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Victor S. FrostDan F. Servey Distinguished Professor
Electrical Engineering and Computer ScienceUniversity of Kansas2335 Irving Hill Dr.
Lawrence, Kansas 66045Phone: (785) 864-4833 FAX:(785) 864-7789
e-mail: [email protected]://www.ittc.ku.edu/
Review of Networking Principles#3
All material copyright 2006Victor S. Frost, All Rights Reserved
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Review of Networking Principles
• Who is communicating?• What is being communicated?• How are network functions
structured?• What is the architecture of the
Internet?• What is being shared?
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Who is Communicating
• Internet
Source: Pew Internet & American Life Project, February 15 – April 6, 2006 Tracking
Survey. N=4,001 adults, 18 and older. Margin of error is ±2% for results based on
the full sample and ±2% for results based on internet users.
Please note that prior to our January 2005 survey, the question used to identify internet users read,
“Do you ever go online to access the Internet or World Wide Web or to send and receive email?” The current
two-part question wordingreads, “Do you use the internet, at least occasionally? and
“Do you send or receive email, at least occasionally?”Last updated April 26, 2006.
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Who is Communicating
47%20%MP3 Player
32%30%Laptop
45%73%Cell Phone
TeenAdult
From: Lee Rainie Director, Pew Internet & American Life Project 5/9/06 How the Internet is Changing Consumer Behavior and Expectations Speech to SOCAP Symposium (Society of Consumer Affairs Professionals in Business)
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What is being Communicated• Entertainment
– Music– Video
• Rea-time• Non Real-time
– Games• Speech• Information
– News– Stock quotes– Education/Training– Product information– Job search– Health information– Government information
• Transactions– Customer-to-Business
• On-line banking• Purchase
products/services– Business-to-Business
• Transducer data– Temperature– Motion– Other…..
• Scientific data
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What is being CommunicatedSource: Pew Internet & American Life Project Tracking surveys
(March 2000 – April 2006). Please note that thewording for some items has been abbreviated. For full question
wording, please refer to the questionnaire.*Prior to January 2005, item wording was slightly different for the
items marked with an asterisk. questionnaires for question wording changes.
**Percentage of internet users who do these activities on a typical day is less than 1%
Last updated: April 26, 2006 – Daily Internet Activities were not asked in the January 2006 or May-June 2005
Tracking Survey.
From: Lee Rainie Director, Pew Internet & American Life Project 5/9/06 How the Internet is Changing Consumer Behavior and
Expectations Speech to SOCAP Symposium (Society of Consumer Affairs Professionals in Business)
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Where are the communicating end-points
• Homes• Offices• Vehicles
– Cars– Trucks
• Trains• Planes• Laboratories• In the field
From: John B. Horrigan, BROADBAND ADOPTION AT HOME IN THE UNITED STATES:GROWING BUT SLOWING, 33rd Annual TELECOMMUNICATIONS POLICY RESEARCH CONFERENCE September 24, 2005
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Networking Basics :Information Flow
InternetAccessMedia
Information Flow
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Network architectures and the Reference Models
• Open systems are build upon a Layered Architecture of the network
• Layered Architecture is the “structuring”of network functions
• Reference models provide:– A conceptual framework to characterize
networks– A mechanism to control/describe the
complexity of networks– Required for open systems
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Internet protocol stack• application: supporting network
applications– FTP, SMTP, STTP
• transport: host-host data transfer– TCP, UDP
• network: routing of datagrams from source to destination– IP, routing protocols
• link: data transfer between neighboring network elements– PPP, Ethernet
• physical: bits “on the wire”
application
transport
network
link
physical
From: Computer Networking: A Top Down Approach Featuring the Internet, 2nd edition. Jim Kurose, Keith Ross Addison-Wesley, July 2002.
