Chapter_8_Multiplexing.ppt 904KB Mar 29 2010 05:03:17 AM

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William Stallings

Data and Computer

Communications

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Frequency Division Multiplexing

 _FDM

 _{Useful bandwidth of medium exceeds}

required bandwidth of channel

 _{Each signal is modulated to a different}

carrier frequency

 _{Carrier frequencies separated so signals}

do not overlap (guard bands)

 _{e.g. broadcast radio}

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Frequency Division Multiplexing

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Analog Carrier Systems

 _{AT&T (USA)}

 _{Hierarchy of FDM schemes}  _Group

 _{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

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Synchronous Time Division

Multiplexing

 _{Data rate of medium exceeds data rate of}

digital signal to be transmitted

 _{Multiple digital signals interleaved in time}  _{May be at bit level of blocks}

 _{Time slots preassigned to sources and}

fixed

 _{Time slots allocated even if no data}  _{Time slots do not have to be evenly}

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TDM Link Control

 _{No headers and tailers}

 _{Data link control protocols not needed}  _{Flow control}

 _{Data rate of multiplexed line is fixed}

 _{If one channel receiver can not 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}

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Framing

 _{No flag or SYNC characters bracketing TDM}

frames

 _{Must provide synchronizing mechanism}  _{Added digit framing}

 _{One control bit added to each TDM frame}  _{Looks 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}

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Pulse Stuffing

 _{Problem - Synchronizing data sources}  _{Clocks in different sources drifting}

 _{Data rates from different sources not related}

by simple rational number

 _{Solution - 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}

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Digital Carrier Systems

 _{Hierarchy of TDM}

 _{USA/Canada/Japan use one system}

 _{ITU-T use a similar (but different) system}  _{US system based on DS-1 format}

 _{Multiplexes 24 channels}

 _{Each frame has 8 bits per channel plus}

one framing bit

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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

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Mixed Data

 _{DS-1 can carry mixed voice and data}

signals

 _{24 channels used}  _{No sync byte}

 _{Can also interleave DS-1 channels}

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ISDN User Network Interface

 _{ISDN allows multiplexing of devices over}

single ISDN line

 _{Two interfaces}

 _{Basic ISDN Interface}

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Basic ISDN Interface (1)

 _{Digital data exchanged between subscriber}

and NTE - Full Duplex

 _{Separate physical line for each direction}

 _{Pseudoternary coding scheme}

 _{1=no voltage, 0=positive or negative}

750mV +/-10%

 _{Data rate 192kbps}

 _{Basic access is two 64kbps B channels and}

one 16kbps D channel

 _{This gives 144kbps multiplexed over 192kbps}

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Basic ISDN Interface (2)

 _{B channel is basic iser channel}  _Data

 _{PCM voice}

 _{Separate logical 64kbps connections o}

different destinations

 _{D channel used for control or data}  _{LAPD frames}

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Primary ISDN

 _{Point to point}

 _{Typically supporting PBX}  _1.544Mbps

 _{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}

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Sonet/SDH

 _{Synchronous Optical Network (ANSI)}

 _{Synchronous Digital Hierarchy (ITU-T)}

 _Compatible

 _{Signal 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}

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Statistical TDM

 _{In Synchronous TDM many slots are}

wasted

 _{Statistical TDM allocates time slots}

dynamically based on demand

 _{Multiplexer scans input lines and collects}

data until frame full

 _{Data rate on line lower than aggregate}

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Performance

 _{Output data rate less than aggregate}

input rates

 _{May cause problems during peak periods}  _{Buffer inputs}

 _{Keep buffer size to minimum to reduce}

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Buffer Size

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Asymmetrical Digital Subscriber

Line

 _ADSL

 _{Link between subscriber and network}  _{Local loop}

 _{Uses currently installed twisted pair cable}  _{Can carry broader spectrum}

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ADSL Design

 _Asymmetric

 _{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}

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Discrete Multitone

 _DMT

 _{Multiple carrier signals at different frequencies}

 _{Some bits on each channel}

 _{4kHz subchannels}

 _{Send test signal and use subchannels with}

better signal to noise ratio

 _{256 downstream subchannels at 4kHz}

(60kbps)

 _15.36MHz

 _{Impairments bring this down to 1.5Mbps to}

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xDSL

 _{High data rate DSL}  _{Single line DSL}

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Required Reading

 _{Stallings chapter 8}  _{Web sites on}