CIS 321 – Data Communications & Networking Chapter 8 – Multiplexing.

CIS 321 –Data Communications & Networking

Chapter 8 – Multiplexing

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Multiplexing

Simultaneous transmission of multiple signals across a single data link

Can utilize higher capacity links without adding additional lines for each device – better utilization of bandwidth

Multiplexer (MUX) on sender’s end; demultiplexer (DEMUX) separates transmission stream and directs signals to intended receiving devices

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Multiplexing vs. no multiplexing

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Multiplexing Techniques

3 basic techniques Frequency-division multiplexing – analog Wave-division multiplexing – analog Time-division multiplexing – digital

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Frequency-Division Multiplexing

Analog technique - when bandwidth of link is greater than combined bandwidth of signals to be transmitted

Signals from each sending device modulate different frequencies

Modulated signals are combined into a single composite signal

Bandwidth ranges are channels through which the signals travel, separated by guard bands

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FDM

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FDM MUX-DEMUX Process

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Analog Hierarchy of Phone Network

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Other Applications of FDM

AM and FM radio broadcasting Each station uses a different carrier frequency,

shifting its signal and multiplexing Receiver filters (tunes) to the frequency desired

Same concept for TV broadcasting and first generation cell phones

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6.2 Wave-Division Multiplexing

Use light signals transmitted through fiber-optic channels

Very narrow bands of light are combined from several sources to make a wider band of light

A prism is used to bend the light beams based on the angle of incidence and frequency

Receiver’s DEMUX separates signals

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WDM

Picture from Cisco Systems, Inc.

MPEG WDM example ©Sydney VisLab

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WDM Applications

Application: SONET network Multiple optical fiber lines are muxed/demuxed

DWDM (dense WDM) allows muxing of large numbers of channels by spacing channels closer to one another to achieve greater efficiency

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6.3 Time-Division Multiplexing (TDM)

Process of combining digital signals from several sources whereby each connection occupies a portion of time in the link

Link is sectioned by time rather than frequency

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TDM

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Time Slots and Frames

Data flow of each connection is divided into units Link combines one unit of each connection to

make a frame n input connections n time slots Data rate of link must be n times the duration of a

time slot to guarantee flow of data Time slots are grouped into frames; one complete

cycle of time slots; each slot dedicated to one device

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TDM

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Interleaving

Process of taking a specific amount of data from each device in a regular order

May be done by bit, byte, or any other data unit

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Synchronizing

Framing bits are used to alleviate timing inconsistencies that may be introduced

Usually one or two synchronization bits are added to beginning and end of each frame that allows the DEMUX to synchronize with the incoming stream so it can separate time slots accurately

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Framing Bits example

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Bit Padding

Used when differing speeds of devices are not integer multiples of each other (e.g. device A may be five and a half times as fast as device B)

MUX adds extra bits to a device’s source stream to force speed relationships between devices into integer multiples of each other

Extra bits are then discarded at the DEMUX

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Digital Signal (DS) Service

Hierarchy of digital signals DS-0 – single channel of 64 Kbps DS-1 – single service or 24 DS-0 channels multiplexed to

yield 1.544 Mbps DS-2 – single service or 4 DS-1 channels or 96 DS-0

channels to yield 6.312 Mbps DS-3 – single service, 7 DS-2 channels, 28 DS-1

channels, or 672 DS-0 channels to yield 44.376 Mbps DS-4 – 6 DS-3 channels, 42 DS-2 channels, 168 DS-1

channels, 4032 DS-0 channels to yield 274.176 Mbps

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DS Hierarchy

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T Lines

Capacity lines matching data rates of DS-1 to DS-4 services

Service Line Rate (Mbps) Voice Channels

DS-1 T-1 1.544 24

DS-2 T-2 6.312 96

DS-3 T-3 44.736 672

DS-4 T-4 274.176 4032

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T Lines

Digital lines designed for digital data, voice, or audio May be used for regular analog if sampled then

multiplexed using TDM

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T-1 line for multiplexing telephone lines

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T-1 frame structure

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Inverse TDM

Takes data stream from one high-speed line and breaks into portions and sends over several lower-speed lines simultaneously

Used in bandwidth-on-demand where channels can be used for several applications requiring different transmission rates (i.e. voice, data, video)

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Inverse Multiplexing

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More TDM Applications

Second-generation cell phone companies Divides into bands and uses FDM to combine Then uses TDM for sharing