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MAC Protocols for Ad Hoc Wireless Networks. Wireless Interference. A radio interface either transmits or receives (half-duplex). A receiver must get a minimum SINR for successful reception This means no other transmitter (interferer) in vicinity. If untrue -> collision (SINR insufficient). - PowerPoint PPT Presentation
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MAC ProtocolsMAC Protocolsfor Ad Hoc Wireless for Ad Hoc Wireless NetworksNetworks
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Wireless InterferenceWireless Interference
A radio interface either transmits or receives (half-duplex).
A receiver must get a minimum SINR for successful receptionThis means no other transmitter (interferer) in
vicinity. If untrue -> collision (SINR insufficient).
Quintessential MAC problemSchedule transmissions on links conflict-free.
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Time Division Multiple Access Time Division Multiple Access (TDMA)(TDMA) Use slotted time. Schedule conflicting transmissions at
different time slots. Problem equivalent to graph coloring
Optimal solution is computationally hard. Significant research since the days of
packet radio. Often not deemed practical
Hard to compute good schedules in a distributed fashion.
Schedule needs to be traffic dependent. Need synchronized clocks in hardware to implement
slots
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Carrier Sense Multiple Access Carrier Sense Multiple Access (CSMA)(CSMA) Transmit when ready
Use a combination of carrier-sense and randomization to avoid conflict.
Not foolproof. Carrier sense not foolproof
Propagation delay (also a problem in wireline).Can sense only at transmitter; but collision
happens at receiver (a wireless problem).
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Virtual Carrier SensingVirtual Carrier Sensing
Any node hearing RTS or CTS sets up their NAV (network allocation vector) until end of ACK.
NAV set -> node silent (act as if carrier busy).
A B C D
RTSRTS
CTS
DATA
ACK
E
CTS
DATAACK
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802.11 Timeline802.11 Timeline
If carrier busy (physical or virtual), schedule transmission after a random backoff when carrier is free.
Average backoff interval is doubled for each failed attempt.
t
SIFS
DIFSACK
Defer access
Nodes that hear transmitter
Receiver
TransmitterDATA
Randombackoff
RTS
CTS
SIFS SIFS
NAV (RTS)NAV (CTS)
DIFS
Nodes that hear receiver Another
transfer
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Hidden and Exposed Terminal Problems Hidden and Exposed Terminal Problems (Revisited)(Revisited) In Ad Hoc networks, HTP and ETP would
happen frequently. Conventional CSMA severely suffer from both HTP and ETP !
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Design Goals of MAC Protocol for AHWNDesign Goals of MAC Protocol for AHWN
Distributed operation QoS support for real-time traffic Low access delay Bandwidth efficiency Fair allocation of BW to nodes Low control overhead Minimize the effects of hidden and exposed terminal
problems Scalable Efficient power control mechanism Adaptive data rate control, taking into consideration
of network load and neighbor status Try to use of directional antennas for reducing
interference, increasing spectrum reuse, and reducing power consumption
Time synchronization for BW reservation
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Classification of MAC Protocols for Classification of MAC Protocols for AHWNAHWN
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Contention-basedContention-basedSender-initiatedSender-initiatedSingle-channel ProtocolsSingle-channel Protocols
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MACA: Multiple Access Collision MACA: Multiple Access Collision AvoidanceAvoidance Proposed by Phil Karn (1990) as an alternative to
the CSMA Inspired by the CSMA/CA method
Extend and Enhance the CA part of the CSMA/CA – Every one overhearing CTS knows just how long to wait to avoid collision.
Get rid of the CS in CSMA/CA and become MACA. Lack of carrier doesn’t always mean it’s OK to transmit Presence of carrier doesn’t always mean it’s bad to transmit It’s too hard to build a good DCD (Data Carrier Detect) circuit
MACA uses signaling packets for CA RTS/CTS Contain: sender address, receiver address, packet size
If a packet transmitted is lost, use BEB algorithm Variants of this method are used in IEEE 802.11
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Data
MACA exampleMACA example MACA avoids HTP MACA avoids ETP
S1 S2R
RTS
CTSCTS
Data
R1 S2S1
CTS
RTSRTS
Data
R2
Vulnerable period is known to C by
CTS
Overhearing RTS
No CTS
RTSRTS
CTS
Data DataMACA could greatly relieve both problems,
but not completely solve them.
