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1、Data and Computer Communications,Chapter 8 Multiplexing,为充分利用通信干线的传输容量，常采用多路复用技术。多路器(复用器)组合N路输入线路上的数据,通过一条高容量的数据链路传输；分路器接收到复用的数据流,按通道分离(分路)这些数据,把它们送到适当的输出线路上。,多路复用,几种类型的多路复用技术,频分多路复用(FDM) 在无线电和电视机技术中广为使用； 同步时分多路复用(TDM) 时分多路复用的特殊情况,常被称为TDM,普遍用于多路复用数字语音流的传输； 统计TDM 通过在多路器上增加一些复杂处理提高同步TDM的效率,各种熟悉的名称包括：统
2、计TDM、异步TDM、智能TDM，统计TDM的名称指出了它的主要特性。,频分多路复用,(a) 发送器,(b) 复合信号的谱(正f),频分多路复用,(c) 接收器,频分复用,波分复用：FDM的变形,时分复用,复用：共享使用公共信道,DEMUX,复用器,解复用器,共享信道,Multiplexing,8.1Frequency Division Multiplexing(1),FDM频分复用 Useful bandwidth of medium exceeds required bandwidth of channel 在传输媒体的有效带宽超出信道需要的带宽时,就可以使用FDM Each signal
3、 is modulated to a different carrier frequency 每一个信号调制到不同的载波频率,Frequency Division Multiplexing(2),Carrier frequencies separated so signals do not overlap (guard bands) 载波频率要相互隔离使信号不重叠(防护频带) e.g. broadcast radio Channel allocated even if no data,Frequency Division Multiplexing Diagram,FDM System,FDM
4、of Three Voiceband Signals,Analog Carrier Systems(1),AT&T (USA)美国电话电报公司 Hierarchy of FDM schemes Group(基群) 12 voice channels (4kHz each) = 48kHz Range 60kHz to 108kHz,Analog Carrier Systems(2),Supergroup(超群) 60 channel FDM of 5 group signals on carriers between 420kHz and 612 kHz Mastergroup(主群) 10
5、supergroups,8.2 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 pre-assigned to sources and fixed Time slots allocated even if no data Time slots do
6、not have to be evenly distributed amongst sources(不必在数据源间均匀分配时隙),Time Division Multiplexing,TDM System,TDM Link Control,No headers and tailers(传输数据流无首尾部) Data link control protocols not needed Flow control Data rate of multiplexed line is fixed If one channel receiver can not receive data, the other
7、s must carry on The corresponding source must be quenched This leaves empty slots Error control Errors are detected and handled by individual channel systems,Data Link Control on TDM,Framing,No flag or SYNC characters bracketing TDM frames(无标志或SYNC对TDM定界) Must provide synchronizing mechanism Added d
8、igit framing(增加数字组帧) One control bit added to each TDM frame Looks like another channel - “control channel” Identifiable bit pattern used on control channel e.g. alternating 01010101unlikely on a data channel Can compare incoming bit patterns on each channel with sync pattern,Added digit framing(增加数
9、字组帧),One control bit added to each TDM frame Looks like another channel - “control channel” 每一帧附加一控制位,看起来像另一个信道-控制信道 Identifiable bit pattern used on control channel 控制信道使用可标示位模式 e.g. alternating 01010101unlikely on a data channel 如数据信道不太可能的交替位模式01010101 Can compare incoming bit patterns on each cha
10、nnel with sync pattern,Pulse Stuffing(脉冲填充),Problem - Synchronizing various 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 rate
11、s Stuff extra dummy bits or pulses into each incoming signal until it matches local clock Stuffed pulses inserted at fixed locations in frame and removed at demultiplexer,Solution - Pulse Stuffing,Outgoing data rate (excluding framing bits) higher than sum of incoming rates 输出数据速率(不含帧定位比特)高于进入数据速率和
12、Stuff extra dummy bits or pulses into each incoming signal until it matches local clock 对每一个输入填充额外空比特或脉冲直到匹配本地时钟 Stuffed pulses inserted at fixed locations in frame and removed at demultiplexer 填充脉冲被插入帧中固定位置使分用器能删除之,TDM of Analog and Digital Sources,Digital Carrier Systems,Hierarchy of TDM USA/Canad
13、a/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,Digital Carrier Systems (2),For voice each channel contains one word of digitized data (PCM, 8000 samples
14、 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
15、 for data or systems control 24th channel is sync,Mixed Data,DS-1 can carry mixed voice and data signals 24 channels used No sync byte Can also interleave DS-1 channels Ds-2 is four DS-1 giving 6.312Mbps,ISDN User Network Interface,ISDN allows multiplexing of devices over single ISDN line Two interf
16、aces Basic ISDN Interface Primary ISDN Interface,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 Remaining capacity used for framing and sync,Basic ISDN Interface (2),B channel is basic iser channel Da
