《现代数字通信网》第三章数字传输系统[共72页]

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1、现代数字通信网第三章 数字传输系统北京理工大学电子工程系刘家康Email: L第三章 数字传输系统3.1多路复用3.2 SDH/SONET 3.3 物理层接口 RS-232C3.4 ISDN接口 本章主要内容：3.1多路复用1. 多路复用的概念2. 多路复用的分类：FDM, TDM3. PCM 30/32帧结构4. PCM 24帧结构5. PDH系列和PDH的复用方式复用：允许多个信号在一条数据链路上同时传输的技术。多路复用(Multiplexing)的概念多路复用的分类频分复用：在一条数据链路上多路信号分别调制在不同的载波上同时传输的技术。频分复用(FDM)的原理FDMFDM, Time D

2、omainMultiplexing, Frequency DomainDemultiplexing, Time DomainDemultiplexing, Frequency DomainFDM的工作过程Flash动画演示（08_04.swf）Wavelength-Division Multiplexing (WDM)波分复用WDM HighlightsBasic ideaMultiple beams of light at different frequenciesCarried by optical fiberA form of FDMEach color of light (wavele

3、ngth) carries separate data channelLandmark for WDM Bell Labs in 1997100 beamsEach at 10 Gbps1 Terabit per second (Tbps) on a single fiber!Commercial systems of 160 channels each 10 Gbps now availableLab systems going farthere.g. 256 channels each 39.8 Gbps10+ Tbps in totalOperating over a span of 1

4、00+ kmWDM OperationSame general architecture as other FDM systemsNo. of sources generating laser beams different wavelengthsMultiplexer consolidates sources for xmit over single fiberOptical amplifiers amplify all wavelengths Typically tens of kilometers apartDemux separates channels at destination

5、for multiple receiverse.g. 1550nm wavelen. (192-196THz) range is common, 50GHz spacing per channel (i.e. .4nm per channel)ITU WDM channel spacing (G.692) standard accommodates eighty 50GHz channels!Dense WDM (DWDM)No official or standard definitionImplies more channels, more closely spaced, than WDM

6、Coarse WDM (CWDM)Wider spacing, less channels时分复用：在一条数据链路上多路信号分别在不同的时隙上同时传输的技术。时分复用(TDM)的原理同步同步TDM(Synchronous TDM)TDM, MultiplexingTDM, Demultiplexing异步异步TDM(Asynchronous TDM)TDM的工作过程Flash动画演示（08_12.swf）Flash动画演示（08_17.swf）(asynchronous TDM)PCM 30/32：将一帧分为32个时隙，每个时隙8比特，其中的TS0和TS16分别用作帧同步和信令传输，共计328

7、=256比特，帧速率为256 8 k = 2.048 Mbps。PCM 30/32的帧结构PCM 24：将一帧分为24个时隙，每个时隙8比特，另加1比特的帧同步比特，共计248+1=193比特，帧速率为193 8 k = 1.544 Mbps。PCM 24的帧结构PDH序列(Plesiochronous Digital Hierarchy)3.2 SDH/SONET 1 为什么要提出SDH？2 SDH的帧结构3 SDH的复接4 SDH的优点为什么要提出SDH？a) 传统的PDH序列不能满足高速的要求b) PDH序列不是整数倍的关系 （例如二次群8448kbps不是一次群2048 kbps的四倍

8、，多了256kbps用于码速调整。） 在美国称为SONET（Synchronous Optical NETwork）（同步光纤网）, 在其他国家则称为SDH（Synchronous Digital Hierarchy ）（同步数字序列）SONET/SDHSynchronous Optical Network (SONET)Optical transmission interfaceDeveloped by BellCore, standardized by ANSISynchronous Digital Hierarchy (SDH)ITU-T standard compatible with

9、 SONETSignal HierarchyOC-x rate is optical equivalent of STS-x electrical signalSynch Transport Signal level 1 (STS-1) or Optical Carrier level 1 (OC-1)51.84 Mbps data rate, 50.112 Mbps payload rateCould carry DS-3 or group of lower rate signals (e.g. DS1 or DS2) plus ITU-T TDM carrier rates (e.g. L

10、evel-1 at 2.048 Mbps)Multiple STS-1 are combined into STS-N signal; some common onesOC-3 has 155.52 Mbps data rate, 150.336 payload rateOC-12 has 622.08 Mbps data rate, 601.344 payload rateOC-48 has 2488.32 Mbps data rate, 2405.376 payload rateOC-192 has 9953.28 Mbps data rate, 9621.504 payload rate

