When the BSC doesn’t have any TCH available and queuing is not activated, there is NO TIMER. The rejection is immediate.
But the BSC might or might not send the rejection message to the MS. (this is a parameter called “immediate assignment reject”)
if reject is not sent, the MS will just wait for a very short while, and send again another channel request. The delay between 2 consecutive channel requests is very short.
if reject is sent, then the MS will wait for a longer while (timer is specified in the reject message)., then resend another channel request.
Assignment failure due to congestion are usually immediate, without a timer.
Timers are used to detect a radio failure.
TCH assignment can also be failed due to poor radio link, and in this case, it is the expiry of a timer that triggers this event.