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GSM Information

The GSM Network comprises three parts, Mobile Station (MS) which is similar to a cordless phone with extra features, the Base Transceiver Station (BTS) that controls the connection with the Mobile Station, the Base Station Controller (BSC) that controls multiply Base Transceiver Station's and then the rest of the network covered further below..

Mobile Station (MS)

A Digital Mobile Phone and a SIM card make up the Mobile Station. The SIM (Subscriber Identity Module) is a card that fits into your handset and is one of two sizes - either full size (same size as a credit card) or the smaller plug in version. The SIM microprocessor is based on a silicon chip which is designed to tolerate temperatures between -25 Degrees Celsius and +70 Degrees Celsius, and will also withstand up to 85% humidity. However silicon is fragile and, therefore, if the card is tampered with, physically or electronically, the card will be rendered useless.
The SIM contains all of your identification details, such as your IMSI (International Mobile Subscriber Identity. This is a numeric string, where the first 3 digits represent the country where the SIM is from, the next represent the operator in that specific country. The other digits represent the subscribers identity in his home-network), phone memories, billing information, SMS text messages, pin numbers and international roaming information.

A IMEI (International Mobile Equipment Identity) card is the serial number of the GSM phone that is the equivalent of the ESN number in a Analogue Phone, this is fixed in the phone and cannot be changed. The SIM card contains a IMSI (International Mobile Subscriber Identity) number that identifies the user to the network along with other user and security information.

Base Transceiver Station (BTS)

The Base Transceiver Station consists of a radio transceiver with antenna that covers a single cell. It handles the communications with the MS via radio interface.

BTS are all connected together to allow you to move from one cell to another. The antenna can take on various forms, in the UK lampposts are being used, but normally it has three directional cells.

Base Station Controller (BSC)

The Base Station Controller manages multiple BTS's. It controls the allocation and release of radio channels and handovers between cells.

A series of BTS's are connected to each Base Station Controller, the BSC keeps a eye on each call and decides when to pass the call off to another BTS and to which one.

The Rest of the Network

Several BSC's are controlled by the Mobile service Switching Center (MSC), the MSC works with four databases (HLR, VLR, EIR and the AuC) and together they manage the communications between Mobile Station user and the other network types. Each of the databases has a separate job, these are as follows

Mobile Switching Center (MSC)

The Mobile Switching Center is the interface between the base station system and the switching subsystem of the mobile phone network. Furthermore, the MSC is also the interface between the cellular network and the PSTN. The MSC generates all billing records and ensures that all usage is directed to the appropriate account.

The MSC has a relatively complex task, as unlike a conventional telephone exchange, when GSM subscribers make calls they could be anywhere within the network.

The MSC must ensure that calls are routed through to those subscribers, wherever they are and wherever they move to throughout the duration of each cell. This situation becomes even more complex when two mobile subscribers wish to contact each other from two distant locations.

In order to simplify the subscriber management function, a specific service area is allocated to each MSC. The MSC has to control the switching of tariff to and from the subscribers within it's service area which involves the coordination of all radio resources and the inter cell hand-off activities.

Home Location Register (HLR)

The HLR is the central data base for all the subscribers which contains details on the identity of each subscriber, the services to which they have access and the locations where the subscriber was last registered.

All subscriber administration procedures are communicated to the HLR where the data is stored until it is required by another part of the Public Land Mobile Network (PLMN).

The two key references used to route calls to each subscriber are the International Mobile Subscriber Identity (IMSI) and the Mobile Subscriber Integrated Services Digital Network (MSISDN) number.

The IMSI is the unique number allocated to the subscriber which is stored in the SIM Card and is used by the network for internal communications. When the SIM Card is inserted into a Mobile Equipment it becomes a Mobile Station.

The MSISDN is the subscriber's mobile number which is linked to the IMSI in the HLR. Incoming calls to a subscriber are translated back to the IMSI at the HLR thus enabling them to be delivered to the Mobile Station.

Once the Mobile Station's MSISDN has been used to identify the IMSI, the HLR verifies the subscription records to ensure that the call can be delivered to the last known location of the Mobile Station.

Visitor's Location Register (VLR)

The VLR is a database that is linked to an MSC and temporarily stares information about each Mobile Station within the area served by that MSC.

The information that is temporarily stored in the VLR is sufficient to allow any Mobile Station within that MSC area to make and receive calls. This includes the Mobile Station's identity, the area in which it was last registered and data pertaining to the subscriber and any supplementary services that have been selected by the subscriber.

The MSC refers to the VLR each time that a Mobile Station attempts to make a call in order to verify that the request can be fulfilled. This process is to establish that no call restrictions or call barring instructions are in place.

Equipment Identity Register (EIR)

The EIR ensures that all Mobile Equipment's are valid and authorised to function on the PLMN. Three categories exist on the EIR, a white list, a gray list and a black list.

The white list comprises the IMEI ranges of all the Mobile Equipment's that have been approved by any one of the three European, GSM approval centers.

Any Mobile Equipment that appears on the gray list will be allowed to function but will trigger an alert to the network operator. This facility allows the network operator to identify any subscriber that is using a lost or stolen Mobile Equipment.

Mobiles that are lost or stolen can be blacklisted which will prevent them from functioning on the home PLMN or on other PLMNs around the world.

Central Equipment Identity Register (CEIR)

A central EIR is managed by the MoU Permanent Secretariat in Dublin, Ireland. Every MoU member is committed to linking their network's EIR to the CEIR by January 1995.

The advantage in having the CEIR concept is that it empowers each network operator to restrict or prevent the operation of any given MS throughout all PLMNs that are linked up to the CEIR.

Authentication Center (AUC)

The authentication center is used to validate the SIM Card being used by the Mobile Station. Secret information that is held in the AUC and which is also contained within the SIM Card is used to perform a complex mathematical calculation. Authentication occurs if the results of these two calculations agree.

• SMSC (SMS Center or Service Center), the SMSC handled all the SMS messages that are sent. The messages are sent on a data channel so you can receive them whilst on a call.

• GMSC (Gateway MSC), is a gateway switch where the call is directed when setting up a call to a GSM user. The GMSC looks for the subscriber by interrogating the right HLR which then interrogates the VLR and routes the incoming call towards the MSC where the subscriber can be reached.