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High SDCCH Fail on VSAT cells

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  • #52643 Reply
    Rosa
    Guest

    Hi. On our network, the rate of SDCCH fails on every VSAT cell is 50%. We have smaller values on the terrestrial cells though. I would like to know what is the cause, and any hints on howto optimise this indicator on these cells. We are using ALcatel Equipment.

    #52644 Reply
    pix
    Guest

    rosa,

    sdcch assignment failure rate due to radio problems = 50% is NORMAL if your ABIS (or ATER) are over VSAT.

    There is nothing you can do : the MS sends a CHANNEL REQUEST, but by the time the BSS replies with an immediate assignment message, the MS has already sent a second CHANNEL REQUEST. This is an internal MS timer, which is too short. You can’t change it.

    Therefore, consider this value “50%” as your reference for VSAT cells.

    #52645 Reply
    peyo
    Guest

    You can change time between repetition for chan_req according to ccch_conf, tx_integer. see 3GPP 4.08

    but there’s a limitation which should be (if i’m not wrong) with cch_conf (non_combined) and tx_integer =32.

    That way time between rach repetition is:

    1 RACCH slot = 4,62 ms
    Tx_Integer = T = 32 RACCH slots = 0,147 sec
    S = 217 RACCH slots = 1,002 sec for non_combiend bcch+ tx_integer=32
    Consequently time between repeated Chan_req if no IA is received is between:
    1,002 sec (= S * 4,62) up to 1,144 sec (S + T – 1 = 248 * 4,62)

    but it can still be too short.

    #52646 Reply
    peyo
    Guest

    my bad,

    I’m talking about radio interface and note Abis. Forget that

    #52647 Reply
    pix
    Guest

    peyo,
    thanks for the calculations ๐Ÿ™‚ i put these settings on a VSAT cell, but it didn’t change anything. It looks like 1.5s is still too short for the reply. But wait a second… the value for S cannot be as big as 217, can it ??

    And anyway, you’re right. The problem lies on the RACH repetition on the air interface. The channel request will then be transmitted on the abis interface. So your proposition is 100% valid !

    But conclusion is still the same : for VSAT cells in Alcatel systems, a SAFR radio = 50% is N.O.R.M.A.L.

    It’s not my conclusion, tests have been done by TAC. it’s worldwidely accepted.

    #52648 Reply
    Peyo
    Guest

    Pix: yes, S=217 is the maximum value you can have (for tx_integer =32 and non_combined BCCH)

    the only thing that may make a difference here is that this setting also increase T3126 minimu value (which is the timer started by MS after last chan_req is sent . min T3126 = 2S+T =249 *4.62 = 1.15s which is only 6 ms longer than maximum time between retransmission and it can be up to 5s so there’s is probably no gain.

    I agree with you that there’s nothing to do with this.

    #52649 Reply
    pix
    Guest

    peyo,

    ok, long time i didn’t check out these parameters… T3126 cannot be tuned, it is automatically set to a value computed by the MS, according to the value of S and T, correct?

    value of T3126=2S+T = 2.1s (you did an error in your computation ? or i misunderstood ?)

    well, anyway, T3126 doesn’t help here, because it doesn’t control the repetition of Ch. Req. but just how long it should wait after the last Ch. Req.

    thanks a lot for your in-depth analysis, it’s a good “memory refresh” for me ๐Ÿ™‚

    #52650 Reply
    Peyo
    Guest

    Pix: I’ve made a mistake ๐Ÿ™‚

    yes it’s minimum for T3126= 2.15 secondes

    I don’t deeply know T3126 but from 4.08:

    The minimum value of this timer is equal to the time taken by T+2S slots of the mobile station’s RACH. S and T are defined in section 3.3.1.2. The maximum value of this timer is 5 seconds.

    to be honest i don’t understand this maximum value of 5s , it cannot be reached by tuning S and T. If you have any explanation to provide it would be great.

    #52651 Reply
    pix
    Guest

    was gonna ask you the same question ๐Ÿ™‚ IMO it is depending on the SW implementation in the MS. some vendors might put it to 5s, fixed.

    let’s forget about it, it’s really not important at all !! it doesn’t impact the BSS in any way.

    #52652 Reply
    Ayan
    Guest

    Hi Pix,

    In that case, what will be the possible reasons if a MS sometimes needs to make a few dials before he get through to a receiving MS if A.bis is over satellite? As first call will always get destination MS is unreacheable or out of coverage area? Which timer cause this problem?

    #52653 Reply
    pix
    Guest

    ayan,
    i would say the number of channel requests retransmissions you authorize for the MS is equal to 0.

    it should be equal to 2.

    this is a BSS parameter.

    #52654 Reply
    Ayan
    Guest

    Thanks Pix,

    But it should be 2 otherwise sdcch assignment failure rate will not be 50% right? And this 50% failure does not mean MS need to make two tries? It is not related.

    #52655 Reply
    pix
    Guest

    ayan, the subscriber does one dial, and the MS will send 3 channel requests in a row (with some time between them in order to wait for an answer).

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