l SO that it can be located for incoming calls. The phone then identifies itself periodically after this in case it moves from one cell to another. It also repeats this action if it detects it has moved from one cell to another. 262 Newnes Guide to Radio and Communications Technology The other function that has to be undertaken is that of handover. If the mobile handset moves from one cell to another it needs to be able to undertake a number of actions SO that the handset communicates with a second base station instead of the first. This handover needs to be implemented very quickly SO that there is no interruption to the communication otherwise the users will notice a disruption to their conversation. This handover is co-ordinated by the base station controllers and the mobile switching centres. For handover within a cluster local switching centres can be used to control this but when transfer takes place between clusters it is handled by switching centres further up the chain. The system monitors the signal strength of the mobile. When it is falling in strength and the mobile signal can be heard better in the adjacent cell, the system arranges for it to swap to a clear channel supported by the new cell. When the change takes place the mobile swaps its frequency and the data from the base station is switched from the old base station to the new one and to the correct channel. All of this takes a fraction of a second and normally goes totally unnoticed by the user, especially when digital systems are used. Analogue systems The first mobile phone systems to be introduced were analogue. They were introduced at a time when IC technology was not as far advanced as it is today, and even then the first phones to be introduced were very large by comparison to those used today. In view of this the modulation schemes were relatively simple using FM for the voice channel and a form of frequency shift keying for the data channel. When compared to the systems in use today the spectrum usage was not particularly efficient. Th