Multiplexing

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Contents

FDMBell Systems’s FDM

Synchronous TDMT1, T3

Statistical TDMMultiple Access: FDMA, TDMA, CDMA

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Multiplexing/Demultiplexing

Multiplexing is the process of combining two or more signals and transmitting them over a single transmission link

Demultiplexing is the reverse process of separating the multiplexed signals at the receiving end of the transmission link

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Multiplexing

Two approaches to achieve greater efficiency in the use of transmission services:

Multiplexing: several information sources share a large transmission capacity

Compression: reduces the number of bits required to represent a given amount of information

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Why multiplexing?

The higher the data rate, the more cost-effective the transmission facility

Data communication devices require relatively modest data rate support

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Multiplexing Example

Long-distance telephone networking Traffic from subscribers is routed and switched from

source to destination, with each trunk carrying multiplexed traffic from a variety of sources

By multiplexing, tens of voice channels can be combined on pairs of wire, hundreds on coaxial cable and thousands on coaxial cable, microwave and fiber optics

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FDM/TDM

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FDM

A number of signals can be carried simultaneously if each signal is modulated onto a different carrier frequency

The carrier frequencies are sufficiently separated so that the bandwidths of the signals do not overlap

Each modulated signal requires a certain bandwidth centered around its carrier frequency, referred to as a channel

The greater the bandwidth, the greater the number of signals that can be multiplexed

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FDM

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Hierarchy of the Bell Systems’s FDM

FDM of 4 kHz voice signalsMultiple TV signals can be frequency-division

multiplexed on a cable, each with a bandwidth of 6 MHz

Dozens of video signals can be simultaneously carried using FDM on a coaxial cable (500 MHz)

Hierarchy of the Bell Systems’s FDM

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Hierarchy of the Bell Systems’s FDM

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Hierarchy of the Bell Systems’s FDM

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Synchronous TDM

PCM (Pulse Code Modulation): An example of TDM signals

Analog signals from several sources are digitized and interleaved to form a PCM signal

The time-division multiplexed PCM signal is then transmitted onto a single frequency channel

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Time Division Multiplexing(also called STDM –Synchronous Time Division Multiplexing)

Multiplexern linksrate r bpseach 1 link, rate nr bps

Frame:

Time “slots” are reserved

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TDM PCM Signal

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PCM Signals

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Digital Multiplex Hierarchy

The basis of the TDM hierarchy is the DS-1 transmission format which multiplexes 24 channels:T-1 Carrier

T-1 facility was first introduced by AT&T in the 1960s

The Bell Operating Companies began to tariff T-1 services in 1984

The most common use of T-1 facilities is for leased dedicated transmission between customer premises

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Digital Multiplex Hierarchy

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Digital Multiplex Hierarchy

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Digital Multiplex Hierarchy: T-1

T-1 carrier provides a data rate of 1.544 Mbps and is capable of supporting the DS-1 multiplex format

Each channel contains one word (8 bits) of digitized voice data

Each frame contains 8 bits per channel plus a framing bit for 24*8+1=193 bits

Each frame repeats 8000 times per second for a data rate 8000*193=1.544 Mbps

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Digital Multiplex Hierarchy: T-1

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Digital Multiplex Hierarchy: T-1

Superframe: T1 carrier frames are tx in groups of 12The data transmitted by a synchronous TDM are

organized into frames, each of which contains a cycle of time slots

The set of time slots dedicated to one source, from frame to frame is called a channel

Synchronous TDM is called synchronous not because synchronous transmission is used but because the time slots are preassigned to sources and are fixed

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Digital Multiplex Hierarchy: T-1

Bit robbing: LSB of every 6th

and 12th frameTo tx supervisory information:

on-hook/off-hook condition, dialing, ringing, busy status

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Digital Multiplex Hierarchy: T-3

T-3 FacilitiesEach higher level of the TDM hierarchy is formed by

multiplexing signals from a lower level or by combination of those signals plus input at the appropriate data rate from other sources

Almost 30 times the capacity of T-1 lines, 44.7 Mbps (DS 3)

T-3 facilities are attractive for building backbone wide-area networks for high-volume users

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Statistical TDM

The use of synchronous TDM for a group of devices is extremely inefficient

In statistical multiplexer, there are more attached devices than time slots available within a frame for transmission

Each device (I/O line) has a buffer associated with it

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Statistical TDM

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Statistical TDM

For input, the multiplexer scans the input buffers, collect the data until a frame is filled, and then send the frame

For output, the multiplexier receives a frame and distributes the slots of data to the appropriate output buffers

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Statistical TDM

The statistical multiplexer does not send empty slots when there are no data to send

Since data arrive from and are distributed to I/O lines unpredictably, address information is required to ensure proper delivery

Although the average aggregate input may be less than the multiplexed line capacity, there may be peak periods when the input exceeds capacity

The solution to this problem is to include a buffer in the multiplexer to hold temporary excess input

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Statistical TDM

There is a trade-off between the size of the buffer and the data rate of the line

The trade-off is one between system response time and the speed of the multiplexed line

Overflow probability is a function of buffer sizeSynchronous multiplexers still have an important

role in long distance private and public networks

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Multiple Access

How to organize the channels for multiple users?Channelization

FDMA

TDMA

SSMA: Spread Spectrum

SDMA

u1

u2

u3

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FDMA

FDMA works best for analog signalContinuous transmission scheme

CH 1

CH 2CH 3

CH N..

t

f

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TDMA

TDMA system tx data in a buffer-and-burst method tx is not continuous: low battery consumptionTDMA works for digital systemsGSM: TDMA

CH 1

t

fCH

2. . . CH

NCH

1CH

2

Frame 1 Frame 2

CH 1

t

f. . . CH

8CH

1. . . CH

8CH

1

CH 1

t

fCH

1CH

1

200kHztx

rx

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Basic Structure of time slotsUSDC 25 frames/sec

6 slots/frameGSM 26 frames/120msec

8 slots/frame

Sync bit: receivers need to be synchronized for each data burstEqualization bit: training bitsGuard (blank bits): buffer between TSs to compensate for the time

delay between the mobile and the base stationRamp (blank bits): start/stop (power up/down) bits, only at uplink

TS1 TS2 TSN

Syncbit

Signalingor Control

INFO Guard/Ramp

TSt

Equalizationbit

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Summary

Bell Systems’s FDM (Channel group, Supergroup): analog process

Synchronous TDM (T1, T3): digital processStatistical TDMCellular Network: FDMA, TDMA