Cellular Telephony

Call Handling

Unlike a landline (a wire-connected telephone fixed in one place), a mobile cellular telephone sends and receives calls using signals carried by radio waves. Different geographical areas are divided into distinct but overlapping areas called cells, each served by a different transmitting and receiving antenna. Cells vary greatly in size, from a few blocks or less in urban centers to entire towns in quieter rural areas.

When a cellular phone user makes a call, the phone sends out a radio signal to the nearest cellular antenna. This transmits the signal to a centralized call-distribution building called a Mobile Telephone Switching Office (MTSO) that can, in turn, send the signal either to the ordinary phone network (the PSTN—Public Switched Telephone Network) or to another cellular phone on the same network. Calls from the PSTN are routed in the opposite direction via the nearest cell to the cellular phone. As cellular phone users move around, the signals they send out may be picked up by more than one antenna in more than one cell. When users cross the boundary from one cell to another, calls they are making or receiving are “handed off” from one cell to the other, usually without any interruption in conversation.

Analog and Digital

Ordinary telephones send signals in analog form (variations in speech are transmitted down a copper wire by a fluctuating electric current). Computerized equipment, by contrast, is more suited to sending information in digital form (continuously varying speech signals are encoded as binary data). The first cellular phones used only analog technology; more recent devices have used a mixture of analog and digital, and the latest systems are entirely digital.

Phone calls are transmitted by radio waves using a mathematical technique called modulation, in which the shape of a radio wave (called the carrier) is changed (modulated) so that it incorporates the speech or data signal. Analog and digital phone systems differ in the type of modulation they use; the number of phone calls a system can handle at once depends on how effectively those calls can be kept separate within the limited band of radio frequencies available to the phone system, which in turn depends on the type of modulation used. The original analog phone network used in the United States was called Analog Mobile [Page 66]Phone System (AMPS). It used a system of modulation called Frequency Division Multiple Access (FDMA), so that each different phone call was carried by a different frequency of radio waves in a band between 800 and 900 MHz. But such a system rapidly runs out of frequencies as more and more users try to send calls.

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