks Multiple channel access
1-persistent CSMA Non-persistent CSMA
There will always be a collision iftwo nodes want to retransmit(usually you stop transmissionattempts after few tries)
Random backoff reduces probabilityof collisions Waste idle time if thebackoff time is too long
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 44
Random Access CSMA variations (contrsquo)
Ad hoc Sensor Networks Multiple channel access
p-persistent CSMA CSMA CD (Collision Detection)
A good tradeoff between non-persistent and 1-persistent CSMA
Same with CSMA however a stationalso listen to the carrier whiletransmitting to see if the transmissioncollides with someone elsetransmission It can be used in listen-while-talk capable channels (fullduplex) In single radio channels thetransmission need to be interrupted inorder to sense the channel
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 45
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 46
Random Access CSMA (contrsquo)bull How to select probability p
ndash Assume that M nodes have a packet to send and the medium is busy then Mp is theexpected number of nodes that will attempt to transmit once the medium becomesidle If Mp gt 1 then a collision is expected to occur Therefore the network mustmake sure that Mp lt 1 to avoid collision where M is the maximum number of nodesthat can be active at a time
bull Problems1 The mean packet delay increases exponently with increasing offered load2 CSMA protocols sense the carrier but sensing the carrier does not always releasestrue information about the status of the wireless channel There are two problems thatare unique to wireless channels (different than wireline channels)ndash Hidden terminal problemndash Exposed terminal problem
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 47
Hidden terminal problembull A sends to B C cannot receive Abull C wants to send to B C senses a
ldquofreerdquo medium (CS fails)bull collision at B A cannot receive the
collision (CD fails)bull A is ldquohiddenrdquo for C
Ad hoc Sensor Networks Multiple channel access
Random Access hidden and exposed terminal problemExposed terminal problem
bull B sends to A C wants to send to Dbull C has to wait CS signals a medium
in usebull since A is outside the radio range of
C waiting is not necessarybull C is ldquoexposedrdquo to B
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 48
Random Access MACAWhen a station wants to transmit databull It sends an RTS (Ready-to-Send) packet to the intended receiver
ndash The RTS packet contains the length of the data that needs to be transmittedndash Any station other than the intended recipient hearing RTS defers transmission for atime duration equal to the end of the corresponding CTS reception
bull The receiver sends back CTS (Clear-to-Send) packet back to sender if it isavailable to receivendash The CTS packet contains the length of the data that original sender wants totransmitndash Any station other than the original RTS sender hearing CTS defers transmissionuntil the data is sent
bull The original sender upon reception of the CTS starts transmitting
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 49
Hidden terminal problem
Ad hoc Sensor Networks Multiple channel access
Random Access MACA (contrsquo)Exposed terminal problem
Waiting time of node X is muchsmaller than waiting time of node C
bull C defers transmission upon hearing BrsquosRTS until B could get CTS from Abull After that C can start transmission to DFor that it first sends an RTSbull C is not longer exposed to the datatransmission of B
Problem It does not address the collision of RTS packet
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 50
Random Access reliabilitybull Wireless links are prone to have errors High packet loss rate detrimental to
transport-layer performance Mechanisms needed to reduce packet loss rateexperienced by upper layers
bull When node B receives a data packet from node A node B sends anAcknowledgement (Ack) This approach adopted in many protocols
bull If node A fails to receive an Ack it will retransmit the packet
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 51
Random Access MACAW (CSMACA) bull Physical carrier sense and virtual carrier sense using Network Allocation
Vector (NAV)ndash NAV is updated based on overheard RTSCTSDATAACK packets each of whichspecified duration of a pending transmissionndash Nodes stay silent when carrier sensed (physicalvirtual)
bull Backoff intervals used to reduce collision probabilityndash When transmitting a packet choose a backoff interval in the range [0w] where wis contention windowndash Count down the backoff interval when channel is idlendash Count down is suspended if channel becomes busyndash When backoff interval reaches 0 transmit RTS
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 52
Random Access CSMACA (contrsquo) bull The time spent counting down backoff intervals is a part of MAC overhead
ndash Choosing a large w leads to large backoff intervals and can result in larger overheadndash Choosing a small w leads to a larger number of collisions (when two nodes countdown to 0 simultaneously)ndash Since the number of nodes attempting to transmit simultaneously may change withtime some mechanism to manage contention is needed
bull IEEE 80211 DCF contention window w is chosen dynamically depending oncollision occurrence (binary exponential backoff)ndash When a node fails to receive CTS in response to its RTS it increases the contentionwindowndash w is doubled (up to an upper bound)ndash When a node successfully completes a data transfer it restores w to wmin
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 53
Random Access fairnessbull Simplest definition of fairness all nodes should receive equal bandwidthbull An example of unfairness
ndash Assume that initially A and B both choose a backoff interval in range [031] buttheir RTSs collidendash Nodes A and B then choose from range [063]ndash Node A chooses 4 slots and B choose 60 slotsndash After A transmits a packet it next chooses from range [031]ndash It is possible that A may transmit several packets before B transmits its first packet
Ad hoc Sensor Networks Multiple channel access
Unfairness occurs when one node has backed off much more than some other node
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 54
Random Access fairness (contrsquo) MACAW solution for fairnessbull When a node transmits a packet it appends value w to the packet all nodes
hearing value w use it for their future transmission attemptsbull Since w is an indication of the level of congestion in the vicinity of a specific
receiver node MACAW proposes maintaining w independently for eachreceiver
bull Using per-receiver wi is particularly useful in multi-hop environments sincecongestion level at different receivers can be very different
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 55
Random Access energy efficiency bull Since many mobile hosts are operated by batteries MAC
protocols which conserve energy are of interestbull Two approaches to reduce energy consumption
ndash Power save turn off wireless interface when desirablendash Power control reduce transmit power
bull Power control has some more potential benefitsndash Reduced interference (in ad hoc networks) eg it improves ALOHAefficiency where user with high power can capture a packet even if there is acollisionndash Increased spatial reuse (in infrastructured wireless networks) eg it is usedin CDMA to maintain target SIR of voice and data users or generally it canbe used to maintain target SIR for different user classes (admission control)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 56
Random Access power control bull When C transmits to D at a high power level B cannot receive Arsquos
transmission due to interference from Cbull If C reduces transmit power it can still communicate with D
ndash Reduces energy consumption at node Cndash Allows B to receive Arsquos transmission (spatial reuse)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 57
bull A special wireless ad hoc networkndash Large number of nodesndash Battery poweredndash Topology and density changendash Nodes for a common taskndash In-network data processing
bull Sensor-net applicationsndash Sensor-triggered bursty trafficndash Can often tolerate some delay
Ad hoc Sensor Networks Multiple channel access
Characteristics of Sensor Network
Scalabilty and self configurationEnergy efficiencyAdaptivityFairness not importantLatency
AdaptivityEnergy-delay tradoff
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 58
Ad hoc Sensor Networks Multiple channel access
Wireless sensor node and sensor network
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 59
MAC for WSN attributes bull Attributes
ndash Collision avoidance (Basic task)ndash Energy efficiency (most primary in WSN)ndash Scalability and adaptivityndash Channel utilizationndash Latencyndash Throughputndash Fairness
bull What causes energy wastendash Collisionsndash Control packet overheadndash Overhearing unnecessary trafficndash Long idle time (it consumes 50-100 of the power)
Ad hoc Sensor Networks Multiple channel access
Primary
Secondary
Dominant factor
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 60
MAC for WSN
bull Contention-based protocols need to work hard in all directions for energysavingsndash Reduce idle listening ndash support low duty cyclendash Better collision avoidancendash Reduce control overheadndash Avoid unnecessary overhearing
Ad hoc Sensor Networks Multiple channel access
Scheduled Protocols Contention ProtocolsCollisions No YesEnergy efficiency Good BadScalability and adaptivity Bad GoodMulti-hop communication Difficult EasyTime synchronization Strict Loose or not required
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 61
MAC for WSN classification
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 62
MAC for WSN SMAC (2002)bull It has been developed by Ye Heidemann and Estrinbull Tradeoffs between (latency fairness) and energybull Major components in S-MAC
ndash Periodic listen and sleep turn off radio when sleeping the reduced dutycycle is ~ 10 (eg 120ms on 12s off)ndash Collision avoidance is based on contention similarly to IEEE 80211 adhoc modendash Overhearing avoidance the node is sleeping when neighbors talkndash Massage passing long message is fragmented amp sent in burst operationwith extended Tx time and re-transmit occurs immediately
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 63
Summarybull The algorithms and protocols that enable sharing by multiple users and
controls the access to the wireless channel from different users are calledMAC
bull There are two types of wireless networks Infrastructured wireless networksand ad hoc wireless networks
bull We also want our wireless system to have high user capacity and we canidentify 10 methods for increasing it
bull There are several MACs with no absolute advantage it should be tailoredto the system requirements
bull There are primary concerns for designing a WSN MAC (collisionavoidance energy efficiency scalability and adaptivity) while thesecondary concerns are as follows (channel utilization latency throughputfairness)
bull Next lecture Routing protocols
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 18
Ad hoc Sensor Networks Multiple channel access
Multiple Access FDMA (contrsquo)bull Capacity of FDMA systems
Nc = (B-2Bguard)Wwhere B is the total spectrum allocation Bguard is the guard band allocated atthe edge of the spectrum band W is the bandwidth of an individual channel
bull Erlang B and Erlang C system assumesndashinfinite population of sources which jointly offer traffic to Nc channelndashcall attempts arrive following a Poisson process so call arrivals are independentThe average arrival rate is λndashmessage length (holding times) are exponentially distributed (Markovian system)The average call length is hndashthe total amount of traffic offered in erlangs L= λhndashIf a user is rejected his next call attempt is made statistically independent of theprevious attempt (Erlang B)ndashIf all the channels are busy when a request arrives from a user the request isqueued An unlimited number of requests may be held in the queue (Erlang C)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 19
Ad hoc Sensor Networks Multiple channel access
bull Statictical measures of offered load Ncndash In Erlang B system the probability of call
blocking
ndash In Erlang C system the probabilty of waiting
ndash And the average wait time
c
c
cBLOCK
0
Pr
N
Nk
k
LN
Lk
=
=
sum
c
cc
wait 1
c0c
Pr 1
N
NkN
k
LL LL N kN
minus
=
=⎛ ⎞
+ minus⎜ ⎟⎝ ⎠
sum
wait waitc
Pr htN L
=minus
Multiple Access FDMA (contrsquo)Recall from
Queuing theory
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 20
Ad hoc Sensor Networks Multiple channel access
Multiple Access example of Erlang B systembull In an Erlang B system 40 channels are available A blocking probabilty of
less than 1 is required What is the traffic that can be serve if there is oneoperator or three operators
bull (Solution L(1 operator)gtL(3 operators))bull (Note The number of users increases faster than linearly with the number of
available channels The difference between actual increase and linearincrease is called the trunking gain It is preferable to have a large pool ofavailable channels that serves all users (a single operator) The reasons fornot choosing this approach are political not technical)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 21
Ad hoc Sensor Networks Multiple channel access
Multiple Access TDMAbull The allocated radio spectrum for the system is divided into time slots
ndash In each slot a user can transmit or receivendash A user occupies a cyclically repeating slotsndash A channel is logically defined as a particular time slot that repeats with some
periodbull TDMA systems buffer the data until its turn (time slot) comes to transmitbull In TDMATDD half of the slots in the frame is used for uplink channels the
other is used for downlink channelsbull In TDMAFDD a different carrier frequency is used for uplink or downlink
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 22
Ad hoc Sensor Networks Multiple channel access
Multiple Access TDMA (contrsquo)bull Preamble contains address and synchronization info to identify base
station and mobiles to each otherbull Guard times are used to allow synchronization of the receivers between
different slots and framesndash Different mobiles may have different propagation delays to a base station
because of different distances
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 23
Multiple Access TDMA (contrsquo)bull Each frame contains overhead bits and data bits Efficiency of frame is
defined as the percentage of data (information) bits to the total frame size inbits
η =(1 ndash boverheadbtotal) 100 where btotal= Tf R is the total number of bits in a frame Tf is the frameduration in sec boverhead is the number of overhead bits (GSM efficiency 73)
bull TDMA is usually combined with FDMAndash Neighboring cells are allocated and using different carrier frequencies (FDMA)
Inside a cell TDMA can be used Cells may be re-using the same frequency if theyare far from each-other
ndash There may be more than one channel allocated and used inside each cell Eachcarrier frequency (radio channel) may be using TDMA to further multiplex moreuser (ie having TDMA logical channels inside radio channels) For example inGSM each radio channel has 200KHz bandwidth and has 8 time slots (8 logicalchannels) Hence GSM is using FDMA combined with TDMA
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 24
Multiple Access TDMA (contrsquo)
Ad hoc Sensor Networks Multiple channel access
GSM (Europa) IS-54 (USA) PDC (Japan) DECT
Bit Rate 2708 Kbps 486 Kbps 42 Kbps 1152 Mbps
Bandwidth 200 KHz 30 KHz 25 KHz 1728 MHz
Time Slot 0577 ms 67 ms 67 ms 0417 ms
Upstream slots per frame 8 3 3 12
Duplexing FDD FDD FDD TDD
Efficiency 73 80 80 67
Modulation GMSK π4 DQPSK π4 DQPSK GMSK
Adaptive equalized Mandatory Mandatory Optional None
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 25
Multiple Access TDMA (contrsquo)bull Enables the sharing of a single radio channel among N usersbull Requires high data-rate per radio channel to support N users
simultaneously which requires adaptive equalizers to be usedin multipath environments
bull Transmission occurs in bursts (not continually) It enablespower saving by going to sleep modes in unrelated slots Itdiscontinues transmission and also enables mobile assistedhandoff
bull It equires synchronization of the receivers Need guard bits andsync bits which occurs large overhead per slot
bull Allocation of slots to mobile users should not be uniform Itmay depend on the traffic requirement of mobiles This bringsextra flexibility and efficiency compared to FDMA systems
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2620111015 TAacuteMOP ndash 412-082AKMR-2009-0006 26
Ad hoc Sensor Networks Multiple channel access
Multiple Access spread spectrum multiple accessbull SSMA uses signals that have transmission bandwidth that is
several orders of magnitude larger than minimum requiredRF bandwidth
bull It providesndash Immunity to multipath interferencendash Robust multiple access
bull Two techniquesndash Frequency Hopped Multiple Access (FHMA)ndash Direct Sequence Multiple Access (DSMA) or Code Division
Multiple Access (CDMA)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2720111015 TAacuteMOP ndash 412-082AKMR-2009-0006 27
Ad hoc Sensor Networks Multiple channel access
Multiple Access FHMAbull The carrier frequency of users are varied in a pseudo-random
fashionndash Each user is using a narrowband channel (spectrum) at a specific
instance of timendash The random change in frequency make the change of using the same
narrowband channel very lowbull The sender receiver change frequency (calling hopping)
using the same pseudo-random sequence hence they aresynchronized
bull Rate of hopping versus Symbol ratendash If hopping rate is greather Fast Frequency Hoppingndash If symbol rate is greater Slow Frequency Hopping
bull GSM and Bluetooth are example systems
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2820111015 TAacuteMOP ndash 412-082AKMR-2009-0006 28
Ad hoc Sensor Networks Multiple channel access
Multiple Access FHMA (contrsquo)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2920111015 TAacuteMOP ndash 412-082AKMR-2009-0006 29
Ad hoc Sensor Networks Multiple channel access
Multiple Access CDMAbull The narrowband message signal is multiplied by a very large bandwidth
signal called spreading signal (code) before modulation and transmissionover the air This is called spreading
bull Spreading signal use a pseudo-noise (PN) sequence (a pseudo-randomsequence) called codeword which are orthogonal (low autocorrelation)
bull The receiver correlator distinguishes the senders signal by examining thewideband signal with the same time-synchronized spreading code(despreading)
bull Advantagesndash Low power spectral densityndash Interference limited operationndash Privacyndash Reduction of multipath affects by using a larger spectrumndash Users can start their transmission at any time (random access possible)ndash Cell capacity is soft and higher than TDMA and FDMAndash No frequency management and no equalizers and quard time needed
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3020111015 TAacuteMOP ndash 412-082AKMR-2009-0006 30
Ad hoc Sensor Networks Multiple channel access
Multiple Access near far problem and power controlbull At a receiver the signals may come from various sources (multiuser) The
strongest signal usually captures the modulator The other signals areconsidered as noise Each source may have different distances to the basestation
bull In CDMA we want a BS to receive CDMA coded signals from variousmobile users at the same time
ndash Therefore the receiver power at the BS for all mobile users should be close toeach other This requires power control at the mobiles
bull Power Control BS monitors the RSSI (Received Signal Strength Indicator)values from different mobiles and then sends power change commands to themobiles over a forward channel The mobiles then adjust their transmitpower
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3120111015 TAacuteMOP ndash 412-082AKMR-2009-0006 31
Ad hoc Sensor Networks Multiple channel access
Multiple Access hybrid spread spectrum techniquesbull FDMACDMA
ndash Available wideband spectrum is divided into a number narrowband radio channels CDMAis employed inside each channel
bull DSFHMAndash The signals are spread using spreading codes (direct sequence signals are obtained) but
these signal are not transmitted over a constant carrier frequency they are transmitted over afrequency hopping carrier frequency
bull Time Division CDMA (TCDMA)ndash Each cell is using a different spreading code (CDMA employed between cells) that is
conveyed to the mobiles in its rangendash Inside each cell (inside a CDMA channel) TDMA is employed to multiplex multiple users
bull Time Division Frequency Hoppingndash At each time slot the user is hopped to a new frequency according to a pseudo-random
hopping sequencendash Employed in severe co-interference and multi-path environmentsndash Bluetooth and GSM are using this technique
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3220111015 TAacuteMOP ndash 412-082AKMR-2009-0006 32
Ad hoc Sensor Networks Multiple channel access
Multiple Access comparison TDMAFDMACDMA
Techniques TDMA FDMA CDMA SDMA
Idea
segment sending time into disjoint time-slots
Segment the frequency band into disjoint sub-bands
Spread the spectrum using orthogonal codes
Segment space into sectors
Signal separation
Synchronization in the time domain
Filtering in the frequency domain
Code Directed antennas
Advantage
Established flexible
Simple established robust
Flexible less frequency planning needed soft handover
Simple increases capacity
DisadvantageQuard space synchronization difficult
Inflexible scarce source
Complex receiver power control
inflexible
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3320111015 TAacuteMOP ndash 412-082AKMR-2009-0006 33
Ad hoc Sensor Networks Multiple channel access
User capacity per cell
The user capacity C is defined as the total number of active users percell that the system can support while meeting a common BERconstrain For orthogonal multiple access (as FDMA and TDMA)
C = Ncellwhere Ncell is the number of channels assigned to any given cellThe total number of orthogonal channels of bandwidth Bs that can becreated from a total system bandwidth of B is B Bs The reuse factorsatisfies N = (B Bs) Ncell this implies
C = (B Bs) N
Recall from Chapter 1
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3420111015 TAacuteMOP ndash 412-082AKMR-2009-0006 34
Ad hoc Sensor Networks Multiple channel access
10 methods for increasing capacity1 Increasing the amount of spectrum used very expensive2 More efficient modulation format and coding3 Better source coding4 Adaptive modulation and coding5 Discontinuous transmission6 Multiuser detection CDMA systems7 Reduction of cell radius effective but very expensive smaller
cells require more handovers8 Use of sector cells tripled cells have tripled BS antennas9 Use of an overlay structure10 Multiple antennas diversity MIMO systems SDMA
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3520111015 TAacuteMOP ndash 412-082AKMR-2009-0006 35
Ad hoc Sensor Networks Multiple channel access
bull Distributed operation is requiredbull Synchronization is required in TDMA-based systemsbull Hidden terminals are nodes hidden from a senderbull Exposed terminals are exposed nodes preventing a sender
from sendingbull Throughput needs to be maximizedbull Access delay should be minimizedbull Fairness refers to provide an equal share to all competing
nodesbull Real-time traffic support is required for voice video and
real-time databull Resource reservation is required for QoSbull Ability to measure resource availability handles the
resourcesbull Capability for power control reduces the energy
consumption
Random access in adhoc networks
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3620111015 TAacuteMOP ndash 412-082AKMR-2009-0006 36
Ad hoc Sensor Networks Multiple channel access
Random accessbull All random access techniques are based on packetized data or packet radio where user
date is collected into packets of M bitsbull Collision if packets from different users overlap in timebull The transmission time of a packet is τ = M R where R [bps] the data ratebull Analysis of random access techniques assumes that the users generate packets according
to memoryless Poisson process at a rate λ [packets per unit time] the probability that thenumber of packet arrivals in a time period [0t] denoted by X(t) is equal to integer k isgiven by
Pr(X(t)=k) = (λt)k k e-λt
bull The traffic load is defined as L = λ τ If L gt 1 means on avarage more packets arrive inthe system over a given time period than can be transmitted in that period so systemswith Lgt1 are unstable
bull The throughput is defined as the ratio of the average number of packets successfullytransmitted in any given time interval divided by the number of attempted transmissionsin that interval
T = L Pr(succesful packet transmission)bull The effective data rate of the system is RT
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3720111015 TAacuteMOP ndash 412-082AKMR-2009-0006 37
Ad hoc Sensor Networks Multiple channel access
Random access (contrsquo)bull We will examine the following attributes
ndash Channel utilization (Throughput)ndash Latencyndash Collision avoidance (Hidden and exposed terminals problem)ndash Reliability (ACK)ndash Energy efficiency (power control)ndash Fairnessndash Throughput needs to be maximized
bull We will discuss the following techniquesndash Aloha and slotted Alohandash CSMA Protocols (1-persistent non-persistent p-persistent CSMA CSMACD)ndash MACAndash MACAWndash Energy efficient MAC in wireless sensor networks (eg SMAC)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 38
Random Access ALOHA (1970)bull ALOHA was developed for a wireless system at the University of Hawaii
(Abramson et al)ndash Multiple remote stations plus one base stationndash Frame transmissions are made at one frequency from a remote station to the basestation the base station re-broadcasts frames on another frequency Contention is foraccess to the base station
bull Frames can be sent at any time to the base stationndash If no acknowledgment (ACK) is received assume that the frame is lost andretransmit after waiting a random amount of timendash If more than one station broadcasts on the base station access frequency at the sametime ndash a collision ndash interference will destroy the frames
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 39
Random Access ALOHA (contrsquo)
bull The probability that no packets generated during the tima [-τ τ] is given by (nocollision)
Pr(X(t)=0) = (λ2τ)0 0 e-2λτ = e-2Lwith corresponding throughput
T = L e-2L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 05 05e 018
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 40
Random Access slotted ALOHA (1971)bull Improvement Time is slotted and a packet can only be transmitted at the beginning of
one slot Thus it can reduce the collision duration
bull The probability that no packets generated during the time [0 τ] is given by (nocollision)
Pr(X(t)=0) = (λτ)0 0 e-λτ = e-Lwith corresponding throughput T = L e-L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 1 e 037
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 41
bull Slotted Aloha has double the maximumthroughput as pure Aloha and achievesthis maximum at a higher offered loadThe effective data rate is still less than40 of the raw transmission rate
bull Aloha does not listen to the carrier beforetransmission more sophisticatedtechniques are needed to increaseefficiency
bull Further problem the delayndashDelay
ndashThe mean packet delay
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
( ) ( )delayPr 1 e ekL Lkτ minus minus= = minus
( )delay1
1 e e ekL L L
kMPD E kτ
infinminus minus
=
= = minus =sum
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 42
Random Access Carrier Sense Multiple Accessbull CSMA (Carrier Sense Multiple Access) listen to the carrier before transmission
and transmits if channel is idlebull Detection delay and propagation delay are two important parameters
ndash Detection delay time required to sense the carrier and decide if it is idle or busyndash Propagation delay distancespeed_of_ligth The time required for bit to travel
from transmitter to the receiver
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 43
Random Access CSMA variations
Ad hoc Sensor Networks Multiple channel access
1-persistent CSMA Non-persistent CSMA
There will always be a collision iftwo nodes want to retransmit(usually you stop transmissionattempts after few tries)
Random backoff reduces probabilityof collisions Waste idle time if thebackoff time is too long
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 44
Random Access CSMA variations (contrsquo)
Ad hoc Sensor Networks Multiple channel access
p-persistent CSMA CSMA CD (Collision Detection)
A good tradeoff between non-persistent and 1-persistent CSMA
Same with CSMA however a stationalso listen to the carrier whiletransmitting to see if the transmissioncollides with someone elsetransmission It can be used in listen-while-talk capable channels (fullduplex) In single radio channels thetransmission need to be interrupted inorder to sense the channel
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 45
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 46
Random Access CSMA (contrsquo)bull How to select probability p
ndash Assume that M nodes have a packet to send and the medium is busy then Mp is theexpected number of nodes that will attempt to transmit once the medium becomesidle If Mp gt 1 then a collision is expected to occur Therefore the network mustmake sure that Mp lt 1 to avoid collision where M is the maximum number of nodesthat can be active at a time
bull Problems1 The mean packet delay increases exponently with increasing offered load2 CSMA protocols sense the carrier but sensing the carrier does not always releasestrue information about the status of the wireless channel There are two problems thatare unique to wireless channels (different than wireline channels)ndash Hidden terminal problemndash Exposed terminal problem
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 47
Hidden terminal problembull A sends to B C cannot receive Abull C wants to send to B C senses a
ldquofreerdquo medium (CS fails)bull collision at B A cannot receive the
collision (CD fails)bull A is ldquohiddenrdquo for C
Ad hoc Sensor Networks Multiple channel access
Random Access hidden and exposed terminal problemExposed terminal problem
bull B sends to A C wants to send to Dbull C has to wait CS signals a medium
in usebull since A is outside the radio range of
C waiting is not necessarybull C is ldquoexposedrdquo to B
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 48
Random Access MACAWhen a station wants to transmit databull It sends an RTS (Ready-to-Send) packet to the intended receiver
ndash The RTS packet contains the length of the data that needs to be transmittedndash Any station other than the intended recipient hearing RTS defers transmission for atime duration equal to the end of the corresponding CTS reception
bull The receiver sends back CTS (Clear-to-Send) packet back to sender if it isavailable to receivendash The CTS packet contains the length of the data that original sender wants totransmitndash Any station other than the original RTS sender hearing CTS defers transmissionuntil the data is sent
bull The original sender upon reception of the CTS starts transmitting
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 49
Hidden terminal problem
Ad hoc Sensor Networks Multiple channel access
Random Access MACA (contrsquo)Exposed terminal problem
Waiting time of node X is muchsmaller than waiting time of node C
bull C defers transmission upon hearing BrsquosRTS until B could get CTS from Abull After that C can start transmission to DFor that it first sends an RTSbull C is not longer exposed to the datatransmission of B
Problem It does not address the collision of RTS packet
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 50
Random Access reliabilitybull Wireless links are prone to have errors High packet loss rate detrimental to
transport-layer performance Mechanisms needed to reduce packet loss rateexperienced by upper layers
bull When node B receives a data packet from node A node B sends anAcknowledgement (Ack) This approach adopted in many protocols
bull If node A fails to receive an Ack it will retransmit the packet
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 51
Random Access MACAW (CSMACA) bull Physical carrier sense and virtual carrier sense using Network Allocation
Vector (NAV)ndash NAV is updated based on overheard RTSCTSDATAACK packets each of whichspecified duration of a pending transmissionndash Nodes stay silent when carrier sensed (physicalvirtual)
bull Backoff intervals used to reduce collision probabilityndash When transmitting a packet choose a backoff interval in the range [0w] where wis contention windowndash Count down the backoff interval when channel is idlendash Count down is suspended if channel becomes busyndash When backoff interval reaches 0 transmit RTS
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 52
Random Access CSMACA (contrsquo) bull The time spent counting down backoff intervals is a part of MAC overhead
ndash Choosing a large w leads to large backoff intervals and can result in larger overheadndash Choosing a small w leads to a larger number of collisions (when two nodes countdown to 0 simultaneously)ndash Since the number of nodes attempting to transmit simultaneously may change withtime some mechanism to manage contention is needed
bull IEEE 80211 DCF contention window w is chosen dynamically depending oncollision occurrence (binary exponential backoff)ndash When a node fails to receive CTS in response to its RTS it increases the contentionwindowndash w is doubled (up to an upper bound)ndash When a node successfully completes a data transfer it restores w to wmin
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 53
Random Access fairnessbull Simplest definition of fairness all nodes should receive equal bandwidthbull An example of unfairness
ndash Assume that initially A and B both choose a backoff interval in range [031] buttheir RTSs collidendash Nodes A and B then choose from range [063]ndash Node A chooses 4 slots and B choose 60 slotsndash After A transmits a packet it next chooses from range [031]ndash It is possible that A may transmit several packets before B transmits its first packet
Ad hoc Sensor Networks Multiple channel access
Unfairness occurs when one node has backed off much more than some other node
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 54
Random Access fairness (contrsquo) MACAW solution for fairnessbull When a node transmits a packet it appends value w to the packet all nodes
hearing value w use it for their future transmission attemptsbull Since w is an indication of the level of congestion in the vicinity of a specific
receiver node MACAW proposes maintaining w independently for eachreceiver
bull Using per-receiver wi is particularly useful in multi-hop environments sincecongestion level at different receivers can be very different
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 55
Random Access energy efficiency bull Since many mobile hosts are operated by batteries MAC
protocols which conserve energy are of interestbull Two approaches to reduce energy consumption
ndash Power save turn off wireless interface when desirablendash Power control reduce transmit power
bull Power control has some more potential benefitsndash Reduced interference (in ad hoc networks) eg it improves ALOHAefficiency where user with high power can capture a packet even if there is acollisionndash Increased spatial reuse (in infrastructured wireless networks) eg it is usedin CDMA to maintain target SIR of voice and data users or generally it canbe used to maintain target SIR for different user classes (admission control)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 56
Random Access power control bull When C transmits to D at a high power level B cannot receive Arsquos
transmission due to interference from Cbull If C reduces transmit power it can still communicate with D
ndash Reduces energy consumption at node Cndash Allows B to receive Arsquos transmission (spatial reuse)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 57
bull A special wireless ad hoc networkndash Large number of nodesndash Battery poweredndash Topology and density changendash Nodes for a common taskndash In-network data processing
bull Sensor-net applicationsndash Sensor-triggered bursty trafficndash Can often tolerate some delay
Ad hoc Sensor Networks Multiple channel access
Characteristics of Sensor Network
Scalabilty and self configurationEnergy efficiencyAdaptivityFairness not importantLatency
AdaptivityEnergy-delay tradoff
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 58
Ad hoc Sensor Networks Multiple channel access
Wireless sensor node and sensor network
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 59
MAC for WSN attributes bull Attributes
ndash Collision avoidance (Basic task)ndash Energy efficiency (most primary in WSN)ndash Scalability and adaptivityndash Channel utilizationndash Latencyndash Throughputndash Fairness
bull What causes energy wastendash Collisionsndash Control packet overheadndash Overhearing unnecessary trafficndash Long idle time (it consumes 50-100 of the power)
Ad hoc Sensor Networks Multiple channel access
Primary
Secondary
Dominant factor
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 60
MAC for WSN
bull Contention-based protocols need to work hard in all directions for energysavingsndash Reduce idle listening ndash support low duty cyclendash Better collision avoidancendash Reduce control overheadndash Avoid unnecessary overhearing
Ad hoc Sensor Networks Multiple channel access
Scheduled Protocols Contention ProtocolsCollisions No YesEnergy efficiency Good BadScalability and adaptivity Bad GoodMulti-hop communication Difficult EasyTime synchronization Strict Loose or not required
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 61
MAC for WSN classification
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 62
MAC for WSN SMAC (2002)bull It has been developed by Ye Heidemann and Estrinbull Tradeoffs between (latency fairness) and energybull Major components in S-MAC
ndash Periodic listen and sleep turn off radio when sleeping the reduced dutycycle is ~ 10 (eg 120ms on 12s off)ndash Collision avoidance is based on contention similarly to IEEE 80211 adhoc modendash Overhearing avoidance the node is sleeping when neighbors talkndash Massage passing long message is fragmented amp sent in burst operationwith extended Tx time and re-transmit occurs immediately
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 63
Summarybull The algorithms and protocols that enable sharing by multiple users and
controls the access to the wireless channel from different users are calledMAC
bull There are two types of wireless networks Infrastructured wireless networksand ad hoc wireless networks
bull We also want our wireless system to have high user capacity and we canidentify 10 methods for increasing it
bull There are several MACs with no absolute advantage it should be tailoredto the system requirements
bull There are primary concerns for designing a WSN MAC (collisionavoidance energy efficiency scalability and adaptivity) while thesecondary concerns are as follows (channel utilization latency throughputfairness)
bull Next lecture Routing protocols
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 19
Ad hoc Sensor Networks Multiple channel access
bull Statictical measures of offered load Ncndash In Erlang B system the probability of call
blocking
ndash In Erlang C system the probabilty of waiting
ndash And the average wait time
c
c
cBLOCK
0
Pr
N
Nk
k
LN
Lk
=
=
sum
c
cc
wait 1
c0c
Pr 1
N
NkN
k
LL LL N kN
minus
=
=⎛ ⎞
+ minus⎜ ⎟⎝ ⎠
sum
wait waitc
Pr htN L
=minus
Multiple Access FDMA (contrsquo)Recall from
Queuing theory
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 20
Ad hoc Sensor Networks Multiple channel access
Multiple Access example of Erlang B systembull In an Erlang B system 40 channels are available A blocking probabilty of
less than 1 is required What is the traffic that can be serve if there is oneoperator or three operators
bull (Solution L(1 operator)gtL(3 operators))bull (Note The number of users increases faster than linearly with the number of
available channels The difference between actual increase and linearincrease is called the trunking gain It is preferable to have a large pool ofavailable channels that serves all users (a single operator) The reasons fornot choosing this approach are political not technical)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 21
Ad hoc Sensor Networks Multiple channel access
Multiple Access TDMAbull The allocated radio spectrum for the system is divided into time slots
ndash In each slot a user can transmit or receivendash A user occupies a cyclically repeating slotsndash A channel is logically defined as a particular time slot that repeats with some
periodbull TDMA systems buffer the data until its turn (time slot) comes to transmitbull In TDMATDD half of the slots in the frame is used for uplink channels the
other is used for downlink channelsbull In TDMAFDD a different carrier frequency is used for uplink or downlink
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 22
Ad hoc Sensor Networks Multiple channel access
Multiple Access TDMA (contrsquo)bull Preamble contains address and synchronization info to identify base
station and mobiles to each otherbull Guard times are used to allow synchronization of the receivers between
different slots and framesndash Different mobiles may have different propagation delays to a base station
because of different distances
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 23
Multiple Access TDMA (contrsquo)bull Each frame contains overhead bits and data bits Efficiency of frame is
defined as the percentage of data (information) bits to the total frame size inbits
η =(1 ndash boverheadbtotal) 100 where btotal= Tf R is the total number of bits in a frame Tf is the frameduration in sec boverhead is the number of overhead bits (GSM efficiency 73)
bull TDMA is usually combined with FDMAndash Neighboring cells are allocated and using different carrier frequencies (FDMA)
Inside a cell TDMA can be used Cells may be re-using the same frequency if theyare far from each-other
ndash There may be more than one channel allocated and used inside each cell Eachcarrier frequency (radio channel) may be using TDMA to further multiplex moreuser (ie having TDMA logical channels inside radio channels) For example inGSM each radio channel has 200KHz bandwidth and has 8 time slots (8 logicalchannels) Hence GSM is using FDMA combined with TDMA
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 24
Multiple Access TDMA (contrsquo)
Ad hoc Sensor Networks Multiple channel access
GSM (Europa) IS-54 (USA) PDC (Japan) DECT
Bit Rate 2708 Kbps 486 Kbps 42 Kbps 1152 Mbps
Bandwidth 200 KHz 30 KHz 25 KHz 1728 MHz
Time Slot 0577 ms 67 ms 67 ms 0417 ms
Upstream slots per frame 8 3 3 12
Duplexing FDD FDD FDD TDD
Efficiency 73 80 80 67
Modulation GMSK π4 DQPSK π4 DQPSK GMSK
Adaptive equalized Mandatory Mandatory Optional None
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 25
Multiple Access TDMA (contrsquo)bull Enables the sharing of a single radio channel among N usersbull Requires high data-rate per radio channel to support N users
simultaneously which requires adaptive equalizers to be usedin multipath environments
bull Transmission occurs in bursts (not continually) It enablespower saving by going to sleep modes in unrelated slots Itdiscontinues transmission and also enables mobile assistedhandoff
bull It equires synchronization of the receivers Need guard bits andsync bits which occurs large overhead per slot
bull Allocation of slots to mobile users should not be uniform Itmay depend on the traffic requirement of mobiles This bringsextra flexibility and efficiency compared to FDMA systems
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2620111015 TAacuteMOP ndash 412-082AKMR-2009-0006 26
Ad hoc Sensor Networks Multiple channel access
Multiple Access spread spectrum multiple accessbull SSMA uses signals that have transmission bandwidth that is
several orders of magnitude larger than minimum requiredRF bandwidth
bull It providesndash Immunity to multipath interferencendash Robust multiple access
bull Two techniquesndash Frequency Hopped Multiple Access (FHMA)ndash Direct Sequence Multiple Access (DSMA) or Code Division
Multiple Access (CDMA)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2720111015 TAacuteMOP ndash 412-082AKMR-2009-0006 27
Ad hoc Sensor Networks Multiple channel access
Multiple Access FHMAbull The carrier frequency of users are varied in a pseudo-random
fashionndash Each user is using a narrowband channel (spectrum) at a specific
instance of timendash The random change in frequency make the change of using the same
narrowband channel very lowbull The sender receiver change frequency (calling hopping)
using the same pseudo-random sequence hence they aresynchronized
bull Rate of hopping versus Symbol ratendash If hopping rate is greather Fast Frequency Hoppingndash If symbol rate is greater Slow Frequency Hopping
bull GSM and Bluetooth are example systems
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2820111015 TAacuteMOP ndash 412-082AKMR-2009-0006 28
Ad hoc Sensor Networks Multiple channel access
Multiple Access FHMA (contrsquo)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2920111015 TAacuteMOP ndash 412-082AKMR-2009-0006 29
Ad hoc Sensor Networks Multiple channel access
Multiple Access CDMAbull The narrowband message signal is multiplied by a very large bandwidth
signal called spreading signal (code) before modulation and transmissionover the air This is called spreading
bull Spreading signal use a pseudo-noise (PN) sequence (a pseudo-randomsequence) called codeword which are orthogonal (low autocorrelation)
bull The receiver correlator distinguishes the senders signal by examining thewideband signal with the same time-synchronized spreading code(despreading)
bull Advantagesndash Low power spectral densityndash Interference limited operationndash Privacyndash Reduction of multipath affects by using a larger spectrumndash Users can start their transmission at any time (random access possible)ndash Cell capacity is soft and higher than TDMA and FDMAndash No frequency management and no equalizers and quard time needed
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3020111015 TAacuteMOP ndash 412-082AKMR-2009-0006 30
Ad hoc Sensor Networks Multiple channel access
Multiple Access near far problem and power controlbull At a receiver the signals may come from various sources (multiuser) The
strongest signal usually captures the modulator The other signals areconsidered as noise Each source may have different distances to the basestation
bull In CDMA we want a BS to receive CDMA coded signals from variousmobile users at the same time
ndash Therefore the receiver power at the BS for all mobile users should be close toeach other This requires power control at the mobiles
bull Power Control BS monitors the RSSI (Received Signal Strength Indicator)values from different mobiles and then sends power change commands to themobiles over a forward channel The mobiles then adjust their transmitpower
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3120111015 TAacuteMOP ndash 412-082AKMR-2009-0006 31
Ad hoc Sensor Networks Multiple channel access
Multiple Access hybrid spread spectrum techniquesbull FDMACDMA
ndash Available wideband spectrum is divided into a number narrowband radio channels CDMAis employed inside each channel
bull DSFHMAndash The signals are spread using spreading codes (direct sequence signals are obtained) but
these signal are not transmitted over a constant carrier frequency they are transmitted over afrequency hopping carrier frequency
bull Time Division CDMA (TCDMA)ndash Each cell is using a different spreading code (CDMA employed between cells) that is
conveyed to the mobiles in its rangendash Inside each cell (inside a CDMA channel) TDMA is employed to multiplex multiple users
bull Time Division Frequency Hoppingndash At each time slot the user is hopped to a new frequency according to a pseudo-random
hopping sequencendash Employed in severe co-interference and multi-path environmentsndash Bluetooth and GSM are using this technique
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3220111015 TAacuteMOP ndash 412-082AKMR-2009-0006 32
Ad hoc Sensor Networks Multiple channel access
Multiple Access comparison TDMAFDMACDMA
Techniques TDMA FDMA CDMA SDMA
Idea
segment sending time into disjoint time-slots
Segment the frequency band into disjoint sub-bands
Spread the spectrum using orthogonal codes
Segment space into sectors
Signal separation
Synchronization in the time domain
Filtering in the frequency domain
Code Directed antennas
Advantage
Established flexible
Simple established robust
Flexible less frequency planning needed soft handover
Simple increases capacity
DisadvantageQuard space synchronization difficult
Inflexible scarce source
Complex receiver power control
inflexible
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3320111015 TAacuteMOP ndash 412-082AKMR-2009-0006 33
Ad hoc Sensor Networks Multiple channel access
User capacity per cell
The user capacity C is defined as the total number of active users percell that the system can support while meeting a common BERconstrain For orthogonal multiple access (as FDMA and TDMA)
C = Ncellwhere Ncell is the number of channels assigned to any given cellThe total number of orthogonal channels of bandwidth Bs that can becreated from a total system bandwidth of B is B Bs The reuse factorsatisfies N = (B Bs) Ncell this implies
C = (B Bs) N
Recall from Chapter 1
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3420111015 TAacuteMOP ndash 412-082AKMR-2009-0006 34
Ad hoc Sensor Networks Multiple channel access
10 methods for increasing capacity1 Increasing the amount of spectrum used very expensive2 More efficient modulation format and coding3 Better source coding4 Adaptive modulation and coding5 Discontinuous transmission6 Multiuser detection CDMA systems7 Reduction of cell radius effective but very expensive smaller
cells require more handovers8 Use of sector cells tripled cells have tripled BS antennas9 Use of an overlay structure10 Multiple antennas diversity MIMO systems SDMA
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3520111015 TAacuteMOP ndash 412-082AKMR-2009-0006 35
Ad hoc Sensor Networks Multiple channel access
bull Distributed operation is requiredbull Synchronization is required in TDMA-based systemsbull Hidden terminals are nodes hidden from a senderbull Exposed terminals are exposed nodes preventing a sender
from sendingbull Throughput needs to be maximizedbull Access delay should be minimizedbull Fairness refers to provide an equal share to all competing
nodesbull Real-time traffic support is required for voice video and
real-time databull Resource reservation is required for QoSbull Ability to measure resource availability handles the
resourcesbull Capability for power control reduces the energy
consumption
Random access in adhoc networks
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3620111015 TAacuteMOP ndash 412-082AKMR-2009-0006 36
Ad hoc Sensor Networks Multiple channel access
Random accessbull All random access techniques are based on packetized data or packet radio where user
date is collected into packets of M bitsbull Collision if packets from different users overlap in timebull The transmission time of a packet is τ = M R where R [bps] the data ratebull Analysis of random access techniques assumes that the users generate packets according
to memoryless Poisson process at a rate λ [packets per unit time] the probability that thenumber of packet arrivals in a time period [0t] denoted by X(t) is equal to integer k isgiven by
Pr(X(t)=k) = (λt)k k e-λt
bull The traffic load is defined as L = λ τ If L gt 1 means on avarage more packets arrive inthe system over a given time period than can be transmitted in that period so systemswith Lgt1 are unstable
bull The throughput is defined as the ratio of the average number of packets successfullytransmitted in any given time interval divided by the number of attempted transmissionsin that interval
T = L Pr(succesful packet transmission)bull The effective data rate of the system is RT
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3720111015 TAacuteMOP ndash 412-082AKMR-2009-0006 37
Ad hoc Sensor Networks Multiple channel access
Random access (contrsquo)bull We will examine the following attributes
ndash Channel utilization (Throughput)ndash Latencyndash Collision avoidance (Hidden and exposed terminals problem)ndash Reliability (ACK)ndash Energy efficiency (power control)ndash Fairnessndash Throughput needs to be maximized
bull We will discuss the following techniquesndash Aloha and slotted Alohandash CSMA Protocols (1-persistent non-persistent p-persistent CSMA CSMACD)ndash MACAndash MACAWndash Energy efficient MAC in wireless sensor networks (eg SMAC)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 38
Random Access ALOHA (1970)bull ALOHA was developed for a wireless system at the University of Hawaii
(Abramson et al)ndash Multiple remote stations plus one base stationndash Frame transmissions are made at one frequency from a remote station to the basestation the base station re-broadcasts frames on another frequency Contention is foraccess to the base station
bull Frames can be sent at any time to the base stationndash If no acknowledgment (ACK) is received assume that the frame is lost andretransmit after waiting a random amount of timendash If more than one station broadcasts on the base station access frequency at the sametime ndash a collision ndash interference will destroy the frames
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 39
Random Access ALOHA (contrsquo)
bull The probability that no packets generated during the tima [-τ τ] is given by (nocollision)
Pr(X(t)=0) = (λ2τ)0 0 e-2λτ = e-2Lwith corresponding throughput
T = L e-2L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 05 05e 018
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 40
Random Access slotted ALOHA (1971)bull Improvement Time is slotted and a packet can only be transmitted at the beginning of
one slot Thus it can reduce the collision duration
bull The probability that no packets generated during the time [0 τ] is given by (nocollision)
Pr(X(t)=0) = (λτ)0 0 e-λτ = e-Lwith corresponding throughput T = L e-L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 1 e 037
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 41
bull Slotted Aloha has double the maximumthroughput as pure Aloha and achievesthis maximum at a higher offered loadThe effective data rate is still less than40 of the raw transmission rate
bull Aloha does not listen to the carrier beforetransmission more sophisticatedtechniques are needed to increaseefficiency
bull Further problem the delayndashDelay
ndashThe mean packet delay
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
( ) ( )delayPr 1 e ekL Lkτ minus minus= = minus
( )delay1
1 e e ekL L L
kMPD E kτ
infinminus minus
=
= = minus =sum
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 42
Random Access Carrier Sense Multiple Accessbull CSMA (Carrier Sense Multiple Access) listen to the carrier before transmission
and transmits if channel is idlebull Detection delay and propagation delay are two important parameters
ndash Detection delay time required to sense the carrier and decide if it is idle or busyndash Propagation delay distancespeed_of_ligth The time required for bit to travel
from transmitter to the receiver
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 43
Random Access CSMA variations
Ad hoc Sensor Networks Multiple channel access
1-persistent CSMA Non-persistent CSMA
There will always be a collision iftwo nodes want to retransmit(usually you stop transmissionattempts after few tries)
Random backoff reduces probabilityof collisions Waste idle time if thebackoff time is too long
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 44
Random Access CSMA variations (contrsquo)
Ad hoc Sensor Networks Multiple channel access
p-persistent CSMA CSMA CD (Collision Detection)
A good tradeoff between non-persistent and 1-persistent CSMA
Same with CSMA however a stationalso listen to the carrier whiletransmitting to see if the transmissioncollides with someone elsetransmission It can be used in listen-while-talk capable channels (fullduplex) In single radio channels thetransmission need to be interrupted inorder to sense the channel
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 45
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 46
Random Access CSMA (contrsquo)bull How to select probability p
ndash Assume that M nodes have a packet to send and the medium is busy then Mp is theexpected number of nodes that will attempt to transmit once the medium becomesidle If Mp gt 1 then a collision is expected to occur Therefore the network mustmake sure that Mp lt 1 to avoid collision where M is the maximum number of nodesthat can be active at a time
bull Problems1 The mean packet delay increases exponently with increasing offered load2 CSMA protocols sense the carrier but sensing the carrier does not always releasestrue information about the status of the wireless channel There are two problems thatare unique to wireless channels (different than wireline channels)ndash Hidden terminal problemndash Exposed terminal problem
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 47
Hidden terminal problembull A sends to B C cannot receive Abull C wants to send to B C senses a
ldquofreerdquo medium (CS fails)bull collision at B A cannot receive the
collision (CD fails)bull A is ldquohiddenrdquo for C
Ad hoc Sensor Networks Multiple channel access
Random Access hidden and exposed terminal problemExposed terminal problem
bull B sends to A C wants to send to Dbull C has to wait CS signals a medium
in usebull since A is outside the radio range of
C waiting is not necessarybull C is ldquoexposedrdquo to B
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 48
Random Access MACAWhen a station wants to transmit databull It sends an RTS (Ready-to-Send) packet to the intended receiver
ndash The RTS packet contains the length of the data that needs to be transmittedndash Any station other than the intended recipient hearing RTS defers transmission for atime duration equal to the end of the corresponding CTS reception
bull The receiver sends back CTS (Clear-to-Send) packet back to sender if it isavailable to receivendash The CTS packet contains the length of the data that original sender wants totransmitndash Any station other than the original RTS sender hearing CTS defers transmissionuntil the data is sent
bull The original sender upon reception of the CTS starts transmitting
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 49
Hidden terminal problem
Ad hoc Sensor Networks Multiple channel access
Random Access MACA (contrsquo)Exposed terminal problem
Waiting time of node X is muchsmaller than waiting time of node C
bull C defers transmission upon hearing BrsquosRTS until B could get CTS from Abull After that C can start transmission to DFor that it first sends an RTSbull C is not longer exposed to the datatransmission of B
Problem It does not address the collision of RTS packet
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 50
Random Access reliabilitybull Wireless links are prone to have errors High packet loss rate detrimental to
transport-layer performance Mechanisms needed to reduce packet loss rateexperienced by upper layers
bull When node B receives a data packet from node A node B sends anAcknowledgement (Ack) This approach adopted in many protocols
bull If node A fails to receive an Ack it will retransmit the packet
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 51
Random Access MACAW (CSMACA) bull Physical carrier sense and virtual carrier sense using Network Allocation
Vector (NAV)ndash NAV is updated based on overheard RTSCTSDATAACK packets each of whichspecified duration of a pending transmissionndash Nodes stay silent when carrier sensed (physicalvirtual)
bull Backoff intervals used to reduce collision probabilityndash When transmitting a packet choose a backoff interval in the range [0w] where wis contention windowndash Count down the backoff interval when channel is idlendash Count down is suspended if channel becomes busyndash When backoff interval reaches 0 transmit RTS
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 52
Random Access CSMACA (contrsquo) bull The time spent counting down backoff intervals is a part of MAC overhead
ndash Choosing a large w leads to large backoff intervals and can result in larger overheadndash Choosing a small w leads to a larger number of collisions (when two nodes countdown to 0 simultaneously)ndash Since the number of nodes attempting to transmit simultaneously may change withtime some mechanism to manage contention is needed
bull IEEE 80211 DCF contention window w is chosen dynamically depending oncollision occurrence (binary exponential backoff)ndash When a node fails to receive CTS in response to its RTS it increases the contentionwindowndash w is doubled (up to an upper bound)ndash When a node successfully completes a data transfer it restores w to wmin
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 53
Random Access fairnessbull Simplest definition of fairness all nodes should receive equal bandwidthbull An example of unfairness
ndash Assume that initially A and B both choose a backoff interval in range [031] buttheir RTSs collidendash Nodes A and B then choose from range [063]ndash Node A chooses 4 slots and B choose 60 slotsndash After A transmits a packet it next chooses from range [031]ndash It is possible that A may transmit several packets before B transmits its first packet
Ad hoc Sensor Networks Multiple channel access
Unfairness occurs when one node has backed off much more than some other node
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 54
Random Access fairness (contrsquo) MACAW solution for fairnessbull When a node transmits a packet it appends value w to the packet all nodes
hearing value w use it for their future transmission attemptsbull Since w is an indication of the level of congestion in the vicinity of a specific
receiver node MACAW proposes maintaining w independently for eachreceiver
bull Using per-receiver wi is particularly useful in multi-hop environments sincecongestion level at different receivers can be very different
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 55
Random Access energy efficiency bull Since many mobile hosts are operated by batteries MAC
protocols which conserve energy are of interestbull Two approaches to reduce energy consumption
ndash Power save turn off wireless interface when desirablendash Power control reduce transmit power
bull Power control has some more potential benefitsndash Reduced interference (in ad hoc networks) eg it improves ALOHAefficiency where user with high power can capture a packet even if there is acollisionndash Increased spatial reuse (in infrastructured wireless networks) eg it is usedin CDMA to maintain target SIR of voice and data users or generally it canbe used to maintain target SIR for different user classes (admission control)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 56
Random Access power control bull When C transmits to D at a high power level B cannot receive Arsquos
transmission due to interference from Cbull If C reduces transmit power it can still communicate with D
ndash Reduces energy consumption at node Cndash Allows B to receive Arsquos transmission (spatial reuse)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 57
bull A special wireless ad hoc networkndash Large number of nodesndash Battery poweredndash Topology and density changendash Nodes for a common taskndash In-network data processing
bull Sensor-net applicationsndash Sensor-triggered bursty trafficndash Can often tolerate some delay
Ad hoc Sensor Networks Multiple channel access
Characteristics of Sensor Network
Scalabilty and self configurationEnergy efficiencyAdaptivityFairness not importantLatency
AdaptivityEnergy-delay tradoff
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 58
Ad hoc Sensor Networks Multiple channel access
Wireless sensor node and sensor network
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 59
MAC for WSN attributes bull Attributes
ndash Collision avoidance (Basic task)ndash Energy efficiency (most primary in WSN)ndash Scalability and adaptivityndash Channel utilizationndash Latencyndash Throughputndash Fairness
bull What causes energy wastendash Collisionsndash Control packet overheadndash Overhearing unnecessary trafficndash Long idle time (it consumes 50-100 of the power)
Ad hoc Sensor Networks Multiple channel access
Primary
Secondary
Dominant factor
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 60
MAC for WSN
bull Contention-based protocols need to work hard in all directions for energysavingsndash Reduce idle listening ndash support low duty cyclendash Better collision avoidancendash Reduce control overheadndash Avoid unnecessary overhearing
Ad hoc Sensor Networks Multiple channel access
Scheduled Protocols Contention ProtocolsCollisions No YesEnergy efficiency Good BadScalability and adaptivity Bad GoodMulti-hop communication Difficult EasyTime synchronization Strict Loose or not required
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 61
MAC for WSN classification
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 62
MAC for WSN SMAC (2002)bull It has been developed by Ye Heidemann and Estrinbull Tradeoffs between (latency fairness) and energybull Major components in S-MAC
ndash Periodic listen and sleep turn off radio when sleeping the reduced dutycycle is ~ 10 (eg 120ms on 12s off)ndash Collision avoidance is based on contention similarly to IEEE 80211 adhoc modendash Overhearing avoidance the node is sleeping when neighbors talkndash Massage passing long message is fragmented amp sent in burst operationwith extended Tx time and re-transmit occurs immediately
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 63
Summarybull The algorithms and protocols that enable sharing by multiple users and
controls the access to the wireless channel from different users are calledMAC
bull There are two types of wireless networks Infrastructured wireless networksand ad hoc wireless networks
bull We also want our wireless system to have high user capacity and we canidentify 10 methods for increasing it
bull There are several MACs with no absolute advantage it should be tailoredto the system requirements
bull There are primary concerns for designing a WSN MAC (collisionavoidance energy efficiency scalability and adaptivity) while thesecondary concerns are as follows (channel utilization latency throughputfairness)
bull Next lecture Routing protocols
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 20
Ad hoc Sensor Networks Multiple channel access
Multiple Access example of Erlang B systembull In an Erlang B system 40 channels are available A blocking probabilty of
less than 1 is required What is the traffic that can be serve if there is oneoperator or three operators
bull (Solution L(1 operator)gtL(3 operators))bull (Note The number of users increases faster than linearly with the number of
available channels The difference between actual increase and linearincrease is called the trunking gain It is preferable to have a large pool ofavailable channels that serves all users (a single operator) The reasons fornot choosing this approach are political not technical)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 21
Ad hoc Sensor Networks Multiple channel access
Multiple Access TDMAbull The allocated radio spectrum for the system is divided into time slots
ndash In each slot a user can transmit or receivendash A user occupies a cyclically repeating slotsndash A channel is logically defined as a particular time slot that repeats with some
periodbull TDMA systems buffer the data until its turn (time slot) comes to transmitbull In TDMATDD half of the slots in the frame is used for uplink channels the
other is used for downlink channelsbull In TDMAFDD a different carrier frequency is used for uplink or downlink
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 22
Ad hoc Sensor Networks Multiple channel access
Multiple Access TDMA (contrsquo)bull Preamble contains address and synchronization info to identify base
station and mobiles to each otherbull Guard times are used to allow synchronization of the receivers between
different slots and framesndash Different mobiles may have different propagation delays to a base station
because of different distances
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 23
Multiple Access TDMA (contrsquo)bull Each frame contains overhead bits and data bits Efficiency of frame is
defined as the percentage of data (information) bits to the total frame size inbits
η =(1 ndash boverheadbtotal) 100 where btotal= Tf R is the total number of bits in a frame Tf is the frameduration in sec boverhead is the number of overhead bits (GSM efficiency 73)
bull TDMA is usually combined with FDMAndash Neighboring cells are allocated and using different carrier frequencies (FDMA)
Inside a cell TDMA can be used Cells may be re-using the same frequency if theyare far from each-other
ndash There may be more than one channel allocated and used inside each cell Eachcarrier frequency (radio channel) may be using TDMA to further multiplex moreuser (ie having TDMA logical channels inside radio channels) For example inGSM each radio channel has 200KHz bandwidth and has 8 time slots (8 logicalchannels) Hence GSM is using FDMA combined with TDMA
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 24
Multiple Access TDMA (contrsquo)
Ad hoc Sensor Networks Multiple channel access
GSM (Europa) IS-54 (USA) PDC (Japan) DECT
Bit Rate 2708 Kbps 486 Kbps 42 Kbps 1152 Mbps
Bandwidth 200 KHz 30 KHz 25 KHz 1728 MHz
Time Slot 0577 ms 67 ms 67 ms 0417 ms
Upstream slots per frame 8 3 3 12
Duplexing FDD FDD FDD TDD
Efficiency 73 80 80 67
Modulation GMSK π4 DQPSK π4 DQPSK GMSK
Adaptive equalized Mandatory Mandatory Optional None
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 25
Multiple Access TDMA (contrsquo)bull Enables the sharing of a single radio channel among N usersbull Requires high data-rate per radio channel to support N users
simultaneously which requires adaptive equalizers to be usedin multipath environments
bull Transmission occurs in bursts (not continually) It enablespower saving by going to sleep modes in unrelated slots Itdiscontinues transmission and also enables mobile assistedhandoff
bull It equires synchronization of the receivers Need guard bits andsync bits which occurs large overhead per slot
bull Allocation of slots to mobile users should not be uniform Itmay depend on the traffic requirement of mobiles This bringsextra flexibility and efficiency compared to FDMA systems
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2620111015 TAacuteMOP ndash 412-082AKMR-2009-0006 26
Ad hoc Sensor Networks Multiple channel access
Multiple Access spread spectrum multiple accessbull SSMA uses signals that have transmission bandwidth that is
several orders of magnitude larger than minimum requiredRF bandwidth
bull It providesndash Immunity to multipath interferencendash Robust multiple access
bull Two techniquesndash Frequency Hopped Multiple Access (FHMA)ndash Direct Sequence Multiple Access (DSMA) or Code Division
Multiple Access (CDMA)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2720111015 TAacuteMOP ndash 412-082AKMR-2009-0006 27
Ad hoc Sensor Networks Multiple channel access
Multiple Access FHMAbull The carrier frequency of users are varied in a pseudo-random
fashionndash Each user is using a narrowband channel (spectrum) at a specific
instance of timendash The random change in frequency make the change of using the same
narrowband channel very lowbull The sender receiver change frequency (calling hopping)
using the same pseudo-random sequence hence they aresynchronized
bull Rate of hopping versus Symbol ratendash If hopping rate is greather Fast Frequency Hoppingndash If symbol rate is greater Slow Frequency Hopping
bull GSM and Bluetooth are example systems
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2820111015 TAacuteMOP ndash 412-082AKMR-2009-0006 28
Ad hoc Sensor Networks Multiple channel access
Multiple Access FHMA (contrsquo)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2920111015 TAacuteMOP ndash 412-082AKMR-2009-0006 29
Ad hoc Sensor Networks Multiple channel access
Multiple Access CDMAbull The narrowband message signal is multiplied by a very large bandwidth
signal called spreading signal (code) before modulation and transmissionover the air This is called spreading
bull Spreading signal use a pseudo-noise (PN) sequence (a pseudo-randomsequence) called codeword which are orthogonal (low autocorrelation)
bull The receiver correlator distinguishes the senders signal by examining thewideband signal with the same time-synchronized spreading code(despreading)
bull Advantagesndash Low power spectral densityndash Interference limited operationndash Privacyndash Reduction of multipath affects by using a larger spectrumndash Users can start their transmission at any time (random access possible)ndash Cell capacity is soft and higher than TDMA and FDMAndash No frequency management and no equalizers and quard time needed
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3020111015 TAacuteMOP ndash 412-082AKMR-2009-0006 30
Ad hoc Sensor Networks Multiple channel access
Multiple Access near far problem and power controlbull At a receiver the signals may come from various sources (multiuser) The
strongest signal usually captures the modulator The other signals areconsidered as noise Each source may have different distances to the basestation
bull In CDMA we want a BS to receive CDMA coded signals from variousmobile users at the same time
ndash Therefore the receiver power at the BS for all mobile users should be close toeach other This requires power control at the mobiles
bull Power Control BS monitors the RSSI (Received Signal Strength Indicator)values from different mobiles and then sends power change commands to themobiles over a forward channel The mobiles then adjust their transmitpower
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3120111015 TAacuteMOP ndash 412-082AKMR-2009-0006 31
Ad hoc Sensor Networks Multiple channel access
Multiple Access hybrid spread spectrum techniquesbull FDMACDMA
ndash Available wideband spectrum is divided into a number narrowband radio channels CDMAis employed inside each channel
bull DSFHMAndash The signals are spread using spreading codes (direct sequence signals are obtained) but
these signal are not transmitted over a constant carrier frequency they are transmitted over afrequency hopping carrier frequency
bull Time Division CDMA (TCDMA)ndash Each cell is using a different spreading code (CDMA employed between cells) that is
conveyed to the mobiles in its rangendash Inside each cell (inside a CDMA channel) TDMA is employed to multiplex multiple users
bull Time Division Frequency Hoppingndash At each time slot the user is hopped to a new frequency according to a pseudo-random
hopping sequencendash Employed in severe co-interference and multi-path environmentsndash Bluetooth and GSM are using this technique
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3220111015 TAacuteMOP ndash 412-082AKMR-2009-0006 32
Ad hoc Sensor Networks Multiple channel access
Multiple Access comparison TDMAFDMACDMA
Techniques TDMA FDMA CDMA SDMA
Idea
segment sending time into disjoint time-slots
Segment the frequency band into disjoint sub-bands
Spread the spectrum using orthogonal codes
Segment space into sectors
Signal separation
Synchronization in the time domain
Filtering in the frequency domain
Code Directed antennas
Advantage
Established flexible
Simple established robust
Flexible less frequency planning needed soft handover
Simple increases capacity
DisadvantageQuard space synchronization difficult
Inflexible scarce source
Complex receiver power control
inflexible
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3320111015 TAacuteMOP ndash 412-082AKMR-2009-0006 33
Ad hoc Sensor Networks Multiple channel access
User capacity per cell
The user capacity C is defined as the total number of active users percell that the system can support while meeting a common BERconstrain For orthogonal multiple access (as FDMA and TDMA)
C = Ncellwhere Ncell is the number of channels assigned to any given cellThe total number of orthogonal channels of bandwidth Bs that can becreated from a total system bandwidth of B is B Bs The reuse factorsatisfies N = (B Bs) Ncell this implies
C = (B Bs) N
Recall from Chapter 1
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3420111015 TAacuteMOP ndash 412-082AKMR-2009-0006 34
Ad hoc Sensor Networks Multiple channel access
10 methods for increasing capacity1 Increasing the amount of spectrum used very expensive2 More efficient modulation format and coding3 Better source coding4 Adaptive modulation and coding5 Discontinuous transmission6 Multiuser detection CDMA systems7 Reduction of cell radius effective but very expensive smaller
cells require more handovers8 Use of sector cells tripled cells have tripled BS antennas9 Use of an overlay structure10 Multiple antennas diversity MIMO systems SDMA
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3520111015 TAacuteMOP ndash 412-082AKMR-2009-0006 35
Ad hoc Sensor Networks Multiple channel access
bull Distributed operation is requiredbull Synchronization is required in TDMA-based systemsbull Hidden terminals are nodes hidden from a senderbull Exposed terminals are exposed nodes preventing a sender
from sendingbull Throughput needs to be maximizedbull Access delay should be minimizedbull Fairness refers to provide an equal share to all competing
nodesbull Real-time traffic support is required for voice video and
real-time databull Resource reservation is required for QoSbull Ability to measure resource availability handles the
resourcesbull Capability for power control reduces the energy
consumption
Random access in adhoc networks
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3620111015 TAacuteMOP ndash 412-082AKMR-2009-0006 36
Ad hoc Sensor Networks Multiple channel access
Random accessbull All random access techniques are based on packetized data or packet radio where user
date is collected into packets of M bitsbull Collision if packets from different users overlap in timebull The transmission time of a packet is τ = M R where R [bps] the data ratebull Analysis of random access techniques assumes that the users generate packets according
to memoryless Poisson process at a rate λ [packets per unit time] the probability that thenumber of packet arrivals in a time period [0t] denoted by X(t) is equal to integer k isgiven by
Pr(X(t)=k) = (λt)k k e-λt
bull The traffic load is defined as L = λ τ If L gt 1 means on avarage more packets arrive inthe system over a given time period than can be transmitted in that period so systemswith Lgt1 are unstable
