As the telephone user moves or from one cell area to another, the exchange automatically commands the handset and a cell site with a stronger signal (from the handset) to go to a new radio channels. When the handset responds through the new cell-site, the exchange switches the connection to the new cell-site.

Almost all mobile phones use cellular technology, including GSM, TDMA & CDMA and the old analog mobile phone systems. Hence, many people use the term "cell phone" to mean any mobile telephone system. The exception to mobile phones using cellular technology are satellite phones.

Old systems predating the cellular principle may still be in use in places. The most notable real hold-out is that many amateur radio operators maintain phone patches in their clubs' VHF repeaters.

There are a number of different digital cellular technologies; these include: GSM &  TDMA.

TDMA/GSM
TDMA is the access method used by the Electronics Industry Alliance and the Telecommunications Industry Association for Interim Standard 54 (IS-54) and Interim Standard 136 (IS-136). Using TDMA, a narrow band that is 30 kHz wide and 6.7 milliseconds long is split time-wise into three time slots.

Narrow band means "channels" in the traditional sense.

Each conversation gets the radio for one-third of the time. This is possible because voice data that has been converted to digital information is compressed so that it takes up significantly less transmission space. Therefore, TDMA has three times the capacity of an analog system using the same number of channels. TDMA systems operate in either the 800-MHz (IS-54) or 1900-MHz (IS-136) frequency bands.

GSM is the international standard in Europe, Australia and much of Asia and Africa. In covered areas, cell-phone users can buy one phone that will work anywhere where the standard is supported. To connect to the specific service providers in these different countries, GSM users simply switch subscriber identification module (SIM) cards. SIM cards are small removable disks that slip in and out of GSM cell phones. They store all the connection data and identification numbers you need to access a particular wireless service provider.

Unfortunately, the 1900-MHz GSM phones used in the United States are not compatible with the international system. If you live in the United States and need to have cell-phone access when you're overseas, the easiest thing to do is to buy a GSM 900MHz/1800MHz cell phone for traveling.

TDMA splits a frequency into time slots.




The structure of a GSM network