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Data and Computer Communications

Part Two Data communications

Chapter 8 Multiplexing

Main Contents 8.1 frequency-division multiplexing FDM 频分复用8.2 Synchronous Time Division Multiplexing 同步时分复用8.3 Statistical Time Division Multiplexing 统计时分复用8.4 Asymmetrical Digital Subscriber Line ADSL 不对称数字用户线路

Key PointsFDM, TDM FDM system synchronous TDM and statistical TDM ADSL 2

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Multiplexing 复用，多路复用

The two common forms of multiplexing : frequency-division multiplexing (FDM) Time-division multiplexing (TDM)

★1. FDM and TDM 频分复用和时分复用

ab b ca tc d

frequency-division

Time-division

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8.1 Frequency Division Multiplexing P243

FDM Used for analog signals Each signal is modulated to a different carrier

frequency

Each requires a certain bandwidth centered around the carrier frequency-channel

The channels are separated by guard bands

e.g. broadcast radio and cable TV

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Frequency Division Multiplexing Diagram

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FDM of Three Voice band Signals

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Sender

Receiver

FDM System

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(USA)

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Wavelength Division Multiplexing 波分复用 Multiple beams of light at different frequency Carried by optical fiber Each color of light (wavelength) carries separate

data channel

1997 Bell Labs Commercial systems of 160 channels of 10 Gbps

now available

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8.2 Synchronous Time Division Multiplexing P250 TDM Used for digital signals

Multiple digital signals interleaved （交替） in time

Time slots （时隙） preassigned to sources and fixed

Time slots allocated even if no data

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Time Division Multiplexing

★ 2. Synchronous TDM and Statistical TDM 同步时分复用和统计时分复用Synchronous

A

B

C

D

a

b

c

d

t

t

t

t④

③

②

①a

c

b

时分复用用户

ab b ca t

4 个时分复用帧

#1

c d

#2 #3 #4

★ 2. Synchronous TDM and Statistical TDM 同步时分复用和统计时分复用

Statistical

用户

A

B

C

D

a

b

c

d

t

t

t

t

t

3 个 STDM 帧

#1

④

③

②

①a

c

b

ab b ca c d

#2 #3

统计时分复用

TDMSystem

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DS-1 Transmission Format

Each frame contains 24 channels; 8 bits per channel ; plus a framing bit 24×8 ＋ 1 ＝ 193bits/frame

Data Rate: 193bits×8000 帧 = 1.544Mbps Data Rate of each channel: 8bits×8000 帧 /s=64kbps

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Sonet/SDH P257 Compatible 兼容

SONET ： Synchronous Optical Network (ANSI) 同步光纤网络 STS_1 一级 :51.84Mb/s STS_3 3×51.84Mb/s=155.52Mb/s

SDH: Synchronous Digital Hierarchy (ITU-T) 同步数字系列 STM-1 155.52Mb/s STM-4 4×155.52=622.08Mb/s

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8.3 Statistical TDM 统计时分复用 P260 In Synchronous TDM many slots are wasted

Allocates time slots dynamically based on demand

n I/O lines, but only k (k<n) time slots available on the TDM frame.

Address information is required to assure proper delivery

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8.4 Asymmetrical Digital Subscriber Line P267

★ 3. ADSL（非对称用户数字线路） Greater capacity downstream than

upstreamLink between subscriber and network

Local loop

Lowest 25kHz for voice Plain old telephone service (POTS) 传统电话业务

Use echo cancellation or FDM to give two bands A smaller upstream band A larger downstream band

Use FDM within bands

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ADSL Channel Configuration

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Discrete Multitone P269 DMT （离散多音） Multiple carrier signals at different frequencies

a number of 4kHz subchannels

Send test signal to determine the signal to noise ratio Assign more bits to subchannels with better

transmission qualities

256 downstream subchannels at 4kHz (60kbps) 15.36MHz (256×60kb/s) Impairments bring this down to 1.5Mbps to 9Mbps

( 损伤）

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Discrete Multitone

What is the difference between frequency multiplexing and frequency modulation?

频分复用调频

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Reflection questions and exercises

Analog data, analog signal P159 ， P163

调幅AM

调相 PM

调频 FM

载波

信号

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Terms and Concepts

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Terms and Concepts

CDMA 码分多址码分多址 code division multiple access ， CDMA对每个节点分配不同的编码。 TDM 和 FDM 分别对节点分配时隙和频率。每个节点用它唯一的编码来对它要发送的数据进行编码。发送方要发送的每一个比特都通过乘以一个信号（编码）的比特来进行编码。接收方使用发送方的编码，从聚合的信号中恢复一个特定的发送方发送的数据。CDMA 协议类似于让聚会客人使用多种语言来谈论；人们实际上会沉浸在他们能理解的语言的谈话中，而过滤了其余的谈话。

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Practice and Application

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伪随机序列是具有某种随机特性的确定的序列。29