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William Stallings Data and Computer Communications Chapter 8 Multiplexing.

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1 William Stallings Data and Computer Communications Chapter 8 Multiplexing


3 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 no data

4 Frequency Division Multiplexing Diagram

5 FDM System

6 FDM of Three Voiceband Signals

7 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

8 Synchronous Time Division Multiplexing zData rate of medium exceeds data rate of digital signal to be transmitted zMultiple digital signals interleaved in time zMay be at bit level of blocks zTime slots preassigned to sources and fixed zTime slots allocated even if no data zTime slots do not have to be evenly distributed amongst sources

9 Time Division Multiplexing

10 TDM System

11 TDM Link Control zNo headers and tailers zData link control protocols not needed zFlow control yData rate of multiplexed line is fixed yIf one channel receiver can not 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

12 Data Link Control on TDM

13 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 01010101…unlikely on a data channel yCan compare incoming bit patterns on each channel with sync pattern

14 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

15 TDM of Analog and Digital Sources

16 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

17 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

18 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

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

20 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

21 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

22 Frame Structure

23 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 using HDB3

24 Primary ISDN Frame Formats

25 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. 2.048Mbps) yMultiple STS-1 combined into STS-N signal yITU-T lowest rate is 155.52Mbps (STM-1)

26 SONET Frame Format

27 SONET STS-1 Overhead Octets

28 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

29 Statistical TDM Frame Formats

30 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

31 Buffer Size and Delay

32 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

33 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

34 ADSL Channel Configuration

35 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

36 DMT Transmitter

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

38 Required Reading zStallings chapter 8 zWeb sites on yADSL ySONET

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