Data and Computer Communications

Chapter 8Multiplexing

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

::

fsc1

Bsc1

fsc2

Bsc2

fscN

BscNB

f

| Mc(f) |

0

(a) 发送器

(b) 复合信号的谱 ( 正 f)

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 fscN

m2(t)

m1(t)

mN (t)

: :

s(t)

(c) 接收器

频分复用

CH2

CH1

CH3

原始带宽

CH1

CH2

CH3

频分后带宽

MUXCH1 CH2 CH3

带宽复用

f

波分复用： FDM 的变形

F2

F1

F3

光谱

F1 F2 F3

共享光纤的光谱

光纤 2

光纤 3

光纤 1

共享光纤

采用无源设备，更可靠

棱柱 / 衍射光栅

时分复用

A2

A1

A3

原始信号

D2

D1

D3

数字化信号

MUX

复用后数据

时隙1 2 3 4

D3D2D1

复用：共享使用公共信道

DEMUX

复用器 解复用器

共享信道MUX

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 FDMEach 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 radioChannel allocated even if no data

Frequency Division MultiplexingDiagram

FDM System

FDM of Three Voiceband Signals

Analog Carrier Systems(1)AT&T (USA) 美国电话电报公司Hierarchy of FDM schemesGroup( 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 MultiplexingData rate of medium exceeds data rate of

digital signal to be transmittedMultiple digital signals interleaved in timeMay be at bit level of blocksTime slots pre-assigned to sources and

fixedTime slots allocated even if no dataTime 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 ControlNo headers and tailers( 传输数据流无首尾部 )Data link control protocols not neededFlow 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

FramingNo flag or SYNC characters bracketing

TDM frames( 无标志或 SYNC 对 TDM 定界 )Must provide synchronizing mechanismAdded digit framing( 增加数字组帧 )

One control bit added to each TDM frameLooks like another channel - “control channel”

Identifiable bit pattern used on control channel e.g. alternating 01010101…unlikely on a data

channel Can compare incoming bit patterns on each

channel with sync pattern

Added digit framing( 增加数字组帧 )

One control bit added to each TDM frameLooks 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 01010101…unlikely 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

Pulse Stuffing( 脉冲填充 )Problem - Synchronizing various data

sourcesClocks in different sources driftingData rates from different sources not

related by simple rational numberSolution - 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 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 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 SystemsHierarchy of TDMUSA/Canada/Japan use one systemITU-T use a similar (but different) systemUS system based on DS-1 formatMultiplexes 24 channelsEach frame has 8 bits per channel plus

one framing bit193 bits per frame

Digital Carrier Systems (2)For voice each channel contains one word of

digitized data (PCM, 8000 samples 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 for data or systems control

24th channel is sync

Mixed DataDS-1 can carry mixed voice and data

signals24 channels usedNo sync byteCan also interleave DS-1 channels

Ds-2 is four DS-1 giving 6.312Mbps

ISDN User Network InterfaceISDN allows multiplexing of devices over

single ISDN lineTwo interfaces

Basic ISDN Interface Primary ISDN Interface

Basic ISDN Interface (1)Digital data exchanged between subscriber

and NTE - Full DuplexSeparate physical line for each directionPseudoternary coding scheme

1=no voltage, 0=positive or negative 750mV +/-10%

Data rate 192kbpsBasic access is two 64kbps B channels and

one 16kbps D channelThis gives 144kbps multiplexed over 192kbpsRemaining capacity used for framing and sync

Basic ISDN Interface (2)B channel is basic iser channelDataPCM voiceSeparate logical 64kbps connections o

different destinationsD channel used for control or data

LAPD frames

Each frame 48 bits longOne frame every 250s

Frame Structure

Primary ISDNPoint to pointTypically supporting PBX1.544Mbps

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 Frame Formats

Sonet/SDHSynchronous Optical Network (ANSI)Synchronous Digital Hierarchy (ITU-T)CompatibleSignal 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) Multiple STS-1 combined into STS-N signal ITU-T lowest rate is 155.52Mbps (STM-1)

SDH(SDH( 同步数字体系同步数字体系 ) ) 和 和 SONET(SONET( 同步光网络同步光网络 )) 技术要点技术要点

由标准的网络单元组成 统一的 NNI ，简化了信号的互通、传输、复用、交叉连接和交换 标准的信息速率等级 标准的光接口 特殊的复用结构， PDH 、 B-ISDN 信号都可以进入 环形网络结构，可靠，路由灵活 帧中有大量开销位，软件进行网络配置和控制 使用单个主时钟来同步

核心特点 同步复用 标准光接口 强大的网络管理能力

速率等级速率等级SONET SDH 比特率 (Mbps)STS-1/OC-1 51.84STS-3/OC-3 STM-1 155.52STS-9/OC-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 2488.32

STS = Synchronous Transport Signal 同步传输信号OC = Optical Carrier 光载波STM = Synchronous Transport Module 同步传输模块

SONET 系统 包含交换机、复用器和中继器包含交换机、复用器和中继器

sectionsection section section

line line

path

复用器 中继器 中继器复用器 复用器

Section ： 段，任意两个相邻设备之间的光纤line: 线路， 两个复用器之间 path: 通道，源站和目的站之间（端到端）

SONET Frame Format

SONET STS-1 Overhead Octets

8.3 Statistical TDMIn Synchronous TDM many slots are

wastedStatistical TDM allocates time slots

dynamically based on demandMultiplexer scans input lines and collects

data until frame fullData rate on line lower than aggregate

rates of input lines

Statistical TDM Frame Formats

PerformanceOutput data rate less than aggregate

input ratesMay cause problems during peak periods

Buffer inputs Keep buffer size to minimum to reduce delay

Buffer Size and Delay

8.4 Asymmetrical Digital Subscriber LineADSLLink between subscriber and network

Local loop

Uses currently installed twisted pair cable Can carry broader spectrum 1 MHz or more

ADSL DesignAsymmetric

Greater capacity downstream than upstream

Frequency division multiplexing Lowest 25kHz for voice

Plain old telephone service (POTS)

Use echo cancellation or FDM to give two bands

Use FDM within bands

Range 5.5km

ADSL Channel Configuration

Discrete MultitoneDMTMultiple carrier signals at different frequenciesSome bits on each channel4kHz subchannelsSend test signal and use subchannels with

better signal to noise ratio256 downstream subchannels at 4kHz

(60kbps) 15.36MHz Impairments bring this down to 1.5Mbps to 9Mbps

DMT Transmitter

xDSLHigh data rate DSLSingle line DSLVery high data rate DSL