Data and Computer Communications

Chapter 8Multiplexing Techniques

Transmission Efficiency: MultiplexingSeveral data sources share a common

transmission medium, with each source having its own channel

Line sharing saves transmission costsHigher data rates mean more cost-

effective transmissionMost individual data sources require

relatively low data rates

Transmission Efficiency: Data compressionReduces the size of data files to move

more information with fewer bitsUsed for transmission and for storage

e.g. ZIP

Often combined with multiplexing to increase efficiency

Alternate Approaches to Terminal SupportDirect point-to-point links Multidrop lineMultiplexer Integrated MUX function in host

Direct Point-to-Point

Multidrop Line

Multiplexer

Integrated MUX in Host

Why multiplexing

share the use of a common channel

DEMUX

Multiplexer Demultiplexer

shared channel

MUX

Multiplexing

Types of Multiplexer

FDM (Frequency Division Multiplexer)

TDM (Time Division Multiplexer)

Frequency Division MultiplexingFDMUseful bandwidth of medium exceeds

required bandwidth of channelEach signal is modulated to a different

carrier frequencyCarrier frequencies separated so signals

do not overlap (guard bands)e.g. broadcast radioChannel allocated even if there is no data

Frequency Division MultiplexingRequires analog signaling & transmissionBandwidth = sum of inputs + guardbandsModulates signals so that each occupies a

different frequency bandStandard for radio broadcasting, analog

telephone network, and television (broadcast, cable, & satellite)

Frequency Division Multiplex

CH2

CH1

CH3

original bandwidth

CH1

CH2

CH3

bandwidths raised infrequency

MUXCH1 CH2 CH3

bandwidths multiplexedinto one channel

f

Frequency Division MultiplexingDiagram

FDM System

FDM of Three Voiceband Signals

Only the lower sideband is used

Analog Carrier SystemsAT&T (USA)Hierarchy of FDM schemesGroup

12 voice channels (4kHz each) = 48kHz Range 60kHz to 108kHz

Supergroup 60 channel FDM of 5 group signals on carriers between

420kHz and 612 kHz

Mastergroup 10 supergroups

Synchronous Time Division MultiplexingData rate of medium exceeds data rate of

digital signal to be transmittedMultiple digital signals interleaved in timeTime slots pre-assigned to sources and

fixedTime slots allocated even if there is no

dataTime slots do not have to be evenly

distributed amongst sources

Synchronous Time-Division Multiplexing (TDM)Used in digital transmissionRequires data rate of the medium to

exceed data rate of signals to be transmitted

Signals “take turns” over mediumSlices of data are organized into frames

Synchronous TDM and PSTN

Used in the modern digital telephone system US, Canada, Japan: DS-1 (T-1), DS-3 (T-3), ... Europe, elsewhere: E-1, E3, … These are listed in table 8.3 Page 249

Data rate of 1.544MbpsUses PCM to digitize voice transmission at

8K samples/sec, frame length of 193bits (8000x193=1.544 Mbps=T1)

Time Division Multiplex

A2

A1

A3

original signal

D2

D1

D3

digitized signal

MUX

data filled in time slot

time slot1 2 3 4

Time Division Multiplexing

TDM System

TDM Link ControlNo headers and trailersData link control protocols not neededFlow control

Data rate of multiplexed line is fixed If one channel receiver cannot receive data, the

others must carry on The corresponding source must be quenched This leaves empty slots

Error control Errors are detected and handled by individual

channel systems

Data Link Control on TDM

FramingNo flag or SYNC characters bracketing

TDM framesMust provide synchronizing mechanismAdded digit framing

One control bit added to each TDM frameLooks like another channel - “control channel”

Identifiable bit pattern used on control channel e.g. alternating 01010101…unlikely on a data

channel Can compare incoming bit patterns on each

channel with sync pattern

Pulse StuffingProblem - Synchronizing data sourcesClocks in different sources driftingData rates from different sources not

related by simple rational numberSolution - Pulse Stuffing

Outgoing data rate (excluding framing bits) higher than sum of incoming rates

Stuff extra dummy bits or pulses into each incoming signal until it matches local clock

