The analog voice channel is a radio channel that uses FM modulation to transfer voice (audio) signals. The analog voice channel also sends digital messages during brief periods by muting the voice signal.

The CDMA system uses advanced mobile phone service (AMPS) radio channels to allow compatibility to 1st generation analog cellular system. An AMP is an analog cellular communications system that uses Frequency-Division Multiple Access (FDMA) for control and Frequency Division Duplex (FDD) for two-way (full duplex) communication. The AMPS radio channel types include 30 kHz FSK control channels and 30 kHz voice channels. It operates in the 825 MHz to 890 MHz range.

In addition to the user voice information, some control information is transferred on the analog voice channel. This control information includes a channel supervisory audio tone (SAT) tone and control messages (blank and burst).

The SAT tone is sent from the base station to the mobile device and the mobile device retransmits the SAT tone back to the base station. The reception of a SAT tone at the base station ensures a radio path is complete from the base station, to the mobile and back to the base station. This is similar to the off-hook loop current in a wired telephone.

When control messages are sent on the analog voice channel, they briefly replace (mute) the audio signal (for approximately 1/10th of a second). These control messages include channel handoff, shore messages, and power level changes.

Figure 1.5 shows that an AMPS...