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GPRS (Inter SGSN RA update) - 25

Inter SGSN RA update [Under GPRS >> Inside PS]
» GPRS description - 23.060 «
» Layer 3 - 24.007 «

In earlier article, we checked Intra SGSN RA update. In case of Inter SGSN RA update, a different procedure need to be followed. The obvious reason is LLC layer will now be on different SGSN. This mean, LLC has to be reset and established again (through which SNDCP XID negotiation can happen). The new SGSN may not support QoS given by earlier SGSN, in which case new SGSN need to perform PDP context modification procedure.

Let us have a overall look at how it goes inside MS PS.

SN SM GMM LLC GRR AIR

Phy layer does the cell reselection and informs GRR. As seen in GMM article, TBF if any in progress is abandoned (LLC takes care of continuity). GRR indicate it to GMM, say in GMMRR-Cell-Reselect-Ind.
GMM checks if RA is same as earlier (stored) RA. If yes, it suspends LLC procedures (sending of SN PDUs if any) and starts RA Update procedure. GMM may also inform about new cell/RA information to MN.
GMMRR-Cell-Reselect-Ind
[Cell ID, RA, GPRS support etc.]
LLGMM-SUSPEND-REQ
LL-UNITDATA-REQ
[GMM PDU = RA Update]
RA update contain old TLLI, old RAI. Of course, this message is not ciphered (new RA may be in different SGSN, so different LLC).
GRR-DATA-REQ
[LLC UI frame]
UL TBF

Network may start authenticaion and ciphering procedures at this point as detailed in One-phase access cont. article.

Network now start configuring LLC with LLC XID frame.
DL TBF

GRR-UNITDATA-IND
[LLC XID frame]

LLC informs SNDCP to reset itself. Also, it returns XID for confirm negotiation.
This indicate to SNDCP that N PDU transfer is to be suspended and SNDCP XID negotiation to take place.
LL-RESET-IND
GRR-UNITDATA-REQ
[LLC XID frame]
UL TBF

Network now start LLC establishment (assuming ack mode).
DL TBF

GRR-UNITDATA-IND
[LLC SABM frame with SNDCP XID message]

LL-EST-IND
[SNDCP XID message]
LL-EST-RES
[SNDCP XID negotiated]
GRR-UNITDATA-REQ
[LLC UA frame]
UL TBF

LLC is established. Network GMM now sends RA update accept.
DL TBF

GRR-DATA-IND
[LLC I frame]
LLGMM-UNITDATA-IND
[GMM PDU = RA update Accept]
This contain new TLLI, last N PDU numbers (for active PDP contexts) that has been received by SGSN. This is to be passed to SNDCP.


SNSM-SEQUENCE-IND
[Rx N PDU number, NSAPI, old TLLI]
This primitive is sent for all NSAPIs (active PDP contexts) in LLC ack mode. SNDCP then need to re-transmit all segments for N PDUs which are not acknowledged. SNDCP responds back with N PDU numbers which it has received from network so that network peer layer can do similar.
SNSM-SEQUENCE-RES
[Rx N PDU number, NSAPI, old TLLI]

Later procedure is similar to normal intra SGSN RA update.
GMM sets LLC and GRR for new TLLI and then sends RA Update Complete message.
LLGMM-ASSIGN-REQ
[new TLLI, Kc, Ciphering algo]
GMMRR-ASSIGN-REQ
[new TLLI]
LLGMM-UNITDATA-REQ
[GMM PDU = RA Update Complete]
This contain Rx N PDU numbers
GRR-DATA-REQ
[LLC UI frame]
UL TBF
GMM can now resume LLC (SN PDU transfer). Also it may inform new RAI, new Cell id to MN layer - which in turn may indicate it to application layer.
RA Update Complete message allows network to make its GRR,LLC, and GMM ready for both receiving and transmitting on new TLLI.
LLGMM-RESUME-REQ

GMM may request LLC/GRR to unassign old TLLI.
Unassign old TLLI
Unassign old TLLI
Network (new SGSN) may start PDP context modification procedure as it will have different traffic conditions, configurations etc.
This complete inter SGSN RA update procedure.

References: None.

Copyright © Samir Amberkar 2010§

Inside PS - Cell/RA update « GPRS Index .