GSM Network Architecture

GSM primer includes:
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The GSM network architecture provided a simple and yet effective architecture to provide the services needed for a 2G cellular or mobile communications system.

Although the 5G network architecture has moved on significantly since 2G GSM was introduced, the 5G network architecture owes a lot to its predecessor. Understanding how the early systems works, also gives a significant insight into understanding the later ones.

There were four main elements to the overall GSM network architecture and these could often be further split. Elements like the base station controller, MSC, AuC, HLR, VLR and the like are brought together to form the overall system.

The 2G GSM network architecture, although now superseded gives an excellent introduction into some of the basic capabilities required to set up a mobile communications phone network and how all the entities operate together.

GSM network architecture elements

In order that the GSM system operates together as a complete system, the overall network architecture brings together a series of data network identities, each with several elements.

The GSM network architecture is defined in the GSM specifications and it can be grouped into four main areas:

• Network and Switching Subsystem (NSS)
• Base-Station Subsystem (BSS)
• Mobile station (MS)
• Operation and Support Subsystem (OSS)

The different elements of the GSM network operate together and the user is not aware of the different entities within the system.

As the GSM network is defined but the specifications and standards, it enables the system to operate reliably together regardless of the supplier of the different elements.

A basic diagram of the overall system architecture for the 2G GSM mobile communications system includes four major elements which are shown below:

Within this diagram the different network areas can be seen - they are grouped into the four areas that provide different functionality, but all operate to enable reliable mobile communications to be achieved.

The overall network architecture provided to be very successful and was developed further to enable 2G evolution to carry data and then with further evolutions to allow 3G to be established.

Network Switching Subsystem (NSS)

The GSM system architecture contains a variety of different elements, and is often termed the core network. It is essentially a data network with a various entities that provide the main control and interfacing for the whole mobile network. The major elements within the core network include:

    Mobile Services Switching Centre (MSC):

The main element within the core network area of the overall GSM network architecture is the Mobile switching Services Centre (MSC).

The MSC acts like a normal switching node within a PSTN or ISDN, but also provides additional functionality to enable the requirements of a mobile user to be supported. These include registration, authentication, call location, inter-MSC handovers and call routing to a mobile subscriber.

It also provides an interface to the PSTN so that the mobile communications calls can be routed from the mobile network to a phone connected to a landline. Interfaces to other MSCs are provided to enable calls to be made to mobiles on different networks.

    Home Location Register (HLR):

This database contains all the administrative information about each subscriber along with their last known location. In this way, the GSM network is able to route calls to the relevant base station for the MS.

When a user switches on their phone, the phone registers with the network and from this it is possible to determine which BTS it communicates with so that incoming calls can be routed appropriately. Even when the phone is not active (but switched on) it re-registers periodically to ensure that the network (HLR) is aware of its latest position.

There is one HLR per network, although it may be distributed across various sub-centres to for operational reasons.

    Visitor Location Register (VLR):

This contains selected information from the HLR that enables the selected services for the individual subscriber to be provided. The VLR can be implemented as a separate entity, but it is commonly realised as an integral part of the MSC, rather than a separate entity. In this way access is made faster and more convenient.

    Equipment Identity Register (EIR):

The EIR is the entity that decides whether a given mobile equipment may be allowed onto the network. Each mobile equipment has a number known as the International Mobile Equipment Identity.

This number, as mentioned above, is installed in the equipment and is checked by the network during registration. Dependent upon the information held in the EIR, the mobile may be allocated one of three states - allowed onto the network, barred access, or monitored in case its problems.

    Authentication Centre (AuC):

The AuC is a protected database that contains the secret key also contained in the user's SIM card. It is used for authentication and for ciphering on the radio channel.

    Gateway Mobile Switching Centre (GMSC):

The GMSC is the point to which a ME terminating call is initially routed, without any knowledge of the MS's location. The GMSC is thus in charge of obtaining the MSRN (Mobile Station Roaming Number) from the HLR based on the MSISDN (Mobile Station ISDN number, the "directory number" of a MS) and routing the call to the correct visited MSC.

The "MSC" part of the term GMSC is misleading, since the gateway operation does not require any linking to an MSC.

    SMS Gateway (SMS-G):

The SMS-G or SMS gateway is the term that is used to collectively describe the two Short Message Services Gateways defined in the GSM standards.

