DataandComputerCommunications实用教案

1、为充分利用通信干线的传输容量为充分利用通信干线的传输容量(rngling)(rngling)，常采用多路复用技术。，常采用多路复用技术。多路器多路器( (复用器复用器) )组合组合N N路输入线路上的数路输入线路上的数据据, ,通过一条高容量通过一条高容量(rngling)(rngling)的数的数据链路传输；分路器接收到复用的数据据链路传输；分路器接收到复用的数据流流, ,按通道分离按通道分离( (分路分路) )这些数据这些数据, ,把它们把它们送到适当的输出线路上。送到适当的输出线路上。多路复用第1页/共52页第一页，共53页。几种类型几种类型(lixng)(lixng)的多路复的多路复用

2、技术用技术 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(shx)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。第2页/共52页第二页，共53页。频分多路复用频分多路复用子载波fsc1m1(t)Ssc1(t)子载波fsc2m2(t)Ssc2(t)子载波fscNmN(t)SscN(t)发送器fcmc(t)s(t)=FDM:NoImagefsc1Bsc1fsc2Bsc2fs

3、cNBscNBf| Mc(f) |0(a) 发送器(b) 复合(fh)信号的谱(正f)第3页/共52页第三页，共53页。频分多路复用频分多路复用带通滤波器,fsc1接收器带通滤波器,fsc2带通滤波器,fscN解调器,fsc1解调器,fsc2解调器,fscNm2(t)m1(t)mN (t): :s(t)(c) 接收器第4页/共52页第四页，共53页。频分复用频分复用(f yn(f yn) )CH2CH1CH3原始带宽原始带宽CH1CH2CH3频分后带宽频分后带宽MUXCH1 CH2 CH3带宽复用带宽复用f第5页/共52页第五页，共53页。波分复用波分复用(f yn(f yn) )：FDMFD

4、M的变形的变形F2F1F3 光谱光谱F1F2F3共享光纤的光谱共享光纤的光谱光纤光纤2光纤光纤3光纤光纤1共享光纤共享光纤 采用无源设备，更可靠采用无源设备，更可靠棱柱棱柱/衍射光栅衍射光栅第6页/共52页第六页，共53页。时分复用时分复用A2A1A3原始信号D2D1D3数字化信号MUX复用后数据复用后数据时隙时隙1234D3D2D1第7页/共52页第七页，共53页。复用复用(f yn(f yn) )：共享使用公共信道：共享使用公共信道 z DEMUX复用器复用器解复用器解复用器共享信道共享信道(xn do)MUX第8页/共52页第八页，共53页。Multiplexing第9页/共52页第九页

5、，共53页。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(y )信号调制到不同的载波频率第10页/共52页第十页，共53页。Frequency Division Multiplexing(2) Carrier frequencies sep

6、arated so signals do not overlap (guard bands) 载波频率要相互隔离使信号(xnho)不重叠(防护频带) e.g. broadcast radio Channel allocated even if no data第11页/共52页第十一页，共53页。Frequency Division MultiplexingDiagram第12页/共52页第十二页，共53页。FDM System第13页/共52页第十三页，共53页。FDM of Three Voiceband Signals第14页/共52页第十四页，共53页。Analog Carrier Sy

7、stems(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第15页/共52页第十五页，共53页。Analog Carrier Systems(2) Supergroup(超群(choqn) 60 channel FDM of 5 group signals on carriers between 420kHz and 612 kHz Mastergroup(主群) 10 supergroups第16页/共52页

8、第十六页，共53页。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 no

9、t have to be evenly distributed amongst sources(不必在数据源间均匀分配(fnpi)时隙)第17页/共52页第十七页，共53页。Time Division Multiplexing第18页/共52页第十八页，共53页。TDM System第19页/共52页第十九页，共53页。TDM Link Control No headers and tailers(传输数据流无首尾部(wi b) Data link control protocols not needed Flow control Data rate of multiplexed line i

10、s 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第20页/共52页第二十页，共53页。Data Link Control on TDM第21页/共52页第二十一页，共53页。Framing No flag o

11、r SYNC characters bracketing TDM frames(无标志(biozh)或SYNC对TDM定界) Must provide synchronizing mechanism Added digit 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