bull The throughput is defined as the ratio of the average number of packets successfullytransmitted in any given time interval divided by the number of attempted transmissionsin that interval
T = L Pr(succesful packet transmission)bull The effective data rate of the system is RT
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3720111015 TAacuteMOP ndash 412-082AKMR-2009-0006 37
Ad hoc Sensor Networks Multiple channel access
Random access (contrsquo)bull We will examine the following attributes
ndash Channel utilization (Throughput)ndash Latencyndash Collision avoidance (Hidden and exposed terminals problem)ndash Reliability (ACK)ndash Energy efficiency (power control)ndash Fairnessndash Throughput needs to be maximized
bull We will discuss the following techniquesndash Aloha and slotted Alohandash CSMA Protocols (1-persistent non-persistent p-persistent CSMA CSMACD)ndash MACAndash MACAWndash Energy efficient MAC in wireless sensor networks (eg SMAC)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 38
Random Access ALOHA (1970)bull ALOHA was developed for a wireless system at the University of Hawaii
(Abramson et al)ndash Multiple remote stations plus one base stationndash Frame transmissions are made at one frequency from a remote station to the basestation the base station re-broadcasts frames on another frequency Contention is foraccess to the base station
bull Frames can be sent at any time to the base stationndash If no acknowledgment (ACK) is received assume that the frame is lost andretransmit after waiting a random amount of timendash If more than one station broadcasts on the base station access frequency at the sametime ndash a collision ndash interference will destroy the frames
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 39
Random Access ALOHA (contrsquo)
bull The probability that no packets generated during the tima [-τ τ] is given by (nocollision)
Pr(X(t)=0) = (λ2τ)0 0 e-2λτ = e-2Lwith corresponding throughput
T = L e-2L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 05 05e 018
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 40
Random Access slotted ALOHA (1971)bull Improvement Time is slotted and a packet can only be transmitted at the beginning of
one slot Thus it can reduce the collision duration
bull The probability that no packets generated during the time [0 τ] is given by (nocollision)
Pr(X(t)=0) = (λτ)0 0 e-λτ = e-Lwith corresponding throughput T = L e-L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 1 e 037
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 41
bull Slotted Aloha has double the maximumthroughput as pure Aloha and achievesthis maximum at a higher offered loadThe effective data rate is still less than40 of the raw transmission rate
bull Aloha does not listen to the carrier beforetransmission more sophisticatedtechniques are needed to increaseefficiency
bull Further problem the delayndashDelay
ndashThe mean packet delay
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
( ) ( )delayPr 1 e ekL Lkτ minus minus= = minus
( )delay1
1 e e ekL L L
kMPD E kτ
infinminus minus
=
= = minus =sum
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 42
Random Access Carrier Sense Multiple Accessbull CSMA (Carrier Sense Multiple Access) listen to the carrier before transmission
and transmits if channel is idlebull Detection delay and propagation delay are two important parameters
ndash Detection delay time required to sense the carrier and decide if it is idle or busyndash Propagation delay distancespeed_of_ligth The time required for bit to travel
from transmitter to the receiver
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 43
Random Access CSMA variations
Ad hoc Sensor Networks Multiple channel access
1-persistent CSMA Non-persistent CSMA
There will always be a collision iftwo nodes want to retransmit(usually you stop transmissionattempts after few tries)
Random backoff reduces probabilityof collisions Waste idle time if thebackoff time is too long
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 44
Random Access CSMA variations (contrsquo)
Ad hoc Sensor Networks Multiple channel access
p-persistent CSMA CSMA CD (Collision Detection)
A good tradeoff between non-persistent and 1-persistent CSMA
Same with CSMA however a stationalso listen to the carrier whiletransmitting to see if the transmissioncollides with someone elsetransmission It can be used in listen-while-talk capable channels (fullduplex) In single radio channels thetransmission need to be interrupted inorder to sense the channel
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 45
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 46
Random Access CSMA (contrsquo)bull How to select probability p
ndash Assume that M nodes have a packet to send and the medium is busy then Mp is theexpected number of nodes that will attempt to transmit once the medium becomesidle If Mp gt 1 then a collision is expected to occur Therefore the network mustmake sure that Mp lt 1 to avoid collision where M is the maximum number of nodesthat can be active at a time
bull Problems1 The mean packet delay increases exponently with increasing offered load2 CSMA protocols sense the carrier but sensing the carrier does not always releasestrue information about the status of the wireless channel There are two problems thatare unique to wireless channels (different than wireline channels)ndash Hidden terminal problemndash Exposed terminal problem
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 47
Hidden terminal problembull A sends to B C cannot receive Abull C wants to send to B C senses a
ldquofreerdquo medium (CS fails)bull collision at B A cannot receive the
collision (CD fails)bull A is ldquohiddenrdquo for C
Ad hoc Sensor Networks Multiple channel access
Random Access hidden and exposed terminal problemExposed terminal problem
bull B sends to A C wants to send to Dbull C has to wait CS signals a medium
in usebull since A is outside the radio range of
C waiting is not necessarybull C is ldquoexposedrdquo to B
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 48
Random Access MACAWhen a station wants to transmit databull It sends an RTS (Ready-to-Send) packet to the intended receiver
ndash The RTS packet contains the length of the data that needs to be transmittedndash Any station other than the intended recipient hearing RTS defers transmission for atime duration equal to the end of the corresponding CTS reception
bull The receiver sends back CTS (Clear-to-Send) packet back to sender if it isavailable to receivendash The CTS packet contains the length of the data that original sender wants totransmitndash Any station other than the original RTS sender hearing CTS defers transmissionuntil the data is sent
bull The original sender upon reception of the CTS starts transmitting
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 49
Hidden terminal problem
Ad hoc Sensor Networks Multiple channel access
Random Access MACA (contrsquo)Exposed terminal problem
Waiting time of node X is muchsmaller than waiting time of node C
bull C defers transmission upon hearing BrsquosRTS until B could get CTS from Abull After that C can start transmission to DFor that it first sends an RTSbull C is not longer exposed to the datatransmission of B
Problem It does not address the collision of RTS packet
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 50
Random Access reliabilitybull Wireless links are prone to have errors High packet loss rate detrimental to
transport-layer performance Mechanisms needed to reduce packet loss rateexperienced by upper layers
bull When node B receives a data packet from node A node B sends anAcknowledgement (Ack) This approach adopted in many protocols
bull If node A fails to receive an Ack it will retransmit the packet
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 51
Random Access MACAW (CSMACA) bull Physical carrier sense and virtual carrier sense using Network Allocation
Vector (NAV)ndash NAV is updated based on overheard RTSCTSDATAACK packets each of whichspecified duration of a pending transmissionndash Nodes stay silent when carrier sensed (physicalvirtual)
bull Backoff intervals used to reduce collision probabilityndash When transmitting a packet choose a backoff interval in the range [0w] where wis contention windowndash Count down the backoff interval when channel is idlendash Count down is suspended if channel becomes busyndash When backoff interval reaches 0 transmit RTS
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 52
Random Access CSMACA (contrsquo) bull The time spent counting down backoff intervals is a part of MAC overhead
ndash Choosing a large w leads to large backoff intervals and can result in larger overheadndash Choosing a small w leads to a larger number of collisions (when two nodes countdown to 0 simultaneously)ndash Since the number of nodes attempting to transmit simultaneously may change withtime some mechanism to manage contention is needed
bull IEEE 80211 DCF contention window w is chosen dynamically depending oncollision occurrence (binary exponential backoff)ndash When a node fails to receive CTS in response to its RTS it increases the contentionwindowndash w is doubled (up to an upper bound)ndash When a node successfully completes a data transfer it restores w to wmin
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 53
Random Access fairnessbull Simplest definition of fairness all nodes should receive equal bandwidthbull An example of unfairness
ndash Assume that initially A and B both choose a backoff interval in range [031] buttheir RTSs collidendash Nodes A and B then choose from range [063]ndash Node A chooses 4 slots and B choose 60 slotsndash After A transmits a packet it next chooses from range [031]ndash It is possible that A may transmit several packets before B transmits its first packet
Ad hoc Sensor Networks Multiple channel access
Unfairness occurs when one node has backed off much more than some other node
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 54
Random Access fairness (contrsquo) MACAW solution for fairnessbull When a node transmits a packet it appends value w to the packet all nodes
hearing value w use it for their future transmission attemptsbull Since w is an indication of the level of congestion in the vicinity of a specific
receiver node MACAW proposes maintaining w independently for eachreceiver
bull Using per-receiver wi is particularly useful in multi-hop environments sincecongestion level at different receivers can be very different
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 55
Random Access energy efficiency bull Since many mobile hosts are operated by batteries MAC
protocols which conserve energy are of interestbull Two approaches to reduce energy consumption
ndash Power save turn off wireless interface when desirablendash Power control reduce transmit power
bull Power control has some more potential benefitsndash Reduced interference (in ad hoc networks) eg it improves ALOHAefficiency where user with high power can capture a packet even if there is acollisionndash Increased spatial reuse (in infrastructured wireless networks) eg it is usedin CDMA to maintain target SIR of voice and data users or generally it canbe used to maintain target SIR for different user classes (admission control)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 56
Random Access power control bull When C transmits to D at a high power level B cannot receive Arsquos
transmission due to interference from Cbull If C reduces transmit power it can still communicate with D
ndash Reduces energy consumption at node Cndash Allows B to receive Arsquos transmission (spatial reuse)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 57
bull A special wireless ad hoc networkndash Large number of nodesndash Battery poweredndash Topology and density changendash Nodes for a common taskndash In-network data processing
bull Sensor-net applicationsndash Sensor-triggered bursty trafficndash Can often tolerate some delay
Ad hoc Sensor Networks Multiple channel access
Characteristics of Sensor Network
Scalabilty and self configurationEnergy efficiencyAdaptivityFairness not importantLatency
AdaptivityEnergy-delay tradoff
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 58
Ad hoc Sensor Networks Multiple channel access
Wireless sensor node and sensor network
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 59
MAC for WSN attributes bull Attributes
ndash Collision avoidance (Basic task)ndash Energy efficiency (most primary in WSN)ndash Scalability and adaptivityndash Channel utilizationndash Latencyndash Throughputndash Fairness
bull What causes energy wastendash Collisionsndash Control packet overheadndash Overhearing unnecessary trafficndash Long idle time (it consumes 50-100 of the power)
Ad hoc Sensor Networks Multiple channel access
Primary
Secondary
Dominant factor
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 60
MAC for WSN
bull Contention-based protocols need to work hard in all directions for energysavingsndash Reduce idle listening ndash support low duty cyclendash Better collision avoidancendash Reduce control overheadndash Avoid unnecessary overhearing
Ad hoc Sensor Networks Multiple channel access
Scheduled Protocols Contention ProtocolsCollisions No YesEnergy efficiency Good BadScalability and adaptivity Bad GoodMulti-hop communication Difficult EasyTime synchronization Strict Loose or not required
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 61
MAC for WSN classification
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 62
MAC for WSN SMAC (2002)bull It has been developed by Ye Heidemann and Estrinbull Tradeoffs between (latency fairness) and energybull Major components in S-MAC
ndash Periodic listen and sleep turn off radio when sleeping the reduced dutycycle is ~ 10 (eg 120ms on 12s off)ndash Collision avoidance is based on contention similarly to IEEE 80211 adhoc modendash Overhearing avoidance the node is sleeping when neighbors talkndash Massage passing long message is fragmented amp sent in burst operationwith extended Tx time and re-transmit occurs immediately
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 63
Summarybull The algorithms and protocols that enable sharing by multiple users and
controls the access to the wireless channel from different users are calledMAC
bull There are two types of wireless networks Infrastructured wireless networksand ad hoc wireless networks
bull We also want our wireless system to have high user capacity and we canidentify 10 methods for increasing it
bull There are several MACs with no absolute advantage it should be tailoredto the system requirements
bull There are primary concerns for designing a WSN MAC (collisionavoidance energy efficiency scalability and adaptivity) while thesecondary concerns are as follows (channel utilization latency throughputfairness)
bull Next lecture Routing protocols
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 21
Ad hoc Sensor Networks Multiple channel access
Multiple Access TDMAbull The allocated radio spectrum for the system is divided into time slots
ndash In each slot a user can transmit or receivendash A user occupies a cyclically repeating slotsndash A channel is logically defined as a particular time slot that repeats with some
periodbull TDMA systems buffer the data until its turn (time slot) comes to transmitbull In TDMATDD half of the slots in the frame is used for uplink channels the
other is used for downlink channelsbull In TDMAFDD a different carrier frequency is used for uplink or downlink
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 22
Ad hoc Sensor Networks Multiple channel access
Multiple Access TDMA (contrsquo)bull Preamble contains address and synchronization info to identify base
station and mobiles to each otherbull Guard times are used to allow synchronization of the receivers between
different slots and framesndash Different mobiles may have different propagation delays to a base station
because of different distances
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 23
Multiple Access TDMA (contrsquo)bull Each frame contains overhead bits and data bits Efficiency of frame is
defined as the percentage of data (information) bits to the total frame size inbits
η =(1 ndash boverheadbtotal) 100 where btotal= Tf R is the total number of bits in a frame Tf is the frameduration in sec boverhead is the number of overhead bits (GSM efficiency 73)
bull TDMA is usually combined with FDMAndash Neighboring cells are allocated and using different carrier frequencies (FDMA)
Inside a cell TDMA can be used Cells may be re-using the same frequency if theyare far from each-other
ndash There may be more than one channel allocated and used inside each cell Eachcarrier frequency (radio channel) may be using TDMA to further multiplex moreuser (ie having TDMA logical channels inside radio channels) For example inGSM each radio channel has 200KHz bandwidth and has 8 time slots (8 logicalchannels) Hence GSM is using FDMA combined with TDMA
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 24
Multiple Access TDMA (contrsquo)
Ad hoc Sensor Networks Multiple channel access
GSM (Europa) IS-54 (USA) PDC (Japan) DECT
Bit Rate 2708 Kbps 486 Kbps 42 Kbps 1152 Mbps
Bandwidth 200 KHz 30 KHz 25 KHz 1728 MHz
Time Slot 0577 ms 67 ms 67 ms 0417 ms
Upstream slots per frame 8 3 3 12
Duplexing FDD FDD FDD TDD
Efficiency 73 80 80 67
Modulation GMSK π4 DQPSK π4 DQPSK GMSK
Adaptive equalized Mandatory Mandatory Optional None
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 25
Multiple Access TDMA (contrsquo)bull Enables the sharing of a single radio channel among N usersbull Requires high data-rate per radio channel to support N users
simultaneously which requires adaptive equalizers to be usedin multipath environments
bull Transmission occurs in bursts (not continually) It enablespower saving by going to sleep modes in unrelated slots Itdiscontinues transmission and also enables mobile assistedhandoff
bull It equires synchronization of the receivers Need guard bits andsync bits which occurs large overhead per slot
bull Allocation of slots to mobile users should not be uniform Itmay depend on the traffic requirement of mobiles This bringsextra flexibility and efficiency compared to FDMA systems
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2620111015 TAacuteMOP ndash 412-082AKMR-2009-0006 26
Ad hoc Sensor Networks Multiple channel access
Multiple Access spread spectrum multiple accessbull SSMA uses signals that have transmission bandwidth that is
several orders of magnitude larger than minimum requiredRF bandwidth
bull It providesndash Immunity to multipath interferencendash Robust multiple access
bull Two techniquesndash Frequency Hopped Multiple Access (FHMA)ndash Direct Sequence Multiple Access (DSMA) or Code Division
Multiple Access (CDMA)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2720111015 TAacuteMOP ndash 412-082AKMR-2009-0006 27
Ad hoc Sensor Networks Multiple channel access
Multiple Access FHMAbull The carrier frequency of users are varied in a pseudo-random
fashionndash Each user is using a narrowband channel (spectrum) at a specific
instance of timendash The random change in frequency make the change of using the same
narrowband channel very lowbull The sender receiver change frequency (calling hopping)
using the same pseudo-random sequence hence they aresynchronized
bull Rate of hopping versus Symbol ratendash If hopping rate is greather Fast Frequency Hoppingndash If symbol rate is greater Slow Frequency Hopping
bull GSM and Bluetooth are example systems
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2820111015 TAacuteMOP ndash 412-082AKMR-2009-0006 28
Ad hoc Sensor Networks Multiple channel access
Multiple Access FHMA (contrsquo)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2920111015 TAacuteMOP ndash 412-082AKMR-2009-0006 29
Ad hoc Sensor Networks Multiple channel access
Multiple Access CDMAbull The narrowband message signal is multiplied by a very large bandwidth
signal called spreading signal (code) before modulation and transmissionover the air This is called spreading
bull Spreading signal use a pseudo-noise (PN) sequence (a pseudo-randomsequence) called codeword which are orthogonal (low autocorrelation)
bull The receiver correlator distinguishes the senders signal by examining thewideband signal with the same time-synchronized spreading code(despreading)
bull Advantagesndash Low power spectral densityndash Interference limited operationndash Privacyndash Reduction of multipath affects by using a larger spectrumndash Users can start their transmission at any time (random access possible)ndash Cell capacity is soft and higher than TDMA and FDMAndash No frequency management and no equalizers and quard time needed
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3020111015 TAacuteMOP ndash 412-082AKMR-2009-0006 30
Ad hoc Sensor Networks Multiple channel access
Multiple Access near far problem and power controlbull At a receiver the signals may come from various sources (multiuser) The
strongest signal usually captures the modulator The other signals areconsidered as noise Each source may have different distances to the basestation
bull In CDMA we want a BS to receive CDMA coded signals from variousmobile users at the same time
ndash Therefore the receiver power at the BS for all mobile users should be close toeach other This requires power control at the mobiles
bull Power Control BS monitors the RSSI (Received Signal Strength Indicator)values from different mobiles and then sends power change commands to themobiles over a forward channel The mobiles then adjust their transmitpower
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3120111015 TAacuteMOP ndash 412-082AKMR-2009-0006 31
Ad hoc Sensor Networks Multiple channel access
Multiple Access hybrid spread spectrum techniquesbull FDMACDMA
ndash Available wideband spectrum is divided into a number narrowband radio channels CDMAis employed inside each channel
bull DSFHMAndash The signals are spread using spreading codes (direct sequence signals are obtained) but
these signal are not transmitted over a constant carrier frequency they are transmitted over afrequency hopping carrier frequency
bull Time Division CDMA (TCDMA)ndash Each cell is using a different spreading code (CDMA employed between cells) that is
conveyed to the mobiles in its rangendash Inside each cell (inside a CDMA channel) TDMA is employed to multiplex multiple users
bull Time Division Frequency Hoppingndash At each time slot the user is hopped to a new frequency according to a pseudo-random
hopping sequencendash Employed in severe co-interference and multi-path environmentsndash Bluetooth and GSM are using this technique
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3220111015 TAacuteMOP ndash 412-082AKMR-2009-0006 32
Ad hoc Sensor Networks Multiple channel access
Multiple Access comparison TDMAFDMACDMA
Techniques TDMA FDMA CDMA SDMA
Idea
segment sending time into disjoint time-slots
Segment the frequency band into disjoint sub-bands
Spread the spectrum using orthogonal codes
Segment space into sectors
Signal separation
Synchronization in the time domain
Filtering in the frequency domain
Code Directed antennas
Advantage
Established flexible
Simple established robust
Flexible less frequency planning needed soft handover
Simple increases capacity
DisadvantageQuard space synchronization difficult
Inflexible scarce source
Complex receiver power control
inflexible
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3320111015 TAacuteMOP ndash 412-082AKMR-2009-0006 33
Ad hoc Sensor Networks Multiple channel access
User capacity per cell
The user capacity C is defined as the total number of active users percell that the system can support while meeting a common BERconstrain For orthogonal multiple access (as FDMA and TDMA)
C = Ncellwhere Ncell is the number of channels assigned to any given cellThe total number of orthogonal channels of bandwidth Bs that can becreated from a total system bandwidth of B is B Bs The reuse factorsatisfies N = (B Bs) Ncell this implies
C = (B Bs) N
Recall from Chapter 1
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3420111015 TAacuteMOP ndash 412-082AKMR-2009-0006 34
Ad hoc Sensor Networks Multiple channel access
10 methods for increasing capacity1 Increasing the amount of spectrum used very expensive2 More efficient modulation format and coding3 Better source coding4 Adaptive modulation and coding5 Discontinuous transmission6 Multiuser detection CDMA systems7 Reduction of cell radius effective but very expensive smaller
cells require more handovers8 Use of sector cells tripled cells have tripled BS antennas9 Use of an overlay structure10 Multiple antennas diversity MIMO systems SDMA
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3520111015 TAacuteMOP ndash 412-082AKMR-2009-0006 35
Ad hoc Sensor Networks Multiple channel access
bull Distributed operation is requiredbull Synchronization is required in TDMA-based systemsbull Hidden terminals are nodes hidden from a senderbull Exposed terminals are exposed nodes preventing a sender
from sendingbull Throughput needs to be maximizedbull Access delay should be minimizedbull Fairness refers to provide an equal share to all competing
nodesbull Real-time traffic support is required for voice video and
real-time databull Resource reservation is required for QoSbull Ability to measure resource availability handles the
resourcesbull Capability for power control reduces the energy
consumption
Random access in adhoc networks
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3620111015 TAacuteMOP ndash 412-082AKMR-2009-0006 36
Ad hoc Sensor Networks Multiple channel access
Random accessbull All random access techniques are based on packetized data or packet radio where user
date is collected into packets of M bitsbull Collision if packets from different users overlap in timebull The transmission time of a packet is τ = M R where R [bps] the data ratebull Analysis of random access techniques assumes that the users generate packets according
to memoryless Poisson process at a rate λ [packets per unit time] the probability that thenumber of packet arrivals in a time period [0t] denoted by X(t) is equal to integer k isgiven by
Pr(X(t)=k) = (λt)k k e-λt
bull The traffic load is defined as L = λ τ If L gt 1 means on avarage more packets arrive inthe system over a given time period than can be transmitted in that period so systemswith Lgt1 are unstable
bull The throughput is defined as the ratio of the average number of packets successfullytransmitted in any given time interval divided by the number of attempted transmissionsin that interval
T = L Pr(succesful packet transmission)bull The effective data rate of the system is RT
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3720111015 TAacuteMOP ndash 412-082AKMR-2009-0006 37
Ad hoc Sensor Networks Multiple channel access
Random access (contrsquo)bull We will examine the following attributes
ndash Channel utilization (Throughput)ndash Latencyndash Collision avoidance (Hidden and exposed terminals problem)ndash Reliability (ACK)ndash Energy efficiency (power control)ndash Fairnessndash Throughput needs to be maximized
bull We will discuss the following techniquesndash Aloha and slotted Alohandash CSMA Protocols (1-persistent non-persistent p-persistent CSMA CSMACD)ndash MACAndash MACAWndash Energy efficient MAC in wireless sensor networks (eg SMAC)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 38
Random Access ALOHA (1970)bull ALOHA was developed for a wireless system at the University of Hawaii
(Abramson et al)ndash Multiple remote stations plus one base stationndash Frame transmissions are made at one frequency from a remote station to the basestation the base station re-broadcasts frames on another frequency Contention is foraccess to the base station
bull Frames can be sent at any time to the base stationndash If no acknowledgment (ACK) is received assume that the frame is lost andretransmit after waiting a random amount of timendash If more than one station broadcasts on the base station access frequency at the sametime ndash a collision ndash interference will destroy the frames
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 39
Random Access ALOHA (contrsquo)
bull The probability that no packets generated during the tima [-τ τ] is given by (nocollision)
Pr(X(t)=0) = (λ2τ)0 0 e-2λτ = e-2Lwith corresponding throughput
T = L e-2L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 05 05e 018
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 40
Random Access slotted ALOHA (1971)bull Improvement Time is slotted and a packet can only be transmitted at the beginning of
one slot Thus it can reduce the collision duration
bull The probability that no packets generated during the time [0 τ] is given by (nocollision)
Pr(X(t)=0) = (λτ)0 0 e-λτ = e-Lwith corresponding throughput T = L e-L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 1 e 037
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 41
bull Slotted Aloha has double the maximumthroughput as pure Aloha and achievesthis maximum at a higher offered loadThe effective data rate is still less than40 of the raw transmission rate
bull Aloha does not listen to the carrier beforetransmission more sophisticatedtechniques are needed to increaseefficiency
bull Further problem the delayndashDelay
ndashThe mean packet delay
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
( ) ( )delayPr 1 e ekL Lkτ minus minus= = minus
( )delay1
1 e e ekL L L
kMPD E kτ
infinminus minus
=
= = minus =sum
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 42
Random Access Carrier Sense Multiple Accessbull CSMA (Carrier Sense Multiple Access) listen to the carrier before transmission
and transmits if channel is idlebull Detection delay and propagation delay are two important parameters
ndash Detection delay time required to sense the carrier and decide if it is idle or busyndash Propagation delay distancespeed_of_ligth The time required for bit to travel
from transmitter to the receiver
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 43
Random Access CSMA variations
Ad hoc Sensor Networks Multiple channel access
1-persistent CSMA Non-persistent CSMA
There will always be a collision iftwo nodes want to retransmit(usually you stop transmissionattempts after few tries)
Random backoff reduces probabilityof collisions Waste idle time if thebackoff time is too long
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 44
Random Access CSMA variations (contrsquo)
Ad hoc Sensor Networks Multiple channel access
p-persistent CSMA CSMA CD (Collision Detection)
A good tradeoff between non-persistent and 1-persistent CSMA
Same with CSMA however a stationalso listen to the carrier whiletransmitting to see if the transmissioncollides with someone elsetransmission It can be used in listen-while-talk capable channels (fullduplex) In single radio channels thetransmission need to be interrupted inorder to sense the channel
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 45
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 46
Random Access CSMA (contrsquo)bull How to select probability p
ndash Assume that M nodes have a packet to send and the medium is busy then Mp is theexpected number of nodes that will attempt to transmit once the medium becomesidle If Mp gt 1 then a collision is expected to occur Therefore the network mustmake sure that Mp lt 1 to avoid collision where M is the maximum number of nodesthat can be active at a time
bull Problems1 The mean packet delay increases exponently with increasing offered load2 CSMA protocols sense the carrier but sensing the carrier does not always releasestrue information about the status of the wireless channel There are two problems thatare unique to wireless channels (different than wireline channels)ndash Hidden terminal problemndash Exposed terminal problem
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 47
Hidden terminal problembull A sends to B C cannot receive Abull C wants to send to B C senses a
ldquofreerdquo medium (CS fails)bull collision at B A cannot receive the
collision (CD fails)bull A is ldquohiddenrdquo for C
Ad hoc Sensor Networks Multiple channel access
Random Access hidden and exposed terminal problemExposed terminal problem
bull B sends to A C wants to send to Dbull C has to wait CS signals a medium
in usebull since A is outside the radio range of
C waiting is not necessarybull C is ldquoexposedrdquo to B
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 48
Random Access MACAWhen a station wants to transmit databull It sends an RTS (Ready-to-Send) packet to the intended receiver
ndash The RTS packet contains the length of the data that needs to be transmittedndash Any station other than the intended recipient hearing RTS defers transmission for atime duration equal to the end of the corresponding CTS reception
bull The receiver sends back CTS (Clear-to-Send) packet back to sender if it isavailable to receivendash The CTS packet contains the length of the data that original sender wants totransmitndash Any station other than the original RTS sender hearing CTS defers transmissionuntil the data is sent
bull The original sender upon reception of the CTS starts transmitting
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 49
Hidden terminal problem
Ad hoc Sensor Networks Multiple channel access
Random Access MACA (contrsquo)Exposed terminal problem
Waiting time of node X is muchsmaller than waiting time of node C
bull C defers transmission upon hearing BrsquosRTS until B could get CTS from Abull After that C can start transmission to DFor that it first sends an RTSbull C is not longer exposed to the datatransmission of B
Problem It does not address the collision of RTS packet
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 50
Random Access reliabilitybull Wireless links are prone to have errors High packet loss rate detrimental to
transport-layer performance Mechanisms needed to reduce packet loss rateexperienced by upper layers
bull When node B receives a data packet from node A node B sends anAcknowledgement (Ack) This approach adopted in many protocols
bull If node A fails to receive an Ack it will retransmit the packet
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 51
Random Access MACAW (CSMACA) bull Physical carrier sense and virtual carrier sense using Network Allocation
Vector (NAV)ndash NAV is updated based on overheard RTSCTSDATAACK packets each of whichspecified duration of a pending transmissionndash Nodes stay silent when carrier sensed (physicalvirtual)
bull Backoff intervals used to reduce collision probabilityndash When transmitting a packet choose a backoff interval in the range [0w] where wis contention windowndash Count down the backoff interval when channel is idlendash Count down is suspended if channel becomes busyndash When backoff interval reaches 0 transmit RTS
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 52
Random Access CSMACA (contrsquo) bull The time spent counting down backoff intervals is a part of MAC overhead
ndash Choosing a large w leads to large backoff intervals and can result in larger overheadndash Choosing a small w leads to a larger number of collisions (when two nodes countdown to 0 simultaneously)ndash Since the number of nodes attempting to transmit simultaneously may change withtime some mechanism to manage contention is needed
bull IEEE 80211 DCF contention window w is chosen dynamically depending oncollision occurrence (binary exponential backoff)ndash When a node fails to receive CTS in response to its RTS it increases the contentionwindowndash w is doubled (up to an upper bound)ndash When a node successfully completes a data transfer it restores w to wmin
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 53
Random Access fairnessbull Simplest definition of fairness all nodes should receive equal bandwidthbull An example of unfairness
ndash Assume that initially A and B both choose a backoff interval in range [031] buttheir RTSs collidendash Nodes A and B then choose from range [063]ndash Node A chooses 4 slots and B choose 60 slotsndash After A transmits a packet it next chooses from range [031]ndash It is possible that A may transmit several packets before B transmits its first packet
Ad hoc Sensor Networks Multiple channel access
Unfairness occurs when one node has backed off much more than some other node
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 54
Random Access fairness (contrsquo) MACAW solution for fairnessbull When a node transmits a packet it appends value w to the packet all nodes
hearing value w use it for their future transmission attemptsbull Since w is an indication of the level of congestion in the vicinity of a specific
receiver node MACAW proposes maintaining w independently for eachreceiver
bull Using per-receiver wi is particularly useful in multi-hop environments sincecongestion level at different receivers can be very different
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 55
Random Access energy efficiency bull Since many mobile hosts are operated by batteries MAC
protocols which conserve energy are of interestbull Two approaches to reduce energy consumption
ndash Power save turn off wireless interface when desirablendash Power control reduce transmit power
bull Power control has some more potential benefitsndash Reduced interference (in ad hoc networks) eg it improves ALOHAefficiency where user with high power can capture a packet even if there is acollisionndash Increased spatial reuse (in infrastructured wireless networks) eg it is usedin CDMA to maintain target SIR of voice and data users or generally it canbe used to maintain target SIR for different user classes (admission control)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 56
Random Access power control bull When C transmits to D at a high power level B cannot receive Arsquos
transmission due to interference from Cbull If C reduces transmit power it can still communicate with D
ndash Reduces energy consumption at node Cndash Allows B to receive Arsquos transmission (spatial reuse)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 57
bull A special wireless ad hoc networkndash Large number of nodesndash Battery poweredndash Topology and density changendash Nodes for a common taskndash In-network data processing
bull Sensor-net applicationsndash Sensor-triggered bursty trafficndash Can often tolerate some delay
Ad hoc Sensor Networks Multiple channel access
Characteristics of Sensor Network
Scalabilty and self configurationEnergy efficiencyAdaptivityFairness not importantLatency
AdaptivityEnergy-delay tradoff
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 58
Ad hoc Sensor Networks Multiple channel access
Wireless sensor node and sensor network
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 59
MAC for WSN attributes bull Attributes
ndash Collision avoidance (Basic task)ndash Energy efficiency (most primary in WSN)ndash Scalability and adaptivityndash Channel utilizationndash Latencyndash Throughputndash Fairness
bull What causes energy wastendash Collisionsndash Control packet overheadndash Overhearing unnecessary trafficndash Long idle time (it consumes 50-100 of the power)
Ad hoc Sensor Networks Multiple channel access
Primary
Secondary
Dominant factor
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 60
MAC for WSN
bull Contention-based protocols need to work hard in all directions for energysavingsndash Reduce idle listening ndash support low duty cyclendash Better collision avoidancendash Reduce control overheadndash Avoid unnecessary overhearing
Ad hoc Sensor Networks Multiple channel access
Scheduled Protocols Contention ProtocolsCollisions No YesEnergy efficiency Good BadScalability and adaptivity Bad GoodMulti-hop communication Difficult EasyTime synchronization Strict Loose or not required
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 61
MAC for WSN classification
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 62
MAC for WSN SMAC (2002)bull It has been developed by Ye Heidemann and Estrinbull Tradeoffs between (latency fairness) and energybull Major components in S-MAC
ndash Periodic listen and sleep turn off radio when sleeping the reduced dutycycle is ~ 10 (eg 120ms on 12s off)ndash Collision avoidance is based on contention similarly to IEEE 80211 adhoc modendash Overhearing avoidance the node is sleeping when neighbors talkndash Massage passing long message is fragmented amp sent in burst operationwith extended Tx time and re-transmit occurs immediately
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 63
Summarybull The algorithms and protocols that enable sharing by multiple users and
controls the access to the wireless channel from different users are calledMAC
bull There are two types of wireless networks Infrastructured wireless networksand ad hoc wireless networks
bull We also want our wireless system to have high user capacity and we canidentify 10 methods for increasing it
bull There are several MACs with no absolute advantage it should be tailoredto the system requirements
bull There are primary concerns for designing a WSN MAC (collisionavoidance energy efficiency scalability and adaptivity) while thesecondary concerns are as follows (channel utilization latency throughputfairness)
bull Next lecture Routing protocols
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 22
Ad hoc Sensor Networks Multiple channel access
Multiple Access TDMA (contrsquo)bull Preamble contains address and synchronization info to identify base
station and mobiles to each otherbull Guard times are used to allow synchronization of the receivers between
different slots and framesndash Different mobiles may have different propagation delays to a base station
because of different distances
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 23
Multiple Access TDMA (contrsquo)bull Each frame contains overhead bits and data bits Efficiency of frame is
defined as the percentage of data (information) bits to the total frame size inbits
η =(1 ndash boverheadbtotal) 100 where btotal= Tf R is the total number of bits in a frame Tf is the frameduration in sec boverhead is the number of overhead bits (GSM efficiency 73)
bull TDMA is usually combined with FDMAndash Neighboring cells are allocated and using different carrier frequencies (FDMA)
Inside a cell TDMA can be used Cells may be re-using the same frequency if theyare far from each-other
ndash There may be more than one channel allocated and used inside each cell Eachcarrier frequency (radio channel) may be using TDMA to further multiplex moreuser (ie having TDMA logical channels inside radio channels) For example inGSM each radio channel has 200KHz bandwidth and has 8 time slots (8 logicalchannels) Hence GSM is using FDMA combined with TDMA
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 24
Multiple Access TDMA (contrsquo)
Ad hoc Sensor Networks Multiple channel access
GSM (Europa) IS-54 (USA) PDC (Japan) DECT
Bit Rate 2708 Kbps 486 Kbps 42 Kbps 1152 Mbps
Bandwidth 200 KHz 30 KHz 25 KHz 1728 MHz
Time Slot 0577 ms 67 ms 67 ms 0417 ms
Upstream slots per frame 8 3 3 12
Duplexing FDD FDD FDD TDD
Efficiency 73 80 80 67
Modulation GMSK π4 DQPSK π4 DQPSK GMSK
Adaptive equalized Mandatory Mandatory Optional None
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 25
Multiple Access TDMA (contrsquo)bull Enables the sharing of a single radio channel among N usersbull Requires high data-rate per radio channel to support N users
simultaneously which requires adaptive equalizers to be usedin multipath environments
bull Transmission occurs in bursts (not continually) It enablespower saving by going to sleep modes in unrelated slots Itdiscontinues transmission and also enables mobile assistedhandoff
bull It equires synchronization of the receivers Need guard bits andsync bits which occurs large overhead per slot
bull Allocation of slots to mobile users should not be uniform Itmay depend on the traffic requirement of mobiles This bringsextra flexibility and efficiency compared to FDMA systems
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2620111015 TAacuteMOP ndash 412-082AKMR-2009-0006 26
Ad hoc Sensor Networks Multiple channel access
Multiple Access spread spectrum multiple accessbull SSMA uses signals that have transmission bandwidth that is
several orders of magnitude larger than minimum requiredRF bandwidth
bull It providesndash Immunity to multipath interferencendash Robust multiple access
bull Two techniquesndash Frequency Hopped Multiple Access (FHMA)ndash Direct Sequence Multiple Access (DSMA) or Code Division
Multiple Access (CDMA)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2720111015 TAacuteMOP ndash 412-082AKMR-2009-0006 27
Ad hoc Sensor Networks Multiple channel access
Multiple Access FHMAbull The carrier frequency of users are varied in a pseudo-random
fashionndash Each user is using a narrowband channel (spectrum) at a specific
instance of timendash The random change in frequency make the change of using the same
narrowband channel very lowbull The sender receiver change frequency (calling hopping)
using the same pseudo-random sequence hence they aresynchronized
bull Rate of hopping versus Symbol ratendash If hopping rate is greather Fast Frequency Hoppingndash If symbol rate is greater Slow Frequency Hopping
bull GSM and Bluetooth are example systems
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2820111015 TAacuteMOP ndash 412-082AKMR-2009-0006 28
Ad hoc Sensor Networks Multiple channel access
Multiple Access FHMA (contrsquo)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2920111015 TAacuteMOP ndash 412-082AKMR-2009-0006 29
Ad hoc Sensor Networks Multiple channel access
Multiple Access CDMAbull The narrowband message signal is multiplied by a very large bandwidth
signal called spreading signal (code) before modulation and transmissionover the air This is called spreading
bull Spreading signal use a pseudo-noise (PN) sequence (a pseudo-randomsequence) called codeword which are orthogonal (low autocorrelation)
bull The receiver correlator distinguishes the senders signal by examining thewideband signal with the same time-synchronized spreading code(despreading)
bull Advantagesndash Low power spectral densityndash Interference limited operationndash Privacyndash Reduction of multipath affects by using a larger spectrumndash Users can start their transmission at any time (random access possible)ndash Cell capacity is soft and higher than TDMA and FDMAndash No frequency management and no equalizers and quard time needed
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3020111015 TAacuteMOP ndash 412-082AKMR-2009-0006 30
Ad hoc Sensor Networks Multiple channel access
Multiple Access near far problem and power controlbull At a receiver the signals may come from various sources (multiuser) The
strongest signal usually captures the modulator The other signals areconsidered as noise Each source may have different distances to the basestation
bull In CDMA we want a BS to receive CDMA coded signals from variousmobile users at the same time
ndash Therefore the receiver power at the BS for all mobile users should be close toeach other This requires power control at the mobiles
bull Power Control BS monitors the RSSI (Received Signal Strength Indicator)values from different mobiles and then sends power change commands to themobiles over a forward channel The mobiles then adjust their transmitpower
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3120111015 TAacuteMOP ndash 412-082AKMR-2009-0006 31
Ad hoc Sensor Networks Multiple channel access
Multiple Access hybrid spread spectrum techniquesbull FDMACDMA
ndash Available wideband spectrum is divided into a number narrowband radio channels CDMAis employed inside each channel
bull DSFHMAndash The signals are spread using spreading codes (direct sequence signals are obtained) but
these signal are not transmitted over a constant carrier frequency they are transmitted over afrequency hopping carrier frequency
bull Time Division CDMA (TCDMA)ndash Each cell is using a different spreading code (CDMA employed between cells) that is
conveyed to the mobiles in its rangendash Inside each cell (inside a CDMA channel) TDMA is employed to multiplex multiple users
bull Time Division Frequency Hoppingndash At each time slot the user is hopped to a new frequency according to a pseudo-random
hopping sequencendash Employed in severe co-interference and multi-path environmentsndash Bluetooth and GSM are using this technique
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3220111015 TAacuteMOP ndash 412-082AKMR-2009-0006 32
Ad hoc Sensor Networks Multiple channel access
Multiple Access comparison TDMAFDMACDMA
Techniques TDMA FDMA CDMA SDMA
Idea
segment sending time into disjoint time-slots
Segment the frequency band into disjoint sub-bands
Spread the spectrum using orthogonal codes
Segment space into sectors
Signal separation
Synchronization in the time domain
Filtering in the frequency domain
Code Directed antennas
Advantage
Established flexible
Simple established robust
Flexible less frequency planning needed soft handover
Simple increases capacity
DisadvantageQuard space synchronization difficult
Inflexible scarce source
Complex receiver power control
inflexible
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3320111015 TAacuteMOP ndash 412-082AKMR-2009-0006 33
Ad hoc Sensor Networks Multiple channel access
User capacity per cell
The user capacity C is defined as the total number of active users percell that the system can support while meeting a common BERconstrain For orthogonal multiple access (as FDMA and TDMA)
C = Ncellwhere Ncell is the number of channels assigned to any given cellThe total number of orthogonal channels of bandwidth Bs that can becreated from a total system bandwidth of B is B Bs The reuse factorsatisfies N = (B Bs) Ncell this implies
C = (B Bs) N
Recall from Chapter 1
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3420111015 TAacuteMOP ndash 412-082AKMR-2009-0006 34
Ad hoc Sensor Networks Multiple channel access
10 methods for increasing capacity1 Increasing the amount of spectrum used very expensive2 More efficient modulation format and coding3 Better source coding4 Adaptive modulation and coding5 Discontinuous transmission6 Multiuser detection CDMA systems7 Reduction of cell radius effective but very expensive smaller
cells require more handovers8 Use of sector cells tripled cells have tripled BS antennas9 Use of an overlay structure10 Multiple antennas diversity MIMO systems SDMA
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3520111015 TAacuteMOP ndash 412-082AKMR-2009-0006 35
Ad hoc Sensor Networks Multiple channel access
bull Distributed operation is requiredbull Synchronization is required in TDMA-based systemsbull Hidden terminals are nodes hidden from a senderbull Exposed terminals are exposed nodes preventing a sender
from sendingbull Throughput needs to be maximizedbull Access delay should be minimizedbull Fairness refers to provide an equal share to all competing
nodesbull Real-time traffic support is required for voice video and
real-time databull Resource reservation is required for QoSbull Ability to measure resource availability handles the
resourcesbull Capability for power control reduces the energy
consumption
Random access in adhoc networks
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3620111015 TAacuteMOP ndash 412-082AKMR-2009-0006 36
Ad hoc Sensor Networks Multiple channel access
Random accessbull All random access techniques are based on packetized data or packet radio where user
date is collected into packets of M bitsbull Collision if packets from different users overlap in timebull The transmission time of a packet is τ = M R where R [bps] the data ratebull Analysis of random access techniques assumes that the users generate packets according
to memoryless Poisson process at a rate λ [packets per unit time] the probability that thenumber of packet arrivals in a time period [0t] denoted by X(t) is equal to integer k isgiven by
Pr(X(t)=k) = (λt)k k e-λt
bull The traffic load is defined as L = λ τ If L gt 1 means on avarage more packets arrive inthe system over a given time period than can be transmitted in that period so systemswith Lgt1 are unstable
bull The throughput is defined as the ratio of the average number of packets successfullytransmitted in any given time interval divided by the number of attempted transmissionsin that interval
T = L Pr(succesful packet transmission)bull The effective data rate of the system is RT
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3720111015 TAacuteMOP ndash 412-082AKMR-2009-0006 37
Ad hoc Sensor Networks Multiple channel access
Random access (contrsquo)bull We will examine the following attributes
ndash Channel utilization (Throughput)ndash Latencyndash Collision avoidance (Hidden and exposed terminals problem)ndash Reliability (ACK)ndash Energy efficiency (power control)ndash Fairnessndash Throughput needs to be maximized
bull We will discuss the following techniquesndash Aloha and slotted Alohandash CSMA Protocols (1-persistent non-persistent p-persistent CSMA CSMACD)ndash MACAndash MACAWndash Energy efficient MAC in wireless sensor networks (eg SMAC)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 38
Random Access ALOHA (1970)bull ALOHA was developed for a wireless system at the University of Hawaii
(Abramson et al)ndash Multiple remote stations plus one base stationndash Frame transmissions are made at one frequency from a remote station to the basestation the base station re-broadcasts frames on another frequency Contention is foraccess to the base station
bull Frames can be sent at any time to the base stationndash If no acknowledgment (ACK) is received assume that the frame is lost andretransmit after waiting a random amount of timendash If more than one station broadcasts on the base station access frequency at the sametime ndash a collision ndash interference will destroy the frames
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 39
Random Access ALOHA (contrsquo)
bull The probability that no packets generated during the tima [-τ τ] is given by (nocollision)
Pr(X(t)=0) = (λ2τ)0 0 e-2λτ = e-2Lwith corresponding throughput
T = L e-2L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 05 05e 018
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 40
Random Access slotted ALOHA (1971)bull Improvement Time is slotted and a packet can only be transmitted at the beginning of
one slot Thus it can reduce the collision duration
bull The probability that no packets generated during the time [0 τ] is given by (nocollision)
Pr(X(t)=0) = (λτ)0 0 e-λτ = e-Lwith corresponding throughput T = L e-L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 1 e 037
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 41
bull Slotted Aloha has double the maximumthroughput as pure Aloha and achievesthis maximum at a higher offered loadThe effective data rate is still less than40 of the raw transmission rate
bull Aloha does not listen to the carrier beforetransmission more sophisticatedtechniques are needed to increaseefficiency
bull Further problem the delayndashDelay
ndashThe mean packet delay
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
( ) ( )delayPr 1 e ekL Lkτ minus minus= = minus
( )delay1
1 e e ekL L L
kMPD E kτ
infinminus minus
=
= = minus =sum
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 42
Random Access Carrier Sense Multiple Accessbull CSMA (Carrier Sense Multiple Access) listen to the carrier before transmission
and transmits if channel is idlebull Detection delay and propagation delay are two important parameters
ndash Detection delay time required to sense the carrier and decide if it is idle or busyndash Propagation delay distancespeed_of_ligth The time required for bit to travel
from transmitter to the receiver
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 43
Random Access CSMA variations
Ad hoc Sensor Networks Multiple channel access
1-persistent CSMA Non-persistent CSMA
There will always be a collision iftwo nodes want to retransmit(usually you stop transmissionattempts after few tries)
Random backoff reduces probabilityof collisions Waste idle time if thebackoff time is too long
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 44
Random Access CSMA variations (contrsquo)
Ad hoc Sensor Networks Multiple channel access
p-persistent CSMA CSMA CD (Collision Detection)
A good tradeoff between non-persistent and 1-persistent CSMA
Same with CSMA however a stationalso listen to the carrier whiletransmitting to see if the transmissioncollides with someone elsetransmission It can be used in listen-while-talk capable channels (fullduplex) In single radio channels thetransmission need to be interrupted inorder to sense the channel
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 45
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 46
Random Access CSMA (contrsquo)bull How to select probability p
ndash Assume that M nodes have a packet to send and the medium is busy then Mp is theexpected number of nodes that will attempt to transmit once the medium becomesidle If Mp gt 1 then a collision is expected to occur Therefore the network mustmake sure that Mp lt 1 to avoid collision where M is the maximum number of nodesthat can be active at a time
bull Problems1 The mean packet delay increases exponently with increasing offered load2 CSMA protocols sense the carrier but sensing the carrier does not always releasestrue information about the status of the wireless channel There are two problems thatare unique to wireless channels (different than wireline channels)ndash Hidden terminal problemndash Exposed terminal problem
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 47
Hidden terminal problembull A sends to B C cannot receive Abull C wants to send to B C senses a
ldquofreerdquo medium (CS fails)bull collision at B A cannot receive the
collision (CD fails)bull A is ldquohiddenrdquo for C
Ad hoc Sensor Networks Multiple channel access
Random Access hidden and exposed terminal problemExposed terminal problem
bull B sends to A C wants to send to Dbull C has to wait CS signals a medium
in usebull since A is outside the radio range of
C waiting is not necessarybull C is ldquoexposedrdquo to B
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 48
Random Access MACAWhen a station wants to transmit databull It sends an RTS (Ready-to-Send) packet to the intended receiver
ndash The RTS packet contains the length of the data that needs to be transmittedndash Any station other than the intended recipient hearing RTS defers transmission for atime duration equal to the end of the corresponding CTS reception
bull The receiver sends back CTS (Clear-to-Send) packet back to sender if it isavailable to receivendash The CTS packet contains the length of the data that original sender wants totransmitndash Any station other than the original RTS sender hearing CTS defers transmissionuntil the data is sent
bull The original sender upon reception of the CTS starts transmitting
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 49
Hidden terminal problem
Ad hoc Sensor Networks Multiple channel access
Random Access MACA (contrsquo)Exposed terminal problem
Waiting time of node X is muchsmaller than waiting time of node C
bull C defers transmission upon hearing BrsquosRTS until B could get CTS from Abull After that C can start transmission to DFor that it first sends an RTSbull C is not longer exposed to the datatransmission of B
Problem It does not address the collision of RTS packet
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 50
Random Access reliabilitybull Wireless links are prone to have errors High packet loss rate detrimental to
transport-layer performance Mechanisms needed to reduce packet loss rateexperienced by upper layers
bull When node B receives a data packet from node A node B sends anAcknowledgement (Ack) This approach adopted in many protocols
bull If node A fails to receive an Ack it will retransmit the packet
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 51
Random Access MACAW (CSMACA) bull Physical carrier sense and virtual carrier sense using Network Allocation
Vector (NAV)ndash NAV is updated based on overheard RTSCTSDATAACK packets each of whichspecified duration of a pending transmissionndash Nodes stay silent when carrier sensed (physicalvirtual)
bull Backoff intervals used to reduce collision probabilityndash When transmitting a packet choose a backoff interval in the range [0w] where wis contention windowndash Count down the backoff interval when channel is idlendash Count down is suspended if channel becomes busyndash When backoff interval reaches 0 transmit RTS
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 52
Random Access CSMACA (contrsquo) bull The time spent counting down backoff intervals is a part of MAC overhead
ndash Choosing a large w leads to large backoff intervals and can result in larger overheadndash Choosing a small w leads to a larger number of collisions (when two nodes countdown to 0 simultaneously)ndash Since the number of nodes attempting to transmit simultaneously may change withtime some mechanism to manage contention is needed
bull IEEE 80211 DCF contention window w is chosen dynamically depending oncollision occurrence (binary exponential backoff)ndash When a node fails to receive CTS in response to its RTS it increases the contentionwindowndash w is doubled (up to an upper bound)ndash When a node successfully completes a data transfer it restores w to wmin
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 53
Random Access fairnessbull Simplest definition of fairness all nodes should receive equal bandwidthbull An example of unfairness
ndash Assume that initially A and B both choose a backoff interval in range [031] buttheir RTSs collidendash Nodes A and B then choose from range [063]ndash Node A chooses 4 slots and B choose 60 slotsndash After A transmits a packet it next chooses from range [031]ndash It is possible that A may transmit several packets before B transmits its first packet
Ad hoc Sensor Networks Multiple channel access
Unfairness occurs when one node has backed off much more than some other node
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 54
Random Access fairness (contrsquo) MACAW solution for fairnessbull When a node transmits a packet it appends value w to the packet all nodes
hearing value w use it for their future transmission attemptsbull Since w is an indication of the level of congestion in the vicinity of a specific
receiver node MACAW proposes maintaining w independently for eachreceiver
bull Using per-receiver wi is particularly useful in multi-hop environments sincecongestion level at different receivers can be very different
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 55
Random Access energy efficiency bull Since many mobile hosts are operated by batteries MAC
protocols which conserve energy are of interestbull Two approaches to reduce energy consumption
ndash Power save turn off wireless interface when desirablendash Power control reduce transmit power
bull Power control has some more potential benefitsndash Reduced interference (in ad hoc networks) eg it improves ALOHAefficiency where user with high power can capture a packet even if there is acollisionndash Increased spatial reuse (in infrastructured wireless networks) eg it is usedin CDMA to maintain target SIR of voice and data users or generally it canbe used to maintain target SIR for different user classes (admission control)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 56
Random Access power control bull When C transmits to D at a high power level B cannot receive Arsquos
transmission due to interference from Cbull If C reduces transmit power it can still communicate with D
ndash Reduces energy consumption at node Cndash Allows B to receive Arsquos transmission (spatial reuse)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 57
bull A special wireless ad hoc networkndash Large number of nodesndash Battery poweredndash Topology and density changendash Nodes for a common taskndash In-network data processing
bull Sensor-net applicationsndash Sensor-triggered bursty trafficndash Can often tolerate some delay
Ad hoc Sensor Networks Multiple channel access
Characteristics of Sensor Network
Scalabilty and self configurationEnergy efficiencyAdaptivityFairness not importantLatency
AdaptivityEnergy-delay tradoff
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 58
Ad hoc Sensor Networks Multiple channel access
Wireless sensor node and sensor network
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 59
MAC for WSN attributes bull Attributes
ndash Collision avoidance (Basic task)ndash Energy efficiency (most primary in WSN)ndash Scalability and adaptivityndash Channel utilizationndash Latencyndash Throughputndash Fairness
bull What causes energy wastendash Collisionsndash Control packet overheadndash Overhearing unnecessary trafficndash Long idle time (it consumes 50-100 of the power)
Ad hoc Sensor Networks Multiple channel access
Primary
Secondary
Dominant factor
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 60
MAC for WSN
bull Contention-based protocols need to work hard in all directions for energysavingsndash Reduce idle listening ndash support low duty cyclendash Better collision avoidancendash Reduce control overheadndash Avoid unnecessary overhearing
Ad hoc Sensor Networks Multiple channel access
Scheduled Protocols Contention ProtocolsCollisions No YesEnergy efficiency Good BadScalability and adaptivity Bad GoodMulti-hop communication Difficult EasyTime synchronization Strict Loose or not required
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 61
MAC for WSN classification
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 62
MAC for WSN SMAC (2002)bull It has been developed by Ye Heidemann and Estrinbull Tradeoffs between (latency fairness) and energybull Major components in S-MAC
ndash Periodic listen and sleep turn off radio when sleeping the reduced dutycycle is ~ 10 (eg 120ms on 12s off)ndash Collision avoidance is based on contention similarly to IEEE 80211 adhoc modendash Overhearing avoidance the node is sleeping when neighbors talkndash Massage passing long message is fragmented amp sent in burst operationwith extended Tx time and re-transmit occurs immediately
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 63
Summarybull The algorithms and protocols that enable sharing by multiple users and
controls the access to the wireless channel from different users are calledMAC
bull There are two types of wireless networks Infrastructured wireless networksand ad hoc wireless networks
bull We also want our wireless system to have high user capacity and we canidentify 10 methods for increasing it
bull There are several MACs with no absolute advantage it should be tailoredto the system requirements
bull There are primary concerns for designing a WSN MAC (collisionavoidance energy efficiency scalability and adaptivity) while thesecondary concerns are as follows (channel utilization latency throughputfairness)
bull Next lecture Routing protocols
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 23
Multiple Access TDMA (contrsquo)bull Each frame contains overhead bits and data bits Efficiency of frame is
defined as the percentage of data (information) bits to the total frame size inbits
η =(1 ndash boverheadbtotal) 100 where btotal= Tf R is the total number of bits in a frame Tf is the frameduration in sec boverhead is the number of overhead bits (GSM efficiency 73)
bull TDMA is usually combined with FDMAndash Neighboring cells are allocated and using different carrier frequencies (FDMA)
Inside a cell TDMA can be used Cells may be re-using the same frequency if theyare far from each-other
ndash There may be more than one channel allocated and used inside each cell Eachcarrier frequency (radio channel) may be using TDMA to further multiplex moreuser (ie having TDMA logical channels inside radio channels) For example inGSM each radio channel has 200KHz bandwidth and has 8 time slots (8 logicalchannels) Hence GSM is using FDMA combined with TDMA
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 24
Multiple Access TDMA (contrsquo)
Ad hoc Sensor Networks Multiple channel access
GSM (Europa) IS-54 (USA) PDC (Japan) DECT
Bit Rate 2708 Kbps 486 Kbps 42 Kbps 1152 Mbps
Bandwidth 200 KHz 30 KHz 25 KHz 1728 MHz
Time Slot 0577 ms 67 ms 67 ms 0417 ms
Upstream slots per frame 8 3 3 12
Duplexing FDD FDD FDD TDD
Efficiency 73 80 80 67
Modulation GMSK π4 DQPSK π4 DQPSK GMSK
Adaptive equalized Mandatory Mandatory Optional None
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 25
Multiple Access TDMA (contrsquo)bull Enables the sharing of a single radio channel among N usersbull Requires high data-rate per radio channel to support N users
simultaneously which requires adaptive equalizers to be usedin multipath environments
bull Transmission occurs in bursts (not continually) It enablespower saving by going to sleep modes in unrelated slots Itdiscontinues transmission and also enables mobile assistedhandoff
bull It equires synchronization of the receivers Need guard bits andsync bits which occurs large overhead per slot
bull Allocation of slots to mobile users should not be uniform Itmay depend on the traffic requirement of mobiles This bringsextra flexibility and efficiency compared to FDMA systems
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2620111015 TAacuteMOP ndash 412-082AKMR-2009-0006 26
Ad hoc Sensor Networks Multiple channel access
Multiple Access spread spectrum multiple accessbull SSMA uses signals that have transmission bandwidth that is
several orders of magnitude larger than minimum requiredRF bandwidth
bull It providesndash Immunity to multipath interferencendash Robust multiple access
bull Two techniquesndash Frequency Hopped Multiple Access (FHMA)ndash Direct Sequence Multiple Access (DSMA) or Code Division
Multiple Access (CDMA)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2720111015 TAacuteMOP ndash 412-082AKMR-2009-0006 27
Ad hoc Sensor Networks Multiple channel access
Multiple Access FHMAbull The carrier frequency of users are varied in a pseudo-random
fashionndash Each user is using a narrowband channel (spectrum) at a specific
instance of timendash The random change in frequency make the change of using the same
narrowband channel very lowbull The sender receiver change frequency (calling hopping)
using the same pseudo-random sequence hence they aresynchronized
bull Rate of hopping versus Symbol ratendash If hopping rate is greather Fast Frequency Hoppingndash If symbol rate is greater Slow Frequency Hopping
bull GSM and Bluetooth are example systems
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2820111015 TAacuteMOP ndash 412-082AKMR-2009-0006 28
Ad hoc Sensor Networks Multiple channel access
Multiple Access FHMA (contrsquo)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2920111015 TAacuteMOP ndash 412-082AKMR-2009-0006 29
Ad hoc Sensor Networks Multiple channel access
Multiple Access CDMAbull The narrowband message signal is multiplied by a very large bandwidth
signal called spreading signal (code) before modulation and transmissionover the air This is called spreading
bull Spreading signal use a pseudo-noise (PN) sequence (a pseudo-randomsequence) called codeword which are orthogonal (low autocorrelation)
bull The receiver correlator distinguishes the senders signal by examining thewideband signal with the same time-synchronized spreading code(despreading)
bull Advantagesndash Low power spectral densityndash Interference limited operationndash Privacyndash Reduction of multipath affects by using a larger spectrumndash Users can start their transmission at any time (random access possible)ndash Cell capacity is soft and higher than TDMA and FDMAndash No frequency management and no equalizers and quard time needed
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3020111015 TAacuteMOP ndash 412-082AKMR-2009-0006 30
Ad hoc Sensor Networks Multiple channel access
Multiple Access near far problem and power controlbull At a receiver the signals may come from various sources (multiuser) The
strongest signal usually captures the modulator The other signals areconsidered as noise Each source may have different distances to the basestation
bull In CDMA we want a BS to receive CDMA coded signals from variousmobile users at the same time
ndash Therefore the receiver power at the BS for all mobile users should be close toeach other This requires power control at the mobiles
bull Power Control BS monitors the RSSI (Received Signal Strength Indicator)values from different mobiles and then sends power change commands to themobiles over a forward channel The mobiles then adjust their transmitpower
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3120111015 TAacuteMOP ndash 412-082AKMR-2009-0006 31
Ad hoc Sensor Networks Multiple channel access
Multiple Access hybrid spread spectrum techniquesbull FDMACDMA
ndash Available wideband spectrum is divided into a number narrowband radio channels CDMAis employed inside each channel
bull DSFHMAndash The signals are spread using spreading codes (direct sequence signals are obtained) but
these signal are not transmitted over a constant carrier frequency they are transmitted over afrequency hopping carrier frequency
bull Time Division CDMA (TCDMA)ndash Each cell is using a different spreading code (CDMA employed between cells) that is
conveyed to the mobiles in its rangendash Inside each cell (inside a CDMA channel) TDMA is employed to multiplex multiple users
bull Time Division Frequency Hoppingndash At each time slot the user is hopped to a new frequency according to a pseudo-random
hopping sequencendash Employed in severe co-interference and multi-path environmentsndash Bluetooth and GSM are using this technique
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3220111015 TAacuteMOP ndash 412-082AKMR-2009-0006 32
Ad hoc Sensor Networks Multiple channel access
Multiple Access comparison TDMAFDMACDMA
Techniques TDMA FDMA CDMA SDMA
Idea
segment sending time into disjoint time-slots
Segment the frequency band into disjoint sub-bands
Spread the spectrum using orthogonal codes
Segment space into sectors
Signal separation
Synchronization in the time domain
Filtering in the frequency domain
Code Directed antennas
Advantage
Established flexible
Simple established robust
Flexible less frequency planning needed soft handover
Simple increases capacity
DisadvantageQuard space synchronization difficult
Inflexible scarce source
Complex receiver power control
inflexible
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3320111015 TAacuteMOP ndash 412-082AKMR-2009-0006 33
Ad hoc Sensor Networks Multiple channel access
User capacity per cell
The user capacity C is defined as the total number of active users percell that the system can support while meeting a common BERconstrain For orthogonal multiple access (as FDMA and TDMA)
C = Ncellwhere Ncell is the number of channels assigned to any given cellThe total number of orthogonal channels of bandwidth Bs that can becreated from a total system bandwidth of B is B Bs The reuse factorsatisfies N = (B Bs) Ncell this implies
C = (B Bs) N
Recall from Chapter 1
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3420111015 TAacuteMOP ndash 412-082AKMR-2009-0006 34
Ad hoc Sensor Networks Multiple channel access
10 methods for increasing capacity1 Increasing the amount of spectrum used very expensive2 More efficient modulation format and coding3 Better source coding4 Adaptive modulation and coding5 Discontinuous transmission6 Multiuser detection CDMA systems7 Reduction of cell radius effective but very expensive smaller
cells require more handovers8 Use of sector cells tripled cells have tripled BS antennas9 Use of an overlay structure10 Multiple antennas diversity MIMO systems SDMA
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3520111015 TAacuteMOP ndash 412-082AKMR-2009-0006 35
Ad hoc Sensor Networks Multiple channel access
bull Distributed operation is requiredbull Synchronization is required in TDMA-based systemsbull Hidden terminals are nodes hidden from a senderbull Exposed terminals are exposed nodes preventing a sender
from sendingbull Throughput needs to be maximizedbull Access delay should be minimizedbull Fairness refers to provide an equal share to all competing
nodesbull Real-time traffic support is required for voice video and
real-time databull Resource reservation is required for QoSbull Ability to measure resource availability handles the
resourcesbull Capability for power control reduces the energy
consumption
Random access in adhoc networks
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3620111015 TAacuteMOP ndash 412-082AKMR-2009-0006 36
Ad hoc Sensor Networks Multiple channel access
Random accessbull All random access techniques are based on packetized data or packet radio where user
date is collected into packets of M bitsbull Collision if packets from different users overlap in timebull The transmission time of a packet is τ = M R where R [bps] the data ratebull Analysis of random access techniques assumes that the users generate packets according
to memoryless Poisson process at a rate λ [packets per unit time] the probability that thenumber of packet arrivals in a time period [0t] denoted by X(t) is equal to integer k isgiven by
Pr(X(t)=k) = (λt)k k e-λt
bull The traffic load is defined as L = λ τ If L gt 1 means on avarage more packets arrive inthe system over a given time period than can be transmitted in that period so systemswith Lgt1 are unstable
bull The throughput is defined as the ratio of the average number of packets successfullytransmitted in any given time interval divided by the number of attempted transmissionsin that interval
T = L Pr(succesful packet transmission)bull The effective data rate of the system is RT
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3720111015 TAacuteMOP ndash 412-082AKMR-2009-0006 37
Ad hoc Sensor Networks Multiple channel access
Random access (contrsquo)bull We will examine the following attributes
ndash Channel utilization (Throughput)ndash Latencyndash Collision avoidance (Hidden and exposed terminals problem)ndash Reliability (ACK)ndash Energy efficiency (power control)ndash Fairnessndash Throughput needs to be maximized
bull We will discuss the following techniquesndash Aloha and slotted Alohandash CSMA Protocols (1-persistent non-persistent p-persistent CSMA CSMACD)ndash MACAndash MACAWndash Energy efficient MAC in wireless sensor networks (eg SMAC)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 38
Random Access ALOHA (1970)bull ALOHA was developed for a wireless system at the University of Hawaii
(Abramson et al)ndash Multiple remote stations plus one base stationndash Frame transmissions are made at one frequency from a remote station to the basestation the base station re-broadcasts frames on another frequency Contention is foraccess to the base station
bull Frames can be sent at any time to the base stationndash If no acknowledgment (ACK) is received assume that the frame is lost andretransmit after waiting a random amount of timendash If more than one station broadcasts on the base station access frequency at the sametime ndash a collision ndash interference will destroy the frames
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 39
Random Access ALOHA (contrsquo)
bull The probability that no packets generated during the tima [-τ τ] is given by (nocollision)
Pr(X(t)=0) = (λ2τ)0 0 e-2λτ = e-2Lwith corresponding throughput
T = L e-2L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 05 05e 018
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 40
