The main structure of GSM system
The main components of the GSM digital cellular communication system can be divided into mobile stations, base station subsystems, and network subsystems. The base station subsystem (referred to as the base station BS) is composed of a base transceiver station (BTS) and a base station controller (BSC); the network subsystem is composed of a mobile switching center (MSC) and an operation and maintenance center (OMC) and a home location register (HLR), It consists of access location register (VLR), authentication center (AUC) and equipment identification register (EIR).
A mobile station (MS) is a portable station (mobile phone) or a vehicle-mounted station. Can also be equipped with terminal equipment (TE) or terminal adapter (TA). The mobile station is a physical device. It must also contain a subscriber identification module (SIM). The SIM card and hardware devices together form the mobile station. Without a SIM card, MS cannot access the GSM network (except emergency services).
Base Transceiver Station (BTS) includes various hardware and software required for wireless transmission, such as transmitters, receivers, antennas required to support various upper cell structures (such as omnidirectional, sector, star, and chain), connection The interface circuit of the base station controller and the detection and control device required by the transceiver station.
The base station controller (BSC) is the connection point between the base transceiver station and the mobile switching center, and also provides an interface for the exchange of information between the base transceiver station and the operation and maintenance center. A base station controller usually controls several base transceiver stations. Its main functions are to manage wireless channels, establish and remove call and communication links, and control the handover of mobile stations in this control area.
The mobile switching center (MSC) is the core of the cellular communication network, and its main function is to control and manage the mobile users located in the control area of ​​the MSC. E.g:
1) Channel management and allocation;
2) Call processing and control;
3) Control of handoff and roaming;
4) Registration and management of user location information;
5) Registration and management of user numbers and mobile device numbers;
6) Control of service type;
7) Implement user authentication;
8) Provide link interfaces for connecting other MSCs in the system and other public communication networks, such as the Public Switched Telecommunications Network (PSTN), Integrated Services Digital Network (ISDN) and Public Data Network (PDN), to ensure that users are transferring or roaming In the process of achieving gap-free service.
It can be seen that the functions of the MSC are similar to the fixed network switching equipment (such as call connection and information exchange), and also have special requirements (such as radio resource management and user mobility control).
Home Location Register (HLR) is a database used to store local user location information. In a cellular communication network, several HLRs are usually set, and each user must be registered in a certain HLR (equivalent to the user's origin). The content of registration is divided into two categories: one is permanent parameters, such as user number, mobile device number, access priority, predetermined service type and confidential parameters; the other is temporary and needs to be updated at any time The parameters, that is, the relevant parameters of the user's current location, even if the user roams outside the area served by the HLR, the HLR must register the location information transmitted by the area. The purpose of this is to ensure that when any mobile user who does not know which region is called, the mobile user's home location register can know which region he is in at that time, and then establish a communication link.
Visiting location registration (VLR) is a database used to store the location information of visiting users. A VLR usually serves one MSC control area, and can also serve several adjacent MSC control areas. When a mobile user roams to a new MSC control area, it must apply for registration with the VLR in that area. The VLR needs to query relevant parameters from the user's HLR, assign a new roaming number (MSRN) to the user, and notify the HLR to modify the user's location information, preparing to provide routing information for other users to call the mobile user. If the mobile user moves from one VLR service area to another VLR service area, the HLR notifies the original VLR after deleting the user's location information, and deletes the mobile user's location information.
The role of the authentication center (AUC) is to reliably identify the user's identity, allowing only authorized users to access the network and obtain services. * Equipment identification register (EIR) is a database that stores mobile station device parameters. It is used to identify and monitor mobile devices and reject non-mobile stations from accessing the network.
The mission of the Operation and Maintenance Center (OMC) is to monitor and operate the entire network, such as system self-test, alarm and activation of backup equipment, system fault diagnosis and processing, traffic statistics and billing data recording and Transmission, and collection, analysis and display of various data.
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