ters with the local base station. If the user terminal is somewhere other than its home system, then this infor- mation is sent to the home system, a process called roaming discussed in Sec. 12.5.5. If the user terminal is turned off or out of communication range, the system detects this and, on PCS networks, routes the call to the customer's system-based voice mail. Once the system knows where the user terminal is located, at least to the nearest cell, it sends a paging message to tell the user terminal that it has a call. The user ter- minal then initiates communication with the system the same way it does when the mobile subscriber is making the call. There is another reason for the user terminal to detect and recognize the system. Forming a CDMA radio link takes time for the receiver to become synchronized with the transmitter. From a cold start with no prior timing information, the synchroniza- tion process can take half a minute, but from a warm start where the receiver already has some information about the CDMA timing sequence, the same synchronization process only takes about 1 second. A user pressing the SEND key expects to wait a sec- ond or two, but a 30-second wait to connect a phone call is out of the question. Instead, 2The early cellular telephone service was still a high-priced luxury item. Our AMPS trial cus- tomers in Chicago drove mostly Cadillacs, Lincolns, BMWs, and Porches. Superscript(3)The MSC was called a mobile telephone switching office (MTSO) in the early cellular days. 4We are sure that somewhere out there are base stations that also do telephone switching. For our purposes, we would consider those to be both base station and MSC colocated. Also, in GSM the BSC administers several base stations, and it can execute a handoff (called a handover in Eu- rope) between two of its own cells. 5In the Bell System days, this was often referred to as plain old telephone service (POTS), and the local switch was called a class 5 switch. Downloaded from Digital Engineering Library @ McGraw-Hill (w