11、ITU-T lowest rate is STM-1 = STS-3 = OC-3SDH/SONET的组成SONET的基本单位是STS(Synchronous Transport Signal level), 又称为OC(Optical Carrier level), STS-1的速率是51.84 MbpsSDH的基本单位是STM(Synchronous Transfer Module), STM-1的速率为155.520 Mbps。而且，STM-n = n STM-1 = n 155.520 MbpsSONET systemConsists of switches, multiplexers

12、, and repeaters, all connected by fiberExample path from source to destination shown belowFiber from one device to another is a sectionA run between two MUXs (perhaps with repeaters in middle) is a lineConnection between source and destination (perhaps with MUXs and repeaters in middle) is a pathTop

13、ology can be a mesh, but often a dual ring SONET physical layerSDH/SONET的帧结构原理 STS-1/OC-1的速率：908 9 8k = 51.84 MbpsSONET frame formatBuilding block is STS-1 = OC-1 frame810 octets transmitted once every 125us 810 x 8000 frames/s = 6.48 MB/s = 51.84 Mb/sViewed as matrix of 9 rows of 90 octets each Fir

14、st 3 columns (27 octets) are overheads 9 of 27 for section-related overheads Generated/checked at start/end of each sections 18 of 27 for line-related overheads Generated/checked at start/end of each line Remainder is payloads 9 x 87 = 783 octetss Except one column of path overheads Injected somewhe

15、re in payloads Line-related overhead contains pointer to its locations Indicates start of Synchronous Payload Envelope (i.e. user data)s More on next two slidesSDH/SONET的帧结构STM-1的速率：2708 9 8k = 155.520 MbpsSONET Frame Format Section overhead: for single point-to-point fiber run Line overhead: for mu

16、ltiplexing multiple data streams (tributaries) onto single line and demultiplexing at other end Path overhead: for end-to-end issuesTransmitted row by row (LeftRight, TopBottom)Lowest rate in ITU-T SDH is STM-1 155.52 Mb/s; thus STM-1 OC-3, STM-4 OC12, etc.SONET STS-1 Overhead OctetsSDH的复接 SDH的复接等级序

17、列速率SDH的优点 a) 同步性b) 易于上下话路c) 易于管理d) 自愈性(Self Healing)3.3物理层接口 RS-232C 同步与异步传输方式接口的系统结构RS-232C标准 空MODEM的接法 同步与异步传输方式同步传输需要时钟线，特点是高速异步传输不需要时钟，只要两根数据线就可以了数据结构是：n表示方法： 1200-7N1 n表示1200波特，7位数据，无奇偶校验，1位停止位 起始位信息位停止位空闲位AsynchronousData transmitted one character at a time5 to 8 bitsTiming only needs maintain

18、ing within each characterResynchronization with start of each characterAsynchronous TransmissionReceiver slower or fasterthan transmitter Data rate is 10 kbps (and thus each bit has duration of 100ms) Receiver here is 6% too fast (i.e. 6ms per bit time)Asynchronous - BehaviorBehaviorIn a steady stre

19、am, interval between characters is uniform (length of stop element)In idle state, receiver looks for transition 1-to-0Then samples next subsequent intervals (char length)Afterwards, looks for 1-to-0 transition for next characterBasic featuresSimpleCheapOverhead of 2 or 3 bits per char (20%)Good for

20、data with large gaps (keyboard)接口的系统结构与典型配置 Line ConfigurationTopologyPhysical arrangement of stations on mediumPoint-to-pointMultipoint Computer and terminals, local-area networkHalf-duplexOnly one station may transmit at a timeRequires one data pathFull-duplexSimultaneous transmission and receptio

21、n between two stationsRequires two data pathsTraditional ConfigurationsInterfacingData processing devices (a.k.a. data terminal equipment, DTE) do not usually include data transmission facilitiesNeed interface called data circuit terminating equipment (DCE)e.g. modem, NICDCE transmits bits on medium

22、DCE communicates data and control info with DTEDone over interchange circuitsClear interface standards requiredRS-232-C的四种主要特性 机械特性 电气特性 功能特性 规程特性 Characteristics of InterfaceMechanicalConnection plugs ElectricalVoltage, timing, encodingFunctionalData, control, timing, groundingProceduralSequence of

23、 eventsV.24/EIA-232-FITU-T v.24Only specifies functional and proceduralReferences other standards for electrical and mechanicalEIA-232-F (USA)RS-232Mechanical ISO 2110Electrical v.28Functional v.24Procedural v.24RS-232-C的机械特性(D型25针) RS-232-C的机械特性(D型9针) 针脚序号简称对应的25针序号1CD82TD23RD34DTR205GND76DSR67RTS4

