DataandComputerCommunications.ppt

1、Data and Computer Communications,Chapter 8 Multiplexing,為充分利用通信干線的傳輸容量，常采用多路復(fù)用技術(shù)。多路器(復(fù)用器)組合N路輸入線路上的數(shù)據(jù),通過一條高容量的數(shù)據(jù)鏈路傳輸；分路器接收到復(fù)用的數(shù)據(jù)流,按通道分離(分路)這些數(shù)據(jù),把它們送到適當(dāng)?shù)妮敵鼍€路上。,多路復(fù)用,幾種類型的多路復(fù)用技術(shù),頻分多路復(fù)用(FDM) 在無線電和電視機(jī)技術(shù)中廣為使用； 同步時(shí)分多路復(fù)用(TDM) 時(shí)分多路復(fù)用的特殊情況,常被稱為TDM,普遍用于多路復(fù)用數(shù)字語音流的傳輸； 統(tǒng)計(jì)TDM 通過在多路器上增加一些復(fù)雜處理提高同步TDM的效率,各種熟悉的名稱包括：統(tǒng)

2、計(jì)TDM、異步TDM、智能TDM，統(tǒng)計(jì)TDM的名稱指出了它的主要特性。,頻分多路復(fù)用,(a) 發(fā)送器,(b) 復(fù)合信號(hào)的譜(正f),頻分多路復(fù)用,(c) 接收器,頻分復(fù)用,波分復(fù)用：FDM的變形,時(shí)分復(fù)用,復(fù)用：共享使用公共信道,DEMUX,復(fù)用器,解復(fù)用器,共享信道,Multiplexing,8.1Frequency Division Multiplexing(1),FDM頻分復(fù)用 Useful bandwidth of medium exceeds required bandwidth of channel 在傳輸媒體的有效帶寬超出信道需要的帶寬時(shí),就可以使用FDM Each signal

3、 is modulated to a different carrier frequency 每一個(gè)信號(hào)調(diào)制到不同的載波頻率,Frequency Division Multiplexing(2),Carrier frequencies separated so signals do not overlap (guard bands) 載波頻率要相互隔離使信號(hào)不重疊(防護(hù)頻帶) e.g. broadcast radio Channel allocated even if no data,Frequency Division MultiplexingDiagram,FDM System,FDM o

4、f Three Voiceband Signals,Analog Carrier Systems(1),AT&T (USA)美國(guó)電話電報(bào)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 s

5、upergroups,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 n

6、ot have to be evenly distributed amongst sources(不必在數(shù)據(jù)源間均勻分配時(shí)隙),Time Division Multiplexing,TDM System,TDM Link Control,No headers and tailers(傳輸數(shù)據(jù)流無首尾部) 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

7、 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(無標(biāo)志或SYNC對(duì)TDM定界) Must provide synchronizing mechanism Added di

8、git framing(增加數(shù)字組幀) 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(增加數(shù)字

9、組幀),One control bit added to each TDM frame Looks like another channel - “control channel” 每一幀附加一控制位,看起來像另一個(gè)信道-控制信道 Identifiable bit pattern used on control channel 控制信道使用可標(biāo)示位模式 e.g. alternating 01010101unlikely on a data channel 如數(shù)據(jù)信道不太可能的交替位模式01010101 Can compare incoming bit patterns on each chan

10、nel with sync pattern,Pulse Stuffing(脈沖填充),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 rates

11、 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 輸出數(shù)據(jù)速率(不含幀定位比特)高于進(jìn)入數(shù)據(jù)速率和 S

12、tuff extra dummy bits or pulses into each incoming signal until it matches local clock 對(duì)每一個(gè)輸入填充額外空比特或脈沖直到匹配本地時(shí)鐘 Stuffed pulses inserted at fixed locations in frame and removed at demultiplexer 填充脈沖被插入幀中固定位置使分用器能刪除之,TDM of Analog and Digital Sources,Digital Carrier Systems,Hierarchy of TDM USA/Canada

13、/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 8000 x193 = 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 64kb

17、ps 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 Data PCM voice Separate logical 64kbps connections o different destinations D channel used for control or data LAPD

18、 frames Each frame 48 bits long One frame every 250s,Frame Structure,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 Line coding is AMI usingHDB3,Primary ISDN Fra

19、me Formats,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

20、) Multiple STS-1 combined into STS-N signal ITU-T lowest rate is 155.52Mbps (STM-1),SDH(同步數(shù)字體系) 和 SONET(同步光網(wǎng)絡(luò)),技術(shù)要點(diǎn) 由標(biāo)準(zhǔn)的網(wǎng)絡(luò)單元組成 統(tǒng)一的NNI，簡(jiǎn)化了信號(hào)的互通、傳輸、復(fù)用、交叉連接和交換 標(biāo)準(zhǔn)的信息速率等級(jí) 標(biāo)準(zhǔn)的光接口 特殊的復(fù)用結(jié)構(gòu)，PDH、B-ISDN信號(hào)都可以進(jìn)入 環(huán)形網(wǎng)絡(luò)結(jié)構(gòu)，可靠，路由靈活 幀中有大量開銷位，軟件進(jìn)行網(wǎng)絡(luò)配置和控制 使用單個(gè)主時(shí)鐘來同步 核心特點(diǎn) 同步復(fù)用 標(biāo)準(zhǔn)光接口 強(qiáng)大的網(wǎng)絡(luò)管理能力,速率等級(jí),SONETSDH比特率 (Mbps) STS

21、-1/OC-1 51.84 STS-3/OC-3 STM-1 155.52 STS-9/OC-9 STM-3 466.56 STS-12/OC-12 STM-4 622.08 STS-18/OC-18 STM-6 933.12 STS-24/OC-24 STM-8 1244.16 STS-36/OC-36 STM-12 1866.24 STS-48/OC-48 STM-16 2488.32 STS = Synchronous Transport Signal 同步傳輸信號(hào) OC = Optical Carrier 光載波 STM = Synchronous Transport Module 同

22、步傳輸模塊,SONET 系統(tǒng),包含交換機(jī)、復(fù)用器和中繼器,Section： 段，任意兩個(gè)相鄰設(shè)備之間的光纖 line: 線路， 兩個(gè)復(fù)用器之間 path: 通道，源站和目的站之間（端到端）,SONET Frame Format,SONET STS-1 Overhead Octets,8.3 Statistical TDM,In Synchronous TDM many slots are wasted Statistical TDM allocates time slots dynamically based on demand Multiplexer scans input lines an

23、d collects data until frame full Data rate on line lower than aggregate rates of input lines,Statistical TDM Frame Formats,Performance,Output data rate less than aggregate input rates May cause problems during peak periods Buffer inputs Keep buffer size to minimum to reduce delay,Buffer Size and Delay,8.4 Asymmetrical Digital Subscriber L