Cellular page
A cellular telephone is basically a two-way walkie-talkie that acts like a telephone. With a walkie-talkie, you either talk or you listen; with a cell phone, you can talk and listen at the same time. You can dial a number to place a call. You can receive calls. You can do fancy things like three-way calls, conference calls, call hold, and voice mail.
All conversations on cell phones are unprotected and can be intercepted. Don't think of a cell phone as a telephone, think of it as a radio. Sure, there are laws to protect you against illegal eavesdroppers, but obviously these laws are hard to enforce since it's hard to catch someone listening in on your call. Be careful of what you say (or transmit with a modem) on a cell phone!
Cellular phone systems can be analog of digital. Older systems are analog and newer systems are digital.
Each cell phone identifies itself to the cellular system each time it places or receives a call so that the cellular system can verify it is a valid paying customer. The cell phone's identity includes the phone number that is assigned by the service provider.
Many different types of systems have provided city-wide two-way communications, such as radio systems for taxis or the police. In these systems, a single antenna is located near the center of the city. Each two-way conversation occupied one channel so if there were 100 channels in the city, only 100 simultaneous conversations could be held. To cover the entire city, the antenna is placed on a tall building and emits a very strong signal.
In the cellular phone system, the city is divided into smaller sections, or cells. Each cell contains its own antenna and uses only a subset of all the channels. Each antenna is lower in height and emits a much weaker signal so that the same subset of channels can be used in a cell somewhere else within the city. For example, within a typical cellular system, every seventh cell uses the same set of channels.
The predominant cellular system operating in the US is the Advanced Mobile Phone System, AMPS. There are two minor variations of AMPS: EAMPS and NAMPS. They are all basically the same and were the basis for the European analog cellular system,
There are three variations of the analog cellular system in operation in the US:
All three systems have 42 control channels that are for setting up calls; the remaining channels are for voice conversations. All three systems are also analog systems. The systems are listed in chronological order and are backward compatible; that is, the infrastructure is designed so that older phones work on the newer systems.
There are three digital cellular systems being considered to supplement (and eventually replace) the current analog cellular system, AMPS, in the US:
As with the evolution of the analog cellular systems, these digital cellular systems will be backward compatible with the existing analog cellular systems,AMPS. Manufacturers typically offer "dual-mode" phones that operate in, for example, AMPS and CDMA; the phone dynamically uses whichever system provides the best signal.
The NAMPS were the standards untile the digital cellular service was aggreed upon.