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Contention resolution in case of CS call

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    Could anybody please tell me why in case of GSM CS call, there is no concept of “contention resolution” which happens during a GPRS uplink TBF establishment ?



    there is such a concept ! if 2 MS send a channel request at the same time, the BTS will reply at some point, and in this message there will be an in fo which clearly identify one unique MS (based on rach id, or something like this…)
    Sorry i totally forgot the details… but i know for sure it exists !



    Hi Pix,
    Thanks for replying.

    That same procedure applies to GPRS access over CCCH also. The BTS sends back the random reference received in RACH as well as the frame no. in which it was received. From these pieces of information, the MS can uniquely recognize its response.
    But, in worst case, these pieces of information (random ref and frame no.) could also be used by two MS simultaneously. Then it becomes an issue to recognize whom the response belongs to.

    So, here GPRS has a solution to this through its contention resolution mechanism. I was wondering why there is no such thing in CS.

    One thing, I can think of the fact that every CS call may undergo an authentication phase (MM) before proceeding to the real conversation phase. In case of GPRS, sessions are bursty and MM authentication is not required with every little TBF establishment. Hence is this procedure called contention resolution in GPRS.

    Please comment.



    Hi Girish,

    after the access on CCCH and moved to the dedicated channel, the first message sent by the mobile is a SABM with:
    – Location Update Request
    – CM Service Request

    In both cases the mobile identity is present there.
    This message is sent back by UA and received by the mobile.

    If a second mobile was involved in that access, at that point it can stop and move away.

    You can find details here:
    3GPP 44.018, par. 3.1.5


    Hi fede,
    Thanks. I think that answers my query.

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