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