AMPS channels are controlled by the MSC. The traffic channel assignment process was described in the preceding subsections. However, there are other aspects of RRM, including power control and handoff.
Power control is handled by monitoring the received signal strength of the reverse channel at the base station, which in turn passes this and other channel quality information to the MSC. The MSC evaluates this data, including a trend analysis, to determine whether the mobile should increase or reduce its power level or be handed-off to another cell. AMPS defines eight power levels in 4 dB steps. This power level control is a means of controlling the local access point to the network for a mobile station.
Cell handoff is handled in a BCHO manner, as discussed in Section 2.4. The system controls handoffs by transmitting the SAT in-band on the forward channel. This tone is filtered by the mobile-it is outside the range of the audible channel-before reaching the subscriber's ear, and is reflected back to the system in-band on the reverse channel.
The base station filters the reflected SAT and evaluates the quality of the reflected tone. The base station forwards SAT quality information on to the MSC. Based on RSS and SAT data, the MSC determines whether or not to initiate cell handoff procedures.
Cell handoff procedures include having neighboring cells' base stations monitor the mobile's reverse channel and evaluating the received signal strength. If another base station "hears" the mobile better than the current base station, the mobile is instructed to move to a new channel pair via a "blank and burst" message transmitted in-band by the base station. The mobile then tunes its RF transceivers to the channel pair instructed. All of these steps are orchestrated by the MSC.
The "blank and burst" message sounds like static to the ear of the subscriber and is momentarily disruptive to the conversation taking place. It is also highly destructive of any data transfer which could be occurring at that point in time via modems on the cellular channel. This is one of the reasons that cellular has historically been a harsh environment for mobile data users.
Intelligent algorithms are used to prevent unnecessary and premature handoffs, especially for non-moving mobiles, mobiles located in poor in-cell coverage areas, mobiles traveling along the border between cells and situations in which no cellular channels are available beyond the cell's boundary