Explain in detail access channels and operational techniques.
4. Explain in detail access channels and operational techniques.
Access channels:
In mobile-originating calls, the mobile unit scans its 21 set-up channels and chooses the strongest one. Because each set-up channel is associated with one cell, the strongest set-up channel indicates which cell is to serve the mobile-originating calls. Th. mobile unit detects the system information transmitted from the cell site. Also, the mobile unit monitors the Busy/Idle status bits over the desired forward setup channel. When the idle bits are received, the mobile unit can use the corresponding reverse set-up channel to initiate a call. Frequently only one system operates in a given city; for instance, block B system might be operating and the mobile unit could be set to “preferable A system.” When the mobile unit first scans the 21 set-up channels in block A, two conditions can occur.
1. If no set-up channels of block A are operational, the mobile unit automatically switches to block B.
2. If a strong set-up signal strength is received but no message can be detected, then the scanner chooses the second strongest set-up channel. If the message still cannot be detected, the mobile unit switches to block B and scans to block B set-up channels.
The operational functions are described as follows:
1. Power of a forward set-up channel [or forward control channel (FOCC)]: The power of the set-up channel can be varied in order to control the number of incoming calls served by the cell. The number of mobile-originating calls is limited by the number of voice channels in each cell site, when the traffic is heavy, most voice channels are occupied and the power of the set-up channel should be reduced in order to reduce the coverage of the cell for the incoming calls originating from the mobile unit. This will force the mobile units to originate calls from other cell sites, assuming that all cells are adequately overlapped.
2. The set-up channel received level: The setup channel threshold level is determined in order to control the reception at the reverse control channel (RECC). If the received power level is greater than the given set-up threshold level, the call request will be taken.
3. Change power at the mobile unit: When the mobile unit monitors the strongest signal strength from all Set-up channels and selects that channel to receive the messages, there are three types of message.
a. Mobile station control message. This message is used for paging and consists of one, two, or four words -DCC, MIN, SCC and VMAX.
b. System parameter overhead message. This message contains two words, including DCC, SID, CMAX, or CPA.
c. Control-filler message. This message may be sent with a system parameter overhead message, CMAC—a control mobile attenuation code (seven levels).
4. Direct call retry. When a cell site has no available voice channels, it can send a direct call retry message through the set-up channel. The mobile unit will initiate, the call from a neighbouring cell which is on the list of neighboring cells in the direct call-retry message.
