Cellular networks rely on relatively short-range transmitter/ receiver (transceiver) base stations that serve small sections (or cells) of a larger service area. Mobile telephone users communicate by acquiring a frequency or time slot in the cell in which they are located.
Amaster switching center called the “mobile transport serving office” (MTSO) links calls between users in different cells and acts as a gateway to the PSTN. Figure C-5 illustrates the link from the MTSO to the base stations in each cell. The MTSO also has links to local telephone central offices so that cellular users can communicate with users of conventional phones.
Cell Sites Cell boundaries are neither uniform nor constant. The usage density in the area, as well as the landscape, the presence of major sources of interference (e.g., power lines, buildings), and the location of competing carrier cells, contributes to the definition of cell size. Cellular boundaries change continuously, with no limit to the number of frequencies available for transmission of cellular calls in an area.
As the density of cellular usage increases, individual cells are split to expand capacity. By dividing a service area into small cells with limited-range transceivers, each cellular system can reuse the same frequencies many times. Technologies such as Code Division Multiple Access (CDMA) and Expanded Time Division Multiple Access (E-TDMA) promise further capacity gains in the future.
Master Switching Center In a typical cellular network, the master-switching center operates similar to a telephone central office and provides links to other offices. The switching center supports trunk lines to the base stations that establish the cells in the service area. Each base station supports a specific number of simultaneous calls—from 3 to 15, depending on the underlying technology (i.e., CDMA, TDMA, or some derivative).
Transmission Channels Most cellular systems provide two types of channels: a control channel and a traffic channel. The base station and mobile station use the control channel to support incoming and outgoing calls, monitor signal quality, and register when a user moves into a new zone. The traffic channel is used only when the station is off-hook and actually involved in a call.
The control and traffic channels are divided into time slots. When the user initiates access to the control channel to place a call, the mobile station randomly selects a subslot in a general- use time slot to reach the system; the system then assigns a time slot to the traffic channel.
For an incoming call to a mobile station, the base station initiates conversations on the control channel by addressing the mobile station in a time slot, which at the same time reserves that time slot for the station’s reply. If a user’s call attempt collides with another user’s call attempt, both instruments automatically reselect a subslot and try again. After repeated collisions, if no time slots are available within a predetermined time, the system rejects service requests for incoming and outgoing calls.
When a mobile telephone user places a call, the cell in which the user is traveling allocates a slot for the call. The call slot allows the user access through the base station to the master switching center, essentially providing an extension on which the call can be placed.
The master switching center, through an element of the user-to-base-station connection, continuously monitors the quality of the call signal and transfers the call to another base station when the signal quality reaches an unacceptable level due to the distance traveled by the user, obstructions, and/or interference. If the user travels outside the system altogether, the master switching center terminates the call as soon as the signal quality deteriorates to an unacceptable level.
Cellular Telephones Cellular telephones incorporate a combination of multiaccess digital communications technology and traditional telephone technology and are designed to appear to the user as familiar residential or business telephone equipment. Manufacturers use miniaturization and digital signal processing technology to make cellular phones feature rich yet compact and economical.
Cellular instruments consist of a transceiver, an analog/ digital converter, and a supervisory/control system that manages calls and coordinates service with both the base station and the master switching center. Cellular telephones can be powered from a variety of sources, including vehicle batteries, ac adapters, and rechargeable battery sets.
Traditional cellular instrument types include hand-held, transportable, and car telephones. However, advances in cellular technology are creating additional types of telephones, including modular and pocket phones. The trend in cellular instruments is toward multipurpose transportable telephones. There are dual-mode cellular phones that can be used with in-building wireless Private Branch Exchanges (PBXs) as well as with the outside cellular service.
The handset registers itself with an in-building base station and takes its commands from the wireless PBX. For out-of-building calling, the handset registers with the nearest cell site transceiver. Aside from convenience, an added benefit of the dual-mode phone is that calls made off the corporate premises can be aggregated with business calls made at home or on the road for the purpose of achieving a discounted rate on all calls.