The original analog AMPS consisted of three major components, which remain
the basic components of a cellular system today:
- Mobile telephone switching office (MTSO)
- Base stations
- Cellular phones or other devices
In some cellular systems, functions may reside in the base station
rather than the MTSO. The terminology may differ slightly, but most cellular
systems include these three components, as shown in Figure
. The new 3G
technologies have additional components to interface with the Internet or an
intranet, using IP.
The basic idea behind cellular communications
divides a serving area into geographic zones, called cells. The bandwidth
within a cell is shared among the users according to a media access control
method. Each cell has a base station in the center. All cellular devices
communicate through this base station. This is similar to a WLAN cell with an
AP. There will be at least one MTSO per cellular system. The MTSO communicates
with the base stations and with the PSTN.
Cells are normally roughly
circular. In general, a smaller cell size means less power is needed, which
leads to smaller and cheaper devices. However, it also means that more cell
sites are required for the complete coverage of an area. When a cell becomes
too crowded, the overloaded cell is split into smaller cells by sectoring the
coverage of the cell into pie shaped segments using directional antennas on the
cell site. Cell sites are many times initially designed to have mutiple sectors
due to projected traffic loading and coverage. This allows more frequency reuse
and therefore, more capacity.