RTS
Data
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MACAW: MACA for Wireless LANsMACAW: MACA for Wireless LANs
Problems in MACA
Enhancement of the MACA by V. Bharghavan (1994) RTS-CTS-DS-DATA-ACK
R1 S2S1
CTS
RTSRTS
Data
Exposed Terminal Situation
R2
Overhearing RTS
No CTS
RTSRTS
CTSData
Back-off
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MACAW Packet Exchange: MACAW Packet Exchange: RTS-CTS-DS-DATA-ACKRTS-CTS-DS-DATA-ACK ACK for the fast error
recovery DS (Data Sending) packet
to ensure successful RTC-CTS dialog to solve exposed terminal
Include RRTS (request for RTS) to inform neighbor sender of tx timing
CTSRTS
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BEB in MACA may starves flows
Back-off counter carried in packet header is copied by receiving node
Reset to min value after every successful transmission
MILD back-off ( x1.5, -1 ) implements per flow fairness
Run back-off algorithm for each queue (per flow)
high volumeof traffic
collisionBEB
MACAW Back-off ModificationMACAW Back-off Modification
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FAMA: Floor Acquisition Multiple FAMA: Floor Acquisition Multiple Access Access C. Fullmer, J. Garcia-Luna-
Aceves (1995) In MACA,
data packets are prone to collisions with RTS packets (because of no CS)
Tx of bursts of packets is not possible
Floor acquisition Floor (channel) is acquired by
means of exchanging control packets (RTS-CTS) before transmission
Refinement of the MACA Duration of RTS >= 2τ (max
channel propagation delay)• To ensures that data packets
are always transmitted without collision
The length of the CTS is made longer than the RTS to deal with HTP of MACA
• The dominating CTS plays the role of Busy Tone
S1 S2R
Data
Hidden Terminal Situation
Data
MACA
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FAMA (Cont’d)FAMA (Cont’d)
2 FAMA protocols RTS-CTS exchange with no CS: ALOHA + RTS/CTS RTS-CTS exchange with non-persistent CS: non-
persistent CSMA• FAMA-NTR (non-persistent transmit request)
FAMA-NTR If channel is busy, sender backs off for a random period
and retry later If channel is idle,
• sender listens to the channel after RTS tx• If no CTS received within 2τ or corrupted, then take
random back-off and retry later• If CTS received, transmit a burst of data packets
Packet burst transmission• Receiver: wait RTS for τ seconds after each data packet
received• Sender: wait CTS for 2 τ seconds after tx RTS
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Contention-basedContention-basedSender-initiatedSender-initiatedMulti-channel ProtocolsMulti-channel Protocols
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BTMA: Busy Tone Multiple AccessBTMA: Busy Tone Multiple Access 2 channels: data/control ch.
Control ch for Tx busy tone signal
Carrier sense on busy tone before transmission.
If idle, turn on busy tone and start Tx
Any other nodes which sense carrier on the incoming data channel also Tx busy tone signal
No other nodes in the 2-hop neighborhood of the Tx node is permitted to simultaneously transmit
Perfect solution. But need a busy tone channel and extra interface. Channel gains on data and busy tone channels may be different.