Random Access slotted ALOHA (1971)bull Improvement Time is slotted and a packet can only be transmitted at the beginning of
one slot Thus it can reduce the collision duration
bull The probability that no packets generated during the time [0 τ] is given by (nocollision)
Pr(X(t)=0) = (λτ)0 0 e-λτ = e-Lwith corresponding throughput T = L e-L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 1 e 037
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 41
bull Slotted Aloha has double the maximumthroughput as pure Aloha and achievesthis maximum at a higher offered loadThe effective data rate is still less than40 of the raw transmission rate
bull Aloha does not listen to the carrier beforetransmission more sophisticatedtechniques are needed to increaseefficiency
bull Further problem the delayndashDelay
ndashThe mean packet delay
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
( ) ( )delayPr 1 e ekL Lkτ minus minus= = minus
( )delay1
1 e e ekL L L
kMPD E kτ
infinminus minus
=
= = minus =sum
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 42
Random Access Carrier Sense Multiple Accessbull CSMA (Carrier Sense Multiple Access) listen to the carrier before transmission
and transmits if channel is idlebull Detection delay and propagation delay are two important parameters
ndash Detection delay time required to sense the carrier and decide if it is idle or busyndash Propagation delay distancespeed_of_ligth The time required for bit to travel
from transmitter to the receiver
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 43
Random Access CSMA variations
Ad hoc Sensor Networks Multiple channel access
1-persistent CSMA Non-persistent CSMA
There will always be a collision iftwo nodes want to retransmit(usually you stop transmissionattempts after few tries)
Random backoff reduces probabilityof collisions Waste idle time if thebackoff time is too long
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 44
Random Access CSMA variations (contrsquo)
Ad hoc Sensor Networks Multiple channel access
p-persistent CSMA CSMA CD (Collision Detection)
A good tradeoff between non-persistent and 1-persistent CSMA
Same with CSMA however a stationalso listen to the carrier whiletransmitting to see if the transmissioncollides with someone elsetransmission It can be used in listen-while-talk capable channels (fullduplex) In single radio channels thetransmission need to be interrupted inorder to sense the channel
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 45
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 46
Random Access CSMA (contrsquo)bull How to select probability p
ndash Assume that M nodes have a packet to send and the medium is busy then Mp is theexpected number of nodes that will attempt to transmit once the medium becomesidle If Mp gt 1 then a collision is expected to occur Therefore the network mustmake sure that Mp lt 1 to avoid collision where M is the maximum number of nodesthat can be active at a time
bull Problems1 The mean packet delay increases exponently with increasing offered load2 CSMA protocols sense the carrier but sensing the carrier does not always releasestrue information about the status of the wireless channel There are two problems thatare unique to wireless channels (different than wireline channels)ndash Hidden terminal problemndash Exposed terminal problem
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 47
Hidden terminal problembull A sends to B C cannot receive Abull C wants to send to B C senses a
ldquofreerdquo medium (CS fails)bull collision at B A cannot receive the
collision (CD fails)bull A is ldquohiddenrdquo for C
Ad hoc Sensor Networks Multiple channel access
Random Access hidden and exposed terminal problemExposed terminal problem
bull B sends to A C wants to send to Dbull C has to wait CS signals a medium
in usebull since A is outside the radio range of
C waiting is not necessarybull C is ldquoexposedrdquo to B
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 48
Random Access MACAWhen a station wants to transmit databull It sends an RTS (Ready-to-Send) packet to the intended receiver
ndash The RTS packet contains the length of the data that needs to be transmittedndash Any station other than the intended recipient hearing RTS defers transmission for atime duration equal to the end of the corresponding CTS reception
bull The receiver sends back CTS (Clear-to-Send) packet back to sender if it isavailable to receivendash The CTS packet contains the length of the data that original sender wants totransmitndash Any station other than the original RTS sender hearing CTS defers transmissionuntil the data is sent
bull The original sender upon reception of the CTS starts transmitting
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 49
Hidden terminal problem
Ad hoc Sensor Networks Multiple channel access
Random Access MACA (contrsquo)Exposed terminal problem
Waiting time of node X is muchsmaller than waiting time of node C
bull C defers transmission upon hearing BrsquosRTS until B could get CTS from Abull After that C can start transmission to DFor that it first sends an RTSbull C is not longer exposed to the datatransmission of B
Problem It does not address the collision of RTS packet
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 50
Random Access reliabilitybull Wireless links are prone to have errors High packet loss rate detrimental to
transport-layer performance Mechanisms needed to reduce packet loss rateexperienced by upper layers
bull When node B receives a data packet from node A node B sends anAcknowledgement (Ack) This approach adopted in many protocols
bull If node A fails to receive an Ack it will retransmit the packet
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 51
Random Access MACAW (CSMACA) bull Physical carrier sense and virtual carrier sense using Network Allocation
Vector (NAV)ndash NAV is updated based on overheard RTSCTSDATAACK packets each of whichspecified duration of a pending transmissionndash Nodes stay silent when carrier sensed (physicalvirtual)
bull Backoff intervals used to reduce collision probabilityndash When transmitting a packet choose a backoff interval in the range [0w] where wis contention windowndash Count down the backoff interval when channel is idlendash Count down is suspended if channel becomes busyndash When backoff interval reaches 0 transmit RTS
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 52
Random Access CSMACA (contrsquo) bull The time spent counting down backoff intervals is a part of MAC overhead
ndash Choosing a large w leads to large backoff intervals and can result in larger overheadndash Choosing a small w leads to a larger number of collisions (when two nodes countdown to 0 simultaneously)ndash Since the number of nodes attempting to transmit simultaneously may change withtime some mechanism to manage contention is needed
bull IEEE 80211 DCF contention window w is chosen dynamically depending oncollision occurrence (binary exponential backoff)ndash When a node fails to receive CTS in response to its RTS it increases the contentionwindowndash w is doubled (up to an upper bound)ndash When a node successfully completes a data transfer it restores w to wmin
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 53
Random Access fairnessbull Simplest definition of fairness all nodes should receive equal bandwidthbull An example of unfairness
ndash Assume that initially A and B both choose a backoff interval in range [031] buttheir RTSs collidendash Nodes A and B then choose from range [063]ndash Node A chooses 4 slots and B choose 60 slotsndash After A transmits a packet it next chooses from range [031]ndash It is possible that A may transmit several packets before B transmits its first packet
Ad hoc Sensor Networks Multiple channel access
Unfairness occurs when one node has backed off much more than some other node
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 54
Random Access fairness (contrsquo) MACAW solution for fairnessbull When a node transmits a packet it appends value w to the packet all nodes
hearing value w use it for their future transmission attemptsbull Since w is an indication of the level of congestion in the vicinity of a specific
receiver node MACAW proposes maintaining w independently for eachreceiver
bull Using per-receiver wi is particularly useful in multi-hop environments sincecongestion level at different receivers can be very different
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 55
Random Access energy efficiency bull Since many mobile hosts are operated by batteries MAC
protocols which conserve energy are of interestbull Two approaches to reduce energy consumption
ndash Power save turn off wireless interface when desirablendash Power control reduce transmit power
bull Power control has some more potential benefitsndash Reduced interference (in ad hoc networks) eg it improves ALOHAefficiency where user with high power can capture a packet even if there is acollisionndash Increased spatial reuse (in infrastructured wireless networks) eg it is usedin CDMA to maintain target SIR of voice and data users or generally it canbe used to maintain target SIR for different user classes (admission control)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 56
Random Access power control bull When C transmits to D at a high power level B cannot receive Arsquos
transmission due to interference from Cbull If C reduces transmit power it can still communicate with D
ndash Reduces energy consumption at node Cndash Allows B to receive Arsquos transmission (spatial reuse)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 57
bull A special wireless ad hoc networkndash Large number of nodesndash Battery poweredndash Topology and density changendash Nodes for a common taskndash In-network data processing
bull Sensor-net applicationsndash Sensor-triggered bursty trafficndash Can often tolerate some delay
Ad hoc Sensor Networks Multiple channel access
Characteristics of Sensor Network
Scalabilty and self configurationEnergy efficiencyAdaptivityFairness not importantLatency
AdaptivityEnergy-delay tradoff
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 58
Ad hoc Sensor Networks Multiple channel access
Wireless sensor node and sensor network
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 59
MAC for WSN attributes bull Attributes
ndash Collision avoidance (Basic task)ndash Energy efficiency (most primary in WSN)ndash Scalability and adaptivityndash Channel utilizationndash Latencyndash Throughputndash Fairness
bull What causes energy wastendash Collisionsndash Control packet overheadndash Overhearing unnecessary trafficndash Long idle time (it consumes 50-100 of the power)
Ad hoc Sensor Networks Multiple channel access
Primary
Secondary
Dominant factor
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 60
MAC for WSN
bull Contention-based protocols need to work hard in all directions for energysavingsndash Reduce idle listening ndash support low duty cyclendash Better collision avoidancendash Reduce control overheadndash Avoid unnecessary overhearing
Ad hoc Sensor Networks Multiple channel access
Scheduled Protocols Contention ProtocolsCollisions No YesEnergy efficiency Good BadScalability and adaptivity Bad GoodMulti-hop communication Difficult EasyTime synchronization Strict Loose or not required
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 61
MAC for WSN classification
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 62
MAC for WSN SMAC (2002)bull It has been developed by Ye Heidemann and Estrinbull Tradeoffs between (latency fairness) and energybull Major components in S-MAC
ndash Periodic listen and sleep turn off radio when sleeping the reduced dutycycle is ~ 10 (eg 120ms on 12s off)ndash Collision avoidance is based on contention similarly to IEEE 80211 adhoc modendash Overhearing avoidance the node is sleeping when neighbors talkndash Massage passing long message is fragmented amp sent in burst operationwith extended Tx time and re-transmit occurs immediately
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 63
Summarybull The algorithms and protocols that enable sharing by multiple users and
controls the access to the wireless channel from different users are calledMAC
bull There are two types of wireless networks Infrastructured wireless networksand ad hoc wireless networks
bull We also want our wireless system to have high user capacity and we canidentify 10 methods for increasing it
bull There are several MACs with no absolute advantage it should be tailoredto the system requirements
bull There are primary concerns for designing a WSN MAC (collisionavoidance energy efficiency scalability and adaptivity) while thesecondary concerns are as follows (channel utilization latency throughputfairness)
bull Next lecture Routing protocols
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 24
Multiple Access TDMA (contrsquo)
Ad hoc Sensor Networks Multiple channel access
GSM (Europa) IS-54 (USA) PDC (Japan) DECT
Bit Rate 2708 Kbps 486 Kbps 42 Kbps 1152 Mbps
Bandwidth 200 KHz 30 KHz 25 KHz 1728 MHz
Time Slot 0577 ms 67 ms 67 ms 0417 ms
Upstream slots per frame 8 3 3 12
Duplexing FDD FDD FDD TDD
Efficiency 73 80 80 67
Modulation GMSK π4 DQPSK π4 DQPSK GMSK
Adaptive equalized Mandatory Mandatory Optional None
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 25
Multiple Access TDMA (contrsquo)bull Enables the sharing of a single radio channel among N usersbull Requires high data-rate per radio channel to support N users
simultaneously which requires adaptive equalizers to be usedin multipath environments
bull Transmission occurs in bursts (not continually) It enablespower saving by going to sleep modes in unrelated slots Itdiscontinues transmission and also enables mobile assistedhandoff
bull It equires synchronization of the receivers Need guard bits andsync bits which occurs large overhead per slot
bull Allocation of slots to mobile users should not be uniform Itmay depend on the traffic requirement of mobiles This bringsextra flexibility and efficiency compared to FDMA systems
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2620111015 TAacuteMOP ndash 412-082AKMR-2009-0006 26
Ad hoc Sensor Networks Multiple channel access
Multiple Access spread spectrum multiple accessbull SSMA uses signals that have transmission bandwidth that is
several orders of magnitude larger than minimum requiredRF bandwidth
bull It providesndash Immunity to multipath interferencendash Robust multiple access
bull Two techniquesndash Frequency Hopped Multiple Access (FHMA)ndash Direct Sequence Multiple Access (DSMA) or Code Division
Multiple Access (CDMA)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2720111015 TAacuteMOP ndash 412-082AKMR-2009-0006 27
Ad hoc Sensor Networks Multiple channel access
Multiple Access FHMAbull The carrier frequency of users are varied in a pseudo-random
fashionndash Each user is using a narrowband channel (spectrum) at a specific
instance of timendash The random change in frequency make the change of using the same
narrowband channel very lowbull The sender receiver change frequency (calling hopping)
using the same pseudo-random sequence hence they aresynchronized
bull Rate of hopping versus Symbol ratendash If hopping rate is greather Fast Frequency Hoppingndash If symbol rate is greater Slow Frequency Hopping
bull GSM and Bluetooth are example systems
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2820111015 TAacuteMOP ndash 412-082AKMR-2009-0006 28
Ad hoc Sensor Networks Multiple channel access
Multiple Access FHMA (contrsquo)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2920111015 TAacuteMOP ndash 412-082AKMR-2009-0006 29
Ad hoc Sensor Networks Multiple channel access
Multiple Access CDMAbull The narrowband message signal is multiplied by a very large bandwidth
signal called spreading signal (code) before modulation and transmissionover the air This is called spreading
bull Spreading signal use a pseudo-noise (PN) sequence (a pseudo-randomsequence) called codeword which are orthogonal (low autocorrelation)
bull The receiver correlator distinguishes the senders signal by examining thewideband signal with the same time-synchronized spreading code(despreading)
bull Advantagesndash Low power spectral densityndash Interference limited operationndash Privacyndash Reduction of multipath affects by using a larger spectrumndash Users can start their transmission at any time (random access possible)ndash Cell capacity is soft and higher than TDMA and FDMAndash No frequency management and no equalizers and quard time needed
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3020111015 TAacuteMOP ndash 412-082AKMR-2009-0006 30
Ad hoc Sensor Networks Multiple channel access
Multiple Access near far problem and power controlbull At a receiver the signals may come from various sources (multiuser) The
strongest signal usually captures the modulator The other signals areconsidered as noise Each source may have different distances to the basestation
bull In CDMA we want a BS to receive CDMA coded signals from variousmobile users at the same time
ndash Therefore the receiver power at the BS for all mobile users should be close toeach other This requires power control at the mobiles
bull Power Control BS monitors the RSSI (Received Signal Strength Indicator)values from different mobiles and then sends power change commands to themobiles over a forward channel The mobiles then adjust their transmitpower
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3120111015 TAacuteMOP ndash 412-082AKMR-2009-0006 31
Ad hoc Sensor Networks Multiple channel access
Multiple Access hybrid spread spectrum techniquesbull FDMACDMA
ndash Available wideband spectrum is divided into a number narrowband radio channels CDMAis employed inside each channel
bull DSFHMAndash The signals are spread using spreading codes (direct sequence signals are obtained) but
these signal are not transmitted over a constant carrier frequency they are transmitted over afrequency hopping carrier frequency
bull Time Division CDMA (TCDMA)ndash Each cell is using a different spreading code (CDMA employed between cells) that is
conveyed to the mobiles in its rangendash Inside each cell (inside a CDMA channel) TDMA is employed to multiplex multiple users
bull Time Division Frequency Hoppingndash At each time slot the user is hopped to a new frequency according to a pseudo-random
hopping sequencendash Employed in severe co-interference and multi-path environmentsndash Bluetooth and GSM are using this technique
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3220111015 TAacuteMOP ndash 412-082AKMR-2009-0006 32
Ad hoc Sensor Networks Multiple channel access
Multiple Access comparison TDMAFDMACDMA
Techniques TDMA FDMA CDMA SDMA
Idea
segment sending time into disjoint time-slots
Segment the frequency band into disjoint sub-bands
Spread the spectrum using orthogonal codes
Segment space into sectors
Signal separation
Synchronization in the time domain
Filtering in the frequency domain
Code Directed antennas
Advantage
Established flexible
Simple established robust
Flexible less frequency planning needed soft handover
Simple increases capacity
DisadvantageQuard space synchronization difficult
Inflexible scarce source
Complex receiver power control
inflexible
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3320111015 TAacuteMOP ndash 412-082AKMR-2009-0006 33
Ad hoc Sensor Networks Multiple channel access
User capacity per cell
The user capacity C is defined as the total number of active users percell that the system can support while meeting a common BERconstrain For orthogonal multiple access (as FDMA and TDMA)
C = Ncellwhere Ncell is the number of channels assigned to any given cellThe total number of orthogonal channels of bandwidth Bs that can becreated from a total system bandwidth of B is B Bs The reuse factorsatisfies N = (B Bs) Ncell this implies
C = (B Bs) N
Recall from Chapter 1
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3420111015 TAacuteMOP ndash 412-082AKMR-2009-0006 34
Ad hoc Sensor Networks Multiple channel access
10 methods for increasing capacity1 Increasing the amount of spectrum used very expensive2 More efficient modulation format and coding3 Better source coding4 Adaptive modulation and coding5 Discontinuous transmission6 Multiuser detection CDMA systems7 Reduction of cell radius effective but very expensive smaller
cells require more handovers8 Use of sector cells tripled cells have tripled BS antennas9 Use of an overlay structure10 Multiple antennas diversity MIMO systems SDMA
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3520111015 TAacuteMOP ndash 412-082AKMR-2009-0006 35
Ad hoc Sensor Networks Multiple channel access
bull Distributed operation is requiredbull Synchronization is required in TDMA-based systemsbull Hidden terminals are nodes hidden from a senderbull Exposed terminals are exposed nodes preventing a sender
from sendingbull Throughput needs to be maximizedbull Access delay should be minimizedbull Fairness refers to provide an equal share to all competing
nodesbull Real-time traffic support is required for voice video and
real-time databull Resource reservation is required for QoSbull Ability to measure resource availability handles the
resourcesbull Capability for power control reduces the energy
consumption
Random access in adhoc networks
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3620111015 TAacuteMOP ndash 412-082AKMR-2009-0006 36
Ad hoc Sensor Networks Multiple channel access
Random accessbull All random access techniques are based on packetized data or packet radio where user
date is collected into packets of M bitsbull Collision if packets from different users overlap in timebull The transmission time of a packet is τ = M R where R [bps] the data ratebull Analysis of random access techniques assumes that the users generate packets according
to memoryless Poisson process at a rate λ [packets per unit time] the probability that thenumber of packet arrivals in a time period [0t] denoted by X(t) is equal to integer k isgiven by
Pr(X(t)=k) = (λt)k k e-λt
bull The traffic load is defined as L = λ τ If L gt 1 means on avarage more packets arrive inthe system over a given time period than can be transmitted in that period so systemswith Lgt1 are unstable
bull The throughput is defined as the ratio of the average number of packets successfullytransmitted in any given time interval divided by the number of attempted transmissionsin that interval
T = L Pr(succesful packet transmission)bull The effective data rate of the system is RT
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3720111015 TAacuteMOP ndash 412-082AKMR-2009-0006 37
Ad hoc Sensor Networks Multiple channel access
Random access (contrsquo)bull We will examine the following attributes
ndash Channel utilization (Throughput)ndash Latencyndash Collision avoidance (Hidden and exposed terminals problem)ndash Reliability (ACK)ndash Energy efficiency (power control)ndash Fairnessndash Throughput needs to be maximized
bull We will discuss the following techniquesndash Aloha and slotted Alohandash CSMA Protocols (1-persistent non-persistent p-persistent CSMA CSMACD)ndash MACAndash MACAWndash Energy efficient MAC in wireless sensor networks (eg SMAC)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 38
Random Access ALOHA (1970)bull ALOHA was developed for a wireless system at the University of Hawaii
(Abramson et al)ndash Multiple remote stations plus one base stationndash Frame transmissions are made at one frequency from a remote station to the basestation the base station re-broadcasts frames on another frequency Contention is foraccess to the base station
bull Frames can be sent at any time to the base stationndash If no acknowledgment (ACK) is received assume that the frame is lost andretransmit after waiting a random amount of timendash If more than one station broadcasts on the base station access frequency at the sametime ndash a collision ndash interference will destroy the frames
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 39
Random Access ALOHA (contrsquo)
bull The probability that no packets generated during the tima [-τ τ] is given by (nocollision)
Pr(X(t)=0) = (λ2τ)0 0 e-2λτ = e-2Lwith corresponding throughput
T = L e-2L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 05 05e 018
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 40
Random Access slotted ALOHA (1971)bull Improvement Time is slotted and a packet can only be transmitted at the beginning of
one slot Thus it can reduce the collision duration
bull The probability that no packets generated during the time [0 τ] is given by (nocollision)
Pr(X(t)=0) = (λτ)0 0 e-λτ = e-Lwith corresponding throughput T = L e-L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 1 e 037
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 41
bull Slotted Aloha has double the maximumthroughput as pure Aloha and achievesthis maximum at a higher offered loadThe effective data rate is still less than40 of the raw transmission rate
bull Aloha does not listen to the carrier beforetransmission more sophisticatedtechniques are needed to increaseefficiency
bull Further problem the delayndashDelay
ndashThe mean packet delay
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
( ) ( )delayPr 1 e ekL Lkτ minus minus= = minus
( )delay1
1 e e ekL L L
kMPD E kτ
infinminus minus
=
= = minus =sum
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 42
Random Access Carrier Sense Multiple Accessbull CSMA (Carrier Sense Multiple Access) listen to the carrier before transmission
and transmits if channel is idlebull Detection delay and propagation delay are two important parameters
ndash Detection delay time required to sense the carrier and decide if it is idle or busyndash Propagation delay distancespeed_of_ligth The time required for bit to travel
from transmitter to the receiver
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 43
Random Access CSMA variations
Ad hoc Sensor Networks Multiple channel access
1-persistent CSMA Non-persistent CSMA
There will always be a collision iftwo nodes want to retransmit(usually you stop transmissionattempts after few tries)
Random backoff reduces probabilityof collisions Waste idle time if thebackoff time is too long
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 44
Random Access CSMA variations (contrsquo)
Ad hoc Sensor Networks Multiple channel access
p-persistent CSMA CSMA CD (Collision Detection)
A good tradeoff between non-persistent and 1-persistent CSMA
Same with CSMA however a stationalso listen to the carrier whiletransmitting to see if the transmissioncollides with someone elsetransmission It can be used in listen-while-talk capable channels (fullduplex) In single radio channels thetransmission need to be interrupted inorder to sense the channel
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 45
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 46
Random Access CSMA (contrsquo)bull How to select probability p
ndash Assume that M nodes have a packet to send and the medium is busy then Mp is theexpected number of nodes that will attempt to transmit once the medium becomesidle If Mp gt 1 then a collision is expected to occur Therefore the network mustmake sure that Mp lt 1 to avoid collision where M is the maximum number of nodesthat can be active at a time
bull Problems1 The mean packet delay increases exponently with increasing offered load2 CSMA protocols sense the carrier but sensing the carrier does not always releasestrue information about the status of the wireless channel There are two problems thatare unique to wireless channels (different than wireline channels)ndash Hidden terminal problemndash Exposed terminal problem
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 47
Hidden terminal problembull A sends to B C cannot receive Abull C wants to send to B C senses a
ldquofreerdquo medium (CS fails)bull collision at B A cannot receive the
collision (CD fails)bull A is ldquohiddenrdquo for C
Ad hoc Sensor Networks Multiple channel access
Random Access hidden and exposed terminal problemExposed terminal problem
bull B sends to A C wants to send to Dbull C has to wait CS signals a medium
in usebull since A is outside the radio range of
C waiting is not necessarybull C is ldquoexposedrdquo to B
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 48
Random Access MACAWhen a station wants to transmit databull It sends an RTS (Ready-to-Send) packet to the intended receiver
ndash The RTS packet contains the length of the data that needs to be transmittedndash Any station other than the intended recipient hearing RTS defers transmission for atime duration equal to the end of the corresponding CTS reception
bull The receiver sends back CTS (Clear-to-Send) packet back to sender if it isavailable to receivendash The CTS packet contains the length of the data that original sender wants totransmitndash Any station other than the original RTS sender hearing CTS defers transmissionuntil the data is sent
bull The original sender upon reception of the CTS starts transmitting
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 49
Hidden terminal problem
Ad hoc Sensor Networks Multiple channel access
Random Access MACA (contrsquo)Exposed terminal problem
Waiting time of node X is muchsmaller than waiting time of node C
bull C defers transmission upon hearing BrsquosRTS until B could get CTS from Abull After that C can start transmission to DFor that it first sends an RTSbull C is not longer exposed to the datatransmission of B
Problem It does not address the collision of RTS packet
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 50
Random Access reliabilitybull Wireless links are prone to have errors High packet loss rate detrimental to
transport-layer performance Mechanisms needed to reduce packet loss rateexperienced by upper layers
bull When node B receives a data packet from node A node B sends anAcknowledgement (Ack) This approach adopted in many protocols
bull If node A fails to receive an Ack it will retransmit the packet
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 51
Random Access MACAW (CSMACA) bull Physical carrier sense and virtual carrier sense using Network Allocation
Vector (NAV)ndash NAV is updated based on overheard RTSCTSDATAACK packets each of whichspecified duration of a pending transmissionndash Nodes stay silent when carrier sensed (physicalvirtual)
bull Backoff intervals used to reduce collision probabilityndash When transmitting a packet choose a backoff interval in the range [0w] where wis contention windowndash Count down the backoff interval when channel is idlendash Count down is suspended if channel becomes busyndash When backoff interval reaches 0 transmit RTS
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 52
Random Access CSMACA (contrsquo) bull The time spent counting down backoff intervals is a part of MAC overhead
ndash Choosing a large w leads to large backoff intervals and can result in larger overheadndash Choosing a small w leads to a larger number of collisions (when two nodes countdown to 0 simultaneously)ndash Since the number of nodes attempting to transmit simultaneously may change withtime some mechanism to manage contention is needed
bull IEEE 80211 DCF contention window w is chosen dynamically depending oncollision occurrence (binary exponential backoff)ndash When a node fails to receive CTS in response to its RTS it increases the contentionwindowndash w is doubled (up to an upper bound)ndash When a node successfully completes a data transfer it restores w to wmin
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 53
Random Access fairnessbull Simplest definition of fairness all nodes should receive equal bandwidthbull An example of unfairness
ndash Assume that initially A and B both choose a backoff interval in range [031] buttheir RTSs collidendash Nodes A and B then choose from range [063]ndash Node A chooses 4 slots and B choose 60 slotsndash After A transmits a packet it next chooses from range [031]ndash It is possible that A may transmit several packets before B transmits its first packet
Ad hoc Sensor Networks Multiple channel access
Unfairness occurs when one node has backed off much more than some other node
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 54
Random Access fairness (contrsquo) MACAW solution for fairnessbull When a node transmits a packet it appends value w to the packet all nodes
hearing value w use it for their future transmission attemptsbull Since w is an indication of the level of congestion in the vicinity of a specific
receiver node MACAW proposes maintaining w independently for eachreceiver
bull Using per-receiver wi is particularly useful in multi-hop environments sincecongestion level at different receivers can be very different
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 55
Random Access energy efficiency bull Since many mobile hosts are operated by batteries MAC
protocols which conserve energy are of interestbull Two approaches to reduce energy consumption
ndash Power save turn off wireless interface when desirablendash Power control reduce transmit power
bull Power control has some more potential benefitsndash Reduced interference (in ad hoc networks) eg it improves ALOHAefficiency where user with high power can capture a packet even if there is acollisionndash Increased spatial reuse (in infrastructured wireless networks) eg it is usedin CDMA to maintain target SIR of voice and data users or generally it canbe used to maintain target SIR for different user classes (admission control)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 56
Random Access power control bull When C transmits to D at a high power level B cannot receive Arsquos
transmission due to interference from Cbull If C reduces transmit power it can still communicate with D
ndash Reduces energy consumption at node Cndash Allows B to receive Arsquos transmission (spatial reuse)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 57
bull A special wireless ad hoc networkndash Large number of nodesndash Battery poweredndash Topology and density changendash Nodes for a common taskndash In-network data processing
bull Sensor-net applicationsndash Sensor-triggered bursty trafficndash Can often tolerate some delay
Ad hoc Sensor Networks Multiple channel access
Characteristics of Sensor Network
Scalabilty and self configurationEnergy efficiencyAdaptivityFairness not importantLatency
AdaptivityEnergy-delay tradoff
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 58
Ad hoc Sensor Networks Multiple channel access
Wireless sensor node and sensor network
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 59
MAC for WSN attributes bull Attributes
ndash Collision avoidance (Basic task)ndash Energy efficiency (most primary in WSN)ndash Scalability and adaptivityndash Channel utilizationndash Latencyndash Throughputndash Fairness
bull What causes energy wastendash Collisionsndash Control packet overheadndash Overhearing unnecessary trafficndash Long idle time (it consumes 50-100 of the power)
Ad hoc Sensor Networks Multiple channel access
Primary
Secondary
Dominant factor
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 60
MAC for WSN
bull Contention-based protocols need to work hard in all directions for energysavingsndash Reduce idle listening ndash support low duty cyclendash Better collision avoidancendash Reduce control overheadndash Avoid unnecessary overhearing
Ad hoc Sensor Networks Multiple channel access
Scheduled Protocols Contention ProtocolsCollisions No YesEnergy efficiency Good BadScalability and adaptivity Bad GoodMulti-hop communication Difficult EasyTime synchronization Strict Loose or not required
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 61
MAC for WSN classification
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 62
MAC for WSN SMAC (2002)bull It has been developed by Ye Heidemann and Estrinbull Tradeoffs between (latency fairness) and energybull Major components in S-MAC
ndash Periodic listen and sleep turn off radio when sleeping the reduced dutycycle is ~ 10 (eg 120ms on 12s off)ndash Collision avoidance is based on contention similarly to IEEE 80211 adhoc modendash Overhearing avoidance the node is sleeping when neighbors talkndash Massage passing long message is fragmented amp sent in burst operationwith extended Tx time and re-transmit occurs immediately
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 63
Summarybull The algorithms and protocols that enable sharing by multiple users and
controls the access to the wireless channel from different users are calledMAC
bull There are two types of wireless networks Infrastructured wireless networksand ad hoc wireless networks
bull We also want our wireless system to have high user capacity and we canidentify 10 methods for increasing it
bull There are several MACs with no absolute advantage it should be tailoredto the system requirements
bull There are primary concerns for designing a WSN MAC (collisionavoidance energy efficiency scalability and adaptivity) while thesecondary concerns are as follows (channel utilization latency throughputfairness)
bull Next lecture Routing protocols
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 25
Multiple Access TDMA (contrsquo)bull Enables the sharing of a single radio channel among N usersbull Requires high data-rate per radio channel to support N users
simultaneously which requires adaptive equalizers to be usedin multipath environments
bull Transmission occurs in bursts (not continually) It enablespower saving by going to sleep modes in unrelated slots Itdiscontinues transmission and also enables mobile assistedhandoff
bull It equires synchronization of the receivers Need guard bits andsync bits which occurs large overhead per slot
bull Allocation of slots to mobile users should not be uniform Itmay depend on the traffic requirement of mobiles This bringsextra flexibility and efficiency compared to FDMA systems
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2620111015 TAacuteMOP ndash 412-082AKMR-2009-0006 26
Ad hoc Sensor Networks Multiple channel access
Multiple Access spread spectrum multiple accessbull SSMA uses signals that have transmission bandwidth that is
several orders of magnitude larger than minimum requiredRF bandwidth
bull It providesndash Immunity to multipath interferencendash Robust multiple access
bull Two techniquesndash Frequency Hopped Multiple Access (FHMA)ndash Direct Sequence Multiple Access (DSMA) or Code Division
Multiple Access (CDMA)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2720111015 TAacuteMOP ndash 412-082AKMR-2009-0006 27
Ad hoc Sensor Networks Multiple channel access
Multiple Access FHMAbull The carrier frequency of users are varied in a pseudo-random
fashionndash Each user is using a narrowband channel (spectrum) at a specific
instance of timendash The random change in frequency make the change of using the same
narrowband channel very lowbull The sender receiver change frequency (calling hopping)
using the same pseudo-random sequence hence they aresynchronized
bull Rate of hopping versus Symbol ratendash If hopping rate is greather Fast Frequency Hoppingndash If symbol rate is greater Slow Frequency Hopping
bull GSM and Bluetooth are example systems
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2820111015 TAacuteMOP ndash 412-082AKMR-2009-0006 28
Ad hoc Sensor Networks Multiple channel access
Multiple Access FHMA (contrsquo)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2920111015 TAacuteMOP ndash 412-082AKMR-2009-0006 29
Ad hoc Sensor Networks Multiple channel access
Multiple Access CDMAbull The narrowband message signal is multiplied by a very large bandwidth
signal called spreading signal (code) before modulation and transmissionover the air This is called spreading
bull Spreading signal use a pseudo-noise (PN) sequence (a pseudo-randomsequence) called codeword which are orthogonal (low autocorrelation)
bull The receiver correlator distinguishes the senders signal by examining thewideband signal with the same time-synchronized spreading code(despreading)
bull Advantagesndash Low power spectral densityndash Interference limited operationndash Privacyndash Reduction of multipath affects by using a larger spectrumndash Users can start their transmission at any time (random access possible)ndash Cell capacity is soft and higher than TDMA and FDMAndash No frequency management and no equalizers and quard time needed
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3020111015 TAacuteMOP ndash 412-082AKMR-2009-0006 30
Ad hoc Sensor Networks Multiple channel access
Multiple Access near far problem and power controlbull At a receiver the signals may come from various sources (multiuser) The
strongest signal usually captures the modulator The other signals areconsidered as noise Each source may have different distances to the basestation
bull In CDMA we want a BS to receive CDMA coded signals from variousmobile users at the same time
ndash Therefore the receiver power at the BS for all mobile users should be close toeach other This requires power control at the mobiles
bull Power Control BS monitors the RSSI (Received Signal Strength Indicator)values from different mobiles and then sends power change commands to themobiles over a forward channel The mobiles then adjust their transmitpower
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3120111015 TAacuteMOP ndash 412-082AKMR-2009-0006 31
Ad hoc Sensor Networks Multiple channel access
Multiple Access hybrid spread spectrum techniquesbull FDMACDMA
ndash Available wideband spectrum is divided into a number narrowband radio channels CDMAis employed inside each channel
bull DSFHMAndash The signals are spread using spreading codes (direct sequence signals are obtained) but
these signal are not transmitted over a constant carrier frequency they are transmitted over afrequency hopping carrier frequency
bull Time Division CDMA (TCDMA)ndash Each cell is using a different spreading code (CDMA employed between cells) that is
conveyed to the mobiles in its rangendash Inside each cell (inside a CDMA channel) TDMA is employed to multiplex multiple users
bull Time Division Frequency Hoppingndash At each time slot the user is hopped to a new frequency according to a pseudo-random
hopping sequencendash Employed in severe co-interference and multi-path environmentsndash Bluetooth and GSM are using this technique
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3220111015 TAacuteMOP ndash 412-082AKMR-2009-0006 32
Ad hoc Sensor Networks Multiple channel access
Multiple Access comparison TDMAFDMACDMA
Techniques TDMA FDMA CDMA SDMA
Idea
segment sending time into disjoint time-slots
Segment the frequency band into disjoint sub-bands
Spread the spectrum using orthogonal codes
Segment space into sectors
Signal separation
Synchronization in the time domain
Filtering in the frequency domain
Code Directed antennas
Advantage
Established flexible
Simple established robust
Flexible less frequency planning needed soft handover
Simple increases capacity
DisadvantageQuard space synchronization difficult
Inflexible scarce source
Complex receiver power control
inflexible
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3320111015 TAacuteMOP ndash 412-082AKMR-2009-0006 33
Ad hoc Sensor Networks Multiple channel access
User capacity per cell
The user capacity C is defined as the total number of active users percell that the system can support while meeting a common BERconstrain For orthogonal multiple access (as FDMA and TDMA)
C = Ncellwhere Ncell is the number of channels assigned to any given cellThe total number of orthogonal channels of bandwidth Bs that can becreated from a total system bandwidth of B is B Bs The reuse factorsatisfies N = (B Bs) Ncell this implies
C = (B Bs) N
Recall from Chapter 1
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3420111015 TAacuteMOP ndash 412-082AKMR-2009-0006 34
Ad hoc Sensor Networks Multiple channel access
10 methods for increasing capacity1 Increasing the amount of spectrum used very expensive2 More efficient modulation format and coding3 Better source coding4 Adaptive modulation and coding5 Discontinuous transmission6 Multiuser detection CDMA systems7 Reduction of cell radius effective but very expensive smaller
cells require more handovers8 Use of sector cells tripled cells have tripled BS antennas9 Use of an overlay structure10 Multiple antennas diversity MIMO systems SDMA
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3520111015 TAacuteMOP ndash 412-082AKMR-2009-0006 35
Ad hoc Sensor Networks Multiple channel access
bull Distributed operation is requiredbull Synchronization is required in TDMA-based systemsbull Hidden terminals are nodes hidden from a senderbull Exposed terminals are exposed nodes preventing a sender
from sendingbull Throughput needs to be maximizedbull Access delay should be minimizedbull Fairness refers to provide an equal share to all competing
nodesbull Real-time traffic support is required for voice video and
real-time databull Resource reservation is required for QoSbull Ability to measure resource availability handles the
resourcesbull Capability for power control reduces the energy
consumption
Random access in adhoc networks
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3620111015 TAacuteMOP ndash 412-082AKMR-2009-0006 36
Ad hoc Sensor Networks Multiple channel access
Random accessbull All random access techniques are based on packetized data or packet radio where user
date is collected into packets of M bitsbull Collision if packets from different users overlap in timebull The transmission time of a packet is τ = M R where R [bps] the data ratebull Analysis of random access techniques assumes that the users generate packets according
to memoryless Poisson process at a rate λ [packets per unit time] the probability that thenumber of packet arrivals in a time period [0t] denoted by X(t) is equal to integer k isgiven by
Pr(X(t)=k) = (λt)k k e-λt
bull The traffic load is defined as L = λ τ If L gt 1 means on avarage more packets arrive inthe system over a given time period than can be transmitted in that period so systemswith Lgt1 are unstable
bull The throughput is defined as the ratio of the average number of packets successfullytransmitted in any given time interval divided by the number of attempted transmissionsin that interval
T = L Pr(succesful packet transmission)bull The effective data rate of the system is RT
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3720111015 TAacuteMOP ndash 412-082AKMR-2009-0006 37
Ad hoc Sensor Networks Multiple channel access
Random access (contrsquo)bull We will examine the following attributes
ndash Channel utilization (Throughput)ndash Latencyndash Collision avoidance (Hidden and exposed terminals problem)ndash Reliability (ACK)ndash Energy efficiency (power control)ndash Fairnessndash Throughput needs to be maximized
bull We will discuss the following techniquesndash Aloha and slotted Alohandash CSMA Protocols (1-persistent non-persistent p-persistent CSMA CSMACD)ndash MACAndash MACAWndash Energy efficient MAC in wireless sensor networks (eg SMAC)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 38
Random Access ALOHA (1970)bull ALOHA was developed for a wireless system at the University of Hawaii
(Abramson et al)ndash Multiple remote stations plus one base stationndash Frame transmissions are made at one frequency from a remote station to the basestation the base station re-broadcasts frames on another frequency Contention is foraccess to the base station
bull Frames can be sent at any time to the base stationndash If no acknowledgment (ACK) is received assume that the frame is lost andretransmit after waiting a random amount of timendash If more than one station broadcasts on the base station access frequency at the sametime ndash a collision ndash interference will destroy the frames
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 39
Random Access ALOHA (contrsquo)
bull The probability that no packets generated during the tima [-τ τ] is given by (nocollision)
Pr(X(t)=0) = (λ2τ)0 0 e-2λτ = e-2Lwith corresponding throughput
T = L e-2L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 05 05e 018
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 40
Random Access slotted ALOHA (1971)bull Improvement Time is slotted and a packet can only be transmitted at the beginning of
one slot Thus it can reduce the collision duration
bull The probability that no packets generated during the time [0 τ] is given by (nocollision)
Pr(X(t)=0) = (λτ)0 0 e-λτ = e-Lwith corresponding throughput T = L e-L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 1 e 037
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 41
bull Slotted Aloha has double the maximumthroughput as pure Aloha and achievesthis maximum at a higher offered loadThe effective data rate is still less than40 of the raw transmission rate
bull Aloha does not listen to the carrier beforetransmission more sophisticatedtechniques are needed to increaseefficiency
bull Further problem the delayndashDelay
ndashThe mean packet delay
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
( ) ( )delayPr 1 e ekL Lkτ minus minus= = minus
( )delay1
1 e e ekL L L
kMPD E kτ
infinminus minus
=
= = minus =sum
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 42
Random Access Carrier Sense Multiple Accessbull CSMA (Carrier Sense Multiple Access) listen to the carrier before transmission
and transmits if channel is idlebull Detection delay and propagation delay are two important parameters
ndash Detection delay time required to sense the carrier and decide if it is idle or busyndash Propagation delay distancespeed_of_ligth The time required for bit to travel
from transmitter to the receiver
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 43
Random Access CSMA variations
Ad hoc Sensor Networks Multiple channel access
1-persistent CSMA Non-persistent CSMA
There will always be a collision iftwo nodes want to retransmit(usually you stop transmissionattempts after few tries)
Random backoff reduces probabilityof collisions Waste idle time if thebackoff time is too long
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 44
Random Access CSMA variations (contrsquo)
Ad hoc Sensor Networks Multiple channel access
p-persistent CSMA CSMA CD (Collision Detection)
A good tradeoff between non-persistent and 1-persistent CSMA
Same with CSMA however a stationalso listen to the carrier whiletransmitting to see if the transmissioncollides with someone elsetransmission It can be used in listen-while-talk capable channels (fullduplex) In single radio channels thetransmission need to be interrupted inorder to sense the channel
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 45
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 46
Random Access CSMA (contrsquo)bull How to select probability p
ndash Assume that M nodes have a packet to send and the medium is busy then Mp is theexpected number of nodes that will attempt to transmit once the medium becomesidle If Mp gt 1 then a collision is expected to occur Therefore the network mustmake sure that Mp lt 1 to avoid collision where M is the maximum number of nodesthat can be active at a time
bull Problems1 The mean packet delay increases exponently with increasing offered load2 CSMA protocols sense the carrier but sensing the carrier does not always releasestrue information about the status of the wireless channel There are two problems thatare unique to wireless channels (different than wireline channels)ndash Hidden terminal problemndash Exposed terminal problem
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 47
Hidden terminal problembull A sends to B C cannot receive Abull C wants to send to B C senses a
ldquofreerdquo medium (CS fails)bull collision at B A cannot receive the
collision (CD fails)bull A is ldquohiddenrdquo for C
Ad hoc Sensor Networks Multiple channel access
Random Access hidden and exposed terminal problemExposed terminal problem
bull B sends to A C wants to send to Dbull C has to wait CS signals a medium
in usebull since A is outside the radio range of
C waiting is not necessarybull C is ldquoexposedrdquo to B
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 48
Random Access MACAWhen a station wants to transmit databull It sends an RTS (Ready-to-Send) packet to the intended receiver
ndash The RTS packet contains the length of the data that needs to be transmittedndash Any station other than the intended recipient hearing RTS defers transmission for atime duration equal to the end of the corresponding CTS reception
bull The receiver sends back CTS (Clear-to-Send) packet back to sender if it isavailable to receivendash The CTS packet contains the length of the data that original sender wants totransmitndash Any station other than the original RTS sender hearing CTS defers transmissionuntil the data is sent
bull The original sender upon reception of the CTS starts transmitting
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 49
Hidden terminal problem
Ad hoc Sensor Networks Multiple channel access
Random Access MACA (contrsquo)Exposed terminal problem
Waiting time of node X is muchsmaller than waiting time of node C
bull C defers transmission upon hearing BrsquosRTS until B could get CTS from Abull After that C can start transmission to DFor that it first sends an RTSbull C is not longer exposed to the datatransmission of B
Problem It does not address the collision of RTS packet
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 50
Random Access reliabilitybull Wireless links are prone to have errors High packet loss rate detrimental to
transport-layer performance Mechanisms needed to reduce packet loss rateexperienced by upper layers
bull When node B receives a data packet from node A node B sends anAcknowledgement (Ack) This approach adopted in many protocols
bull If node A fails to receive an Ack it will retransmit the packet
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 51
Random Access MACAW (CSMACA) bull Physical carrier sense and virtual carrier sense using Network Allocation
Vector (NAV)ndash NAV is updated based on overheard RTSCTSDATAACK packets each of whichspecified duration of a pending transmissionndash Nodes stay silent when carrier sensed (physicalvirtual)
bull Backoff intervals used to reduce collision probabilityndash When transmitting a packet choose a backoff interval in the range [0w] where wis contention windowndash Count down the backoff interval when channel is idlendash Count down is suspended if channel becomes busyndash When backoff interval reaches 0 transmit RTS
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 52
Random Access CSMACA (contrsquo) bull The time spent counting down backoff intervals is a part of MAC overhead
ndash Choosing a large w leads to large backoff intervals and can result in larger overheadndash Choosing a small w leads to a larger number of collisions (when two nodes countdown to 0 simultaneously)ndash Since the number of nodes attempting to transmit simultaneously may change withtime some mechanism to manage contention is needed
bull IEEE 80211 DCF contention window w is chosen dynamically depending oncollision occurrence (binary exponential backoff)ndash When a node fails to receive CTS in response to its RTS it increases the contentionwindowndash w is doubled (up to an upper bound)ndash When a node successfully completes a data transfer it restores w to wmin
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 53
Random Access fairnessbull Simplest definition of fairness all nodes should receive equal bandwidthbull An example of unfairness
ndash Assume that initially A and B both choose a backoff interval in range [031] buttheir RTSs collidendash Nodes A and B then choose from range [063]ndash Node A chooses 4 slots and B choose 60 slotsndash After A transmits a packet it next chooses from range [031]ndash It is possible that A may transmit several packets before B transmits its first packet
Ad hoc Sensor Networks Multiple channel access
Unfairness occurs when one node has backed off much more than some other node
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 54
Random Access fairness (contrsquo) MACAW solution for fairnessbull When a node transmits a packet it appends value w to the packet all nodes
hearing value w use it for their future transmission attemptsbull Since w is an indication of the level of congestion in the vicinity of a specific
receiver node MACAW proposes maintaining w independently for eachreceiver
bull Using per-receiver wi is particularly useful in multi-hop environments sincecongestion level at different receivers can be very different
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 55
Random Access energy efficiency bull Since many mobile hosts are operated by batteries MAC
protocols which conserve energy are of interestbull Two approaches to reduce energy consumption
ndash Power save turn off wireless interface when desirablendash Power control reduce transmit power
bull Power control has some more potential benefitsndash Reduced interference (in ad hoc networks) eg it improves ALOHAefficiency where user with high power can capture a packet even if there is acollisionndash Increased spatial reuse (in infrastructured wireless networks) eg it is usedin CDMA to maintain target SIR of voice and data users or generally it canbe used to maintain target SIR for different user classes (admission control)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 56
Random Access power control bull When C transmits to D at a high power level B cannot receive Arsquos
transmission due to interference from Cbull If C reduces transmit power it can still communicate with D
ndash Reduces energy consumption at node Cndash Allows B to receive Arsquos transmission (spatial reuse)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 57
bull A special wireless ad hoc networkndash Large number of nodesndash Battery poweredndash Topology and density changendash Nodes for a common taskndash In-network data processing
bull Sensor-net applicationsndash Sensor-triggered bursty trafficndash Can often tolerate some delay
Ad hoc Sensor Networks Multiple channel access
Characteristics of Sensor Network
Scalabilty and self configurationEnergy efficiencyAdaptivityFairness not importantLatency
AdaptivityEnergy-delay tradoff
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 58
Ad hoc Sensor Networks Multiple channel access
Wireless sensor node and sensor network
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 59
MAC for WSN attributes bull Attributes
ndash Collision avoidance (Basic task)ndash Energy efficiency (most primary in WSN)ndash Scalability and adaptivityndash Channel utilizationndash Latencyndash Throughputndash Fairness
bull What causes energy wastendash Collisionsndash Control packet overheadndash Overhearing unnecessary trafficndash Long idle time (it consumes 50-100 of the power)
Ad hoc Sensor Networks Multiple channel access
Primary
Secondary
Dominant factor
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 60
MAC for WSN
bull Contention-based protocols need to work hard in all directions for energysavingsndash Reduce idle listening ndash support low duty cyclendash Better collision avoidancendash Reduce control overheadndash Avoid unnecessary overhearing
Ad hoc Sensor Networks Multiple channel access
Scheduled Protocols Contention ProtocolsCollisions No YesEnergy efficiency Good BadScalability and adaptivity Bad GoodMulti-hop communication Difficult EasyTime synchronization Strict Loose or not required
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 61
MAC for WSN classification
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 62
MAC for WSN SMAC (2002)bull It has been developed by Ye Heidemann and Estrinbull Tradeoffs between (latency fairness) and energybull Major components in S-MAC
ndash Periodic listen and sleep turn off radio when sleeping the reduced dutycycle is ~ 10 (eg 120ms on 12s off)ndash Collision avoidance is based on contention similarly to IEEE 80211 adhoc modendash Overhearing avoidance the node is sleeping when neighbors talkndash Massage passing long message is fragmented amp sent in burst operationwith extended Tx time and re-transmit occurs immediately
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 63
Summarybull The algorithms and protocols that enable sharing by multiple users and
controls the access to the wireless channel from different users are calledMAC
bull There are two types of wireless networks Infrastructured wireless networksand ad hoc wireless networks
bull We also want our wireless system to have high user capacity and we canidentify 10 methods for increasing it
bull There are several MACs with no absolute advantage it should be tailoredto the system requirements
bull There are primary concerns for designing a WSN MAC (collisionavoidance energy efficiency scalability and adaptivity) while thesecondary concerns are as follows (channel utilization latency throughputfairness)
bull Next lecture Routing protocols
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2620111015 TAacuteMOP ndash 412-082AKMR-2009-0006 26
Ad hoc Sensor Networks Multiple channel access
Multiple Access spread spectrum multiple accessbull SSMA uses signals that have transmission bandwidth that is
several orders of magnitude larger than minimum requiredRF bandwidth
bull It providesndash Immunity to multipath interferencendash Robust multiple access
bull Two techniquesndash Frequency Hopped Multiple Access (FHMA)ndash Direct Sequence Multiple Access (DSMA) or Code Division
Multiple Access (CDMA)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2720111015 TAacuteMOP ndash 412-082AKMR-2009-0006 27
Ad hoc Sensor Networks Multiple channel access
Multiple Access FHMAbull The carrier frequency of users are varied in a pseudo-random
fashionndash Each user is using a narrowband channel (spectrum) at a specific
instance of timendash The random change in frequency make the change of using the same
narrowband channel very lowbull The sender receiver change frequency (calling hopping)
using the same pseudo-random sequence hence they aresynchronized
bull Rate of hopping versus Symbol ratendash If hopping rate is greather Fast Frequency Hoppingndash If symbol rate is greater Slow Frequency Hopping
bull GSM and Bluetooth are example systems
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2820111015 TAacuteMOP ndash 412-082AKMR-2009-0006 28
Ad hoc Sensor Networks Multiple channel access
Multiple Access FHMA (contrsquo)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2920111015 TAacuteMOP ndash 412-082AKMR-2009-0006 29
Ad hoc Sensor Networks Multiple channel access
Multiple Access CDMAbull The narrowband message signal is multiplied by a very large bandwidth
signal called spreading signal (code) before modulation and transmissionover the air This is called spreading
bull Spreading signal use a pseudo-noise (PN) sequence (a pseudo-randomsequence) called codeword which are orthogonal (low autocorrelation)
bull The receiver correlator distinguishes the senders signal by examining thewideband signal with the same time-synchronized spreading code(despreading)
bull Advantagesndash Low power spectral densityndash Interference limited operationndash Privacyndash Reduction of multipath affects by using a larger spectrumndash Users can start their transmission at any time (random access possible)ndash Cell capacity is soft and higher than TDMA and FDMAndash No frequency management and no equalizers and quard time needed
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3020111015 TAacuteMOP ndash 412-082AKMR-2009-0006 30
Ad hoc Sensor Networks Multiple channel access
Multiple Access near far problem and power controlbull At a receiver the signals may come from various sources (multiuser) The
strongest signal usually captures the modulator The other signals areconsidered as noise Each source may have different distances to the basestation
bull In CDMA we want a BS to receive CDMA coded signals from variousmobile users at the same time
ndash Therefore the receiver power at the BS for all mobile users should be close toeach other This requires power control at the mobiles
bull Power Control BS monitors the RSSI (Received Signal Strength Indicator)values from different mobiles and then sends power change commands to themobiles over a forward channel The mobiles then adjust their transmitpower
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3120111015 TAacuteMOP ndash 412-082AKMR-2009-0006 31
Ad hoc Sensor Networks Multiple channel access
Multiple Access hybrid spread spectrum techniquesbull FDMACDMA
ndash Available wideband spectrum is divided into a number narrowband radio channels CDMAis employed inside each channel
bull DSFHMAndash The signals are spread using spreading codes (direct sequence signals are obtained) but
these signal are not transmitted over a constant carrier frequency they are transmitted over afrequency hopping carrier frequency
bull Time Division CDMA (TCDMA)ndash Each cell is using a different spreading code (CDMA employed between cells) that is
conveyed to the mobiles in its rangendash Inside each cell (inside a CDMA channel) TDMA is employed to multiplex multiple users
bull Time Division Frequency Hoppingndash At each time slot the user is hopped to a new frequency according to a pseudo-random
hopping sequencendash Employed in severe co-interference and multi-path environmentsndash Bluetooth and GSM are using this technique
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3220111015 TAacuteMOP ndash 412-082AKMR-2009-0006 32
Ad hoc Sensor Networks Multiple channel access
Multiple Access comparison TDMAFDMACDMA
Techniques TDMA FDMA CDMA SDMA
Idea
segment sending time into disjoint time-slots
Segment the frequency band into disjoint sub-bands
Spread the spectrum using orthogonal codes
Segment space into sectors
Signal separation
Synchronization in the time domain
Filtering in the frequency domain
Code Directed antennas
Advantage
Established flexible
Simple established robust
Flexible less frequency planning needed soft handover
Simple increases capacity
DisadvantageQuard space synchronization difficult
Inflexible scarce source
Complex receiver power control
inflexible
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3320111015 TAacuteMOP ndash 412-082AKMR-2009-0006 33
Ad hoc Sensor Networks Multiple channel access
User capacity per cell
The user capacity C is defined as the total number of active users percell that the system can support while meeting a common BERconstrain For orthogonal multiple access (as FDMA and TDMA)
C = Ncellwhere Ncell is the number of channels assigned to any given cellThe total number of orthogonal channels of bandwidth Bs that can becreated from a total system bandwidth of B is B Bs The reuse factorsatisfies N = (B Bs) Ncell this implies
C = (B Bs) N
Recall from Chapter 1
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3420111015 TAacuteMOP ndash 412-082AKMR-2009-0006 34
Ad hoc Sensor Networks Multiple channel access
10 methods for increasing capacity1 Increasing the amount of spectrum used very expensive2 More efficient modulation format and coding3 Better source coding4 Adaptive modulation and coding5 Discontinuous transmission6 Multiuser detection CDMA systems7 Reduction of cell radius effective but very expensive smaller
cells require more handovers8 Use of sector cells tripled cells have tripled BS antennas9 Use of an overlay structure10 Multiple antennas diversity MIMO systems SDMA
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3520111015 TAacuteMOP ndash 412-082AKMR-2009-0006 35
Ad hoc Sensor Networks Multiple channel access
bull Distributed operation is requiredbull Synchronization is required in TDMA-based systemsbull Hidden terminals are nodes hidden from a senderbull Exposed terminals are exposed nodes preventing a sender
from sendingbull Throughput needs to be maximizedbull Access delay should be minimizedbull Fairness refers to provide an equal share to all competing
nodesbull Real-time traffic support is required for voice video and
real-time databull Resource reservation is required for QoSbull Ability to measure resource availability handles the
resourcesbull Capability for power control reduces the energy
consumption
Random access in adhoc networks
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3620111015 TAacuteMOP ndash 412-082AKMR-2009-0006 36
Ad hoc Sensor Networks Multiple channel access
Random accessbull All random access techniques are based on packetized data or packet radio where user
date is collected into packets of M bitsbull Collision if packets from different users overlap in timebull The transmission time of a packet is τ = M R where R [bps] the data ratebull Analysis of random access techniques assumes that the users generate packets according
to memoryless Poisson process at a rate λ [packets per unit time] the probability that thenumber of packet arrivals in a time period [0t] denoted by X(t) is equal to integer k isgiven by
Pr(X(t)=k) = (λt)k k e-λt
bull The traffic load is defined as L = λ τ If L gt 1 means on avarage more packets arrive inthe system over a given time period than can be transmitted in that period so systemswith Lgt1 are unstable
bull The throughput is defined as the ratio of the average number of packets successfullytransmitted in any given time interval divided by the number of attempted transmissionsin that interval
T = L Pr(succesful packet transmission)bull The effective data rate of the system is RT
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3720111015 TAacuteMOP ndash 412-082AKMR-2009-0006 37
Ad hoc Sensor Networks Multiple channel access
Random access (contrsquo)bull We will examine the following attributes
ndash Channel utilization (Throughput)ndash Latencyndash Collision avoidance (Hidden and exposed terminals problem)ndash Reliability (ACK)ndash Energy efficiency (power control)ndash Fairnessndash Throughput needs to be maximized
bull We will discuss the following techniquesndash Aloha and slotted Alohandash CSMA Protocols (1-persistent non-persistent p-persistent CSMA CSMACD)ndash MACAndash MACAWndash Energy efficient MAC in wireless sensor networks (eg SMAC)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 38
Random Access ALOHA (1970)bull ALOHA was developed for a wireless system at the University of Hawaii
(Abramson et al)ndash Multiple remote stations plus one base stationndash Frame transmissions are made at one frequency from a remote station to the basestation the base station re-broadcasts frames on another frequency Contention is foraccess to the base station
bull Frames can be sent at any time to the base stationndash If no acknowledgment (ACK) is received assume that the frame is lost andretransmit after waiting a random amount of timendash If more than one station broadcasts on the base station access frequency at the sametime ndash a collision ndash interference will destroy the frames
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 39
Random Access ALOHA (contrsquo)
bull The probability that no packets generated during the tima [-τ τ] is given by (nocollision)
Pr(X(t)=0) = (λ2τ)0 0 e-2λτ = e-2Lwith corresponding throughput
T = L e-2L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 05 05e 018
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 40
Random Access slotted ALOHA (1971)bull Improvement Time is slotted and a packet can only be transmitted at the beginning of
one slot Thus it can reduce the collision duration
bull The probability that no packets generated during the time [0 τ] is given by (nocollision)
Pr(X(t)=0) = (λτ)0 0 e-λτ = e-Lwith corresponding throughput T = L e-L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 1 e 037
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 41
bull Slotted Aloha has double the maximumthroughput as pure Aloha and achievesthis maximum at a higher offered loadThe effective data rate is still less than40 of the raw transmission rate
bull Aloha does not listen to the carrier beforetransmission more sophisticatedtechniques are needed to increaseefficiency
bull Further problem the delayndashDelay
ndashThe mean packet delay
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
( ) ( )delayPr 1 e ekL Lkτ minus minus= = minus
( )delay1
1 e e ekL L L
kMPD E kτ
infinminus minus
=
= = minus =sum
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 42
Random Access Carrier Sense Multiple Accessbull CSMA (Carrier Sense Multiple Access) listen to the carrier before transmission
and transmits if channel is idlebull Detection delay and propagation delay are two important parameters
ndash Detection delay time required to sense the carrier and decide if it is idle or busyndash Propagation delay distancespeed_of_ligth The time required for bit to travel
from transmitter to the receiver
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 43
Random Access CSMA variations
Ad hoc Sensor Networks Multiple channel access
1-persistent CSMA Non-persistent CSMA
There will always be a collision iftwo nodes want to retransmit(usually you stop transmissionattempts after few tries)
Random backoff reduces probabilityof collisions Waste idle time if thebackoff time is too long
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 44
Random Access CSMA variations (contrsquo)
Ad hoc Sensor Networks Multiple channel access
p-persistent CSMA CSMA CD (Collision Detection)
A good tradeoff between non-persistent and 1-persistent CSMA
Same with CSMA however a stationalso listen to the carrier whiletransmitting to see if the transmissioncollides with someone elsetransmission It can be used in listen-while-talk capable channels (fullduplex) In single radio channels thetransmission need to be interrupted inorder to sense the channel
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 45
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 46
Random Access CSMA (contrsquo)bull How to select probability p
ndash Assume that M nodes have a packet to send and the medium is busy then Mp is theexpected number of nodes that will attempt to transmit once the medium becomesidle If Mp gt 1 then a collision is expected to occur Therefore the network mustmake sure that Mp lt 1 to avoid collision where M is the maximum number of nodesthat can be active at a time
bull Problems1 The mean packet delay increases exponently with increasing offered load2 CSMA protocols sense the carrier but sensing the carrier does not always releasestrue information about the status of the wireless channel There are two problems thatare unique to wireless channels (different than wireline channels)ndash Hidden terminal problemndash Exposed terminal problem
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 47
Hidden terminal problembull A sends to B C cannot receive Abull C wants to send to B C senses a
ldquofreerdquo medium (CS fails)bull collision at B A cannot receive the
collision (CD fails)bull A is ldquohiddenrdquo for C
Ad hoc Sensor Networks Multiple channel access
Random Access hidden and exposed terminal problemExposed terminal problem
bull B sends to A C wants to send to Dbull C has to wait CS signals a medium
in usebull since A is outside the radio range of
C waiting is not necessarybull C is ldquoexposedrdquo to B
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 48
Random Access MACAWhen a station wants to transmit databull It sends an RTS (Ready-to-Send) packet to the intended receiver
ndash The RTS packet contains the length of the data that needs to be transmittedndash Any station other than the intended recipient hearing RTS defers transmission for atime duration equal to the end of the corresponding CTS reception
bull The receiver sends back CTS (Clear-to-Send) packet back to sender if it isavailable to receivendash The CTS packet contains the length of the data that original sender wants totransmitndash Any station other than the original RTS sender hearing CTS defers transmissionuntil the data is sent
bull The original sender upon reception of the CTS starts transmitting
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 49
Hidden terminal problem
Ad hoc Sensor Networks Multiple channel access
Random Access MACA (contrsquo)Exposed terminal problem
Waiting time of node X is muchsmaller than waiting time of node C
bull C defers transmission upon hearing BrsquosRTS until B could get CTS from Abull After that C can start transmission to DFor that it first sends an RTSbull C is not longer exposed to the datatransmission of B
Problem It does not address the collision of RTS packet
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 50
Random Access reliabilitybull Wireless links are prone to have errors High packet loss rate detrimental to
transport-layer performance Mechanisms needed to reduce packet loss rateexperienced by upper layers
bull When node B receives a data packet from node A node B sends anAcknowledgement (Ack) This approach adopted in many protocols
bull If node A fails to receive an Ack it will retransmit the packet
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 51
Random Access MACAW (CSMACA) bull Physical carrier sense and virtual carrier sense using Network Allocation
Vector (NAV)ndash NAV is updated based on overheard RTSCTSDATAACK packets each of whichspecified duration of a pending transmissionndash Nodes stay silent when carrier sensed (physicalvirtual)
bull Backoff intervals used to reduce collision probabilityndash When transmitting a packet choose a backoff interval in the range [0w] where wis contention windowndash Count down the backoff interval when channel is idlendash Count down is suspended if channel becomes busyndash When backoff interval reaches 0 transmit RTS
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 52
Random Access CSMACA (contrsquo) bull The time spent counting down backoff intervals is a part of MAC overhead
ndash Choosing a large w leads to large backoff intervals and can result in larger overheadndash Choosing a small w leads to a larger number of collisions (when two nodes countdown to 0 simultaneously)ndash Since the number of nodes attempting to transmit simultaneously may change withtime some mechanism to manage contention is needed
bull IEEE 80211 DCF contention window w is chosen dynamically depending oncollision occurrence (binary exponential backoff)ndash When a node fails to receive CTS in response to its RTS it increases the contentionwindowndash w is doubled (up to an upper bound)ndash When a node successfully completes a data transfer it restores w to wmin
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 53
Random Access fairnessbull Simplest definition of fairness all nodes should receive equal bandwidthbull An example of unfairness
ndash Assume that initially A and B both choose a backoff interval in range [031] buttheir RTSs collidendash Nodes A and B then choose from range [063]ndash Node A chooses 4 slots and B choose 60 slotsndash After A transmits a packet it next chooses from range [031]ndash It is possible that A may transmit several packets before B transmits its first packet
Ad hoc Sensor Networks Multiple channel access
Unfairness occurs when one node has backed off much more than some other node
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 54
Random Access fairness (contrsquo) MACAW solution for fairnessbull When a node transmits a packet it appends value w to the packet all nodes
hearing value w use it for their future transmission attemptsbull Since w is an indication of the level of congestion in the vicinity of a specific
receiver node MACAW proposes maintaining w independently for eachreceiver
bull Using per-receiver wi is particularly useful in multi-hop environments sincecongestion level at different receivers can be very different
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 55
Random Access energy efficiency bull Since many mobile hosts are operated by batteries MAC
