Well, you can imagine a supermarket for instance. The customer wants to pay at a cashier. If all cashiers are busy, then if leaves the supermarket, without paying. He’s not happy, and supermarket is not happy either 🙂
If the supermarket manager allows queuing, the customer will simply queue in a common line, and when he’s the first of the queue, as soon as a cashier is free, he’ll go there.
It’s exactly the same in GSM : during call setup, the mobile performs all the SDCCH signalling (authentication, ciphering, etc.) and then waits for a free TCH in a queue. I hope you understand how it can help reduce congestion now…
The queue is defined by :
1- a max queuing time (T11) : the MS cannot stay more than T11 seconds within the queue. Within this period he is allowed to wait for a TCH, after that, he’s rejected due to congestion.
2- a total amount of MS in the queue : not more than BTS_QUEUE_LENGTH mobiles can wait for a free TCH at the same time. If a MS number “bts_queue_length + 1” is asking for a TCH, it is rejected due to congestion
3- QUEUE_ANYWAY : if the MSC does not allow TCH queuing in the BTS, it can be “forced” by the BSC. But instead of using T11, it will use T11_FORCED.