d on, and the subscriber hears ringing, a busy signal, or whatever the telephone network sends. In the case of an incoming call, the base station sends an alert message via blank and burst, and the cellular telephone rings. 6.6 AMPS Call Maintenance Once the call is set up with a voice link, the mobile telephone monitors SAT. So long as the mobile receives SAT of the correct frequency, it sends the same SAT back to the base station. Otherwise, the mobile telephone mutes the audio signal and waits. If too much time goes by without SAT, the mobile telephone drops the call. The base station monitors the returned SAT. As in the mobile telephone case, mo- mentary loss of the correct SAT cause audio muting, and sustained absence causes a lost call. During the call, the base station directs power levels and handoffs as needed. 6.6.1 AMPS power control On call setup and every 5 seconds, the base station measures the signal strength of each AMPS call. Based on the mobile telephone transmit power, the system can infer the radio signal path gain of the call. AMPS mobile telephones have eight power levels called voice mobile attenuation codes (VMACs). These range from full power (VMAC set to 0) to minimum power (VMAC set to 7), as shown in Table 6.1. Class 1 mobile telephones go up all the way to VMAC = 0, class 2 mobile telephones go up to VMAC = 1, and class 3 mobile tele- phones only go to VMAC = 2. Once the base station determines the radio path, then it can set its own transmit power and tell the mobile telephone what power level to use. This power level is sent using a blank and burst message. The technique of deducing the received power at the mobile telephone from the mo- bile telephone transmit power and the received power at the base station may sound trivial, but it was not obvious at the time. In fact, it was considered subtle enough for TABLE 6.1 Voice Mobile Attenuation Codes VMAC = 0 3.0 W 36 dBm VMAC = 1 1.6 W 32 dBm VMAC = 2 630 mW 28 dBm VMAC = 3 250 mW 24 dBm VMAC = 4 100 mW 20 dBm VMAC = 5 40 mW 16 dBm VM