GSM System Architecture

A Public Land Mobile Network (PLMN), includes the following system entities:

· Mobile-services Switching Center (MSC)
· Home Location Register (HLR)
· Visitor Location Register (VLR)
· Equipment Identity Register (EIR)
· Authentication Center (AUC)
· Base Station System (BSS)
- Base Transceiver Station (BTS)
- Base Station Controller (BSC)
· Mobile Station (MS)
· Operation and Maintenance Center (OMC)
· Other Network Elements

Mobile-services Switching Center:
Mobile-services Switching Center (MSC) 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 MSCs and other system entities, such
as the HLR, in the PLMN.
Functions of the MSC include:
· Call handling that copes with mobile nature of subscribers (e.g., paging)
· Management of required logical radio-link channel during calls
· Management of MSC-BSS signaling protocol
· Handling location registration and ensuring interworking between Mobile Station and VLR
· Control of inter-BSS and inter-MSC handovers
· Acting as a gateway MSC to interrogate the HLR
· Exchange of signaling information with other system entities
· Standard functions of a local exchange switch in the fixed network (example: charging)

Home Location Register:
The Home Location Register (HLR) contains the identities of mobile subscribers
(called International Mobile Subscriber Identities or IMSIs), their service
parameters, and their location information.
In summary, the HLR 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

Visitor Location Register:
The Visitor Location Register (VLR) contains the subscriber parameters and
location information for all mobile subscribers currently located in the
geographical area (i.e., cells) controlled by that VLR.
In summary, the VLR 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

Equipment Identity Register:
The Equipment Identity Register (EIR) is accessed during the equipment
validation procedure when a mobile station accesses the system. It contains the
identity of mobile station equipment (called International Mobile Station
Equipment Identity or IMEI) 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

Authentication Center:
The Authentication Center (AUC):
· 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

Operations and Maintenance Center:
The Operations and Maintenance Center (OMC) 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.

Characteristics of the Base Station System (BSS) are:
· The BSS 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 of Lucent Technologies 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).

Note that in the GSM network implementation of Lucent Technologies, the interface between the BSC and the TRAU is known as the M-interface (which is a proprietary interface).

BSS types are differentiated by the following characteristics:

· The BSS can be an integrated (Intg) BSS or a distributed (Dist) BSS.
An integrated BSS is a BSS, which has the BSC, and BTS functionality located in the same physical unit. In a distributed BSS, the BTS and BSC are physically separated.

· The BSS can have internally (Int) or externally (Ext) located speech transcoding.
Speech transcoding to 64 kbit/s takes place either in the BSC for BSS types 1, 4 and 5, or external to the BSS (i.e. the transcoder is co-located with the MSC) for BSS types 2, 6 and 7. For BSS type 3 transcoding takes place in the BTSs.

· The Abis interface uses multiplexing (Mult) or rate adaptation (RA) on its links.
The first option means that four 16 kbit/s links are multiplexed into one 64 kbit/s channel. The latter option means that no multiplexing of the 16 kbit/s channels takes place.

Description of Mobile Station:

The Mobile Station (MS) 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:

The functions of the Subscriber Identity Module (SIM) are:
· 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:

The Mobile Station (MS) performs the following:
· 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.

Other network elements:

Other optional network elements that the MSC can interface include:

· Billing Center: Each MSC writes call accounting records to local disk memory. The Billing Center periodically polls the disk records of each MSC to collect the billing data for the PLMN.
· Service Center: The Service Center interfaces with the MSCs to provide special services, such as the Short Message Service (SMS), to mobile subscribers in the PLMN.

The Billing Center and Service Center are not a basic part of the GSM system.