Data and Computer Communications

Chapter 8 – Multiplexing

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Multiplexing Multiple channels on 1 physical link Common on long-haul, high capacity, links

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Frequency Division Multiplexing 넓은주파수대역을여러개의좁은채널로분할하여독립적으로사용하는방식

모든사람이같은언어를사용하고, 큰회의실을작은대화방으로나눈다음, 각대화방에들어가서동시에대화를함

통신보호대역 (guard band)을두어채널간간섭을피함 수신측에서는대역통과필터를거친신호만을통과시키므로상호교란을방지할수있음

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FDM Overview

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FDM Overview

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AM/FM 라디오방송 AM 방송할당대역 : 530 ~ 1,700KHz

모든방송국들이이대역을같이사용해야함

각 AM 방송국은 10KHz의대역폭이필요각방송국은서로다른반송파를사용방송국이신호를편이시키고다중화시킴공중으로발산되는신호는이모든신호를합한것수신장치가이모든신호를받아서원하는신호만필터링

FM 방송은 200KHz 더넓은대역이필요 88 ~ 108MHz 대역을사용

TV 방송 각 TV 채널은각각 6MHz의대역폭을가짐

Example

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각 100KHz 대역폭을갖는 5개의채널을 FDM으로다중화할경우필요한대역폭은? 단, 보호대역은 10KHz.

(5 X 100KHz) + (4 X 10KHz) = 540KHz

Example

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1Mbps로전송하는 4개의데이터채널(디지털)이 1MHz의아날로그위성채널을사용하기위한 FDM을설계하라. 1 MHz의위성채널을 4개의채널로분할하면각채널은 250KHz. 따라서, 1 Mbps의각디지털채널에서 4비트가 1Hz로변조되어야함 16-QAM 사용

Example

4비트가하나의신호로인코딩

아날로그위성채널

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FDM Voiceband Example Group

12 voice channels (4khz each) = 48khz 60khz ~ 108khz

Supergroup FDM of 5 group signals supports 60 channels 420khz ~ 612 khz

Mastergroup FDM of 10 supergroups supports 600 channels

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Asymmetrical Digital Subscriber Line (ADSL) Currently installed twisted pair cable Uses frequency division multiplexing

Reserve lowest 25Khz for voice (POTS : Plain Old Telephone Service)

Is asymmetric - bigger downstream than upstream Has a range of up to 5.5km

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Wavelength Division Multiplexing Architecture similar to FDM systems

Multiplexer consolidates laser sources for transmission over single fiber 다중빛소스를단일빛으로결합

프리즘이용 : 임계각과주파수기반

Demux separates channels at the destination 단일빛은다중빛소스로분리

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Synchronous Time Division Multiplexing 시간축에서여러개의시간슬롯을나누어전송매체를공유하는방식

자신에게할당된시간슬롯동안데이터전송

회의실전체를사용하여자기에게할당된시간동안에만이야기를함

통상 FDM에의하여분할된하나의주파수대역을시간축에서다시분할

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TDM System Overview

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Example 네개의 800 bps 채널을 TDM으로다중화하는데각채널마다 1 byte씩교대(interleave)할경우프레임길이와비트시간은얼마인가? 프레임전송율과비트전송율은? 각채널의전송율이 800 bps이므로다중화링크의전송율은 4*800 =

3200 bps 비트시간은 1/3200 s 프레임의크기 = 4*8 = 32 비트 프레임길이는 32 * 1/3200 = 1/100 s 프레임전송율 = 100 frame/s

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Data Link Control on TDMFlow and error control on individual channel

No data link control protocol is needed over multiplexed link

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Framing

One control bit added to each TDM frame Identifiable bit pattern used on control channel

Eg. Alternating 01010101…unlikely on a data channel

Compare incoming bit patterns on each channel with known sync pattern

01010101

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Example

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Example

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Statistical TDM

Statistical TDM allocates time slots dynamically based on demand In synch TDM, many

slots are wasted Multiplexer scans

input lines and collects data until frame full

May have problems during peak periods Must buffer inputs

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FDD & TDD

다중화를사용하여 full duplex를구현하는방식 FDD (Frequency Division Duplex)서로다른 주파수를이용하여양방향전송

TDD (Time Division Duplex)하나의주파수대역을 2개의시분할채널로분할하여양방향전송

송신과수신이시간을달리하기때문에 부가적인지연이발생

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Cable Modems

Dedicate two cable TV channels to data transfer Each channel shared by number of subscribers,

using statistical TDM Downstream

Cable scheduler delivers data in small packets Active subscribers share downstream capacity Headend scheduler notifies subscriber of slots to use

Allocates upstream time slots to subscribers

Upstream User requests timeslots on shared upstream channel

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Example

Allocates upstream time slots to subscribers

User requests timeslots