IMSI attach refers to the procedure used when a mobile device or mobile station joins a GSM network when it turns on and IMSI detach refers to the procedure used to leave or disconnect from a network when the device is turned off.
The Visitor Location Register (VLR) checks its database to determine whether there is an existing record of the particular subscriber.
If no record is found, the VLR communicates with the subscriber's Home Location Register (HLR) and obtains a copy of the subscription information.
The IMSI detach procedure informs the network that the Mobile Station is switched off or is unreachable.
If the radio link quality is poor when IMSI detach occurs, the VLR may not properly receive the IMSI-detach request.
The GSM air-interface, designated Um, transmits network-specific information on specific broadcast channels.
The cancel location procedure can also be initiated with MML commands, with those, for example, that are used for changing the area, or deleting the MS from the HLR.