COE 341: Data & Computer Communications (T062)Dr. Marwan Abu-Amara

Chapter 8:

Multiplexing

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Lecture Contents1. Introduction

2. Multiplexing Types

3. FDM

4. Synchronous TDM

5. Statistical TDM

6. ADSL

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Introduction Multiplexing: A generic term used when more

than one application or connection share the capacity of one link

Objective is to achieve better utilization of resources

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Multiplexing Types

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Frequency Division Multiplexing (FDM)

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FDM Useful bandwidth of medium exceeds required

bandwidth of channel Each signal is modulated to a different carrier

frequency Carrier frequencies separated so signals do not

overlap (guard bands) e.g. broadcast radio

Channel allocated even if no data

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FDM

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FDM Multiplexing Process: Time-Domain View

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FDM Multiplexing Process: Frequency-Domain View

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FDM De-Multiplexing Process: Time-Domain View

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FDM De-Multiplexing Process: Frequency-Domain View

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FDM System – Transmitter

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FDM System – Receiver

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FDM of Three Voiceband Signals

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Analog Carrier Systems Devised by AT&T (USA) Hierarchy of FDM schemes Group

12 voice channels (4kHz each) = 48kHz Range 60kHz to 108kHz

Supergroup 60 channels FDM of 5 group signals on carriers between 420kHz and 612 kHz

Master group 10 supergroups

Jumbo group 6 master groups

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Analog FDM Hierarchy

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Time Division Multiplexing (TDM)

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TDM Data rate of medium exceeds data rate of

digital signal to be transmitted Multiple digital signals interleaved in time May be at bit level or blocks Time slots preassigned to sources and fixed Time slots allocated even if no data Time slots do not have to be evenly

distributed amongst sources

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Time Division Multiplexing

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TDM Frames

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TDM System – Transmitter

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TDM System – Receiver

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TDM Link Control No headers and trailers Data link control protocols not needed for

MUXed line Flow 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 halted This leaves empty slots

Error control Errors are detected and handled by individual

channel systems

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Data Link Control on TDM

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Framing So far, no flag or SYNC characters bracketing

TDM frames on the MUXed line Must provide frame synchronizing mechanism Added digit framing

One control bit added to each TDM frame Looks 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

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Framing in TDM

C 1 2 3 4 C 1 2 3 4 C 1

0 1 0

Four data channels

Control Channel, C010101….

…..

A data ChannelUnlikely to have 010101…. over successive frames

…..

MUXed frame

• RX knows the size of the MUXed frame• It can check each frame bit frame-to-frame for the special pattern until it finds it • Once the position of the control channel is established, RX knows where the channel sequence starts and sync is established with TX

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Pulse Stuffing Problem - Synchronizing data sources

Clocks in different sources drifting Data rates from different sources not related

by simple rational number Solution - 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

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TDM of Analog and Digital Sources

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Digital Carrier Systems Hierarchy of TDM (similar to FDM) USA/Canada/Japan use one system ITU-T use a similar (but different) system US system based on DS-1 format

Multiplexes 24 PCM voice channels Each frame has 8 bits per channel plus one

framing bit 193 bits per frame (24 ch. 8 bits per ch. + 1

framing bit = 193 bits per frame)

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DS Signals

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DS & T Lines Rates

Service LineRate

(Mbps)Voice Channels

DS-1DS-1 T-1T-1 1.5441.544 2424

DS-2DS-2 T-2T-2 6.3126.312 9696

DS-3DS-3 T-3T-3 44.73644.736 672672

DS-4DS-4 T-4T-4 274.176274.176 40324032

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Digital Carrier Systems For voice each channel contains one word of

digitized data (PCM, 8000 samples per second) Data rate 8000 x 193 = 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 DS-1 can carry mixed voice and data signals

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DS-1 Transmission Format

(8000 x 7 bits = 56 kbps)

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T1

Due to 1 framing bitPer frame

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T1 Frames

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E1

E LineRate

(Mbps)Voice

Channels

E-1E-1 2.0482.048 3030

E-2E-2 8.4488.448 120120

E-3E-3 34.36834.368 480480

E-4E-4 139.264139.264 19201920

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Mixed Data

DS-1 can carry mixed voice and data signals 24 channels used No sync byte Can also interleave DS-1 channels

DS-2 is four DS-1 giving 6.312Mbps

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SONET/SDH Synchronous Optical Network (ANSI) Synchronous Digital Hierarchy (ITU-T) Compatible Utilize the large channel capacity of optical fibers Signal Hierarchy

Synchronous Transport Signal level 1 (STS-1) or Optical Carrier level 1 (OC-1) Frame of 810 octets every 125 s: 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)

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SONET/SDH Frame Format

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Statistical TDM

In Synchronous TDM many slots are wasted Statistical TDM allocates time slots dynamically

based on demand Multiplexer scans input lines and collects data

until frame is full Data rate on line lower than aggregate rates of

input lines

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Statistical TDM

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Statistical TDM Frame Formats

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Performance

Output data rate less than aggregate input rates

May cause problems during peak periods Buffer inputs Keep buffer size to minimum to reduce delay

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Buffer Size and Delay Increasing

utilization increases Buffer size delay

Utilization

> 0.8 is undesirable

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ADSL ADSL is an asymmetric communication technology

designed for residential users; it is not suitable for businesses

The existing local loops can handle bandwidths up to 1.1 MHz

ADSL is an adaptive technology. The system uses a data rate based on the condition of the local loop line

Use the following as a rule of thumb XYZ km XYZ Mbps XYZ km XYZ Mbps

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ADSL Hardware

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ADSL Frequency Bands

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DMT