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differences related to the fact that the subscribers move about. These differences are primarily seen in the databases, the HLR and the VLR. The two main parts of the GSM core network are:

• The GSM circuit switched core network is known as the Network and Switching Subsystem or NSS.
• The GSM packet switched core network is GPRS core network which provides packet services.

The two elements above combine to provide GSM services to mobile phone subscribers.

1 HLR

The Home Location Register or HLR is the element where all of the subscribers' data is stored. It contains a database of all an operator's subscribers keyed on

Data stored includes:

• IMSI (the subscriber's identity number - this is the data key)
• MSISDN (the subscriber's phone number)
• GSM services that the subscriber has requested or been given
• Access Points (GPRS) allocated to the subscriber.
• Current Location of subscriber ( VLR and SGSN)

The HLR data is stored for as long as a subscriber remains with the operator, and therefore needs to be persistent, for example on disk drives.

The primary functions of the HLR are:

• to send the subscriber data to a VLR when a subscriber first roams there.
• to send the subscriber data to an SGSN which requests the information during a routing area update procedure
• to broker between the GMSC or SMSC and the subscriber's current VLR in order to allow incoming calls or text messages to be delivered.
• to remove subscriber data from the old VLR when a subscriber has roamed away from it.

2 AUC

The AUC (Authentication Centre) is a function closely associated with the HLR and often built into the HLR. Whereas the HLR deals with authorisation the AUC deals with authentication. Specifically this is authentication of the SIM card. In a standard GSM AUC, a set of numbers is stored:

• the IMSI
• the Ki.

In an AUC designed for UMTS and for use with the USIMUSIM cards are subscriber identity modules for 3G mobile telephony. They are the same physical size as normal 2G GSM SIM cards. we add to these numbers

• a sequence number (SEQN)

The Ki is a shared secret which is used in the AUC and in the SIM to do mutual authentication through the GSM authentication or UMTS authentication mechanisms.

The AUC should be implemented as a crypto box which probably is integrated to the HLR. Proper implementation of security in and around the AUC is a key part of an operator's strategy to avoid SIM cloning .

3 VLR

The Visitors Location Register or VLR is a registry of which subscribers are currently in a particular area. The data stored in the VLR has been received from the HLR, or collected from the MS. In practice, for performance reasons, most vendors integrate the VLR directly to the MSC and, where this is not done, the VLR is very tightly linked with the MSC via a proprietary interface.

Data stored includes:

• IMSI (the subscriber's identity number)
• authentication data
• MSISDN (the subscriber's phone number)
• GSM services that the subscriber is allowed to access
• Access Point (GPRS) subscribed
• the HLR address of the subscriber

The primary functions of the VLR are

• to inform the HLR that a subscriber has arrived in the particular area covered by the VLR
• to track where the subscriber is within the VLR area (location area) when no call is ongoing
• to allow or disallow which services the subscriber may use
• to allocate roaming numbers during the processing of incoming calls

If a subscriber becomes inactive whilst in the area of a VLR then the VLR may delete the subscriber's data after some time and inform the HLR. This would happen when the phone has been switched off and left off or when the subscriber has moved to an area with no coverage for a long time. The case of a subscriber moving to another area is normally handled by the HLR telling the VLR to delete the subscriber's data explicitly.