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GSM System Architecture in Telecommunication System

Dec, 29 2020, 11:35 am [IST]
GSM System Architecture in Telecommunication System

System architecture

As with whole systems in the telecommunication domain, GSM arrives with a hierarchical, Complex system architecture concentrating on many entities, interfaces, and acronyms. A GSM system consists of three subsystems.

  • Radio Subsystem (RSS)
  • Network and Switching Subsystem (NSS), and 
  • Operations Subsystem (OSS)

Each subsystem will be explained in more detail in the following subsections.

Generally, a GSM Customer only notices a very small fraction of the whole network - the mobile station MS and some antenna Masts of the base transceiver stations (BTS).

Radio station

As the name implies, the radio subsystem comprises all radio specific entities, i.e., the mobile station and the base station subsystem.

  • Base station subsystem (BSS): A GSM network comprises many BSSs, each managed by a base station controller (BSC). The BSS performs all functions required to control radio connections to MS, coding and decoding of voice, and rate adaptation from the wireless network part. Besides a BSC, The BSS contains many BTSs.
  • Base transceiver station (BTS): A BTS comprises all radio equipment, i.e., antennas, signal processing, amplifiers necessary for radio transmission. A BTS can form a radio cell or, using sectorized antennas, several cells, and is connected to MS  via the Um interface, and to the BSC via Abis interface. The Um interface contains all mechanisms necessary for wireless transmission.
  • Base station controller (BSC): The BSC manages the BTSs. It reserves radio frequencies, handles the handover from one BTS to another within the BSS, and performs paging of the MS. The BSC also multiplexes the radio channels onto the fixed network connections at the A interface.
  • The mobile station (MS): The MS comprises all user equipment and software required for communication with a GSM network. An MS consist of user-independent hard and software and of the subscriber identity module (SIM), which stores all user-specific data. While an MS can be identified by the international mobile equipment identity (IMEI), a user can personalize any MS using his SIM, i.e., user-specific devices like editing and authentication are based on the SIM, not on the device. Device-specific mechanisms, e.g., theft protection, use the device-specific EI. Typical MSs for GSM 900 Own A transmit power of up to 2 W, whereas for GSM 1800 1 W is enough due to the smaller cell size. Apart from the telephone interface, an MS can also offer other types of interfaces to users with the display, loudspeaker, microphone, and Programmable soft keys.

Network and switching subsystem

The heart of the GSM system is formed by the network and switching subsystem (NSS). The NSS connects the wireless network with standard public networks, performs handovers between different BSSs, comprises functions for worldwide localization of users, and supports changing accounting, and roaming of users between different providers in different countries. The NSS consists of the following switches and databases:

  • Mobile services switching center (MSC): MSCs are high-performance digital ISDN switches. They set up links to other MSCs and the BSCs by the A interface. MSCs thus form the fixed backbone network of a GSM system. Typically, an MSC controls several BSCs of a geographical region.
  • Home location register (HLR): the HLR is is the most important database in a GSM system as its stores are user-relevant information. This comprises static transformation such as the mobile subscriber ISDN number, subscriber services, and the authentication key Ki. Furthermore, dynamic information is needed, e.g., the current location area (LA) of the MS. As soon as an MS leaves its current LA, the information in the HLR is updated. This information is necessary to localize a user in the worldwide GSM networks. All these user-specific information elements only exist once for each user in a single HLR, which also supports charging and accounting.
  • Visitor location register (VLR): the VLR associated with each MSC is a very dynamic database that stores all important information needed for the MS users currently in the LA that is associated with the MSC. If a new MS comes into an LA the VLR is responsible for, it copies all relevant information for this user from the HLR.

Operation subsystem

The third-party of a GSM system, the operation subsystem, contains all functions necessary for network operation and maintenance. The OSS possesses network entities of its own and accesses other entities via SS7 signaling.

  • Operation and maintenance center (OMC): The OMC monitors and controls all other network entities via the O interface. Typical OMC management functions are traffic monitoring, status reports of network entities, subscriber and security Management, or accounting and billing.
  • Authentication Center (AUC): As the radio interface and mobile stations are particularly vulnerable, a separate AuC has been defined to protect user identity and data transmission. The AUC contains the algorithms for authentication as well as the keys for encryption and generates the values needed for user authentication in the HLR.
  • Equipment identity register (EIR): the EIR is a database for all IMESs, i.e., it stores all device identifications registered for this network. As MSs are mobile, they can be easily stolen. With a valid SIM of their own, anyone could use the stolen MS. Thus, the EIR has a blacklist of stolen devices. An MS is useless as soon as the owner has reported a theft.

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