Telecom Tutorials by Samir Amberkar

Inside PS - Cell/RA update [Under GPRS]
» GPRS description - 23.060 «
» Layer 3 - 24.007 «

When GMM is in Ready state, it does cell update for cell change. As seen in GMM article, layer 1 does the cell reselection and informs GRR. TBF if any in progress is abandoned (LLC takes care of continuity).GRR, indicate cell change to GMM. If cell belong to current RA, GMM proceed with Cell Update procedure. If RA is different, GMM starts RA update procedure.

When GMM is in Standby state, it does not do cell update, but only RA update (periodic or upon RA change).

MN

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 no, it triggers Cell Update - which is done by triggering LLC procedures. GMM may also inform about new cell information to MN. GMMRR-Cell-Reselect-Ind [Cell ID, RA, GPRS support etc.] LLGMM-TRIGGER-REQ [Cell update cause, TLLI] New cell information MN may in turn inform it to application layer. On receiving trigger request, LLC sends pending frames to GRR. If no frames are pending, Receive Ready frame is sent for Ack mode and UI frame (empty) if Unack mode. The idea is to make network aware of new cell. UL TBF Network notes down the cell change. GRR takes care of changes in Timing Advance, Power etc as described in RLC/MAC articles.

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MN

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 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 GMM accepts new RA update (assumed that new RA is within same SGSN). DL TBF procedure is done to send RA Update Accept. DL TBF GRR-DATA-IND [LLC UI frame] LL-UNITDATA-IND [GMM PDU = RA Update Accept] RA Update Accept contain new TLLI. GMM now need to inform LLC about new TLLI and Ciphering key (done during Auth/Ciph procedure). Also, RA Update Complete is to be informed to network. LLGMM-ASSIGN-REQ [new TLLI, Kc] LLGMM-UNITDATA-REQ [GMM PDU = RA Update Complete] GRR-DATA-REQ [LLC UI frame] GMM can now resume LLC (SN PDU transfer). GMM may inform new RAI, new Cell id to MN layer. UL TBF LLGMM-RESUME-REQ New cell information MN may in turn inform it to application layer. GMM may request LLC/GRR to unassign old TLLI. Unassign old TLLI Unassign old TLLI This complete the RA update procedure.

References: GPRS in Practice by McGuiggan

Copyright © Samir Amberkar 2010

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