multiplexing function

multiplexing function

In both local and wide area communications, it is almost always the case that the

capacity of the transmission medium exceeds the capacity required for the transmission

of a single signal. To make efficient use of the transmission system, it is desirable

to carry multiple signals on a single medium. This is referred to as multiplexing.

 depicts the multiplexing function in its simplest form. There are n

inputs to a multiplexer. The multiplexer is connected by a single data link to a

demultiplexer. The link is able to carry n separate channels of data. The multiplexer

combines (multiplexes) data from the n input lines and transmits over a highercapacity

data link. The demultiplexer accepts the multiplexed data stream, separates

(demultiplexes) the data according to channel, and delivers them to the appropriate

output lines.

The widespread use of multiplexing in data communications can be explained

by the following:

1. The higher the data rate, the more cost effective the transmission facility. That

is, for a given application and over a given distance, the cost per kbps declines

with an increase in the data rate of the transmission facility. Similarly, the cost

of transmission and receiving equipment, per kbps, declines with increasing

data rate.

2. Most individual data communicating devices require relatively modest data

rate support. For example, for most client/server applications, a data rate of up

to 64 kbps is often more than adequate.

The preceding statements were phrased in terms of data communicating devices.

Similar statements apply to voice communications. That is, the greater the capacity of a

transmission facility, in terms of voice channels, the less the cost per individual voice

channel, and the capacity required for a single voice channel is modest.

Two techniques for multiplexing in telecommunications networks are in common

use: frequency division multiplexing (FDM) and time division multiplexing (TDM).

FDM takes advantage of the fact that the useful bandwidth of the medium

exceeds the required bandwidth of a given signal. A number of signals can be carried

simultaneously if each signal is modulated onto a different carrier frequency and the

carrier frequencies are sufficiently separated so that the bandwidths of the signals do

not overlap. Figure 2.12a depicts a simple case. Six signal sources are fed into a multiplexer

that modulates each signal onto a different frequency (ii,· .. ,16)' Each signal

requires a certain bandwidth centered on its carrier frequency, referred to as a

channel. To prevent interference, the channels are separated by guard bands, which

are unused portions of the spectrum (not shown in the figure).

An example is the multiplexing of voice signals. We mentioned that the useful

spectrum for voice is 300 to 3400 Hz. Thus, a bandwidth of 4 kHz is adequate to

carry the voice signal and provide a guard band. For both North America (AT&T

standard) and internationally (International Telecommunication Union Telecommunication

Standardization Sector [ITU-T] standard), a standard voice multiplexing

scheme is twelve 4-kHz voice channels from 60 to 108 kHz. For higher-capacity

links, both AT&T and ITU-T define larger groupings of 4-kHz channels.

TDM takes advantage of the fact that the achievable bit rate (sometimes,

unfortunately, called bandwidth) of the medium exceeds the required data rate of a

digital signal. Multiple digital signals can be carried on a single transmission path

by interleaving portions of each signal in time. The interleaving can be at the bit

level or in blocks of bytes or larger quantities. For example, the multiplexer in

 has six inputs that might each be, say, 9.6 kbps. A single line with a

capacity of 57.6 kbps could accommodate all six sources. Analogously to FDM, the

sequence of time slots dedicated to a particular source is called a channel. One

cycle of time slots (one per source) is called a frame.