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3 2 11 22 1
3 2 11 22 1
21
Medium
3 2 11 221 2 1
21
2 134 1 2 3 4
End System1
End System2
Network1
2
Physical layer entity
Data link layer entity 3 Network layer entity
3 Network layer entity
Transport layer entity4
Modified from: Leon-Garcia & Widjaja: Communication Networks
Router
End-to-End System
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Layering for the Access Network
• Physical Layer– Responsible for the media interface,
e.g., radio interface, powerline or optical– Characterized by – Capacity in b/s
• Quality in bit error rate
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Layering for the Access Network
• Data Link Layer– Medium Access Control (MAC) sublayer
• Controls transmissions on the physical layer– Link Access Control (LAC)
• Manages the logical link
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Cross-layer protocols
application
transport
network
link
physical
State of the physical layer used for scheduling or adaptive error control
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Cross-layer protocols
application
transport
network
link
physical
State of the link layer used to determine source of end-to-end packet loss, errors or congestion
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Segmentation and Reassembly
• Segmentation Often lower layers breakup packets into smaller elements add overhead to the smaller elements for transmission
• Reassembly combines smaller elements to form packet
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Segmentation and Reassembly• Example:
Hs
MH tH n Higher Layer Packet
MH tH n HL2
Link LayerHL2=Link Layer OverheadHs=Segment OverheadHMac= MAC Overhead
Hs Hs
Hs HMAC Hs HMAC Hs HMAC
Segm
enta
tion
Reas
sem
bly
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Segmentation and Reassembly• Specific Example:
Hs
MH tH n Higher Layer Packet
MH tH n HL2Radio ControlHRLC=Link Layer OverheadHs=RLC OverheadHMac= MAC OverheadHs Hs
Hs HMAC Hs HMAC Hs HMAC
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What’s the Internet: “nuts and bolts” view• millions of connected
computing devices: hosts, end-systems– PCs workstations, servers– PDAs phones, toastersrunning network apps
• communication links– fiber, copper, radio,
satellite– transmission rate =
bandwidth• routers: forward packets
(chunks of data)
local ISP
companynetwork
regional ISP
router workstationserver mobile
From: Computer Networking: A Top Down Approach Featuring the Internet, 2nd edition. Jim Kurose, Keith RossAddison-Wesley, July 2002.
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Architecture of the Internet
From: Computer Networks, A. S. Tannenbaum, 4th Ed, Prentice Hall, 2003
e.g., Sprint or ATT
or cable system
Voice, Image, Data, Video
e.g., Sprint or ATT
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What is being shared?
• There are shared resources in access networks
• Sharing implies some form of access coordination
• Coordination must be based on some perception of the state on the “system”– Which nodes have packets to send?– Do nodes have a common clock?– Is a wireless node in a deep fade?
• In general the more state information the nodes have the more efficient the sharing
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Duplexing
• Transmit and receive information flows also share the physical connection.
• Half Duplex-transmit and then receive
• Full Duplex-transmit and receive at the same time, split physical connection into a– Transmit resources– Receive resources
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Downstream and Upstream
• Upstream and downstream can use different access technologies
• All information flows from the node go toward the Internet
• All information flows destine for the nodes come from the Internet InternetInternet
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What is being shared?
• On the physical connection: resources– Time– Frequency– Transmit Power/Code– Space
• System elements resources– Buffer– Processing
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FDMA and TDMA
FDMA= Frequency Division Multiple Access
frequency
timeTDMA= Time Division Multiple Access
frequency
4 usersExample:
From: Computer Networking: A Top Down Approach Featuring the Internet, 2nd edition. Jim Kurose, Keith RossAddison-Wesley, July 2002.
time
Channel
Time SlotFrame
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FDMA• For N nodes and total Bandwidth = BT
– Bandwidth per user = BT/N• FDM must have guard bands between channels
wastes resources• Frequency Division Duplexing (FDD)
– Downstream on one channel– Upstream on another channel
Frequency
Guard bands
Time
W
12
MM–1
…
Modified from: Leon-Garcia & Widjaja: Communication Networks
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• What if you could eliminate the guard bands?
• Brief introduction to Digital Modulation
• Discussion of OFDM
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Brief introduction to Digital Modulation
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BER Performace
From: L.W. Couch, II, Digital and Analog Communication Systems, Seventh Edition, Prentice Hall Inc., Upper Saddle River, NJ, 2007
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OFDM Upstream/Downstream
From: Erik Dahlman 3G Evolution, ELSEVIER, 2008
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Orthogonal Frequency-division Multiplexing-OFDM
• Multicarrier Modulation• Uses a large number of parallel narrow-
band channels, each on a unique sub carrier• Combats
– Multipath– Narrow-band interference
• Problems– Sensitive of frequency and phase noise– Has large Peak-to-average ratio resulting is
inefficient use of power amplifiers
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• Assume for a base system – Bit rate = R– Channel bandwidth = Nfb @ fc– Using all the channel bandwidth the bit duration
would be 1/R=Tb• Process the R b/s stream into N streams
each at a rate of R/N, now for each stream the symbol time is N*T.
