 the paging list and the amount of signaling traffic. Having a short timeout for registration keeps the paging list current but increases signaling channel traffic with more mobile telephone registrations. Having a longer timeout, on the other hand, reduces signaling channel congestion at the expense of paging more subscribers who are not there to re- ceive their calls. 12.5.3 Call setup The entire process of establishing a link to the subscriber's phone on a call-by-call ba- sis is a traffic issue specific to wireless. A landline telephone has a local loop, a link that is always there, managed by the local exchange office, perhaps with the help of a PBX. The local loop can always be there because the subscriber's phone is always in one place-it is not mobile. A call attempt is made after the user terminal is registered via the paging and access channels. As a result, the user terminal and the system have identified one another. When the user terminal makes a call attempt, the system already knows who the sub- scriber is and where the mobile telephone is. In response to the call attempt, again us- ing paging and access capacity, the terminal and system send several messages back and forth to establish the call itself and its radio environment. 12.5.4 Handoff As an active wireless call moves from one cell sector to another, the call gives up its ra- dio link with the old sector and is connected to the new sector. We call this process a handoff (or handover in GSM terminology). CDMA allows a soft handoff where the call is served simultaneously by two or more cell sectors. Handoff was a new concept in telephony, and it still has few analogues in the land- line network. 7 A landline call can be viewed as a point-to-point connection, but a wire- less call is something more than its connection. The call is maintained while the con- nection changes, following the mobile user terminal. Answering the telephone in the bedroom and running downstairs to the kitchen to talk there is not quite the same thing. Downloaded from D