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DBTMA: Dual BTMADBTMA: Dual BTMA
Control channel for RTS-CTS Busy tones
• BTt : to indicate Tx on data ch• BTr : to indicate Rx on data ch
RTS/CTS-based MAC (MACA and MACAW) block both the forward and backward Tx
But, DBTMA blocks reverse Tx
If no BTr,Tx RTS
If no BTt,Tx CTS
Block other nodes’ Rx
Block other nodes’ Tx
S1 R1S2R2 S3
BTt BTr
Rx cleared
Tx cleared
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Contention-basedContention-basedReceiver-initiated Receiver-initiated ProtocolsProtocols
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Receiver Initiated ProtocolsReceiver Initiated Protocols
Features Receiver polls its neighbor asking for data Reduce the number of control packets More efficient than sender-initiated collision avoidance
Design Issues – How to Poll the neighbors ? Polling Rate
• Whether the polling rate is independent of the data rate at polling nodes
• Independent Polling / Data Driven Polling Intended Audience
• Whether the poll is sent to a particular neighbor or to all neighbors
Intent of a polling packet• Whether the polling packet asks for permission to transmit
as well
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RI-BTMA: Receiver-Initiated BTMARI-BTMA: Receiver-Initiated BTMA
Data packet: preamble (P) + DATA Preamble carries ID of
intended DEST node Data and control
channels are slotted Each slot equal to
preamle Busy tone means
ACK the sender about successful reception of preamble
Block hidden node’s Tx
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MACA-BI (MACA - By Invitation)MACA-BI (MACA - By Invitation) F. Taluci, M. Gerla (1997) CTS RTR (Ready to Receive)
RTR packet carries time interval during DATA Tx Traffic prediction by receiver: Time interval is estimated by
• DATA packet modified to carry control information regarding backlog such as # of packets queued and packet lengths
• Or RTS from sender to declare it backlog, if RTR is not received within a given time period
But, RTR may collide DATA may collide with RTR
RTR
(1) (2)
DATA Hidden terminal:
Blocked fromTransmission
RTR3DATA DATA
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MARCH: Media Access MARCH: Media Access with Reduced Handshakewith Reduced Handshake Does not require any
traffic prediction Neighbor
overhearing CTS transmit CTS to receive DATA RTS is used only for the
first packet of the stream
Route identification CTS contains: MAC-SA,
MAC-DA, RTid
Lower # of control packets improves throughput and reduce E-E delay
MACA MARCH
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RIMA: Receiver Initiated Multiple RIMA: Receiver Initiated Multiple AccessAccess A. Tzamaloukas, J. Garcia-Luna-
Aceves (1999) RIMA-SP (Simple Polling), RIMA-DP (Dual-use Polling), RIMA-BP (Broadcast Polling)
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Contention-based Contention-based Synchronous Protocols Synchronous Protocols with Reservationwith Reservation
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Quality of Service Quality of Service
Difficulties in Quality of Service Support in Ad-hoc Networks No centralized coordinator : ex) IEEE 802.11 PCF (Point
Coordination Function) To guarantee a QoS request, a Distributed/Dynamic
Reservation Scheme is needed. IEEE 802.11.e EDCF (Enhanced DCF)
Provides Differentiated Access; Up to 8 Access Categories (AC)
• A Station should have separate Queues for each AC• Each AC may have different Values for Contention Window and
AIFS
aSlotTime
Backof Window Next FrameBusy Medium
Defer Access
PIFS
SIFS
DIFS/AIFS
DIFS/AIFS
Contention Window
Select slot and DecrementBackoff as long as medium is idle
Immediate accesswhen medium is free>= DIFS/AIFS[i] AIFS[i]
AIFS[j]
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D-PRMA: Distributed Packet RSV D-PRMA: Distributed Packet RSV MAMA Extends PRMA protocol
for voice support in AHWN
Contention only during reservation. Once reserved, CF
Slot-reservation A certain period at the
beginning of each minislot is reserved for CS
First minislot is used to contend the slot; if no node wins, the remaining minislots are used for contention until a contending node wins (RTS/CTS exchange)
Within reserved slot, communication between source and receiver nodes takes place by means of TDD or FDD
Prioritization Contention with
