Types of cellular networks Edit
Within the United States, different types of digital cellular networks abound that follow distinct incompatible sets of standards. The two most dominant types of digital cellular networks are known as
Other common cellular networks include
IDEN networks use a proprietary protocol designed by Motorola, while the others follow standardized open protocols. A digital version of the original analog standard for cellular phone service, called Digital Advanced Mobile Phone Service (D-AMPS), also exist.
How they work Edit
Cellular networks provide coverage based on dividing up a large geographical service area into smaller areas of coverage called cells. Cells play an important role in reuse of radio frequencies in the limited radio spectrum available to allow more calls to occur than otherwise would be possible. Cells are often drawn as circles or hexagons, but real-world cells are irregular in shape because buildings and trees obstruct the radio waves. By deploying enough transmitters or base stations, cellular operators provide continuous coverage wherever their customers are likely to be. Because users often pass through several cells as they travel through a city, a cellular system has to automatically hand off the call from base station to base station. As the user nears the edge of a cell, the system reassigns the user to a new cell by determining which of the other base stations in the area can provide the strongest signal.
The cellular architecture makes efficient use of the spectrum and increases system capacity. In a conventional single-tower system, each channel can only be used by one customer at anyone time. By contrast, a cellular system allows a channel used in one cell to be reused by a different user in another cell, as long as there is enough separation between the cells to minimize interference. Network operators can further increase system capacity by splitting large cells into several smaller ones. The greater the number of cells, the greater the number of users who can use a channel at the same time. In typical systems, cells at highway interchanges or in downtown areas are less than a mile in diameter, while in areas where the traffic is light they may be up to 20 miles across.
The heart of a cellular system is the Mobile Telephone Switching Office (MTSO), which is connected by microwave or landline links to all of the base stations. It is also connected via a high speed digital link to the public switched telephone network. The user’s voice signal is transmitted from the phone through the air to a base station, back to the MTSO, and then through the landline network to its destination. The MTSO is responsible for managing the assignment of radio channels to users. When the user dials a number and presses the “send” button on their phone, the MTSO checks to see if there is a channel available and then assigns the channel. During the call, the MTSO monitors the signal strength to see if it should initiate a handoff to a nearby cell.