12、 a data channel Can compare incoming bit patterns on each channel with sync pattern第22页/共52页第二十二页，共53页。Added digit framing(增加增加(zngji)数字组帧数字组帧) 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 cha

13、nnel 控制信道使用(shyng)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 channel with sync pattern第23页/共52页第二十三页，共53页。Pulse Stuffing(脉冲脉冲(michng)填充填充) Problem - Synchronizing various data sources Clocks in different sources drifti

14、ng 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

15、locations in frame and removed at demultiplexer第24页/共52页第二十四页，共53页。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 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

16、直到(zhdo)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第25页/共52页第二十五页，共53页。TDM of Analog and Digital Sources第26页/共52页第二十六页，共53页。Digital Carrier Systems Hierarchy of TDM USA/Canada/Japan use one system ITU-T use a similar (but different) s

17、ystem 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第27页/共52页第二十七页，共53页。Digital Carrier Systems (2) For voice each channel contains one word of digitized data (PCM, 8000 samples per sec) Data rate 8000 x193 = 1.544Mbps

18、 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 24th channe

19、l is sync第28页/共52页第二十八页，共53页。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第29页/共52页第二十九页，共53页。ISDN User Network Interface ISDN allows multiplexing of devices over single ISDN line Two interfac

20、es Basic ISDN Interface Primary ISDN Interface第30页/共52页第三十页，共53页。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 ac

21、cess is two 64kbps B channels and one 16kbps D channel This gives 144kbps multiplexed over 192kbps Remaining capacity used for framing and sync第31页/共52页第三十一页，共53页。Basic ISDN Interface (2) B channel is basic iser channel Data PCM voice Separate logical 64kbps connections o different destinations D ch

22、annel used for control or data LAPD frames Each frame 48 bits long One frame every 250s第32页/共52页第三十二页，共53页。Frame Structure第33页/共52页第三十三页，共53页。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 standa

23、rds 30 B plus one D channel Line coding is AMI usingHDB3第34页/共52页第三十四页，共53页。Primary ISDN Frame Formats第35页/共52页第三十五页，共53页。Sonet/SDH Synchronous Optical Network (ANSI) Synchronous Digital Hierarchy (ITU-T) Compatible Signal Hierarchy Synchronous Transport Signal level 1 (STS-1) or Optical Carrier lev

24、el 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 ITU-T lowest rate is 155.52Mbps (STM-1)第36页/共52页第三十六页，共53页。第37页/共52页第三十七页，共53页。SONETSDH比特率 (Mbps)STS-1/OC-1 51.84STS-3/OC-3 STM-1 155.52STS-9/OC-

25、9 STM-3 466.56STS-12/OC-12 STM-4 622.08STS-18/OC-18 STM-6 933.12STS-24/OC-24 STM-8 1244.16STS-36/OC-36 STM-12 1866.24STS-48/OC-48 STM-16 2488.32STS = Synchronous Transport Signal 同步传输信号(xnho)OC = Optical Carrier 光载波STM = Synchronous Transport Module 同步传输模块第38页/共52页第三十八页，共53页。SONET 系统(xtng)sectionsec

26、tionsectionsectionlinelinepath复用器复用器中继器中继器中继器中继器复用器复用器复用器复用器Section： 段，任意段，任意(rny)两个相邻设备之间的光纤两个相邻设备之间的光纤line: 线路，线路， 两个复用器之间两个复用器之间 path: 通道，源站和目的站之间（端到端）通道，源站和目的站之间（端到端）第39页/共52页第三十九页，共53页。SONET Frame Format第40页/共52页第四十页，共53页。SONET STS-1 Overhead Octets第41页/共52页第四十一页，共53页。8.3 Statistical TDM In Syn

27、chronous 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 rates of input lines第42页/共52页第四十二页，共53页。Statistical TDM Frame Formats第43页/共52页第四十三页，共53页。Perform

28、ance Output data rate less than aggregate input rates May cause problems during peak periods Buffer inputs Keep buffer size to minimum to reduce delay第44页/共52页第四十四页，共53页。Buffer Size and Delay第45页/共52页第四十五页，共53页。8.4 Asymmetrical Digital Subscriber Line ADSL Link between subscriber and network Local loop Uses currently installed twisted pair cable Can carry broader spectrum 1 MHz or more第46页/共52页第四十六页，共53页。ADSL Design Asymmetric Greater capacity do