传输、复用.ppt

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。 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 频分多路复用 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 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 MultiplexingDiagram FDM System FDM of Three Voiceband Signals Analog Carrier Systems(1) ATT (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 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 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 others 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 p