protocols which conserve energy are of interestbull Two approaches to reduce energy consumption
ndash Power save turn off wireless interface when desirablendash Power control reduce transmit power
bull Power control has some more potential benefitsndash Reduced interference (in ad hoc networks) eg it improves ALOHAefficiency where user with high power can capture a packet even if there is acollisionndash Increased spatial reuse (in infrastructured wireless networks) eg it is usedin CDMA to maintain target SIR of voice and data users or generally it canbe used to maintain target SIR for different user classes (admission control)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 56
Random Access power control bull When C transmits to D at a high power level B cannot receive Arsquos
transmission due to interference from Cbull If C reduces transmit power it can still communicate with D
ndash Reduces energy consumption at node Cndash Allows B to receive Arsquos transmission (spatial reuse)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 57
bull A special wireless ad hoc networkndash Large number of nodesndash Battery poweredndash Topology and density changendash Nodes for a common taskndash In-network data processing
bull Sensor-net applicationsndash Sensor-triggered bursty trafficndash Can often tolerate some delay
Ad hoc Sensor Networks Multiple channel access
Characteristics of Sensor Network
Scalabilty and self configurationEnergy efficiencyAdaptivityFairness not importantLatency
AdaptivityEnergy-delay tradoff
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 58
Ad hoc Sensor Networks Multiple channel access
Wireless sensor node and sensor network
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 59
MAC for WSN attributes bull Attributes
ndash Collision avoidance (Basic task)ndash Energy efficiency (most primary in WSN)ndash Scalability and adaptivityndash Channel utilizationndash Latencyndash Throughputndash Fairness
bull What causes energy wastendash Collisionsndash Control packet overheadndash Overhearing unnecessary trafficndash Long idle time (it consumes 50-100 of the power)
Ad hoc Sensor Networks Multiple channel access
Primary
Secondary
Dominant factor
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 60
MAC for WSN
bull Contention-based protocols need to work hard in all directions for energysavingsndash Reduce idle listening ndash support low duty cyclendash Better collision avoidancendash Reduce control overheadndash Avoid unnecessary overhearing
Ad hoc Sensor Networks Multiple channel access
Scheduled Protocols Contention ProtocolsCollisions No YesEnergy efficiency Good BadScalability and adaptivity Bad GoodMulti-hop communication Difficult EasyTime synchronization Strict Loose or not required
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 61
MAC for WSN classification
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 62
MAC for WSN SMAC (2002)bull It has been developed by Ye Heidemann and Estrinbull Tradeoffs between (latency fairness) and energybull Major components in S-MAC
ndash Periodic listen and sleep turn off radio when sleeping the reduced dutycycle is ~ 10 (eg 120ms on 12s off)ndash Collision avoidance is based on contention similarly to IEEE 80211 adhoc modendash Overhearing avoidance the node is sleeping when neighbors talkndash Massage passing long message is fragmented amp sent in burst operationwith extended Tx time and re-transmit occurs immediately
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 63
Summarybull The algorithms and protocols that enable sharing by multiple users and
controls the access to the wireless channel from different users are calledMAC
bull There are two types of wireless networks Infrastructured wireless networksand ad hoc wireless networks
bull We also want our wireless system to have high user capacity and we canidentify 10 methods for increasing it
bull There are several MACs with no absolute advantage it should be tailoredto the system requirements
bull There are primary concerns for designing a WSN MAC (collisionavoidance energy efficiency scalability and adaptivity) while thesecondary concerns are as follows (channel utilization latency throughputfairness)
bull Next lecture Routing protocols
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2720111015 TAacuteMOP ndash 412-082AKMR-2009-0006 27
Ad hoc Sensor Networks Multiple channel access
Multiple Access FHMAbull The carrier frequency of users are varied in a pseudo-random
fashionndash Each user is using a narrowband channel (spectrum) at a specific
instance of timendash The random change in frequency make the change of using the same
narrowband channel very lowbull The sender receiver change frequency (calling hopping)
using the same pseudo-random sequence hence they aresynchronized
bull Rate of hopping versus Symbol ratendash If hopping rate is greather Fast Frequency Hoppingndash If symbol rate is greater Slow Frequency Hopping
bull GSM and Bluetooth are example systems
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2820111015 TAacuteMOP ndash 412-082AKMR-2009-0006 28
Ad hoc Sensor Networks Multiple channel access
Multiple Access FHMA (contrsquo)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2920111015 TAacuteMOP ndash 412-082AKMR-2009-0006 29
Ad hoc Sensor Networks Multiple channel access
Multiple Access CDMAbull The narrowband message signal is multiplied by a very large bandwidth
signal called spreading signal (code) before modulation and transmissionover the air This is called spreading
bull Spreading signal use a pseudo-noise (PN) sequence (a pseudo-randomsequence) called codeword which are orthogonal (low autocorrelation)
bull The receiver correlator distinguishes the senders signal by examining thewideband signal with the same time-synchronized spreading code(despreading)
bull Advantagesndash Low power spectral densityndash Interference limited operationndash Privacyndash Reduction of multipath affects by using a larger spectrumndash Users can start their transmission at any time (random access possible)ndash Cell capacity is soft and higher than TDMA and FDMAndash No frequency management and no equalizers and quard time needed
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3020111015 TAacuteMOP ndash 412-082AKMR-2009-0006 30
Ad hoc Sensor Networks Multiple channel access
Multiple Access near far problem and power controlbull At a receiver the signals may come from various sources (multiuser) The
strongest signal usually captures the modulator The other signals areconsidered as noise Each source may have different distances to the basestation
bull In CDMA we want a BS to receive CDMA coded signals from variousmobile users at the same time
ndash Therefore the receiver power at the BS for all mobile users should be close toeach other This requires power control at the mobiles
bull Power Control BS monitors the RSSI (Received Signal Strength Indicator)values from different mobiles and then sends power change commands to themobiles over a forward channel The mobiles then adjust their transmitpower
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3120111015 TAacuteMOP ndash 412-082AKMR-2009-0006 31
Ad hoc Sensor Networks Multiple channel access
Multiple Access hybrid spread spectrum techniquesbull FDMACDMA
ndash Available wideband spectrum is divided into a number narrowband radio channels CDMAis employed inside each channel
bull DSFHMAndash The signals are spread using spreading codes (direct sequence signals are obtained) but
these signal are not transmitted over a constant carrier frequency they are transmitted over afrequency hopping carrier frequency
bull Time Division CDMA (TCDMA)ndash Each cell is using a different spreading code (CDMA employed between cells) that is
conveyed to the mobiles in its rangendash Inside each cell (inside a CDMA channel) TDMA is employed to multiplex multiple users
bull Time Division Frequency Hoppingndash At each time slot the user is hopped to a new frequency according to a pseudo-random
hopping sequencendash Employed in severe co-interference and multi-path environmentsndash Bluetooth and GSM are using this technique
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3220111015 TAacuteMOP ndash 412-082AKMR-2009-0006 32
Ad hoc Sensor Networks Multiple channel access
Multiple Access comparison TDMAFDMACDMA
Techniques TDMA FDMA CDMA SDMA
Idea
segment sending time into disjoint time-slots
Segment the frequency band into disjoint sub-bands
Spread the spectrum using orthogonal codes
Segment space into sectors
Signal separation
Synchronization in the time domain
Filtering in the frequency domain
Code Directed antennas
Advantage
Established flexible
Simple established robust
Flexible less frequency planning needed soft handover
Simple increases capacity
DisadvantageQuard space synchronization difficult
Inflexible scarce source
Complex receiver power control
inflexible
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3320111015 TAacuteMOP ndash 412-082AKMR-2009-0006 33
Ad hoc Sensor Networks Multiple channel access
User capacity per cell
The user capacity C is defined as the total number of active users percell that the system can support while meeting a common BERconstrain For orthogonal multiple access (as FDMA and TDMA)
C = Ncellwhere Ncell is the number of channels assigned to any given cellThe total number of orthogonal channels of bandwidth Bs that can becreated from a total system bandwidth of B is B Bs The reuse factorsatisfies N = (B Bs) Ncell this implies
C = (B Bs) N
Recall from Chapter 1
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3420111015 TAacuteMOP ndash 412-082AKMR-2009-0006 34
Ad hoc Sensor Networks Multiple channel access
10 methods for increasing capacity1 Increasing the amount of spectrum used very expensive2 More efficient modulation format and coding3 Better source coding4 Adaptive modulation and coding5 Discontinuous transmission6 Multiuser detection CDMA systems7 Reduction of cell radius effective but very expensive smaller
cells require more handovers8 Use of sector cells tripled cells have tripled BS antennas9 Use of an overlay structure10 Multiple antennas diversity MIMO systems SDMA
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3520111015 TAacuteMOP ndash 412-082AKMR-2009-0006 35
Ad hoc Sensor Networks Multiple channel access
bull Distributed operation is requiredbull Synchronization is required in TDMA-based systemsbull Hidden terminals are nodes hidden from a senderbull Exposed terminals are exposed nodes preventing a sender
from sendingbull Throughput needs to be maximizedbull Access delay should be minimizedbull Fairness refers to provide an equal share to all competing
nodesbull Real-time traffic support is required for voice video and
real-time databull Resource reservation is required for QoSbull Ability to measure resource availability handles the
resourcesbull Capability for power control reduces the energy
consumption
Random access in adhoc networks
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3620111015 TAacuteMOP ndash 412-082AKMR-2009-0006 36
Ad hoc Sensor Networks Multiple channel access
Random accessbull All random access techniques are based on packetized data or packet radio where user
date is collected into packets of M bitsbull Collision if packets from different users overlap in timebull The transmission time of a packet is τ = M R where R [bps] the data ratebull Analysis of random access techniques assumes that the users generate packets according
to memoryless Poisson process at a rate λ [packets per unit time] the probability that thenumber of packet arrivals in a time period [0t] denoted by X(t) is equal to integer k isgiven by
Pr(X(t)=k) = (λt)k k e-λt
bull The traffic load is defined as L = λ τ If L gt 1 means on avarage more packets arrive inthe system over a given time period than can be transmitted in that period so systemswith Lgt1 are unstable
bull The throughput is defined as the ratio of the average number of packets successfullytransmitted in any given time interval divided by the number of attempted transmissionsin that interval
T = L Pr(succesful packet transmission)bull The effective data rate of the system is RT
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3720111015 TAacuteMOP ndash 412-082AKMR-2009-0006 37
Ad hoc Sensor Networks Multiple channel access
Random access (contrsquo)bull We will examine the following attributes
ndash Channel utilization (Throughput)ndash Latencyndash Collision avoidance (Hidden and exposed terminals problem)ndash Reliability (ACK)ndash Energy efficiency (power control)ndash Fairnessndash Throughput needs to be maximized
bull We will discuss the following techniquesndash Aloha and slotted Alohandash CSMA Protocols (1-persistent non-persistent p-persistent CSMA CSMACD)ndash MACAndash MACAWndash Energy efficient MAC in wireless sensor networks (eg SMAC)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 38
Random Access ALOHA (1970)bull ALOHA was developed for a wireless system at the University of Hawaii
(Abramson et al)ndash Multiple remote stations plus one base stationndash Frame transmissions are made at one frequency from a remote station to the basestation the base station re-broadcasts frames on another frequency Contention is foraccess to the base station
bull Frames can be sent at any time to the base stationndash If no acknowledgment (ACK) is received assume that the frame is lost andretransmit after waiting a random amount of timendash If more than one station broadcasts on the base station access frequency at the sametime ndash a collision ndash interference will destroy the frames
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 39
Random Access ALOHA (contrsquo)
bull The probability that no packets generated during the tima [-τ τ] is given by (nocollision)
Pr(X(t)=0) = (λ2τ)0 0 e-2λτ = e-2Lwith corresponding throughput
T = L e-2L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 05 05e 018
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 40
Random Access slotted ALOHA (1971)bull Improvement Time is slotted and a packet can only be transmitted at the beginning of
one slot Thus it can reduce the collision duration
bull The probability that no packets generated during the time [0 τ] is given by (nocollision)
Pr(X(t)=0) = (λτ)0 0 e-λτ = e-Lwith corresponding throughput T = L e-L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 1 e 037
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 41
bull Slotted Aloha has double the maximumthroughput as pure Aloha and achievesthis maximum at a higher offered loadThe effective data rate is still less than40 of the raw transmission rate
bull Aloha does not listen to the carrier beforetransmission more sophisticatedtechniques are needed to increaseefficiency
bull Further problem the delayndashDelay
ndashThe mean packet delay
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
( ) ( )delayPr 1 e ekL Lkτ minus minus= = minus
( )delay1
1 e e ekL L L
kMPD E kτ
infinminus minus
=
= = minus =sum
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 42
Random Access Carrier Sense Multiple Accessbull CSMA (Carrier Sense Multiple Access) listen to the carrier before transmission
and transmits if channel is idlebull Detection delay and propagation delay are two important parameters
ndash Detection delay time required to sense the carrier and decide if it is idle or busyndash Propagation delay distancespeed_of_ligth The time required for bit to travel
from transmitter to the receiver
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 43
Random Access CSMA variations
Ad hoc Sensor Networks Multiple channel access
1-persistent CSMA Non-persistent CSMA
There will always be a collision iftwo nodes want to retransmit(usually you stop transmissionattempts after few tries)
Random backoff reduces probabilityof collisions Waste idle time if thebackoff time is too long
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 44
Random Access CSMA variations (contrsquo)
Ad hoc Sensor Networks Multiple channel access
p-persistent CSMA CSMA CD (Collision Detection)
A good tradeoff between non-persistent and 1-persistent CSMA
Same with CSMA however a stationalso listen to the carrier whiletransmitting to see if the transmissioncollides with someone elsetransmission It can be used in listen-while-talk capable channels (fullduplex) In single radio channels thetransmission need to be interrupted inorder to sense the channel
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 45
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 46
Random Access CSMA (contrsquo)bull How to select probability p
ndash Assume that M nodes have a packet to send and the medium is busy then Mp is theexpected number of nodes that will attempt to transmit once the medium becomesidle If Mp gt 1 then a collision is expected to occur Therefore the network mustmake sure that Mp lt 1 to avoid collision where M is the maximum number of nodesthat can be active at a time
bull Problems1 The mean packet delay increases exponently with increasing offered load2 CSMA protocols sense the carrier but sensing the carrier does not always releasestrue information about the status of the wireless channel There are two problems thatare unique to wireless channels (different than wireline channels)ndash Hidden terminal problemndash Exposed terminal problem
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 47
Hidden terminal problembull A sends to B C cannot receive Abull C wants to send to B C senses a
ldquofreerdquo medium (CS fails)bull collision at B A cannot receive the
collision (CD fails)bull A is ldquohiddenrdquo for C
Ad hoc Sensor Networks Multiple channel access
Random Access hidden and exposed terminal problemExposed terminal problem
bull B sends to A C wants to send to Dbull C has to wait CS signals a medium
in usebull since A is outside the radio range of
C waiting is not necessarybull C is ldquoexposedrdquo to B
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 48
Random Access MACAWhen a station wants to transmit databull It sends an RTS (Ready-to-Send) packet to the intended receiver
ndash The RTS packet contains the length of the data that needs to be transmittedndash Any station other than the intended recipient hearing RTS defers transmission for atime duration equal to the end of the corresponding CTS reception
bull The receiver sends back CTS (Clear-to-Send) packet back to sender if it isavailable to receivendash The CTS packet contains the length of the data that original sender wants totransmitndash Any station other than the original RTS sender hearing CTS defers transmissionuntil the data is sent
bull The original sender upon reception of the CTS starts transmitting
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 49
Hidden terminal problem
Ad hoc Sensor Networks Multiple channel access
Random Access MACA (contrsquo)Exposed terminal problem
Waiting time of node X is muchsmaller than waiting time of node C
bull C defers transmission upon hearing BrsquosRTS until B could get CTS from Abull After that C can start transmission to DFor that it first sends an RTSbull C is not longer exposed to the datatransmission of B
Problem It does not address the collision of RTS packet
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 50
Random Access reliabilitybull Wireless links are prone to have errors High packet loss rate detrimental to
transport-layer performance Mechanisms needed to reduce packet loss rateexperienced by upper layers
bull When node B receives a data packet from node A node B sends anAcknowledgement (Ack) This approach adopted in many protocols
bull If node A fails to receive an Ack it will retransmit the packet
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 51
Random Access MACAW (CSMACA) bull Physical carrier sense and virtual carrier sense using Network Allocation
Vector (NAV)ndash NAV is updated based on overheard RTSCTSDATAACK packets each of whichspecified duration of a pending transmissionndash Nodes stay silent when carrier sensed (physicalvirtual)
bull Backoff intervals used to reduce collision probabilityndash When transmitting a packet choose a backoff interval in the range [0w] where wis contention windowndash Count down the backoff interval when channel is idlendash Count down is suspended if channel becomes busyndash When backoff interval reaches 0 transmit RTS
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 52
Random Access CSMACA (contrsquo) bull The time spent counting down backoff intervals is a part of MAC overhead
ndash Choosing a large w leads to large backoff intervals and can result in larger overheadndash Choosing a small w leads to a larger number of collisions (when two nodes countdown to 0 simultaneously)ndash Since the number of nodes attempting to transmit simultaneously may change withtime some mechanism to manage contention is needed
bull IEEE 80211 DCF contention window w is chosen dynamically depending oncollision occurrence (binary exponential backoff)ndash When a node fails to receive CTS in response to its RTS it increases the contentionwindowndash w is doubled (up to an upper bound)ndash When a node successfully completes a data transfer it restores w to wmin
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 53
Random Access fairnessbull Simplest definition of fairness all nodes should receive equal bandwidthbull An example of unfairness
ndash Assume that initially A and B both choose a backoff interval in range [031] buttheir RTSs collidendash Nodes A and B then choose from range [063]ndash Node A chooses 4 slots and B choose 60 slotsndash After A transmits a packet it next chooses from range [031]ndash It is possible that A may transmit several packets before B transmits its first packet
Ad hoc Sensor Networks Multiple channel access
Unfairness occurs when one node has backed off much more than some other node
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 54
Random Access fairness (contrsquo) MACAW solution for fairnessbull When a node transmits a packet it appends value w to the packet all nodes
hearing value w use it for their future transmission attemptsbull Since w is an indication of the level of congestion in the vicinity of a specific
receiver node MACAW proposes maintaining w independently for eachreceiver
bull Using per-receiver wi is particularly useful in multi-hop environments sincecongestion level at different receivers can be very different
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 55
Random Access energy efficiency bull Since many mobile hosts are operated by batteries MAC
protocols which conserve energy are of interestbull Two approaches to reduce energy consumption
ndash Power save turn off wireless interface when desirablendash Power control reduce transmit power
bull Power control has some more potential benefitsndash Reduced interference (in ad hoc networks) eg it improves ALOHAefficiency where user with high power can capture a packet even if there is acollisionndash Increased spatial reuse (in infrastructured wireless networks) eg it is usedin CDMA to maintain target SIR of voice and data users or generally it canbe used to maintain target SIR for different user classes (admission control)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 56
Random Access power control bull When C transmits to D at a high power level B cannot receive Arsquos
transmission due to interference from Cbull If C reduces transmit power it can still communicate with D
ndash Reduces energy consumption at node Cndash Allows B to receive Arsquos transmission (spatial reuse)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 57
bull A special wireless ad hoc networkndash Large number of nodesndash Battery poweredndash Topology and density changendash Nodes for a common taskndash In-network data processing
bull Sensor-net applicationsndash Sensor-triggered bursty trafficndash Can often tolerate some delay
Ad hoc Sensor Networks Multiple channel access
Characteristics of Sensor Network
Scalabilty and self configurationEnergy efficiencyAdaptivityFairness not importantLatency
AdaptivityEnergy-delay tradoff
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 58
Ad hoc Sensor Networks Multiple channel access
Wireless sensor node and sensor network
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 59
MAC for WSN attributes bull Attributes
ndash Collision avoidance (Basic task)ndash Energy efficiency (most primary in WSN)ndash Scalability and adaptivityndash Channel utilizationndash Latencyndash Throughputndash Fairness
bull What causes energy wastendash Collisionsndash Control packet overheadndash Overhearing unnecessary trafficndash Long idle time (it consumes 50-100 of the power)
Ad hoc Sensor Networks Multiple channel access
Primary
Secondary
Dominant factor
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 60
MAC for WSN
bull Contention-based protocols need to work hard in all directions for energysavingsndash Reduce idle listening ndash support low duty cyclendash Better collision avoidancendash Reduce control overheadndash Avoid unnecessary overhearing
Ad hoc Sensor Networks Multiple channel access
Scheduled Protocols Contention ProtocolsCollisions No YesEnergy efficiency Good BadScalability and adaptivity Bad GoodMulti-hop communication Difficult EasyTime synchronization Strict Loose or not required
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 61
MAC for WSN classification
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 62
MAC for WSN SMAC (2002)bull It has been developed by Ye Heidemann and Estrinbull Tradeoffs between (latency fairness) and energybull Major components in S-MAC
ndash Periodic listen and sleep turn off radio when sleeping the reduced dutycycle is ~ 10 (eg 120ms on 12s off)ndash Collision avoidance is based on contention similarly to IEEE 80211 adhoc modendash Overhearing avoidance the node is sleeping when neighbors talkndash Massage passing long message is fragmented amp sent in burst operationwith extended Tx time and re-transmit occurs immediately
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 63
Summarybull The algorithms and protocols that enable sharing by multiple users and
controls the access to the wireless channel from different users are calledMAC
bull There are two types of wireless networks Infrastructured wireless networksand ad hoc wireless networks
bull We also want our wireless system to have high user capacity and we canidentify 10 methods for increasing it
bull There are several MACs with no absolute advantage it should be tailoredto the system requirements
bull There are primary concerns for designing a WSN MAC (collisionavoidance energy efficiency scalability and adaptivity) while thesecondary concerns are as follows (channel utilization latency throughputfairness)
bull Next lecture Routing protocols
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2820111015 TAacuteMOP ndash 412-082AKMR-2009-0006 28
Ad hoc Sensor Networks Multiple channel access
Multiple Access FHMA (contrsquo)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2920111015 TAacuteMOP ndash 412-082AKMR-2009-0006 29
Ad hoc Sensor Networks Multiple channel access
Multiple Access CDMAbull The narrowband message signal is multiplied by a very large bandwidth
signal called spreading signal (code) before modulation and transmissionover the air This is called spreading
bull Spreading signal use a pseudo-noise (PN) sequence (a pseudo-randomsequence) called codeword which are orthogonal (low autocorrelation)
bull The receiver correlator distinguishes the senders signal by examining thewideband signal with the same time-synchronized spreading code(despreading)
bull Advantagesndash Low power spectral densityndash Interference limited operationndash Privacyndash Reduction of multipath affects by using a larger spectrumndash Users can start their transmission at any time (random access possible)ndash Cell capacity is soft and higher than TDMA and FDMAndash No frequency management and no equalizers and quard time needed
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3020111015 TAacuteMOP ndash 412-082AKMR-2009-0006 30
Ad hoc Sensor Networks Multiple channel access
Multiple Access near far problem and power controlbull At a receiver the signals may come from various sources (multiuser) The
strongest signal usually captures the modulator The other signals areconsidered as noise Each source may have different distances to the basestation
bull In CDMA we want a BS to receive CDMA coded signals from variousmobile users at the same time
ndash Therefore the receiver power at the BS for all mobile users should be close toeach other This requires power control at the mobiles
bull Power Control BS monitors the RSSI (Received Signal Strength Indicator)values from different mobiles and then sends power change commands to themobiles over a forward channel The mobiles then adjust their transmitpower
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3120111015 TAacuteMOP ndash 412-082AKMR-2009-0006 31
Ad hoc Sensor Networks Multiple channel access
Multiple Access hybrid spread spectrum techniquesbull FDMACDMA
ndash Available wideband spectrum is divided into a number narrowband radio channels CDMAis employed inside each channel
bull DSFHMAndash The signals are spread using spreading codes (direct sequence signals are obtained) but
these signal are not transmitted over a constant carrier frequency they are transmitted over afrequency hopping carrier frequency
bull Time Division CDMA (TCDMA)ndash Each cell is using a different spreading code (CDMA employed between cells) that is
conveyed to the mobiles in its rangendash Inside each cell (inside a CDMA channel) TDMA is employed to multiplex multiple users
bull Time Division Frequency Hoppingndash At each time slot the user is hopped to a new frequency according to a pseudo-random
hopping sequencendash Employed in severe co-interference and multi-path environmentsndash Bluetooth and GSM are using this technique
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3220111015 TAacuteMOP ndash 412-082AKMR-2009-0006 32
Ad hoc Sensor Networks Multiple channel access
Multiple Access comparison TDMAFDMACDMA
Techniques TDMA FDMA CDMA SDMA
Idea
segment sending time into disjoint time-slots
Segment the frequency band into disjoint sub-bands
Spread the spectrum using orthogonal codes
Segment space into sectors
Signal separation
Synchronization in the time domain
Filtering in the frequency domain
Code Directed antennas
Advantage
Established flexible
Simple established robust
Flexible less frequency planning needed soft handover
Simple increases capacity
DisadvantageQuard space synchronization difficult
Inflexible scarce source
Complex receiver power control
inflexible
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3320111015 TAacuteMOP ndash 412-082AKMR-2009-0006 33
Ad hoc Sensor Networks Multiple channel access
User capacity per cell
The user capacity C is defined as the total number of active users percell that the system can support while meeting a common BERconstrain For orthogonal multiple access (as FDMA and TDMA)
C = Ncellwhere Ncell is the number of channels assigned to any given cellThe total number of orthogonal channels of bandwidth Bs that can becreated from a total system bandwidth of B is B Bs The reuse factorsatisfies N = (B Bs) Ncell this implies
C = (B Bs) N
Recall from Chapter 1
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3420111015 TAacuteMOP ndash 412-082AKMR-2009-0006 34
Ad hoc Sensor Networks Multiple channel access
10 methods for increasing capacity1 Increasing the amount of spectrum used very expensive2 More efficient modulation format and coding3 Better source coding4 Adaptive modulation and coding5 Discontinuous transmission6 Multiuser detection CDMA systems7 Reduction of cell radius effective but very expensive smaller
cells require more handovers8 Use of sector cells tripled cells have tripled BS antennas9 Use of an overlay structure10 Multiple antennas diversity MIMO systems SDMA
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3520111015 TAacuteMOP ndash 412-082AKMR-2009-0006 35
Ad hoc Sensor Networks Multiple channel access
bull Distributed operation is requiredbull Synchronization is required in TDMA-based systemsbull Hidden terminals are nodes hidden from a senderbull Exposed terminals are exposed nodes preventing a sender
from sendingbull Throughput needs to be maximizedbull Access delay should be minimizedbull Fairness refers to provide an equal share to all competing
nodesbull Real-time traffic support is required for voice video and
real-time databull Resource reservation is required for QoSbull Ability to measure resource availability handles the
resourcesbull Capability for power control reduces the energy
consumption
Random access in adhoc networks
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3620111015 TAacuteMOP ndash 412-082AKMR-2009-0006 36
Ad hoc Sensor Networks Multiple channel access
Random accessbull All random access techniques are based on packetized data or packet radio where user
date is collected into packets of M bitsbull Collision if packets from different users overlap in timebull The transmission time of a packet is τ = M R where R [bps] the data ratebull Analysis of random access techniques assumes that the users generate packets according
to memoryless Poisson process at a rate λ [packets per unit time] the probability that thenumber of packet arrivals in a time period [0t] denoted by X(t) is equal to integer k isgiven by
Pr(X(t)=k) = (λt)k k e-λt
bull The traffic load is defined as L = λ τ If L gt 1 means on avarage more packets arrive inthe system over a given time period than can be transmitted in that period so systemswith Lgt1 are unstable
bull The throughput is defined as the ratio of the average number of packets successfullytransmitted in any given time interval divided by the number of attempted transmissionsin that interval
T = L Pr(succesful packet transmission)bull The effective data rate of the system is RT
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3720111015 TAacuteMOP ndash 412-082AKMR-2009-0006 37
Ad hoc Sensor Networks Multiple channel access
Random access (contrsquo)bull We will examine the following attributes
ndash Channel utilization (Throughput)ndash Latencyndash Collision avoidance (Hidden and exposed terminals problem)ndash Reliability (ACK)ndash Energy efficiency (power control)ndash Fairnessndash Throughput needs to be maximized
bull We will discuss the following techniquesndash Aloha and slotted Alohandash CSMA Protocols (1-persistent non-persistent p-persistent CSMA CSMACD)ndash MACAndash MACAWndash Energy efficient MAC in wireless sensor networks (eg SMAC)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 38
Random Access ALOHA (1970)bull ALOHA was developed for a wireless system at the University of Hawaii
(Abramson et al)ndash Multiple remote stations plus one base stationndash Frame transmissions are made at one frequency from a remote station to the basestation the base station re-broadcasts frames on another frequency Contention is foraccess to the base station
bull Frames can be sent at any time to the base stationndash If no acknowledgment (ACK) is received assume that the frame is lost andretransmit after waiting a random amount of timendash If more than one station broadcasts on the base station access frequency at the sametime ndash a collision ndash interference will destroy the frames
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 39
Random Access ALOHA (contrsquo)
bull The probability that no packets generated during the tima [-τ τ] is given by (nocollision)
Pr(X(t)=0) = (λ2τ)0 0 e-2λτ = e-2Lwith corresponding throughput
T = L e-2L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 05 05e 018
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 40
Random Access slotted ALOHA (1971)bull Improvement Time is slotted and a packet can only be transmitted at the beginning of
one slot Thus it can reduce the collision duration
bull The probability that no packets generated during the time [0 τ] is given by (nocollision)
Pr(X(t)=0) = (λτ)0 0 e-λτ = e-Lwith corresponding throughput T = L e-L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 1 e 037
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 41
bull Slotted Aloha has double the maximumthroughput as pure Aloha and achievesthis maximum at a higher offered loadThe effective data rate is still less than40 of the raw transmission rate
bull Aloha does not listen to the carrier beforetransmission more sophisticatedtechniques are needed to increaseefficiency
bull Further problem the delayndashDelay
ndashThe mean packet delay
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
( ) ( )delayPr 1 e ekL Lkτ minus minus= = minus
( )delay1
1 e e ekL L L
kMPD E kτ
infinminus minus
=
= = minus =sum
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 42
Random Access Carrier Sense Multiple Accessbull CSMA (Carrier Sense Multiple Access) listen to the carrier before transmission
and transmits if channel is idlebull Detection delay and propagation delay are two important parameters
ndash Detection delay time required to sense the carrier and decide if it is idle or busyndash Propagation delay distancespeed_of_ligth The time required for bit to travel
from transmitter to the receiver
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 43
Random Access CSMA variations
Ad hoc Sensor Networks Multiple channel access
1-persistent CSMA Non-persistent CSMA
There will always be a collision iftwo nodes want to retransmit(usually you stop transmissionattempts after few tries)
Random backoff reduces probabilityof collisions Waste idle time if thebackoff time is too long
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 44
Random Access CSMA variations (contrsquo)
Ad hoc Sensor Networks Multiple channel access
p-persistent CSMA CSMA CD (Collision Detection)
A good tradeoff between non-persistent and 1-persistent CSMA
Same with CSMA however a stationalso listen to the carrier whiletransmitting to see if the transmissioncollides with someone elsetransmission It can be used in listen-while-talk capable channels (fullduplex) In single radio channels thetransmission need to be interrupted inorder to sense the channel
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 45
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 46
Random Access CSMA (contrsquo)bull How to select probability p
ndash Assume that M nodes have a packet to send and the medium is busy then Mp is theexpected number of nodes that will attempt to transmit once the medium becomesidle If Mp gt 1 then a collision is expected to occur Therefore the network mustmake sure that Mp lt 1 to avoid collision where M is the maximum number of nodesthat can be active at a time
bull Problems1 The mean packet delay increases exponently with increasing offered load2 CSMA protocols sense the carrier but sensing the carrier does not always releasestrue information about the status of the wireless channel There are two problems thatare unique to wireless channels (different than wireline channels)ndash Hidden terminal problemndash Exposed terminal problem
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 47
Hidden terminal problembull A sends to B C cannot receive Abull C wants to send to B C senses a
ldquofreerdquo medium (CS fails)bull collision at B A cannot receive the
collision (CD fails)bull A is ldquohiddenrdquo for C
Ad hoc Sensor Networks Multiple channel access
Random Access hidden and exposed terminal problemExposed terminal problem
bull B sends to A C wants to send to Dbull C has to wait CS signals a medium
in usebull since A is outside the radio range of
C waiting is not necessarybull C is ldquoexposedrdquo to B
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 48
Random Access MACAWhen a station wants to transmit databull It sends an RTS (Ready-to-Send) packet to the intended receiver
ndash The RTS packet contains the length of the data that needs to be transmittedndash Any station other than the intended recipient hearing RTS defers transmission for atime duration equal to the end of the corresponding CTS reception
bull The receiver sends back CTS (Clear-to-Send) packet back to sender if it isavailable to receivendash The CTS packet contains the length of the data that original sender wants totransmitndash Any station other than the original RTS sender hearing CTS defers transmissionuntil the data is sent
bull The original sender upon reception of the CTS starts transmitting
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 49
Hidden terminal problem
Ad hoc Sensor Networks Multiple channel access
Random Access MACA (contrsquo)Exposed terminal problem
Waiting time of node X is muchsmaller than waiting time of node C
bull C defers transmission upon hearing BrsquosRTS until B could get CTS from Abull After that C can start transmission to DFor that it first sends an RTSbull C is not longer exposed to the datatransmission of B
Problem It does not address the collision of RTS packet
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 50
Random Access reliabilitybull Wireless links are prone to have errors High packet loss rate detrimental to
transport-layer performance Mechanisms needed to reduce packet loss rateexperienced by upper layers
bull When node B receives a data packet from node A node B sends anAcknowledgement (Ack) This approach adopted in many protocols
bull If node A fails to receive an Ack it will retransmit the packet
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 51
Random Access MACAW (CSMACA) bull Physical carrier sense and virtual carrier sense using Network Allocation
Vector (NAV)ndash NAV is updated based on overheard RTSCTSDATAACK packets each of whichspecified duration of a pending transmissionndash Nodes stay silent when carrier sensed (physicalvirtual)
bull Backoff intervals used to reduce collision probabilityndash When transmitting a packet choose a backoff interval in the range [0w] where wis contention windowndash Count down the backoff interval when channel is idlendash Count down is suspended if channel becomes busyndash When backoff interval reaches 0 transmit RTS
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 52
Random Access CSMACA (contrsquo) bull The time spent counting down backoff intervals is a part of MAC overhead
ndash Choosing a large w leads to large backoff intervals and can result in larger overheadndash Choosing a small w leads to a larger number of collisions (when two nodes countdown to 0 simultaneously)ndash Since the number of nodes attempting to transmit simultaneously may change withtime some mechanism to manage contention is needed
bull IEEE 80211 DCF contention window w is chosen dynamically depending oncollision occurrence (binary exponential backoff)ndash When a node fails to receive CTS in response to its RTS it increases the contentionwindowndash w is doubled (up to an upper bound)ndash When a node successfully completes a data transfer it restores w to wmin
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 53
Random Access fairnessbull Simplest definition of fairness all nodes should receive equal bandwidthbull An example of unfairness
ndash Assume that initially A and B both choose a backoff interval in range [031] buttheir RTSs collidendash Nodes A and B then choose from range [063]ndash Node A chooses 4 slots and B choose 60 slotsndash After A transmits a packet it next chooses from range [031]ndash It is possible that A may transmit several packets before B transmits its first packet
Ad hoc Sensor Networks Multiple channel access
Unfairness occurs when one node has backed off much more than some other node
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 54
Random Access fairness (contrsquo) MACAW solution for fairnessbull When a node transmits a packet it appends value w to the packet all nodes
hearing value w use it for their future transmission attemptsbull Since w is an indication of the level of congestion in the vicinity of a specific
receiver node MACAW proposes maintaining w independently for eachreceiver
bull Using per-receiver wi is particularly useful in multi-hop environments sincecongestion level at different receivers can be very different
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 55
Random Access energy efficiency bull Since many mobile hosts are operated by batteries MAC
protocols which conserve energy are of interestbull Two approaches to reduce energy consumption
ndash Power save turn off wireless interface when desirablendash Power control reduce transmit power
bull Power control has some more potential benefitsndash Reduced interference (in ad hoc networks) eg it improves ALOHAefficiency where user with high power can capture a packet even if there is acollisionndash Increased spatial reuse (in infrastructured wireless networks) eg it is usedin CDMA to maintain target SIR of voice and data users or generally it canbe used to maintain target SIR for different user classes (admission control)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 56
Random Access power control bull When C transmits to D at a high power level B cannot receive Arsquos
transmission due to interference from Cbull If C reduces transmit power it can still communicate with D
ndash Reduces energy consumption at node Cndash Allows B to receive Arsquos transmission (spatial reuse)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 57
bull A special wireless ad hoc networkndash Large number of nodesndash Battery poweredndash Topology and density changendash Nodes for a common taskndash In-network data processing
bull Sensor-net applicationsndash Sensor-triggered bursty trafficndash Can often tolerate some delay
Ad hoc Sensor Networks Multiple channel access
Characteristics of Sensor Network
Scalabilty and self configurationEnergy efficiencyAdaptivityFairness not importantLatency
AdaptivityEnergy-delay tradoff
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 58
Ad hoc Sensor Networks Multiple channel access
Wireless sensor node and sensor network
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 59
MAC for WSN attributes bull Attributes
ndash Collision avoidance (Basic task)ndash Energy efficiency (most primary in WSN)ndash Scalability and adaptivityndash Channel utilizationndash Latencyndash Throughputndash Fairness
bull What causes energy wastendash Collisionsndash Control packet overheadndash Overhearing unnecessary trafficndash Long idle time (it consumes 50-100 of the power)
Ad hoc Sensor Networks Multiple channel access
Primary
Secondary
Dominant factor
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 60
MAC for WSN
bull Contention-based protocols need to work hard in all directions for energysavingsndash Reduce idle listening ndash support low duty cyclendash Better collision avoidancendash Reduce control overheadndash Avoid unnecessary overhearing
Ad hoc Sensor Networks Multiple channel access
Scheduled Protocols Contention ProtocolsCollisions No YesEnergy efficiency Good BadScalability and adaptivity Bad GoodMulti-hop communication Difficult EasyTime synchronization Strict Loose or not required
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 61
MAC for WSN classification
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 62
MAC for WSN SMAC (2002)bull It has been developed by Ye Heidemann and Estrinbull Tradeoffs between (latency fairness) and energybull Major components in S-MAC
ndash Periodic listen and sleep turn off radio when sleeping the reduced dutycycle is ~ 10 (eg 120ms on 12s off)ndash Collision avoidance is based on contention similarly to IEEE 80211 adhoc modendash Overhearing avoidance the node is sleeping when neighbors talkndash Massage passing long message is fragmented amp sent in burst operationwith extended Tx time and re-transmit occurs immediately
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 63
Summarybull The algorithms and protocols that enable sharing by multiple users and
controls the access to the wireless channel from different users are calledMAC
bull There are two types of wireless networks Infrastructured wireless networksand ad hoc wireless networks
bull We also want our wireless system to have high user capacity and we canidentify 10 methods for increasing it
bull There are several MACs with no absolute advantage it should be tailoredto the system requirements
bull There are primary concerns for designing a WSN MAC (collisionavoidance energy efficiency scalability and adaptivity) while thesecondary concerns are as follows (channel utilization latency throughputfairness)
bull Next lecture Routing protocols
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 2920111015 TAacuteMOP ndash 412-082AKMR-2009-0006 29
Ad hoc Sensor Networks Multiple channel access
Multiple Access CDMAbull The narrowband message signal is multiplied by a very large bandwidth
signal called spreading signal (code) before modulation and transmissionover the air This is called spreading
bull Spreading signal use a pseudo-noise (PN) sequence (a pseudo-randomsequence) called codeword which are orthogonal (low autocorrelation)
bull The receiver correlator distinguishes the senders signal by examining thewideband signal with the same time-synchronized spreading code(despreading)
bull Advantagesndash Low power spectral densityndash Interference limited operationndash Privacyndash Reduction of multipath affects by using a larger spectrumndash Users can start their transmission at any time (random access possible)ndash Cell capacity is soft and higher than TDMA and FDMAndash No frequency management and no equalizers and quard time needed
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3020111015 TAacuteMOP ndash 412-082AKMR-2009-0006 30
Ad hoc Sensor Networks Multiple channel access
Multiple Access near far problem and power controlbull At a receiver the signals may come from various sources (multiuser) The
strongest signal usually captures the modulator The other signals areconsidered as noise Each source may have different distances to the basestation
bull In CDMA we want a BS to receive CDMA coded signals from variousmobile users at the same time
ndash Therefore the receiver power at the BS for all mobile users should be close toeach other This requires power control at the mobiles
bull Power Control BS monitors the RSSI (Received Signal Strength Indicator)values from different mobiles and then sends power change commands to themobiles over a forward channel The mobiles then adjust their transmitpower
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3120111015 TAacuteMOP ndash 412-082AKMR-2009-0006 31
Ad hoc Sensor Networks Multiple channel access
Multiple Access hybrid spread spectrum techniquesbull FDMACDMA
ndash Available wideband spectrum is divided into a number narrowband radio channels CDMAis employed inside each channel
bull DSFHMAndash The signals are spread using spreading codes (direct sequence signals are obtained) but
these signal are not transmitted over a constant carrier frequency they are transmitted over afrequency hopping carrier frequency
bull Time Division CDMA (TCDMA)ndash Each cell is using a different spreading code (CDMA employed between cells) that is
conveyed to the mobiles in its rangendash Inside each cell (inside a CDMA channel) TDMA is employed to multiplex multiple users
bull Time Division Frequency Hoppingndash At each time slot the user is hopped to a new frequency according to a pseudo-random
hopping sequencendash Employed in severe co-interference and multi-path environmentsndash Bluetooth and GSM are using this technique
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3220111015 TAacuteMOP ndash 412-082AKMR-2009-0006 32
Ad hoc Sensor Networks Multiple channel access
Multiple Access comparison TDMAFDMACDMA
Techniques TDMA FDMA CDMA SDMA
Idea
segment sending time into disjoint time-slots
Segment the frequency band into disjoint sub-bands
Spread the spectrum using orthogonal codes
Segment space into sectors
Signal separation
Synchronization in the time domain
Filtering in the frequency domain
Code Directed antennas
Advantage
Established flexible
Simple established robust
Flexible less frequency planning needed soft handover
Simple increases capacity
DisadvantageQuard space synchronization difficult
Inflexible scarce source
Complex receiver power control
inflexible
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3320111015 TAacuteMOP ndash 412-082AKMR-2009-0006 33
Ad hoc Sensor Networks Multiple channel access
User capacity per cell
The user capacity C is defined as the total number of active users percell that the system can support while meeting a common BERconstrain For orthogonal multiple access (as FDMA and TDMA)
C = Ncellwhere Ncell is the number of channels assigned to any given cellThe total number of orthogonal channels of bandwidth Bs that can becreated from a total system bandwidth of B is B Bs The reuse factorsatisfies N = (B Bs) Ncell this implies
C = (B Bs) N
Recall from Chapter 1
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3420111015 TAacuteMOP ndash 412-082AKMR-2009-0006 34
Ad hoc Sensor Networks Multiple channel access
10 methods for increasing capacity1 Increasing the amount of spectrum used very expensive2 More efficient modulation format and coding3 Better source coding4 Adaptive modulation and coding5 Discontinuous transmission6 Multiuser detection CDMA systems7 Reduction of cell radius effective but very expensive smaller
cells require more handovers8 Use of sector cells tripled cells have tripled BS antennas9 Use of an overlay structure10 Multiple antennas diversity MIMO systems SDMA
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3520111015 TAacuteMOP ndash 412-082AKMR-2009-0006 35
Ad hoc Sensor Networks Multiple channel access
bull Distributed operation is requiredbull Synchronization is required in TDMA-based systemsbull Hidden terminals are nodes hidden from a senderbull Exposed terminals are exposed nodes preventing a sender
from sendingbull Throughput needs to be maximizedbull Access delay should be minimizedbull Fairness refers to provide an equal share to all competing
nodesbull Real-time traffic support is required for voice video and
real-time databull Resource reservation is required for QoSbull Ability to measure resource availability handles the
resourcesbull Capability for power control reduces the energy
consumption
Random access in adhoc networks
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3620111015 TAacuteMOP ndash 412-082AKMR-2009-0006 36
Ad hoc Sensor Networks Multiple channel access
Random accessbull All random access techniques are based on packetized data or packet radio where user
date is collected into packets of M bitsbull Collision if packets from different users overlap in timebull The transmission time of a packet is τ = M R where R [bps] the data ratebull Analysis of random access techniques assumes that the users generate packets according
to memoryless Poisson process at a rate λ [packets per unit time] the probability that thenumber of packet arrivals in a time period [0t] denoted by X(t) is equal to integer k isgiven by
Pr(X(t)=k) = (λt)k k e-λt
bull The traffic load is defined as L = λ τ If L gt 1 means on avarage more packets arrive inthe system over a given time period than can be transmitted in that period so systemswith Lgt1 are unstable
bull The throughput is defined as the ratio of the average number of packets successfullytransmitted in any given time interval divided by the number of attempted transmissionsin that interval
T = L Pr(succesful packet transmission)bull The effective data rate of the system is RT
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3720111015 TAacuteMOP ndash 412-082AKMR-2009-0006 37
Ad hoc Sensor Networks Multiple channel access
Random access (contrsquo)bull We will examine the following attributes
ndash Channel utilization (Throughput)ndash Latencyndash Collision avoidance (Hidden and exposed terminals problem)ndash Reliability (ACK)ndash Energy efficiency (power control)ndash Fairnessndash Throughput needs to be maximized
bull We will discuss the following techniquesndash Aloha and slotted Alohandash CSMA Protocols (1-persistent non-persistent p-persistent CSMA CSMACD)ndash MACAndash MACAWndash Energy efficient MAC in wireless sensor networks (eg SMAC)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 38
Random Access ALOHA (1970)bull ALOHA was developed for a wireless system at the University of Hawaii
(Abramson et al)ndash Multiple remote stations plus one base stationndash Frame transmissions are made at one frequency from a remote station to the basestation the base station re-broadcasts frames on another frequency Contention is foraccess to the base station
bull Frames can be sent at any time to the base stationndash If no acknowledgment (ACK) is received assume that the frame is lost andretransmit after waiting a random amount of timendash If more than one station broadcasts on the base station access frequency at the sametime ndash a collision ndash interference will destroy the frames
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 39
Random Access ALOHA (contrsquo)
bull The probability that no packets generated during the tima [-τ τ] is given by (nocollision)
Pr(X(t)=0) = (λ2τ)0 0 e-2λτ = e-2Lwith corresponding throughput
T = L e-2L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 05 05e 018
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 40
Random Access slotted ALOHA (1971)bull Improvement Time is slotted and a packet can only be transmitted at the beginning of
one slot Thus it can reduce the collision duration
bull The probability that no packets generated during the time [0 τ] is given by (nocollision)
Pr(X(t)=0) = (λτ)0 0 e-λτ = e-Lwith corresponding throughput T = L e-L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 1 e 037
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 41
bull Slotted Aloha has double the maximumthroughput as pure Aloha and achievesthis maximum at a higher offered loadThe effective data rate is still less than40 of the raw transmission rate
bull Aloha does not listen to the carrier beforetransmission more sophisticatedtechniques are needed to increaseefficiency
bull Further problem the delayndashDelay
ndashThe mean packet delay
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
( ) ( )delayPr 1 e ekL Lkτ minus minus= = minus
( )delay1
1 e e ekL L L
kMPD E kτ
infinminus minus
=
= = minus =sum
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 42
Random Access Carrier Sense Multiple Accessbull CSMA (Carrier Sense Multiple Access) listen to the carrier before transmission
and transmits if channel is idlebull Detection delay and propagation delay are two important parameters
ndash Detection delay time required to sense the carrier and decide if it is idle or busyndash Propagation delay distancespeed_of_ligth The time required for bit to travel
from transmitter to the receiver
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 43
Random Access CSMA variations
Ad hoc Sensor Networks Multiple channel access
1-persistent CSMA Non-persistent CSMA
There will always be a collision iftwo nodes want to retransmit(usually you stop transmissionattempts after few tries)
Random backoff reduces probabilityof collisions Waste idle time if thebackoff time is too long
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 44
Random Access CSMA variations (contrsquo)
Ad hoc Sensor Networks Multiple channel access
p-persistent CSMA CSMA CD (Collision Detection)
A good tradeoff between non-persistent and 1-persistent CSMA
Same with CSMA however a stationalso listen to the carrier whiletransmitting to see if the transmissioncollides with someone elsetransmission It can be used in listen-while-talk capable channels (fullduplex) In single radio channels thetransmission need to be interrupted inorder to sense the channel
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 45
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 46
Random Access CSMA (contrsquo)bull How to select probability p
ndash Assume that M nodes have a packet to send and the medium is busy then Mp is theexpected number of nodes that will attempt to transmit once the medium becomesidle If Mp gt 1 then a collision is expected to occur Therefore the network mustmake sure that Mp lt 1 to avoid collision where M is the maximum number of nodesthat can be active at a time
bull Problems1 The mean packet delay increases exponently with increasing offered load2 CSMA protocols sense the carrier but sensing the carrier does not always releasestrue information about the status of the wireless channel There are two problems thatare unique to wireless channels (different than wireline channels)ndash Hidden terminal problemndash Exposed terminal problem
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 47
Hidden terminal problembull A sends to B C cannot receive Abull C wants to send to B C senses a
ldquofreerdquo medium (CS fails)bull collision at B A cannot receive the
collision (CD fails)bull A is ldquohiddenrdquo for C
Ad hoc Sensor Networks Multiple channel access
Random Access hidden and exposed terminal problemExposed terminal problem
bull B sends to A C wants to send to Dbull C has to wait CS signals a medium
in usebull since A is outside the radio range of
C waiting is not necessarybull C is ldquoexposedrdquo to B
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 48
Random Access MACAWhen a station wants to transmit databull It sends an RTS (Ready-to-Send) packet to the intended receiver
ndash The RTS packet contains the length of the data that needs to be transmittedndash Any station other than the intended recipient hearing RTS defers transmission for atime duration equal to the end of the corresponding CTS reception
bull The receiver sends back CTS (Clear-to-Send) packet back to sender if it isavailable to receivendash The CTS packet contains the length of the data that original sender wants totransmitndash Any station other than the original RTS sender hearing CTS defers transmissionuntil the data is sent
bull The original sender upon reception of the CTS starts transmitting
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 49
Hidden terminal problem
Ad hoc Sensor Networks Multiple channel access
Random Access MACA (contrsquo)Exposed terminal problem
Waiting time of node X is muchsmaller than waiting time of node C
bull C defers transmission upon hearing BrsquosRTS until B could get CTS from Abull After that C can start transmission to DFor that it first sends an RTSbull C is not longer exposed to the datatransmission of B
Problem It does not address the collision of RTS packet
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 50
Random Access reliabilitybull Wireless links are prone to have errors High packet loss rate detrimental to
transport-layer performance Mechanisms needed to reduce packet loss rateexperienced by upper layers
bull When node B receives a data packet from node A node B sends anAcknowledgement (Ack) This approach adopted in many protocols
bull If node A fails to receive an Ack it will retransmit the packet
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 51
Random Access MACAW (CSMACA) bull Physical carrier sense and virtual carrier sense using Network Allocation
Vector (NAV)ndash NAV is updated based on overheard RTSCTSDATAACK packets each of whichspecified duration of a pending transmissionndash Nodes stay silent when carrier sensed (physicalvirtual)
bull Backoff intervals used to reduce collision probabilityndash When transmitting a packet choose a backoff interval in the range [0w] where wis contention windowndash Count down the backoff interval when channel is idlendash Count down is suspended if channel becomes busyndash When backoff interval reaches 0 transmit RTS
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 52
Random Access CSMACA (contrsquo) bull The time spent counting down backoff intervals is a part of MAC overhead
ndash Choosing a large w leads to large backoff intervals and can result in larger overheadndash Choosing a small w leads to a larger number of collisions (when two nodes countdown to 0 simultaneously)ndash Since the number of nodes attempting to transmit simultaneously may change withtime some mechanism to manage contention is needed
bull IEEE 80211 DCF contention window w is chosen dynamically depending oncollision occurrence (binary exponential backoff)ndash When a node fails to receive CTS in response to its RTS it increases the contentionwindowndash w is doubled (up to an upper bound)ndash When a node successfully completes a data transfer it restores w to wmin
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 53
Random Access fairnessbull Simplest definition of fairness all nodes should receive equal bandwidthbull An example of unfairness
ndash Assume that initially A and B both choose a backoff interval in range [031] buttheir RTSs collidendash Nodes A and B then choose from range [063]ndash Node A chooses 4 slots and B choose 60 slotsndash After A transmits a packet it next chooses from range [031]ndash It is possible that A may transmit several packets before B transmits its first packet
Ad hoc Sensor Networks Multiple channel access
Unfairness occurs when one node has backed off much more than some other node
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 54
Random Access fairness (contrsquo) MACAW solution for fairnessbull When a node transmits a packet it appends value w to the packet all nodes
hearing value w use it for their future transmission attemptsbull Since w is an indication of the level of congestion in the vicinity of a specific
receiver node MACAW proposes maintaining w independently for eachreceiver
bull Using per-receiver wi is particularly useful in multi-hop environments sincecongestion level at different receivers can be very different
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 55
Random Access energy efficiency bull Since many mobile hosts are operated by batteries MAC
protocols which conserve energy are of interestbull Two approaches to reduce energy consumption
ndash Power save turn off wireless interface when desirablendash Power control reduce transmit power
bull Power control has some more potential benefitsndash Reduced interference (in ad hoc networks) eg it improves ALOHAefficiency where user with high power can capture a packet even if there is acollisionndash Increased spatial reuse (in infrastructured wireless networks) eg it is usedin CDMA to maintain target SIR of voice and data users or generally it canbe used to maintain target SIR for different user classes (admission control)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 56
Random Access power control bull When C transmits to D at a high power level B cannot receive Arsquos
transmission due to interference from Cbull If C reduces transmit power it can still communicate with D
ndash Reduces energy consumption at node Cndash Allows B to receive Arsquos transmission (spatial reuse)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 57
bull A special wireless ad hoc networkndash Large number of nodesndash Battery poweredndash Topology and density changendash Nodes for a common taskndash In-network data processing
bull Sensor-net applicationsndash Sensor-triggered bursty trafficndash Can often tolerate some delay
Ad hoc Sensor Networks Multiple channel access
Characteristics of Sensor Network
Scalabilty and self configurationEnergy efficiencyAdaptivityFairness not importantLatency
AdaptivityEnergy-delay tradoff
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 58
Ad hoc Sensor Networks Multiple channel access
Wireless sensor node and sensor network
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 59
MAC for WSN attributes bull Attributes
ndash Collision avoidance (Basic task)ndash Energy efficiency (most primary in WSN)ndash Scalability and adaptivityndash Channel utilizationndash Latencyndash Throughputndash Fairness
bull What causes energy wastendash Collisionsndash Control packet overheadndash Overhearing unnecessary trafficndash Long idle time (it consumes 50-100 of the power)
Ad hoc Sensor Networks Multiple channel access
Primary
Secondary
Dominant factor
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 60
MAC for WSN
bull Contention-based protocols need to work hard in all directions for energysavingsndash Reduce idle listening ndash support low duty cyclendash Better collision avoidancendash Reduce control overheadndash Avoid unnecessary overhearing
Ad hoc Sensor Networks Multiple channel access
Scheduled Protocols Contention ProtocolsCollisions No YesEnergy efficiency Good BadScalability and adaptivity Bad GoodMulti-hop communication Difficult EasyTime synchronization Strict Loose or not required
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 61
MAC for WSN classification
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 62
MAC for WSN SMAC (2002)bull It has been developed by Ye Heidemann and Estrinbull Tradeoffs between (latency fairness) and energybull Major components in S-MAC
ndash Periodic listen and sleep turn off radio when sleeping the reduced dutycycle is ~ 10 (eg 120ms on 12s off)ndash Collision avoidance is based on contention similarly to IEEE 80211 adhoc modendash Overhearing avoidance the node is sleeping when neighbors talkndash Massage passing long message is fragmented amp sent in burst operationwith extended Tx time and re-transmit occurs immediately
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 63
Summarybull The algorithms and protocols that enable sharing by multiple users and
controls the access to the wireless channel from different users are calledMAC
bull There are two types of wireless networks Infrastructured wireless networksand ad hoc wireless networks
bull We also want our wireless system to have high user capacity and we canidentify 10 methods for increasing it
bull There are several MACs with no absolute advantage it should be tailoredto the system requirements
bull There are primary concerns for designing a WSN MAC (collisionavoidance energy efficiency scalability and adaptivity) while thesecondary concerns are as follows (channel utilization latency throughputfairness)
bull Next lecture Routing protocols
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3020111015 TAacuteMOP ndash 412-082AKMR-2009-0006 30
Ad hoc Sensor Networks Multiple channel access
Multiple Access near far problem and power controlbull At a receiver the signals may come from various sources (multiuser) The
strongest signal usually captures the modulator The other signals areconsidered as noise Each source may have different distances to the basestation
bull In CDMA we want a BS to receive CDMA coded signals from variousmobile users at the same time
ndash Therefore the receiver power at the BS for all mobile users should be close toeach other This requires power control at the mobiles
bull Power Control BS monitors the RSSI (Received Signal Strength Indicator)values from different mobiles and then sends power change commands to themobiles over a forward channel The mobiles then adjust their transmitpower
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3120111015 TAacuteMOP ndash 412-082AKMR-2009-0006 31
Ad hoc Sensor Networks Multiple channel access
Multiple Access hybrid spread spectrum techniquesbull FDMACDMA
ndash Available wideband spectrum is divided into a number narrowband radio channels CDMAis employed inside each channel
bull DSFHMAndash The signals are spread using spreading codes (direct sequence signals are obtained) but
these signal are not transmitted over a constant carrier frequency they are transmitted over afrequency hopping carrier frequency
bull Time Division CDMA (TCDMA)ndash Each cell is using a different spreading code (CDMA employed between cells) that is
conveyed to the mobiles in its rangendash Inside each cell (inside a CDMA channel) TDMA is employed to multiplex multiple users
bull Time Division Frequency Hoppingndash At each time slot the user is hopped to a new frequency according to a pseudo-random
hopping sequencendash Employed in severe co-interference and multi-path environmentsndash Bluetooth and GSM are using this technique
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3220111015 TAacuteMOP ndash 412-082AKMR-2009-0006 32
Ad hoc Sensor Networks Multiple channel access
Multiple Access comparison TDMAFDMACDMA
Techniques TDMA FDMA CDMA SDMA
Idea
segment sending time into disjoint time-slots
Segment the frequency band into disjoint sub-bands
Spread the spectrum using orthogonal codes
Segment space into sectors
Signal separation
Synchronization in the time domain
Filtering in the frequency domain
Code Directed antennas
Advantage
Established flexible
Simple established robust
Flexible less frequency planning needed soft handover
Simple increases capacity
DisadvantageQuard space synchronization difficult
Inflexible scarce source
Complex receiver power control
inflexible
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3320111015 TAacuteMOP ndash 412-082AKMR-2009-0006 33
Ad hoc Sensor Networks Multiple channel access
User capacity per cell
The user capacity C is defined as the total number of active users percell that the system can support while meeting a common BERconstrain For orthogonal multiple access (as FDMA and TDMA)
C = Ncellwhere Ncell is the number of channels assigned to any given cellThe total number of orthogonal channels of bandwidth Bs that can becreated from a total system bandwidth of B is B Bs The reuse factorsatisfies N = (B Bs) Ncell this implies
C = (B Bs) N
Recall from Chapter 1
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3420111015 TAacuteMOP ndash 412-082AKMR-2009-0006 34
Ad hoc Sensor Networks Multiple channel access
10 methods for increasing capacity1 Increasing the amount of spectrum used very expensive2 More efficient modulation format and coding3 Better source coding4 Adaptive modulation and coding5 Discontinuous transmission6 Multiuser detection CDMA systems7 Reduction of cell radius effective but very expensive smaller
cells require more handovers8 Use of sector cells tripled cells have tripled BS antennas9 Use of an overlay structure10 Multiple antennas diversity MIMO systems SDMA
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3520111015 TAacuteMOP ndash 412-082AKMR-2009-0006 35
Ad hoc Sensor Networks Multiple channel access
bull Distributed operation is requiredbull Synchronization is required in TDMA-based systemsbull Hidden terminals are nodes hidden from a senderbull Exposed terminals are exposed nodes preventing a sender
from sendingbull Throughput needs to be maximizedbull Access delay should be minimizedbull Fairness refers to provide an equal share to all competing
nodesbull Real-time traffic support is required for voice video and
real-time databull Resource reservation is required for QoSbull Ability to measure resource availability handles the
resourcesbull Capability for power control reduces the energy
consumption
Random access in adhoc networks
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3620111015 TAacuteMOP ndash 412-082AKMR-2009-0006 36
Ad hoc Sensor Networks Multiple channel access
Random accessbull All random access techniques are based on packetized data or packet radio where user
date is collected into packets of M bitsbull Collision if packets from different users overlap in timebull The transmission time of a packet is τ = M R where R [bps] the data ratebull Analysis of random access techniques assumes that the users generate packets according
to memoryless Poisson process at a rate λ [packets per unit time] the probability that thenumber of packet arrivals in a time period [0t] denoted by X(t) is equal to integer k isgiven by
Pr(X(t)=k) = (λt)k k e-λt
bull The traffic load is defined as L = λ τ If L gt 1 means on avarage more packets arrive inthe system over a given time period than can be transmitted in that period so systemswith Lgt1 are unstable
bull The throughput is defined as the ratio of the average number of packets successfullytransmitted in any given time interval divided by the number of attempted transmissionsin that interval
T = L Pr(succesful packet transmission)bull The effective data rate of the system is RT
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3720111015 TAacuteMOP ndash 412-082AKMR-2009-0006 37
Ad hoc Sensor Networks Multiple channel access
Random access (contrsquo)bull We will examine the following attributes
ndash Channel utilization (Throughput)ndash Latencyndash Collision avoidance (Hidden and exposed terminals problem)ndash Reliability (ACK)ndash Energy efficiency (power control)ndash Fairnessndash Throughput needs to be maximized
bull We will discuss the following techniquesndash Aloha and slotted Alohandash CSMA Protocols (1-persistent non-persistent p-persistent CSMA CSMACD)ndash MACAndash MACAWndash Energy efficient MAC in wireless sensor networks (eg SMAC)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 38
Random Access ALOHA (1970)bull ALOHA was developed for a wireless system at the University of Hawaii
(Abramson et al)ndash Multiple remote stations plus one base stationndash Frame transmissions are made at one frequency from a remote station to the basestation the base station re-broadcasts frames on another frequency Contention is foraccess to the base station
bull Frames can be sent at any time to the base stationndash If no acknowledgment (ACK) is received assume that the frame is lost andretransmit after waiting a random amount of timendash If more than one station broadcasts on the base station access frequency at the sametime ndash a collision ndash interference will destroy the frames
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 39
Random Access ALOHA (contrsquo)
bull The probability that no packets generated during the tima [-τ τ] is given by (nocollision)
Pr(X(t)=0) = (λ2τ)0 0 e-2λτ = e-2Lwith corresponding throughput
T = L e-2L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 05 05e 018
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 40
Random Access slotted ALOHA (1971)bull Improvement Time is slotted and a packet can only be transmitted at the beginning of
one slot Thus it can reduce the collision duration
bull The probability that no packets generated during the time [0 τ] is given by (nocollision)
Pr(X(t)=0) = (λτ)0 0 e-λτ = e-Lwith corresponding throughput T = L e-L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 1 e 037
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 41
bull Slotted Aloha has double the maximumthroughput as pure Aloha and achievesthis maximum at a higher offered loadThe effective data rate is still less than40 of the raw transmission rate
bull Aloha does not listen to the carrier beforetransmission more sophisticatedtechniques are needed to increaseefficiency
bull Further problem the delayndashDelay
ndashThe mean packet delay
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
( ) ( )delayPr 1 e ekL Lkτ minus minus= = minus
( )delay1
1 e e ekL L L
kMPD E kτ
infinminus minus
=
= = minus =sum
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 42
Random Access Carrier Sense Multiple Accessbull CSMA (Carrier Sense Multiple Access) listen to the carrier before transmission
and transmits if channel is idlebull Detection delay and propagation delay are two important parameters
ndash Detection delay time required to sense the carrier and decide if it is idle or busyndash Propagation delay distancespeed_of_ligth The time required for bit to travel
from transmitter to the receiver
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 43
Random Access CSMA variations
Ad hoc Sensor Networks Multiple channel access
1-persistent CSMA Non-persistent CSMA
There will always be a collision iftwo nodes want to retransmit(usually you stop transmissionattempts after few tries)
