GPRS Normal Attach

Figure GPRS Normal Attach

1. The terminal initiates the attach procedure after power on. The message contains the previously used TMSI (Temporary Mobile Subscriber Id). The mobile network identity, the location area and routing area  information is also included in the message.
2. The SGSN (Serving GPRS Support Node) searches for TMSI in its database.
3. No entry is found for the TMSI, so the SGSN uses the old location area information to identify the old SGSN where this terminal was being served.
4. The old SGSN responds with the GPRS mobile's IMSI (International Mobile Subscriber Identity) to the SGSN.
5. The SGSN asks the terminal to identify itself.
6. The terminal responds back.
7. The SGSN authenticates the GPRS mobile by sending a RAND value (a random value).
8. The SIM applies secret GSM algorithms on the RAND and the secret key Ki to obtain the session key Kc and SRES.
9. The computed SRES value is passed to the SGSN.
10. The SGSN then requests the identity of the GPRS mobile.
11. GPRS mobile responds back with the identity.
12. Verify that that GPRS mobile being used by the user is not a stolen one. The IMEI (Internaional Mobile Equipment Identity) obtained from the GPRS mobile is sent to the Equipment Identification Register (EIR).
13. The EIR clears the subscriber and responds back to the SGSN with the status.
14. The SGSN now informs the Home Location Register (HLR) about the new location of the GPRS mobile.
15. The HLR informs the old SGSN that the GPRS mobile has moved to a new location.
16. The old SGSN acknowledges back.
17. The HLR updates the new SGSN with all the subscriber information.
18. The SGSN responds back to the HLR.
19. The HLR now responds back to the SGSN's "Update Location" message.
20. The SGSN responds back to the original GRPS normal attach request from the mobile.
21. The GPRS mobile acknowledges the receipt of "Attach Accept".

Source:
GPRS attach and PDP context activation sequence diagrams
GPRS Attach Procedure