CHANNEL REQUEST

The MS initiates a call by sending a CR message, with an REF. The REF include and establishment cause and a RAND. It is transmitted on the RACH channel. The RACH channel is associated with the CCCH channel which the MS is monitoring while in idle mode. 
The establishment cause have 7 field of the REF specifies:

1. An emergency call
2. Call re-establishment
3. Response to paging
4. MS-originating speech call
5. MS-originating data call
6. Location update
7. Service call, like SMS.....

The MS note the random number and frame number associated with each CR message. These are used by the MS to recognize the response sent form the BSS. This response is sent on the AGCH, which can be monitored by many MS. The MS decodes all message sent on this AGCH, and only accepts a message with a random number and frame number matching one of the last three requests sent. The MS continuous to transmit Channel Request message until it receives a response. If no response is received before the MS has transmitted a predefined number or retries, the MS have two follow:

1. Displays a network error message for all calls except location update.
2. Performs automatic re-selection for location update calls, This means that the MS attempts random access on a different cell.

On receipt of the CR message from the MS, the  BST sends a CR message to the BSC. This message contains the random number sent by the MS, and the TA measured by the BTS.

Under peak load conditions, resource maybe over allocated due to this process.

Below will show how the Immediate Assignments Extended feature work to alleviate.

In order to establish a radio connection on a VGCH between a MS which is in group receive mode on that channel and the BTS, the MS sends an uplink access message with the Subsequent talker uplink request parameter on the voice group call channel. The uplink access message is similar to a CR message but is sent only on the group call channel uplink.

The MS sends and uplink access message when:

1. A subsequent talker uplink is required: The BTS performs any necessary contention resolution and grants the uplink to one MS by sending a VGCS  uplink grant message to the MS in unacknowledged mode on the main signaling link. The BSS send uplink busy message on the main signaling link in all cells of the group call area.
2. There is a replay to an uplink access request.