数据与计算机通信第八章

1、William StallingsData and Computer Communications7th EditionChapter 8Multiplexing主要内容 8.1 Frequency-Division Multiplexing 8.2 Synchronous Time-Division Multiplexing 8.3 Statistical Time-Division Multiplexing 8.4 Asymmetric Digital Subscriber Line 8.5 xDSL 8.6 Recommeded Reading and Web SitesMultiple

2、xing8.1 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 Channel allocated even if no dataFrequency

3、 Division MultiplexingDiagramAnalog Carrier Systems AT&T (USA) Hierarchy (层次) 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 10 supergroupsWavelength Division Multipl

4、exing Multiple beams of light at different frequency Carried by optical fiber A form of FDM Each color of light (wavelength) carries separate data channel 1997 Bell Labs 100 beams Each at 10 Gbps Giving 1 terabit per second (Tbps) Commercial systems of 160 channels of 10 Gbps now available Lab syste

5、ms (Alcatel) 256 channels at 39.8 Gbps each 10.1 Tbps Over 100kmWDM Operation Same general architecture as other FDM Number of sources generating laser beams at different frequencies Multiplexer consolidates sources for transmission over single fiber Optical amplifiers amplify all wavelengths Typica

6、lly tens of km apart Demux separates channels at the destination Mostly 1550nm wavelength range Was 200MHz per channel Now 50GHzDense Wavelength Division Multiplexing DWDM No official or standard definition Implies more channels more closely spaced that WDM 200GHz or less8.2 Synchronous Time Divisio

7、n Multiplexing Data rate of medium exceeds (超过) data rate of digital signal to be transmitted Multiple digital signals interleaved (交叉) 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 distribut

8、ed amongst sourcesTime Division MultiplexingTDM SystemTDM Link Control No headers and trailers Data link control protocols not needed Flow controlData rate of multiplexed line is fixedIf one channel receiver can not receive data, the others must carry onThe corresponding source must be quenched(约束)T

9、his leaves empty slots Error controlErrors are detected and handled by individual channel systemsFraming No flag or SYNC characters bracketing TDM frames Must provide synchronizing mechanism Added digit framingOne control bit added to each TDM frame Looks like another channel - “control channel”Iden

10、tifiable bit pattern used on control channele.g. alternating 01010101unlikely on a data channelCan compare incoming bit patterns on each channel with sync patternPulse Stuffing (脉冲填充) Problem - Synchronizing data sources Clocks in different sources drifting Data rates from different sources not rela

11、ted by simple rational number Solution - Pulse StuffingOutgoing (输出) data rate (excluding framing bits) higher than sum of incoming ratesStuff extra dummy (额外的) bits or pulses into each incoming signal until it matches local clockStuffed pulses inserted at fixed locations in frame and removed at dem

12、ultiplexerTDM of Analog and Digital SourcesDigital 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 193 bits per frame T1载波Digital

13、Carrier Systems (2) T1载波也叫一次群, 它把24路话音信道按时分多路的原理复合在一条1.544 Mb/s的高速信道上。 CCITT有一个类似 1.544 Mb/s的标准。 CCITT还有一个2.048 Mb/s脉冲编码调制载波标准。 CCITT T1载波还可以多路复用到更高级的载波上， 如图所示。 图 8B.25 贝尔系统的T1载波 图 8B.26 多路复用 Digital Carrier Systems (2)Digital Carrier Systems (2) For voice each channel contains one word of digitized

14、 data (PCM, 8000 samples per sec)Data rate 8000 x193 = 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 info Same format for digital data23 channels of data 7 bits per fram

15、e plus indicator bit for data or systems control24th channel is syncMixed Data DS-1 can carry mixed voice and data signals 24 channels used No sync byte Can also interleave DS-1 channelsDs-2 is four DS-1 giving 6.312Mbps E1载波:23个8位一组的数据样本组装成125us的基本帧,其中30个子信道用于传送数据,2个子信道用于传送控制信令,每4帧能提供64个控制位,速率为2.04

16、8Mb/s.ISDN User Network Interface ISDN allows multiplexing of devices over single ISDN line Two interfaces(2B+D)Basic ISDN InterfacePrimary ISDN InterfaceBasic ISDN Interface (1) Digital data exchanged between subscriber(用户) and NTE(网络终端设备) - Full Duplex Separate physical line for each direction Pse

17、udoternary(伪三编码) 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 Remaining capacity used for framing and syncBasic ISDN Interface (2) B channel is basic user c

18、hannel Data PCM voice Separate logical 64kbps connections other different destinations D channel used for control or dataLAPD frames Each frame 48 bits long One frame every 250sPrimary ISDN Point to point Typically supporting PBX 1.544MbpsBased on US DS-1Used on T1 services23 B plus one D channel 2.

19、048MbpsBased on European standards30 B plus one D channelLine coding is AMI usingHDB3DS-1 Transmission FormatSONET (同步光纤网络)/SDH(同步数字体系同步数字体系) Synchronous Optical Network (ANSI) Synchronous Digital Hierarchy (ITU-T) Compatible Signal Hierarchy(信号体系)Synchronous Transport Signal level 1 (STS-1) or Opti

20、cal 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)SONET Frame FormatSONET STS-1 Overhead Octets8.3 Statistical TDM In Synchronous TDM many slots

21、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 rates of input linesStatistical TDM Frame FormatsPerformance Output data rate less than aggregate input rates May cause

22、problems during peak periodsBuffer inputsKeep buffer size to minimum to reduce delayBuffer Size and DelayCable Modem Outline Two channels from cable TV provider dedicated to data transfer One in each direction Each channel shared by number of subscribers Scheme needed to allocate capacity Statistica

23、l TDMCable Modem Operation Downstream Cable scheduler delivers data in small packets If more than one subscriber active, each gets fraction of downstream capacity May get 500kbps to 1.5Mbps Also used to allocate upstream time slots to subscribers Upstream User requests timeslots on shared upstream channel Dedicated slots for this Headend scheduler sends back assignment of future tme slots to subscriberCable Modem Scheme8.4 Asymmetrical Digital Subscriber Line ADSL Link between subscriber and networkLocal loop Uses currently installed twisted pair cableCan carry broader sp