Random backoff reduces probabilityof collisions Waste idle time if thebackoff time is too long
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 44
Random Access CSMA variations (contrsquo)
Ad hoc Sensor Networks Multiple channel access
p-persistent CSMA CSMA CD (Collision Detection)
A good tradeoff between non-persistent and 1-persistent CSMA
Same with CSMA however a stationalso listen to the carrier whiletransmitting to see if the transmissioncollides with someone elsetransmission It can be used in listen-while-talk capable channels (fullduplex) In single radio channels thetransmission need to be interrupted inorder to sense the channel
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 45
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 46
Random Access CSMA (contrsquo)bull How to select probability p
ndash Assume that M nodes have a packet to send and the medium is busy then Mp is theexpected number of nodes that will attempt to transmit once the medium becomesidle If Mp gt 1 then a collision is expected to occur Therefore the network mustmake sure that Mp lt 1 to avoid collision where M is the maximum number of nodesthat can be active at a time
bull Problems1 The mean packet delay increases exponently with increasing offered load2 CSMA protocols sense the carrier but sensing the carrier does not always releasestrue information about the status of the wireless channel There are two problems thatare unique to wireless channels (different than wireline channels)ndash Hidden terminal problemndash Exposed terminal problem
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 47
Hidden terminal problembull A sends to B C cannot receive Abull C wants to send to B C senses a
ldquofreerdquo medium (CS fails)bull collision at B A cannot receive the
collision (CD fails)bull A is ldquohiddenrdquo for C
Ad hoc Sensor Networks Multiple channel access
Random Access hidden and exposed terminal problemExposed terminal problem
bull B sends to A C wants to send to Dbull C has to wait CS signals a medium
in usebull since A is outside the radio range of
C waiting is not necessarybull C is ldquoexposedrdquo to B
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 48
Random Access MACAWhen a station wants to transmit databull It sends an RTS (Ready-to-Send) packet to the intended receiver
ndash The RTS packet contains the length of the data that needs to be transmittedndash Any station other than the intended recipient hearing RTS defers transmission for atime duration equal to the end of the corresponding CTS reception
bull The receiver sends back CTS (Clear-to-Send) packet back to sender if it isavailable to receivendash The CTS packet contains the length of the data that original sender wants totransmitndash Any station other than the original RTS sender hearing CTS defers transmissionuntil the data is sent
bull The original sender upon reception of the CTS starts transmitting
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 49
Hidden terminal problem
Ad hoc Sensor Networks Multiple channel access
Random Access MACA (contrsquo)Exposed terminal problem
Waiting time of node X is muchsmaller than waiting time of node C
bull C defers transmission upon hearing BrsquosRTS until B could get CTS from Abull After that C can start transmission to DFor that it first sends an RTSbull C is not longer exposed to the datatransmission of B
Problem It does not address the collision of RTS packet
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 50
Random Access reliabilitybull Wireless links are prone to have errors High packet loss rate detrimental to
transport-layer performance Mechanisms needed to reduce packet loss rateexperienced by upper layers
bull When node B receives a data packet from node A node B sends anAcknowledgement (Ack) This approach adopted in many protocols
bull If node A fails to receive an Ack it will retransmit the packet
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 51
Random Access MACAW (CSMACA) bull Physical carrier sense and virtual carrier sense using Network Allocation
Vector (NAV)ndash NAV is updated based on overheard RTSCTSDATAACK packets each of whichspecified duration of a pending transmissionndash Nodes stay silent when carrier sensed (physicalvirtual)
bull Backoff intervals used to reduce collision probabilityndash When transmitting a packet choose a backoff interval in the range [0w] where wis contention windowndash Count down the backoff interval when channel is idlendash Count down is suspended if channel becomes busyndash When backoff interval reaches 0 transmit RTS
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 52
Random Access CSMACA (contrsquo) bull The time spent counting down backoff intervals is a part of MAC overhead
ndash Choosing a large w leads to large backoff intervals and can result in larger overheadndash Choosing a small w leads to a larger number of collisions (when two nodes countdown to 0 simultaneously)ndash Since the number of nodes attempting to transmit simultaneously may change withtime some mechanism to manage contention is needed
bull IEEE 80211 DCF contention window w is chosen dynamically depending oncollision occurrence (binary exponential backoff)ndash When a node fails to receive CTS in response to its RTS it increases the contentionwindowndash w is doubled (up to an upper bound)ndash When a node successfully completes a data transfer it restores w to wmin
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 53
Random Access fairnessbull Simplest definition of fairness all nodes should receive equal bandwidthbull An example of unfairness
ndash Assume that initially A and B both choose a backoff interval in range [031] buttheir RTSs collidendash Nodes A and B then choose from range [063]ndash Node A chooses 4 slots and B choose 60 slotsndash After A transmits a packet it next chooses from range [031]ndash It is possible that A may transmit several packets before B transmits its first packet
Ad hoc Sensor Networks Multiple channel access
Unfairness occurs when one node has backed off much more than some other node
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 54
Random Access fairness (contrsquo) MACAW solution for fairnessbull When a node transmits a packet it appends value w to the packet all nodes
hearing value w use it for their future transmission attemptsbull Since w is an indication of the level of congestion in the vicinity of a specific
receiver node MACAW proposes maintaining w independently for eachreceiver
bull Using per-receiver wi is particularly useful in multi-hop environments sincecongestion level at different receivers can be very different
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 55
Random Access energy efficiency bull Since many mobile hosts are operated by batteries MAC
protocols which conserve energy are of interestbull Two approaches to reduce energy consumption
ndash Power save turn off wireless interface when desirablendash Power control reduce transmit power
bull Power control has some more potential benefitsndash Reduced interference (in ad hoc networks) eg it improves ALOHAefficiency where user with high power can capture a packet even if there is acollisionndash Increased spatial reuse (in infrastructured wireless networks) eg it is usedin CDMA to maintain target SIR of voice and data users or generally it canbe used to maintain target SIR for different user classes (admission control)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 56
Random Access power control bull When C transmits to D at a high power level B cannot receive Arsquos
transmission due to interference from Cbull If C reduces transmit power it can still communicate with D
ndash Reduces energy consumption at node Cndash Allows B to receive Arsquos transmission (spatial reuse)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 57
bull A special wireless ad hoc networkndash Large number of nodesndash Battery poweredndash Topology and density changendash Nodes for a common taskndash In-network data processing
bull Sensor-net applicationsndash Sensor-triggered bursty trafficndash Can often tolerate some delay
Ad hoc Sensor Networks Multiple channel access
Characteristics of Sensor Network
Scalabilty and self configurationEnergy efficiencyAdaptivityFairness not importantLatency
AdaptivityEnergy-delay tradoff
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 58
Ad hoc Sensor Networks Multiple channel access
Wireless sensor node and sensor network
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 59
MAC for WSN attributes bull Attributes
ndash Collision avoidance (Basic task)ndash Energy efficiency (most primary in WSN)ndash Scalability and adaptivityndash Channel utilizationndash Latencyndash Throughputndash Fairness
bull What causes energy wastendash Collisionsndash Control packet overheadndash Overhearing unnecessary trafficndash Long idle time (it consumes 50-100 of the power)
Ad hoc Sensor Networks Multiple channel access
Primary
Secondary
Dominant factor
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 60
MAC for WSN
bull Contention-based protocols need to work hard in all directions for energysavingsndash Reduce idle listening ndash support low duty cyclendash Better collision avoidancendash Reduce control overheadndash Avoid unnecessary overhearing
Ad hoc Sensor Networks Multiple channel access
Scheduled Protocols Contention ProtocolsCollisions No YesEnergy efficiency Good BadScalability and adaptivity Bad GoodMulti-hop communication Difficult EasyTime synchronization Strict Loose or not required
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 61
MAC for WSN classification
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 62
MAC for WSN SMAC (2002)bull It has been developed by Ye Heidemann and Estrinbull Tradeoffs between (latency fairness) and energybull Major components in S-MAC
ndash Periodic listen and sleep turn off radio when sleeping the reduced dutycycle is ~ 10 (eg 120ms on 12s off)ndash Collision avoidance is based on contention similarly to IEEE 80211 adhoc modendash Overhearing avoidance the node is sleeping when neighbors talkndash Massage passing long message is fragmented amp sent in burst operationwith extended Tx time and re-transmit occurs immediately
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 63
Summarybull The algorithms and protocols that enable sharing by multiple users and
controls the access to the wireless channel from different users are calledMAC
bull There are two types of wireless networks Infrastructured wireless networksand ad hoc wireless networks
bull We also want our wireless system to have high user capacity and we canidentify 10 methods for increasing it
bull There are several MACs with no absolute advantage it should be tailoredto the system requirements
bull There are primary concerns for designing a WSN MAC (collisionavoidance energy efficiency scalability and adaptivity) while thesecondary concerns are as follows (channel utilization latency throughputfairness)
bull Next lecture Routing protocols
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3120111015 TAacuteMOP ndash 412-082AKMR-2009-0006 31
Ad hoc Sensor Networks Multiple channel access
Multiple Access hybrid spread spectrum techniquesbull FDMACDMA
ndash Available wideband spectrum is divided into a number narrowband radio channels CDMAis employed inside each channel
bull DSFHMAndash The signals are spread using spreading codes (direct sequence signals are obtained) but
these signal are not transmitted over a constant carrier frequency they are transmitted over afrequency hopping carrier frequency
bull Time Division CDMA (TCDMA)ndash Each cell is using a different spreading code (CDMA employed between cells) that is
conveyed to the mobiles in its rangendash Inside each cell (inside a CDMA channel) TDMA is employed to multiplex multiple users
bull Time Division Frequency Hoppingndash At each time slot the user is hopped to a new frequency according to a pseudo-random
hopping sequencendash Employed in severe co-interference and multi-path environmentsndash Bluetooth and GSM are using this technique
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3220111015 TAacuteMOP ndash 412-082AKMR-2009-0006 32
Ad hoc Sensor Networks Multiple channel access
Multiple Access comparison TDMAFDMACDMA
Techniques TDMA FDMA CDMA SDMA
Idea
segment sending time into disjoint time-slots
Segment the frequency band into disjoint sub-bands
Spread the spectrum using orthogonal codes
Segment space into sectors
Signal separation
Synchronization in the time domain
Filtering in the frequency domain
Code Directed antennas
Advantage
Established flexible
Simple established robust
Flexible less frequency planning needed soft handover
Simple increases capacity
DisadvantageQuard space synchronization difficult
Inflexible scarce source
Complex receiver power control
inflexible
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3320111015 TAacuteMOP ndash 412-082AKMR-2009-0006 33
Ad hoc Sensor Networks Multiple channel access
User capacity per cell
The user capacity C is defined as the total number of active users percell that the system can support while meeting a common BERconstrain For orthogonal multiple access (as FDMA and TDMA)
C = Ncellwhere Ncell is the number of channels assigned to any given cellThe total number of orthogonal channels of bandwidth Bs that can becreated from a total system bandwidth of B is B Bs The reuse factorsatisfies N = (B Bs) Ncell this implies
C = (B Bs) N
Recall from Chapter 1
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3420111015 TAacuteMOP ndash 412-082AKMR-2009-0006 34
Ad hoc Sensor Networks Multiple channel access
10 methods for increasing capacity1 Increasing the amount of spectrum used very expensive2 More efficient modulation format and coding3 Better source coding4 Adaptive modulation and coding5 Discontinuous transmission6 Multiuser detection CDMA systems7 Reduction of cell radius effective but very expensive smaller
cells require more handovers8 Use of sector cells tripled cells have tripled BS antennas9 Use of an overlay structure10 Multiple antennas diversity MIMO systems SDMA
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3520111015 TAacuteMOP ndash 412-082AKMR-2009-0006 35
Ad hoc Sensor Networks Multiple channel access
bull Distributed operation is requiredbull Synchronization is required in TDMA-based systemsbull Hidden terminals are nodes hidden from a senderbull Exposed terminals are exposed nodes preventing a sender
from sendingbull Throughput needs to be maximizedbull Access delay should be minimizedbull Fairness refers to provide an equal share to all competing
nodesbull Real-time traffic support is required for voice video and
real-time databull Resource reservation is required for QoSbull Ability to measure resource availability handles the
resourcesbull Capability for power control reduces the energy
consumption
Random access in adhoc networks
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3620111015 TAacuteMOP ndash 412-082AKMR-2009-0006 36
Ad hoc Sensor Networks Multiple channel access
Random accessbull All random access techniques are based on packetized data or packet radio where user
date is collected into packets of M bitsbull Collision if packets from different users overlap in timebull The transmission time of a packet is τ = M R where R [bps] the data ratebull Analysis of random access techniques assumes that the users generate packets according
to memoryless Poisson process at a rate λ [packets per unit time] the probability that thenumber of packet arrivals in a time period [0t] denoted by X(t) is equal to integer k isgiven by
Pr(X(t)=k) = (λt)k k e-λt
bull The traffic load is defined as L = λ τ If L gt 1 means on avarage more packets arrive inthe system over a given time period than can be transmitted in that period so systemswith Lgt1 are unstable
bull The throughput is defined as the ratio of the average number of packets successfullytransmitted in any given time interval divided by the number of attempted transmissionsin that interval
T = L Pr(succesful packet transmission)bull The effective data rate of the system is RT
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3720111015 TAacuteMOP ndash 412-082AKMR-2009-0006 37
Ad hoc Sensor Networks Multiple channel access
Random access (contrsquo)bull We will examine the following attributes
ndash Channel utilization (Throughput)ndash Latencyndash Collision avoidance (Hidden and exposed terminals problem)ndash Reliability (ACK)ndash Energy efficiency (power control)ndash Fairnessndash Throughput needs to be maximized
bull We will discuss the following techniquesndash Aloha and slotted Alohandash CSMA Protocols (1-persistent non-persistent p-persistent CSMA CSMACD)ndash MACAndash MACAWndash Energy efficient MAC in wireless sensor networks (eg SMAC)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 38
Random Access ALOHA (1970)bull ALOHA was developed for a wireless system at the University of Hawaii
(Abramson et al)ndash Multiple remote stations plus one base stationndash Frame transmissions are made at one frequency from a remote station to the basestation the base station re-broadcasts frames on another frequency Contention is foraccess to the base station
bull Frames can be sent at any time to the base stationndash If no acknowledgment (ACK) is received assume that the frame is lost andretransmit after waiting a random amount of timendash If more than one station broadcasts on the base station access frequency at the sametime ndash a collision ndash interference will destroy the frames
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 39
Random Access ALOHA (contrsquo)
bull The probability that no packets generated during the tima [-τ τ] is given by (nocollision)
Pr(X(t)=0) = (λ2τ)0 0 e-2λτ = e-2Lwith corresponding throughput
T = L e-2L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 05 05e 018
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 40
Random Access slotted ALOHA (1971)bull Improvement Time is slotted and a packet can only be transmitted at the beginning of
one slot Thus it can reduce the collision duration
bull The probability that no packets generated during the time [0 τ] is given by (nocollision)
Pr(X(t)=0) = (λτ)0 0 e-λτ = e-Lwith corresponding throughput T = L e-L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 1 e 037
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 41
bull Slotted Aloha has double the maximumthroughput as pure Aloha and achievesthis maximum at a higher offered loadThe effective data rate is still less than40 of the raw transmission rate
bull Aloha does not listen to the carrier beforetransmission more sophisticatedtechniques are needed to increaseefficiency
bull Further problem the delayndashDelay
ndashThe mean packet delay
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
( ) ( )delayPr 1 e ekL Lkτ minus minus= = minus
( )delay1
1 e e ekL L L
kMPD E kτ
infinminus minus
=
= = minus =sum
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 42
Random Access Carrier Sense Multiple Accessbull CSMA (Carrier Sense Multiple Access) listen to the carrier before transmission
and transmits if channel is idlebull Detection delay and propagation delay are two important parameters
ndash Detection delay time required to sense the carrier and decide if it is idle or busyndash Propagation delay distancespeed_of_ligth The time required for bit to travel
from transmitter to the receiver
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 43
Random Access CSMA variations
Ad hoc Sensor Networks Multiple channel access
1-persistent CSMA Non-persistent CSMA
There will always be a collision iftwo nodes want to retransmit(usually you stop transmissionattempts after few tries)
Random backoff reduces probabilityof collisions Waste idle time if thebackoff time is too long
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 44
Random Access CSMA variations (contrsquo)
Ad hoc Sensor Networks Multiple channel access
p-persistent CSMA CSMA CD (Collision Detection)
A good tradeoff between non-persistent and 1-persistent CSMA
Same with CSMA however a stationalso listen to the carrier whiletransmitting to see if the transmissioncollides with someone elsetransmission It can be used in listen-while-talk capable channels (fullduplex) In single radio channels thetransmission need to be interrupted inorder to sense the channel
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 45
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 46
Random Access CSMA (contrsquo)bull How to select probability p
ndash Assume that M nodes have a packet to send and the medium is busy then Mp is theexpected number of nodes that will attempt to transmit once the medium becomesidle If Mp gt 1 then a collision is expected to occur Therefore the network mustmake sure that Mp lt 1 to avoid collision where M is the maximum number of nodesthat can be active at a time
bull Problems1 The mean packet delay increases exponently with increasing offered load2 CSMA protocols sense the carrier but sensing the carrier does not always releasestrue information about the status of the wireless channel There are two problems thatare unique to wireless channels (different than wireline channels)ndash Hidden terminal problemndash Exposed terminal problem
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 47
Hidden terminal problembull A sends to B C cannot receive Abull C wants to send to B C senses a
ldquofreerdquo medium (CS fails)bull collision at B A cannot receive the
collision (CD fails)bull A is ldquohiddenrdquo for C
Ad hoc Sensor Networks Multiple channel access
Random Access hidden and exposed terminal problemExposed terminal problem
bull B sends to A C wants to send to Dbull C has to wait CS signals a medium
in usebull since A is outside the radio range of
C waiting is not necessarybull C is ldquoexposedrdquo to B
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 48
Random Access MACAWhen a station wants to transmit databull It sends an RTS (Ready-to-Send) packet to the intended receiver
ndash The RTS packet contains the length of the data that needs to be transmittedndash Any station other than the intended recipient hearing RTS defers transmission for atime duration equal to the end of the corresponding CTS reception
bull The receiver sends back CTS (Clear-to-Send) packet back to sender if it isavailable to receivendash The CTS packet contains the length of the data that original sender wants totransmitndash Any station other than the original RTS sender hearing CTS defers transmissionuntil the data is sent
bull The original sender upon reception of the CTS starts transmitting
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 49
Hidden terminal problem
Ad hoc Sensor Networks Multiple channel access
Random Access MACA (contrsquo)Exposed terminal problem
Waiting time of node X is muchsmaller than waiting time of node C
bull C defers transmission upon hearing BrsquosRTS until B could get CTS from Abull After that C can start transmission to DFor that it first sends an RTSbull C is not longer exposed to the datatransmission of B
Problem It does not address the collision of RTS packet
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 50
Random Access reliabilitybull Wireless links are prone to have errors High packet loss rate detrimental to
transport-layer performance Mechanisms needed to reduce packet loss rateexperienced by upper layers
bull When node B receives a data packet from node A node B sends anAcknowledgement (Ack) This approach adopted in many protocols
bull If node A fails to receive an Ack it will retransmit the packet
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 51
Random Access MACAW (CSMACA) bull Physical carrier sense and virtual carrier sense using Network Allocation
Vector (NAV)ndash NAV is updated based on overheard RTSCTSDATAACK packets each of whichspecified duration of a pending transmissionndash Nodes stay silent when carrier sensed (physicalvirtual)
bull Backoff intervals used to reduce collision probabilityndash When transmitting a packet choose a backoff interval in the range [0w] where wis contention windowndash Count down the backoff interval when channel is idlendash Count down is suspended if channel becomes busyndash When backoff interval reaches 0 transmit RTS
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 52
Random Access CSMACA (contrsquo) bull The time spent counting down backoff intervals is a part of MAC overhead
ndash Choosing a large w leads to large backoff intervals and can result in larger overheadndash Choosing a small w leads to a larger number of collisions (when two nodes countdown to 0 simultaneously)ndash Since the number of nodes attempting to transmit simultaneously may change withtime some mechanism to manage contention is needed
bull IEEE 80211 DCF contention window w is chosen dynamically depending oncollision occurrence (binary exponential backoff)ndash When a node fails to receive CTS in response to its RTS it increases the contentionwindowndash w is doubled (up to an upper bound)ndash When a node successfully completes a data transfer it restores w to wmin
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 53
Random Access fairnessbull Simplest definition of fairness all nodes should receive equal bandwidthbull An example of unfairness
ndash Assume that initially A and B both choose a backoff interval in range [031] buttheir RTSs collidendash Nodes A and B then choose from range [063]ndash Node A chooses 4 slots and B choose 60 slotsndash After A transmits a packet it next chooses from range [031]ndash It is possible that A may transmit several packets before B transmits its first packet
Ad hoc Sensor Networks Multiple channel access
Unfairness occurs when one node has backed off much more than some other node
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 54
Random Access fairness (contrsquo) MACAW solution for fairnessbull When a node transmits a packet it appends value w to the packet all nodes
hearing value w use it for their future transmission attemptsbull Since w is an indication of the level of congestion in the vicinity of a specific
receiver node MACAW proposes maintaining w independently for eachreceiver
bull Using per-receiver wi is particularly useful in multi-hop environments sincecongestion level at different receivers can be very different
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 55
Random Access energy efficiency bull Since many mobile hosts are operated by batteries MAC
protocols which conserve energy are of interestbull Two approaches to reduce energy consumption
ndash Power save turn off wireless interface when desirablendash Power control reduce transmit power
bull Power control has some more potential benefitsndash Reduced interference (in ad hoc networks) eg it improves ALOHAefficiency where user with high power can capture a packet even if there is acollisionndash Increased spatial reuse (in infrastructured wireless networks) eg it is usedin CDMA to maintain target SIR of voice and data users or generally it canbe used to maintain target SIR for different user classes (admission control)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 56
Random Access power control bull When C transmits to D at a high power level B cannot receive Arsquos
transmission due to interference from Cbull If C reduces transmit power it can still communicate with D
ndash Reduces energy consumption at node Cndash Allows B to receive Arsquos transmission (spatial reuse)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 57
bull A special wireless ad hoc networkndash Large number of nodesndash Battery poweredndash Topology and density changendash Nodes for a common taskndash In-network data processing
bull Sensor-net applicationsndash Sensor-triggered bursty trafficndash Can often tolerate some delay
Ad hoc Sensor Networks Multiple channel access
Characteristics of Sensor Network
Scalabilty and self configurationEnergy efficiencyAdaptivityFairness not importantLatency
AdaptivityEnergy-delay tradoff
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 58
Ad hoc Sensor Networks Multiple channel access
Wireless sensor node and sensor network
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 59
MAC for WSN attributes bull Attributes
ndash Collision avoidance (Basic task)ndash Energy efficiency (most primary in WSN)ndash Scalability and adaptivityndash Channel utilizationndash Latencyndash Throughputndash Fairness
bull What causes energy wastendash Collisionsndash Control packet overheadndash Overhearing unnecessary trafficndash Long idle time (it consumes 50-100 of the power)
Ad hoc Sensor Networks Multiple channel access
Primary
Secondary
Dominant factor
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 60
MAC for WSN
bull Contention-based protocols need to work hard in all directions for energysavingsndash Reduce idle listening ndash support low duty cyclendash Better collision avoidancendash Reduce control overheadndash Avoid unnecessary overhearing
Ad hoc Sensor Networks Multiple channel access
Scheduled Protocols Contention ProtocolsCollisions No YesEnergy efficiency Good BadScalability and adaptivity Bad GoodMulti-hop communication Difficult EasyTime synchronization Strict Loose or not required
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 61
MAC for WSN classification
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 62
MAC for WSN SMAC (2002)bull It has been developed by Ye Heidemann and Estrinbull Tradeoffs between (latency fairness) and energybull Major components in S-MAC
ndash Periodic listen and sleep turn off radio when sleeping the reduced dutycycle is ~ 10 (eg 120ms on 12s off)ndash Collision avoidance is based on contention similarly to IEEE 80211 adhoc modendash Overhearing avoidance the node is sleeping when neighbors talkndash Massage passing long message is fragmented amp sent in burst operationwith extended Tx time and re-transmit occurs immediately
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 63
Summarybull The algorithms and protocols that enable sharing by multiple users and
controls the access to the wireless channel from different users are calledMAC
bull There are two types of wireless networks Infrastructured wireless networksand ad hoc wireless networks
bull We also want our wireless system to have high user capacity and we canidentify 10 methods for increasing it
bull There are several MACs with no absolute advantage it should be tailoredto the system requirements
bull There are primary concerns for designing a WSN MAC (collisionavoidance energy efficiency scalability and adaptivity) while thesecondary concerns are as follows (channel utilization latency throughputfairness)
bull Next lecture Routing protocols
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3220111015 TAacuteMOP ndash 412-082AKMR-2009-0006 32
Ad hoc Sensor Networks Multiple channel access
Multiple Access comparison TDMAFDMACDMA
Techniques TDMA FDMA CDMA SDMA
Idea
segment sending time into disjoint time-slots
Segment the frequency band into disjoint sub-bands
Spread the spectrum using orthogonal codes
Segment space into sectors
Signal separation
Synchronization in the time domain
Filtering in the frequency domain
Code Directed antennas
Advantage
Established flexible
Simple established robust
Flexible less frequency planning needed soft handover
Simple increases capacity
DisadvantageQuard space synchronization difficult
Inflexible scarce source
Complex receiver power control
inflexible
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3320111015 TAacuteMOP ndash 412-082AKMR-2009-0006 33
Ad hoc Sensor Networks Multiple channel access
User capacity per cell
The user capacity C is defined as the total number of active users percell that the system can support while meeting a common BERconstrain For orthogonal multiple access (as FDMA and TDMA)
C = Ncellwhere Ncell is the number of channels assigned to any given cellThe total number of orthogonal channels of bandwidth Bs that can becreated from a total system bandwidth of B is B Bs The reuse factorsatisfies N = (B Bs) Ncell this implies
C = (B Bs) N
Recall from Chapter 1
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3420111015 TAacuteMOP ndash 412-082AKMR-2009-0006 34
Ad hoc Sensor Networks Multiple channel access
10 methods for increasing capacity1 Increasing the amount of spectrum used very expensive2 More efficient modulation format and coding3 Better source coding4 Adaptive modulation and coding5 Discontinuous transmission6 Multiuser detection CDMA systems7 Reduction of cell radius effective but very expensive smaller
cells require more handovers8 Use of sector cells tripled cells have tripled BS antennas9 Use of an overlay structure10 Multiple antennas diversity MIMO systems SDMA
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3520111015 TAacuteMOP ndash 412-082AKMR-2009-0006 35
Ad hoc Sensor Networks Multiple channel access
bull Distributed operation is requiredbull Synchronization is required in TDMA-based systemsbull Hidden terminals are nodes hidden from a senderbull Exposed terminals are exposed nodes preventing a sender
from sendingbull Throughput needs to be maximizedbull Access delay should be minimizedbull Fairness refers to provide an equal share to all competing
nodesbull Real-time traffic support is required for voice video and
real-time databull Resource reservation is required for QoSbull Ability to measure resource availability handles the
resourcesbull Capability for power control reduces the energy
consumption
Random access in adhoc networks
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3620111015 TAacuteMOP ndash 412-082AKMR-2009-0006 36
Ad hoc Sensor Networks Multiple channel access
Random accessbull All random access techniques are based on packetized data or packet radio where user
date is collected into packets of M bitsbull Collision if packets from different users overlap in timebull The transmission time of a packet is τ = M R where R [bps] the data ratebull Analysis of random access techniques assumes that the users generate packets according
to memoryless Poisson process at a rate λ [packets per unit time] the probability that thenumber of packet arrivals in a time period [0t] denoted by X(t) is equal to integer k isgiven by
Pr(X(t)=k) = (λt)k k e-λt
bull The traffic load is defined as L = λ τ If L gt 1 means on avarage more packets arrive inthe system over a given time period than can be transmitted in that period so systemswith Lgt1 are unstable
bull The throughput is defined as the ratio of the average number of packets successfullytransmitted in any given time interval divided by the number of attempted transmissionsin that interval
T = L Pr(succesful packet transmission)bull The effective data rate of the system is RT
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3720111015 TAacuteMOP ndash 412-082AKMR-2009-0006 37
Ad hoc Sensor Networks Multiple channel access
Random access (contrsquo)bull We will examine the following attributes
ndash Channel utilization (Throughput)ndash Latencyndash Collision avoidance (Hidden and exposed terminals problem)ndash Reliability (ACK)ndash Energy efficiency (power control)ndash Fairnessndash Throughput needs to be maximized
bull We will discuss the following techniquesndash Aloha and slotted Alohandash CSMA Protocols (1-persistent non-persistent p-persistent CSMA CSMACD)ndash MACAndash MACAWndash Energy efficient MAC in wireless sensor networks (eg SMAC)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 38
Random Access ALOHA (1970)bull ALOHA was developed for a wireless system at the University of Hawaii
(Abramson et al)ndash Multiple remote stations plus one base stationndash Frame transmissions are made at one frequency from a remote station to the basestation the base station re-broadcasts frames on another frequency Contention is foraccess to the base station
bull Frames can be sent at any time to the base stationndash If no acknowledgment (ACK) is received assume that the frame is lost andretransmit after waiting a random amount of timendash If more than one station broadcasts on the base station access frequency at the sametime ndash a collision ndash interference will destroy the frames
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 39
Random Access ALOHA (contrsquo)
bull The probability that no packets generated during the tima [-τ τ] is given by (nocollision)
Pr(X(t)=0) = (λ2τ)0 0 e-2λτ = e-2Lwith corresponding throughput
T = L e-2L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 05 05e 018
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 40
Random Access slotted ALOHA (1971)bull Improvement Time is slotted and a packet can only be transmitted at the beginning of
one slot Thus it can reduce the collision duration
bull The probability that no packets generated during the time [0 τ] is given by (nocollision)
Pr(X(t)=0) = (λτ)0 0 e-λτ = e-Lwith corresponding throughput T = L e-L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 1 e 037
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 41
bull Slotted Aloha has double the maximumthroughput as pure Aloha and achievesthis maximum at a higher offered loadThe effective data rate is still less than40 of the raw transmission rate
bull Aloha does not listen to the carrier beforetransmission more sophisticatedtechniques are needed to increaseefficiency
bull Further problem the delayndashDelay
ndashThe mean packet delay
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
( ) ( )delayPr 1 e ekL Lkτ minus minus= = minus
( )delay1
1 e e ekL L L
kMPD E kτ
infinminus minus
=
= = minus =sum
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 42
Random Access Carrier Sense Multiple Accessbull CSMA (Carrier Sense Multiple Access) listen to the carrier before transmission
and transmits if channel is idlebull Detection delay and propagation delay are two important parameters
ndash Detection delay time required to sense the carrier and decide if it is idle or busyndash Propagation delay distancespeed_of_ligth The time required for bit to travel
from transmitter to the receiver
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 43
Random Access CSMA variations
Ad hoc Sensor Networks Multiple channel access
1-persistent CSMA Non-persistent CSMA
There will always be a collision iftwo nodes want to retransmit(usually you stop transmissionattempts after few tries)
Random backoff reduces probabilityof collisions Waste idle time if thebackoff time is too long
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 44
Random Access CSMA variations (contrsquo)
Ad hoc Sensor Networks Multiple channel access
p-persistent CSMA CSMA CD (Collision Detection)
A good tradeoff between non-persistent and 1-persistent CSMA
Same with CSMA however a stationalso listen to the carrier whiletransmitting to see if the transmissioncollides with someone elsetransmission It can be used in listen-while-talk capable channels (fullduplex) In single radio channels thetransmission need to be interrupted inorder to sense the channel
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 45
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 46
Random Access CSMA (contrsquo)bull How to select probability p
ndash Assume that M nodes have a packet to send and the medium is busy then Mp is theexpected number of nodes that will attempt to transmit once the medium becomesidle If Mp gt 1 then a collision is expected to occur Therefore the network mustmake sure that Mp lt 1 to avoid collision where M is the maximum number of nodesthat can be active at a time
bull Problems1 The mean packet delay increases exponently with increasing offered load2 CSMA protocols sense the carrier but sensing the carrier does not always releasestrue information about the status of the wireless channel There are two problems thatare unique to wireless channels (different than wireline channels)ndash Hidden terminal problemndash Exposed terminal problem
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 47
Hidden terminal problembull A sends to B C cannot receive Abull C wants to send to B C senses a
ldquofreerdquo medium (CS fails)bull collision at B A cannot receive the
collision (CD fails)bull A is ldquohiddenrdquo for C
Ad hoc Sensor Networks Multiple channel access
Random Access hidden and exposed terminal problemExposed terminal problem
bull B sends to A C wants to send to Dbull C has to wait CS signals a medium
in usebull since A is outside the radio range of
C waiting is not necessarybull C is ldquoexposedrdquo to B
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 48
Random Access MACAWhen a station wants to transmit databull It sends an RTS (Ready-to-Send) packet to the intended receiver
ndash The RTS packet contains the length of the data that needs to be transmittedndash Any station other than the intended recipient hearing RTS defers transmission for atime duration equal to the end of the corresponding CTS reception
bull The receiver sends back CTS (Clear-to-Send) packet back to sender if it isavailable to receivendash The CTS packet contains the length of the data that original sender wants totransmitndash Any station other than the original RTS sender hearing CTS defers transmissionuntil the data is sent
bull The original sender upon reception of the CTS starts transmitting
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 49
Hidden terminal problem
Ad hoc Sensor Networks Multiple channel access
Random Access MACA (contrsquo)Exposed terminal problem
Waiting time of node X is muchsmaller than waiting time of node C
bull C defers transmission upon hearing BrsquosRTS until B could get CTS from Abull After that C can start transmission to DFor that it first sends an RTSbull C is not longer exposed to the datatransmission of B
Problem It does not address the collision of RTS packet
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 50
Random Access reliabilitybull Wireless links are prone to have errors High packet loss rate detrimental to
transport-layer performance Mechanisms needed to reduce packet loss rateexperienced by upper layers
bull When node B receives a data packet from node A node B sends anAcknowledgement (Ack) This approach adopted in many protocols
bull If node A fails to receive an Ack it will retransmit the packet
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 51
Random Access MACAW (CSMACA) bull Physical carrier sense and virtual carrier sense using Network Allocation
Vector (NAV)ndash NAV is updated based on overheard RTSCTSDATAACK packets each of whichspecified duration of a pending transmissionndash Nodes stay silent when carrier sensed (physicalvirtual)
bull Backoff intervals used to reduce collision probabilityndash When transmitting a packet choose a backoff interval in the range [0w] where wis contention windowndash Count down the backoff interval when channel is idlendash Count down is suspended if channel becomes busyndash When backoff interval reaches 0 transmit RTS
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 52
Random Access CSMACA (contrsquo) bull The time spent counting down backoff intervals is a part of MAC overhead
ndash Choosing a large w leads to large backoff intervals and can result in larger overheadndash Choosing a small w leads to a larger number of collisions (when two nodes countdown to 0 simultaneously)ndash Since the number of nodes attempting to transmit simultaneously may change withtime some mechanism to manage contention is needed
bull IEEE 80211 DCF contention window w is chosen dynamically depending oncollision occurrence (binary exponential backoff)ndash When a node fails to receive CTS in response to its RTS it increases the contentionwindowndash w is doubled (up to an upper bound)ndash When a node successfully completes a data transfer it restores w to wmin
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 53
Random Access fairnessbull Simplest definition of fairness all nodes should receive equal bandwidthbull An example of unfairness
ndash Assume that initially A and B both choose a backoff interval in range [031] buttheir RTSs collidendash Nodes A and B then choose from range [063]ndash Node A chooses 4 slots and B choose 60 slotsndash After A transmits a packet it next chooses from range [031]ndash It is possible that A may transmit several packets before B transmits its first packet
Ad hoc Sensor Networks Multiple channel access
Unfairness occurs when one node has backed off much more than some other node
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 54
Random Access fairness (contrsquo) MACAW solution for fairnessbull When a node transmits a packet it appends value w to the packet all nodes
hearing value w use it for their future transmission attemptsbull Since w is an indication of the level of congestion in the vicinity of a specific
receiver node MACAW proposes maintaining w independently for eachreceiver
bull Using per-receiver wi is particularly useful in multi-hop environments sincecongestion level at different receivers can be very different
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 55
Random Access energy efficiency bull Since many mobile hosts are operated by batteries MAC
protocols which conserve energy are of interestbull Two approaches to reduce energy consumption
ndash Power save turn off wireless interface when desirablendash Power control reduce transmit power
bull Power control has some more potential benefitsndash Reduced interference (in ad hoc networks) eg it improves ALOHAefficiency where user with high power can capture a packet even if there is acollisionndash Increased spatial reuse (in infrastructured wireless networks) eg it is usedin CDMA to maintain target SIR of voice and data users or generally it canbe used to maintain target SIR for different user classes (admission control)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 56
Random Access power control bull When C transmits to D at a high power level B cannot receive Arsquos
transmission due to interference from Cbull If C reduces transmit power it can still communicate with D
ndash Reduces energy consumption at node Cndash Allows B to receive Arsquos transmission (spatial reuse)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 57
bull A special wireless ad hoc networkndash Large number of nodesndash Battery poweredndash Topology and density changendash Nodes for a common taskndash In-network data processing
bull Sensor-net applicationsndash Sensor-triggered bursty trafficndash Can often tolerate some delay
Ad hoc Sensor Networks Multiple channel access
Characteristics of Sensor Network
Scalabilty and self configurationEnergy efficiencyAdaptivityFairness not importantLatency
AdaptivityEnergy-delay tradoff
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 58
Ad hoc Sensor Networks Multiple channel access
Wireless sensor node and sensor network
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 59
MAC for WSN attributes bull Attributes
ndash Collision avoidance (Basic task)ndash Energy efficiency (most primary in WSN)ndash Scalability and adaptivityndash Channel utilizationndash Latencyndash Throughputndash Fairness
bull What causes energy wastendash Collisionsndash Control packet overheadndash Overhearing unnecessary trafficndash Long idle time (it consumes 50-100 of the power)
Ad hoc Sensor Networks Multiple channel access
Primary
Secondary
Dominant factor
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 60
MAC for WSN
bull Contention-based protocols need to work hard in all directions for energysavingsndash Reduce idle listening ndash support low duty cyclendash Better collision avoidancendash Reduce control overheadndash Avoid unnecessary overhearing
Ad hoc Sensor Networks Multiple channel access
Scheduled Protocols Contention ProtocolsCollisions No YesEnergy efficiency Good BadScalability and adaptivity Bad GoodMulti-hop communication Difficult EasyTime synchronization Strict Loose or not required
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 61
MAC for WSN classification
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 62
MAC for WSN SMAC (2002)bull It has been developed by Ye Heidemann and Estrinbull Tradeoffs between (latency fairness) and energybull Major components in S-MAC
ndash Periodic listen and sleep turn off radio when sleeping the reduced dutycycle is ~ 10 (eg 120ms on 12s off)ndash Collision avoidance is based on contention similarly to IEEE 80211 adhoc modendash Overhearing avoidance the node is sleeping when neighbors talkndash Massage passing long message is fragmented amp sent in burst operationwith extended Tx time and re-transmit occurs immediately
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 63
Summarybull The algorithms and protocols that enable sharing by multiple users and
controls the access to the wireless channel from different users are calledMAC
bull There are two types of wireless networks Infrastructured wireless networksand ad hoc wireless networks
bull We also want our wireless system to have high user capacity and we canidentify 10 methods for increasing it
bull There are several MACs with no absolute advantage it should be tailoredto the system requirements
bull There are primary concerns for designing a WSN MAC (collisionavoidance energy efficiency scalability and adaptivity) while thesecondary concerns are as follows (channel utilization latency throughputfairness)
bull Next lecture Routing protocols
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3320111015 TAacuteMOP ndash 412-082AKMR-2009-0006 33
Ad hoc Sensor Networks Multiple channel access
User capacity per cell
The user capacity C is defined as the total number of active users percell that the system can support while meeting a common BERconstrain For orthogonal multiple access (as FDMA and TDMA)
C = Ncellwhere Ncell is the number of channels assigned to any given cellThe total number of orthogonal channels of bandwidth Bs that can becreated from a total system bandwidth of B is B Bs The reuse factorsatisfies N = (B Bs) Ncell this implies
C = (B Bs) N
Recall from Chapter 1
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3420111015 TAacuteMOP ndash 412-082AKMR-2009-0006 34
Ad hoc Sensor Networks Multiple channel access
10 methods for increasing capacity1 Increasing the amount of spectrum used very expensive2 More efficient modulation format and coding3 Better source coding4 Adaptive modulation and coding5 Discontinuous transmission6 Multiuser detection CDMA systems7 Reduction of cell radius effective but very expensive smaller
cells require more handovers8 Use of sector cells tripled cells have tripled BS antennas9 Use of an overlay structure10 Multiple antennas diversity MIMO systems SDMA
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3520111015 TAacuteMOP ndash 412-082AKMR-2009-0006 35
Ad hoc Sensor Networks Multiple channel access
bull Distributed operation is requiredbull Synchronization is required in TDMA-based systemsbull Hidden terminals are nodes hidden from a senderbull Exposed terminals are exposed nodes preventing a sender
from sendingbull Throughput needs to be maximizedbull Access delay should be minimizedbull Fairness refers to provide an equal share to all competing
nodesbull Real-time traffic support is required for voice video and
real-time databull Resource reservation is required for QoSbull Ability to measure resource availability handles the
resourcesbull Capability for power control reduces the energy
consumption
Random access in adhoc networks
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3620111015 TAacuteMOP ndash 412-082AKMR-2009-0006 36
Ad hoc Sensor Networks Multiple channel access
Random accessbull All random access techniques are based on packetized data or packet radio where user
date is collected into packets of M bitsbull Collision if packets from different users overlap in timebull The transmission time of a packet is τ = M R where R [bps] the data ratebull Analysis of random access techniques assumes that the users generate packets according
to memoryless Poisson process at a rate λ [packets per unit time] the probability that thenumber of packet arrivals in a time period [0t] denoted by X(t) is equal to integer k isgiven by
Pr(X(t)=k) = (λt)k k e-λt
bull The traffic load is defined as L = λ τ If L gt 1 means on avarage more packets arrive inthe system over a given time period than can be transmitted in that period so systemswith Lgt1 are unstable
bull The throughput is defined as the ratio of the average number of packets successfullytransmitted in any given time interval divided by the number of attempted transmissionsin that interval
T = L Pr(succesful packet transmission)bull The effective data rate of the system is RT
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3720111015 TAacuteMOP ndash 412-082AKMR-2009-0006 37
Ad hoc Sensor Networks Multiple channel access
Random access (contrsquo)bull We will examine the following attributes
ndash Channel utilization (Throughput)ndash Latencyndash Collision avoidance (Hidden and exposed terminals problem)ndash Reliability (ACK)ndash Energy efficiency (power control)ndash Fairnessndash Throughput needs to be maximized
bull We will discuss the following techniquesndash Aloha and slotted Alohandash CSMA Protocols (1-persistent non-persistent p-persistent CSMA CSMACD)ndash MACAndash MACAWndash Energy efficient MAC in wireless sensor networks (eg SMAC)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 38
Random Access ALOHA (1970)bull ALOHA was developed for a wireless system at the University of Hawaii
(Abramson et al)ndash Multiple remote stations plus one base stationndash Frame transmissions are made at one frequency from a remote station to the basestation the base station re-broadcasts frames on another frequency Contention is foraccess to the base station
bull Frames can be sent at any time to the base stationndash If no acknowledgment (ACK) is received assume that the frame is lost andretransmit after waiting a random amount of timendash If more than one station broadcasts on the base station access frequency at the sametime ndash a collision ndash interference will destroy the frames
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 39
Random Access ALOHA (contrsquo)
bull The probability that no packets generated during the tima [-τ τ] is given by (nocollision)
Pr(X(t)=0) = (λ2τ)0 0 e-2λτ = e-2Lwith corresponding throughput
T = L e-2L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 05 05e 018
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 40
Random Access slotted ALOHA (1971)bull Improvement Time is slotted and a packet can only be transmitted at the beginning of
one slot Thus it can reduce the collision duration
bull The probability that no packets generated during the time [0 τ] is given by (nocollision)
Pr(X(t)=0) = (λτ)0 0 e-λτ = e-Lwith corresponding throughput T = L e-L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 1 e 037
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 41
bull Slotted Aloha has double the maximumthroughput as pure Aloha and achievesthis maximum at a higher offered loadThe effective data rate is still less than40 of the raw transmission rate
bull Aloha does not listen to the carrier beforetransmission more sophisticatedtechniques are needed to increaseefficiency
bull Further problem the delayndashDelay
ndashThe mean packet delay
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
( ) ( )delayPr 1 e ekL Lkτ minus minus= = minus
( )delay1
1 e e ekL L L
kMPD E kτ
infinminus minus
=
= = minus =sum
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 42
Random Access Carrier Sense Multiple Accessbull CSMA (Carrier Sense Multiple Access) listen to the carrier before transmission
and transmits if channel is idlebull Detection delay and propagation delay are two important parameters
ndash Detection delay time required to sense the carrier and decide if it is idle or busyndash Propagation delay distancespeed_of_ligth The time required for bit to travel
from transmitter to the receiver
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 43
Random Access CSMA variations
Ad hoc Sensor Networks Multiple channel access
1-persistent CSMA Non-persistent CSMA
There will always be a collision iftwo nodes want to retransmit(usually you stop transmissionattempts after few tries)
Random backoff reduces probabilityof collisions Waste idle time if thebackoff time is too long
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 44
Random Access CSMA variations (contrsquo)
Ad hoc Sensor Networks Multiple channel access
p-persistent CSMA CSMA CD (Collision Detection)
A good tradeoff between non-persistent and 1-persistent CSMA
Same with CSMA however a stationalso listen to the carrier whiletransmitting to see if the transmissioncollides with someone elsetransmission It can be used in listen-while-talk capable channels (fullduplex) In single radio channels thetransmission need to be interrupted inorder to sense the channel
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 45
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 46
Random Access CSMA (contrsquo)bull How to select probability p
ndash Assume that M nodes have a packet to send and the medium is busy then Mp is theexpected number of nodes that will attempt to transmit once the medium becomesidle If Mp gt 1 then a collision is expected to occur Therefore the network mustmake sure that Mp lt 1 to avoid collision where M is the maximum number of nodesthat can be active at a time
bull Problems1 The mean packet delay increases exponently with increasing offered load2 CSMA protocols sense the carrier but sensing the carrier does not always releasestrue information about the status of the wireless channel There are two problems thatare unique to wireless channels (different than wireline channels)ndash Hidden terminal problemndash Exposed terminal problem
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 47
Hidden terminal problembull A sends to B C cannot receive Abull C wants to send to B C senses a
ldquofreerdquo medium (CS fails)bull collision at B A cannot receive the
collision (CD fails)bull A is ldquohiddenrdquo for C
Ad hoc Sensor Networks Multiple channel access
Random Access hidden and exposed terminal problemExposed terminal problem
bull B sends to A C wants to send to Dbull C has to wait CS signals a medium
in usebull since A is outside the radio range of
C waiting is not necessarybull C is ldquoexposedrdquo to B
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 48
Random Access MACAWhen a station wants to transmit databull It sends an RTS (Ready-to-Send) packet to the intended receiver
ndash The RTS packet contains the length of the data that needs to be transmittedndash Any station other than the intended recipient hearing RTS defers transmission for atime duration equal to the end of the corresponding CTS reception
bull The receiver sends back CTS (Clear-to-Send) packet back to sender if it isavailable to receivendash The CTS packet contains the length of the data that original sender wants totransmitndash Any station other than the original RTS sender hearing CTS defers transmissionuntil the data is sent
bull The original sender upon reception of the CTS starts transmitting
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 49
Hidden terminal problem
Ad hoc Sensor Networks Multiple channel access
Random Access MACA (contrsquo)Exposed terminal problem
Waiting time of node X is muchsmaller than waiting time of node C
bull C defers transmission upon hearing BrsquosRTS until B could get CTS from Abull After that C can start transmission to DFor that it first sends an RTSbull C is not longer exposed to the datatransmission of B
Problem It does not address the collision of RTS packet
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 50
Random Access reliabilitybull Wireless links are prone to have errors High packet loss rate detrimental to
transport-layer performance Mechanisms needed to reduce packet loss rateexperienced by upper layers
bull When node B receives a data packet from node A node B sends anAcknowledgement (Ack) This approach adopted in many protocols
bull If node A fails to receive an Ack it will retransmit the packet
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 51
Random Access MACAW (CSMACA) bull Physical carrier sense and virtual carrier sense using Network Allocation
Vector (NAV)ndash NAV is updated based on overheard RTSCTSDATAACK packets each of whichspecified duration of a pending transmissionndash Nodes stay silent when carrier sensed (physicalvirtual)
bull Backoff intervals used to reduce collision probabilityndash When transmitting a packet choose a backoff interval in the range [0w] where wis contention windowndash Count down the backoff interval when channel is idlendash Count down is suspended if channel becomes busyndash When backoff interval reaches 0 transmit RTS
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 52
Random Access CSMACA (contrsquo) bull The time spent counting down backoff intervals is a part of MAC overhead
ndash Choosing a large w leads to large backoff intervals and can result in larger overheadndash Choosing a small w leads to a larger number of collisions (when two nodes countdown to 0 simultaneously)ndash Since the number of nodes attempting to transmit simultaneously may change withtime some mechanism to manage contention is needed
bull IEEE 80211 DCF contention window w is chosen dynamically depending oncollision occurrence (binary exponential backoff)ndash When a node fails to receive CTS in response to its RTS it increases the contentionwindowndash w is doubled (up to an upper bound)ndash When a node successfully completes a data transfer it restores w to wmin
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 53
Random Access fairnessbull Simplest definition of fairness all nodes should receive equal bandwidthbull An example of unfairness
ndash Assume that initially A and B both choose a backoff interval in range [031] buttheir RTSs collidendash Nodes A and B then choose from range [063]ndash Node A chooses 4 slots and B choose 60 slotsndash After A transmits a packet it next chooses from range [031]ndash It is possible that A may transmit several packets before B transmits its first packet
Ad hoc Sensor Networks Multiple channel access
Unfairness occurs when one node has backed off much more than some other node
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 54
Random Access fairness (contrsquo) MACAW solution for fairnessbull When a node transmits a packet it appends value w to the packet all nodes
hearing value w use it for their future transmission attemptsbull Since w is an indication of the level of congestion in the vicinity of a specific
receiver node MACAW proposes maintaining w independently for eachreceiver
bull Using per-receiver wi is particularly useful in multi-hop environments sincecongestion level at different receivers can be very different
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 55
Random Access energy efficiency bull Since many mobile hosts are operated by batteries MAC
protocols which conserve energy are of interestbull Two approaches to reduce energy consumption
ndash Power save turn off wireless interface when desirablendash Power control reduce transmit power
bull Power control has some more potential benefitsndash Reduced interference (in ad hoc networks) eg it improves ALOHAefficiency where user with high power can capture a packet even if there is acollisionndash Increased spatial reuse (in infrastructured wireless networks) eg it is usedin CDMA to maintain target SIR of voice and data users or generally it canbe used to maintain target SIR for different user classes (admission control)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 56
Random Access power control bull When C transmits to D at a high power level B cannot receive Arsquos
transmission due to interference from Cbull If C reduces transmit power it can still communicate with D
ndash Reduces energy consumption at node Cndash Allows B to receive Arsquos transmission (spatial reuse)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 57
bull A special wireless ad hoc networkndash Large number of nodesndash Battery poweredndash Topology and density changendash Nodes for a common taskndash In-network data processing
bull Sensor-net applicationsndash Sensor-triggered bursty trafficndash Can often tolerate some delay
Ad hoc Sensor Networks Multiple channel access
Characteristics of Sensor Network
Scalabilty and self configurationEnergy efficiencyAdaptivityFairness not importantLatency
AdaptivityEnergy-delay tradoff
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 58
Ad hoc Sensor Networks Multiple channel access
Wireless sensor node and sensor network
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 59
MAC for WSN attributes bull Attributes
ndash Collision avoidance (Basic task)ndash Energy efficiency (most primary in WSN)ndash Scalability and adaptivityndash Channel utilizationndash Latencyndash Throughputndash Fairness
bull What causes energy wastendash Collisionsndash Control packet overheadndash Overhearing unnecessary trafficndash Long idle time (it consumes 50-100 of the power)
Ad hoc Sensor Networks Multiple channel access
Primary
Secondary
Dominant factor
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 60
MAC for WSN
bull Contention-based protocols need to work hard in all directions for energysavingsndash Reduce idle listening ndash support low duty cyclendash Better collision avoidancendash Reduce control overheadndash Avoid unnecessary overhearing
Ad hoc Sensor Networks Multiple channel access
Scheduled Protocols Contention ProtocolsCollisions No YesEnergy efficiency Good BadScalability and adaptivity Bad GoodMulti-hop communication Difficult EasyTime synchronization Strict Loose or not required
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 61
MAC for WSN classification
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 62
MAC for WSN SMAC (2002)bull It has been developed by Ye Heidemann and Estrinbull Tradeoffs between (latency fairness) and energybull Major components in S-MAC
ndash Periodic listen and sleep turn off radio when sleeping the reduced dutycycle is ~ 10 (eg 120ms on 12s off)ndash Collision avoidance is based on contention similarly to IEEE 80211 adhoc modendash Overhearing avoidance the node is sleeping when neighbors talkndash Massage passing long message is fragmented amp sent in burst operationwith extended Tx time and re-transmit occurs immediately
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 63
Summarybull The algorithms and protocols that enable sharing by multiple users and
controls the access to the wireless channel from different users are calledMAC
bull There are two types of wireless networks Infrastructured wireless networksand ad hoc wireless networks
bull We also want our wireless system to have high user capacity and we canidentify 10 methods for increasing it
bull There are several MACs with no absolute advantage it should be tailoredto the system requirements
bull There are primary concerns for designing a WSN MAC (collisionavoidance energy efficiency scalability and adaptivity) while thesecondary concerns are as follows (channel utilization latency throughputfairness)
bull Next lecture Routing protocols
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3420111015 TAacuteMOP ndash 412-082AKMR-2009-0006 34
Ad hoc Sensor Networks Multiple channel access
10 methods for increasing capacity1 Increasing the amount of spectrum used very expensive2 More efficient modulation format and coding3 Better source coding4 Adaptive modulation and coding5 Discontinuous transmission6 Multiuser detection CDMA systems7 Reduction of cell radius effective but very expensive smaller
cells require more handovers8 Use of sector cells tripled cells have tripled BS antennas9 Use of an overlay structure10 Multiple antennas diversity MIMO systems SDMA
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3520111015 TAacuteMOP ndash 412-082AKMR-2009-0006 35
Ad hoc Sensor Networks Multiple channel access
bull Distributed operation is requiredbull Synchronization is required in TDMA-based systemsbull Hidden terminals are nodes hidden from a senderbull Exposed terminals are exposed nodes preventing a sender
from sendingbull Throughput needs to be maximizedbull Access delay should be minimizedbull Fairness refers to provide an equal share to all competing
nodesbull Real-time traffic support is required for voice video and
real-time databull Resource reservation is required for QoSbull Ability to measure resource availability handles the
resourcesbull Capability for power control reduces the energy
consumption
Random access in adhoc networks
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3620111015 TAacuteMOP ndash 412-082AKMR-2009-0006 36
Ad hoc Sensor Networks Multiple channel access
Random accessbull All random access techniques are based on packetized data or packet radio where user
date is collected into packets of M bitsbull Collision if packets from different users overlap in timebull The transmission time of a packet is τ = M R where R [bps] the data ratebull Analysis of random access techniques assumes that the users generate packets according
to memoryless Poisson process at a rate λ [packets per unit time] the probability that thenumber of packet arrivals in a time period [0t] denoted by X(t) is equal to integer k isgiven by
Pr(X(t)=k) = (λt)k k e-λt
bull The traffic load is defined as L = λ τ If L gt 1 means on avarage more packets arrive inthe system over a given time period than can be transmitted in that period so systemswith Lgt1 are unstable
bull The throughput is defined as the ratio of the average number of packets successfullytransmitted in any given time interval divided by the number of attempted transmissionsin that interval
T = L Pr(succesful packet transmission)bull The effective data rate of the system is RT
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3720111015 TAacuteMOP ndash 412-082AKMR-2009-0006 37
Ad hoc Sensor Networks Multiple channel access
Random access (contrsquo)bull We will examine the following attributes
ndash Channel utilization (Throughput)ndash Latencyndash Collision avoidance (Hidden and exposed terminals problem)ndash Reliability (ACK)ndash Energy efficiency (power control)ndash Fairnessndash Throughput needs to be maximized
bull We will discuss the following techniquesndash Aloha and slotted Alohandash CSMA Protocols (1-persistent non-persistent p-persistent CSMA CSMACD)ndash MACAndash MACAWndash Energy efficient MAC in wireless sensor networks (eg SMAC)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 38
Random Access ALOHA (1970)bull ALOHA was developed for a wireless system at the University of Hawaii
(Abramson et al)ndash Multiple remote stations plus one base stationndash Frame transmissions are made at one frequency from a remote station to the basestation the base station re-broadcasts frames on another frequency Contention is foraccess to the base station
bull Frames can be sent at any time to the base stationndash If no acknowledgment (ACK) is received assume that the frame is lost andretransmit after waiting a random amount of timendash If more than one station broadcasts on the base station access frequency at the sametime ndash a collision ndash interference will destroy the frames
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 39
Random Access ALOHA (contrsquo)
bull The probability that no packets generated during the tima [-τ τ] is given by (nocollision)
Pr(X(t)=0) = (λ2τ)0 0 e-2λτ = e-2Lwith corresponding throughput
T = L e-2L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 05 05e 018
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 40
Random Access slotted ALOHA (1971)bull Improvement Time is slotted and a packet can only be transmitted at the beginning of
one slot Thus it can reduce the collision duration
bull The probability that no packets generated during the time [0 τ] is given by (nocollision)
Pr(X(t)=0) = (λτ)0 0 e-λτ = e-Lwith corresponding throughput T = L e-L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 1 e 037
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 41
bull Slotted Aloha has double the maximumthroughput as pure Aloha and achievesthis maximum at a higher offered loadThe effective data rate is still less than40 of the raw transmission rate
bull Aloha does not listen to the carrier beforetransmission more sophisticatedtechniques are needed to increaseefficiency
bull Further problem the delayndashDelay
ndashThe mean packet delay
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
( ) ( )delayPr 1 e ekL Lkτ minus minus= = minus
( )delay1
1 e e ekL L L
kMPD E kτ
infinminus minus
=
= = minus =sum
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 42
Random Access Carrier Sense Multiple Accessbull CSMA (Carrier Sense Multiple Access) listen to the carrier before transmission
and transmits if channel is idlebull Detection delay and propagation delay are two important parameters
ndash Detection delay time required to sense the carrier and decide if it is idle or busyndash Propagation delay distancespeed_of_ligth The time required for bit to travel
from transmitter to the receiver
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 43
Random Access CSMA variations
Ad hoc Sensor Networks Multiple channel access
1-persistent CSMA Non-persistent CSMA
There will always be a collision iftwo nodes want to retransmit(usually you stop transmissionattempts after few tries)
Random backoff reduces probabilityof collisions Waste idle time if thebackoff time is too long
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 44
Random Access CSMA variations (contrsquo)
Ad hoc Sensor Networks Multiple channel access
p-persistent CSMA CSMA CD (Collision Detection)
A good tradeoff between non-persistent and 1-persistent CSMA
Same with CSMA however a stationalso listen to the carrier whiletransmitting to see if the transmissioncollides with someone elsetransmission It can be used in listen-while-talk capable channels (fullduplex) In single radio channels thetransmission need to be interrupted inorder to sense the channel
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 45
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 46
Random Access CSMA (contrsquo)bull How to select probability p
ndash Assume that M nodes have a packet to send and the medium is busy then Mp is theexpected number of nodes that will attempt to transmit once the medium becomesidle If Mp gt 1 then a collision is expected to occur Therefore the network mustmake sure that Mp lt 1 to avoid collision where M is the maximum number of nodesthat can be active at a time
bull Problems1 The mean packet delay increases exponently with increasing offered load2 CSMA protocols sense the carrier but sensing the carrier does not always releasestrue information about the status of the wireless channel There are two problems thatare unique to wireless channels (different than wireline channels)ndash Hidden terminal problemndash Exposed terminal problem
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 47
Hidden terminal problembull A sends to B C cannot receive Abull C wants to send to B C senses a
ldquofreerdquo medium (CS fails)bull collision at B A cannot receive the
collision (CD fails)bull A is ldquohiddenrdquo for C
Ad hoc Sensor Networks Multiple channel access
Random Access hidden and exposed terminal problemExposed terminal problem
bull B sends to A C wants to send to Dbull C has to wait CS signals a medium
in usebull since A is outside the radio range of
C waiting is not necessarybull C is ldquoexposedrdquo to B
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 48
Random Access MACAWhen a station wants to transmit databull It sends an RTS (Ready-to-Send) packet to the intended receiver
ndash The RTS packet contains the length of the data that needs to be transmittedndash Any station other than the intended recipient hearing RTS defers transmission for atime duration equal to the end of the corresponding CTS reception
bull The receiver sends back CTS (Clear-to-Send) packet back to sender if it isavailable to receivendash The CTS packet contains the length of the data that original sender wants totransmitndash Any station other than the original RTS sender hearing CTS defers transmissionuntil the data is sent
bull The original sender upon reception of the CTS starts transmitting
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 49
Hidden terminal problem
Ad hoc Sensor Networks Multiple channel access
Random Access MACA (contrsquo)Exposed terminal problem
Waiting time of node X is muchsmaller than waiting time of node C
bull C defers transmission upon hearing BrsquosRTS until B could get CTS from Abull After that C can start transmission to DFor that it first sends an RTSbull C is not longer exposed to the datatransmission of B
Problem It does not address the collision of RTS packet
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 50
Random Access reliabilitybull Wireless links are prone to have errors High packet loss rate detrimental to
transport-layer performance Mechanisms needed to reduce packet loss rateexperienced by upper layers
bull When node B receives a data packet from node A node B sends anAcknowledgement (Ack) This approach adopted in many protocols
bull If node A fails to receive an Ack it will retransmit the packet
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 51
Random Access MACAW (CSMACA) bull Physical carrier sense and virtual carrier sense using Network Allocation
Vector (NAV)ndash NAV is updated based on overheard RTSCTSDATAACK packets each of whichspecified duration of a pending transmissionndash Nodes stay silent when carrier sensed (physicalvirtual)
bull Backoff intervals used to reduce collision probabilityndash When transmitting a packet choose a backoff interval in the range [0w] where wis contention windowndash Count down the backoff interval when channel is idlendash Count down is suspended if channel becomes busyndash When backoff interval reaches 0 transmit RTS
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 52
Random Access CSMACA (contrsquo) bull The time spent counting down backoff intervals is a part of MAC overhead
ndash Choosing a large w leads to large backoff intervals and can result in larger overheadndash Choosing a small w leads to a larger number of collisions (when two nodes countdown to 0 simultaneously)ndash Since the number of nodes attempting to transmit simultaneously may change withtime some mechanism to manage contention is needed
bull IEEE 80211 DCF contention window w is chosen dynamically depending oncollision occurrence (binary exponential backoff)ndash When a node fails to receive CTS in response to its RTS it increases the contentionwindowndash w is doubled (up to an upper bound)ndash When a node successfully completes a data transfer it restores w to wmin
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 53
Random Access fairnessbull Simplest definition of fairness all nodes should receive equal bandwidthbull An example of unfairness
ndash Assume that initially A and B both choose a backoff interval in range [031] buttheir RTSs collidendash Nodes A and B then choose from range [063]ndash Node A chooses 4 slots and B choose 60 slotsndash After A transmits a packet it next chooses from range [031]ndash It is possible that A may transmit several packets before B transmits its first packet
Ad hoc Sensor Networks Multiple channel access
Unfairness occurs when one node has backed off much more than some other node
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 54
Random Access fairness (contrsquo) MACAW solution for fairnessbull When a node transmits a packet it appends value w to the packet all nodes
hearing value w use it for their future transmission attemptsbull Since w is an indication of the level of congestion in the vicinity of a specific
