Serving GPRS Support Node (SGSN) Overview
â–„ Cisco ASR 5000 Series Product Overview
Authentication is mandatory if no MM context exists for the MS/UE:
The SGSN gets a random value (RAND) from the HLR to use as a challenge to the MS/UE.
The SGSN sends a Authentication Request message to the UE containing the random RAND.
The MS/UE contains a SIM that contains a secret key (Ki) shared between it and the HLR called a Individual
Subscriber Key. The UE uses an algorithm to process the RAND and Ki to get the session key (Kc) and the signed
response (SRES).
The MS/UE sends a Authentication Response to the SGSN containing the SRES.
The SGSN updates location information for the MS/UE:
a) The SGSN sends an Update Location message, to the HLR, containing the SGSN number, SGSN address, and IMSI.
b) The HLR sends an Insert Subscriber Data message to the ―new‖ SGSN. It contains subscriber information such as IMSI
and GPRS subscription data.
c) The ―New‖ SGSN validates the MS/UE in new routing area:
If invalid: The SGSN rejects the Attach Request with the appropriate cause code.
If valid: The SGSN creates a new MM context for the MS/UE and sends a Insert Subscriber Data Ack back to the HLR.
d) The HLR sends a Update Location Ack to the SGSN after it successfully clears the old MM context and creates new one
The SGSN sends an Attach Accept message to the MS/UE containing the P-TMSI (included if it is new), VLR TMSI, P-
TMSI Signature, and Radio Priority SMS.
At this point the GPRS Attach is complete and the SGSN begins generating M-CDRs.
If the MS/UE initiates a second call, the procedure is more complex and involves information exchanges and validations
between ―old‖ and ―new‖ SGSNs and ―old‖ and ―new‖ MSC/VLRs. The details of this combined GPRS/IMSI attach
procedure can be found in 3GPP TS23.060.
PDP Context Activation Procedures
The following figure provides a high-level view of the PDP Context Activation procedure performed by the SGSN to
establish PDP contexts for the MS with a BSS-Gb interface connection or a UE with a UTRAN-Iu interface connection.
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