An Introduction to Wireless Technology, Second Edition

to prevent fraud, but today, ways have been found to re-write ESNs and

fraudulently register stolen phones. Fraudulent use of cellular phones is a major

problem in the US and is reaching other parts of the world very rapidly. The

new digital phone technologies will have more sophisticated methods of fraud

prevention.

Different cellular technologies (AMPS, ETACS, NMT) will have slightly different

detailed implementations of the calling process, but the overall sequence is

similar for all technologies. When a subscriber with a cellular phone wishes to

make a call, the phone number is entered on the key pad of the phone and the

SEND button is pressed. This causes the phone to send the dialled digits to the

network via the control channel assuming that the phone is still registered with a

base station. The cellular network will then assign a voice channel for the

connection once the called party′s phone starts to ring.

While the call is in progress, the network can detect if a mobile phone is moving

out of range of a particular base station. The network can then request that the

phone switch channels to a closer base station to continue the call. This is

achieved by the transmission of a short burst of data on the voice channel which

is not passed to the audio circuits of the phone, and thus is not heard by the

subscriber.

When a call is placed to a mobile phone the call is first routed to the MSC

closest to the origin of the call. If there is more than one MSC in the network,

the call will be routed to the MSC that the phone was last registered in. The

MSC will then transmit a paging message to all mobile phones in its area. If the

mobile phone responds, the MSC will assign a voice channel and route the call

to it. If the mobile phone does not respond, the other MSCs will try paging

mobile phones in their areas.

1.2.3 The Move to Digital

Analog mobile telephony served well as a first-generation technology; however,

analog services are now straining to keep up with user demand. Analog

transmissions are less efficient than digital transmissions when it comes to

spectrum utilization. Most analog standards allow low-speed (up to 4.8 Kbps)

data transmission such as fax or file transfer, but interface equipment is

expensive compared to the cost of mobile phones, and performance can be

unreliable (for example, fax only works well when sent from a stationary

terminal). Roaming across national boundaries is only possible where

neighboring countries implement the same standards.

For these reasons, efforts to develop next-generation mobile telephony networks

focus on digital technologies in general and on GSM (Global System for Mobile

Communications, formally called Group Spécial Mobile), a digital transmission

standard accepted by all European countries and many other countries.

Analog cellular phone systems such as AMPS, TACS and NMT use the analog

signal from the microphone to modulate the frequency of the radio carrier wave

directly using frequency modulation. See 3.1.2, “Frequency Modulation (FM)” on

page 59 for details of the way in which frequency modulation works. A digital

cellular phone will convert the analog signal from the microphone into digital

data which will then be used to modulate the carrier. This would normally use

phase modulation. See 3.1.3, “Phase Modulation (PM)” on page 60 for a

description of phase modulation. The analog signal is converted using a device

called a vocoder which will sample the level of the analog signal many times

Chapter 1. Introduction 9