Stuffed pulses inserted at fixed locations in frame and removed at demultiplexer

TDM of Analog and Digital Sources

Digital Carrier SystemsHierarchy of TDMUSA/Canada/Japan use one systemITU-T use a similar (but different) systemUS system based on DS-1 formatMultiplexes 24 channelsEach frame has 8 bits per channel plus

one framing bit193 bits per frame

Digital Carrier Systems (2)For voice each channel contains one word of

digitized data (PCM, 8000 samples per sec) Data rate 8000x193 = 1.544Mbps Five out of six frames have 8 bit PCM samples Sixth frame is 7 bit PCM word plus signaling bit Signaling bits form stream for each channel

containing control and routing info

Same format for digital data 23 channels of data

7 bits per frame plus indicator bit for data or systems control

24th channel is sync

Mixed DataDS-1 can carry mixed voice and data

signals24 channels usedNo sync byteCan also interleave DS-1 channels

Ds-2 is four DS-1 giving 6.312Mbps

ISDN User Network InterfaceISDN allows multiplexing of devices over

single ISDN lineTwo interfaces

Basic ISDN Interface Primary ISDN Interface

Basic ISDN Interface (1)Digital data exchanged between subscriber

and NTE - Full DuplexSeparate physical line for each directionPseudoternary coding scheme

1=no voltage, 0=positive or negative 750mV +/-10%

Data rate 192kbpsBasic access is two 64kbps B channels and

one 16kbps D channelThis gives 144kbps multiplexed over 192kbpsRemaining capacity used for framing and sync

Basic ISDN Interface (2)B channel is basic user channelDataPCM voiceSeparate logical 64kbps connections o

different destinationsD channel used for control or data

LAPD frames

Each frame 48 bits longOne frame every 250s

Frame Structure

Primary ISDNPoint to pointTypically supporting PBX1.544Mbps

Based on US DS-1 Used on T1 services 23 B plus one D channel

2.048Mbps Based on European standards 30 B plus one D channel Line coding is AMI usingHDB3

Primary ISDN Frame Formats

SONET: Synchronous Optical NetworkSpecification for high-speed digital

transfer via optical fiberRates from 51.84Mbps to 13.2GbpsUses Synchronous TDM

Sonet/SDHSynchronous Optical Network (ANSI)Synchronous Digital Hierarchy (ITU-T)CompatibleSignal Hierarchy

Synchronous Transport Signal level 1 (STS-1) or Optical Carrier level 1 (OC-1)

51.84Mbps Carry DS-3 or group of lower rate signals (DS1

DS1C DS2) plus ITU-T rates (e.g. 2.048Mbps) Multiple STS-1 combined into STS-N signal ITU-T lowest rate is 155.52Mbps (STM-1)

SONET Frame Format

SONET STS-1 Overhead Octets

Statistical Time Division Multiplexingrequires digital signaling & transmissiondata rate capacity required is well below

the sum of connected capacitysame concepts as synchronous TDMuses memory buffers to avoid loss of datawidely used for remote communications

with multiple terminalssimilar to medium-sharing done by LANs

Statistical TDMIn Synchronous TDM many slots are

wastedStatistical TDM allocates time slots

dynamically based on demandMultiplexer scans input lines and collects

data until frame fullData rate on line lower than aggregate

rates of input lines

Statistical TDM

A

B

C

D

Data to be sent

t1 t2 t3

A1 B1 C1D1 C2

D2A2 B2

1st cycle 2nd cycle

Synchronous TDMwaste bandwidth

A1 B1 C2B2

1st cycle 2nd cycle

Statistical TDMextra bandwidthavailable

Statistical TDM Frame Formats

PerformanceOutput data rate less than aggregate

input ratesMay cause problems during peak periods

Buffer inputs Keep buffer size to minimum to reduce delay

Buffer Size and Delay

Asymmetrical Digital Subscriber LineADSLLink between subscriber and network

Local loop

Uses currently installed twisted pair cable Can carry broader spectrum 1 MHz or more

ADSL DesignAsymmetric

Greater capacity downstream than upstream

Frequency division multiplexing Lowest 25kHz for voice

Plain old telephone service (POTS)

Use echo cancellation or FDM to give two bands

Use FDM within bands

Range 5.5km

ADSL Channel Configuration

Discrete MultitoneDMTMultiple carrier signals at different frequenciesSome bits on each channel4kHz subchannelsSend test signal and use subchannels with

better signal to noise ratio256 downstream subchannels at 4kHz

(60kbps) 15.36MHz Impairments bring this down to 1.5Mbps to 9Mbps

DMT Transmitter

xDSLHigh data rate DSLSingle line DSLVery high data rate DSL