(Multiplexing (chapter 8

(Multiplexing (chapter 8

اسلاید 1: William Stallings Data and Computer CommunicationsChapter 8Multiplexing

اسلاید 2: Multiplexing

اسلاید 3: 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 no data

اسلاید 4: Frequency Division Multiplexing Diagram

اسلاید 5: FDM System

اسلاید 6: FDM of Three Voiceband Signals

اسلاید 7: Analog Carrier SystemsAT&T (USA)Hierarchy of FDM schemesGroup12 voice channels (4kHz each) = 48kHzRange 60kHz to 108kHzSupergroup60 channelFDM of 5 group signals on carriers between 420kHz and 612 kHzMastergroup10 supergroups

اسلاید 8: Synchronous Time Division MultiplexingData rate of medium exceeds data rate of digital signal to be transmittedMultiple digital signals interleaved in timeMay be at bit level of blocksTime slots preassigned to sources and fixedTime slots allocated even if no dataTime slots do not have to be evenly distributed amongst sources

اسلاید 9: Time Division Multiplexing

اسلاید 10: TDM System

اسلاید 11: TDM Link ControlNo headers and tailersData link control protocols not neededFlow controlData rate of multiplexed line is fixedIf one channel receiver can not receive data, the others must carry onThe corresponding source must be quenchedThis leaves empty slotsError controlErrors are detected and handled by individual channel systems

اسلاید 12: Data Link Control on TDM

اسلاید 13: FramingNo flag or SYNC characters bracketing TDM framesMust provide synchronizing mechanismAdded digit framingOne control bit added to each TDM frameLooks like another channel - “control channel”Identifiable bit pattern used on control channele.g. alternating 01010101…unlikely on a data channelCan compare incoming bit patterns on each channel with sync pattern

اسلاید 14: Pulse StuffingProblem - Synchronizing data sourcesClocks in different sources driftingData rates from different sources not related by simple rational numberSolution - Pulse StuffingOutgoing data rate (excluding framing bits) higher than sum of incoming ratesStuff extra dummy bits or pulses into each incoming signal until it matches local clockStuffed pulses inserted at fixed locations in frame and removed at demultiplexer

اسلاید 15: TDM of Analog and Digital Sources

اسلاید 16: 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

اسلاید 17: Digital Carrier Systems (2)For voice each channel contains one word of digitized data (PCM, 8000 samples per sec)Data rate 8000x193 = 1.544MbpsFive out of six frames have 8 bit PCM samplesSixth frame is 7 bit PCM word plus signaling bitSignaling bits form stream for each channel containing control and routing infoSame format for digital data23 channels of data7 bits per frame plus indicator bit for data or systems control24th channel is sync

اسلاید 18: Mixed DataDS-1 can carry mixed voice and data signals24 channels usedNo sync byteCan also interleave DS-1 channelsDs-2 is four DS-1 giving 6.312Mbps

اسلاید 19: ISDN User Network InterfaceISDN allows multiplexing of devices over single ISDN lineTwo interfacesBasic ISDN InterfacePrimary ISDN Interface

اسلاید 20: Basic ISDN Interface (1)Digital data exchanged between subscriber and NTE - Full DuplexSeparate physical line for each directionPseudoternary coding scheme1=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

اسلاید 21: Basic ISDN Interface (2)B channel is basic iser channelDataPCM voiceSeparate logical 64kbps connections o different destinationsD channel used for control or dataLAPD framesEach frame 48 bits longOne frame every 250s

اسلاید 22: Frame Structure

اسلاید 23: Primary ISDNPoint to pointTypically supporting PBX1.544MbpsBased on US DS-1Used on T1 services23 B plus one D channel2.048MbpsBased on European standards30 B plus one D channelLine coding is AMI usingHDB3

اسلاید 24: Primary ISDN Frame Formats

اسلاید 25: Sonet/SDHSynchronous Optical Network (ANSI)Synchronous Digital Hierarchy (ITU-T)CompatibleSignal HierarchySynchronous Transport Signal level 1 (STS-1) or Optical Carrier level 1 (OC-1)51.84MbpsCarry 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 signalITU-T lowest rate is 155.52Mbps (STM-1)

اسلاید 26: SONET Frame Format

اسلاید 27: SONET STS-1 Overhead Octets

اسلاید 28: 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

اسلاید 29: Statistical TDM Frame Formats

اسلاید 30: PerformanceOutput data rate less than aggregate input ratesMay cause problems during peak periodsBuffer inputsKeep buffer size to minimum to reduce delay

اسلاید 31: Buffer Size and Delay

اسلاید 32: Asymmetrical Digital Subscriber LineADSLLink between subscriber and networkLocal loopUses currently installed twisted pair cableCan carry broader spectrum1 MHz or more

اسلاید 33: ADSL DesignAsymmetricGreater capacity downstream than upstreamFrequency division multiplexingLowest 25kHz for voicePlain old telephone service (POTS)Use echo cancellation or FDM to give two bandsUse FDM within bandsRange 5.5km

اسلاید 34: ADSL Channel Configuration

اسلاید 35: 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)15.36MHzImpairments bring this down to 1.5Mbps to 9Mbps

اسلاید 36: DMT Transmitter

اسلاید 37: xDSLHigh data rate DSLSingle line DSLVery high data rate DSL

اسلاید 38: Required ReadingStallings chapter 8Web sites on ADSLSONET