IMSI attach

In a GSM network, when a Mobile Station or (MS) is switched on, the International Mobile Subscriber Identity, or IMSI attach procedure is executed. This procedure is required for the MSC/VLR to register the mobile station in the network. If the MS has changed Location area (LA) while it was powered off the IMSI attach procedure will lead to a Location update.

IMSI attach procedure

When the MS is switched on, it searches for a mobile network to connect to. Once the mobile identifies the desired network, it sends a message to the network to indicate that it has entered in to idle state. 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 and obtains a copy of the subscription information. The obtained information is stored in the database of the VLR. Then an acknowledge message is sent to the MS.