IMSI detach

IMSI detach is the process of detaching a Mobile Station from the mobile network to which it was connected. The IMSI detach procedure informs the network that the Mobile Station is switched off or is unreachable.

IMSI detach procedure

At power off the Mobile Station requests a signaling channel and once the channel is assigned sends a IMSI detach message to the VLR. The VLR has information about the subscribers who are in the area served by it. When VLR receives the IMSI detach message the corresponding IMSI is marked as detached by setting the IMSI detach flag. However the HLR is not informed about this. The VLR does not acknowledge the MS about the IMSI detach.
If the radio link quality is poor at the time of IMSI detach the VLR may not properly receive the IMSI detach request. Since an acknowledgment message is not sent to the Mobile Station it does not make further attempts to send IMSI detach messages. Therefore the GSM network considers the Mobile Station to be still attached.

Implicit IMSI detach

In the GSM air interface network specific information are broadcast on specific broadcast channels. This information include whether periodic Location update is enabled. If it is enabled the Mobile Station must send location update requests at time intervals specified by the network. As stated above when the Mobile Station was switched off without properly completing the IMSI detach procedure if periodic Location update is in use the network will soon detect that the Mobile Station as switched off or unreachable. In this situation the VLR performs an implicit IMSI detach.