receiver node MACAW proposes maintaining w independently for eachreceiver
bull Using per-receiver wi is particularly useful in multi-hop environments sincecongestion level at different receivers can be very different
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 55
Random Access energy efficiency bull Since many mobile hosts are operated by batteries MAC
protocols which conserve energy are of interestbull Two approaches to reduce energy consumption
ndash Power save turn off wireless interface when desirablendash Power control reduce transmit power
bull Power control has some more potential benefitsndash Reduced interference (in ad hoc networks) eg it improves ALOHAefficiency where user with high power can capture a packet even if there is acollisionndash Increased spatial reuse (in infrastructured wireless networks) eg it is usedin CDMA to maintain target SIR of voice and data users or generally it canbe used to maintain target SIR for different user classes (admission control)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 56
Random Access power control bull When C transmits to D at a high power level B cannot receive Arsquos
transmission due to interference from Cbull If C reduces transmit power it can still communicate with D
ndash Reduces energy consumption at node Cndash Allows B to receive Arsquos transmission (spatial reuse)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 57
bull A special wireless ad hoc networkndash Large number of nodesndash Battery poweredndash Topology and density changendash Nodes for a common taskndash In-network data processing
bull Sensor-net applicationsndash Sensor-triggered bursty trafficndash Can often tolerate some delay
Ad hoc Sensor Networks Multiple channel access
Characteristics of Sensor Network
Scalabilty and self configurationEnergy efficiencyAdaptivityFairness not importantLatency
AdaptivityEnergy-delay tradoff
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 58
Ad hoc Sensor Networks Multiple channel access
Wireless sensor node and sensor network
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 59
MAC for WSN attributes bull Attributes
ndash Collision avoidance (Basic task)ndash Energy efficiency (most primary in WSN)ndash Scalability and adaptivityndash Channel utilizationndash Latencyndash Throughputndash Fairness
bull What causes energy wastendash Collisionsndash Control packet overheadndash Overhearing unnecessary trafficndash Long idle time (it consumes 50-100 of the power)
Ad hoc Sensor Networks Multiple channel access
Primary
Secondary
Dominant factor
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 60
MAC for WSN
bull Contention-based protocols need to work hard in all directions for energysavingsndash Reduce idle listening ndash support low duty cyclendash Better collision avoidancendash Reduce control overheadndash Avoid unnecessary overhearing
Ad hoc Sensor Networks Multiple channel access
Scheduled Protocols Contention ProtocolsCollisions No YesEnergy efficiency Good BadScalability and adaptivity Bad GoodMulti-hop communication Difficult EasyTime synchronization Strict Loose or not required
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 61
MAC for WSN classification
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 62
MAC for WSN SMAC (2002)bull It has been developed by Ye Heidemann and Estrinbull Tradeoffs between (latency fairness) and energybull Major components in S-MAC
ndash Periodic listen and sleep turn off radio when sleeping the reduced dutycycle is ~ 10 (eg 120ms on 12s off)ndash Collision avoidance is based on contention similarly to IEEE 80211 adhoc modendash Overhearing avoidance the node is sleeping when neighbors talkndash Massage passing long message is fragmented amp sent in burst operationwith extended Tx time and re-transmit occurs immediately
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 63
Summarybull The algorithms and protocols that enable sharing by multiple users and
controls the access to the wireless channel from different users are calledMAC
bull There are two types of wireless networks Infrastructured wireless networksand ad hoc wireless networks
bull We also want our wireless system to have high user capacity and we canidentify 10 methods for increasing it
bull There are several MACs with no absolute advantage it should be tailoredto the system requirements
bull There are primary concerns for designing a WSN MAC (collisionavoidance energy efficiency scalability and adaptivity) while thesecondary concerns are as follows (channel utilization latency throughputfairness)
bull Next lecture Routing protocols
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3520111015 TAacuteMOP ndash 412-082AKMR-2009-0006 35
Ad hoc Sensor Networks Multiple channel access
bull Distributed operation is requiredbull Synchronization is required in TDMA-based systemsbull Hidden terminals are nodes hidden from a senderbull Exposed terminals are exposed nodes preventing a sender
from sendingbull Throughput needs to be maximizedbull Access delay should be minimizedbull Fairness refers to provide an equal share to all competing
nodesbull Real-time traffic support is required for voice video and
real-time databull Resource reservation is required for QoSbull Ability to measure resource availability handles the
resourcesbull Capability for power control reduces the energy
consumption
Random access in adhoc networks
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3620111015 TAacuteMOP ndash 412-082AKMR-2009-0006 36
Ad hoc Sensor Networks Multiple channel access
Random accessbull All random access techniques are based on packetized data or packet radio where user
date is collected into packets of M bitsbull Collision if packets from different users overlap in timebull The transmission time of a packet is τ = M R where R [bps] the data ratebull Analysis of random access techniques assumes that the users generate packets according
to memoryless Poisson process at a rate λ [packets per unit time] the probability that thenumber of packet arrivals in a time period [0t] denoted by X(t) is equal to integer k isgiven by
Pr(X(t)=k) = (λt)k k e-λt
bull The traffic load is defined as L = λ τ If L gt 1 means on avarage more packets arrive inthe system over a given time period than can be transmitted in that period so systemswith Lgt1 are unstable
bull The throughput is defined as the ratio of the average number of packets successfullytransmitted in any given time interval divided by the number of attempted transmissionsin that interval
T = L Pr(succesful packet transmission)bull The effective data rate of the system is RT
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3720111015 TAacuteMOP ndash 412-082AKMR-2009-0006 37
Ad hoc Sensor Networks Multiple channel access
Random access (contrsquo)bull We will examine the following attributes
ndash Channel utilization (Throughput)ndash Latencyndash Collision avoidance (Hidden and exposed terminals problem)ndash Reliability (ACK)ndash Energy efficiency (power control)ndash Fairnessndash Throughput needs to be maximized
bull We will discuss the following techniquesndash Aloha and slotted Alohandash CSMA Protocols (1-persistent non-persistent p-persistent CSMA CSMACD)ndash MACAndash MACAWndash Energy efficient MAC in wireless sensor networks (eg SMAC)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 38
Random Access ALOHA (1970)bull ALOHA was developed for a wireless system at the University of Hawaii
(Abramson et al)ndash Multiple remote stations plus one base stationndash Frame transmissions are made at one frequency from a remote station to the basestation the base station re-broadcasts frames on another frequency Contention is foraccess to the base station
bull Frames can be sent at any time to the base stationndash If no acknowledgment (ACK) is received assume that the frame is lost andretransmit after waiting a random amount of timendash If more than one station broadcasts on the base station access frequency at the sametime ndash a collision ndash interference will destroy the frames
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 39
Random Access ALOHA (contrsquo)
bull The probability that no packets generated during the tima [-τ τ] is given by (nocollision)
Pr(X(t)=0) = (λ2τ)0 0 e-2λτ = e-2Lwith corresponding throughput
T = L e-2L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 05 05e 018
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 40
Random Access slotted ALOHA (1971)bull Improvement Time is slotted and a packet can only be transmitted at the beginning of
one slot Thus it can reduce the collision duration
bull The probability that no packets generated during the time [0 τ] is given by (nocollision)
Pr(X(t)=0) = (λτ)0 0 e-λτ = e-Lwith corresponding throughput T = L e-L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 1 e 037
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 41
bull Slotted Aloha has double the maximumthroughput as pure Aloha and achievesthis maximum at a higher offered loadThe effective data rate is still less than40 of the raw transmission rate
bull Aloha does not listen to the carrier beforetransmission more sophisticatedtechniques are needed to increaseefficiency
bull Further problem the delayndashDelay
ndashThe mean packet delay
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
( ) ( )delayPr 1 e ekL Lkτ minus minus= = minus
( )delay1
1 e e ekL L L
kMPD E kτ
infinminus minus
=
= = minus =sum
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 42
Random Access Carrier Sense Multiple Accessbull CSMA (Carrier Sense Multiple Access) listen to the carrier before transmission
and transmits if channel is idlebull Detection delay and propagation delay are two important parameters
ndash Detection delay time required to sense the carrier and decide if it is idle or busyndash Propagation delay distancespeed_of_ligth The time required for bit to travel
from transmitter to the receiver
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 43
Random Access CSMA variations
Ad hoc Sensor Networks Multiple channel access
1-persistent CSMA Non-persistent CSMA
There will always be a collision iftwo nodes want to retransmit(usually you stop transmissionattempts after few tries)
Random backoff reduces probabilityof collisions Waste idle time if thebackoff time is too long
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 44
Random Access CSMA variations (contrsquo)
Ad hoc Sensor Networks Multiple channel access
p-persistent CSMA CSMA CD (Collision Detection)
A good tradeoff between non-persistent and 1-persistent CSMA
Same with CSMA however a stationalso listen to the carrier whiletransmitting to see if the transmissioncollides with someone elsetransmission It can be used in listen-while-talk capable channels (fullduplex) In single radio channels thetransmission need to be interrupted inorder to sense the channel
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 45
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 46
Random Access CSMA (contrsquo)bull How to select probability p
ndash Assume that M nodes have a packet to send and the medium is busy then Mp is theexpected number of nodes that will attempt to transmit once the medium becomesidle If Mp gt 1 then a collision is expected to occur Therefore the network mustmake sure that Mp lt 1 to avoid collision where M is the maximum number of nodesthat can be active at a time
bull Problems1 The mean packet delay increases exponently with increasing offered load2 CSMA protocols sense the carrier but sensing the carrier does not always releasestrue information about the status of the wireless channel There are two problems thatare unique to wireless channels (different than wireline channels)ndash Hidden terminal problemndash Exposed terminal problem
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 47
Hidden terminal problembull A sends to B C cannot receive Abull C wants to send to B C senses a
ldquofreerdquo medium (CS fails)bull collision at B A cannot receive the
collision (CD fails)bull A is ldquohiddenrdquo for C
Ad hoc Sensor Networks Multiple channel access
Random Access hidden and exposed terminal problemExposed terminal problem
bull B sends to A C wants to send to Dbull C has to wait CS signals a medium
in usebull since A is outside the radio range of
C waiting is not necessarybull C is ldquoexposedrdquo to B
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 48
Random Access MACAWhen a station wants to transmit databull It sends an RTS (Ready-to-Send) packet to the intended receiver
ndash The RTS packet contains the length of the data that needs to be transmittedndash Any station other than the intended recipient hearing RTS defers transmission for atime duration equal to the end of the corresponding CTS reception
bull The receiver sends back CTS (Clear-to-Send) packet back to sender if it isavailable to receivendash The CTS packet contains the length of the data that original sender wants totransmitndash Any station other than the original RTS sender hearing CTS defers transmissionuntil the data is sent
bull The original sender upon reception of the CTS starts transmitting
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 49
Hidden terminal problem
Ad hoc Sensor Networks Multiple channel access
Random Access MACA (contrsquo)Exposed terminal problem
Waiting time of node X is muchsmaller than waiting time of node C
bull C defers transmission upon hearing BrsquosRTS until B could get CTS from Abull After that C can start transmission to DFor that it first sends an RTSbull C is not longer exposed to the datatransmission of B
Problem It does not address the collision of RTS packet
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 50
Random Access reliabilitybull Wireless links are prone to have errors High packet loss rate detrimental to
transport-layer performance Mechanisms needed to reduce packet loss rateexperienced by upper layers
bull When node B receives a data packet from node A node B sends anAcknowledgement (Ack) This approach adopted in many protocols
bull If node A fails to receive an Ack it will retransmit the packet
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 51
Random Access MACAW (CSMACA) bull Physical carrier sense and virtual carrier sense using Network Allocation
Vector (NAV)ndash NAV is updated based on overheard RTSCTSDATAACK packets each of whichspecified duration of a pending transmissionndash Nodes stay silent when carrier sensed (physicalvirtual)
bull Backoff intervals used to reduce collision probabilityndash When transmitting a packet choose a backoff interval in the range [0w] where wis contention windowndash Count down the backoff interval when channel is idlendash Count down is suspended if channel becomes busyndash When backoff interval reaches 0 transmit RTS
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 52
Random Access CSMACA (contrsquo) bull The time spent counting down backoff intervals is a part of MAC overhead
ndash Choosing a large w leads to large backoff intervals and can result in larger overheadndash Choosing a small w leads to a larger number of collisions (when two nodes countdown to 0 simultaneously)ndash Since the number of nodes attempting to transmit simultaneously may change withtime some mechanism to manage contention is needed
bull IEEE 80211 DCF contention window w is chosen dynamically depending oncollision occurrence (binary exponential backoff)ndash When a node fails to receive CTS in response to its RTS it increases the contentionwindowndash w is doubled (up to an upper bound)ndash When a node successfully completes a data transfer it restores w to wmin
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 53
Random Access fairnessbull Simplest definition of fairness all nodes should receive equal bandwidthbull An example of unfairness
ndash Assume that initially A and B both choose a backoff interval in range [031] buttheir RTSs collidendash Nodes A and B then choose from range [063]ndash Node A chooses 4 slots and B choose 60 slotsndash After A transmits a packet it next chooses from range [031]ndash It is possible that A may transmit several packets before B transmits its first packet
Ad hoc Sensor Networks Multiple channel access
Unfairness occurs when one node has backed off much more than some other node
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 54
Random Access fairness (contrsquo) MACAW solution for fairnessbull When a node transmits a packet it appends value w to the packet all nodes
hearing value w use it for their future transmission attemptsbull Since w is an indication of the level of congestion in the vicinity of a specific
receiver node MACAW proposes maintaining w independently for eachreceiver
bull Using per-receiver wi is particularly useful in multi-hop environments sincecongestion level at different receivers can be very different
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 55
Random Access energy efficiency bull Since many mobile hosts are operated by batteries MAC
protocols which conserve energy are of interestbull Two approaches to reduce energy consumption
ndash Power save turn off wireless interface when desirablendash Power control reduce transmit power
bull Power control has some more potential benefitsndash Reduced interference (in ad hoc networks) eg it improves ALOHAefficiency where user with high power can capture a packet even if there is acollisionndash Increased spatial reuse (in infrastructured wireless networks) eg it is usedin CDMA to maintain target SIR of voice and data users or generally it canbe used to maintain target SIR for different user classes (admission control)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 56
Random Access power control bull When C transmits to D at a high power level B cannot receive Arsquos
transmission due to interference from Cbull If C reduces transmit power it can still communicate with D
ndash Reduces energy consumption at node Cndash Allows B to receive Arsquos transmission (spatial reuse)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 57
bull A special wireless ad hoc networkndash Large number of nodesndash Battery poweredndash Topology and density changendash Nodes for a common taskndash In-network data processing
bull Sensor-net applicationsndash Sensor-triggered bursty trafficndash Can often tolerate some delay
Ad hoc Sensor Networks Multiple channel access
Characteristics of Sensor Network
Scalabilty and self configurationEnergy efficiencyAdaptivityFairness not importantLatency
AdaptivityEnergy-delay tradoff
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 58
Ad hoc Sensor Networks Multiple channel access
Wireless sensor node and sensor network
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 59
MAC for WSN attributes bull Attributes
ndash Collision avoidance (Basic task)ndash Energy efficiency (most primary in WSN)ndash Scalability and adaptivityndash Channel utilizationndash Latencyndash Throughputndash Fairness
bull What causes energy wastendash Collisionsndash Control packet overheadndash Overhearing unnecessary trafficndash Long idle time (it consumes 50-100 of the power)
Ad hoc Sensor Networks Multiple channel access
Primary
Secondary
Dominant factor
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 60
MAC for WSN
bull Contention-based protocols need to work hard in all directions for energysavingsndash Reduce idle listening ndash support low duty cyclendash Better collision avoidancendash Reduce control overheadndash Avoid unnecessary overhearing
Ad hoc Sensor Networks Multiple channel access
Scheduled Protocols Contention ProtocolsCollisions No YesEnergy efficiency Good BadScalability and adaptivity Bad GoodMulti-hop communication Difficult EasyTime synchronization Strict Loose or not required
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 61
MAC for WSN classification
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 62
MAC for WSN SMAC (2002)bull It has been developed by Ye Heidemann and Estrinbull Tradeoffs between (latency fairness) and energybull Major components in S-MAC
ndash Periodic listen and sleep turn off radio when sleeping the reduced dutycycle is ~ 10 (eg 120ms on 12s off)ndash Collision avoidance is based on contention similarly to IEEE 80211 adhoc modendash Overhearing avoidance the node is sleeping when neighbors talkndash Massage passing long message is fragmented amp sent in burst operationwith extended Tx time and re-transmit occurs immediately
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 63
Summarybull The algorithms and protocols that enable sharing by multiple users and
controls the access to the wireless channel from different users are calledMAC
bull There are two types of wireless networks Infrastructured wireless networksand ad hoc wireless networks
bull We also want our wireless system to have high user capacity and we canidentify 10 methods for increasing it
bull There are several MACs with no absolute advantage it should be tailoredto the system requirements
bull There are primary concerns for designing a WSN MAC (collisionavoidance energy efficiency scalability and adaptivity) while thesecondary concerns are as follows (channel utilization latency throughputfairness)
bull Next lecture Routing protocols
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3620111015 TAacuteMOP ndash 412-082AKMR-2009-0006 36
Ad hoc Sensor Networks Multiple channel access
Random accessbull All random access techniques are based on packetized data or packet radio where user
date is collected into packets of M bitsbull Collision if packets from different users overlap in timebull The transmission time of a packet is τ = M R where R [bps] the data ratebull Analysis of random access techniques assumes that the users generate packets according
to memoryless Poisson process at a rate λ [packets per unit time] the probability that thenumber of packet arrivals in a time period [0t] denoted by X(t) is equal to integer k isgiven by
Pr(X(t)=k) = (λt)k k e-λt
bull The traffic load is defined as L = λ τ If L gt 1 means on avarage more packets arrive inthe system over a given time period than can be transmitted in that period so systemswith Lgt1 are unstable
bull The throughput is defined as the ratio of the average number of packets successfullytransmitted in any given time interval divided by the number of attempted transmissionsin that interval
T = L Pr(succesful packet transmission)bull The effective data rate of the system is RT
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3720111015 TAacuteMOP ndash 412-082AKMR-2009-0006 37
Ad hoc Sensor Networks Multiple channel access
Random access (contrsquo)bull We will examine the following attributes
ndash Channel utilization (Throughput)ndash Latencyndash Collision avoidance (Hidden and exposed terminals problem)ndash Reliability (ACK)ndash Energy efficiency (power control)ndash Fairnessndash Throughput needs to be maximized
bull We will discuss the following techniquesndash Aloha and slotted Alohandash CSMA Protocols (1-persistent non-persistent p-persistent CSMA CSMACD)ndash MACAndash MACAWndash Energy efficient MAC in wireless sensor networks (eg SMAC)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 38
Random Access ALOHA (1970)bull ALOHA was developed for a wireless system at the University of Hawaii
(Abramson et al)ndash Multiple remote stations plus one base stationndash Frame transmissions are made at one frequency from a remote station to the basestation the base station re-broadcasts frames on another frequency Contention is foraccess to the base station
bull Frames can be sent at any time to the base stationndash If no acknowledgment (ACK) is received assume that the frame is lost andretransmit after waiting a random amount of timendash If more than one station broadcasts on the base station access frequency at the sametime ndash a collision ndash interference will destroy the frames
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 39
Random Access ALOHA (contrsquo)
bull The probability that no packets generated during the tima [-τ τ] is given by (nocollision)
Pr(X(t)=0) = (λ2τ)0 0 e-2λτ = e-2Lwith corresponding throughput
T = L e-2L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 05 05e 018
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 40
Random Access slotted ALOHA (1971)bull Improvement Time is slotted and a packet can only be transmitted at the beginning of
one slot Thus it can reduce the collision duration
bull The probability that no packets generated during the time [0 τ] is given by (nocollision)
Pr(X(t)=0) = (λτ)0 0 e-λτ = e-Lwith corresponding throughput T = L e-L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 1 e 037
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 41
bull Slotted Aloha has double the maximumthroughput as pure Aloha and achievesthis maximum at a higher offered loadThe effective data rate is still less than40 of the raw transmission rate
bull Aloha does not listen to the carrier beforetransmission more sophisticatedtechniques are needed to increaseefficiency
bull Further problem the delayndashDelay
ndashThe mean packet delay
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
( ) ( )delayPr 1 e ekL Lkτ minus minus= = minus
( )delay1
1 e e ekL L L
kMPD E kτ
infinminus minus
=
= = minus =sum
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 42
Random Access Carrier Sense Multiple Accessbull CSMA (Carrier Sense Multiple Access) listen to the carrier before transmission
and transmits if channel is idlebull Detection delay and propagation delay are two important parameters
ndash Detection delay time required to sense the carrier and decide if it is idle or busyndash Propagation delay distancespeed_of_ligth The time required for bit to travel
from transmitter to the receiver
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 43
Random Access CSMA variations
Ad hoc Sensor Networks Multiple channel access
1-persistent CSMA Non-persistent CSMA
There will always be a collision iftwo nodes want to retransmit(usually you stop transmissionattempts after few tries)
Random backoff reduces probabilityof collisions Waste idle time if thebackoff time is too long
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 44
Random Access CSMA variations (contrsquo)
Ad hoc Sensor Networks Multiple channel access
p-persistent CSMA CSMA CD (Collision Detection)
A good tradeoff between non-persistent and 1-persistent CSMA
Same with CSMA however a stationalso listen to the carrier whiletransmitting to see if the transmissioncollides with someone elsetransmission It can be used in listen-while-talk capable channels (fullduplex) In single radio channels thetransmission need to be interrupted inorder to sense the channel
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 45
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 46
Random Access CSMA (contrsquo)bull How to select probability p
ndash Assume that M nodes have a packet to send and the medium is busy then Mp is theexpected number of nodes that will attempt to transmit once the medium becomesidle If Mp gt 1 then a collision is expected to occur Therefore the network mustmake sure that Mp lt 1 to avoid collision where M is the maximum number of nodesthat can be active at a time
bull Problems1 The mean packet delay increases exponently with increasing offered load2 CSMA protocols sense the carrier but sensing the carrier does not always releasestrue information about the status of the wireless channel There are two problems thatare unique to wireless channels (different than wireline channels)ndash Hidden terminal problemndash Exposed terminal problem
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 47
Hidden terminal problembull A sends to B C cannot receive Abull C wants to send to B C senses a
ldquofreerdquo medium (CS fails)bull collision at B A cannot receive the
collision (CD fails)bull A is ldquohiddenrdquo for C
Ad hoc Sensor Networks Multiple channel access
Random Access hidden and exposed terminal problemExposed terminal problem
bull B sends to A C wants to send to Dbull C has to wait CS signals a medium
in usebull since A is outside the radio range of
C waiting is not necessarybull C is ldquoexposedrdquo to B
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 48
Random Access MACAWhen a station wants to transmit databull It sends an RTS (Ready-to-Send) packet to the intended receiver
ndash The RTS packet contains the length of the data that needs to be transmittedndash Any station other than the intended recipient hearing RTS defers transmission for atime duration equal to the end of the corresponding CTS reception
bull The receiver sends back CTS (Clear-to-Send) packet back to sender if it isavailable to receivendash The CTS packet contains the length of the data that original sender wants totransmitndash Any station other than the original RTS sender hearing CTS defers transmissionuntil the data is sent
bull The original sender upon reception of the CTS starts transmitting
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 49
Hidden terminal problem
Ad hoc Sensor Networks Multiple channel access
Random Access MACA (contrsquo)Exposed terminal problem
Waiting time of node X is muchsmaller than waiting time of node C
bull C defers transmission upon hearing BrsquosRTS until B could get CTS from Abull After that C can start transmission to DFor that it first sends an RTSbull C is not longer exposed to the datatransmission of B
Problem It does not address the collision of RTS packet
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 50
Random Access reliabilitybull Wireless links are prone to have errors High packet loss rate detrimental to
transport-layer performance Mechanisms needed to reduce packet loss rateexperienced by upper layers
bull When node B receives a data packet from node A node B sends anAcknowledgement (Ack) This approach adopted in many protocols
bull If node A fails to receive an Ack it will retransmit the packet
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 51
Random Access MACAW (CSMACA) bull Physical carrier sense and virtual carrier sense using Network Allocation
Vector (NAV)ndash NAV is updated based on overheard RTSCTSDATAACK packets each of whichspecified duration of a pending transmissionndash Nodes stay silent when carrier sensed (physicalvirtual)
bull Backoff intervals used to reduce collision probabilityndash When transmitting a packet choose a backoff interval in the range [0w] where wis contention windowndash Count down the backoff interval when channel is idlendash Count down is suspended if channel becomes busyndash When backoff interval reaches 0 transmit RTS
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 52
Random Access CSMACA (contrsquo) bull The time spent counting down backoff intervals is a part of MAC overhead
ndash Choosing a large w leads to large backoff intervals and can result in larger overheadndash Choosing a small w leads to a larger number of collisions (when two nodes countdown to 0 simultaneously)ndash Since the number of nodes attempting to transmit simultaneously may change withtime some mechanism to manage contention is needed
bull IEEE 80211 DCF contention window w is chosen dynamically depending oncollision occurrence (binary exponential backoff)ndash When a node fails to receive CTS in response to its RTS it increases the contentionwindowndash w is doubled (up to an upper bound)ndash When a node successfully completes a data transfer it restores w to wmin
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 53
Random Access fairnessbull Simplest definition of fairness all nodes should receive equal bandwidthbull An example of unfairness
ndash Assume that initially A and B both choose a backoff interval in range [031] buttheir RTSs collidendash Nodes A and B then choose from range [063]ndash Node A chooses 4 slots and B choose 60 slotsndash After A transmits a packet it next chooses from range [031]ndash It is possible that A may transmit several packets before B transmits its first packet
Ad hoc Sensor Networks Multiple channel access
Unfairness occurs when one node has backed off much more than some other node
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 54
Random Access fairness (contrsquo) MACAW solution for fairnessbull When a node transmits a packet it appends value w to the packet all nodes
hearing value w use it for their future transmission attemptsbull Since w is an indication of the level of congestion in the vicinity of a specific
receiver node MACAW proposes maintaining w independently for eachreceiver
bull Using per-receiver wi is particularly useful in multi-hop environments sincecongestion level at different receivers can be very different
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 55
Random Access energy efficiency bull Since many mobile hosts are operated by batteries MAC
protocols which conserve energy are of interestbull Two approaches to reduce energy consumption
ndash Power save turn off wireless interface when desirablendash Power control reduce transmit power
bull Power control has some more potential benefitsndash Reduced interference (in ad hoc networks) eg it improves ALOHAefficiency where user with high power can capture a packet even if there is acollisionndash Increased spatial reuse (in infrastructured wireless networks) eg it is usedin CDMA to maintain target SIR of voice and data users or generally it canbe used to maintain target SIR for different user classes (admission control)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 56
Random Access power control bull When C transmits to D at a high power level B cannot receive Arsquos
transmission due to interference from Cbull If C reduces transmit power it can still communicate with D
ndash Reduces energy consumption at node Cndash Allows B to receive Arsquos transmission (spatial reuse)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 57
bull A special wireless ad hoc networkndash Large number of nodesndash Battery poweredndash Topology and density changendash Nodes for a common taskndash In-network data processing
bull Sensor-net applicationsndash Sensor-triggered bursty trafficndash Can often tolerate some delay
Ad hoc Sensor Networks Multiple channel access
Characteristics of Sensor Network
Scalabilty and self configurationEnergy efficiencyAdaptivityFairness not importantLatency
AdaptivityEnergy-delay tradoff
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 58
Ad hoc Sensor Networks Multiple channel access
Wireless sensor node and sensor network
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 59
MAC for WSN attributes bull Attributes
ndash Collision avoidance (Basic task)ndash Energy efficiency (most primary in WSN)ndash Scalability and adaptivityndash Channel utilizationndash Latencyndash Throughputndash Fairness
bull What causes energy wastendash Collisionsndash Control packet overheadndash Overhearing unnecessary trafficndash Long idle time (it consumes 50-100 of the power)
Ad hoc Sensor Networks Multiple channel access
Primary
Secondary
Dominant factor
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 60
MAC for WSN
bull Contention-based protocols need to work hard in all directions for energysavingsndash Reduce idle listening ndash support low duty cyclendash Better collision avoidancendash Reduce control overheadndash Avoid unnecessary overhearing
Ad hoc Sensor Networks Multiple channel access
Scheduled Protocols Contention ProtocolsCollisions No YesEnergy efficiency Good BadScalability and adaptivity Bad GoodMulti-hop communication Difficult EasyTime synchronization Strict Loose or not required
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 61
MAC for WSN classification
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 62
MAC for WSN SMAC (2002)bull It has been developed by Ye Heidemann and Estrinbull Tradeoffs between (latency fairness) and energybull Major components in S-MAC
ndash Periodic listen and sleep turn off radio when sleeping the reduced dutycycle is ~ 10 (eg 120ms on 12s off)ndash Collision avoidance is based on contention similarly to IEEE 80211 adhoc modendash Overhearing avoidance the node is sleeping when neighbors talkndash Massage passing long message is fragmented amp sent in burst operationwith extended Tx time and re-transmit occurs immediately
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 63
Summarybull The algorithms and protocols that enable sharing by multiple users and
controls the access to the wireless channel from different users are calledMAC
bull There are two types of wireless networks Infrastructured wireless networksand ad hoc wireless networks
bull We also want our wireless system to have high user capacity and we canidentify 10 methods for increasing it
bull There are several MACs with no absolute advantage it should be tailoredto the system requirements
bull There are primary concerns for designing a WSN MAC (collisionavoidance energy efficiency scalability and adaptivity) while thesecondary concerns are as follows (channel utilization latency throughputfairness)
bull Next lecture Routing protocols
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 3720111015 TAacuteMOP ndash 412-082AKMR-2009-0006 37
Ad hoc Sensor Networks Multiple channel access
Random access (contrsquo)bull We will examine the following attributes
ndash Channel utilization (Throughput)ndash Latencyndash Collision avoidance (Hidden and exposed terminals problem)ndash Reliability (ACK)ndash Energy efficiency (power control)ndash Fairnessndash Throughput needs to be maximized
bull We will discuss the following techniquesndash Aloha and slotted Alohandash CSMA Protocols (1-persistent non-persistent p-persistent CSMA CSMACD)ndash MACAndash MACAWndash Energy efficient MAC in wireless sensor networks (eg SMAC)
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 38
Random Access ALOHA (1970)bull ALOHA was developed for a wireless system at the University of Hawaii
(Abramson et al)ndash Multiple remote stations plus one base stationndash Frame transmissions are made at one frequency from a remote station to the basestation the base station re-broadcasts frames on another frequency Contention is foraccess to the base station
bull Frames can be sent at any time to the base stationndash If no acknowledgment (ACK) is received assume that the frame is lost andretransmit after waiting a random amount of timendash If more than one station broadcasts on the base station access frequency at the sametime ndash a collision ndash interference will destroy the frames
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 39
Random Access ALOHA (contrsquo)
bull The probability that no packets generated during the tima [-τ τ] is given by (nocollision)
Pr(X(t)=0) = (λ2τ)0 0 e-2λτ = e-2Lwith corresponding throughput
T = L e-2L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 05 05e 018
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 40
Random Access slotted ALOHA (1971)bull Improvement Time is slotted and a packet can only be transmitted at the beginning of
one slot Thus it can reduce the collision duration
bull The probability that no packets generated during the time [0 τ] is given by (nocollision)
Pr(X(t)=0) = (λτ)0 0 e-λτ = e-Lwith corresponding throughput T = L e-L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 1 e 037
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 41
bull Slotted Aloha has double the maximumthroughput as pure Aloha and achievesthis maximum at a higher offered loadThe effective data rate is still less than40 of the raw transmission rate
bull Aloha does not listen to the carrier beforetransmission more sophisticatedtechniques are needed to increaseefficiency
bull Further problem the delayndashDelay
ndashThe mean packet delay
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
( ) ( )delayPr 1 e ekL Lkτ minus minus= = minus
( )delay1
1 e e ekL L L
kMPD E kτ
infinminus minus
=
= = minus =sum
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 42
Random Access Carrier Sense Multiple Accessbull CSMA (Carrier Sense Multiple Access) listen to the carrier before transmission
and transmits if channel is idlebull Detection delay and propagation delay are two important parameters
ndash Detection delay time required to sense the carrier and decide if it is idle or busyndash Propagation delay distancespeed_of_ligth The time required for bit to travel
from transmitter to the receiver
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 43
Random Access CSMA variations
Ad hoc Sensor Networks Multiple channel access
1-persistent CSMA Non-persistent CSMA
There will always be a collision iftwo nodes want to retransmit(usually you stop transmissionattempts after few tries)
Random backoff reduces probabilityof collisions Waste idle time if thebackoff time is too long
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 44
Random Access CSMA variations (contrsquo)
Ad hoc Sensor Networks Multiple channel access
p-persistent CSMA CSMA CD (Collision Detection)
A good tradeoff between non-persistent and 1-persistent CSMA
Same with CSMA however a stationalso listen to the carrier whiletransmitting to see if the transmissioncollides with someone elsetransmission It can be used in listen-while-talk capable channels (fullduplex) In single radio channels thetransmission need to be interrupted inorder to sense the channel
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 45
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 46
Random Access CSMA (contrsquo)bull How to select probability p
ndash Assume that M nodes have a packet to send and the medium is busy then Mp is theexpected number of nodes that will attempt to transmit once the medium becomesidle If Mp gt 1 then a collision is expected to occur Therefore the network mustmake sure that Mp lt 1 to avoid collision where M is the maximum number of nodesthat can be active at a time
bull Problems1 The mean packet delay increases exponently with increasing offered load2 CSMA protocols sense the carrier but sensing the carrier does not always releasestrue information about the status of the wireless channel There are two problems thatare unique to wireless channels (different than wireline channels)ndash Hidden terminal problemndash Exposed terminal problem
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 47
Hidden terminal problembull A sends to B C cannot receive Abull C wants to send to B C senses a
ldquofreerdquo medium (CS fails)bull collision at B A cannot receive the
collision (CD fails)bull A is ldquohiddenrdquo for C
Ad hoc Sensor Networks Multiple channel access
Random Access hidden and exposed terminal problemExposed terminal problem
bull B sends to A C wants to send to Dbull C has to wait CS signals a medium
in usebull since A is outside the radio range of
C waiting is not necessarybull C is ldquoexposedrdquo to B
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 48
Random Access MACAWhen a station wants to transmit databull It sends an RTS (Ready-to-Send) packet to the intended receiver
ndash The RTS packet contains the length of the data that needs to be transmittedndash Any station other than the intended recipient hearing RTS defers transmission for atime duration equal to the end of the corresponding CTS reception
bull The receiver sends back CTS (Clear-to-Send) packet back to sender if it isavailable to receivendash The CTS packet contains the length of the data that original sender wants totransmitndash Any station other than the original RTS sender hearing CTS defers transmissionuntil the data is sent
bull The original sender upon reception of the CTS starts transmitting
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 49
Hidden terminal problem
Ad hoc Sensor Networks Multiple channel access
Random Access MACA (contrsquo)Exposed terminal problem
Waiting time of node X is muchsmaller than waiting time of node C
bull C defers transmission upon hearing BrsquosRTS until B could get CTS from Abull After that C can start transmission to DFor that it first sends an RTSbull C is not longer exposed to the datatransmission of B
Problem It does not address the collision of RTS packet
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 50
Random Access reliabilitybull Wireless links are prone to have errors High packet loss rate detrimental to
transport-layer performance Mechanisms needed to reduce packet loss rateexperienced by upper layers
bull When node B receives a data packet from node A node B sends anAcknowledgement (Ack) This approach adopted in many protocols
bull If node A fails to receive an Ack it will retransmit the packet
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 51
Random Access MACAW (CSMACA) bull Physical carrier sense and virtual carrier sense using Network Allocation
Vector (NAV)ndash NAV is updated based on overheard RTSCTSDATAACK packets each of whichspecified duration of a pending transmissionndash Nodes stay silent when carrier sensed (physicalvirtual)
bull Backoff intervals used to reduce collision probabilityndash When transmitting a packet choose a backoff interval in the range [0w] where wis contention windowndash Count down the backoff interval when channel is idlendash Count down is suspended if channel becomes busyndash When backoff interval reaches 0 transmit RTS
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 52
Random Access CSMACA (contrsquo) bull The time spent counting down backoff intervals is a part of MAC overhead
ndash Choosing a large w leads to large backoff intervals and can result in larger overheadndash Choosing a small w leads to a larger number of collisions (when two nodes countdown to 0 simultaneously)ndash Since the number of nodes attempting to transmit simultaneously may change withtime some mechanism to manage contention is needed
bull IEEE 80211 DCF contention window w is chosen dynamically depending oncollision occurrence (binary exponential backoff)ndash When a node fails to receive CTS in response to its RTS it increases the contentionwindowndash w is doubled (up to an upper bound)ndash When a node successfully completes a data transfer it restores w to wmin
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 53
Random Access fairnessbull Simplest definition of fairness all nodes should receive equal bandwidthbull An example of unfairness
ndash Assume that initially A and B both choose a backoff interval in range [031] buttheir RTSs collidendash Nodes A and B then choose from range [063]ndash Node A chooses 4 slots and B choose 60 slotsndash After A transmits a packet it next chooses from range [031]ndash It is possible that A may transmit several packets before B transmits its first packet
Ad hoc Sensor Networks Multiple channel access
Unfairness occurs when one node has backed off much more than some other node
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 54
Random Access fairness (contrsquo) MACAW solution for fairnessbull When a node transmits a packet it appends value w to the packet all nodes
hearing value w use it for their future transmission attemptsbull Since w is an indication of the level of congestion in the vicinity of a specific
receiver node MACAW proposes maintaining w independently for eachreceiver
bull Using per-receiver wi is particularly useful in multi-hop environments sincecongestion level at different receivers can be very different
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 55
Random Access energy efficiency bull Since many mobile hosts are operated by batteries MAC
protocols which conserve energy are of interestbull Two approaches to reduce energy consumption
ndash Power save turn off wireless interface when desirablendash Power control reduce transmit power
bull Power control has some more potential benefitsndash Reduced interference (in ad hoc networks) eg it improves ALOHAefficiency where user with high power can capture a packet even if there is acollisionndash Increased spatial reuse (in infrastructured wireless networks) eg it is usedin CDMA to maintain target SIR of voice and data users or generally it canbe used to maintain target SIR for different user classes (admission control)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 56
Random Access power control bull When C transmits to D at a high power level B cannot receive Arsquos
transmission due to interference from Cbull If C reduces transmit power it can still communicate with D
ndash Reduces energy consumption at node Cndash Allows B to receive Arsquos transmission (spatial reuse)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 57
bull A special wireless ad hoc networkndash Large number of nodesndash Battery poweredndash Topology and density changendash Nodes for a common taskndash In-network data processing
bull Sensor-net applicationsndash Sensor-triggered bursty trafficndash Can often tolerate some delay
Ad hoc Sensor Networks Multiple channel access
Characteristics of Sensor Network
Scalabilty and self configurationEnergy efficiencyAdaptivityFairness not importantLatency
AdaptivityEnergy-delay tradoff
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 58
Ad hoc Sensor Networks Multiple channel access
Wireless sensor node and sensor network
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 59
MAC for WSN attributes bull Attributes
ndash Collision avoidance (Basic task)ndash Energy efficiency (most primary in WSN)ndash Scalability and adaptivityndash Channel utilizationndash Latencyndash Throughputndash Fairness
bull What causes energy wastendash Collisionsndash Control packet overheadndash Overhearing unnecessary trafficndash Long idle time (it consumes 50-100 of the power)
Ad hoc Sensor Networks Multiple channel access
Primary
Secondary
Dominant factor
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 60
MAC for WSN
bull Contention-based protocols need to work hard in all directions for energysavingsndash Reduce idle listening ndash support low duty cyclendash Better collision avoidancendash Reduce control overheadndash Avoid unnecessary overhearing
Ad hoc Sensor Networks Multiple channel access
Scheduled Protocols Contention ProtocolsCollisions No YesEnergy efficiency Good BadScalability and adaptivity Bad GoodMulti-hop communication Difficult EasyTime synchronization Strict Loose or not required
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 61
MAC for WSN classification
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 62
MAC for WSN SMAC (2002)bull It has been developed by Ye Heidemann and Estrinbull Tradeoffs between (latency fairness) and energybull Major components in S-MAC
ndash Periodic listen and sleep turn off radio when sleeping the reduced dutycycle is ~ 10 (eg 120ms on 12s off)ndash Collision avoidance is based on contention similarly to IEEE 80211 adhoc modendash Overhearing avoidance the node is sleeping when neighbors talkndash Massage passing long message is fragmented amp sent in burst operationwith extended Tx time and re-transmit occurs immediately
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 63
Summarybull The algorithms and protocols that enable sharing by multiple users and
controls the access to the wireless channel from different users are calledMAC
bull There are two types of wireless networks Infrastructured wireless networksand ad hoc wireless networks
bull We also want our wireless system to have high user capacity and we canidentify 10 methods for increasing it
bull There are several MACs with no absolute advantage it should be tailoredto the system requirements
bull There are primary concerns for designing a WSN MAC (collisionavoidance energy efficiency scalability and adaptivity) while thesecondary concerns are as follows (channel utilization latency throughputfairness)
bull Next lecture Routing protocols
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 38
Random Access ALOHA (1970)bull ALOHA was developed for a wireless system at the University of Hawaii
(Abramson et al)ndash Multiple remote stations plus one base stationndash Frame transmissions are made at one frequency from a remote station to the basestation the base station re-broadcasts frames on another frequency Contention is foraccess to the base station
bull Frames can be sent at any time to the base stationndash If no acknowledgment (ACK) is received assume that the frame is lost andretransmit after waiting a random amount of timendash If more than one station broadcasts on the base station access frequency at the sametime ndash a collision ndash interference will destroy the frames
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 39
Random Access ALOHA (contrsquo)
bull The probability that no packets generated during the tima [-τ τ] is given by (nocollision)
Pr(X(t)=0) = (λ2τ)0 0 e-2λτ = e-2Lwith corresponding throughput
T = L e-2L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 05 05e 018
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 40
Random Access slotted ALOHA (1971)bull Improvement Time is slotted and a packet can only be transmitted at the beginning of
one slot Thus it can reduce the collision duration
bull The probability that no packets generated during the time [0 τ] is given by (nocollision)
Pr(X(t)=0) = (λτ)0 0 e-λτ = e-Lwith corresponding throughput T = L e-L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 1 e 037
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 41
bull Slotted Aloha has double the maximumthroughput as pure Aloha and achievesthis maximum at a higher offered loadThe effective data rate is still less than40 of the raw transmission rate
bull Aloha does not listen to the carrier beforetransmission more sophisticatedtechniques are needed to increaseefficiency
bull Further problem the delayndashDelay
ndashThe mean packet delay
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
( ) ( )delayPr 1 e ekL Lkτ minus minus= = minus
( )delay1
1 e e ekL L L
kMPD E kτ
infinminus minus
=
= = minus =sum
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 42
Random Access Carrier Sense Multiple Accessbull CSMA (Carrier Sense Multiple Access) listen to the carrier before transmission
and transmits if channel is idlebull Detection delay and propagation delay are two important parameters
ndash Detection delay time required to sense the carrier and decide if it is idle or busyndash Propagation delay distancespeed_of_ligth The time required for bit to travel
from transmitter to the receiver
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 43
Random Access CSMA variations
Ad hoc Sensor Networks Multiple channel access
1-persistent CSMA Non-persistent CSMA
There will always be a collision iftwo nodes want to retransmit(usually you stop transmissionattempts after few tries)
Random backoff reduces probabilityof collisions Waste idle time if thebackoff time is too long
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 44
Random Access CSMA variations (contrsquo)
Ad hoc Sensor Networks Multiple channel access
p-persistent CSMA CSMA CD (Collision Detection)
A good tradeoff between non-persistent and 1-persistent CSMA
Same with CSMA however a stationalso listen to the carrier whiletransmitting to see if the transmissioncollides with someone elsetransmission It can be used in listen-while-talk capable channels (fullduplex) In single radio channels thetransmission need to be interrupted inorder to sense the channel
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 45
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 46
Random Access CSMA (contrsquo)bull How to select probability p
ndash Assume that M nodes have a packet to send and the medium is busy then Mp is theexpected number of nodes that will attempt to transmit once the medium becomesidle If Mp gt 1 then a collision is expected to occur Therefore the network mustmake sure that Mp lt 1 to avoid collision where M is the maximum number of nodesthat can be active at a time
bull Problems1 The mean packet delay increases exponently with increasing offered load2 CSMA protocols sense the carrier but sensing the carrier does not always releasestrue information about the status of the wireless channel There are two problems thatare unique to wireless channels (different than wireline channels)ndash Hidden terminal problemndash Exposed terminal problem
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 47
Hidden terminal problembull A sends to B C cannot receive Abull C wants to send to B C senses a
ldquofreerdquo medium (CS fails)bull collision at B A cannot receive the
collision (CD fails)bull A is ldquohiddenrdquo for C
Ad hoc Sensor Networks Multiple channel access
Random Access hidden and exposed terminal problemExposed terminal problem
bull B sends to A C wants to send to Dbull C has to wait CS signals a medium
in usebull since A is outside the radio range of
C waiting is not necessarybull C is ldquoexposedrdquo to B
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 48
Random Access MACAWhen a station wants to transmit databull It sends an RTS (Ready-to-Send) packet to the intended receiver
ndash The RTS packet contains the length of the data that needs to be transmittedndash Any station other than the intended recipient hearing RTS defers transmission for atime duration equal to the end of the corresponding CTS reception
bull The receiver sends back CTS (Clear-to-Send) packet back to sender if it isavailable to receivendash The CTS packet contains the length of the data that original sender wants totransmitndash Any station other than the original RTS sender hearing CTS defers transmissionuntil the data is sent
bull The original sender upon reception of the CTS starts transmitting
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 49
Hidden terminal problem
Ad hoc Sensor Networks Multiple channel access
Random Access MACA (contrsquo)Exposed terminal problem
Waiting time of node X is muchsmaller than waiting time of node C
bull C defers transmission upon hearing BrsquosRTS until B could get CTS from Abull After that C can start transmission to DFor that it first sends an RTSbull C is not longer exposed to the datatransmission of B
Problem It does not address the collision of RTS packet
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 50
Random Access reliabilitybull Wireless links are prone to have errors High packet loss rate detrimental to
transport-layer performance Mechanisms needed to reduce packet loss rateexperienced by upper layers
bull When node B receives a data packet from node A node B sends anAcknowledgement (Ack) This approach adopted in many protocols
bull If node A fails to receive an Ack it will retransmit the packet
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 51
Random Access MACAW (CSMACA) bull Physical carrier sense and virtual carrier sense using Network Allocation
Vector (NAV)ndash NAV is updated based on overheard RTSCTSDATAACK packets each of whichspecified duration of a pending transmissionndash Nodes stay silent when carrier sensed (physicalvirtual)
bull Backoff intervals used to reduce collision probabilityndash When transmitting a packet choose a backoff interval in the range [0w] where wis contention windowndash Count down the backoff interval when channel is idlendash Count down is suspended if channel becomes busyndash When backoff interval reaches 0 transmit RTS
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 52
Random Access CSMACA (contrsquo) bull The time spent counting down backoff intervals is a part of MAC overhead
ndash Choosing a large w leads to large backoff intervals and can result in larger overheadndash Choosing a small w leads to a larger number of collisions (when two nodes countdown to 0 simultaneously)ndash Since the number of nodes attempting to transmit simultaneously may change withtime some mechanism to manage contention is needed
bull IEEE 80211 DCF contention window w is chosen dynamically depending oncollision occurrence (binary exponential backoff)ndash When a node fails to receive CTS in response to its RTS it increases the contentionwindowndash w is doubled (up to an upper bound)ndash When a node successfully completes a data transfer it restores w to wmin
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 53
Random Access fairnessbull Simplest definition of fairness all nodes should receive equal bandwidthbull An example of unfairness
ndash Assume that initially A and B both choose a backoff interval in range [031] buttheir RTSs collidendash Nodes A and B then choose from range [063]ndash Node A chooses 4 slots and B choose 60 slotsndash After A transmits a packet it next chooses from range [031]ndash It is possible that A may transmit several packets before B transmits its first packet
Ad hoc Sensor Networks Multiple channel access
Unfairness occurs when one node has backed off much more than some other node
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 54
Random Access fairness (contrsquo) MACAW solution for fairnessbull When a node transmits a packet it appends value w to the packet all nodes
hearing value w use it for their future transmission attemptsbull Since w is an indication of the level of congestion in the vicinity of a specific
receiver node MACAW proposes maintaining w independently for eachreceiver
bull Using per-receiver wi is particularly useful in multi-hop environments sincecongestion level at different receivers can be very different
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 55
Random Access energy efficiency bull Since many mobile hosts are operated by batteries MAC
protocols which conserve energy are of interestbull Two approaches to reduce energy consumption
ndash Power save turn off wireless interface when desirablendash Power control reduce transmit power
bull Power control has some more potential benefitsndash Reduced interference (in ad hoc networks) eg it improves ALOHAefficiency where user with high power can capture a packet even if there is acollisionndash Increased spatial reuse (in infrastructured wireless networks) eg it is usedin CDMA to maintain target SIR of voice and data users or generally it canbe used to maintain target SIR for different user classes (admission control)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 56
Random Access power control bull When C transmits to D at a high power level B cannot receive Arsquos
transmission due to interference from Cbull If C reduces transmit power it can still communicate with D
ndash Reduces energy consumption at node Cndash Allows B to receive Arsquos transmission (spatial reuse)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 57
bull A special wireless ad hoc networkndash Large number of nodesndash Battery poweredndash Topology and density changendash Nodes for a common taskndash In-network data processing
bull Sensor-net applicationsndash Sensor-triggered bursty trafficndash Can often tolerate some delay
Ad hoc Sensor Networks Multiple channel access
Characteristics of Sensor Network
Scalabilty and self configurationEnergy efficiencyAdaptivityFairness not importantLatency
AdaptivityEnergy-delay tradoff
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 58
Ad hoc Sensor Networks Multiple channel access
Wireless sensor node and sensor network
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 59
MAC for WSN attributes bull Attributes
ndash Collision avoidance (Basic task)ndash Energy efficiency (most primary in WSN)ndash Scalability and adaptivityndash Channel utilizationndash Latencyndash Throughputndash Fairness
bull What causes energy wastendash Collisionsndash Control packet overheadndash Overhearing unnecessary trafficndash Long idle time (it consumes 50-100 of the power)
Ad hoc Sensor Networks Multiple channel access
Primary
Secondary
Dominant factor
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 60
MAC for WSN
bull Contention-based protocols need to work hard in all directions for energysavingsndash Reduce idle listening ndash support low duty cyclendash Better collision avoidancendash Reduce control overheadndash Avoid unnecessary overhearing
Ad hoc Sensor Networks Multiple channel access
Scheduled Protocols Contention ProtocolsCollisions No YesEnergy efficiency Good BadScalability and adaptivity Bad GoodMulti-hop communication Difficult EasyTime synchronization Strict Loose or not required
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 61
MAC for WSN classification
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 62
MAC for WSN SMAC (2002)bull It has been developed by Ye Heidemann and Estrinbull Tradeoffs between (latency fairness) and energybull Major components in S-MAC
ndash Periodic listen and sleep turn off radio when sleeping the reduced dutycycle is ~ 10 (eg 120ms on 12s off)ndash Collision avoidance is based on contention similarly to IEEE 80211 adhoc modendash Overhearing avoidance the node is sleeping when neighbors talkndash Massage passing long message is fragmented amp sent in burst operationwith extended Tx time and re-transmit occurs immediately
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 63
Summarybull The algorithms and protocols that enable sharing by multiple users and
controls the access to the wireless channel from different users are calledMAC
bull There are two types of wireless networks Infrastructured wireless networksand ad hoc wireless networks
bull We also want our wireless system to have high user capacity and we canidentify 10 methods for increasing it
bull There are several MACs with no absolute advantage it should be tailoredto the system requirements
bull There are primary concerns for designing a WSN MAC (collisionavoidance energy efficiency scalability and adaptivity) while thesecondary concerns are as follows (channel utilization latency throughputfairness)
bull Next lecture Routing protocols
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 39
Random Access ALOHA (contrsquo)
bull The probability that no packets generated during the tima [-τ τ] is given by (nocollision)
Pr(X(t)=0) = (λ2τ)0 0 e-2λτ = e-2Lwith corresponding throughput
T = L e-2L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 05 05e 018
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 40
Random Access slotted ALOHA (1971)bull Improvement Time is slotted and a packet can only be transmitted at the beginning of
one slot Thus it can reduce the collision duration
bull The probability that no packets generated during the time [0 τ] is given by (nocollision)
Pr(X(t)=0) = (λτ)0 0 e-λτ = e-Lwith corresponding throughput T = L e-L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 1 e 037
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 41
bull Slotted Aloha has double the maximumthroughput as pure Aloha and achievesthis maximum at a higher offered loadThe effective data rate is still less than40 of the raw transmission rate
bull Aloha does not listen to the carrier beforetransmission more sophisticatedtechniques are needed to increaseefficiency
bull Further problem the delayndashDelay
ndashThe mean packet delay
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
( ) ( )delayPr 1 e ekL Lkτ minus minus= = minus
( )delay1
1 e e ekL L L
kMPD E kτ
infinminus minus
=
= = minus =sum
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 42
Random Access Carrier Sense Multiple Accessbull CSMA (Carrier Sense Multiple Access) listen to the carrier before transmission
and transmits if channel is idlebull Detection delay and propagation delay are two important parameters
ndash Detection delay time required to sense the carrier and decide if it is idle or busyndash Propagation delay distancespeed_of_ligth The time required for bit to travel
from transmitter to the receiver
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 43
Random Access CSMA variations
Ad hoc Sensor Networks Multiple channel access
1-persistent CSMA Non-persistent CSMA
There will always be a collision iftwo nodes want to retransmit(usually you stop transmissionattempts after few tries)
Random backoff reduces probabilityof collisions Waste idle time if thebackoff time is too long
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 44
Random Access CSMA variations (contrsquo)
Ad hoc Sensor Networks Multiple channel access
p-persistent CSMA CSMA CD (Collision Detection)
A good tradeoff between non-persistent and 1-persistent CSMA
Same with CSMA however a stationalso listen to the carrier whiletransmitting to see if the transmissioncollides with someone elsetransmission It can be used in listen-while-talk capable channels (fullduplex) In single radio channels thetransmission need to be interrupted inorder to sense the channel
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 45
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 46
Random Access CSMA (contrsquo)bull How to select probability p
ndash Assume that M nodes have a packet to send and the medium is busy then Mp is theexpected number of nodes that will attempt to transmit once the medium becomesidle If Mp gt 1 then a collision is expected to occur Therefore the network mustmake sure that Mp lt 1 to avoid collision where M is the maximum number of nodesthat can be active at a time
bull Problems1 The mean packet delay increases exponently with increasing offered load2 CSMA protocols sense the carrier but sensing the carrier does not always releasestrue information about the status of the wireless channel There are two problems thatare unique to wireless channels (different than wireline channels)ndash Hidden terminal problemndash Exposed terminal problem
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 47
Hidden terminal problembull A sends to B C cannot receive Abull C wants to send to B C senses a
ldquofreerdquo medium (CS fails)bull collision at B A cannot receive the
collision (CD fails)bull A is ldquohiddenrdquo for C
Ad hoc Sensor Networks Multiple channel access
Random Access hidden and exposed terminal problemExposed terminal problem
bull B sends to A C wants to send to Dbull C has to wait CS signals a medium
in usebull since A is outside the radio range of
C waiting is not necessarybull C is ldquoexposedrdquo to B
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 48
Random Access MACAWhen a station wants to transmit databull It sends an RTS (Ready-to-Send) packet to the intended receiver
ndash The RTS packet contains the length of the data that needs to be transmittedndash Any station other than the intended recipient hearing RTS defers transmission for atime duration equal to the end of the corresponding CTS reception
bull The receiver sends back CTS (Clear-to-Send) packet back to sender if it isavailable to receivendash The CTS packet contains the length of the data that original sender wants totransmitndash Any station other than the original RTS sender hearing CTS defers transmissionuntil the data is sent
bull The original sender upon reception of the CTS starts transmitting
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 49
Hidden terminal problem
Ad hoc Sensor Networks Multiple channel access
Random Access MACA (contrsquo)Exposed terminal problem
Waiting time of node X is muchsmaller than waiting time of node C
bull C defers transmission upon hearing BrsquosRTS until B could get CTS from Abull After that C can start transmission to DFor that it first sends an RTSbull C is not longer exposed to the datatransmission of B
Problem It does not address the collision of RTS packet
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 50
Random Access reliabilitybull Wireless links are prone to have errors High packet loss rate detrimental to
transport-layer performance Mechanisms needed to reduce packet loss rateexperienced by upper layers
bull When node B receives a data packet from node A node B sends anAcknowledgement (Ack) This approach adopted in many protocols
bull If node A fails to receive an Ack it will retransmit the packet
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 51
Random Access MACAW (CSMACA) bull Physical carrier sense and virtual carrier sense using Network Allocation
Vector (NAV)ndash NAV is updated based on overheard RTSCTSDATAACK packets each of whichspecified duration of a pending transmissionndash Nodes stay silent when carrier sensed (physicalvirtual)
bull Backoff intervals used to reduce collision probabilityndash When transmitting a packet choose a backoff interval in the range [0w] where wis contention windowndash Count down the backoff interval when channel is idlendash Count down is suspended if channel becomes busyndash When backoff interval reaches 0 transmit RTS
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 52
Random Access CSMACA (contrsquo) bull The time spent counting down backoff intervals is a part of MAC overhead
ndash Choosing a large w leads to large backoff intervals and can result in larger overheadndash Choosing a small w leads to a larger number of collisions (when two nodes countdown to 0 simultaneously)ndash Since the number of nodes attempting to transmit simultaneously may change withtime some mechanism to manage contention is needed
bull IEEE 80211 DCF contention window w is chosen dynamically depending oncollision occurrence (binary exponential backoff)ndash When a node fails to receive CTS in response to its RTS it increases the contentionwindowndash w is doubled (up to an upper bound)ndash When a node successfully completes a data transfer it restores w to wmin
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 53
Random Access fairnessbull Simplest definition of fairness all nodes should receive equal bandwidthbull An example of unfairness
ndash Assume that initially A and B both choose a backoff interval in range [031] buttheir RTSs collidendash Nodes A and B then choose from range [063]ndash Node A chooses 4 slots and B choose 60 slotsndash After A transmits a packet it next chooses from range [031]ndash It is possible that A may transmit several packets before B transmits its first packet
Ad hoc Sensor Networks Multiple channel access
Unfairness occurs when one node has backed off much more than some other node
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 54
Random Access fairness (contrsquo) MACAW solution for fairnessbull When a node transmits a packet it appends value w to the packet all nodes
hearing value w use it for their future transmission attemptsbull Since w is an indication of the level of congestion in the vicinity of a specific
receiver node MACAW proposes maintaining w independently for eachreceiver
bull Using per-receiver wi is particularly useful in multi-hop environments sincecongestion level at different receivers can be very different
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 55
Random Access energy efficiency bull Since many mobile hosts are operated by batteries MAC
protocols which conserve energy are of interestbull Two approaches to reduce energy consumption
ndash Power save turn off wireless interface when desirablendash Power control reduce transmit power
bull Power control has some more potential benefitsndash Reduced interference (in ad hoc networks) eg it improves ALOHAefficiency where user with high power can capture a packet even if there is acollisionndash Increased spatial reuse (in infrastructured wireless networks) eg it is usedin CDMA to maintain target SIR of voice and data users or generally it canbe used to maintain target SIR for different user classes (admission control)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 56
Random Access power control bull When C transmits to D at a high power level B cannot receive Arsquos
transmission due to interference from Cbull If C reduces transmit power it can still communicate with D
ndash Reduces energy consumption at node Cndash Allows B to receive Arsquos transmission (spatial reuse)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 57
bull A special wireless ad hoc networkndash Large number of nodesndash Battery poweredndash Topology and density changendash Nodes for a common taskndash In-network data processing
bull Sensor-net applicationsndash Sensor-triggered bursty trafficndash Can often tolerate some delay
Ad hoc Sensor Networks Multiple channel access
Characteristics of Sensor Network
Scalabilty and self configurationEnergy efficiencyAdaptivityFairness not importantLatency
AdaptivityEnergy-delay tradoff
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 58
Ad hoc Sensor Networks Multiple channel access
Wireless sensor node and sensor network
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 59
MAC for WSN attributes bull Attributes
ndash Collision avoidance (Basic task)ndash Energy efficiency (most primary in WSN)ndash Scalability and adaptivityndash Channel utilizationndash Latencyndash Throughputndash Fairness
bull What causes energy wastendash Collisionsndash Control packet overheadndash Overhearing unnecessary trafficndash Long idle time (it consumes 50-100 of the power)
Ad hoc Sensor Networks Multiple channel access
Primary
Secondary
Dominant factor
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 60
MAC for WSN
bull Contention-based protocols need to work hard in all directions for energysavingsndash Reduce idle listening ndash support low duty cyclendash Better collision avoidancendash Reduce control overheadndash Avoid unnecessary overhearing
Ad hoc Sensor Networks Multiple channel access
Scheduled Protocols Contention ProtocolsCollisions No YesEnergy efficiency Good BadScalability and adaptivity Bad GoodMulti-hop communication Difficult EasyTime synchronization Strict Loose or not required
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 61
MAC for WSN classification
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 62
MAC for WSN SMAC (2002)bull It has been developed by Ye Heidemann and Estrinbull Tradeoffs between (latency fairness) and energybull Major components in S-MAC
ndash Periodic listen and sleep turn off radio when sleeping the reduced dutycycle is ~ 10 (eg 120ms on 12s off)ndash Collision avoidance is based on contention similarly to IEEE 80211 adhoc modendash Overhearing avoidance the node is sleeping when neighbors talkndash Massage passing long message is fragmented amp sent in burst operationwith extended Tx time and re-transmit occurs immediately
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 63
Summarybull The algorithms and protocols that enable sharing by multiple users and
controls the access to the wireless channel from different users are calledMAC
bull There are two types of wireless networks Infrastructured wireless networksand ad hoc wireless networks
bull We also want our wireless system to have high user capacity and we canidentify 10 methods for increasing it
bull There are several MACs with no absolute advantage it should be tailoredto the system requirements
bull There are primary concerns for designing a WSN MAC (collisionavoidance energy efficiency scalability and adaptivity) while thesecondary concerns are as follows (channel utilization latency throughputfairness)
bull Next lecture Routing protocols
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 40
Random Access slotted ALOHA (1971)bull Improvement Time is slotted and a packet can only be transmitted at the beginning of
one slot Thus it can reduce the collision duration
bull The probability that no packets generated during the time [0 τ] is given by (nocollision)
Pr(X(t)=0) = (λτ)0 0 e-λτ = e-Lwith corresponding throughput T = L e-L
bull The maximum throughput
Ad hoc Sensor Networks Multiple channel access
max
1max max0 1 e 037
L L
dT L TdL
minus
=
= rarr = rarr = asymp
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 41
bull Slotted Aloha has double the maximumthroughput as pure Aloha and achievesthis maximum at a higher offered loadThe effective data rate is still less than40 of the raw transmission rate
bull Aloha does not listen to the carrier beforetransmission more sophisticatedtechniques are needed to increaseefficiency
bull Further problem the delayndashDelay
ndashThe mean packet delay
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
( ) ( )delayPr 1 e ekL Lkτ minus minus= = minus
( )delay1
