Seizing a Reverse Access Channel

When an idle mobile MS needs to access the system (send a message), it has to seize a reverse access channel. This is done in the following series of steps.

Step f. The mobile scans the access channels, locks on to the strongest one, reads an overhead message, and examines parameter RCF. If RCF = 0, the mobile sets its transmitter to maximum power, and starts step 3. If RCF = 1, it goes to step 2.

Step 2. The MS reads a control filler word CFW (Fig. 12.4-1), sets its transmit power to the value in CMAC, and, if WFOM = 1, also reads another overhead message, updates its parameters, and then goes to step 3.

Step 3. Any idle MS in a cell can seize an access channel. In order to minimize "collisions" (simultaneous seizure of an access channel by more than one mobile), the mobile first examines the BI (busy-idle) bits on the forward access channel, which indicate whether the associated reverse channel is idle (12.2.2).

If the channel is busy, the mobile waits for a random time (0-200 ms), and repeats this step. Up to NBUSY-PGR or NBUSY-OTHER busy occurrences are allowed, for respectively page response and other messages. If the channel is idle, the MS goes to step 4.

Step 4. The MS seizes the channel, and starts its transmission. In systems with BIS = 0, the mobile transmits the entire message. If BIS = 1, the system has a second defense against "collisions." The mobile then has to keep monitoring the BI bits on FOCC. BI turning to busy before 56 message bits have been sent indicates a collision, and BI not changing to busy after 104 bits have been sent indicates that the message is not being received. In either case, the mobile stops transmitting. The number of allowed RECC seizures is MAXSZTR-PGR or MAXSZTR-OTHER. As long as this limit is not exceeded, the mobile waits for a random time, and returns to step 3.

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