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GPRS – Channel Request and RSL link

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  • #53074 Reply
    Olivier
    Guest

    Hi,

    I’m currently looking at how the PCU processes a CHANNEL REQUEST PDU from a gprs device for allocating an uplink TBF.

    As the PCU normally resides with the BSC, is it correct to think that the CHANNEL REQUEST should be received over the RSL link? If so, will it come in as an 08.58 DATA INDICATION? The 08.58 CHANNEL REQUIRED doesn’t give any indication that the 04.08 CHANNEL REQUEST is for a TBF, as opposed to an SDCCH…

    Another question is, does the BSC need to activate a PDCH (08.58 CHANNEL ACTIVATION) before that PDCH can be used for a TBF?
    If so, when should this PDCH be de-activated?

    Thanks for your help!

    O.

    #53075 Reply
    Pix
    Guest

    04.18 CHANNEL REQUEST for GPRS / EGPRS request

    and 04.60 PACKET RESOURCE REQUEST message from an EGPRS capable MS (for establishment of a concurrent TBF)

    The Channel Request is sent over the RSL, that’s correct.

    Regarding the PDCH, it is highly depending on the vendor you’re using. The PCU can be located in the BSC, in the SGSN, or even be a standalone equipment (= MFS, in alcatel).
    The PDCH allocation / deallocation can be performed by the PCU, but it is not “standard”. So there are many solutions to activate or deactivate a PDCH. It is safe to assume the PCU is always in charge of (de)allocating the PDCH’s, but it might need the confirmation of the BSC.

    You are correct when you say the PDCH should be allocated before being used by the TBF.
    The deallocation can be done with a timer, after the release of the TBF.

    Regards,
    Pix

    #53076 Reply
    Olivier
    Guest

    Thanks Pix.

    What’s not clear to me is, will the channel request show up on the RSL link encapsulated within an 08.58 DATA INDICATION PDU?

    My understanding for GSM is that the BTS always intercepts the 04.08 CHANNEL REQUEST and converts it into an 08.58 CHANNEL REQUIRED on the RSL link.

    Will the BTS do something different if the CHANNEL REQUEST indicates a packet data related cause?

    The other thing as well is that it seems that if the BTS is not configured with PCCCH (but only CCCH), then the UPLINK PACKET ASSIGNMENT should be sent back to the MS within an 04.08 IMMEDIATE ASSIGNMENT (rest octets). Who is then responsible for generating this IMMEDIATE ASSIGNMENT, the BSC or the PCU?…

    If the PCU is not physically located on the BSC, then how can the PCU have access to the RSL link?

    Sorry for all the confusion, there is a lot of new concepts for me here. I would have mostly experience with GSM, not GPRS.

    Thanks again for your assistance,

    O.

    O.

    #53077 Reply
    pix
    Guest

    hello olivier,

    “Will the BTS do something different if the CHANNEL REQUEST indicates a packet data related cause?”

    nothing different : the (e)gprs channel request is sent as a channel required in the RSL. The content of the channel required will indicate to the BSC that the channel required is for (e)GPRS.
    I can’t find anything about “data indication (pdu)” in this case.

    “Who is then responsible for generating this IMMEDIATE ASSIGNMENT, the BSC or the PCU?…”

    The PCU generates a Channel Assignment, which is sent to the BSC. The BSC conveys it as a “Immediate Assignment Command” to the BTS. The BTS sends a “Immediate Assignment” on the PCH or on the AGCH, depending on the DRX mode of the mobile.

    “how can the PCU have access to the RSL link?”

    If the PCU is external to the BSC, then it is connected to the BSC via a dedicated interface (called ATER PS). The BSC will route all GPRS-related message from BTS (RSL or not) to the PCU, quite transparently.

    As i said before, keep in mind the PCU implementation is totally vendor dependent. The solution I’m describing here might not apply for Ericsson, Nokia, Huawei, etc.

    Thanks for your questions, they are interesting ! Not very useful on the field, unless you decide to build your own PCU ๐Ÿ˜€ but interesting to understand how the system works.

    Cheers

    #53078 Reply
    Olivier
    Guest

    >> Not very useful on the field, unless you decide to build your own PCU ๐Ÿ˜€
    ๐Ÿ™‚

    >> The content of the channel required will indicate to the BSC that the channel required is for (e)GPRS.

    I’m looking at the definition of the CHANNEL REQUIRED. Am I correct to think that the information regarding the type of channel request is in the “RA” field of the Request Reference information element of the 08.58 CHANNEL REQUIRED?

    #53079 Reply
    pix
    Guest

    Hi olivier,

    the channel required is an abis message –> a vendor can modify quite extensively the 3GPP in order to fit its needs.

    For example, in case of an 11-bits access burst (= egprs channel request, with a radom access ref of 11 bits), the RA field is not big enough to carry the whole info (only 8 bits !)

    So an optional field at the end of the Channel Required might be used.

    It’s vendor dependent.

    Cheers,
    Pix

    #53080 Reply
    RF
    Guest

    Dear PIX
    can you please explain more about egprs channel request, with a radom access ref of 11 bits and would be it effect on UL throughput.

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