1 e e ekL L L
kMPD E kτ
infinminus minus
=
= = minus =sum
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 42
Random Access Carrier Sense Multiple Accessbull CSMA (Carrier Sense Multiple Access) listen to the carrier before transmission
and transmits if channel is idlebull Detection delay and propagation delay are two important parameters
ndash Detection delay time required to sense the carrier and decide if it is idle or busyndash Propagation delay distancespeed_of_ligth The time required for bit to travel
from transmitter to the receiver
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 43
Random Access CSMA variations
Ad hoc Sensor Networks Multiple channel access
1-persistent CSMA Non-persistent CSMA
There will always be a collision iftwo nodes want to retransmit(usually you stop transmissionattempts after few tries)
Random backoff reduces probabilityof collisions Waste idle time if thebackoff time is too long
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 44
Random Access CSMA variations (contrsquo)
Ad hoc Sensor Networks Multiple channel access
p-persistent CSMA CSMA CD (Collision Detection)
A good tradeoff between non-persistent and 1-persistent CSMA
Same with CSMA however a stationalso listen to the carrier whiletransmitting to see if the transmissioncollides with someone elsetransmission It can be used in listen-while-talk capable channels (fullduplex) In single radio channels thetransmission need to be interrupted inorder to sense the channel
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 45
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 46
Random Access CSMA (contrsquo)bull How to select probability p
ndash Assume that M nodes have a packet to send and the medium is busy then Mp is theexpected number of nodes that will attempt to transmit once the medium becomesidle If Mp gt 1 then a collision is expected to occur Therefore the network mustmake sure that Mp lt 1 to avoid collision where M is the maximum number of nodesthat can be active at a time
bull Problems1 The mean packet delay increases exponently with increasing offered load2 CSMA protocols sense the carrier but sensing the carrier does not always releasestrue information about the status of the wireless channel There are two problems thatare unique to wireless channels (different than wireline channels)ndash Hidden terminal problemndash Exposed terminal problem
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 47
Hidden terminal problembull A sends to B C cannot receive Abull C wants to send to B C senses a
ldquofreerdquo medium (CS fails)bull collision at B A cannot receive the
collision (CD fails)bull A is ldquohiddenrdquo for C
Ad hoc Sensor Networks Multiple channel access
Random Access hidden and exposed terminal problemExposed terminal problem
bull B sends to A C wants to send to Dbull C has to wait CS signals a medium
in usebull since A is outside the radio range of
C waiting is not necessarybull C is ldquoexposedrdquo to B
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 48
Random Access MACAWhen a station wants to transmit databull It sends an RTS (Ready-to-Send) packet to the intended receiver
ndash The RTS packet contains the length of the data that needs to be transmittedndash Any station other than the intended recipient hearing RTS defers transmission for atime duration equal to the end of the corresponding CTS reception
bull The receiver sends back CTS (Clear-to-Send) packet back to sender if it isavailable to receivendash The CTS packet contains the length of the data that original sender wants totransmitndash Any station other than the original RTS sender hearing CTS defers transmissionuntil the data is sent
bull The original sender upon reception of the CTS starts transmitting
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 49
Hidden terminal problem
Ad hoc Sensor Networks Multiple channel access
Random Access MACA (contrsquo)Exposed terminal problem
Waiting time of node X is muchsmaller than waiting time of node C
bull C defers transmission upon hearing BrsquosRTS until B could get CTS from Abull After that C can start transmission to DFor that it first sends an RTSbull C is not longer exposed to the datatransmission of B
Problem It does not address the collision of RTS packet
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 50
Random Access reliabilitybull Wireless links are prone to have errors High packet loss rate detrimental to
transport-layer performance Mechanisms needed to reduce packet loss rateexperienced by upper layers
bull When node B receives a data packet from node A node B sends anAcknowledgement (Ack) This approach adopted in many protocols
bull If node A fails to receive an Ack it will retransmit the packet
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 51
Random Access MACAW (CSMACA) bull Physical carrier sense and virtual carrier sense using Network Allocation
Vector (NAV)ndash NAV is updated based on overheard RTSCTSDATAACK packets each of whichspecified duration of a pending transmissionndash Nodes stay silent when carrier sensed (physicalvirtual)
bull Backoff intervals used to reduce collision probabilityndash When transmitting a packet choose a backoff interval in the range [0w] where wis contention windowndash Count down the backoff interval when channel is idlendash Count down is suspended if channel becomes busyndash When backoff interval reaches 0 transmit RTS
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 52
Random Access CSMACA (contrsquo) bull The time spent counting down backoff intervals is a part of MAC overhead
ndash Choosing a large w leads to large backoff intervals and can result in larger overheadndash Choosing a small w leads to a larger number of collisions (when two nodes countdown to 0 simultaneously)ndash Since the number of nodes attempting to transmit simultaneously may change withtime some mechanism to manage contention is needed
bull IEEE 80211 DCF contention window w is chosen dynamically depending oncollision occurrence (binary exponential backoff)ndash When a node fails to receive CTS in response to its RTS it increases the contentionwindowndash w is doubled (up to an upper bound)ndash When a node successfully completes a data transfer it restores w to wmin
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 53
Random Access fairnessbull Simplest definition of fairness all nodes should receive equal bandwidthbull An example of unfairness
ndash Assume that initially A and B both choose a backoff interval in range [031] buttheir RTSs collidendash Nodes A and B then choose from range [063]ndash Node A chooses 4 slots and B choose 60 slotsndash After A transmits a packet it next chooses from range [031]ndash It is possible that A may transmit several packets before B transmits its first packet
Ad hoc Sensor Networks Multiple channel access
Unfairness occurs when one node has backed off much more than some other node
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 54
Random Access fairness (contrsquo) MACAW solution for fairnessbull When a node transmits a packet it appends value w to the packet all nodes
hearing value w use it for their future transmission attemptsbull Since w is an indication of the level of congestion in the vicinity of a specific
receiver node MACAW proposes maintaining w independently for eachreceiver
bull Using per-receiver wi is particularly useful in multi-hop environments sincecongestion level at different receivers can be very different
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 55
Random Access energy efficiency bull Since many mobile hosts are operated by batteries MAC
protocols which conserve energy are of interestbull Two approaches to reduce energy consumption
ndash Power save turn off wireless interface when desirablendash Power control reduce transmit power
bull Power control has some more potential benefitsndash Reduced interference (in ad hoc networks) eg it improves ALOHAefficiency where user with high power can capture a packet even if there is acollisionndash Increased spatial reuse (in infrastructured wireless networks) eg it is usedin CDMA to maintain target SIR of voice and data users or generally it canbe used to maintain target SIR for different user classes (admission control)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 56
Random Access power control bull When C transmits to D at a high power level B cannot receive Arsquos
transmission due to interference from Cbull If C reduces transmit power it can still communicate with D
ndash Reduces energy consumption at node Cndash Allows B to receive Arsquos transmission (spatial reuse)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 57
bull A special wireless ad hoc networkndash Large number of nodesndash Battery poweredndash Topology and density changendash Nodes for a common taskndash In-network data processing
bull Sensor-net applicationsndash Sensor-triggered bursty trafficndash Can often tolerate some delay
Ad hoc Sensor Networks Multiple channel access
Characteristics of Sensor Network
Scalabilty and self configurationEnergy efficiencyAdaptivityFairness not importantLatency
AdaptivityEnergy-delay tradoff
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 58
Ad hoc Sensor Networks Multiple channel access
Wireless sensor node and sensor network
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 59
MAC for WSN attributes bull Attributes
ndash Collision avoidance (Basic task)ndash Energy efficiency (most primary in WSN)ndash Scalability and adaptivityndash Channel utilizationndash Latencyndash Throughputndash Fairness
bull What causes energy wastendash Collisionsndash Control packet overheadndash Overhearing unnecessary trafficndash Long idle time (it consumes 50-100 of the power)
Ad hoc Sensor Networks Multiple channel access
Primary
Secondary
Dominant factor
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 60
MAC for WSN
bull Contention-based protocols need to work hard in all directions for energysavingsndash Reduce idle listening ndash support low duty cyclendash Better collision avoidancendash Reduce control overheadndash Avoid unnecessary overhearing
Ad hoc Sensor Networks Multiple channel access
Scheduled Protocols Contention ProtocolsCollisions No YesEnergy efficiency Good BadScalability and adaptivity Bad GoodMulti-hop communication Difficult EasyTime synchronization Strict Loose or not required
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 61
MAC for WSN classification
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 62
MAC for WSN SMAC (2002)bull It has been developed by Ye Heidemann and Estrinbull Tradeoffs between (latency fairness) and energybull Major components in S-MAC
ndash Periodic listen and sleep turn off radio when sleeping the reduced dutycycle is ~ 10 (eg 120ms on 12s off)ndash Collision avoidance is based on contention similarly to IEEE 80211 adhoc modendash Overhearing avoidance the node is sleeping when neighbors talkndash Massage passing long message is fragmented amp sent in burst operationwith extended Tx time and re-transmit occurs immediately
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 63
Summarybull The algorithms and protocols that enable sharing by multiple users and
controls the access to the wireless channel from different users are calledMAC
bull There are two types of wireless networks Infrastructured wireless networksand ad hoc wireless networks
bull We also want our wireless system to have high user capacity and we canidentify 10 methods for increasing it
bull There are several MACs with no absolute advantage it should be tailoredto the system requirements
bull There are primary concerns for designing a WSN MAC (collisionavoidance energy efficiency scalability and adaptivity) while thesecondary concerns are as follows (channel utilization latency throughputfairness)
bull Next lecture Routing protocols
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 41
bull Slotted Aloha has double the maximumthroughput as pure Aloha and achievesthis maximum at a higher offered loadThe effective data rate is still less than40 of the raw transmission rate
bull Aloha does not listen to the carrier beforetransmission more sophisticatedtechniques are needed to increaseefficiency
bull Further problem the delayndashDelay
ndashThe mean packet delay
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
( ) ( )delayPr 1 e ekL Lkτ minus minus= = minus
( )delay1
1 e e ekL L L
kMPD E kτ
infinminus minus
=
= = minus =sum
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 42
Random Access Carrier Sense Multiple Accessbull CSMA (Carrier Sense Multiple Access) listen to the carrier before transmission
and transmits if channel is idlebull Detection delay and propagation delay are two important parameters
ndash Detection delay time required to sense the carrier and decide if it is idle or busyndash Propagation delay distancespeed_of_ligth The time required for bit to travel
from transmitter to the receiver
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 43
Random Access CSMA variations
Ad hoc Sensor Networks Multiple channel access
1-persistent CSMA Non-persistent CSMA
There will always be a collision iftwo nodes want to retransmit(usually you stop transmissionattempts after few tries)
Random backoff reduces probabilityof collisions Waste idle time if thebackoff time is too long
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 44
Random Access CSMA variations (contrsquo)
Ad hoc Sensor Networks Multiple channel access
p-persistent CSMA CSMA CD (Collision Detection)
A good tradeoff between non-persistent and 1-persistent CSMA
Same with CSMA however a stationalso listen to the carrier whiletransmitting to see if the transmissioncollides with someone elsetransmission It can be used in listen-while-talk capable channels (fullduplex) In single radio channels thetransmission need to be interrupted inorder to sense the channel
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 45
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 46
Random Access CSMA (contrsquo)bull How to select probability p
ndash Assume that M nodes have a packet to send and the medium is busy then Mp is theexpected number of nodes that will attempt to transmit once the medium becomesidle If Mp gt 1 then a collision is expected to occur Therefore the network mustmake sure that Mp lt 1 to avoid collision where M is the maximum number of nodesthat can be active at a time
bull Problems1 The mean packet delay increases exponently with increasing offered load2 CSMA protocols sense the carrier but sensing the carrier does not always releasestrue information about the status of the wireless channel There are two problems thatare unique to wireless channels (different than wireline channels)ndash Hidden terminal problemndash Exposed terminal problem
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 47
Hidden terminal problembull A sends to B C cannot receive Abull C wants to send to B C senses a
ldquofreerdquo medium (CS fails)bull collision at B A cannot receive the
collision (CD fails)bull A is ldquohiddenrdquo for C
Ad hoc Sensor Networks Multiple channel access
Random Access hidden and exposed terminal problemExposed terminal problem
bull B sends to A C wants to send to Dbull C has to wait CS signals a medium
in usebull since A is outside the radio range of
C waiting is not necessarybull C is ldquoexposedrdquo to B
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 48
Random Access MACAWhen a station wants to transmit databull It sends an RTS (Ready-to-Send) packet to the intended receiver
ndash The RTS packet contains the length of the data that needs to be transmittedndash Any station other than the intended recipient hearing RTS defers transmission for atime duration equal to the end of the corresponding CTS reception
bull The receiver sends back CTS (Clear-to-Send) packet back to sender if it isavailable to receivendash The CTS packet contains the length of the data that original sender wants totransmitndash Any station other than the original RTS sender hearing CTS defers transmissionuntil the data is sent
bull The original sender upon reception of the CTS starts transmitting
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 49
Hidden terminal problem
Ad hoc Sensor Networks Multiple channel access
Random Access MACA (contrsquo)Exposed terminal problem
Waiting time of node X is muchsmaller than waiting time of node C
bull C defers transmission upon hearing BrsquosRTS until B could get CTS from Abull After that C can start transmission to DFor that it first sends an RTSbull C is not longer exposed to the datatransmission of B
Problem It does not address the collision of RTS packet
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 50
Random Access reliabilitybull Wireless links are prone to have errors High packet loss rate detrimental to
transport-layer performance Mechanisms needed to reduce packet loss rateexperienced by upper layers
bull When node B receives a data packet from node A node B sends anAcknowledgement (Ack) This approach adopted in many protocols
bull If node A fails to receive an Ack it will retransmit the packet
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 51
Random Access MACAW (CSMACA) bull Physical carrier sense and virtual carrier sense using Network Allocation
Vector (NAV)ndash NAV is updated based on overheard RTSCTSDATAACK packets each of whichspecified duration of a pending transmissionndash Nodes stay silent when carrier sensed (physicalvirtual)
bull Backoff intervals used to reduce collision probabilityndash When transmitting a packet choose a backoff interval in the range [0w] where wis contention windowndash Count down the backoff interval when channel is idlendash Count down is suspended if channel becomes busyndash When backoff interval reaches 0 transmit RTS
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 52
Random Access CSMACA (contrsquo) bull The time spent counting down backoff intervals is a part of MAC overhead
ndash Choosing a large w leads to large backoff intervals and can result in larger overheadndash Choosing a small w leads to a larger number of collisions (when two nodes countdown to 0 simultaneously)ndash Since the number of nodes attempting to transmit simultaneously may change withtime some mechanism to manage contention is needed
bull IEEE 80211 DCF contention window w is chosen dynamically depending oncollision occurrence (binary exponential backoff)ndash When a node fails to receive CTS in response to its RTS it increases the contentionwindowndash w is doubled (up to an upper bound)ndash When a node successfully completes a data transfer it restores w to wmin
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 53
Random Access fairnessbull Simplest definition of fairness all nodes should receive equal bandwidthbull An example of unfairness
ndash Assume that initially A and B both choose a backoff interval in range [031] buttheir RTSs collidendash Nodes A and B then choose from range [063]ndash Node A chooses 4 slots and B choose 60 slotsndash After A transmits a packet it next chooses from range [031]ndash It is possible that A may transmit several packets before B transmits its first packet
Ad hoc Sensor Networks Multiple channel access
Unfairness occurs when one node has backed off much more than some other node
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 54
Random Access fairness (contrsquo) MACAW solution for fairnessbull When a node transmits a packet it appends value w to the packet all nodes
hearing value w use it for their future transmission attemptsbull Since w is an indication of the level of congestion in the vicinity of a specific
receiver node MACAW proposes maintaining w independently for eachreceiver
bull Using per-receiver wi is particularly useful in multi-hop environments sincecongestion level at different receivers can be very different
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 55
Random Access energy efficiency bull Since many mobile hosts are operated by batteries MAC
protocols which conserve energy are of interestbull Two approaches to reduce energy consumption
ndash Power save turn off wireless interface when desirablendash Power control reduce transmit power
bull Power control has some more potential benefitsndash Reduced interference (in ad hoc networks) eg it improves ALOHAefficiency where user with high power can capture a packet even if there is acollisionndash Increased spatial reuse (in infrastructured wireless networks) eg it is usedin CDMA to maintain target SIR of voice and data users or generally it canbe used to maintain target SIR for different user classes (admission control)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 56
Random Access power control bull When C transmits to D at a high power level B cannot receive Arsquos
transmission due to interference from Cbull If C reduces transmit power it can still communicate with D
ndash Reduces energy consumption at node Cndash Allows B to receive Arsquos transmission (spatial reuse)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 57
bull A special wireless ad hoc networkndash Large number of nodesndash Battery poweredndash Topology and density changendash Nodes for a common taskndash In-network data processing
bull Sensor-net applicationsndash Sensor-triggered bursty trafficndash Can often tolerate some delay
Ad hoc Sensor Networks Multiple channel access
Characteristics of Sensor Network
Scalabilty and self configurationEnergy efficiencyAdaptivityFairness not importantLatency
AdaptivityEnergy-delay tradoff
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 58
Ad hoc Sensor Networks Multiple channel access
Wireless sensor node and sensor network
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 59
MAC for WSN attributes bull Attributes
ndash Collision avoidance (Basic task)ndash Energy efficiency (most primary in WSN)ndash Scalability and adaptivityndash Channel utilizationndash Latencyndash Throughputndash Fairness
bull What causes energy wastendash Collisionsndash Control packet overheadndash Overhearing unnecessary trafficndash Long idle time (it consumes 50-100 of the power)
Ad hoc Sensor Networks Multiple channel access
Primary
Secondary
Dominant factor
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 60
MAC for WSN
bull Contention-based protocols need to work hard in all directions for energysavingsndash Reduce idle listening ndash support low duty cyclendash Better collision avoidancendash Reduce control overheadndash Avoid unnecessary overhearing
Ad hoc Sensor Networks Multiple channel access
Scheduled Protocols Contention ProtocolsCollisions No YesEnergy efficiency Good BadScalability and adaptivity Bad GoodMulti-hop communication Difficult EasyTime synchronization Strict Loose or not required
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 61
MAC for WSN classification
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 62
MAC for WSN SMAC (2002)bull It has been developed by Ye Heidemann and Estrinbull Tradeoffs between (latency fairness) and energybull Major components in S-MAC
ndash Periodic listen and sleep turn off radio when sleeping the reduced dutycycle is ~ 10 (eg 120ms on 12s off)ndash Collision avoidance is based on contention similarly to IEEE 80211 adhoc modendash Overhearing avoidance the node is sleeping when neighbors talkndash Massage passing long message is fragmented amp sent in burst operationwith extended Tx time and re-transmit occurs immediately
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 63
Summarybull The algorithms and protocols that enable sharing by multiple users and
controls the access to the wireless channel from different users are calledMAC
bull There are two types of wireless networks Infrastructured wireless networksand ad hoc wireless networks
bull We also want our wireless system to have high user capacity and we canidentify 10 methods for increasing it
bull There are several MACs with no absolute advantage it should be tailoredto the system requirements
bull There are primary concerns for designing a WSN MAC (collisionavoidance energy efficiency scalability and adaptivity) while thesecondary concerns are as follows (channel utilization latency throughputfairness)
bull Next lecture Routing protocols
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 42
Random Access Carrier Sense Multiple Accessbull CSMA (Carrier Sense Multiple Access) listen to the carrier before transmission
and transmits if channel is idlebull Detection delay and propagation delay are two important parameters
ndash Detection delay time required to sense the carrier and decide if it is idle or busyndash Propagation delay distancespeed_of_ligth The time required for bit to travel
from transmitter to the receiver
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 43
Random Access CSMA variations
Ad hoc Sensor Networks Multiple channel access
1-persistent CSMA Non-persistent CSMA
There will always be a collision iftwo nodes want to retransmit(usually you stop transmissionattempts after few tries)
Random backoff reduces probabilityof collisions Waste idle time if thebackoff time is too long
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 44
Random Access CSMA variations (contrsquo)
Ad hoc Sensor Networks Multiple channel access
p-persistent CSMA CSMA CD (Collision Detection)
A good tradeoff between non-persistent and 1-persistent CSMA
Same with CSMA however a stationalso listen to the carrier whiletransmitting to see if the transmissioncollides with someone elsetransmission It can be used in listen-while-talk capable channels (fullduplex) In single radio channels thetransmission need to be interrupted inorder to sense the channel
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 45
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 46
Random Access CSMA (contrsquo)bull How to select probability p
ndash Assume that M nodes have a packet to send and the medium is busy then Mp is theexpected number of nodes that will attempt to transmit once the medium becomesidle If Mp gt 1 then a collision is expected to occur Therefore the network mustmake sure that Mp lt 1 to avoid collision where M is the maximum number of nodesthat can be active at a time
bull Problems1 The mean packet delay increases exponently with increasing offered load2 CSMA protocols sense the carrier but sensing the carrier does not always releasestrue information about the status of the wireless channel There are two problems thatare unique to wireless channels (different than wireline channels)ndash Hidden terminal problemndash Exposed terminal problem
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 47
Hidden terminal problembull A sends to B C cannot receive Abull C wants to send to B C senses a
ldquofreerdquo medium (CS fails)bull collision at B A cannot receive the
collision (CD fails)bull A is ldquohiddenrdquo for C
Ad hoc Sensor Networks Multiple channel access
Random Access hidden and exposed terminal problemExposed terminal problem
bull B sends to A C wants to send to Dbull C has to wait CS signals a medium
in usebull since A is outside the radio range of
C waiting is not necessarybull C is ldquoexposedrdquo to B
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 48
Random Access MACAWhen a station wants to transmit databull It sends an RTS (Ready-to-Send) packet to the intended receiver
ndash The RTS packet contains the length of the data that needs to be transmittedndash Any station other than the intended recipient hearing RTS defers transmission for atime duration equal to the end of the corresponding CTS reception
bull The receiver sends back CTS (Clear-to-Send) packet back to sender if it isavailable to receivendash The CTS packet contains the length of the data that original sender wants totransmitndash Any station other than the original RTS sender hearing CTS defers transmissionuntil the data is sent
bull The original sender upon reception of the CTS starts transmitting
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 49
Hidden terminal problem
Ad hoc Sensor Networks Multiple channel access
Random Access MACA (contrsquo)Exposed terminal problem
Waiting time of node X is muchsmaller than waiting time of node C
bull C defers transmission upon hearing BrsquosRTS until B could get CTS from Abull After that C can start transmission to DFor that it first sends an RTSbull C is not longer exposed to the datatransmission of B
Problem It does not address the collision of RTS packet
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 50
Random Access reliabilitybull Wireless links are prone to have errors High packet loss rate detrimental to
transport-layer performance Mechanisms needed to reduce packet loss rateexperienced by upper layers
bull When node B receives a data packet from node A node B sends anAcknowledgement (Ack) This approach adopted in many protocols
bull If node A fails to receive an Ack it will retransmit the packet
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 51
Random Access MACAW (CSMACA) bull Physical carrier sense and virtual carrier sense using Network Allocation
Vector (NAV)ndash NAV is updated based on overheard RTSCTSDATAACK packets each of whichspecified duration of a pending transmissionndash Nodes stay silent when carrier sensed (physicalvirtual)
bull Backoff intervals used to reduce collision probabilityndash When transmitting a packet choose a backoff interval in the range [0w] where wis contention windowndash Count down the backoff interval when channel is idlendash Count down is suspended if channel becomes busyndash When backoff interval reaches 0 transmit RTS
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 52
Random Access CSMACA (contrsquo) bull The time spent counting down backoff intervals is a part of MAC overhead
ndash Choosing a large w leads to large backoff intervals and can result in larger overheadndash Choosing a small w leads to a larger number of collisions (when two nodes countdown to 0 simultaneously)ndash Since the number of nodes attempting to transmit simultaneously may change withtime some mechanism to manage contention is needed
bull IEEE 80211 DCF contention window w is chosen dynamically depending oncollision occurrence (binary exponential backoff)ndash When a node fails to receive CTS in response to its RTS it increases the contentionwindowndash w is doubled (up to an upper bound)ndash When a node successfully completes a data transfer it restores w to wmin
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 53
Random Access fairnessbull Simplest definition of fairness all nodes should receive equal bandwidthbull An example of unfairness
ndash Assume that initially A and B both choose a backoff interval in range [031] buttheir RTSs collidendash Nodes A and B then choose from range [063]ndash Node A chooses 4 slots and B choose 60 slotsndash After A transmits a packet it next chooses from range [031]ndash It is possible that A may transmit several packets before B transmits its first packet
Ad hoc Sensor Networks Multiple channel access
Unfairness occurs when one node has backed off much more than some other node
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 54
Random Access fairness (contrsquo) MACAW solution for fairnessbull When a node transmits a packet it appends value w to the packet all nodes
hearing value w use it for their future transmission attemptsbull Since w is an indication of the level of congestion in the vicinity of a specific
receiver node MACAW proposes maintaining w independently for eachreceiver
bull Using per-receiver wi is particularly useful in multi-hop environments sincecongestion level at different receivers can be very different
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 55
Random Access energy efficiency bull Since many mobile hosts are operated by batteries MAC
protocols which conserve energy are of interestbull Two approaches to reduce energy consumption
ndash Power save turn off wireless interface when desirablendash Power control reduce transmit power
bull Power control has some more potential benefitsndash Reduced interference (in ad hoc networks) eg it improves ALOHAefficiency where user with high power can capture a packet even if there is acollisionndash Increased spatial reuse (in infrastructured wireless networks) eg it is usedin CDMA to maintain target SIR of voice and data users or generally it canbe used to maintain target SIR for different user classes (admission control)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 56
Random Access power control bull When C transmits to D at a high power level B cannot receive Arsquos
transmission due to interference from Cbull If C reduces transmit power it can still communicate with D
ndash Reduces energy consumption at node Cndash Allows B to receive Arsquos transmission (spatial reuse)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 57
bull A special wireless ad hoc networkndash Large number of nodesndash Battery poweredndash Topology and density changendash Nodes for a common taskndash In-network data processing
bull Sensor-net applicationsndash Sensor-triggered bursty trafficndash Can often tolerate some delay
Ad hoc Sensor Networks Multiple channel access
Characteristics of Sensor Network
Scalabilty and self configurationEnergy efficiencyAdaptivityFairness not importantLatency
AdaptivityEnergy-delay tradoff
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 58
Ad hoc Sensor Networks Multiple channel access
Wireless sensor node and sensor network
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 59
MAC for WSN attributes bull Attributes
ndash Collision avoidance (Basic task)ndash Energy efficiency (most primary in WSN)ndash Scalability and adaptivityndash Channel utilizationndash Latencyndash Throughputndash Fairness
bull What causes energy wastendash Collisionsndash Control packet overheadndash Overhearing unnecessary trafficndash Long idle time (it consumes 50-100 of the power)
Ad hoc Sensor Networks Multiple channel access
Primary
Secondary
Dominant factor
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 60
MAC for WSN
bull Contention-based protocols need to work hard in all directions for energysavingsndash Reduce idle listening ndash support low duty cyclendash Better collision avoidancendash Reduce control overheadndash Avoid unnecessary overhearing
Ad hoc Sensor Networks Multiple channel access
Scheduled Protocols Contention ProtocolsCollisions No YesEnergy efficiency Good BadScalability and adaptivity Bad GoodMulti-hop communication Difficult EasyTime synchronization Strict Loose or not required
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 61
MAC for WSN classification
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 62
MAC for WSN SMAC (2002)bull It has been developed by Ye Heidemann and Estrinbull Tradeoffs between (latency fairness) and energybull Major components in S-MAC
ndash Periodic listen and sleep turn off radio when sleeping the reduced dutycycle is ~ 10 (eg 120ms on 12s off)ndash Collision avoidance is based on contention similarly to IEEE 80211 adhoc modendash Overhearing avoidance the node is sleeping when neighbors talkndash Massage passing long message is fragmented amp sent in burst operationwith extended Tx time and re-transmit occurs immediately
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 63
Summarybull The algorithms and protocols that enable sharing by multiple users and
controls the access to the wireless channel from different users are calledMAC
bull There are two types of wireless networks Infrastructured wireless networksand ad hoc wireless networks
bull We also want our wireless system to have high user capacity and we canidentify 10 methods for increasing it
bull There are several MACs with no absolute advantage it should be tailoredto the system requirements
bull There are primary concerns for designing a WSN MAC (collisionavoidance energy efficiency scalability and adaptivity) while thesecondary concerns are as follows (channel utilization latency throughputfairness)
bull Next lecture Routing protocols
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 43
Random Access CSMA variations
Ad hoc Sensor Networks Multiple channel access
1-persistent CSMA Non-persistent CSMA
There will always be a collision iftwo nodes want to retransmit(usually you stop transmissionattempts after few tries)
Random backoff reduces probabilityof collisions Waste idle time if thebackoff time is too long
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 44
Random Access CSMA variations (contrsquo)
Ad hoc Sensor Networks Multiple channel access
p-persistent CSMA CSMA CD (Collision Detection)
A good tradeoff between non-persistent and 1-persistent CSMA
Same with CSMA however a stationalso listen to the carrier whiletransmitting to see if the transmissioncollides with someone elsetransmission It can be used in listen-while-talk capable channels (fullduplex) In single radio channels thetransmission need to be interrupted inorder to sense the channel
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 45
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 46
Random Access CSMA (contrsquo)bull How to select probability p
ndash Assume that M nodes have a packet to send and the medium is busy then Mp is theexpected number of nodes that will attempt to transmit once the medium becomesidle If Mp gt 1 then a collision is expected to occur Therefore the network mustmake sure that Mp lt 1 to avoid collision where M is the maximum number of nodesthat can be active at a time
bull Problems1 The mean packet delay increases exponently with increasing offered load2 CSMA protocols sense the carrier but sensing the carrier does not always releasestrue information about the status of the wireless channel There are two problems thatare unique to wireless channels (different than wireline channels)ndash Hidden terminal problemndash Exposed terminal problem
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 47
Hidden terminal problembull A sends to B C cannot receive Abull C wants to send to B C senses a
ldquofreerdquo medium (CS fails)bull collision at B A cannot receive the
collision (CD fails)bull A is ldquohiddenrdquo for C
Ad hoc Sensor Networks Multiple channel access
Random Access hidden and exposed terminal problemExposed terminal problem
bull B sends to A C wants to send to Dbull C has to wait CS signals a medium
in usebull since A is outside the radio range of
C waiting is not necessarybull C is ldquoexposedrdquo to B
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 48
Random Access MACAWhen a station wants to transmit databull It sends an RTS (Ready-to-Send) packet to the intended receiver
ndash The RTS packet contains the length of the data that needs to be transmittedndash Any station other than the intended recipient hearing RTS defers transmission for atime duration equal to the end of the corresponding CTS reception
bull The receiver sends back CTS (Clear-to-Send) packet back to sender if it isavailable to receivendash The CTS packet contains the length of the data that original sender wants totransmitndash Any station other than the original RTS sender hearing CTS defers transmissionuntil the data is sent
bull The original sender upon reception of the CTS starts transmitting
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 49
Hidden terminal problem
Ad hoc Sensor Networks Multiple channel access
Random Access MACA (contrsquo)Exposed terminal problem
Waiting time of node X is muchsmaller than waiting time of node C
bull C defers transmission upon hearing BrsquosRTS until B could get CTS from Abull After that C can start transmission to DFor that it first sends an RTSbull C is not longer exposed to the datatransmission of B
Problem It does not address the collision of RTS packet
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 50
Random Access reliabilitybull Wireless links are prone to have errors High packet loss rate detrimental to
transport-layer performance Mechanisms needed to reduce packet loss rateexperienced by upper layers
bull When node B receives a data packet from node A node B sends anAcknowledgement (Ack) This approach adopted in many protocols
bull If node A fails to receive an Ack it will retransmit the packet
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 51
Random Access MACAW (CSMACA) bull Physical carrier sense and virtual carrier sense using Network Allocation
Vector (NAV)ndash NAV is updated based on overheard RTSCTSDATAACK packets each of whichspecified duration of a pending transmissionndash Nodes stay silent when carrier sensed (physicalvirtual)
bull Backoff intervals used to reduce collision probabilityndash When transmitting a packet choose a backoff interval in the range [0w] where wis contention windowndash Count down the backoff interval when channel is idlendash Count down is suspended if channel becomes busyndash When backoff interval reaches 0 transmit RTS
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 52
Random Access CSMACA (contrsquo) bull The time spent counting down backoff intervals is a part of MAC overhead
ndash Choosing a large w leads to large backoff intervals and can result in larger overheadndash Choosing a small w leads to a larger number of collisions (when two nodes countdown to 0 simultaneously)ndash Since the number of nodes attempting to transmit simultaneously may change withtime some mechanism to manage contention is needed
bull IEEE 80211 DCF contention window w is chosen dynamically depending oncollision occurrence (binary exponential backoff)ndash When a node fails to receive CTS in response to its RTS it increases the contentionwindowndash w is doubled (up to an upper bound)ndash When a node successfully completes a data transfer it restores w to wmin
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 53
Random Access fairnessbull Simplest definition of fairness all nodes should receive equal bandwidthbull An example of unfairness
ndash Assume that initially A and B both choose a backoff interval in range [031] buttheir RTSs collidendash Nodes A and B then choose from range [063]ndash Node A chooses 4 slots and B choose 60 slotsndash After A transmits a packet it next chooses from range [031]ndash It is possible that A may transmit several packets before B transmits its first packet
Ad hoc Sensor Networks Multiple channel access
Unfairness occurs when one node has backed off much more than some other node
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 54
Random Access fairness (contrsquo) MACAW solution for fairnessbull When a node transmits a packet it appends value w to the packet all nodes
hearing value w use it for their future transmission attemptsbull Since w is an indication of the level of congestion in the vicinity of a specific
receiver node MACAW proposes maintaining w independently for eachreceiver
bull Using per-receiver wi is particularly useful in multi-hop environments sincecongestion level at different receivers can be very different
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 55
Random Access energy efficiency bull Since many mobile hosts are operated by batteries MAC
protocols which conserve energy are of interestbull Two approaches to reduce energy consumption
ndash Power save turn off wireless interface when desirablendash Power control reduce transmit power
bull Power control has some more potential benefitsndash Reduced interference (in ad hoc networks) eg it improves ALOHAefficiency where user with high power can capture a packet even if there is acollisionndash Increased spatial reuse (in infrastructured wireless networks) eg it is usedin CDMA to maintain target SIR of voice and data users or generally it canbe used to maintain target SIR for different user classes (admission control)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 56
Random Access power control bull When C transmits to D at a high power level B cannot receive Arsquos
transmission due to interference from Cbull If C reduces transmit power it can still communicate with D
ndash Reduces energy consumption at node Cndash Allows B to receive Arsquos transmission (spatial reuse)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 57
bull A special wireless ad hoc networkndash Large number of nodesndash Battery poweredndash Topology and density changendash Nodes for a common taskndash In-network data processing
bull Sensor-net applicationsndash Sensor-triggered bursty trafficndash Can often tolerate some delay
Ad hoc Sensor Networks Multiple channel access
Characteristics of Sensor Network
Scalabilty and self configurationEnergy efficiencyAdaptivityFairness not importantLatency
AdaptivityEnergy-delay tradoff
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 58
Ad hoc Sensor Networks Multiple channel access
Wireless sensor node and sensor network
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 59
MAC for WSN attributes bull Attributes
ndash Collision avoidance (Basic task)ndash Energy efficiency (most primary in WSN)ndash Scalability and adaptivityndash Channel utilizationndash Latencyndash Throughputndash Fairness
bull What causes energy wastendash Collisionsndash Control packet overheadndash Overhearing unnecessary trafficndash Long idle time (it consumes 50-100 of the power)
Ad hoc Sensor Networks Multiple channel access
Primary
Secondary
Dominant factor
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 60
MAC for WSN
bull Contention-based protocols need to work hard in all directions for energysavingsndash Reduce idle listening ndash support low duty cyclendash Better collision avoidancendash Reduce control overheadndash Avoid unnecessary overhearing
Ad hoc Sensor Networks Multiple channel access
Scheduled Protocols Contention ProtocolsCollisions No YesEnergy efficiency Good BadScalability and adaptivity Bad GoodMulti-hop communication Difficult EasyTime synchronization Strict Loose or not required
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 61
MAC for WSN classification
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 62
MAC for WSN SMAC (2002)bull It has been developed by Ye Heidemann and Estrinbull Tradeoffs between (latency fairness) and energybull Major components in S-MAC
ndash Periodic listen and sleep turn off radio when sleeping the reduced dutycycle is ~ 10 (eg 120ms on 12s off)ndash Collision avoidance is based on contention similarly to IEEE 80211 adhoc modendash Overhearing avoidance the node is sleeping when neighbors talkndash Massage passing long message is fragmented amp sent in burst operationwith extended Tx time and re-transmit occurs immediately
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 63
Summarybull The algorithms and protocols that enable sharing by multiple users and
controls the access to the wireless channel from different users are calledMAC
bull There are two types of wireless networks Infrastructured wireless networksand ad hoc wireless networks
bull We also want our wireless system to have high user capacity and we canidentify 10 methods for increasing it
bull There are several MACs with no absolute advantage it should be tailoredto the system requirements
bull There are primary concerns for designing a WSN MAC (collisionavoidance energy efficiency scalability and adaptivity) while thesecondary concerns are as follows (channel utilization latency throughputfairness)
bull Next lecture Routing protocols
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 44
Random Access CSMA variations (contrsquo)
Ad hoc Sensor Networks Multiple channel access
p-persistent CSMA CSMA CD (Collision Detection)
A good tradeoff between non-persistent and 1-persistent CSMA
Same with CSMA however a stationalso listen to the carrier whiletransmitting to see if the transmissioncollides with someone elsetransmission It can be used in listen-while-talk capable channels (fullduplex) In single radio channels thetransmission need to be interrupted inorder to sense the channel
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 45
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 46
Random Access CSMA (contrsquo)bull How to select probability p
ndash Assume that M nodes have a packet to send and the medium is busy then Mp is theexpected number of nodes that will attempt to transmit once the medium becomesidle If Mp gt 1 then a collision is expected to occur Therefore the network mustmake sure that Mp lt 1 to avoid collision where M is the maximum number of nodesthat can be active at a time
bull Problems1 The mean packet delay increases exponently with increasing offered load2 CSMA protocols sense the carrier but sensing the carrier does not always releasestrue information about the status of the wireless channel There are two problems thatare unique to wireless channels (different than wireline channels)ndash Hidden terminal problemndash Exposed terminal problem
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 47
Hidden terminal problembull A sends to B C cannot receive Abull C wants to send to B C senses a
ldquofreerdquo medium (CS fails)bull collision at B A cannot receive the
collision (CD fails)bull A is ldquohiddenrdquo for C
Ad hoc Sensor Networks Multiple channel access
Random Access hidden and exposed terminal problemExposed terminal problem
bull B sends to A C wants to send to Dbull C has to wait CS signals a medium
in usebull since A is outside the radio range of
C waiting is not necessarybull C is ldquoexposedrdquo to B
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 48
Random Access MACAWhen a station wants to transmit databull It sends an RTS (Ready-to-Send) packet to the intended receiver
ndash The RTS packet contains the length of the data that needs to be transmittedndash Any station other than the intended recipient hearing RTS defers transmission for atime duration equal to the end of the corresponding CTS reception
bull The receiver sends back CTS (Clear-to-Send) packet back to sender if it isavailable to receivendash The CTS packet contains the length of the data that original sender wants totransmitndash Any station other than the original RTS sender hearing CTS defers transmissionuntil the data is sent
bull The original sender upon reception of the CTS starts transmitting
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 49
Hidden terminal problem
Ad hoc Sensor Networks Multiple channel access
Random Access MACA (contrsquo)Exposed terminal problem
Waiting time of node X is muchsmaller than waiting time of node C
bull C defers transmission upon hearing BrsquosRTS until B could get CTS from Abull After that C can start transmission to DFor that it first sends an RTSbull C is not longer exposed to the datatransmission of B
Problem It does not address the collision of RTS packet
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 50
Random Access reliabilitybull Wireless links are prone to have errors High packet loss rate detrimental to
transport-layer performance Mechanisms needed to reduce packet loss rateexperienced by upper layers
bull When node B receives a data packet from node A node B sends anAcknowledgement (Ack) This approach adopted in many protocols
bull If node A fails to receive an Ack it will retransmit the packet
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 51
Random Access MACAW (CSMACA) bull Physical carrier sense and virtual carrier sense using Network Allocation
Vector (NAV)ndash NAV is updated based on overheard RTSCTSDATAACK packets each of whichspecified duration of a pending transmissionndash Nodes stay silent when carrier sensed (physicalvirtual)
bull Backoff intervals used to reduce collision probabilityndash When transmitting a packet choose a backoff interval in the range [0w] where wis contention windowndash Count down the backoff interval when channel is idlendash Count down is suspended if channel becomes busyndash When backoff interval reaches 0 transmit RTS
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 52
Random Access CSMACA (contrsquo) bull The time spent counting down backoff intervals is a part of MAC overhead
ndash Choosing a large w leads to large backoff intervals and can result in larger overheadndash Choosing a small w leads to a larger number of collisions (when two nodes countdown to 0 simultaneously)ndash Since the number of nodes attempting to transmit simultaneously may change withtime some mechanism to manage contention is needed
bull IEEE 80211 DCF contention window w is chosen dynamically depending oncollision occurrence (binary exponential backoff)ndash When a node fails to receive CTS in response to its RTS it increases the contentionwindowndash w is doubled (up to an upper bound)ndash When a node successfully completes a data transfer it restores w to wmin
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 53
Random Access fairnessbull Simplest definition of fairness all nodes should receive equal bandwidthbull An example of unfairness
ndash Assume that initially A and B both choose a backoff interval in range [031] buttheir RTSs collidendash Nodes A and B then choose from range [063]ndash Node A chooses 4 slots and B choose 60 slotsndash After A transmits a packet it next chooses from range [031]ndash It is possible that A may transmit several packets before B transmits its first packet
Ad hoc Sensor Networks Multiple channel access
Unfairness occurs when one node has backed off much more than some other node
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 54
Random Access fairness (contrsquo) MACAW solution for fairnessbull When a node transmits a packet it appends value w to the packet all nodes
hearing value w use it for their future transmission attemptsbull Since w is an indication of the level of congestion in the vicinity of a specific
receiver node MACAW proposes maintaining w independently for eachreceiver
bull Using per-receiver wi is particularly useful in multi-hop environments sincecongestion level at different receivers can be very different
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 55
Random Access energy efficiency bull Since many mobile hosts are operated by batteries MAC
protocols which conserve energy are of interestbull Two approaches to reduce energy consumption
ndash Power save turn off wireless interface when desirablendash Power control reduce transmit power
bull Power control has some more potential benefitsndash Reduced interference (in ad hoc networks) eg it improves ALOHAefficiency where user with high power can capture a packet even if there is acollisionndash Increased spatial reuse (in infrastructured wireless networks) eg it is usedin CDMA to maintain target SIR of voice and data users or generally it canbe used to maintain target SIR for different user classes (admission control)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 56
Random Access power control bull When C transmits to D at a high power level B cannot receive Arsquos
transmission due to interference from Cbull If C reduces transmit power it can still communicate with D
ndash Reduces energy consumption at node Cndash Allows B to receive Arsquos transmission (spatial reuse)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 57
bull A special wireless ad hoc networkndash Large number of nodesndash Battery poweredndash Topology and density changendash Nodes for a common taskndash In-network data processing
bull Sensor-net applicationsndash Sensor-triggered bursty trafficndash Can often tolerate some delay
Ad hoc Sensor Networks Multiple channel access
Characteristics of Sensor Network
Scalabilty and self configurationEnergy efficiencyAdaptivityFairness not importantLatency
AdaptivityEnergy-delay tradoff
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 58
Ad hoc Sensor Networks Multiple channel access
Wireless sensor node and sensor network
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 59
MAC for WSN attributes bull Attributes
ndash Collision avoidance (Basic task)ndash Energy efficiency (most primary in WSN)ndash Scalability and adaptivityndash Channel utilizationndash Latencyndash Throughputndash Fairness
bull What causes energy wastendash Collisionsndash Control packet overheadndash Overhearing unnecessary trafficndash Long idle time (it consumes 50-100 of the power)
Ad hoc Sensor Networks Multiple channel access
Primary
Secondary
Dominant factor
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 60
MAC for WSN
bull Contention-based protocols need to work hard in all directions for energysavingsndash Reduce idle listening ndash support low duty cyclendash Better collision avoidancendash Reduce control overheadndash Avoid unnecessary overhearing
Ad hoc Sensor Networks Multiple channel access
Scheduled Protocols Contention ProtocolsCollisions No YesEnergy efficiency Good BadScalability and adaptivity Bad GoodMulti-hop communication Difficult EasyTime synchronization Strict Loose or not required
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 61
MAC for WSN classification
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 62
MAC for WSN SMAC (2002)bull It has been developed by Ye Heidemann and Estrinbull Tradeoffs between (latency fairness) and energybull Major components in S-MAC
ndash Periodic listen and sleep turn off radio when sleeping the reduced dutycycle is ~ 10 (eg 120ms on 12s off)ndash Collision avoidance is based on contention similarly to IEEE 80211 adhoc modendash Overhearing avoidance the node is sleeping when neighbors talkndash Massage passing long message is fragmented amp sent in burst operationwith extended Tx time and re-transmit occurs immediately
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 63
Summarybull The algorithms and protocols that enable sharing by multiple users and
controls the access to the wireless channel from different users are calledMAC
bull There are two types of wireless networks Infrastructured wireless networksand ad hoc wireless networks
bull We also want our wireless system to have high user capacity and we canidentify 10 methods for increasing it
bull There are several MACs with no absolute advantage it should be tailoredto the system requirements
bull There are primary concerns for designing a WSN MAC (collisionavoidance energy efficiency scalability and adaptivity) while thesecondary concerns are as follows (channel utilization latency throughputfairness)
bull Next lecture Routing protocols
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 45
Ad hoc Sensor Networks Multiple channel access
Random Access throughput
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 46
Random Access CSMA (contrsquo)bull How to select probability p
ndash Assume that M nodes have a packet to send and the medium is busy then Mp is theexpected number of nodes that will attempt to transmit once the medium becomesidle If Mp gt 1 then a collision is expected to occur Therefore the network mustmake sure that Mp lt 1 to avoid collision where M is the maximum number of nodesthat can be active at a time
bull Problems1 The mean packet delay increases exponently with increasing offered load2 CSMA protocols sense the carrier but sensing the carrier does not always releasestrue information about the status of the wireless channel There are two problems thatare unique to wireless channels (different than wireline channels)ndash Hidden terminal problemndash Exposed terminal problem
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 47
Hidden terminal problembull A sends to B C cannot receive Abull C wants to send to B C senses a
ldquofreerdquo medium (CS fails)bull collision at B A cannot receive the
collision (CD fails)bull A is ldquohiddenrdquo for C
Ad hoc Sensor Networks Multiple channel access
Random Access hidden and exposed terminal problemExposed terminal problem
bull B sends to A C wants to send to Dbull C has to wait CS signals a medium
in usebull since A is outside the radio range of
C waiting is not necessarybull C is ldquoexposedrdquo to B
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 48
Random Access MACAWhen a station wants to transmit databull It sends an RTS (Ready-to-Send) packet to the intended receiver
ndash The RTS packet contains the length of the data that needs to be transmittedndash Any station other than the intended recipient hearing RTS defers transmission for atime duration equal to the end of the corresponding CTS reception
bull The receiver sends back CTS (Clear-to-Send) packet back to sender if it isavailable to receivendash The CTS packet contains the length of the data that original sender wants totransmitndash Any station other than the original RTS sender hearing CTS defers transmissionuntil the data is sent
bull The original sender upon reception of the CTS starts transmitting
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 49
Hidden terminal problem
Ad hoc Sensor Networks Multiple channel access
Random Access MACA (contrsquo)Exposed terminal problem
Waiting time of node X is muchsmaller than waiting time of node C
bull C defers transmission upon hearing BrsquosRTS until B could get CTS from Abull After that C can start transmission to DFor that it first sends an RTSbull C is not longer exposed to the datatransmission of B
Problem It does not address the collision of RTS packet
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 50
Random Access reliabilitybull Wireless links are prone to have errors High packet loss rate detrimental to
transport-layer performance Mechanisms needed to reduce packet loss rateexperienced by upper layers
bull When node B receives a data packet from node A node B sends anAcknowledgement (Ack) This approach adopted in many protocols
bull If node A fails to receive an Ack it will retransmit the packet
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 51
Random Access MACAW (CSMACA) bull Physical carrier sense and virtual carrier sense using Network Allocation
Vector (NAV)ndash NAV is updated based on overheard RTSCTSDATAACK packets each of whichspecified duration of a pending transmissionndash Nodes stay silent when carrier sensed (physicalvirtual)
bull Backoff intervals used to reduce collision probabilityndash When transmitting a packet choose a backoff interval in the range [0w] where wis contention windowndash Count down the backoff interval when channel is idlendash Count down is suspended if channel becomes busyndash When backoff interval reaches 0 transmit RTS
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 52
Random Access CSMACA (contrsquo) bull The time spent counting down backoff intervals is a part of MAC overhead
ndash Choosing a large w leads to large backoff intervals and can result in larger overheadndash Choosing a small w leads to a larger number of collisions (when two nodes countdown to 0 simultaneously)ndash Since the number of nodes attempting to transmit simultaneously may change withtime some mechanism to manage contention is needed
bull IEEE 80211 DCF contention window w is chosen dynamically depending oncollision occurrence (binary exponential backoff)ndash When a node fails to receive CTS in response to its RTS it increases the contentionwindowndash w is doubled (up to an upper bound)ndash When a node successfully completes a data transfer it restores w to wmin
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 53
Random Access fairnessbull Simplest definition of fairness all nodes should receive equal bandwidthbull An example of unfairness
ndash Assume that initially A and B both choose a backoff interval in range [031] buttheir RTSs collidendash Nodes A and B then choose from range [063]ndash Node A chooses 4 slots and B choose 60 slotsndash After A transmits a packet it next chooses from range [031]ndash It is possible that A may transmit several packets before B transmits its first packet
Ad hoc Sensor Networks Multiple channel access
Unfairness occurs when one node has backed off much more than some other node
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 54
Random Access fairness (contrsquo) MACAW solution for fairnessbull When a node transmits a packet it appends value w to the packet all nodes
hearing value w use it for their future transmission attemptsbull Since w is an indication of the level of congestion in the vicinity of a specific
receiver node MACAW proposes maintaining w independently for eachreceiver
bull Using per-receiver wi is particularly useful in multi-hop environments sincecongestion level at different receivers can be very different
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 55
Random Access energy efficiency bull Since many mobile hosts are operated by batteries MAC
protocols which conserve energy are of interestbull Two approaches to reduce energy consumption
ndash Power save turn off wireless interface when desirablendash Power control reduce transmit power
bull Power control has some more potential benefitsndash Reduced interference (in ad hoc networks) eg it improves ALOHAefficiency where user with high power can capture a packet even if there is acollisionndash Increased spatial reuse (in infrastructured wireless networks) eg it is usedin CDMA to maintain target SIR of voice and data users or generally it canbe used to maintain target SIR for different user classes (admission control)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 56
Random Access power control bull When C transmits to D at a high power level B cannot receive Arsquos
transmission due to interference from Cbull If C reduces transmit power it can still communicate with D
ndash Reduces energy consumption at node Cndash Allows B to receive Arsquos transmission (spatial reuse)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 57
bull A special wireless ad hoc networkndash Large number of nodesndash Battery poweredndash Topology and density changendash Nodes for a common taskndash In-network data processing
bull Sensor-net applicationsndash Sensor-triggered bursty trafficndash Can often tolerate some delay
Ad hoc Sensor Networks Multiple channel access
Characteristics of Sensor Network
Scalabilty and self configurationEnergy efficiencyAdaptivityFairness not importantLatency
AdaptivityEnergy-delay tradoff
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 58
Ad hoc Sensor Networks Multiple channel access
Wireless sensor node and sensor network
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 59
MAC for WSN attributes bull Attributes
ndash Collision avoidance (Basic task)ndash Energy efficiency (most primary in WSN)ndash Scalability and adaptivityndash Channel utilizationndash Latencyndash Throughputndash Fairness
bull What causes energy wastendash Collisionsndash Control packet overheadndash Overhearing unnecessary trafficndash Long idle time (it consumes 50-100 of the power)
Ad hoc Sensor Networks Multiple channel access
Primary
Secondary
Dominant factor
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 60
MAC for WSN
bull Contention-based protocols need to work hard in all directions for energysavingsndash Reduce idle listening ndash support low duty cyclendash Better collision avoidancendash Reduce control overheadndash Avoid unnecessary overhearing
Ad hoc Sensor Networks Multiple channel access
Scheduled Protocols Contention ProtocolsCollisions No YesEnergy efficiency Good BadScalability and adaptivity Bad GoodMulti-hop communication Difficult EasyTime synchronization Strict Loose or not required
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 61
MAC for WSN classification
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 62
MAC for WSN SMAC (2002)bull It has been developed by Ye Heidemann and Estrinbull Tradeoffs between (latency fairness) and energybull Major components in S-MAC
ndash Periodic listen and sleep turn off radio when sleeping the reduced dutycycle is ~ 10 (eg 120ms on 12s off)ndash Collision avoidance is based on contention similarly to IEEE 80211 adhoc modendash Overhearing avoidance the node is sleeping when neighbors talkndash Massage passing long message is fragmented amp sent in burst operationwith extended Tx time and re-transmit occurs immediately
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 63
Summarybull The algorithms and protocols that enable sharing by multiple users and
controls the access to the wireless channel from different users are calledMAC
bull There are two types of wireless networks Infrastructured wireless networksand ad hoc wireless networks
bull We also want our wireless system to have high user capacity and we canidentify 10 methods for increasing it
bull There are several MACs with no absolute advantage it should be tailoredto the system requirements
bull There are primary concerns for designing a WSN MAC (collisionavoidance energy efficiency scalability and adaptivity) while thesecondary concerns are as follows (channel utilization latency throughputfairness)
bull Next lecture Routing protocols
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 46
Random Access CSMA (contrsquo)bull How to select probability p
ndash Assume that M nodes have a packet to send and the medium is busy then Mp is theexpected number of nodes that will attempt to transmit once the medium becomesidle If Mp gt 1 then a collision is expected to occur Therefore the network mustmake sure that Mp lt 1 to avoid collision where M is the maximum number of nodesthat can be active at a time
bull Problems1 The mean packet delay increases exponently with increasing offered load2 CSMA protocols sense the carrier but sensing the carrier does not always releasestrue information about the status of the wireless channel There are two problems thatare unique to wireless channels (different than wireline channels)ndash Hidden terminal problemndash Exposed terminal problem
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 47
Hidden terminal problembull A sends to B C cannot receive Abull C wants to send to B C senses a
ldquofreerdquo medium (CS fails)bull collision at B A cannot receive the
collision (CD fails)bull A is ldquohiddenrdquo for C
Ad hoc Sensor Networks Multiple channel access
Random Access hidden and exposed terminal problemExposed terminal problem
bull B sends to A C wants to send to Dbull C has to wait CS signals a medium
in usebull since A is outside the radio range of
C waiting is not necessarybull C is ldquoexposedrdquo to B
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 48
Random Access MACAWhen a station wants to transmit databull It sends an RTS (Ready-to-Send) packet to the intended receiver
ndash The RTS packet contains the length of the data that needs to be transmittedndash Any station other than the intended recipient hearing RTS defers transmission for atime duration equal to the end of the corresponding CTS reception
bull The receiver sends back CTS (Clear-to-Send) packet back to sender if it isavailable to receivendash The CTS packet contains the length of the data that original sender wants totransmitndash Any station other than the original RTS sender hearing CTS defers transmissionuntil the data is sent
bull The original sender upon reception of the CTS starts transmitting
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 49
Hidden terminal problem
Ad hoc Sensor Networks Multiple channel access
Random Access MACA (contrsquo)Exposed terminal problem
Waiting time of node X is muchsmaller than waiting time of node C
bull C defers transmission upon hearing BrsquosRTS until B could get CTS from Abull After that C can start transmission to DFor that it first sends an RTSbull C is not longer exposed to the datatransmission of B
Problem It does not address the collision of RTS packet
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 50
Random Access reliabilitybull Wireless links are prone to have errors High packet loss rate detrimental to
transport-layer performance Mechanisms needed to reduce packet loss rateexperienced by upper layers
bull When node B receives a data packet from node A node B sends anAcknowledgement (Ack) This approach adopted in many protocols
bull If node A fails to receive an Ack it will retransmit the packet
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 51
Random Access MACAW (CSMACA) bull Physical carrier sense and virtual carrier sense using Network Allocation
Vector (NAV)ndash NAV is updated based on overheard RTSCTSDATAACK packets each of whichspecified duration of a pending transmissionndash Nodes stay silent when carrier sensed (physicalvirtual)
bull Backoff intervals used to reduce collision probabilityndash When transmitting a packet choose a backoff interval in the range [0w] where wis contention windowndash Count down the backoff interval when channel is idlendash Count down is suspended if channel becomes busyndash When backoff interval reaches 0 transmit RTS
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 52
Random Access CSMACA (contrsquo) bull The time spent counting down backoff intervals is a part of MAC overhead
ndash Choosing a large w leads to large backoff intervals and can result in larger overheadndash Choosing a small w leads to a larger number of collisions (when two nodes countdown to 0 simultaneously)ndash Since the number of nodes attempting to transmit simultaneously may change withtime some mechanism to manage contention is needed
bull IEEE 80211 DCF contention window w is chosen dynamically depending oncollision occurrence (binary exponential backoff)ndash When a node fails to receive CTS in response to its RTS it increases the contentionwindowndash w is doubled (up to an upper bound)ndash When a node successfully completes a data transfer it restores w to wmin
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 53
Random Access fairnessbull Simplest definition of fairness all nodes should receive equal bandwidthbull An example of unfairness
ndash Assume that initially A and B both choose a backoff interval in range [031] buttheir RTSs collidendash Nodes A and B then choose from range [063]ndash Node A chooses 4 slots and B choose 60 slotsndash After A transmits a packet it next chooses from range [031]ndash It is possible that A may transmit several packets before B transmits its first packet
Ad hoc Sensor Networks Multiple channel access
Unfairness occurs when one node has backed off much more than some other node
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 54
Random Access fairness (contrsquo) MACAW solution for fairnessbull When a node transmits a packet it appends value w to the packet all nodes
hearing value w use it for their future transmission attemptsbull Since w is an indication of the level of congestion in the vicinity of a specific
receiver node MACAW proposes maintaining w independently for eachreceiver
bull Using per-receiver wi is particularly useful in multi-hop environments sincecongestion level at different receivers can be very different
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 55
Random Access energy efficiency bull Since many mobile hosts are operated by batteries MAC
protocols which conserve energy are of interestbull Two approaches to reduce energy consumption
ndash Power save turn off wireless interface when desirablendash Power control reduce transmit power
bull Power control has some more potential benefitsndash Reduced interference (in ad hoc networks) eg it improves ALOHAefficiency where user with high power can capture a packet even if there is acollisionndash Increased spatial reuse (in infrastructured wireless networks) eg it is usedin CDMA to maintain target SIR of voice and data users or generally it canbe used to maintain target SIR for different user classes (admission control)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 56
Random Access power control bull When C transmits to D at a high power level B cannot receive Arsquos
transmission due to interference from Cbull If C reduces transmit power it can still communicate with D
ndash Reduces energy consumption at node Cndash Allows B to receive Arsquos transmission (spatial reuse)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 57
bull A special wireless ad hoc networkndash Large number of nodesndash Battery poweredndash Topology and density changendash Nodes for a common taskndash In-network data processing
bull Sensor-net applicationsndash Sensor-triggered bursty trafficndash Can often tolerate some delay
Ad hoc Sensor Networks Multiple channel access
Characteristics of Sensor Network
Scalabilty and self configurationEnergy efficiencyAdaptivityFairness not importantLatency
AdaptivityEnergy-delay tradoff
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 58
Ad hoc Sensor Networks Multiple channel access
Wireless sensor node and sensor network
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 59
MAC for WSN attributes bull Attributes
ndash Collision avoidance (Basic task)ndash Energy efficiency (most primary in WSN)ndash Scalability and adaptivityndash Channel utilizationndash Latencyndash Throughputndash Fairness
bull What causes energy wastendash Collisionsndash Control packet overheadndash Overhearing unnecessary trafficndash Long idle time (it consumes 50-100 of the power)
Ad hoc Sensor Networks Multiple channel access
Primary
Secondary
Dominant factor
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 60
MAC for WSN
bull Contention-based protocols need to work hard in all directions for energysavingsndash Reduce idle listening ndash support low duty cyclendash Better collision avoidancendash Reduce control overheadndash Avoid unnecessary overhearing
Ad hoc Sensor Networks Multiple channel access
Scheduled Protocols Contention ProtocolsCollisions No YesEnergy efficiency Good BadScalability and adaptivity Bad GoodMulti-hop communication Difficult EasyTime synchronization Strict Loose or not required
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 61
MAC for WSN classification
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 62
MAC for WSN SMAC (2002)bull It has been developed by Ye Heidemann and Estrinbull Tradeoffs between (latency fairness) and energybull Major components in S-MAC
ndash Periodic listen and sleep turn off radio when sleeping the reduced dutycycle is ~ 10 (eg 120ms on 12s off)ndash Collision avoidance is based on contention similarly to IEEE 80211 adhoc modendash Overhearing avoidance the node is sleeping when neighbors talkndash Massage passing long message is fragmented amp sent in burst operationwith extended Tx time and re-transmit occurs immediately
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 63
Summarybull The algorithms and protocols that enable sharing by multiple users and
controls the access to the wireless channel from different users are calledMAC
bull There are two types of wireless networks Infrastructured wireless networksand ad hoc wireless networks
bull We also want our wireless system to have high user capacity and we canidentify 10 methods for increasing it
bull There are several MACs with no absolute advantage it should be tailoredto the system requirements
bull There are primary concerns for designing a WSN MAC (collisionavoidance energy efficiency scalability and adaptivity) while thesecondary concerns are as follows (channel utilization latency throughputfairness)
bull Next lecture Routing protocols
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 47
Hidden terminal problembull A sends to B C cannot receive Abull C wants to send to B C senses a
ldquofreerdquo medium (CS fails)bull collision at B A cannot receive the
collision (CD fails)bull A is ldquohiddenrdquo for C
Ad hoc Sensor Networks Multiple channel access
Random Access hidden and exposed terminal problemExposed terminal problem
bull B sends to A C wants to send to Dbull C has to wait CS signals a medium
in usebull since A is outside the radio range of
C waiting is not necessarybull C is ldquoexposedrdquo to B
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 48
Random Access MACAWhen a station wants to transmit databull It sends an RTS (Ready-to-Send) packet to the intended receiver
