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UMTS (Inter-SGSN RA/LA update) - 25

Inter-SGSN combined RA/LA update [Under UMTS]
» MM/GMM - 24.008 «
» GPRS description - 23.060 «
» UTRAN Iu interface - 25.410 «

NAS procedures for IMSI attach (Location update), GPRS attach, Combined attach, RA update remain similar to GSM/GPRS [TS 24.008]. You may also check TS 23.060 for related end-to-end signaling. As an example, let us take a scenario of Inter-SGSN combined RA/LA update.

RRC in idle mode, monitoring broadcast information and informing it to NAS GMM detect RA change. It requests RRC connection.

RRC Connection Establishment procedure.

RRC Initial Direct Transfer [GMM - RA Update Request]
SCCP Connection Request and Confirm
SCCP connection is established (can be called Iu signaling connection).
[GMM - RA Update Request] over RANAP
RA Update Request is sent to new SGSN (via new RNC). It contain old RAI, old P-TMSI etc. New SGSN require UE's current information related to MM, PDP Context, PDCP (pending Sequence number) etc. from old SGSN.
SGSN Context Request over GTP
SGSN Context Response over GTP
New SGSN now invokes authentication/ciphering, first step of which is to get this information from HLR (MAP messages - Send Authentication Info and Ack).

Second step is to authenticate UE.
RRC Downlink Direct Transfer [MM - Authentication & Ciphering Request]
UE calculates (authentication response) RES value using mentioned authentication on given random number. Ciphering key is also generated.
This contain random number and authentication algorithm type that is to be used.
MM - Authentication & Ciphering Response (over RRC UDT message)
If authentication is successful, next step for new SGSN is to inform old SGSN so that old SGSN can clear up particular UE data. Before clearing it forwards pending user packets to new SGSN. If authentication had failed, new SGSN would have sent reject indication to old SGSN.
SGSN Context Ack over GTP
New SGSN is in control now, but it need to inform GGSN about it (SGSN knows which GGSN to contact from PDP context).
Forward packets
Update PDP Context Request over GTP
GGSN updates its own data; now packets arriving for UE will go to new SGSN.
SGSN (network operation mode I) now need to update CS domain about location change. First step is to tell HLR about updated location. In turn HLR provide PS/GPRS subscriber data for UE. If this step fail, SGSN can reject RA update.
Update PDP Context Response over GTP
HLR - Update Location over MAP
HLR - Cancel Location
HLR - Insert Subscriber Data
Next step for SGSN is to inform (new) MSC/HLR about changed location - which in turn updates HLR and gets CS/GSM subscriber data for UE. MSC/VLR provide new TMSI.
HLR - Update Location Ack
MSC/VLR - Location Update Request
SGSN is now ready to accept RA update.
MSC/VLR - Location Update Accept
This contain P TMSI, VLR TMSI, PDCP sequence number (acknowledged PDUs). UE resets PDCP (discards acknowledged PDUs) and confirm SGSN.
RA Update Accept (over RRC DDT)
RA Update Complete (over RRC UDT)
SGSN informs MSC/VLR that TMSI reallocation is complete. It may as well release Iu signaling connection with RNC.
MSC/VLR - TMSI Realloc Complete
SCCP Released
RRC Connection Release
RRC Connection Release Complete
RRC Connection released. RRC return to idle state and continue idle state procedures. MM/GMM remain in attached/ready state.

This complete Inter SGSN combined RA/LA update procedure.

References: UMTS by Sanchez and Thioune, and WCDMA for UMTS by Holma and Toskala.

Copyright © Samir Amberkar 2010§

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