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Chapter 11

Multiplexing and

Demultiplexing (Channelization)

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The Concept of Multiplexing

• Multiplexing to refer to the combination of information streams from multiple sources for transmission over a shared medium– Multiplexor is a mechanism that implements the concept

• Demultiplexing to refer to the separation of a combination back into separate information streams– Demultiplexor to refer to a mechanism that implements the concept

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The Basic Types of Multiplexing • There are four basic approaches to multiplexing that each have a set of

variations and implementations– Frequency Division Multiplexing (FDM)– Wavelength Division Multiplexing (WDM)– Time Division Multiplexing (TDM)– Code Division Multiplexing (CDM)

• TDM and FDM are widely used• WDM is a form of FDM used for optical fiber• CDM is a mathematical approach used in cell phone mechanisms

FDM –Basic Operation • Multiple signals can be

modulated using different frequencies

• Signals with different frequencies can be Multiplexed together

• The multiplexed signal has a center frequency and bandwidth– Larger than total BW of

all multiplexed signals

SummedAnalogSignals

Subcarriers

Shifted in frequency

No overlap

B=Total BW

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Using a Range of Frequencies Per Channel

• To increase the overall data rate– a sender divides the frequency range of the channel into K carriers– and sends 1/K of the data over each carrier

• A sender can perform FDM within an allocated channel– Sometimes, the term subchannel allocation refers to the subdivision

• To increase immunity to interference– a sender uses a technique known as spread spectrum

• Various forms are suggested, but basic idea is– divide the range of the channel into K carriers– transmit the same data over multiple channels– allow a receiver to use a copy of the data that arrives with fewest errors

• The scheme works well in cases where noise is likely to interfere with some frequencies at a given time

FDM –Example

• Transmitted TV signal– Total BW is 6 MHZ

• Audio carrier operating at f0+5.75 MHz (fca)

• Color subcarrier operating at fcc at f0+4.799545 MHz

• Video subcarrier operating at fvc at f0+1.25 MHz

• CATV has a bandwidth of about 500 MHZ – Many channels can be

multiplexed together!

FDM –Multiplexing three voice signals

• Voice signal has a range of 300-3400 KHz

• Recall FM

Lower Sideband

Upper Sideband

tfftfftftftx

tftx

mcmcc

m

c

)(2cos)(2cos2cos2cos)(

2cos)](1[

−+++→=

+

πππππ

SubCarrierFreq.

s1

Fc-Bw Fc+Bw

Upper sideband is filtered

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Frequency Division Multiplexing

Guard band Concept

Guard band Concept

Analog Carrier Systems• Long-distance links use an FDM hierarchy• Implemented by AT&T (USA) and ITU-T

(International) variants– Group

• 12 voice channels (4kHz each) = 48kHz• in range 60kHz to 108kHz

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

– Mastergroup• FDM of 10 supergroups supports 600 channels

– so original signal can be modulated many times

Hierarchical FDM

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Analog Carrier Systems

4KHz each

Range

Wavelength Division Multiplexing

• FDM with multiple beams of light at different frequency

• carried over optical fibre links– commercial systems with 160 channels of

10 Gbps– lab demo of 256 channels @ 39.8 Gbps

• architecture similar to other FDM systems– multiplexer consolidates laser sources

(1550nm) for transmission over single fibre– Optical amplifiers amplify all wavelengths– Demux separates channels at the

destination• also have Dense Wavelength Division

Multiplexing (DWDM)

Synchronous Time Division Multiplexing• The original time division multiplexing.• The multiplexor accepts input from attached devices in a

round-robin fashion and transmit the data in a never ending pattern.

• T-1 and ISDN telephone lines are common examples of synchronous time division multiplexing.