ndash The RTS packet contains the length of the data that needs to be transmittedndash Any station other than the intended recipient hearing RTS defers transmission for atime duration equal to the end of the corresponding CTS reception
bull The receiver sends back CTS (Clear-to-Send) packet back to sender if it isavailable to receivendash The CTS packet contains the length of the data that original sender wants totransmitndash Any station other than the original RTS sender hearing CTS defers transmissionuntil the data is sent
bull The original sender upon reception of the CTS starts transmitting
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 49
Hidden terminal problem
Ad hoc Sensor Networks Multiple channel access
Random Access MACA (contrsquo)Exposed terminal problem
Waiting time of node X is muchsmaller than waiting time of node C
bull C defers transmission upon hearing BrsquosRTS until B could get CTS from Abull After that C can start transmission to DFor that it first sends an RTSbull C is not longer exposed to the datatransmission of B
Problem It does not address the collision of RTS packet
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 50
Random Access reliabilitybull Wireless links are prone to have errors High packet loss rate detrimental to
transport-layer performance Mechanisms needed to reduce packet loss rateexperienced by upper layers
bull When node B receives a data packet from node A node B sends anAcknowledgement (Ack) This approach adopted in many protocols
bull If node A fails to receive an Ack it will retransmit the packet
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 51
Random Access MACAW (CSMACA) bull Physical carrier sense and virtual carrier sense using Network Allocation
Vector (NAV)ndash NAV is updated based on overheard RTSCTSDATAACK packets each of whichspecified duration of a pending transmissionndash Nodes stay silent when carrier sensed (physicalvirtual)
bull Backoff intervals used to reduce collision probabilityndash When transmitting a packet choose a backoff interval in the range [0w] where wis contention windowndash Count down the backoff interval when channel is idlendash Count down is suspended if channel becomes busyndash When backoff interval reaches 0 transmit RTS
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 52
Random Access CSMACA (contrsquo) bull The time spent counting down backoff intervals is a part of MAC overhead
ndash Choosing a large w leads to large backoff intervals and can result in larger overheadndash Choosing a small w leads to a larger number of collisions (when two nodes countdown to 0 simultaneously)ndash Since the number of nodes attempting to transmit simultaneously may change withtime some mechanism to manage contention is needed
bull IEEE 80211 DCF contention window w is chosen dynamically depending oncollision occurrence (binary exponential backoff)ndash When a node fails to receive CTS in response to its RTS it increases the contentionwindowndash w is doubled (up to an upper bound)ndash When a node successfully completes a data transfer it restores w to wmin
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 53
Random Access fairnessbull Simplest definition of fairness all nodes should receive equal bandwidthbull An example of unfairness
ndash Assume that initially A and B both choose a backoff interval in range [031] buttheir RTSs collidendash Nodes A and B then choose from range [063]ndash Node A chooses 4 slots and B choose 60 slotsndash After A transmits a packet it next chooses from range [031]ndash It is possible that A may transmit several packets before B transmits its first packet
Ad hoc Sensor Networks Multiple channel access
Unfairness occurs when one node has backed off much more than some other node
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 54
Random Access fairness (contrsquo) MACAW solution for fairnessbull When a node transmits a packet it appends value w to the packet all nodes
hearing value w use it for their future transmission attemptsbull Since w is an indication of the level of congestion in the vicinity of a specific
receiver node MACAW proposes maintaining w independently for eachreceiver
bull Using per-receiver wi is particularly useful in multi-hop environments sincecongestion level at different receivers can be very different
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 55
Random Access energy efficiency bull Since many mobile hosts are operated by batteries MAC
protocols which conserve energy are of interestbull Two approaches to reduce energy consumption
ndash Power save turn off wireless interface when desirablendash Power control reduce transmit power
bull Power control has some more potential benefitsndash Reduced interference (in ad hoc networks) eg it improves ALOHAefficiency where user with high power can capture a packet even if there is acollisionndash Increased spatial reuse (in infrastructured wireless networks) eg it is usedin CDMA to maintain target SIR of voice and data users or generally it canbe used to maintain target SIR for different user classes (admission control)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 56
Random Access power control bull When C transmits to D at a high power level B cannot receive Arsquos
transmission due to interference from Cbull If C reduces transmit power it can still communicate with D
ndash Reduces energy consumption at node Cndash Allows B to receive Arsquos transmission (spatial reuse)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 57
bull A special wireless ad hoc networkndash Large number of nodesndash Battery poweredndash Topology and density changendash Nodes for a common taskndash In-network data processing
bull Sensor-net applicationsndash Sensor-triggered bursty trafficndash Can often tolerate some delay
Ad hoc Sensor Networks Multiple channel access
Characteristics of Sensor Network
Scalabilty and self configurationEnergy efficiencyAdaptivityFairness not importantLatency
AdaptivityEnergy-delay tradoff
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 58
Ad hoc Sensor Networks Multiple channel access
Wireless sensor node and sensor network
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 59
MAC for WSN attributes bull Attributes
ndash Collision avoidance (Basic task)ndash Energy efficiency (most primary in WSN)ndash Scalability and adaptivityndash Channel utilizationndash Latencyndash Throughputndash Fairness
bull What causes energy wastendash Collisionsndash Control packet overheadndash Overhearing unnecessary trafficndash Long idle time (it consumes 50-100 of the power)
Ad hoc Sensor Networks Multiple channel access
Primary
Secondary
Dominant factor
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 60
MAC for WSN
bull Contention-based protocols need to work hard in all directions for energysavingsndash Reduce idle listening ndash support low duty cyclendash Better collision avoidancendash Reduce control overheadndash Avoid unnecessary overhearing
Ad hoc Sensor Networks Multiple channel access
Scheduled Protocols Contention ProtocolsCollisions No YesEnergy efficiency Good BadScalability and adaptivity Bad GoodMulti-hop communication Difficult EasyTime synchronization Strict Loose or not required
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 61
MAC for WSN classification
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 62
MAC for WSN SMAC (2002)bull It has been developed by Ye Heidemann and Estrinbull Tradeoffs between (latency fairness) and energybull Major components in S-MAC
ndash Periodic listen and sleep turn off radio when sleeping the reduced dutycycle is ~ 10 (eg 120ms on 12s off)ndash Collision avoidance is based on contention similarly to IEEE 80211 adhoc modendash Overhearing avoidance the node is sleeping when neighbors talkndash Massage passing long message is fragmented amp sent in burst operationwith extended Tx time and re-transmit occurs immediately
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 63
Summarybull The algorithms and protocols that enable sharing by multiple users and
controls the access to the wireless channel from different users are calledMAC
bull There are two types of wireless networks Infrastructured wireless networksand ad hoc wireless networks
bull We also want our wireless system to have high user capacity and we canidentify 10 methods for increasing it
bull There are several MACs with no absolute advantage it should be tailoredto the system requirements
bull There are primary concerns for designing a WSN MAC (collisionavoidance energy efficiency scalability and adaptivity) while thesecondary concerns are as follows (channel utilization latency throughputfairness)
bull Next lecture Routing protocols
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 48
Random Access MACAWhen a station wants to transmit databull It sends an RTS (Ready-to-Send) packet to the intended receiver
ndash The RTS packet contains the length of the data that needs to be transmittedndash Any station other than the intended recipient hearing RTS defers transmission for atime duration equal to the end of the corresponding CTS reception
bull The receiver sends back CTS (Clear-to-Send) packet back to sender if it isavailable to receivendash The CTS packet contains the length of the data that original sender wants totransmitndash Any station other than the original RTS sender hearing CTS defers transmissionuntil the data is sent
bull The original sender upon reception of the CTS starts transmitting
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 49
Hidden terminal problem
Ad hoc Sensor Networks Multiple channel access
Random Access MACA (contrsquo)Exposed terminal problem
Waiting time of node X is muchsmaller than waiting time of node C
bull C defers transmission upon hearing BrsquosRTS until B could get CTS from Abull After that C can start transmission to DFor that it first sends an RTSbull C is not longer exposed to the datatransmission of B
Problem It does not address the collision of RTS packet
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 50
Random Access reliabilitybull Wireless links are prone to have errors High packet loss rate detrimental to
transport-layer performance Mechanisms needed to reduce packet loss rateexperienced by upper layers
bull When node B receives a data packet from node A node B sends anAcknowledgement (Ack) This approach adopted in many protocols
bull If node A fails to receive an Ack it will retransmit the packet
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 51
Random Access MACAW (CSMACA) bull Physical carrier sense and virtual carrier sense using Network Allocation
Vector (NAV)ndash NAV is updated based on overheard RTSCTSDATAACK packets each of whichspecified duration of a pending transmissionndash Nodes stay silent when carrier sensed (physicalvirtual)
bull Backoff intervals used to reduce collision probabilityndash When transmitting a packet choose a backoff interval in the range [0w] where wis contention windowndash Count down the backoff interval when channel is idlendash Count down is suspended if channel becomes busyndash When backoff interval reaches 0 transmit RTS
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 52
Random Access CSMACA (contrsquo) bull The time spent counting down backoff intervals is a part of MAC overhead
ndash Choosing a large w leads to large backoff intervals and can result in larger overheadndash Choosing a small w leads to a larger number of collisions (when two nodes countdown to 0 simultaneously)ndash Since the number of nodes attempting to transmit simultaneously may change withtime some mechanism to manage contention is needed
bull IEEE 80211 DCF contention window w is chosen dynamically depending oncollision occurrence (binary exponential backoff)ndash When a node fails to receive CTS in response to its RTS it increases the contentionwindowndash w is doubled (up to an upper bound)ndash When a node successfully completes a data transfer it restores w to wmin
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 53
Random Access fairnessbull Simplest definition of fairness all nodes should receive equal bandwidthbull An example of unfairness
ndash Assume that initially A and B both choose a backoff interval in range [031] buttheir RTSs collidendash Nodes A and B then choose from range [063]ndash Node A chooses 4 slots and B choose 60 slotsndash After A transmits a packet it next chooses from range [031]ndash It is possible that A may transmit several packets before B transmits its first packet
Ad hoc Sensor Networks Multiple channel access
Unfairness occurs when one node has backed off much more than some other node
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 54
Random Access fairness (contrsquo) MACAW solution for fairnessbull When a node transmits a packet it appends value w to the packet all nodes
hearing value w use it for their future transmission attemptsbull Since w is an indication of the level of congestion in the vicinity of a specific
receiver node MACAW proposes maintaining w independently for eachreceiver
bull Using per-receiver wi is particularly useful in multi-hop environments sincecongestion level at different receivers can be very different
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 55
Random Access energy efficiency bull Since many mobile hosts are operated by batteries MAC
protocols which conserve energy are of interestbull Two approaches to reduce energy consumption
ndash Power save turn off wireless interface when desirablendash Power control reduce transmit power
bull Power control has some more potential benefitsndash Reduced interference (in ad hoc networks) eg it improves ALOHAefficiency where user with high power can capture a packet even if there is acollisionndash Increased spatial reuse (in infrastructured wireless networks) eg it is usedin CDMA to maintain target SIR of voice and data users or generally it canbe used to maintain target SIR for different user classes (admission control)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 56
Random Access power control bull When C transmits to D at a high power level B cannot receive Arsquos
transmission due to interference from Cbull If C reduces transmit power it can still communicate with D
ndash Reduces energy consumption at node Cndash Allows B to receive Arsquos transmission (spatial reuse)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 57
bull A special wireless ad hoc networkndash Large number of nodesndash Battery poweredndash Topology and density changendash Nodes for a common taskndash In-network data processing
bull Sensor-net applicationsndash Sensor-triggered bursty trafficndash Can often tolerate some delay
Ad hoc Sensor Networks Multiple channel access
Characteristics of Sensor Network
Scalabilty and self configurationEnergy efficiencyAdaptivityFairness not importantLatency
AdaptivityEnergy-delay tradoff
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 58
Ad hoc Sensor Networks Multiple channel access
Wireless sensor node and sensor network
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 59
MAC for WSN attributes bull Attributes
ndash Collision avoidance (Basic task)ndash Energy efficiency (most primary in WSN)ndash Scalability and adaptivityndash Channel utilizationndash Latencyndash Throughputndash Fairness
bull What causes energy wastendash Collisionsndash Control packet overheadndash Overhearing unnecessary trafficndash Long idle time (it consumes 50-100 of the power)
Ad hoc Sensor Networks Multiple channel access
Primary
Secondary
Dominant factor
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 60
MAC for WSN
bull Contention-based protocols need to work hard in all directions for energysavingsndash Reduce idle listening ndash support low duty cyclendash Better collision avoidancendash Reduce control overheadndash Avoid unnecessary overhearing
Ad hoc Sensor Networks Multiple channel access
Scheduled Protocols Contention ProtocolsCollisions No YesEnergy efficiency Good BadScalability and adaptivity Bad GoodMulti-hop communication Difficult EasyTime synchronization Strict Loose or not required
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 61
MAC for WSN classification
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 62
MAC for WSN SMAC (2002)bull It has been developed by Ye Heidemann and Estrinbull Tradeoffs between (latency fairness) and energybull Major components in S-MAC
ndash Periodic listen and sleep turn off radio when sleeping the reduced dutycycle is ~ 10 (eg 120ms on 12s off)ndash Collision avoidance is based on contention similarly to IEEE 80211 adhoc modendash Overhearing avoidance the node is sleeping when neighbors talkndash Massage passing long message is fragmented amp sent in burst operationwith extended Tx time and re-transmit occurs immediately
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 63
Summarybull The algorithms and protocols that enable sharing by multiple users and
controls the access to the wireless channel from different users are calledMAC
bull There are two types of wireless networks Infrastructured wireless networksand ad hoc wireless networks
bull We also want our wireless system to have high user capacity and we canidentify 10 methods for increasing it
bull There are several MACs with no absolute advantage it should be tailoredto the system requirements
bull There are primary concerns for designing a WSN MAC (collisionavoidance energy efficiency scalability and adaptivity) while thesecondary concerns are as follows (channel utilization latency throughputfairness)
bull Next lecture Routing protocols
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 49
Hidden terminal problem
Ad hoc Sensor Networks Multiple channel access
Random Access MACA (contrsquo)Exposed terminal problem
Waiting time of node X is muchsmaller than waiting time of node C
bull C defers transmission upon hearing BrsquosRTS until B could get CTS from Abull After that C can start transmission to DFor that it first sends an RTSbull C is not longer exposed to the datatransmission of B
Problem It does not address the collision of RTS packet
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 50
Random Access reliabilitybull Wireless links are prone to have errors High packet loss rate detrimental to
transport-layer performance Mechanisms needed to reduce packet loss rateexperienced by upper layers
bull When node B receives a data packet from node A node B sends anAcknowledgement (Ack) This approach adopted in many protocols
bull If node A fails to receive an Ack it will retransmit the packet
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 51
Random Access MACAW (CSMACA) bull Physical carrier sense and virtual carrier sense using Network Allocation
Vector (NAV)ndash NAV is updated based on overheard RTSCTSDATAACK packets each of whichspecified duration of a pending transmissionndash Nodes stay silent when carrier sensed (physicalvirtual)
bull Backoff intervals used to reduce collision probabilityndash When transmitting a packet choose a backoff interval in the range [0w] where wis contention windowndash Count down the backoff interval when channel is idlendash Count down is suspended if channel becomes busyndash When backoff interval reaches 0 transmit RTS
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 52
Random Access CSMACA (contrsquo) bull The time spent counting down backoff intervals is a part of MAC overhead
ndash Choosing a large w leads to large backoff intervals and can result in larger overheadndash Choosing a small w leads to a larger number of collisions (when two nodes countdown to 0 simultaneously)ndash Since the number of nodes attempting to transmit simultaneously may change withtime some mechanism to manage contention is needed
bull IEEE 80211 DCF contention window w is chosen dynamically depending oncollision occurrence (binary exponential backoff)ndash When a node fails to receive CTS in response to its RTS it increases the contentionwindowndash w is doubled (up to an upper bound)ndash When a node successfully completes a data transfer it restores w to wmin
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 53
Random Access fairnessbull Simplest definition of fairness all nodes should receive equal bandwidthbull An example of unfairness
ndash Assume that initially A and B both choose a backoff interval in range [031] buttheir RTSs collidendash Nodes A and B then choose from range [063]ndash Node A chooses 4 slots and B choose 60 slotsndash After A transmits a packet it next chooses from range [031]ndash It is possible that A may transmit several packets before B transmits its first packet
Ad hoc Sensor Networks Multiple channel access
Unfairness occurs when one node has backed off much more than some other node
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 54
Random Access fairness (contrsquo) MACAW solution for fairnessbull When a node transmits a packet it appends value w to the packet all nodes
hearing value w use it for their future transmission attemptsbull Since w is an indication of the level of congestion in the vicinity of a specific
receiver node MACAW proposes maintaining w independently for eachreceiver
bull Using per-receiver wi is particularly useful in multi-hop environments sincecongestion level at different receivers can be very different
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 55
Random Access energy efficiency bull Since many mobile hosts are operated by batteries MAC
protocols which conserve energy are of interestbull Two approaches to reduce energy consumption
ndash Power save turn off wireless interface when desirablendash Power control reduce transmit power
bull Power control has some more potential benefitsndash Reduced interference (in ad hoc networks) eg it improves ALOHAefficiency where user with high power can capture a packet even if there is acollisionndash Increased spatial reuse (in infrastructured wireless networks) eg it is usedin CDMA to maintain target SIR of voice and data users or generally it canbe used to maintain target SIR for different user classes (admission control)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 56
Random Access power control bull When C transmits to D at a high power level B cannot receive Arsquos
transmission due to interference from Cbull If C reduces transmit power it can still communicate with D
ndash Reduces energy consumption at node Cndash Allows B to receive Arsquos transmission (spatial reuse)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 57
bull A special wireless ad hoc networkndash Large number of nodesndash Battery poweredndash Topology and density changendash Nodes for a common taskndash In-network data processing
bull Sensor-net applicationsndash Sensor-triggered bursty trafficndash Can often tolerate some delay
Ad hoc Sensor Networks Multiple channel access
Characteristics of Sensor Network
Scalabilty and self configurationEnergy efficiencyAdaptivityFairness not importantLatency
AdaptivityEnergy-delay tradoff
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 58
Ad hoc Sensor Networks Multiple channel access
Wireless sensor node and sensor network
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 59
MAC for WSN attributes bull Attributes
ndash Collision avoidance (Basic task)ndash Energy efficiency (most primary in WSN)ndash Scalability and adaptivityndash Channel utilizationndash Latencyndash Throughputndash Fairness
bull What causes energy wastendash Collisionsndash Control packet overheadndash Overhearing unnecessary trafficndash Long idle time (it consumes 50-100 of the power)
Ad hoc Sensor Networks Multiple channel access
Primary
Secondary
Dominant factor
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 60
MAC for WSN
bull Contention-based protocols need to work hard in all directions for energysavingsndash Reduce idle listening ndash support low duty cyclendash Better collision avoidancendash Reduce control overheadndash Avoid unnecessary overhearing
Ad hoc Sensor Networks Multiple channel access
Scheduled Protocols Contention ProtocolsCollisions No YesEnergy efficiency Good BadScalability and adaptivity Bad GoodMulti-hop communication Difficult EasyTime synchronization Strict Loose or not required
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 61
MAC for WSN classification
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 62
MAC for WSN SMAC (2002)bull It has been developed by Ye Heidemann and Estrinbull Tradeoffs between (latency fairness) and energybull Major components in S-MAC
ndash Periodic listen and sleep turn off radio when sleeping the reduced dutycycle is ~ 10 (eg 120ms on 12s off)ndash Collision avoidance is based on contention similarly to IEEE 80211 adhoc modendash Overhearing avoidance the node is sleeping when neighbors talkndash Massage passing long message is fragmented amp sent in burst operationwith extended Tx time and re-transmit occurs immediately
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 63
Summarybull The algorithms and protocols that enable sharing by multiple users and
controls the access to the wireless channel from different users are calledMAC
bull There are two types of wireless networks Infrastructured wireless networksand ad hoc wireless networks
bull We also want our wireless system to have high user capacity and we canidentify 10 methods for increasing it
bull There are several MACs with no absolute advantage it should be tailoredto the system requirements
bull There are primary concerns for designing a WSN MAC (collisionavoidance energy efficiency scalability and adaptivity) while thesecondary concerns are as follows (channel utilization latency throughputfairness)
bull Next lecture Routing protocols
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 50
Random Access reliabilitybull Wireless links are prone to have errors High packet loss rate detrimental to
transport-layer performance Mechanisms needed to reduce packet loss rateexperienced by upper layers
bull When node B receives a data packet from node A node B sends anAcknowledgement (Ack) This approach adopted in many protocols
bull If node A fails to receive an Ack it will retransmit the packet
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 51
Random Access MACAW (CSMACA) bull Physical carrier sense and virtual carrier sense using Network Allocation
Vector (NAV)ndash NAV is updated based on overheard RTSCTSDATAACK packets each of whichspecified duration of a pending transmissionndash Nodes stay silent when carrier sensed (physicalvirtual)
bull Backoff intervals used to reduce collision probabilityndash When transmitting a packet choose a backoff interval in the range [0w] where wis contention windowndash Count down the backoff interval when channel is idlendash Count down is suspended if channel becomes busyndash When backoff interval reaches 0 transmit RTS
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 52
Random Access CSMACA (contrsquo) bull The time spent counting down backoff intervals is a part of MAC overhead
ndash Choosing a large w leads to large backoff intervals and can result in larger overheadndash Choosing a small w leads to a larger number of collisions (when two nodes countdown to 0 simultaneously)ndash Since the number of nodes attempting to transmit simultaneously may change withtime some mechanism to manage contention is needed
bull IEEE 80211 DCF contention window w is chosen dynamically depending oncollision occurrence (binary exponential backoff)ndash When a node fails to receive CTS in response to its RTS it increases the contentionwindowndash w is doubled (up to an upper bound)ndash When a node successfully completes a data transfer it restores w to wmin
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 53
Random Access fairnessbull Simplest definition of fairness all nodes should receive equal bandwidthbull An example of unfairness
ndash Assume that initially A and B both choose a backoff interval in range [031] buttheir RTSs collidendash Nodes A and B then choose from range [063]ndash Node A chooses 4 slots and B choose 60 slotsndash After A transmits a packet it next chooses from range [031]ndash It is possible that A may transmit several packets before B transmits its first packet
Ad hoc Sensor Networks Multiple channel access
Unfairness occurs when one node has backed off much more than some other node
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 54
Random Access fairness (contrsquo) MACAW solution for fairnessbull When a node transmits a packet it appends value w to the packet all nodes
hearing value w use it for their future transmission attemptsbull Since w is an indication of the level of congestion in the vicinity of a specific
receiver node MACAW proposes maintaining w independently for eachreceiver
bull Using per-receiver wi is particularly useful in multi-hop environments sincecongestion level at different receivers can be very different
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 55
Random Access energy efficiency bull Since many mobile hosts are operated by batteries MAC
protocols which conserve energy are of interestbull Two approaches to reduce energy consumption
ndash Power save turn off wireless interface when desirablendash Power control reduce transmit power
bull Power control has some more potential benefitsndash Reduced interference (in ad hoc networks) eg it improves ALOHAefficiency where user with high power can capture a packet even if there is acollisionndash Increased spatial reuse (in infrastructured wireless networks) eg it is usedin CDMA to maintain target SIR of voice and data users or generally it canbe used to maintain target SIR for different user classes (admission control)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 56
Random Access power control bull When C transmits to D at a high power level B cannot receive Arsquos
transmission due to interference from Cbull If C reduces transmit power it can still communicate with D
ndash Reduces energy consumption at node Cndash Allows B to receive Arsquos transmission (spatial reuse)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 57
bull A special wireless ad hoc networkndash Large number of nodesndash Battery poweredndash Topology and density changendash Nodes for a common taskndash In-network data processing
bull Sensor-net applicationsndash Sensor-triggered bursty trafficndash Can often tolerate some delay
Ad hoc Sensor Networks Multiple channel access
Characteristics of Sensor Network
Scalabilty and self configurationEnergy efficiencyAdaptivityFairness not importantLatency
AdaptivityEnergy-delay tradoff
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 58
Ad hoc Sensor Networks Multiple channel access
Wireless sensor node and sensor network
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 59
MAC for WSN attributes bull Attributes
ndash Collision avoidance (Basic task)ndash Energy efficiency (most primary in WSN)ndash Scalability and adaptivityndash Channel utilizationndash Latencyndash Throughputndash Fairness
bull What causes energy wastendash Collisionsndash Control packet overheadndash Overhearing unnecessary trafficndash Long idle time (it consumes 50-100 of the power)
Ad hoc Sensor Networks Multiple channel access
Primary
Secondary
Dominant factor
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 60
MAC for WSN
bull Contention-based protocols need to work hard in all directions for energysavingsndash Reduce idle listening ndash support low duty cyclendash Better collision avoidancendash Reduce control overheadndash Avoid unnecessary overhearing
Ad hoc Sensor Networks Multiple channel access
Scheduled Protocols Contention ProtocolsCollisions No YesEnergy efficiency Good BadScalability and adaptivity Bad GoodMulti-hop communication Difficult EasyTime synchronization Strict Loose or not required
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 61
MAC for WSN classification
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 62
MAC for WSN SMAC (2002)bull It has been developed by Ye Heidemann and Estrinbull Tradeoffs between (latency fairness) and energybull Major components in S-MAC
ndash Periodic listen and sleep turn off radio when sleeping the reduced dutycycle is ~ 10 (eg 120ms on 12s off)ndash Collision avoidance is based on contention similarly to IEEE 80211 adhoc modendash Overhearing avoidance the node is sleeping when neighbors talkndash Massage passing long message is fragmented amp sent in burst operationwith extended Tx time and re-transmit occurs immediately
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 63
Summarybull The algorithms and protocols that enable sharing by multiple users and
controls the access to the wireless channel from different users are calledMAC
bull There are two types of wireless networks Infrastructured wireless networksand ad hoc wireless networks
bull We also want our wireless system to have high user capacity and we canidentify 10 methods for increasing it
bull There are several MACs with no absolute advantage it should be tailoredto the system requirements
bull There are primary concerns for designing a WSN MAC (collisionavoidance energy efficiency scalability and adaptivity) while thesecondary concerns are as follows (channel utilization latency throughputfairness)
bull Next lecture Routing protocols
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 51
Random Access MACAW (CSMACA) bull Physical carrier sense and virtual carrier sense using Network Allocation
Vector (NAV)ndash NAV is updated based on overheard RTSCTSDATAACK packets each of whichspecified duration of a pending transmissionndash Nodes stay silent when carrier sensed (physicalvirtual)
bull Backoff intervals used to reduce collision probabilityndash When transmitting a packet choose a backoff interval in the range [0w] where wis contention windowndash Count down the backoff interval when channel is idlendash Count down is suspended if channel becomes busyndash When backoff interval reaches 0 transmit RTS
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 52
Random Access CSMACA (contrsquo) bull The time spent counting down backoff intervals is a part of MAC overhead
ndash Choosing a large w leads to large backoff intervals and can result in larger overheadndash Choosing a small w leads to a larger number of collisions (when two nodes countdown to 0 simultaneously)ndash Since the number of nodes attempting to transmit simultaneously may change withtime some mechanism to manage contention is needed
bull IEEE 80211 DCF contention window w is chosen dynamically depending oncollision occurrence (binary exponential backoff)ndash When a node fails to receive CTS in response to its RTS it increases the contentionwindowndash w is doubled (up to an upper bound)ndash When a node successfully completes a data transfer it restores w to wmin
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 53
Random Access fairnessbull Simplest definition of fairness all nodes should receive equal bandwidthbull An example of unfairness
ndash Assume that initially A and B both choose a backoff interval in range [031] buttheir RTSs collidendash Nodes A and B then choose from range [063]ndash Node A chooses 4 slots and B choose 60 slotsndash After A transmits a packet it next chooses from range [031]ndash It is possible that A may transmit several packets before B transmits its first packet
Ad hoc Sensor Networks Multiple channel access
Unfairness occurs when one node has backed off much more than some other node
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 54
Random Access fairness (contrsquo) MACAW solution for fairnessbull When a node transmits a packet it appends value w to the packet all nodes
hearing value w use it for their future transmission attemptsbull Since w is an indication of the level of congestion in the vicinity of a specific
receiver node MACAW proposes maintaining w independently for eachreceiver
bull Using per-receiver wi is particularly useful in multi-hop environments sincecongestion level at different receivers can be very different
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 55
Random Access energy efficiency bull Since many mobile hosts are operated by batteries MAC
protocols which conserve energy are of interestbull Two approaches to reduce energy consumption
ndash Power save turn off wireless interface when desirablendash Power control reduce transmit power
bull Power control has some more potential benefitsndash Reduced interference (in ad hoc networks) eg it improves ALOHAefficiency where user with high power can capture a packet even if there is acollisionndash Increased spatial reuse (in infrastructured wireless networks) eg it is usedin CDMA to maintain target SIR of voice and data users or generally it canbe used to maintain target SIR for different user classes (admission control)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 56
Random Access power control bull When C transmits to D at a high power level B cannot receive Arsquos
transmission due to interference from Cbull If C reduces transmit power it can still communicate with D
ndash Reduces energy consumption at node Cndash Allows B to receive Arsquos transmission (spatial reuse)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 57
bull A special wireless ad hoc networkndash Large number of nodesndash Battery poweredndash Topology and density changendash Nodes for a common taskndash In-network data processing
bull Sensor-net applicationsndash Sensor-triggered bursty trafficndash Can often tolerate some delay
Ad hoc Sensor Networks Multiple channel access
Characteristics of Sensor Network
Scalabilty and self configurationEnergy efficiencyAdaptivityFairness not importantLatency
AdaptivityEnergy-delay tradoff
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 58
Ad hoc Sensor Networks Multiple channel access
Wireless sensor node and sensor network
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 59
MAC for WSN attributes bull Attributes
ndash Collision avoidance (Basic task)ndash Energy efficiency (most primary in WSN)ndash Scalability and adaptivityndash Channel utilizationndash Latencyndash Throughputndash Fairness
bull What causes energy wastendash Collisionsndash Control packet overheadndash Overhearing unnecessary trafficndash Long idle time (it consumes 50-100 of the power)
Ad hoc Sensor Networks Multiple channel access
Primary
Secondary
Dominant factor
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 60
MAC for WSN
bull Contention-based protocols need to work hard in all directions for energysavingsndash Reduce idle listening ndash support low duty cyclendash Better collision avoidancendash Reduce control overheadndash Avoid unnecessary overhearing
Ad hoc Sensor Networks Multiple channel access
Scheduled Protocols Contention ProtocolsCollisions No YesEnergy efficiency Good BadScalability and adaptivity Bad GoodMulti-hop communication Difficult EasyTime synchronization Strict Loose or not required
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 61
MAC for WSN classification
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 62
MAC for WSN SMAC (2002)bull It has been developed by Ye Heidemann and Estrinbull Tradeoffs between (latency fairness) and energybull Major components in S-MAC
ndash Periodic listen and sleep turn off radio when sleeping the reduced dutycycle is ~ 10 (eg 120ms on 12s off)ndash Collision avoidance is based on contention similarly to IEEE 80211 adhoc modendash Overhearing avoidance the node is sleeping when neighbors talkndash Massage passing long message is fragmented amp sent in burst operationwith extended Tx time and re-transmit occurs immediately
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 63
Summarybull The algorithms and protocols that enable sharing by multiple users and
controls the access to the wireless channel from different users are calledMAC
bull There are two types of wireless networks Infrastructured wireless networksand ad hoc wireless networks
bull We also want our wireless system to have high user capacity and we canidentify 10 methods for increasing it
bull There are several MACs with no absolute advantage it should be tailoredto the system requirements
bull There are primary concerns for designing a WSN MAC (collisionavoidance energy efficiency scalability and adaptivity) while thesecondary concerns are as follows (channel utilization latency throughputfairness)
bull Next lecture Routing protocols
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 52
Random Access CSMACA (contrsquo) bull The time spent counting down backoff intervals is a part of MAC overhead
ndash Choosing a large w leads to large backoff intervals and can result in larger overheadndash Choosing a small w leads to a larger number of collisions (when two nodes countdown to 0 simultaneously)ndash Since the number of nodes attempting to transmit simultaneously may change withtime some mechanism to manage contention is needed
bull IEEE 80211 DCF contention window w is chosen dynamically depending oncollision occurrence (binary exponential backoff)ndash When a node fails to receive CTS in response to its RTS it increases the contentionwindowndash w is doubled (up to an upper bound)ndash When a node successfully completes a data transfer it restores w to wmin
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 53
Random Access fairnessbull Simplest definition of fairness all nodes should receive equal bandwidthbull An example of unfairness
ndash Assume that initially A and B both choose a backoff interval in range [031] buttheir RTSs collidendash Nodes A and B then choose from range [063]ndash Node A chooses 4 slots and B choose 60 slotsndash After A transmits a packet it next chooses from range [031]ndash It is possible that A may transmit several packets before B transmits its first packet
Ad hoc Sensor Networks Multiple channel access
Unfairness occurs when one node has backed off much more than some other node
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 54
Random Access fairness (contrsquo) MACAW solution for fairnessbull When a node transmits a packet it appends value w to the packet all nodes
hearing value w use it for their future transmission attemptsbull Since w is an indication of the level of congestion in the vicinity of a specific
receiver node MACAW proposes maintaining w independently for eachreceiver
bull Using per-receiver wi is particularly useful in multi-hop environments sincecongestion level at different receivers can be very different
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 55
Random Access energy efficiency bull Since many mobile hosts are operated by batteries MAC
protocols which conserve energy are of interestbull Two approaches to reduce energy consumption
ndash Power save turn off wireless interface when desirablendash Power control reduce transmit power
bull Power control has some more potential benefitsndash Reduced interference (in ad hoc networks) eg it improves ALOHAefficiency where user with high power can capture a packet even if there is acollisionndash Increased spatial reuse (in infrastructured wireless networks) eg it is usedin CDMA to maintain target SIR of voice and data users or generally it canbe used to maintain target SIR for different user classes (admission control)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 56
Random Access power control bull When C transmits to D at a high power level B cannot receive Arsquos
transmission due to interference from Cbull If C reduces transmit power it can still communicate with D
ndash Reduces energy consumption at node Cndash Allows B to receive Arsquos transmission (spatial reuse)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 57
bull A special wireless ad hoc networkndash Large number of nodesndash Battery poweredndash Topology and density changendash Nodes for a common taskndash In-network data processing
bull Sensor-net applicationsndash Sensor-triggered bursty trafficndash Can often tolerate some delay
Ad hoc Sensor Networks Multiple channel access
Characteristics of Sensor Network
Scalabilty and self configurationEnergy efficiencyAdaptivityFairness not importantLatency
AdaptivityEnergy-delay tradoff
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 58
Ad hoc Sensor Networks Multiple channel access
Wireless sensor node and sensor network
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 59
MAC for WSN attributes bull Attributes
ndash Collision avoidance (Basic task)ndash Energy efficiency (most primary in WSN)ndash Scalability and adaptivityndash Channel utilizationndash Latencyndash Throughputndash Fairness
bull What causes energy wastendash Collisionsndash Control packet overheadndash Overhearing unnecessary trafficndash Long idle time (it consumes 50-100 of the power)
Ad hoc Sensor Networks Multiple channel access
Primary
Secondary
Dominant factor
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 60
MAC for WSN
bull Contention-based protocols need to work hard in all directions for energysavingsndash Reduce idle listening ndash support low duty cyclendash Better collision avoidancendash Reduce control overheadndash Avoid unnecessary overhearing
Ad hoc Sensor Networks Multiple channel access
Scheduled Protocols Contention ProtocolsCollisions No YesEnergy efficiency Good BadScalability and adaptivity Bad GoodMulti-hop communication Difficult EasyTime synchronization Strict Loose or not required
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 61
MAC for WSN classification
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 62
MAC for WSN SMAC (2002)bull It has been developed by Ye Heidemann and Estrinbull Tradeoffs between (latency fairness) and energybull Major components in S-MAC
ndash Periodic listen and sleep turn off radio when sleeping the reduced dutycycle is ~ 10 (eg 120ms on 12s off)ndash Collision avoidance is based on contention similarly to IEEE 80211 adhoc modendash Overhearing avoidance the node is sleeping when neighbors talkndash Massage passing long message is fragmented amp sent in burst operationwith extended Tx time and re-transmit occurs immediately
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 63
Summarybull The algorithms and protocols that enable sharing by multiple users and
controls the access to the wireless channel from different users are calledMAC
bull There are two types of wireless networks Infrastructured wireless networksand ad hoc wireless networks
bull We also want our wireless system to have high user capacity and we canidentify 10 methods for increasing it
bull There are several MACs with no absolute advantage it should be tailoredto the system requirements
bull There are primary concerns for designing a WSN MAC (collisionavoidance energy efficiency scalability and adaptivity) while thesecondary concerns are as follows (channel utilization latency throughputfairness)
bull Next lecture Routing protocols
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 53
Random Access fairnessbull Simplest definition of fairness all nodes should receive equal bandwidthbull An example of unfairness
ndash Assume that initially A and B both choose a backoff interval in range [031] buttheir RTSs collidendash Nodes A and B then choose from range [063]ndash Node A chooses 4 slots and B choose 60 slotsndash After A transmits a packet it next chooses from range [031]ndash It is possible that A may transmit several packets before B transmits its first packet
Ad hoc Sensor Networks Multiple channel access
Unfairness occurs when one node has backed off much more than some other node
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 54
Random Access fairness (contrsquo) MACAW solution for fairnessbull When a node transmits a packet it appends value w to the packet all nodes
hearing value w use it for their future transmission attemptsbull Since w is an indication of the level of congestion in the vicinity of a specific
receiver node MACAW proposes maintaining w independently for eachreceiver
bull Using per-receiver wi is particularly useful in multi-hop environments sincecongestion level at different receivers can be very different
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 55
Random Access energy efficiency bull Since many mobile hosts are operated by batteries MAC
protocols which conserve energy are of interestbull Two approaches to reduce energy consumption
ndash Power save turn off wireless interface when desirablendash Power control reduce transmit power
bull Power control has some more potential benefitsndash Reduced interference (in ad hoc networks) eg it improves ALOHAefficiency where user with high power can capture a packet even if there is acollisionndash Increased spatial reuse (in infrastructured wireless networks) eg it is usedin CDMA to maintain target SIR of voice and data users or generally it canbe used to maintain target SIR for different user classes (admission control)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 56
Random Access power control bull When C transmits to D at a high power level B cannot receive Arsquos
transmission due to interference from Cbull If C reduces transmit power it can still communicate with D
ndash Reduces energy consumption at node Cndash Allows B to receive Arsquos transmission (spatial reuse)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 57
bull A special wireless ad hoc networkndash Large number of nodesndash Battery poweredndash Topology and density changendash Nodes for a common taskndash In-network data processing
bull Sensor-net applicationsndash Sensor-triggered bursty trafficndash Can often tolerate some delay
Ad hoc Sensor Networks Multiple channel access
Characteristics of Sensor Network
Scalabilty and self configurationEnergy efficiencyAdaptivityFairness not importantLatency
AdaptivityEnergy-delay tradoff
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 58
Ad hoc Sensor Networks Multiple channel access
Wireless sensor node and sensor network
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 59
MAC for WSN attributes bull Attributes
ndash Collision avoidance (Basic task)ndash Energy efficiency (most primary in WSN)ndash Scalability and adaptivityndash Channel utilizationndash Latencyndash Throughputndash Fairness
bull What causes energy wastendash Collisionsndash Control packet overheadndash Overhearing unnecessary trafficndash Long idle time (it consumes 50-100 of the power)
Ad hoc Sensor Networks Multiple channel access
Primary
Secondary
Dominant factor
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 60
MAC for WSN
bull Contention-based protocols need to work hard in all directions for energysavingsndash Reduce idle listening ndash support low duty cyclendash Better collision avoidancendash Reduce control overheadndash Avoid unnecessary overhearing
Ad hoc Sensor Networks Multiple channel access
Scheduled Protocols Contention ProtocolsCollisions No YesEnergy efficiency Good BadScalability and adaptivity Bad GoodMulti-hop communication Difficult EasyTime synchronization Strict Loose or not required
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 61
MAC for WSN classification
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 62
MAC for WSN SMAC (2002)bull It has been developed by Ye Heidemann and Estrinbull Tradeoffs between (latency fairness) and energybull Major components in S-MAC
ndash Periodic listen and sleep turn off radio when sleeping the reduced dutycycle is ~ 10 (eg 120ms on 12s off)ndash Collision avoidance is based on contention similarly to IEEE 80211 adhoc modendash Overhearing avoidance the node is sleeping when neighbors talkndash Massage passing long message is fragmented amp sent in burst operationwith extended Tx time and re-transmit occurs immediately
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 63
Summarybull The algorithms and protocols that enable sharing by multiple users and
controls the access to the wireless channel from different users are calledMAC
bull There are two types of wireless networks Infrastructured wireless networksand ad hoc wireless networks
bull We also want our wireless system to have high user capacity and we canidentify 10 methods for increasing it
bull There are several MACs with no absolute advantage it should be tailoredto the system requirements
bull There are primary concerns for designing a WSN MAC (collisionavoidance energy efficiency scalability and adaptivity) while thesecondary concerns are as follows (channel utilization latency throughputfairness)
bull Next lecture Routing protocols
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 54
Random Access fairness (contrsquo) MACAW solution for fairnessbull When a node transmits a packet it appends value w to the packet all nodes
hearing value w use it for their future transmission attemptsbull Since w is an indication of the level of congestion in the vicinity of a specific
receiver node MACAW proposes maintaining w independently for eachreceiver
bull Using per-receiver wi is particularly useful in multi-hop environments sincecongestion level at different receivers can be very different
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 55
Random Access energy efficiency bull Since many mobile hosts are operated by batteries MAC
protocols which conserve energy are of interestbull Two approaches to reduce energy consumption
ndash Power save turn off wireless interface when desirablendash Power control reduce transmit power
bull Power control has some more potential benefitsndash Reduced interference (in ad hoc networks) eg it improves ALOHAefficiency where user with high power can capture a packet even if there is acollisionndash Increased spatial reuse (in infrastructured wireless networks) eg it is usedin CDMA to maintain target SIR of voice and data users or generally it canbe used to maintain target SIR for different user classes (admission control)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 56
Random Access power control bull When C transmits to D at a high power level B cannot receive Arsquos
transmission due to interference from Cbull If C reduces transmit power it can still communicate with D
ndash Reduces energy consumption at node Cndash Allows B to receive Arsquos transmission (spatial reuse)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 57
bull A special wireless ad hoc networkndash Large number of nodesndash Battery poweredndash Topology and density changendash Nodes for a common taskndash In-network data processing
bull Sensor-net applicationsndash Sensor-triggered bursty trafficndash Can often tolerate some delay
Ad hoc Sensor Networks Multiple channel access
Characteristics of Sensor Network
Scalabilty and self configurationEnergy efficiencyAdaptivityFairness not importantLatency
AdaptivityEnergy-delay tradoff
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 58
Ad hoc Sensor Networks Multiple channel access
Wireless sensor node and sensor network
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 59
MAC for WSN attributes bull Attributes
ndash Collision avoidance (Basic task)ndash Energy efficiency (most primary in WSN)ndash Scalability and adaptivityndash Channel utilizationndash Latencyndash Throughputndash Fairness
bull What causes energy wastendash Collisionsndash Control packet overheadndash Overhearing unnecessary trafficndash Long idle time (it consumes 50-100 of the power)
Ad hoc Sensor Networks Multiple channel access
Primary
Secondary
Dominant factor
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 60
MAC for WSN
bull Contention-based protocols need to work hard in all directions for energysavingsndash Reduce idle listening ndash support low duty cyclendash Better collision avoidancendash Reduce control overheadndash Avoid unnecessary overhearing
Ad hoc Sensor Networks Multiple channel access
Scheduled Protocols Contention ProtocolsCollisions No YesEnergy efficiency Good BadScalability and adaptivity Bad GoodMulti-hop communication Difficult EasyTime synchronization Strict Loose or not required
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 61
MAC for WSN classification
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 62
MAC for WSN SMAC (2002)bull It has been developed by Ye Heidemann and Estrinbull Tradeoffs between (latency fairness) and energybull Major components in S-MAC
ndash Periodic listen and sleep turn off radio when sleeping the reduced dutycycle is ~ 10 (eg 120ms on 12s off)ndash Collision avoidance is based on contention similarly to IEEE 80211 adhoc modendash Overhearing avoidance the node is sleeping when neighbors talkndash Massage passing long message is fragmented amp sent in burst operationwith extended Tx time and re-transmit occurs immediately
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 63
Summarybull The algorithms and protocols that enable sharing by multiple users and
controls the access to the wireless channel from different users are calledMAC
bull There are two types of wireless networks Infrastructured wireless networksand ad hoc wireless networks
bull We also want our wireless system to have high user capacity and we canidentify 10 methods for increasing it
bull There are several MACs with no absolute advantage it should be tailoredto the system requirements
bull There are primary concerns for designing a WSN MAC (collisionavoidance energy efficiency scalability and adaptivity) while thesecondary concerns are as follows (channel utilization latency throughputfairness)
bull Next lecture Routing protocols
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 55
Random Access energy efficiency bull Since many mobile hosts are operated by batteries MAC
protocols which conserve energy are of interestbull Two approaches to reduce energy consumption
ndash Power save turn off wireless interface when desirablendash Power control reduce transmit power
bull Power control has some more potential benefitsndash Reduced interference (in ad hoc networks) eg it improves ALOHAefficiency where user with high power can capture a packet even if there is acollisionndash Increased spatial reuse (in infrastructured wireless networks) eg it is usedin CDMA to maintain target SIR of voice and data users or generally it canbe used to maintain target SIR for different user classes (admission control)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 56
Random Access power control bull When C transmits to D at a high power level B cannot receive Arsquos
transmission due to interference from Cbull If C reduces transmit power it can still communicate with D
ndash Reduces energy consumption at node Cndash Allows B to receive Arsquos transmission (spatial reuse)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 57
bull A special wireless ad hoc networkndash Large number of nodesndash Battery poweredndash Topology and density changendash Nodes for a common taskndash In-network data processing
bull Sensor-net applicationsndash Sensor-triggered bursty trafficndash Can often tolerate some delay
Ad hoc Sensor Networks Multiple channel access
Characteristics of Sensor Network
Scalabilty and self configurationEnergy efficiencyAdaptivityFairness not importantLatency
AdaptivityEnergy-delay tradoff
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 58
Ad hoc Sensor Networks Multiple channel access
Wireless sensor node and sensor network
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 59
MAC for WSN attributes bull Attributes
ndash Collision avoidance (Basic task)ndash Energy efficiency (most primary in WSN)ndash Scalability and adaptivityndash Channel utilizationndash Latencyndash Throughputndash Fairness
bull What causes energy wastendash Collisionsndash Control packet overheadndash Overhearing unnecessary trafficndash Long idle time (it consumes 50-100 of the power)
Ad hoc Sensor Networks Multiple channel access
Primary
Secondary
Dominant factor
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 60
MAC for WSN
bull Contention-based protocols need to work hard in all directions for energysavingsndash Reduce idle listening ndash support low duty cyclendash Better collision avoidancendash Reduce control overheadndash Avoid unnecessary overhearing
Ad hoc Sensor Networks Multiple channel access
Scheduled Protocols Contention ProtocolsCollisions No YesEnergy efficiency Good BadScalability and adaptivity Bad GoodMulti-hop communication Difficult EasyTime synchronization Strict Loose or not required
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 61
MAC for WSN classification
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 62
MAC for WSN SMAC (2002)bull It has been developed by Ye Heidemann and Estrinbull Tradeoffs between (latency fairness) and energybull Major components in S-MAC
ndash Periodic listen and sleep turn off radio when sleeping the reduced dutycycle is ~ 10 (eg 120ms on 12s off)ndash Collision avoidance is based on contention similarly to IEEE 80211 adhoc modendash Overhearing avoidance the node is sleeping when neighbors talkndash Massage passing long message is fragmented amp sent in burst operationwith extended Tx time and re-transmit occurs immediately
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 63
Summarybull The algorithms and protocols that enable sharing by multiple users and
controls the access to the wireless channel from different users are calledMAC
bull There are two types of wireless networks Infrastructured wireless networksand ad hoc wireless networks
bull We also want our wireless system to have high user capacity and we canidentify 10 methods for increasing it
bull There are several MACs with no absolute advantage it should be tailoredto the system requirements
bull There are primary concerns for designing a WSN MAC (collisionavoidance energy efficiency scalability and adaptivity) while thesecondary concerns are as follows (channel utilization latency throughputfairness)
bull Next lecture Routing protocols
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 56
Random Access power control bull When C transmits to D at a high power level B cannot receive Arsquos
transmission due to interference from Cbull If C reduces transmit power it can still communicate with D
ndash Reduces energy consumption at node Cndash Allows B to receive Arsquos transmission (spatial reuse)
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 57
bull A special wireless ad hoc networkndash Large number of nodesndash Battery poweredndash Topology and density changendash Nodes for a common taskndash In-network data processing
bull Sensor-net applicationsndash Sensor-triggered bursty trafficndash Can often tolerate some delay
Ad hoc Sensor Networks Multiple channel access
Characteristics of Sensor Network
Scalabilty and self configurationEnergy efficiencyAdaptivityFairness not importantLatency
AdaptivityEnergy-delay tradoff
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 58
Ad hoc Sensor Networks Multiple channel access
Wireless sensor node and sensor network
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 59
MAC for WSN attributes bull Attributes
ndash Collision avoidance (Basic task)ndash Energy efficiency (most primary in WSN)ndash Scalability and adaptivityndash Channel utilizationndash Latencyndash Throughputndash Fairness
bull What causes energy wastendash Collisionsndash Control packet overheadndash Overhearing unnecessary trafficndash Long idle time (it consumes 50-100 of the power)
Ad hoc Sensor Networks Multiple channel access
Primary
Secondary
Dominant factor
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 60
MAC for WSN
bull Contention-based protocols need to work hard in all directions for energysavingsndash Reduce idle listening ndash support low duty cyclendash Better collision avoidancendash Reduce control overheadndash Avoid unnecessary overhearing
Ad hoc Sensor Networks Multiple channel access
Scheduled Protocols Contention ProtocolsCollisions No YesEnergy efficiency Good BadScalability and adaptivity Bad GoodMulti-hop communication Difficult EasyTime synchronization Strict Loose or not required
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 61
MAC for WSN classification
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 62
MAC for WSN SMAC (2002)bull It has been developed by Ye Heidemann and Estrinbull Tradeoffs between (latency fairness) and energybull Major components in S-MAC
ndash Periodic listen and sleep turn off radio when sleeping the reduced dutycycle is ~ 10 (eg 120ms on 12s off)ndash Collision avoidance is based on contention similarly to IEEE 80211 adhoc modendash Overhearing avoidance the node is sleeping when neighbors talkndash Massage passing long message is fragmented amp sent in burst operationwith extended Tx time and re-transmit occurs immediately
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 63
Summarybull The algorithms and protocols that enable sharing by multiple users and
controls the access to the wireless channel from different users are calledMAC
bull There are two types of wireless networks Infrastructured wireless networksand ad hoc wireless networks
bull We also want our wireless system to have high user capacity and we canidentify 10 methods for increasing it
bull There are several MACs with no absolute advantage it should be tailoredto the system requirements
bull There are primary concerns for designing a WSN MAC (collisionavoidance energy efficiency scalability and adaptivity) while thesecondary concerns are as follows (channel utilization latency throughputfairness)
bull Next lecture Routing protocols
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 57
bull A special wireless ad hoc networkndash Large number of nodesndash Battery poweredndash Topology and density changendash Nodes for a common taskndash In-network data processing
bull Sensor-net applicationsndash Sensor-triggered bursty trafficndash Can often tolerate some delay
Ad hoc Sensor Networks Multiple channel access
Characteristics of Sensor Network
Scalabilty and self configurationEnergy efficiencyAdaptivityFairness not importantLatency
AdaptivityEnergy-delay tradoff
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 58
Ad hoc Sensor Networks Multiple channel access
Wireless sensor node and sensor network
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 59
MAC for WSN attributes bull Attributes
ndash Collision avoidance (Basic task)ndash Energy efficiency (most primary in WSN)ndash Scalability and adaptivityndash Channel utilizationndash Latencyndash Throughputndash Fairness
bull What causes energy wastendash Collisionsndash Control packet overheadndash Overhearing unnecessary trafficndash Long idle time (it consumes 50-100 of the power)
Ad hoc Sensor Networks Multiple channel access
Primary
Secondary
Dominant factor
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 60
MAC for WSN
bull Contention-based protocols need to work hard in all directions for energysavingsndash Reduce idle listening ndash support low duty cyclendash Better collision avoidancendash Reduce control overheadndash Avoid unnecessary overhearing
Ad hoc Sensor Networks Multiple channel access
Scheduled Protocols Contention ProtocolsCollisions No YesEnergy efficiency Good BadScalability and adaptivity Bad GoodMulti-hop communication Difficult EasyTime synchronization Strict Loose or not required
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 61
MAC for WSN classification
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 62
MAC for WSN SMAC (2002)bull It has been developed by Ye Heidemann and Estrinbull Tradeoffs between (latency fairness) and energybull Major components in S-MAC
ndash Periodic listen and sleep turn off radio when sleeping the reduced dutycycle is ~ 10 (eg 120ms on 12s off)ndash Collision avoidance is based on contention similarly to IEEE 80211 adhoc modendash Overhearing avoidance the node is sleeping when neighbors talkndash Massage passing long message is fragmented amp sent in burst operationwith extended Tx time and re-transmit occurs immediately
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 63
Summarybull The algorithms and protocols that enable sharing by multiple users and
controls the access to the wireless channel from different users are calledMAC
bull There are two types of wireless networks Infrastructured wireless networksand ad hoc wireless networks
bull We also want our wireless system to have high user capacity and we canidentify 10 methods for increasing it
bull There are several MACs with no absolute advantage it should be tailoredto the system requirements
bull There are primary concerns for designing a WSN MAC (collisionavoidance energy efficiency scalability and adaptivity) while thesecondary concerns are as follows (channel utilization latency throughputfairness)
bull Next lecture Routing protocols
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 58
Ad hoc Sensor Networks Multiple channel access
Wireless sensor node and sensor network
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 59
MAC for WSN attributes bull Attributes
ndash Collision avoidance (Basic task)ndash Energy efficiency (most primary in WSN)ndash Scalability and adaptivityndash Channel utilizationndash Latencyndash Throughputndash Fairness
bull What causes energy wastendash Collisionsndash Control packet overheadndash Overhearing unnecessary trafficndash Long idle time (it consumes 50-100 of the power)
Ad hoc Sensor Networks Multiple channel access
Primary
Secondary
Dominant factor
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 60
MAC for WSN
bull Contention-based protocols need to work hard in all directions for energysavingsndash Reduce idle listening ndash support low duty cyclendash Better collision avoidancendash Reduce control overheadndash Avoid unnecessary overhearing
Ad hoc Sensor Networks Multiple channel access
Scheduled Protocols Contention ProtocolsCollisions No YesEnergy efficiency Good BadScalability and adaptivity Bad GoodMulti-hop communication Difficult EasyTime synchronization Strict Loose or not required
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 61
MAC for WSN classification
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 62
MAC for WSN SMAC (2002)bull It has been developed by Ye Heidemann and Estrinbull Tradeoffs between (latency fairness) and energybull Major components in S-MAC
ndash Periodic listen and sleep turn off radio when sleeping the reduced dutycycle is ~ 10 (eg 120ms on 12s off)ndash Collision avoidance is based on contention similarly to IEEE 80211 adhoc modendash Overhearing avoidance the node is sleeping when neighbors talkndash Massage passing long message is fragmented amp sent in burst operationwith extended Tx time and re-transmit occurs immediately
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 63
Summarybull The algorithms and protocols that enable sharing by multiple users and
controls the access to the wireless channel from different users are calledMAC
bull There are two types of wireless networks Infrastructured wireless networksand ad hoc wireless networks
bull We also want our wireless system to have high user capacity and we canidentify 10 methods for increasing it
bull There are several MACs with no absolute advantage it should be tailoredto the system requirements
bull There are primary concerns for designing a WSN MAC (collisionavoidance energy efficiency scalability and adaptivity) while thesecondary concerns are as follows (channel utilization latency throughputfairness)
bull Next lecture Routing protocols
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 59
MAC for WSN attributes bull Attributes
ndash Collision avoidance (Basic task)ndash Energy efficiency (most primary in WSN)ndash Scalability and adaptivityndash Channel utilizationndash Latencyndash Throughputndash Fairness
bull What causes energy wastendash Collisionsndash Control packet overheadndash Overhearing unnecessary trafficndash Long idle time (it consumes 50-100 of the power)
Ad hoc Sensor Networks Multiple channel access
Primary
Secondary
Dominant factor
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 60
MAC for WSN
bull Contention-based protocols need to work hard in all directions for energysavingsndash Reduce idle listening ndash support low duty cyclendash Better collision avoidancendash Reduce control overheadndash Avoid unnecessary overhearing
Ad hoc Sensor Networks Multiple channel access
Scheduled Protocols Contention ProtocolsCollisions No YesEnergy efficiency Good BadScalability and adaptivity Bad GoodMulti-hop communication Difficult EasyTime synchronization Strict Loose or not required
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 61
MAC for WSN classification
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 62
MAC for WSN SMAC (2002)bull It has been developed by Ye Heidemann and Estrinbull Tradeoffs between (latency fairness) and energybull Major components in S-MAC
ndash Periodic listen and sleep turn off radio when sleeping the reduced dutycycle is ~ 10 (eg 120ms on 12s off)ndash Collision avoidance is based on contention similarly to IEEE 80211 adhoc modendash Overhearing avoidance the node is sleeping when neighbors talkndash Massage passing long message is fragmented amp sent in burst operationwith extended Tx time and re-transmit occurs immediately
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 63
Summarybull The algorithms and protocols that enable sharing by multiple users and
controls the access to the wireless channel from different users are calledMAC
bull There are two types of wireless networks Infrastructured wireless networksand ad hoc wireless networks
bull We also want our wireless system to have high user capacity and we canidentify 10 methods for increasing it
bull There are several MACs with no absolute advantage it should be tailoredto the system requirements
bull There are primary concerns for designing a WSN MAC (collisionavoidance energy efficiency scalability and adaptivity) while thesecondary concerns are as follows (channel utilization latency throughputfairness)
bull Next lecture Routing protocols
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 60
MAC for WSN
bull Contention-based protocols need to work hard in all directions for energysavingsndash Reduce idle listening ndash support low duty cyclendash Better collision avoidancendash Reduce control overheadndash Avoid unnecessary overhearing
Ad hoc Sensor Networks Multiple channel access
Scheduled Protocols Contention ProtocolsCollisions No YesEnergy efficiency Good BadScalability and adaptivity Bad GoodMulti-hop communication Difficult EasyTime synchronization Strict Loose or not required
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 61
MAC for WSN classification
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 62
MAC for WSN SMAC (2002)bull It has been developed by Ye Heidemann and Estrinbull Tradeoffs between (latency fairness) and energybull Major components in S-MAC
ndash Periodic listen and sleep turn off radio when sleeping the reduced dutycycle is ~ 10 (eg 120ms on 12s off)ndash Collision avoidance is based on contention similarly to IEEE 80211 adhoc modendash Overhearing avoidance the node is sleeping when neighbors talkndash Massage passing long message is fragmented amp sent in burst operationwith extended Tx time and re-transmit occurs immediately
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 63
Summarybull The algorithms and protocols that enable sharing by multiple users and
controls the access to the wireless channel from different users are calledMAC
bull There are two types of wireless networks Infrastructured wireless networksand ad hoc wireless networks
bull We also want our wireless system to have high user capacity and we canidentify 10 methods for increasing it
bull There are several MACs with no absolute advantage it should be tailoredto the system requirements
bull There are primary concerns for designing a WSN MAC (collisionavoidance energy efficiency scalability and adaptivity) while thesecondary concerns are as follows (channel utilization latency throughputfairness)
bull Next lecture Routing protocols
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 61
MAC for WSN classification
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 62
MAC for WSN SMAC (2002)bull It has been developed by Ye Heidemann and Estrinbull Tradeoffs between (latency fairness) and energybull Major components in S-MAC
ndash Periodic listen and sleep turn off radio when sleeping the reduced dutycycle is ~ 10 (eg 120ms on 12s off)ndash Collision avoidance is based on contention similarly to IEEE 80211 adhoc modendash Overhearing avoidance the node is sleeping when neighbors talkndash Massage passing long message is fragmented amp sent in burst operationwith extended Tx time and re-transmit occurs immediately
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 63
Summarybull The algorithms and protocols that enable sharing by multiple users and
controls the access to the wireless channel from different users are calledMAC
bull There are two types of wireless networks Infrastructured wireless networksand ad hoc wireless networks
bull We also want our wireless system to have high user capacity and we canidentify 10 methods for increasing it
bull There are several MACs with no absolute advantage it should be tailoredto the system requirements
bull There are primary concerns for designing a WSN MAC (collisionavoidance energy efficiency scalability and adaptivity) while thesecondary concerns are as follows (channel utilization latency throughputfairness)
bull Next lecture Routing protocols
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 62
MAC for WSN SMAC (2002)bull It has been developed by Ye Heidemann and Estrinbull Tradeoffs between (latency fairness) and energybull Major components in S-MAC
ndash Periodic listen and sleep turn off radio when sleeping the reduced dutycycle is ~ 10 (eg 120ms on 12s off)ndash Collision avoidance is based on contention similarly to IEEE 80211 adhoc modendash Overhearing avoidance the node is sleeping when neighbors talkndash Massage passing long message is fragmented amp sent in burst operationwith extended Tx time and re-transmit occurs immediately
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 63
Summarybull The algorithms and protocols that enable sharing by multiple users and
controls the access to the wireless channel from different users are calledMAC
bull There are two types of wireless networks Infrastructured wireless networksand ad hoc wireless networks
bull We also want our wireless system to have high user capacity and we canidentify 10 methods for increasing it
bull There are several MACs with no absolute advantage it should be tailoredto the system requirements
bull There are primary concerns for designing a WSN MAC (collisionavoidance energy efficiency scalability and adaptivity) while thesecondary concerns are as follows (channel utilization latency throughputfairness)
bull Next lecture Routing protocols
Ad hoc Sensor Networks Multiple channel access
20111015 TAacuteMOP ndash 412-082AKMR-2009-0006 63
Summarybull The algorithms and protocols that enable sharing by multiple users and
controls the access to the wireless channel from different users are calledMAC
bull There are two types of wireless networks Infrastructured wireless networksand ad hoc wireless networks
bull We also want our wireless system to have high user capacity and we canidentify 10 methods for increasing it
bull There are several MACs with no absolute advantage it should be tailoredto the system requirements
bull There are primary concerns for designing a WSN MAC (collisionavoidance energy efficiency scalability and adaptivity) while thesecondary concerns are as follows (channel utilization latency throughputfairness)
bull Next lecture Routing protocols
Ad hoc Sensor Networks Multiple channel access
![Apotheker Zielpreismodell · pzn 300222 omebeta 20 kmr [betapharm] 41,92 € 33,74 82,25 € pzn 2171148 omeprazol 20mg volkspharma kmr [volkspharm](https://img.pdfslide.net/doc/110x75/5e1efa0b2cd4a04def4d3254/apotheker-pzn-300222-omebeta-20-kmr-betapharm-4192-a-3374-8225-a-pzn-2171148.jpg)
