Data and Computer Communications Chapter 8 Multiplexing Techniques

Transmission Efficiency: Multiplexing zSeveral data sources share a common transmission medium, with each source having its own channel zLine sharing saves transmission costs zHigher data rates mean more cost-effective transmission zMost individual data sources require relatively low data rates

Transmission Efficiency: Data compression zReduces the size of data files to move more information with fewer bits zUsed for transmission and for storage ye.g. ZIP zOften combined with multiplexing to increase efficiency

Alternate Approaches to Terminal Support zDirect point-to-point links zMultidrop line zMultiplexer zIntegrated MUX function in host

Direct Point-to-Point

Multidrop Line

Multiplexer

Integrated MUX in Host

Why multiplexing zshare the use of a common channel DEMUX Multiplexer Demultiplexer shared channel MUX

Multiplexing

Types of Multiplexer zFDM (Frequency Division Multiplexer) TDM (Time Division Multiplexer)

Frequency Division Multiplexing zFDM zUseful bandwidth of medium exceeds required bandwidth of channel zEach signal is modulated to a different carrier frequency zCarrier frequencies separated so signals do not overlap (guard bands) ze.g. broadcast radio zChannel allocated even if there is no data

Frequency Division Multiplexing zRequires analog signaling & transmission zBandwidth = sum of inputs + guardbands zModulates signals so that each occupies a different frequency band zStandard for radio broadcasting, analog telephone network, and television (broadcast, cable, & satellite)

Frequency Division Multiplex CH2 CH1 CH3 original bandwidth CH1 CH2 CH3 bandwidths raised in frequency MUX CH1 CH2CH3 bandwidths multiplexed into one channel f

Frequency Division Multiplexing Diagram

FDM System

FDM of Three Voiceband Signals Only the lower sideband is used

Analog Carrier Systems zAT&T (USA) zHierarchy of FDM schemes zGroup y12 voice channels (4kHz each) = 48kHz yRange 60kHz to 108kHz zSupergroup y60 channel yFDM of 5 group signals on carriers between 420kHz and 612 kHz zMastergroup y10 supergroups

Synchronous Time Division Multiplexing zData rate of medium exceeds data rate of digital signal to be transmitted zMultiple digital signals interleaved in time zTime slots pre-assigned to sources and fixed zTime slots allocated even if there is no data zTime slots do not have to be evenly distributed amongst sources

Synchronous Time-Division Multiplexing (TDM) zUsed in digital transmission zRequires data rate of the medium to exceed data rate of signals to be transmitted zSignals “take turns” over medium zSlices of data are organized into frames

Synchronous TDM and PSTN zUsed in the modern digital telephone system yUS, Canada, Japan: DS-1 (T-1), DS-3 (T-3),... yEurope, elsewhere: E-1, E3, … yThese are listed in table 8.3 Page 249 zData rate of 1.544Mbps zUses 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 slot

Time Division Multiplexing

TDM System

TDM Link Control zNo headers and trailers zData link control protocols not needed zFlow control yData rate of multiplexed line is fixed yIf one channel receiver cannot receive data, the others must carry on yThe corresponding source must be quenched yThis leaves empty slots zError control yErrors are detected and handled by individual channel systems

Data Link Control on TDM

Framing zNo flag or SYNC characters bracketing TDM frames zMust provide synchronizing mechanism zAdded digit framing yOne control bit added to each TDM frame xLooks like another channel - “control channel” yIdentifiable bit pattern used on control channel ye.g. alternating …unlikely on a data channel yCan compare incoming bit patterns on each channel with sync pattern

Pulse Stuffing zProblem - Synchronizing data sources zClocks in different sources drifting zData rates from different sources not related by simple rational number zSolution - Pulse Stuffing yOutgoing data rate (excluding framing bits) higher than sum of incoming rates yStuff extra dummy bits or pulses into each incoming signal until it matches local clock yStuffed pulses inserted at fixed locations in frame and removed at demultiplexer

TDM of Analog and Digital Sources

Digital Carrier Systems zHierarchy of TDM zUSA/Canada/Japan use one system zITU-T use a similar (but different) system zUS system based on DS-1 format zMultiplexes 24 channels zEach frame has 8 bits per channel plus one framing bit z193 bits per frame

