Registration and Routing Area Update In the CS mode of operation, we use the Location Updating Request both for the initial registration and for location updates. This is different in the PS mode of operation: GPRS AttachA GPRS Attach is used to establish a GMM context between the UE and the 3G-SGSN. It can be used for a - normal GPRS attach, which is an IMSI attach for ps services, or a- combined GPRS attach for a combined cs and ps IMSI attach. This is only possible with NMO I.
GPRS Session Management Session management is only possible after the establishment of the GMM context between the UE and the network. The session management is then responsible to handle PDP contexts. This incorporates PDP context activation, PDP context modification, and PDP context deactivation procedures. In the TS24.008 you can find all session management messages, which are transmitted over the radio interface. The peer entities are the UE and the SGSN. The UE send the session management message ACTIVATE PDP CONTEXT REQUEST to the SGSN to activate a PDP context. In parallel, it starts the timer T3380 (30 = seconds). The IE contained in this message are sketched in the figure on the right hand side. The SGSN is analysing the message. The APN is analysed to determine a suitable GGSN. The QoS requested by the UE may be modified. The SGSN may even reject the subscriber‘s request. If the SGSN accepts the UE‘s request, it has to establish a GTP tunnel to the selected GGSN by sending the message CREATE PDP CONTEXT REQUEST. The GGSN analysis the message in order to establish a connection to the external PDN. It returns the message CREATE PDP CONTEXT RESPONSE to the SGSN. This message holds among others the Negotiated QoS and the GPRS Charging Identifier. Based on the Negotiated QoS, the SGSN determines the a radio priority level for the UE. The SGSN returns the message ACTIVATE PDP CONTEXT ACCEPT. Upon its reception, the UE stops the timer T3380. If the QoS requested derivates from the Negotiated QoS, the UE either accepts it, or initiates the PDP context deactivation procedure. Upon expiry of T3380, the UE transmits the ACTIVATE PDP CONTEXT REQUEST message again. Retransmission is possible up to four times.