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call drop due to expiry of T200 timer

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  • #54378 Reply
    silvia
    Guest

    i looked for the descriptio of this t200 all over the internet. i found a real bulky piece of equipment used for current generation. there is also something that looks like a cassette cleaner tape that beems every time a signal is sent. i am not satified with it. would you please describe a t200 for me? or give me a straight definition?
    thanks, i know you are overwhelmed.

    #54379 Reply
    Pix
    Guest

    Hi,

    T200 :
    Supervision of acknowledgements on the Abis, by the BSC and by the BTS.

    When BTS sends a message on Abis to the BSC, the BSC should reply with an acknowledgment.

    The BTS waits T200 ms (def T200 = 300ms).

    If ack is received before T200, then fine. Life goes on.

    If T200 expires (no ack received), then BTS sends the initial message one more time.

    It will then wait T200, for the ACK.

    Still no ack ? Then the process is repeated N200 times (N200 = 12, by default).

    So 12 attempts to send 1 message, still no ack received… then the Abis link is assumed to be dead, and all communications are lost in the BTS. TRX, Abis and BSC boards are reset (depends on vendor behaviour).

    #54380 Reply
    Pix
    Guest

    By the way, instead of asking your question, you could check the previous page (same thread), I already posted the answer..

    pix – 28 Oct 2008

    Cheers.

    #54381 Reply
    krish
    Guest

    Hi Pix,

    i want to know that this T200 will start after immediate assignement and at that time this entire process will continue on sdcch ?

    #54382 Reply
    Pix
    Guest

    T200 is on the abis, it doesn’t directly impact the Air interface nor the call setup procedure. All signalling msg on abis are acknowledged.

    #54383 Reply
    silvia
    Guest

    Is T200 equipment a hand held piece or how does it look like. Where can I find information on such an important item. I like to learn more about the equipment.

    help me out would you?

    #54384 Reply
    Pix
    Guest

    Sylvia,

    t200 is a timer.. it’s just a timer, it’s not a physical equipment.

    t200 is started in BTS and BSC, each time they send an Abis msg.

    #54385 Reply
    KRISH
    Guest

    Hey pix,

    I think T200 is on bts level its lapdm and on bsc level its lapd ok so if u modify on bsc level then it will automatic modify on bts level?

    and second one if u are increasing the timer value does it will affect the call set up time?

    and last i want to know is it start after immediate assignement then on which channel this process will continue sdcch or ?

    #54386 Reply
    Pan
    Guest

    Dear all! Pix, allow me to help you.
    In 3GPP 44.006 any interested person can to find information about T200 equipment (it isn’t advertisment:).
    T200 is data link (layer 2) timer. It is triggered on BSC side and on MS side (There is no Layer2 in BTS, only Layer1 – modulation and channel coding).
    T200 supervises the acknowledged transfer of layer 3 messages (e.g. location updating, setup and many others) by data link layer. T200 expires when there is no acknowledgment from peer layer2 (BSC or MS) during T200 ms.
    If T200 expired N200 times then layer2 indicates to layer3 about message transfer failure.
    Values of T200 for different channel types (SDCCH, SACCH, FACCH) are implementation dependent and must be chosen according recommendations in 44.006.
    Values of N200 (from 44.006):
    – 5 for use on SACCH;
    – 23 for use on SDCCH;
    – 34 for use on FACCH/full rate;
    – 48 for use on E FACCH/full rate;
    – 29 for use on FACCH/half rate.

    #54387 Reply
    Pix
    Guest

    Pan, help very much appreciated 🙂

    I’m still confused about layer 2… no T200 in BTS..?
    LapDm is on air interface, LapD on Abis interface… each of those protocols use their own T200 timers, aren’t they ? And BTS is indeed having a role.

    well, i’m actually quite sure of this, because the BTS monitors the Abis/LapD availability based on T200 timer. (quite sure = almost sure, i’m not stubborn and i’m always glad to be proven wrong)
    If BTS detects N200 x T200 expiries, then it will restart its communication board.

    #54388 Reply
    Pan
    Guest

    Pix, I am also in bit doubt about T200 accomodation. Certainly T200 supervises the acknowledged transfer between MS and Network on the radio path (not on A-bis). But where is it accomodated on network side? in BSC or BTS? I see in 08.58 and came to conclusion that most likely than not that it is supervised by BTS on network side. And after T200 has expired N200 times then BTS sends ERROR INDication to BSC but it isn’t resets communication board because problem may be on MS side or radiointerface

    #54389 Reply
    Pix
    Guest

    Hi Pan, I will copy some stuff from our documents (note: i’m not talking about LapDm, but LapD !). Basically, it clearly says that the BTS and BSC are both in charge of layer 2 supervision.

    ” —
    Lapd is a protocol that operates at the layer2, Datalink layer of the OSI architecture to transfer the information between Layer3 entities (in BSC, BTS and TSC) across the user network interface.

    On the BTS side each TRX handles the LAPD protocol for the RSL signalling link of this TRX and the OMU handles the LAPD protocol for the OML signalling link of the BTS.

    In BSC, the LAPD protocol (Layer 1 and Layer 2) is handled in a TCU.

    The reset procedure (Data Link reset in LAPD specification) is performed either by the TRX or the TCU or the OMU mainly when a layer 2 frame has been
    transmitted N200 times without acknowledgement.
    —”

    The lapdm is handled between MS and BTS, it doesn’t do over the abis :
    ” —
    The Link Access Procedure on the Dm channel, LAPDm, is the data link layer protocol, (layer 2 of the OSI model), used between the BTS and the MS, on the air interface.
    — ”

    Ok, so for once, I was closer to the truth 🙂 Perhaps the 3GPP doesn’t describe each protocol well enough and those are Alcatel Lucent implementation.

    #54390 Reply
    Pan
    Guest

    Yes, Pix.
    I agree with you that T200 timer should be realized on A-bis interface between BSC and BTS. As well as on each separate Layer2 link (i.e. MS-BTS and BTS-BSC).
    When I said about T200 I implied only MS-Network interface as in 04.06.
    In general T200 had come to mobile network from ISDN Layer2 specification ETS 300 125.

    #54391 Reply
    Pix
    Guest

    I thought I was a 3gpp geek, but you’re way more into it than me :))

    have a good weekend y’all,
    pix

    #54392 Reply
    Pan
    Guest

    Thank you, Pix! The same to you:)

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