Digital Carrier Systems (2) zFor voice each channel contains one word of digitized data (PCM, 8000 samples per sec) yData rate 8000x193 = 1.544Mbps yFive out of six frames have 8 bit PCM samples ySixth frame is 7 bit PCM word plus signaling bit ySignaling bits form stream for each channel containing control and routing info zSame format for digital data y23 channels of data x7 bits per frame plus indicator bit for data or systems control y24th channel is sync

Mixed Data zDS-1 can carry mixed voice and data signals z24 channels used zNo sync byte zCan also interleave DS-1 channels yDs-2 is four DS-1 giving 6.312Mbps

ISDN User Network Interface zISDN allows multiplexing of devices over single ISDN line zTwo interfaces yBasic ISDN Interface yPrimary ISDN Interface

Basic ISDN Interface (1) zDigital data exchanged between subscriber and NTE - Full Duplex zSeparate physical line for each direction zPseudoternary coding scheme y1=no voltage, 0=positive or negative 750mV +/- 10% zData rate 192kbps zBasic access is two 64kbps B channels and one 16kbps D channel zThis gives 144kbps multiplexed over 192kbps zRemaining capacity used for framing and sync

Basic ISDN Interface (2) zB channel is basic user channel zData zPCM voice zSeparate logical 64kbps connections o different destinations zD channel used for control or data yLAPD frames zEach frame 48 bits long zOne frame every 250  s

Frame Structure

Primary ISDN zPoint to point zTypically supporting PBX z1.544Mbps yBased on US DS-1 yUsed on T1 services y23 B plus one D channel z2.048Mbps yBased on European standards y30 B plus one D channel yLine coding is AMI usingHDB3

Primary ISDN Frame Formats

SONET: Synchronous Optical Network zSpecification for high-speed digital transfer via optical fiber zRates from 51.84Mbps to 13.2Gbps zUses Synchronous TDM

Sonet/SDH zSynchronous Optical Network (ANSI) zSynchronous Digital Hierarchy (ITU-T) zCompatible zSignal Hierarchy ySynchronous Transport Signal level 1 (STS-1) or Optical Carrier level 1 (OC-1) y51.84Mbps yCarry DS-3 or group of lower rate signals (DS1 DS1C DS2) plus ITU-T rates (e.g Mbps) yMultiple STS-1 combined into STS-N signal yITU-T lowest rate is Mbps (STM-1)

SONET Frame Format

SONET STS-1 Overhead Octets

Statistical Time Division Multiplexing zrequires digital signaling & transmission zdata rate capacity required is well below the sum of connected capacity zsame concepts as synchronous TDM zuses memory buffers to avoid loss of data zwidely used for remote communications with multiple terminals zsimilar to medium-sharing done by LANs

Statistical TDM zIn Synchronous TDM many slots are wasted zStatistical TDM allocates time slots dynamically based on demand zMultiplexer scans input lines and collects data until frame full zData rate on line lower than aggregate rates of input lines

Statistical TDM A B C D Data to be sent t 1 t 2 t 3 A1A1 B1B1 C1C1 D1 C2C2 D2 A2 B2B2 1st cycle 2nd cycle Synchronous TDM waste bandwidth A1A1 B1B1 C2C2 B2B2 1 st cycle 2 nd cycle Statistical TDM extra bandwidth available

Statistical TDM Frame Formats

Performance zOutput data rate less than aggregate input rates zMay cause problems during peak periods yBuffer inputs yKeep buffer size to minimum to reduce delay

Buffer Size and Delay

Asymmetrical Digital Subscriber Line zADSL zLink between subscriber and network yLocal loop zUses currently installed twisted pair cable yCan carry broader spectrum y1 MHz or more

ADSL Design zAsymmetric yGreater capacity downstream than upstream zFrequency division multiplexing yLowest 25kHz for voice xPlain old telephone service (POTS) yUse echo cancellation or FDM to give two bands yUse FDM within bands zRange 5.5km

ADSL Channel Configuration

Discrete Multitone zDMT zMultiple carrier signals at different frequencies zSome bits on each channel z4kHz subchannels zSend test signal and use subchannels with better signal to noise ratio z256 downstream subchannels at 4kHz (60kbps) y15.36MHz yImpairments bring this down to 1.5Mbps to 9Mbps

DMT Transmitter

xDSL zHigh data rate DSL zSingle line DSL zVery high data rate DSL