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12.4 AMPS MESSAGE FORMATS AND PARAMETERS 343Figure 12.3-4. Power change and handoff. RVC, Reverse Voice Channel; FVC, Forward VoiceChannel.Power Change. A MSC monitors the mobile signal strengths, received by thebase stations on their reverse voice channels. When it decides that a mobileshould change its transmit power, it sends a “change power” Order (a) that includesa new VMAC value. The mobile then adjusts its transmit power and returns a“change power” confirmation message (b).Handoff. In a handoff (see Section 12.1.7), the MSC sends a Handoff message (c).The message includes the new channel number CHAN and attenuation code VMAC.The mobile acknowledges with a 50-ms on-hook signal (d) and turns off its transmitter.It then tunes to the new channel, sets the new power level, and turns thetransmitter on again.12.4 AMPS MESSAGE FORMATS AND PARAMETERSAMPS distinguishes two groups of messages, overhead parameter messages andmobile control messages. Overhead parameter messages are broadcast to allmobiles on the Forward Control Channels (FOCCs). Mobile control messagescontain call-control messages for individual mobiles and—depending on themessage—are sent on forward or backward control, or voice channels (FOCC,RECC, FVC, and RVC).Before exploring the individual messages, two general aspects of AMPS messagewords need to be mentioned—see Fig. 12.4-1. In the first place, each word has a12-bit P (parity) field, which is used for error control (Section 12.2.2). In thesecond place, the developers of AMPS, anticipating future additions to signalingin mobile systems, included a number of reserved (RSVD) fields in messagewords. In AMPS, a mobile or MSC sets the RSVD fields in its outgoing messages