review of networking principles #3

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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

…


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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


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• 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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• 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

...


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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


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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


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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


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


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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


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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


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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


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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• 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


One Queue perNode

Server

Algorithm Selects Node

Contention AlgorithmUsed in upstream to allocate resources

review of networking principles #3

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