• Note because the symbol time has increased its susceptibility to multi-path induces ISI is decreased
Orthogonal Frequency-division Multiplexing-OFDM
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Orthogonal Frequency-division Multiplexing-OFDM
• The data is “distributed” over the N sub-carriers in a special way, specifically, the frequencies are selected to be orthogonal
• Different types of modulation can be used, e.g., QPSK
• Equalizers may not be needed when using OFDM
From: W. Stallings, Wireless Communications & Networks, Pearson 2005
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Orthogonal Frequency-division Multiplexing-OFDM
• Examples:– IEEE 802.11a/g uses 52 subcarriers– IEEE 802.16 uses 512 subcarriers– Powerline systems
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TDMA• Peak transmission rate = R• Each station transmits at R bps for 1/M of the time• Frames must have bit patterns to indicate “start of frame”
or have a guard time account propagation delayswastes resources
• Time Division Duplexing (TDD)– Downstream on one time slot– Upstream on another time slot
• Stations must be synchronized to common clock
1
Time
Guard time
One cycle
12 3 MW
Frequency
...
Modified from: Leon-Garcia & Widjaja: Communication Networks
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Sharing Power:Code Division Multiple Access
• Each bit in original signal is represented by multiple symbols in the transmitted signal, these symbols are called “chips”
• Coding gain = # chips/bit• The pattern of chips is called the “spreading code”• Because the chip rate >> bit rate the transmitted
signal uses more bandwidth, i.e., the signal is signal across a wider frequency band
• Spread is in direct proportion to number of chips used
• Spreading codes have special properties.
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Example
Modified from: W. Stallings, Wireless Communications & Networks, Pearson 2005
• Spreadingcode = 0110…….
• Receiver must– know the
spreading code and
– be in sync
Bit
Chip
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Channelization: CDMA• Code Division Multiple Access
– Channels determined by a code used in modulation and demodulation
• Stations transmit over entire frequency band all of the time
Time
1
2
3
W
Frequency
From: Leon-Garcia & Widjaja: Communication Networks
UsersEach with Unique code
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Property of Spreading Codes• Each channel uses a different pseudorandom code• Codes should have low cross-correlation
– If they differ in approximately half the bits the correlation between codes is close to zero and the effect at the output of each other’s receiver is small
• As number of users increases, effect of other users on a given receiver increases as additive noise– this is why this is considered sharing power
• CDMA has gradual increase in BER due to noise as number of users is increased
• Interference between channels can be eliminated is codes are selected so they are orthogonal and if receivers and transmitters are synchronized
From: Leon-Garcia & Widjaja: Communication Networks
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Example: CDMA with 3 users
• Assume three users share same medium• Users are synchronized & use different 4-bit
orthogonal codes: {-1,-1,-1,-1}, {-1, +1,-1,+1}, {-1,-1,+1,+1}, {-1,+1,+1,-1},
+1 -1 +1
User 1 x
-1 -1 +1
User 2 x
User 3 x
+1 +1 -1 SharedMedium
+
Receiver
From: Leon-Garcia & Widjaja: Communication Networks
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Channel 1: 110 -> +1+1-1 -> (-1,-1,-1,-1),(-1,-1,-1,-1),(+1,+1,+1,+1)Channel 2: 010 -> -1+1-1 -> (+1,-1,+1,-1),(-1,+1,-1,+1),(+1,-1,+1,-1)Channel 3: 001 -> -1-1+1 -> (+1,+1,-1,-1),(+1,+1,-1,-1),(-1,-1,+1,+1)Sum Signal: (+1,-1,-1,-3),(-1,+1,-3,-1),(+1,-1,+3,+1)
Channel 1
Channel 2
Channel 3
Sum Signal
Sum signal is input to receiver
From: Leon-Garcia & Widjaja: Communication Networks
Bit time Chip time
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Example: Receiver for Station 2
• Each receiver takes sum signal and integrates by code sequence of desired transmitter
• Integrate over T seconds to smooth out noise
x
SharedMedium
+
Decoding signal from station 2
Integrate over T sec
From: Leon-Garcia & Widjaja: Communication Networks
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Sum Signal: (+1,-1,-1,-3),(-1,+1,-3,-1),(+1,-1,+3,+1)Channel 2 Sequence: (-1,+1,-1,+1),(-1,+1,-1,+1),(-1,+1,-1,+1)Correlator Output: (-1,-1,+1,-3),(+1,+1,+3,-1),(-1,-1,-3,+1)Integrated Output: -4, +4, -4Binary Output: 0, 1, 0
Decoding at Receiver 2
Sum Signal
Channel 2Sequence
CorrelatorOutput
IntegratorOutput
-4
+4
-4
Sum Signal
Channel 2Sequence
CorrelatorOutput
IntegratorOutput
-4
+4
-4
From: Leon-Garcia & Widjaja: Communication Networks
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Math on CDMA
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Frequency Hopping Spread Spectrum
• Signal is broadcast over seemingly random series of radio frequencies– A number of channels
allocated for the FH signal– Width of each channel
corresponds to bandwidth of input signal
• Signal hops from frequency to frequency at fixed intervals– Transmitter operates in one
channel at a time– Bits are transmitted using
some encoding scheme– At each successive interval, a
new carrier frequency is selected
Modified from: W. Stallings, Wireless Communications & Networks, Pearson 2005
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Practical considerations for Spread Spectrum Systems
• Problems with orthogonal codes– There are a limited number of orthogonal codes– Obtaining synchronization for all users is complex.
