nel is used for the transmission of operations and maintenance messages between cellular telephones and cell sites. If the phone is within the operating area of a cell site, the site registers the presence of the phone within its operating area, notifies the MTSO to which all the cells in an area are connected, and tracks its position-based on signal strength-as it moves around the cell. When the user wants to place a call, he or she simply pushes the right buttons, which create simulated touch-tone sounds. Once dialing is com- plete, the user pushes the Send button, which causes the handset to transmit the buffered digits across the control channel to the local cell. The local cell hands the call off to the MTSO. The MTSO analyzes the digits, instructs the handset and the cell to use a particular set of fre- quencies for the call, and routes the call to the appropriate destination. MTSOs are interconnected to the wireline network and, therefore, can terminate calls at any location, including to another cellular user. If driving while talking, the user may approach a cell boundary. The MTSO, which tracks the relative signal strength of each user as they move among the various cells within its domain, will effect handoff of a call from one cell to another if the user's movement (based on signal strength) indicates that they are approaching a cell boundary. Access Methods The original analog cellular telephony systems relied on a technique called frequency-division multiple access (FDMA) as the access and fre- quency sharing scheme between mobile users and the cellular network. In FDMA systems, the available spectrum is divided into channels that are assigned to users on demand. One or more of the channels are reserved and set aside as control channels, used to transmit mainte- nance and operations information between the mobile phone and the network. Each conversation requires two 30 kHz channels-one for the for- ward, or base-to-mobile direction, and one for the reverse, or mobile-to- base station direction. This