1. Subsystems and network elements in GSM

The GSM network is called Public Land Mobile Network (PLMN). It is organised in three subsystems:

  • Base Station Subsystem (BSS
  • Network Switching Subsystem (NSS)
  • Network Management Subsystem (NMS)

The three subsystems, different network elements, and their respective tasks are presented in the following.

Figure 1.           The three subsystems of GSM

1.1                                 Network Switching Subsystem (NSS)

The Network Switching Subsystem (NSS) contains the network elements MSC, GMSC, VLR, HLR, AC and EIR.

Figure 2.           The Network Switching Subsystem (NSS)

The main functions of NSS are:
Call control
This identifies the subscriber, establishes a call, and clears the connection after the conversation is over.
This collects the charging information about a call (the numbers of the caller and the called subscriber, the time and type of the transaction, etc.) and transfers it to the Billing Centre.
Mobility management
This maintains information about the subscriber's location.
This applies to interfaces with the BSS and PSTN.
Subscriber data handling
This is the permanent data storage in the HLR and temporary storage of relevant data in the VLR.

1.1.1                              Mobile services Switching Centre (MSC)

The MSC is responsible for controlling calls in the mobile network. It identifies the origin and destination of a call (mobile station or fixed telephone), as well as the type of a call.

The MSC is responsible for several important tasks, such as the following.

Call control

MSC identifies the type of call, the destination, and the origin of a call. It also sets up, supervises, and clears connections.

Initiation of paging

Paging is the process of locating a particular mobile station in case of a mobile terminated call (a call to a mobile station).

Charging data collection

The MSC generates CDRs, Charging Data Records, which contain information about the subscribers’ usage of the network.

1.1.2                              Gateway Mobile services Switching Centre (GMSC)

The GMSC is responsible for the same tasks as the MSC, except for paging. It is needed in case of mobile terminated calls. In fixed networks, a call is established to the local exchange, to which the telephone is connected to. But in GSM, the MSC, which is serving the MS, changes with the subscriber’s mobility. Therefore, in a mobile terminated call, the call is set up to a well defined exchange in the subscriber’s home PLMN. This exchange is called GMSC. The GMSC than interacts with a database called Home Location Register, which holds the information about the MSC, which is currently serving the MS. The process of requesting location information from the HLR is called HLR Interrogation. Given the information about the serving MSC, the GMSC then continues the call establishment process.
In many real life implementations, the MSC functionality and the GMSC functionality are implemented in the same equipment, which is then just called MSC. Many operators use GMSCs for breakout to external networks such as PSTNs.

1.1.3                             Visitor Location Register (VLR)

In the Nokia implementation, Visitor Location Register (VLR) is integrated with the MSC cabinet. VLR is a database which contains information about subscribers currently being in the service area of the MSC/VLR, such as:

·                Identification numbers of the subscribers

Security information for authentication of the SIM card and for ciphering

·                Services that the subscriber can use

The VLR carries out location registrations and updates. When a mobile station comes to a new MSC/VLR serving area, it must register itself in the VLR, in other words perform a location update. Please note that a mobile subscriber must always be registered in a VLR in order to use the services of the network. Also the mobile stations located in the own network is always registered in a VLR.

The VLR database is temporary, in the sense that the data is held as long as the subscriber is within its service area. It also contains the address to every subscriber's Home Location Register, which is the next network element to be discussed.

1.1.4                            Home Location Register (HLR)

HLR maintains a permanent register of the subscribers. For instance the subscriber identity numbers and the subscribed services can be found here. In addition to the fixed data, the HLR also keeps track of the current location of its customers. As you will see later, the GMSC asks for routing information from the HLR if a call is to be set up to a mobile station (mobile terminated call).

In the Nokia implementation, the two network elements, Authentication Centre (AC) and Equipment Identity Register (EIR), are located in the Nokia DX200 HLR.

1.1.5                              Authentication Centre (AC)

The Authentication Centre provides security information to the network, so that we can verify the SIM cards (authentication between the mobile station and the VLR, and cipher the information transmitted in the air interface (between the MS and the Base Transceiver Station)). The Authentication Centre supports the VLR's work by issuing so-called authentication triplets upon request.

1.1.6                              Equipment Identity Register (EIR)

As for AC, the Equipment Identity Register is used for security reasons. But while the AC provides information for verifying the SIM cards, the EIR is responsible for IMEI checking (checking the validity of the mobile equipment). When this optional network element is in use, the mobile station is requested to provide the International Mobile Equipment Identity (IMEI) number. The EIR contains three lists:

A mobile equipment in the white list is allowed to operate normally.

If we suspect that a mobile equipment is faulty, we can monitor the use of it. It is then placed in the grey list.

If the mobile equipment is reported stolen, or it is otherwise not allowed to operate in the network, it is placed in the black list.

Note that IMEI checking is an optional procedure, so it is up to the operator to define if and when IMEI checking is performed. (Some operators do not even implement the EIR at all.)                            Mobile Number Portability - Service Routing Register

In many countries mobile networks are required to support Mobile Number Portability solutions. The feature allows mobile users switch from one operator to another without having to change the number of their mobile subscription. However with subscribers moving from one network to another without changing numbers, it becomes difficult for the MSCs to track which HLRs need to be queried to find subscribers since the normal MSISDN analysis may not result in an accurate HLR address.

To solve this problem many of today's networks use a solution to improve the storing of the subscribers' information to reduce the load of updating the MSCs and excessive signalling.  The network is enhanced with one extra database called the Service Routing Register that serves the MNP functionality. All subscribers are registered here with the address of their HLR. 

Figure 3.           MNP architecture