• CDMA systems require synchronization– Code sync– Bit sync– Often carrier sync
• Example assumed that all signals arrive with the same power thus CDMA systems require power control– Base station monitors power from each node and send
explicit commands to increase/decrease the transmit power
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Other Spread Spectrum Facts
• Hybrids: Direct Sequence/Frequency hop systems
• Spread Spectrum systems were originally developed for:– Anti-jamming (AJ)– Low probability of intercept (LPI)
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Comparison FDMA, TDMA, CDMA, OFDM
Need common code
Inherent
Average Power
EasyGraceful
Degradation
Code
Continuous
PN codes
Orthogonal codes
CDMA
Need common frequency band
Inherent
Average Power
Complex
Frequency Band
Continue
Time dispersion leading to cyclic-prefix insertion
High order modulation per
carrier
OFDM
Need common time slot
Need common frequency band
Broadcast capability
NoNoResistance to multipath
Peak PowerAverage PowerTransmit Power
ComplexComplexEase of adding new users
Time slotFrequency BandResource assigned
Need to wait for time slot (buffer)
ContinuousTransmission
Guard timesGuard bandsPractical consideration
No timing errors(complex for
distributed users)
No guard bandsFor maximum resource utilization
TDMAFDMACharacteristic
Modified from: B. Bing, “Broadband Wireless Access”, Kluwer Academic Press, 2000
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Sharing Space
• Radiated power drops off as a function of distance – 1/D2 for free space– 1/Dn for other environments, e.g., k=4
for the common two-ray model• Inter-channel interference will result
if users are assigned the same frequency and are “too close”
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Sharing Space• Defining “too close” for a one dimensional case*
Following: M. Schwartz, “Mobile Wireless Communications” Cambridge Press, 2005
Desired signalMSC transmitting PT watts
on channel i
R1 Meters R2 Meters
Interfering signalMSC transmitting PT watts
on channel i
n-2
n-1
n-2T
n-1T
-n2T
-n1T
RR (SIR) Ratio ceinterferen-to-Signal
RP phone cell at received power signal gInterferinRP phone cell at received power signal Desired
RPRP phone cell at received Power Total
=
=
=
+=
SIR>Minimum threshold for proper operation
SIR>7-12 dB for GSM
power ginterferin normalizedP where
P
RSIR general In
int
int
-n
=
=∑
Cell phone
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Sharing Space• Example 1-d model 4-cell reuse• Total number of Channels = N• Each cell uses N/4 different channels• So three cells separating the same set of interfering
channels• Consider first tier interferes
Worst case location for cell phone using a channel from group 1
D Meters
1 2 3 4 1 2 3 41 2 3 4R Meters
dB 23SIR 3n assuming
8RD HereR)(DR)(D
R P
RSIR nn
-n
int
-n
=
==
++−== −−∑
Analysis assumes power control
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Sharing Space
Modified from: W. Stallings, Wireless Communications & Networks, Pearson 2005
Cellular systems most oftenanalyzed with an Hexagonal
pattern
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Sharing Space
• Reducing interference and increasing capacity with sectoring using directional antennas
From: T.S. Rappaport Wireless Communications Principles and Practice, 2nd Edition
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Sharing Space
• Sharing in the spatial domain– Distance– Angle
• More on space sharing later….
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System element resources
Packet Buffershared by all
downstream nodes
InternetInternet
Processor
Power limiteddevices
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Sharing Buffers: Statistical Multiplexing
• No Dedicated path• Address information is added to the Packet• Store if output port is busy• Trade off delay for blocking• If message is corrupted then retransmit entire
Packet
Store&
Forward
Store&
Forward
Store&
Forward
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Sharing Buffers: Statistical Multiplexing
• Time measured from entering the queue to completion of clocking on to the link is the delay
• Load = ρ = ΤCλλ = arrival rate in packets/sec
and ΤC = L/R=time to clock packetL= Ave packet length in bits,R=line rate in b/s
• Under common assumptions:– Message lengths have a
exponential probability density function
– Interarrival times have a exponential probability density function
ρ−=
1CTDelayAverage
M/M/1 Delay
05
101520253035
0 0.2 0.4 0.6 0.8 1
Load
Del
ay
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InternetInternetPower limited
devices
Sharing Buffers: Statistical Multiplexing
One Queue perNode
Server
Algorithm Selects Node
Contention AlgorithmUsed in upstream to allocate resources