cally mobile telephones. They can initiate calls from anywhere, they can receive calls anywhere, and they move around during a call. When they initiate calls, subscribers expect prompt service with their full feature capabilities. And subscribers expect to receive calls wherever the wireless telephone systems are compatible with their home service. 12.5.1 Tromboning Most mobile users probably have a story similar to this one: Meeting a New York friend in Krakow, one of us (Rosenberg) called a mutual acquaintance back in Long Island. We dialed his local New York telephone number, and the wireless system in Krakow routed the call to New York for handling. It turned out that the mutual acquaintance was in Italy that day, so we need not have worried about calling too early and waking him up. The New York system routed the call to Italy, and we enjoyed our conversation. The call, however, was a bit excessive in its use of facilities because it had two superfluous transoceanic links. This kind of routing, out to some far away place and almost back again, is called tromboning because the route looks like a trombone. Tromboning is ex- pensive, and newer switching and signaling technologies are being developed to mini- mize it. A landline number with call forwarding would have operated in the same way. This is the case where the shift from location-based telephony to personal telephony has much the same effect as the addition of mobility to the telephone system provided by wireless networks. The difference is that in the case of wireless telephony, subscribers are using their own telephone numbers at distant locations. Mobile telephone engi- neers face the problem of tromboning frequently because mobile telephones encourage their users to be mobile in their telephone usage. 12.5.2 User terminal registration The establishment of location and identity of user terminals on wireless networks is a continuing process that consumes radio and transport resources and which has no ana- logue in the wireline network. The system has 