24、8CTS59RI22Mechanical SpecificationRS-232-C的功能特性RS-232-C的电气特性逻辑电平 CMOS 3 -15V 表示逻辑“1”； +3 +15V表示逻辑“0”； 3+3V 是中间状态，可抗噪声旁路电容 2500 pF开路电压 25 V通信速率 19.2 kbps最大距离 15m规程特性 握手过程举例：第一步：准备 1-7；7-1；第二步：准备好20-6；6-20；第三步：建立连接4-4；5-5；8-8；第四步：数据传输 2-2；3-3；24-24；17-17第五步：拆除链路4-4 off; 5-5 off; 8-8 off参见动画06_12.swfLo

25、cal and Remote LoopbackLoopback control is useful as a tool for fault isolationProcedural SpecificationExample: asynchronous private line modemWhen turned on and ready, modem (DCE) asserts DCE ReadyWhen DTE ready to send data, it asserts Request to Send (RTS) Also inhibits receive mode in half duple

26、xModem responds when ready by asserting Clear to Send (CTS)DTE sends data over Transmitted Data lineWhen data arrives from remote modem, local modem asserts Received Line Signal Detector and delivers data to its DTE on Received Data lineDTE Ready and Ring Indicator needed also if telephone networkDi

27、al-Up Operation (1)6: DCE ReadyDial-Up Operation (2)8: Received Line Signal DetectorDial-Up Operation (3)RS-232-C空MODEM的连接方法 三线连接11根线连接3.4 ISDN的物理层接口接口标准采用RJ-45标准，共8根线，实际使用6根线。ISDN物理层接口的帧结构采用TDM技术实现2B+D的数据传输ISDNIntegrated Services Digital Networka.k.a. Narrowband ISDNIntended for worldwide public te

28、lecomm networkYet to achieve universal deploymentMajor technical achievement is frame relayBasic characteristicsSupports voice and data, switched and non-switched apps.Reliance on 64kbps connections (then the std. rate for dig. voice)One or more B channels (64kbps each) as principal channel for circ

29、uit switching, packet switching, or dedicated (leased) circuits4 kinds of connections over B channel: circuit switched, packet switched, frame mode, semi-permanent (leased line)D channel for control signaling (e.g. call setup)ExamplesBasic service, small business or residence (2 B channels, 1 D chan

30、nel)Primary service, digital PBX or LANs (23 or 30 B channels, 1 D channel)ISDN Connection FeaturesISDN Physical InterfaceConnection between terminal equipment (equiv. to DTE) and network terminating equipment (equiv. to DCE)ISO 8877Cables terminate in matching connectors with 8 contactsTransmit/rec

31、eive lines carry both data and controlISDN Electrical SpecificationISDN uses balanced transmissionCarried on line consisting of two conductors, e.g. twisted pairSignals sent as current down one conductor and back the otherDifferential signaling Not to be confused with differential encoding! Value de

32、pends on direction of voltageTolerates more noise and generates lessAs opposed to unbalanced transmissione.g. RS-232 uses single signal line and groundISDN data encoding depends on data rateBasic rate is 192 kbps and uses pseudoternary encoding Primary rate uses B8ZS (1.544 Mbps) or HDB3 (2.048 Mbps

33、) with no particular advantage between第三章小结多路复用是提高信道利用率的技术多路复用分为FDM和TDM。PCM 30/32帧结构的标准速率为2MbpsSDH/SONET是高速数据传输的接口标准SDH中STM-1的速率是155.520 MbpsRS-232-C是串行通信物理层接口标准ISDN的物理层采用TDM技术作业1 (上交时间：2004年4月7日)在以下两道题目中任选一题。(a) 串行通信概述要求：对物理层协议的串行通信技术进行综述介绍，主要内容包括串行通信的发展历史、协议的主要内容、存在的技术标准以及串行通信技术的发展前景等。字数不得少于2000中文字符。(b) 串行通信编程要求：对串行通信协议的软件实现，实现的目标是通过MODEM的点到点的数据传输，如果不具备条件，可以采用空MODEM方式实现。主要内容包括硬件环境、软件平台、源程序说明、使用说明、以及源程序详细代码。第四章 电话通信网电路交换与电话网络信令方式与七号信令系统话务理论路由控制流量控制综合业务数字网ISDN简介内容预告