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GPRS (Inside PS - GPRS Attach) - 19

Inside PS - GPRS Attach [Under GPRS]
» GPRS description - 23.060 «
» Layer 3 - 24.007 «
» AT commands - 27.007 «

Let us start with GPRS attach procedure.

MN GMM LLC GRR AIR
GPRS attach requested with AT command +CGATT=1. Adaptation (MN) layer requests attach to GMM.
GMMREG-ATTACH-REQ
Attach type=GPRS or combined attach.
LLC Unack mode and RLC Ack mode for GMM messages so GMM sends its PDU in UNITDATA-REQ
LL-UNITDATA-REQ
Primitive contain GMM PDU Attach Request - containing old TLLI, RAI etc. Ciphering off (GMM messages Attach Request, Ident Req/Resp, Auth & Ciph Request/Resp are *not* ciphered.)
GRR-DATA-REQ
RLC Ack mode for GMM messages
Primitive contain LLC PDU (UI frame) - which contain GMM PDU.
TBF
One-phase access, RLC/MAC (Uplink) TBF procedure.
Refer TBF article - 1-phase.

Network GMM layer receives Attach Request and may trigger Security procedures. To do this, RLC/MAC (Downlink) TBF procedure is performed.
TBF
RLC/MAC (Downlink) TBF procedure.
Refer TBF article - 1-phase cont..
GRR-DATA-IND
[LLC UI frame]
LL-UNITDATA-IND [GMM PDU Identification Request]

LL-UNITDATA-REQ [GMM PDU Identification Response]
Identification response would be IMSI. Also oldTLLI provided.
GRR-DATA-REQ
[LLC UI frame]
TBF
One-phase access, RLC/MAC (Uplink) TBF procedure.

Network GMM layer receives Identification Response. Now it may trigger Authentication & Ciphering request.
TBF
RLC/MAC (Downlink) TBF procedure.
GRR-DATA-IND
[LLC UI frame]
GMM now talk to SIM to execute security algorithms and generate responses/keys. This is same as for GSM (3GPP 11.11) 1.
LL-UNITDATA-IND [GMM PDU Authentication & Ciphering Request]
LL-UNITDATA-REQ [GMM PDU Authentication & Ciphering Response]
Auth/Ciph response would be SRES given by SIM.
GRR-DATA-REQ
[LLC UI frame]
TBF
One-phase access, RLC/MAC (Uplink) TBF procedure.
LLGMM-ASSIGN-REQ
Contain Kc and the type of cihpering algorithm to be used.
If Authentication response is ok, network triggers LLC XID negotiation.
TBF
RLC/MAC (Downlink) TBF procedure.
GRR-UNITDATA-IND
[LLC XID frame]
GRR-UNITDATA-REQ
[LLC XID frame]
TBF
RLC/MAC (Uplink) TBF procedure.

Network GMM layer accepts Attach Request with Attach Accept. This would be now Ciphered.
TBF
RLC/MAC (Downlink) TBF procedure.
GRR-DATA-IND
[LLC UI frame]
LLC-UNITDATA-IND
[GMM PDU Attach Accept]
Attach Accept contain new PTMSI (from which new TLLI is calculated). GMM need to inform about attach to MN, give this TLLI to GRR and send Attach Complete to network.
GMMREG-ATTACH-CNF
No receiving this confirmation, Adaptation layer gives AT OK to application layer.
LLGMM-ASSIGN-REQ
[new TLLI]
GMMRR-ASSIGN-REQ
[new TLLI]
LL-UNITDATA-REQ
[GMM PDU Attach Complete]
Attach Complete contain new TLLI.
GRR-DATA-REQ
[LLC UI frame]
TBF
GMM may need to send request to LLC and GRR for "unassigning" old TLLI".
One-phase access, RLC/MAC (Uplink) TBF procedure.
This complete the GPRS Attach procedure.

1: Network give RANDom number which ME passes to SIM in RUN GSM ALGORITHM command, SIM runs a alogorithm and provides SRES (Signed RESponse) and Kc (Cryptographic key, used by the ciphering alogorithm A5) to ME Network runs the same algorithm on same RAND, resulting SRES should match with what given by MS.

References: GPRS in Practice by McGuiggan

Copyright © Samir Amberkar 2010§

Inside PS - SAP diagram « GPRS Index » Inside PS - MS PDP Act