Cellular defined
Four key components make up most cellular radio systems: the cellular layout itself, a carefully engineered network of radio base stations and antennas, base station controllers which manage several base stations at a time, and a mobile switch, which gathers traffic from dozens of cells and passes it on to the public switched telephone network.
All analog and digital mobiles use a network of base stations and antennas to cover a large area. The area a base station covers is called a cell, the spot where the base station and antennas are located is called a cell site. Viewed on a diagram, the small territory covered by each base station appears like a cell in a honeycomb, hence the name cellular. Cell sizes range from sixth tenths of a mile to thirty miles in radius for cellular (1km to 50km). GSM and PCS use much smaller cells, no more than 6 miles (10km) across. A large carrier may use hundreds of cells.
Each cell site's radio base station uses a computerized 800 or 1900 megahertz transceiver with an antenna to provide coverage. Each base station uses carefully chosen frequencies to reduce interference with neighboring cells. Narrowly directed sites cover tunnels, subways and specific roadways. The area served depends on topography, population, and traffic. In some PCS and GSM systems, a base station hierarchy exists, with pico cells covering building interiors, microcells covering selected outdoor areas, and macrocells providing more extensive coverage to wider areas. See the Ericsson diagram below.
The macro cell controls the cells overlaid beneath it. A macro cell often built first to provide coverage and smaller cells built to provide capacity.
Macario describes a business park or college campus as a typical situation. In those cases a macrocell provides overall coverage, especially to fast moving mobiles like those in cars. A microcell might provide coverage to slow moving people between large buildings and a piconet might cover an individual lobby or the floor of a convention center.
Steve Punter, of the excellent Steve's Toronto Area Cellular/PCS Site Guide, http://www.arcx.com/sites/ (external link) says that typically microcells are employed along the sides of busy highways or on street corners. Steve sent in pictures of two typical microcells in the Toronto area:
[Microcell 1 (70K)] [Microcell2 (71K)
Base station equipment by itself is nothing without a means to manage it. In GSM and PCS 1900 that's done by a base station controller or BSC. As Nokia puts it, a base station controller "is a high-capacity switch which provides total overview and control of radio functions, such as handover, management of radio network resources and handling of cell configuration data. It also controls radio frequency power levels in the RBSs, and in the mobile phones. Base station controllers also set transceiver configurations and frequencies for each cell." Depending on the complexity and capacity of a carrier's system, there may be several base station controllers.
These BSCs react and coordinate with a mobile telecommunication switching office or MTSO, sometimes called, too, a MSC or mobile switching center. With AMPS or D-AMPs, however, the mobile switch controls the entire network. In either case, the mobile switch interacts with distant databases and the public switched telephone network or PSTN. It checks that a customer has a valid account before letting a call go through, delivers subscriber services like Caller ID, and pages the mobile when a call comes in. Among many other administrative duties. Learn more about cellular switches by checking out this small graphic. Also, if you want to see pictures of a "mobile" mobile switching center, (a Motorola EMX 100 Plus Cellular Switch) go to Michael Hart's excellent site (external link)[Link not working right now]
How does this work out in the real world? Consider Omnipoint's PCS network for the greater New York city area. To cover the 63,000-square-mile service area, Ericsson says Omnipoint installed over 500 cell sites, with their attendant base stations and antennas, three mobile switching centers, one home location register, and one service control point. (The latter two are network resources.) The New York Times says the entire system cost $680 million dollars, although they didn't say if that included Omnipoint's discounted operating license. Now that we've seen what makes up a cellular network, let's discuss the idea that makes that makes those networks possible: frequency reuse.]
Dual band IS-136 Ericsson RBS 884 base station