Turn Off Register Register Confirmed Registered Attempted Page Received Connect Voice Paged Attempted Figure 16.7 High-level wireless call setup state diagram. equipment vendor. Without going too far into the details, let us go over some of these cases: A mobile telephone may not detect the system paging data stream. Perhaps the radio en- vironment is too hostile, or perhaps the system's radio interface is not a technology the mobile telephone can support. A Global System for Mobility (GSM) telephone should not recognize a code division multiple access (CDMA) paging stream. The user terminal's registration can be rejected because the telephone is not working cor- rectly or because it is corrupted by radio noise. The system may determine that the telephone is not to be served. This could be the result of a customer not paying the bill or a cellular telephone reported stolen. However, the reg- istration may still be completed because wireless systems should still be able to give emer- gency service (911 in the United States) to telephones even when they are otherwise out of service. The system acknowledgment of the registration also can fail, and there is no two-way com- munication between the system and the mobile telephone. There is another curious kind of registration error created by the type of wireless tele- phone fraud called cloning. A telephone may be cloned, and the copy will have the same registration information as the original user terminal. Thus a registration attempt may appear with the same serial number and telephone number as another registration. The system operator may decide to serve one, both, or neither, but the system does have to with duplicate registrations. Registration has to be renewed periodically. Otherwise, a user terminal would remain in the system database long after the subscriber shut it off or moved too far outside the 6 Aside from fraudulent usage, there are also combinations of two cellular telephones with the same number so that an incoming call pages both and both telephones