All GPRS mobility procedures require a TBF connection at the RLC/MAC layer between the MS and the PCU.
When an MS needs to signal its presence to the network in order to access to GPRS services, it performs an IMSI attach procedure for GPRS services.
Network-Initiated Normal GPRS Detach When an SGSN wishes to IMSI detach a given MS for GPRS services, it initiates a GPRS-detach procedure.
The procedure is ended upon the receipt of DETACH ACCEPT message by the SGSN, as illustrated in Figure 7.5.
During this procedure a MM context is created between the MS and the SGSN.
There are two types of GPRS attach procedures: Note that by default, the IMSI-attach procedure is referred to as the attach procedure for circuit-switched services.
Figure 7.2 describes a combined GPRS/IMSI attach procedure in a cell that supports GPRS in network operation mode I.
The difference with the previous scenario is that the new SGSN sends an IMSI attach request to the MSC/VLR via the Gs interface as soon as the new SGSN receives data related to the GPRS subscriber from the HLR.
This implies that the network must know the identifier of the GPRS LA indicating where the MS is located.
If the MS is both IMSI and GPRS attached, the SGSN sends an explicit request to the MSC/VLR to deactivate the association between the SGSN and the MSC/VLR. In a cell supporting PCCCH channels and operating in network operation modes I or III, an MS both IMSI and GPRS attached in GMM STANDBY state can be paged by the network if the SGSN receives a PDU for it.
When the MS receives a paging for packet-switched services, it requests of the BSS a TBF establishment in order to send any LLC frame to the SGSN.
MS-Initiated Combined GPRS Detach When an MS both IMSI and GPRS attached wishes to perform a GPRS detach in a cell that supports GPRS in network operation mode I, it initiates a combined detach procedure to the SGSN.
The latter sends an explicit request to the MSC/VLR to deactivate the association between SGSN and MSC/VLR in order that circuit-switched incoming calls are no longer routed to SGSN. The same scenario is used for an MS both IMSI and GPRS attached wishing to be IMSI detached or both IMSI and GPRS detached in network operation mode I.
This context contains information such as the IMSI, the P_TMSI, the RAI, and the CI.