GSM core network Guide, Meaning , Facts, Information and Description

The GSM Core network is at the heart of a GSM system, the most common mobile phone system in use in the world. It exists to allow mobile phones to work fully with each other and with the wider telecommunications network. It closely resembles a fixed telephone network. There are, however, considerable 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.

Table of contents

1 HLR 2 AUC 3 VLR 4 MSC 4.1 GMSC 4.2 Visited MSC 4.3 Other MSC terms 5 EIR 6 Other support functions 6.4 SMSC 6.5 MMSC 6.6 Lawful interception functions 7 See also 8 External links

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 IMSI and MSISDN.

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.

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 USIM 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.

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.

MSC

The MSC (Mobile Switching Centre) is effectively a sophisticated telephone exchange which provides circuit switched services to mobile phones in a certain geographical area. Typically this means voice and fax services, but often also includes dial up data services.

In the GSM mobile phone system, in contrast with earlier analogue services, fax and data information is sent directly digitally encoded to the MSC. Only at the MSC is this re-coded into an "analogue" signal (although actually this will almost certainly mean sound encoded digitally as PCM signal in a 64k timeslot, known as a DS0 in America).

Tasks of the MSC include

• delivering calls to subscribers as they arrive based on information from the VLR
• connecting outgoing calls to other mobile subscribers or the PSTN.
• delivering SMSs from subscribers to the SMSC and vice versa
• arranging handovers from BSC to BSC
• carrying out handovers from this MSC to another
• supporting supplementary services such as conference calls or call hold.

There are various different names for MSCs in different contexts which reflects their complex role in the network, all of these terms though could refer to the same MSC, but doing different things at different times.

GMSC

The Gateway MSC is the MSC to which calls arrive from external networks. The term is only valid in the context of one call since any MSC may provide both the gateway function and the Visited MSC function, however, some manufactures design dedicated high capacity MSCs which do not have any BSCss connected to them. These MSCs will then be the Gateway MSC in many of the calls they handle.

Visited MSC

The Visited MSC is the MSC where a customer is currently located. The VLR associated with this MSC will have the subscriber's data in it.

Other MSC terms

The Anchor MSC is the MSC from which a Handover has been initiated. The Target MSC is the MSC toward which a Handover should take place. An MSC Server is a part of the redesigned MSC concept starting from 3GPP Release 5.

EIR

The EIR (Equipment Identity Register) is often integrated to the HLR. The EIR keeps a list of mobile phones (identified by their IMEI) which are to be banned from the network or monitored. This is designed to allow tracking of stolen mobile phones. In theory all data about all stolen mobile phones should be distributed to all EIRs in the world through a Central EIR. It is clear, however, that there are some countries where this is not in operation. The EIR data does not have to change in real time, which means that this function can be less distributed than the function of the HLR.

Other support functions

Connected more or less directly to the GSM core network are many other functions.

SMSC

The SMSC (Short Message Service Centre) supports the sending of text messages.

MMSC

The MMSC (Multimedia Messaging System Centre) supports the sending of multimedia messages (e.g. Images, Audio, Video and their combinations) to (or from) MMS-enabled Handsets.

Lawful interception functions

According to US law, which has also been copied into many other countries, especially in Europe, all telecommunications equipment must provide facilities for monitoring the calls of selected users. There must be some level of support for this built into any of the different elements. The concept of lawful interception is also known, following the relevant US law, as CALEA.

See also

The GPRS Core Network works in parallel with the rest of the GSM core network.

External links

• the 3GPP, the standardisation body for GSM and UMTS

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