The GPRS Detach procedure allows:
an MS to inform the network that it does not want to access the SGSN-based services any longer; and
the network to inform an MS that it does not have access to the SGSN-based services any more.
The Detach function allows an MS to inform the network that it wants to make a GPRS and/or IMSI detach, and it allows the network to inform an MS that it has been GPRS-detached or IMSI-detached by the network.
The different types of detach are:
GPRS detach; and
combined GPRS / IMSI detach (MS-initiated only).
The MS is detached either explicitly or implicitly:
Explicit detach: The network or the MS explicitly requests detach.
Implicit detach: The network detaches the MS, without notifying the MS, after the implicit detach timer expired, or after an irrecoverable radio error causes disconnection of the logical link.
In the explicit detach case, a Detach Request (Cause) is sent by the SGSN to the MS, or by the MS to the SGSN.
The MS can make an IMSI detach in one of two ways depending on whether it is GPRS-attached or not:
A GPRS-attached MS sends a Detach Request message to the SGSN, indicating an IMSI detach. This can be made in combination with GPRS detach.
An MS that is not GPRS-attached makes the IMSI detach as already defined in A/Gb mode or Iu mode.
In the Mobile-originated Detach Request message there is an indication to tell if the detach is due to switch off or not. The indication is needed to know whether a Detach Accept message should be returned or not.
In the network-originated Detach Request message there may be an indication to tell the MS that it is requested to initiate GPRS Attach and PDP Context Activation procedures for the previously activated PDP contexts.
The Purge function allows an SGSN to inform the HLR that it has deleted the MM and PDP contexts of a detached MS. The SGSN may, as an implementation option, delete the MM and PDP contexts of an MS immediately after the implicit or explicit detach of the MS. Alternatively, the SGSN may keep for some time the MM and PDP contexts and the authentication triplets of the detached MS, so that the contexts can be reused at a later GPRS attach without accessing the HLR.
When the SGSN deletes the MM and PDP contexts, it shall initiate the Purge procedure as illustrated in Figure 26
After deleting the MM and PDP contexts of a detached MS, the SGSN sends a Purge MS (IMSI) message to the HLR.
The HLR sets the MS Purged for GPRS flag and acknowledges with a Purge MS Ack message.