Timers

To avoid collisions in the BTS when more than one mobile attempts to access the system, for example when they are paged in one message; the Aloha procedure is used to spread out the accesses. This procedure makes use of timer T3120 in the MS.

When the MS receives a Paging message or when it decides to access the system for other reasons, the MS generates a random number between 0 and 8 and an integral value defined by the parameter called TX minus 1. Then it starts timer T3120, which expires after this random number of TDMA-frames. When the timer expires, the MS sends the random access burst with the Channel Request.



The MS restarts T3120 and waits for an order (Immediate Assignment) from the network to go to an SDCCH where the signaling can continue. According to GSM spec. phase 1, the MS waits 250 ms in the case of a non-combined SDCCH and 350 ms in the case of a combined CCCH/SDCCH, plus a new random number of TDMA- frames. The new random number is generated in the MS, this time ranging between 0 and TX. The parameter TX, which is broadcast on the BCCH, has a
maximum value of 50.
In GSM specification phase 2, the time that the MS must wait after the restart depends on the non-combined or combined SDCCH and the value of TX.


If the MS does not receive an Immediate Assignment from the network before T3120 expires, it sends a new random access burst with a channel request. The number of times the MS can attempt to access the system is limited. The MS knows what the limit is from the system information read on the BCCH.

T3120 is stopped when the Immediate Assignment message is received.

In case of an overload situation the BSC can send an "Immediate Assignment Reject" message to the MS. In that case the MS will not re-send the "Channel Request" message and will inform the user that the network is not accessible at the moment.

On the network side there are two timers:
• T3113 which starts when Paging Request is sent and stops when Paging Response
is received.
• T3101, which supervises the allocation of a channel and hence starts when the
Immediate Assignment message is sent and stops when the SABM frame is received.

If T3113 expires before the Paging Response is received, the Paging Request message can be sent again according to an exchange parameter set by the operator. If T3101 expires however, the request is canceled and the channel released.