The two gateways handle messages directed in different directions. The SMS-GMSC (Short Message Service Gateway Mobile Switching Centre) is for short messages being sent to an ME.

The SMS-IWMSC (Short Message Service Inter-Working Mobile Switching Centre) is used for short messages originated with a mobile on that network. The SMS-GMSC role is similar to that of the GMSC, whereas the SMS-IWMSC provides a fixed access point to the Short Message Service Centre.

These entities were the main ones used within the GSM network. They were typically co-located, but often the overall core network was distributed around the country where the network was located. This gave some resilience in case of failure.

Although the GSM system was essential a voice system, the core network was a data network as all signals were handled digitally.

Base Station Subsystem (BSS)

The Base Station Subsystem (BSS) section of the 2G GSM network architecture that is fundamentally associated with communicating with the mobiles on the network.

It consists of two elements:

• Base Transceiver Station (BTS):   The BTS used in a GSM network comprises the radio transmitter receivers, and their associated antennas that transmit and receive to directly communicate with the mobiles. The BTS is the defining element for each cell. The BTS communicates with the mobiles and the interface between the two is known as the Um interface with its associated protocols.
• Base Station Controller (BSC):   The BSC forms the next stage back into the GSM network. It controls a group of BTSs, and is often co-located with one of the BTSs in its group. It manages the radio resources and controls items such as handover within the group of BTSs, allocates channels and the like. It communicates with the BTSs over what is termed the Abis interface.

The base station subsystem element of the GSM network utilised the radio access technology to enable a number of users to access the system concurrently. Each channel supported up to eight users and by enabling a base station to have several channels, a large number of subscribers could be accommodated by each base station.

Base stations are carefully located by the network provider to enable complete coverage of an area. The area being covered bay a base station often being referred to as a cell.

As it is not possible to prevent overlap of the signals into the adjacent cells, channels used in one cell are not used in the next. In this way interference which would reduce call quality is reduced whilst still maintaining sufficient frequency re-use.

It is important to have the different BTSs linked with the BSS and the BSSs linked back to the core network.

A variety of technologies were used to achieve this. As data rates used within he GSM network were relatively low, E1 or T1 lines were often used, especially for linking the BSS back to the core network.

As more data was required with increasing usage of the GSM network, and also as other cellular technologies like 3G became more widespread, many links used carrier grade Ethernet.

Often remote BTSs were linked using small microwave links as this could reduce the need for the installation of specific lines if none were available. As base stations often needed to be located to provide good coverage rather than in areas where lines could be installed, the microwave link option provided an attractive method for providing a data link for the network.

Mobile station

Mobile stations (MS), mobile equipment (ME) or as they are most widely known, cell or mobile phones are the section of a GSM mobile communications network that the user sees and operates. In recent years their size has fallen dramatically while the level of functionality has greatly increased. A further advantage is that the time between charges has significantly increased.

There are a number of elements to the cell phone, although the two main elements are the main hardware and the SIM.

The hardware itself contains the main elements of the mobile phone including the display, case, battery, and the electronics used to generate the signal, and process the data receiver and to be transmitted.

The mobile station, or ME also contains a number known as the International Mobile Equipment Identity (IMEI). This is installed in the phone at manufacture and "cannot" be changed. It is accessed by the network during registration to check whether the equipment has been reported as stolen.

The SIM or Subscriber Identity Module contains the information that provides the identity of the user to the network. It contains are variety of information including a number known as the International Mobile Subscriber Identity (IMSI). As this is included in the SIM, and it means that by moving the SIM card from one mobile to another, the user could easily change mobiles. The ease of changing mobiles whilst keeping the same number meant that people would regularly upgrade, thereby creating a further revenue stream for network providers and helping to increase the overall financial success of GSM.

Operation and Support Subsystem (OSS)

The OSS or operation support subsystem is an element within the overall GSM mobile communications network architecture that is connected to components of the NSS and the BSC. It is used to control and monitor the overall GSM network and it is also used to control the traffic load of the BSS. It must be noted that as the number of BS increases with the scaling of the subscriber population some of the maintenance tasks are transferred to the BTS, allowing savings in the cost of ownership of the system.

The 2G GSM network architecture follows a logical method of operation. It is far simpler than current mobile phone network architectures such as the 5G network architecture which uses software defined entities to enable very flexible operation.

However the 2G GSM architecture does show the voice and operational basic functions that are needed and how they fit together. As the GSM system was all digital, the network was a data network.

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