An Introduction to Wireless Technology, Second Edition

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This equipment is also very expensive because of its analog nature and its

sensitivity to tuning. It requires a large amount of labor to install and

maintain. Another factor contributing to the overall expense is the difficulty

in applying large scale integration techniques to analog systems. Analog

equipment tends to have many more separate components than comparable

digital systems (digital ones have more circuits but are packed together into

a single component). This leads to a higher cost for analog technology,

although at first the opposite was true.

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It requires retuning and maintenance whenever the physical channel

changes (for example, is rerouted or repaired, etc.).

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It is customary to use two channels per conversation, one in either direction.

A reasonable estimate of “good” channel use by this technique (for voice

traffic) is 10%.

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The equipment is extremely modular and a failure in one element most often

does not affect the operation of the rest of the system.

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AMPS and ETACS cellular networks are good examples of FDM techniques.

3.2.2 Time Division Multiplexing (TDM)

This technique is shown in Figure 30. With TDM, many signals take turns at

using the same high-speed transmission link. Each signal is allocated a time

interval or a “frame” in which to transmit. “Frames” are transmitted over a

single high-speed channel. Within each frame there are many slots. A

low-speed channel is allocated one (or more) time slots within a high-speed

frame. Thus a 2.048 Mbps channel can be subdivided into 32 subchannels of 64

Kbps. The start of each frame is signalled by some unique coding which allows

the sender and the receiver to agree on where the beginning of the frame is.

The synchronization coding is sometimes a special (unique) bit stream (as when

SDLC or BSC traditional data transmission is used) but with digital transmission

it is usually signaled by some special state in the underlying Pulse Code

Modulation (PCM) coding. (The most common one is called a “code violation”.)

For more information on PCM see 3.1.4, “Pulse Code Modulation (PCM)” on

page 61.

Attaching equipment is assigned a slot into which data can be inserted or read.

Thus while the medium can run at a very high speed, each attachment point can

operate at a much lower data rate.

Figure 30. Time Division Multiplexing Principle

TDM is now the most common method used in telephone systems for carrying

multiple calls over microwave and other radio links.

Chapter 3. Radio Technology 73