- The Mobile Station (MS)
- The Base Station Subsystem (BSS)
- The Network Switching Subsystem (NSS)
- The Operation Support Subsystem (OSS)
The additional components of the GSM architecture comprise of databases and messaging systems functions:
- Home Location Register (HLR)
- Visitor Location Register (VLR)
- Equipment Identity Register (EIR)
- Authentication Center (AuC)
- SMS Serving Center (SMS SC)
- Gateway MSC (GMSC)
- Chargeback Center (CBC)
- Transcoder and Adaptation Unit (TRAU)
The MS and the BSS communicate across the Um interface. It is also known as the air interface or the radio link. The BSS communicates with the Network Service Switching (NSS) center across the A interface.
GSM network areas
In a GSM network, the following areas are defined:- Cell : Cell is the basic service area; one BTS covers one cell. Each cell is given a Cell Global Identity (CGI), a number that uniquely identifies the cell.
- Location Area : A group of cells form a Location Area (LA). This is the area that is paged when a subscriber gets an incoming call. Each LA is assigned a Location Area Identity (LAI). Each LA is served by one or more BSCs.
- MSC/VLR Service Area : The area covered by one MSC is called the MSC/VLR service area.
- PLMN : The area covered by one network operator is called the Public Land Mobile Network (PLMN). A PLMN can contain one or more MSCs.GSM
1.) Mobile Station
The mobile station
(MS) consists of the mobile equipment (the terminal) and a smart card called
the Subscriber Identity Module (SIM). The SIM provides personal mobility, so
that the user can have access to subscribed services irrespective of a specific
terminal. By inserting the SIM card into another GSM terminal, the user is able
to receive calls at that terminal, make calls from that terminal, and receive
other subscribed services.
The mobile equipment is uniquely identified by the
International Mobile Equipment Identity (IMEI). The SIM card contains the
International Mobile Subscriber Identity (IMSI) used to identify the subscriber
to the system, a secret key for authentication, and other information. The IMEI
and the IMSI are independent, thereby allowing personal mobility. The SIM card
may be protected against unauthorized use by a password or personal identity
number.
2.) Mobile
Switching Centre
The central component
of the Network Subsystem is the Mobile
services Switching Center (MSC). It acts like a normal switching node of the
PSTN or ISDN, and additionally provides all the functionality needed to handle
a mobile subscriber, such as registration, authentication, location updating,
handovers, and call routing to a roaming subscriber. These services are
provided in conjuction with several functional
entities, which together form the Network Subsystem. The MSC provides the
connection to the fixed networks (such as the PSTN or ISDN). Signalling between functional entities in the Network
Subsystem uses Signalling System Number 7 (SS7), used
for trunk signalling in ISDN and widely used in
current public networks.
The Home Location Register (HLR) and Visitor Location Register (VLR),
together with the MSC, provide the call-routing and roaming capabilities of
GSM. The HLR contains all the administrative information of each subscriber
registered in the corresponding GSM network, along with the current location of
the mobile. The location of the mobile is typically in the form of the signalling address of the VLR associated with the mobile
station. The actual routing procedure will be described later. There is
logically one HLR per GSM network, although it may be implemented as a
distributed database. The Visitor Location Register (VLR) contains selected administrative information from the HLR, necessary for call control and provision of the subscribed services, for each mobile currently located in the geographical area controlled by the VLR. Although each functional entity can be implemented as an independent unit, all manufacturers of switching equipment to date implement the VLR together with the MSC, so that the geographical area controlled by the MSC corresponds to that controlled by the VLR, thus simplifying the signalling required. Note that the MSC contains no information about particular mobile stations --- this information is stored in the location registers.
The other two registers are used for authentication and security purposes. The Equipment Identity Register (EIR) is a database that contains a list of all valid mobile equipment on the network, where each mobile station is identified by its International Mobile Equipment Identity (IMEI). An IMEI is marked as invalid if it has been reported stolen or is not type approved. The Authentication Center (AuC) is a protected database that stores a copy of the secret key stored in each subscriber's SIM card, which is used for authentication and encryption over the radio channel.
3.) Base Station Subsystem (BSS)
The Base Station Subsystem is composed of two parts, the
Base Transceiver Station (BTS) and the Base Station Controller (BSC). These
communicate across the standardized Abis interface,
allowing (as in the rest of the system) operation between components made by
different suppliers.
The Base Transceiver Station houses the radio tranceivers that define a cell and handles the radio-link
protocols with the Mobile Station. In a large urban area, there will
potentially be a large number of BTSs deployed, thus
the requirements for a BTS are ruggedness, reliability, portability, and
minimum cost.
The Base Station Controller manages the radio resources for one or more BTSs. It handles radio-channel setup, frequency hopping,
and handovers, as described below. The BSC is the connection between the mobile
station and the Mobile service Switching Center (MSC). 