GSM: The Network and Switching Subsystem

The Network and Switching Subsystem
Its main role is to manage the communications between the mobile users and other users, such as mobile users, ISDN users, fixed telephony users, etc. It also includes data bases needed in order to store information about the subscribers and to manage their mobility. The different components of the NSS are described below.

Mobile Service Switching Center (MSC)
It is the central component of the NSS. The MSC performs the switching functions of the network. It also provides connection to other networks.

The switching point for PCM data from the cellular network to the Public Switching Telephone Network (PSTN). The basic purpose is to route calls hunting free circuits, making connection and releasing them.

Visitor Location Register (VLR)
The VLR contains information from a subscriber's HLR necessary in order to provide the subscribed services to visiting users. When a subscriber enters the covering area of a new MSC, the VLR associated to this MSC will request information about the new subscriber to its corresponding HLR. The VLR will then have enough information in order to assure the subscribed services without needing to ask the HLR each time a communication is established.

The VLR is always implemented together with a MSC; so the area under control of the MSC is also the area under control of the VLR.

Home Location Register (HLR)
The HLR is considered as a very important database that stores information of the subscribers belonging to the covering area of a MSC. It also stores the current location of these subscribers and the services to which they have access. The location of the subscriber corresponds to the SS7 address of the Visitor Location Register (VLR) associated to the terminal.

Gateway Mobile services Switching Center (GMSC)
A gateway is a node interconnecting two networks. The GMSC is the interface between the mobile cellular network and the PSTN. It is in charge of routing calls from the fixed network towards a GSM user. The GMSC is often implemented in the same machines as the MSC.

The Authentication Center (AC)
The AC register is used for security purposes. It provides the parameters needed for authentication and encryption functions. These parameters help to verify the user's identity.

The Equipment Identity Register (EIR)
The EIR is also used for security purposes. It is a register containing information about the mobile equipments. More particularly, it contains a list of all valid terminals. A terminal is identified by its International Mobile Equipment Identity (IMEI). The EIR allows then to forbid calls from stolen or unauthorized terminals (e.g., a terminal which does not respect the specifications concerning the output RF power).

The GSM Interworking Unit (GIWU)
The GIWU corresponds to an interface to various networks for data communications. During these communications, the transmission of speech and data can be alternated.


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