IMSI detach is the process of detaching a MS 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.
At power-down the MS requests a signaling channel. Once assigned, the MS sends an IMSI detach message to the VLR. When the VLR receives the IMSI detach-message, the corresponding IMSI is marked as detached by setting the IMSI detach flag. The HLR is not informed of this and the VLR does not acknowledge the MS about the IMSI detach.
If the radio link quality is poor when IMSI detach occurs, the VLR may not properly receive the IMSI-detach request. Since an acknowledgment message is not sent to the MS, it does not make further attempts to send IMSI detach messages. Therefore the GSM network considers the MS to be still attached.
Read more about this topic: GSM Procedures
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