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Random Access Failure

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  • #51748 Reply
    trucdu
    Guest

    Hi all

    I want to know, what are the reasons
    of Random Access Failure. I have cell near the sea and Cssr values are very bad and RACH Failure values are very high. What can I do? Thanks for helping.

    #51749 Reply
    pix
    Guest

    CSSR values should not be impacted by RACH failures (depending on which formula you use for CSSR)

    RACH failure might be due to the use of ABIS over SATELLITE, is it your case ?

    otherwise, you should check BCCH/BSIC couples, and also TCH/BSIC couples

    finally, it might be due to electromagnetic interference : are there military radars around, or military ships ? what about alarm systems ?
    the only way to know : perform an ABIS trace on this BTS.

    Regards
    pix

    #51750 Reply
    Ibrahim
    Guest

    Hi Pix,

    Basically, there are two CSSR’s formula:
    1. Radio CSSR that takes only TCH & SDCCH resources under consideration.
    2. CSSR_Immediate that is the multiplication of immediate assignment by Radio CSSR. So, RACH failuer would not have an impact on the 1st one, but it does for the 2nd one.

    Regards,

    Ibrahim

    #51751 Reply
    MKT
    Guest

    RACH failure might be due to the use of ABIS over SATELLITE

    How does it goes?

    Can you explain it pix.

    Regards

    #51752 Reply
    pix
    Guest

    for alcatel vendor, the access failure are due to abis satellite (and not “might be due”)

    for other vendors, i don’t know.

    it is due to delay : the 1st channel request is answered by an immediate assigmnment from the BTS, but in the meanwhile the MS has already requested a 2nd channel request.
    The 1st Channel Req is answered (no ffailure), but the second is NOT answered –> failure

    for other vendor i don’t know how it works, possibly they will not “count” this kind of failures, even if they occur.

    regards,
    pix

    #51753 Reply
    peyo
    Guest

    is your cssr formula like:

    rach_success_rate * tch_assignment_success_rate* (1-sdcch_drop)*100

    do you have access to Maximum value for Timing advance ? if yes what is the value?

    #51754 Reply
    pix
    Guest

    maximum value of timing advance is 64 (35km).

    #51755 Reply
    MKT
    Guest

    MS sends an access request to BTS over RACH. BTS receives that request. And further action starts….

    BTS and BSC will now talk and then..

    IMM_ASS will be send by the BTS to MS over AGCH.

    You mean to say that time between sending the RACH and getting IMM_ASS is more then the repeat request time for RACH. And hence the MS sends the second request.

    I have one doubt in this….

    1. Can’t we change/modify the time between the successive RACH requests. If yes, then case is over as per the theory you gave for RACH failure.

    2. If MS is sending a RACH request and BTS is easily receiving that request then,

    how can you say that it is a case of request failure or RACH failure. The failure actually is in the assignment. So don’t you think that we should term this as AGCH failure?

    #51756 Reply
    pix
    Guest

    mkt,

    1/ the MS timer is internal, it cannot be set at BSS level.

    2/ a channel request shall be answered with an Imm Assignment. The 2nd ch req. is actually answered with an imm assignment (sorry, i didn’t make that clear in my previous reply)

    But this I.A. is not received by the MS (MS is already on a SDCCH).

    It is therefore counted as a SDCCH assignment failure due to radio problems, in alcatel systems. In other systems, i don’t know if it is counted, and if it is, i don’t know how it is counted.

    #51757 Reply
    MKT
    Guest

    Thanks..

    So, we can say that

    due to delay on A-bis, we can have
    SDCCH ASSIGNMENT FAILUE.

    But…

    I don’t think that we have the RACH FAILURE counter incremented by this.

    Regards..

    #51758 Reply
    pix
    Guest

    hi mkt,

    i’m talking about alcat. systems : we don’t have RACH failure indicators. Only sdcch assign failure.

    #51759 Reply
    Lvin
    Guest

    dear Pix
    hi
    i have some cell have low random access success rate between(20 to 60)% it must over (98 %)our system ericsson and two bands (900 and 1800 )MHz.
    i dont know what is the problem!
    i chnaged the frequencies and BSIC.

    please help me to solve them
    thank you very much…

    #51760 Reply
    SHELDON
    Guest

    Hi Lvin,

    Check your MAXTA and TALIM settings. If they’re too low, you can have RACH failures. Also check for faulty TRXs.

    Regards,
    Matthew

    #51761 Reply
    Lvin
    Guest

    Dear Sheldon
    hi

    according to what i change the MAXRET , TALIM and ACCMIN?
    are Urban and city area the same?how much the range of these parameters for urban and city center?

    because i checked BSIC and BCCH for the they are correct.

    thank you very much for reply

    FYI
    our system ericsson-900 and 1800 GSM bands.

    #51762 Reply
    Neeraj
    Guest

    i am getting #DIV/0 in SD DROP & Random Access Success Rate.
    Can you have any solution or suggestion

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