I'm on the train

5th May 2000 at 01:00
(Photograph) - The first mobile telephone call was made from Trafalgar Square on New Year's Eve 1985. Five years later there were more than half a million mobile subscribers in the UK. That figure has since increased to more than 24 million users, four in ten of the population. Mobile telephones y or cell phones have evolved dramatically from large power-hungry analogue devices to the current generation of palm-sized digital handsets, which can now incorporate internet access.

People tend to think of mobiles as little more than sophisticated two-way radios.

Both use radio signals but the similarity ends there. Radios communicate directly with each other on a common frequency that can be heard by anyone who tunes in to that frequency y enthusiasts can eavesdrop on air traffic control or police communications. Mobiles can only operate through a network maintained by a service provider (in the UK these are BT Cellnet, Vodafone, Orange and One-2-One). A call is likely to pass through a multiplicity of links y radio, microwave, and a fixed network of fibre-optic or copper cables.

Mobile handsets cannot communicate with each other independently.

When the handset is first switched on, it locks on to the nearest base station which contacts the home location register to check that the telephone is still valid and to note its location. Calls can be routed to the mobile by reference to the HLR, which also records any messages and forwards outgoing call information to the service provider for billing.

During a conversation, the link between a base station and a handset requires a pair of frequencies (one for transmit and one for receive), referred to as a duplex voice channel. (A "channel" is a means of communicating a signal.) The total number of individual calls is vast compared to the number of frequencies allocated for use by mobile phone network channels y for instance, Vodafone has only 55 chanels for 8 million customers in the UK. The principal solution is to divide the country into thousands of overlapping cells, with a base station (ie a mobile mast) at the centre of each. If a mobile moves between cells during a call (for instance, when you are on a train), the old base station "hands over" to the new one. New frequencies are assigned without any interruption to the conversation. So long as adjacent cells avoid using the same frequencies, the allocated channels can be used over and over again throughout the length of the country.

The other device for increasing call capacity only arrived with the digital network, known as Global System for Mobile Communications (GSM), first licensed in 1 992.

Digital cell phone technology compresses continuous speech into tiny bursts of digital data 0.577 milliseconds long, each one containing the voice signal for about 1 0 times its duration. With precise synchronisation the data bursts can be transmitted to the base station in allocated "time slots". This allows six voice channels to use the same frequency simultaneously. At the base station each data burst is decompressed for onward transmission over the network, with a negligible delay. (See panel.) Radio signals travel at the velocity of light (300 million metres per second) which means that there is a small but significant delay as the data bursts travel from the mobile handset to the base station. As the range increases, the handset must compensate for the increasing delay by transmitting its data bursts slightly earlier so that they arrive at the base station in the allocated slot.

By the time the handset is 35km from the base station the necessary advance (about 0.1 ins) is too great for the digital compression to handle, which effectively imposes a limit on the range of a handset.

Words and research Christopher Lambton Illustration Patrick Lewis Sources Vodafone; BT Celinet.

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