probability p• For first minislot, p=1 for
voice, p < 1 for data• For remaining minislots, p
< 1 for voice and data Only if a voice node wins,
reserve the same slot in each subsequent frame
• Receiver transmit BI through RTS/BI part of minislot 1 (eliminate HTP)
• Sender transmit BI through CTS/BI part of minislot 1
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CATA: CA Time AllocationCATA: CA Time Allocation Supports unicast, broadcast, and multicast
Works well with simple single-channel half-duplex radios Minislots
CMS1: receiver tx SR (slot rsv) packet to sender CMS2: sender tx RTS (for uni/broad/multicast session) Unicast session
• CMS3: Receiver tx CTS (rsv the same slot in subsequent frames)• CMS4: if sender sense idle, rsv was successful. Tx packets during
DMS Multicast session
• CMS3: Receiver remains idle. And listen• CMS4:
– Receiver: If listen anything during CMS3, tx NTS (not-to-send) packet to sender
– Sender: If receive NTS or noise, reservation had failed. Otherwise, reservation was successful. Tx packets during DMS
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HRMA: Hop RSV MAHRMA: Hop RSV MA Mulitichannel MAC
protocol based on simple half-duplex, very slow FHSS radios Reserve a FH Guarantee collision-free data
tx Time slot reservation where
each time slot is assigned a separate frequency channel
Frequency: slot fo : synchronizing frequency
for synchnorizing slot (fi, fi*), i=1,2, …, M slots
• fi: used for HR, RTS, CTS, data packet tx
• fi*: used for sending and receivng ACK packets
Each time slot divided into Synchronizing period: all idle
nodes exchange synchronization information with freq fo
HR (hop rsv) period• If hear HR packet, random
back-off RTS/CTS period
• If free, RTS/CTS exchange• If source hear CTS, successful
reservation of the current hop
Merging of subnets
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Contention-based Contention-based Asynchronous Protocols Asynchronous Protocols with Reservationwith Reservation
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MACA/PR (Piggyback Reservation)MACA/PR (Piggyback Reservation) C. Lin, M. Gerla (1999) BW reservation for real-time
packet Each node maintains a
reservation table (RT) that records all the reserved tx and rx slots/windows of all nodes within its transmission range
Periodically exchanges RT (overcome HTP)
For a non real-time packet, MACAW-based MAC is used
For a real-time traffic, slots are periodically guaranteed at each links on the path (per superframe/CYCLE)
The first data packet is transmitted just as best-effort packet, but reservation information is piggy-backed
Receiver node updates it RT, and pigg-backs the rsv confirmation information on ACK packet
QoS routing protocol used in MACA/PR: DSDV routing protocol
Adv: Does not require global synchronization among nodes
Drawback: A free slot can be reserved only if it can fit RTS-CTS-DATA-ACK exchange
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RTMAC: Real-Time MACRTMAC: Real-Time MAC Real-time extension of IEEE
802.11 DCF RTS, CTS, ACK for best-effort
packets ResvRTS, ResvCTS, ResvACK for
real-time packets• IFS for real-time packet = ½ DIFS
BW reservation Reserves a variable length
connection-slot (a set of resv-slot) on successive superframes
Each node maintains a RT containing information such as sender id, receiver id, starting/ending times of reservation
No time synchronization is assumed
• Superframe may not strictly align with the other nodes
• Protocol uses relative time for reservation
– Relative time + local clock absolute time
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RTMAC (Cont’d)RTMAC (Cont’d) 3-way handshake for reservation process
ResvRTS-ResvCTS-ResvACK if reservation OK If receiver rx ResvRTS on a slot reserved by neighbor,
• does not responde (because ACK/NACK packet may cause collision)
If receiver rx ResvRTS on a free slot, but requested connection-slot is not free on reveiver,
• tx negative CTS (resvNCTS) back to sender Reservation release
Sender broadcasts the release RTS (ResvRelRTS)• Nodes hearing this packet update their RT in order to free
the connection Receiver node respondes by broadcasting ResvRelCTS
packet• Receiver’s neighbor nodes update their RT in order to free
the connection QoS routing protocol
An extension of DSDV routing protocol