IMSI attach

In a GSM network, when a Mobile Station

Mobile Station

The mobile station comprises all user equipment and software needed for communication with a mobile network.The mobile station refers to global system connected to the mobile network, i.e. mobile phone or mobile computer connected using a mobile broadband adapter. This is the terminology of 2G...

 (MS) is switched on, the International Mobile Subscriber Identity

International Mobile Subscriber Identity

An International Mobile Subscriber Identity or IMSI is a unique identification associated with all GSM and UMTS network mobile phone users. It is stored as a 64 bit field in the SIM inside the phone and is sent by the phone to the network...

 (IMSI) attach procedure is executed. This procedure is required for the Mobile Switching Center (MSC) and Visitor Location Register (VLR) to register the MS 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.

When the MS is switched on, it searches for a mobile network to connect to. Once the MS identifies its desired network, it sends a message to the network to indicate that it has entered into an 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 (HLR) 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.

IMSI detach

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.

Implicit IMSI detach

The GSM air-interface, designated Um, transmits network-specific information on specific broadcast channels. This information includes whether the periodic location update is enabled. If enabled, then the MS must send location update requests at time intervals specified by the network. If the MS is switched off, having not properly completed the IMSI detach procedure, the network will consider the MS as switched off or unreachable if no location update is made. In this situation the VLR performs an implicit IMSI detach.

Location update

This procedure is used to update the location of the Mobile Station in the network.

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