An Introduction to Wireless Technology, Second Edition

developed by some manufacturers (for example, Microcom Network Protocol 10

or MNP 10). A special adapter is required to connect a PSTN modem to a

cellular phone. Some manufacturers tap into the handset-transceiver connection

(for example, CelJack from Telular) and others provide connections for their own

cellular phones (Motorola). In the US, Spectrum Cellular offers a range of

equipment for connection to the cellular phone network including fax machines

and special modems.

It can sometimes be difficult to get type-approval for these kind of solutions in

some countries, as the requirements of a data connection can be different to

those for a voice-only system. There are no standards for data-over-cellular and

the national approval authorities may not even allow these solutions in some

countries.

2.3 Digital Cellular Telephony

Digital cellular telephony is based on the same network concept as analog

cellular telephony with base stations and mobile stations. The move to digital,

as described in 1.2.3, “The Move to Digital” on page 9, led to the development of

different systems in Europe, Japan, and the US. It had been recognized for

some time that the analog cellular telephone systems did not make efficient use

of the available radio spectrum. In any voice conversation on an analog

network, the whole channel has to be dedicated to the end-to-end connection.

Most conversations consist of a small amount of time when information is

actually being transmitted, and the rest of the available time is silence - between

words, waiting for the other party to respond, pauses for breath, and thinking

time. A digital system can use this “dead time” to allow other conversations to

use the same radio channel. This is called Time Division Multiple Access

(TDMA).

Using digital technology it is also possible to compress speech by making some

assumptions about speech waveforms. In addition to using the “dead time” for

other voice calls, compressing speech allows even more users to share the

same channel. GSM in Europe can have up to eight two-way calls in the same

pair of radio channels. Future developments will be able to double this within

the next few years. With the analog cellular network capacity quickly becoming

saturated, it is not surprising that a great deal of development effort has gone

into digital cellular.

One other major advantage of digital cellular is the quality of the voice call.

Because the digital data stream can have error correction built in, interference

and other short breaks in transmission do not result in any loss of quality. If the

error correction mechanism cannot recover the lost data, then a short period of

silence will ensue. Listening to a digital cellular conversation compared to

listening to an analog phone can be likened to the difference between a compact

disk recording and a vinyl record. In fact, many of the same techniques are

used in digital cellular as are used in the production of CDs.

The last significant advantage of digital cellular is the inherent security against

casual eavesdropping. With analog cellular, a standard FM radio receiver

capable of covering the cellular channels can be tuned to receive an analog

cellular phone conversation. No special equipment is needed and a radio

“scanner” can be readily purchased at an affordable price. The scanner may

only be able to receive the channel being transmitted by the cellular base

station, but both halves of the conversation can usually be heard due to the fact

Chapter 2. Wireless Methodologies 37