Science / Medicine : A Weekly Roundup of News, Features and Commentary : HOW IT WORKS : The Cellular Telephone

Demand for cellular telephones has grown rapidly since their introduction four years ago--the number of cellular users nationally has now surpassed the 1-million mark. There are now two companies serving Southern California--PacTel Cellular and Los Angeles Cellular. PacTel’s system, covering Los Angeles, Orange, Riverside, San Bernardino and Ventura counties, is the nation’s largest in terms of the number of cell sites, covering more than 9,200 square miles. Together, the companies serve 110,000 users.

Here is how the mobile cellular phones work:

Cellular phones are powered by the car battery or portable battery pack.

The PacTel network uses radio frequencies in the 800-megahertz range of the UHF band set aside by the FCC for cellular use. The instant you turn on the ignition, your cellular phone, in its cradle, begins repeatedly scanning all the channels--even if you are not using your phone. In this manner, you may receive incoming calls.

Cellular technology works by dividing a city into smaller geographic areas called cells, each served by one or more radio transmitters. These are low-powered base stations, consisting of a radio transceiver and a bank of computerized equipment, with antennas atop communication towers of various heights.

Such cell sites are designed to accommodate thousands of callers because the same radio frequency can be used simultaneously throughout the system. There are about 45 channels assigned to every cell site, although there may be more or less depending on demand in any given area.

CALLING ON A CELLULAR PHONE LAND TO CAR 1. A caller dials a seven-digit number. 2. The call goes through the underground phone lines to the local telephone company central office and, from there, on to a Mobile Telephone Switching Office.* 3. Computers in the MTSO page all channels in the entire system. 4. When the MTSO computer determines which cell site is closest to the car being called, the computer sends the call via wireline facilities to that cell site. The cell site transmits the call via radio frequency to the car’s cellular phone and it rings. When a car phone user calls a land--based phone number, the procedure is reversed. CAR TO CAR A. One car phone user calls another. B. The call is transmitted via radio frequency to the closest cell site. C. The cell site transmits the call via wireline facilities to the MTSO. D. Domputers in the MTSO send out a paging signal from every cell site in the entire system. E. When the MTSO determines which cell site is closest to the car being called, then transmits the call via wireline and/or microwave facilities to that cell site. That cell site transmits the call via radio frequency to the other car’s cellular phone and it rings. * Digital switching will replace analog in 1988 throuhgout PacTel’s system because generally it costs less to operate. L.A. Cellular already uses digital switching, Tracy Crowe / Los Angeles Times.