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The SDCCH assignment fail from the LU.

Viewing 15 posts - 1 through 15 (of 37 total)
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  • #51154

    We want to reduce the SDCCH Assignment fail in our network. After we took a look on the cause of assignment fail,
    we found that the most was come from the Location update.
    We found in the protocol message that the failure always occurred after the AUTREQ and CICMD.

    Could you please tell me that what is the cause of SDCCH failure from the Location update?
    Thank you very much in advance. ๐Ÿ™‚


    Hello Sky,

    I’m surprised you can say the problem occurs only during Location Update : during a Call Setup, the SDCCH phase is almost the same (Ciphering & authentication)

    The problem you are describing is not a SDCCH assignment failure, but it is a SDCCH drop. So it is probably linked to a radio problem, such as interference or bad coverage, which occurs on the TRX that is carrying the SDCCH timeslot.

    If the problem is only on this TRX and not on the others, then it is probably a faulty TRX.


    Hi Sky, what is your default value for timer T200 ? It sounds like a Layer 2 problem. Greetings Z.


    Hello Zbigniew,

    Thanks for your recommend.

    We are using the T200=300ms corresponds to Abis terrestrial link. (in Alcatel have only 2 choices, Terrestrial=300ms and Satellite=1000ms.)

    More information and more question about this problem.

    We found the time after the CLUPD(send from BSC to MSC) before the CLCMD(send from the MSC to the BSC has the cause value=call control) is equal about 11s.

    From this it look like the time out behavier.

    Could anyone please tell me that which timer should it be?


    i don’t really want to disturb anymore… ๐Ÿ™‚


    Why not, Pix??? Please!!! ๐Ÿ™‚


    Hi Sky, I recommend to set the timer T200 to 1.000 ms. We noticed the same problem in Nokia and Siemens BSS. Greetings Z.


    Motorola BSS?
    it might be “Phantom RACH”


    We found that the 11-12s time out is the T3109 which is related to the Radio link timeout.

    So it probably linked to the radio problem like Pix said.

    We are investigating and finding out the solution.

    Any recommend are welcome!!! ๐Ÿ™‚



    Can You tell me is this site border site? if yes please check the bsc borders and try to decrease coverage for this site to have less LU In your networks.


    Try increase the CRH( Cell reselection Hysteris) on the border cells . This will decreasse the ping pong around the LA Border and so the sdcch faiures will decrease

    Zaki Ryne

    Dear all,

    I am working as a network performance team Engineer. During drive test we have 2 cell phones adjusted which call one another(master & slave phone) continuously. But when they crosses the location border, the call drops.

    Can anybody give me a suggestion what actually happens in the border? Why do they always drop?


    Have you looked at the messages logged in the drive test (layer 3)
    – what happens just before the drop ?
    – what about qoS statistics in serving and target cells ? do you notice something in terms of handover failures or call drops ?

    i would suspect that the two location areas are located in different MSC’s, right ?

    you could check neighbour definition and LAC definition in the different MSCs (can’t tell you more than this, you must check with NSS engineers)

    you can also ensure that both cells are using the same ciphering algorithms, as it can lead to a drop when switching ciphering in dedicated mode (as I experienced, but it might not be true with every vendor !)

    if the cells are from same MSC, but different BSC, then there is probably something you need to investigate on the A interface.

    Altogether, you can investigate further with detailed QoS indicators and/or A interface traces, and simply have a look to your layer 3 messages in the drive test.


    Zaki Ryne

    Dear Pix,

    Thanks for the suggestion. I will try them out.

    Best regards



    Dear all,
    I work with BSS Alcatel. We have a serious problem with Immediate Assignment: for 20% of cells it`s ~ 85-90%.The problem connects to counter MC149 – expired T3101 (3 sec), i.e. within 3 sec MS doesn`t acknowledge occupation of SDCCH. We have changed BCCH/BSIC to chek the version with Phantom RACH and BCCH-interference, have changed all equipment in the cell (trxs & ANc) but – without any results ๐Ÿ™
    Do anybody have similar problem?

Viewing 15 posts - 1 through 15 (of 37 total)
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