ellular telephone ring but then does receive no- tification of voice mail from someone who called just minutes before. The network is de- signed to prevent this kind of frustration whenever possible, but momentary failure of the link across the air interface or unrecognized movement of the mobile terminal from one serving area to another can create these failures to connect a call coming into a mo- bile telephone. In the Internet/ethernet world of today, the notion of a broadcast page to a specific user does not seem strange. In the world of telephony, however, the concept of calling a telephone that might be anywhere or might be nowhere was a new and strange concept. The switch also has to respond to calls initiated by subscribers. When the user ter- minal starts a call, it sends a radio signal to the base station. This signal is similar to the signal sent in response to a page. 9 11.4.3 Roaming Since subscribers can roam from system to system, the MSC finds itself part of a net- work that keeps track of all its telephones all over the world. The home and visiting systems have to communicate on several levels. 11.4.3.1 Defining home and roam. Even the terms home and roaming have differ- ent meanings at different levels of the cellular system. In the ANSI-41 standard, a user terminal is on its home system when it is being serviced by any base station served by its home MSC, and it is roaming when it is registered at any other MSC. There is an intermediate level of the network that is a system (containing one or more MSCs and their base stations) all identified by one system identification (SID). (See Chap. 15 for details.) For some purposes, roaming may refer to being on any sys- tem with a SID other than the SID of the home system. Call management is particu- larly difficult when the equipment on the system on which the user terminal is regis- tered is different from the equipment on the home system. For example, hypothetically, a subscriber might subscribe to voice-mail service but be unable to reach his or her