Synchronous Time Division Multiplexing -TDM

TDM –Basic Operation

Round Robin

Round Robin

Why is synchronization needed? observe that a synchronous TDM sends one slot after another without any indication of the output to which a given slot occurs

A demultiplexor cannot tell where a slot begins

a slight difference in the clocksused to time bits can cause a demultiplexor to misinterpret the bit stream

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Framing Used in the Telephone System Version of TDM

• To prevent misinterpretation, the version of TDM used in the phone system includes an extra framing channel as input

• Instead of taking a complete slot, framing inserts a single bit in the stream on each round

• A demultiplexor extracts data from the framing channel and checks for alternating 0 and 1 bits

Example –TDM of analog and digital sources with different transmission rates

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Conversing Analog

To Digital

Sampler A/DConverter

Analog

Example –Only one station is transmitting!

Another Example

-Byte interleaving-Characters are shuffled! -The receiver reassembles each channel

TDM Hierarchy: Digital Carrier Systems• long-distance links use an TDM hierarchy• AT&T (USA) and ITU-T variants• US system based on DS-1 format• can carry mixed voice and data signals• 24 channels used for total data rate 1.544Mbps• each voice channel contains one word of digitized

data (PCM, 8000 samples per sec)• same format for 56kbps digital data• can interleave DS-1 channels for higher rates

– e.g., DS-2 is four DS-1 at 6.312Mbps

Digital System Level StandardDS-Standards

Carrier-Level T- Standards

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DS-1 Transmission Format

; Bit 8 indicates voice or data

(6x8KHz or 5x9.6KHz or 10x4.8 KHz) – The first bit is used to indicate the subrate

1/8000=125 usec125 usec / 193 = 0.6477 usec0.6477 x 8 = 5.18 usec

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The Problem with Synchronous TDM: Unfilled Slots

• Synchronous TDM works well if each source produces data at a uniform, fixed rate equal to 1/N of the capacity of the shared medium

• Many sources generate data in bursts, with idle time between bursts

• In practice, a slot cannot be empty because the underlying system must continue to transmit data– the slot is assigned a value (such as zero)– and an extra bit is set to indicate that the value is invalid

Statistical TDM

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More Efficient!

Comparing Synchronous and Statistical TDM Frame Format

http://www.trendcomms.com/multimedia/training/broadband%20networks/web/main/m2/temari/seccio9/t1e1.htm

Each slot has channel ID and possibly message length information

CDM

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Code Division Multiplexing (CDM)

• CDM used in parts of the cellular telephone system and for some satellite communication– The specific version of CDM used in cell phones is known as Code

Division Multi-Access (CDMA)• Frequency hopping: Each user regularly hops between different frequencies

(hoping pattern)

• CDM does not rely on physical properties– such as frequency or time

• CDM relies on an interesting mathematical idea– values from orthogonal vector spaces can be combined and

separated without interference

• Each sender is assigned a unique binary code Ci– that is known as a chip sequence– chip sequences are selected to be orthogonal vectors – (i.e., the dot product of any two chip sequences is zero)

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Code Division Multiplexing

• The first step consists of converting the binary values into vectors that use -1 to represent 0:

• If we think of the resulting values as a sequence of signal strengths to be transmitted at the same time– the resulting signal will be the sum of the two signals

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Code Division Multiplexing

• A receiver treats the sequence as a vector– computes the product of the vector and the chip sequence– treats the result as a sequence, and converts the result to binary by

interpreting positive values as binary 1 and negative values as 0

• Thus, receiver number 1 computes:

• Interpreting the result as a sequence produces: (2 -2 2 -2)– which becomes the binary value: (1 0 1 0)– note that 1010 is the correct value of V1

– receiver 2 will extract V2 from the same transmission

CDM General Operation –Power Level

Cell Phone Communications Using CDMA

http://www.celltrek.com/sub03_technologies/sub03_08.php

Advantages and Disadvantages

TDM Link Control

• no headers and trailers• data link control protocols not needed• flow control

– data rate of multiplexed line is fixed– if one channel receiver can not receive data, the others

must carry on– corresponding source must be quenched– leaving empty slots

• error control– errors detected & handled on individual channel