It’s basically an ISDN (International Subscriber Digital Number) Switch, coordinating and setting up calls to and from MSs. It performs the switching functions for all mobile stations located in the geographic area covered by its assigned BSSs.
Functions performed include interfacing with the Public Switched Telephone Network (PSTN) as well as with the other MSC’s and other system entities, such as the HLR, in the PLMN (Public Land Mobile Network).
Functions
• Call handling
• Management of required logical radio-link channel during calls
• Management of MSC-BSS signaling protocol
• Handling location registration and ensuring inter-working between Mobile Station and VLR
• Control of inter-BSS and inter-MSC handovers
• Acting as a gateway MSC to interrogate the HLR.
HLR – Home Location Register
It’s a database used to store permanent and semi-permanent subscriber data. The HLR will always know in which location area the MS is (assuming the MS is in a coverage area), and this data is used to locate an MS in the event of a MS terminating call set-up. It contains the identities of mobile subscribers called IMSIs (International Mobile Subscriber Identities), their service parameters, and their location information.
It contains,
• Identity of mobile subscriber
• ISDN directory number of mobile station
• Subscription information on teleservices and bearer services
• Service restrictions (if any)
• Supplementary services
• Location information for call routing
VLR - Visitor Location Register:
The VLR contains all the subscriber data, both permanent and temporary, which are necessary to control a MS in the MSC’s coverage area. It contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area (i.e., cells) controlled by that VLR.
It contains,
• Identity of mobile subscriber
• Any temporary mobile subscriber identity
• ISDN directory number of mobile
• A directory number to route calls to a roaming station
• Location area where the mobile station is registered
• Copy of (part of) the subscriber data from the HLR
EIR - Equipment Identity Register:
Its database contains information on the MS and its capabilities. The IMEI (International Mobile Station Equipment Identity) is used to interrogate the EIR. It is accessed during the equipment validation procedure when a mobile station accesses the system. It contains the identity of mobile station equipment which may be valid, suspect, or known to be fraudulent.
This contains:
• White or Valid list - List of valid MS equipment identities
• Grey or Monitored list - List of suspected mobiles under observation
• Black or prohibited list - List of mobiles for which service is barred
AuC - Authentication Center:
Its database contains the subscriber authentication keys and the algorithm required to calculate the authentication parameters to be transferred to the HLR.
• Contains subscriber authentication data called Authentication Keys (Ki)
• Generates security related parameters needed to authorize service using Ki
• Generates unique data pattern called a CipherKey (Kc) needed for encrypting user speech and data
OMC - Operations and Maintenance Center:
It is the centralized maintenance and diagnostic heart of the Base Station System (BSS). It allows the network provider to operate, administer, and monitor the functioning of the BSS.
BSS (Base Station Subsystem) - It’s a combination of a BSC and one or more BTSs.
Characteristics of the BSS are:
• It is responsible for communicating with mobile stations in cell areas.
• One BSC controls one or more BTSs and can perform inter-BTS and intra-BTS handovers.
• The BTS serves one or more cells in the cellular network and contains one or more TRXs (Transceivers or radio units).
• The TRX serves full duplex communications to the MS.
• In the GSM network implementation, the BSC includes the TRAU (Transcoder/Rate Adapter Unit). The TRAU adapts the transmission bit rate of the A-interface (64 kbit/s) to the Abis-interface (16 kbit/s).
MS - Mobile Station:
It represents the terminal equipment used by the wireless subscriber supported by the GSM Wireless system. The MS consists of two entities, each with its own identity:
• Subscriber Identity Module (SIM)
• Mobile equipment
The SIM may be a removable module. A subscriber with an appropriate SIM can access the system using various mobile equipments. The equipment identity is not linked to a particular subscriber. Validity checks made on the MS equipment are performed independently of the authentication checks made on the MS subscriber information.
Functions of a SIM:
• Authentication of the validity of the MS when accessing the network
• User authentication
• Storage of subscriber-related information, which can be: data fixed during administrative phase (e.g., subscriber identification), and temporary network data (e.g., cell location identity).
Functions of a Mobile Station:
• Radio transmission termination
• Radio channel management
• Speech encoding/decoding
• Radio link error protection
• Flow control of data
• Rate adaptation of user data to the radio link
• Mobility management
• Performance measurements of radio link
Types of Mobile Stations:
Mobile stations can come in different power classes, which define the maximum RF power level that the unit can transmit. For GSM-900 there is five powers classes, for GSM-1800 there are three power classes. The mobile station maximum output power is specified in GSM.
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