GSM NETWORK SWITCHING SYSTEM
GSM is a complex system providing many facilities. It performs a number of complicated tasks including switching/connection of mobile subscribers, connection /management of subscribers to the PSTN, interrogation and collation of the user database and the general management of the location registers for the system — in addition to the task of collecting call billing data.
To achieve these complicated tasks the Network Switching System is broken down into the following major components:
· Gateway Mobile Switching Centre (GMSC)
· Mobile Switching Centre (MSC)
· Home Location Register (HLR)
· Visitor Location Register (VLR)
· Authentication Register (AuC)
· Equipment Identity Register (EIR)
Gateway Mobile Switching Centre (GMSC)
The Gateway Mobile Switching Centre (GMSC) is the point to which a MS terminating call is initially routed, without any knowledge of the MS’s location. The GMSC is thus responsible to obtain the Mobile Station Roaming Number (MSRN) from the HLR based on the Mobile Station ISDN number (MSISDN), the “directory number” of a MS, and route the call to the correct visited MSC. The MSC part of GMSC however is misleading, since the gateway operation does not function as an “MSC” but purely acts as the interface between the parent mobile network and other systems such as other mobile networks and PSTN.
Mobile Switching Centre (MSC)
The Mobile Switching Centres (MSC) provide all the call switching for the system in the same way a standard telephone exchange would, and to give some idea of the capability of each unit — one MSC can support a community of approximately one million inhabitants.
The system would normally support a number of MSC’s and the geographical location of each MSC will reflect the function of the individual Switch. It is the service provider, who decides on the functionality of the switch, however, all MSC’s are capable of the following:
Call Processing — Includes control of data/voice call set-up, inter-BSS/inter-MSC handovers and control of mobility management (subscriber validation and location).
Operations and Maintenance Support — Includes database management, traffic metering and measurement, and a man-machine interface.
Internet work Inter working (Gateway) — Manages the interface between the GSM
Network and PSTN.
Billing — Collects call billing data.
Home Location Register (HLR)
The Home Location Register (HLR) is controlled and updated by the service provider. Its function is to hold a subscriber profile for all subscribers held a GSM Public Land Mobile Network (PLMN). The subscriber database is decentralised, as a protection measure, with a number of HLRs operating within the system, however, only one copy of each subscriber profile is held, allowing each HLR to handle only a portion of the total database.
Although the service provider manages the database, it is the MSC and VLRs that normally perform interrogation, gaining access via the International Mobile Subscriber Identity (IMSI) or the Mobile Station International Services Digital Network (MSISDN). Each subscriber profile contains the following information:
· Subscriber Identity (IMSI and MSISDN)
· Current subscriber VLR (current locations)
· Supplementary services (if activated)
· Supplementary service information (i.e. current forwarding number, call barring)
· Subscriber status (registered/de-registered)
· Authentication key and Authentication Centre (AUC) functionality
· Mobile Subscriber Roaming Number (MSRN)
For Roaming purposes, MSCs and VLRs from other networks will interrogate data to allow inter-system and inter-country roaming. This interrogation occurs once during affiliation to the foreign or additional network, however, after a given period (decided by the service provider) the information will be timed out and re-requested via the MSCs and VLRs.
Visitor Location Register (VLR)
Unlike the HLR (updated by the service provider/system) the Visitor Location Register (VLR) gains its information purely from the system. This information is drawn from:
The “home” network HLR and HLRs of a roamed network.
When required, a temporary subscriber profile is generated with the following additional data added:
· Mobile status (busy/free/no answer etc)
· Location Area Identity (LAI)
· Temporary Mobile Subscriber Identity (TMSI)
· Mobile Station Roaming Number (MSRN)
It is appreciated that a large amount of time can be used updating information across the system or a number of systems. The VLRs ability to create temporary duplicate subscriber profile, with additional data, cuts the need for constant interrogation and therefore unnecessary “clogging” of the system, an important issue in the management of a busy system. The number of VLRs contained within a system will depend on the size and geographical area of the network; however, a VLR is normally found at each MSC.
Location Area Identity (LAI)
Cells within the PLMN are grouped together into geographical areas. Each area is assigned a Location Area Identity (LAI), a location area may typically contain 30 cells. Each VLR controls a number of LAIs and as a subscriber moves from one VLR to another, the VLR address is updated at the HLR.
Temporary Mobile Subscriber Identity (TMSI)
A Temporary Mobile Subscriber Identity (TMSI) is an MSC-VLR specific alias that is designed to maintain user confidentiality. It is assigned only after successful subscriber authentication. Under certain conditions (such as traffic system disruption and malfunctioning of the system), the MSC can direct individual TMSI’s to provide the MSC with their IMSI. In all cases, the ME will always respond to IMSI even if a TMSI has been assigned. VLR must be capable of correlating the IMSI of an MS with its current TMSI.
Since the TMSI has only local significance (that is, within the VLR and the area controlled by the VLR, the structure of this can be chosen by each administration in order to meet local needs. In order to avoid a double allocation of TMSI’s after a restart of a VLR, some part of the TMSI may be related to time. The TMSI is stored on the SIM card and in the VLR and it consists of 4 Octets maximum, which are determined by the service provider.
Mobile Subscriber Roaming Number (MSRN)
The MSRN is allocated on a temporary basis when the MS roams into another numbering area. The MSRN is numbered used by the HLR for re-routing calls to the MS. It is assigned upon demand by the HLR on a per-call basis. The MSRN for PSTN/ISDN routing shall have the same structure as international ISDN numbers in the area in which the MSRN is allocated. The HLR knows in what MSC/VLR service area the subscriber is located. At the reception of the MSRN, HLR sends it to the Gateway MSC (GMSC), which can now route the call to the MSCNLR exchange where the called subscriber is currently registered.
Equipment Identity Register (EIR)
The Equipment Identity Register (EIR) is a centralised database of all ME registered with the service provider. To ensure all networks remain fully informed EIRs could be interrogated by any system, irrespective of the service provider, via the MSC, with validation of the ME being achieved by successful authentication of the IMEI against the database held in the EIR.
The database splits the IMEIs into 3 categories:
· White List — Contains those IMEIs associated with approved MSs.
· Black List — Contains IMEIs of MS that have been reported stolen or which are to be denied service for some other reason (server software fault).
· Grey List — Contains IMEIs of MS that have problems (faulty software or all out going calls barred). These are not, however, sufficiently significant to warrant a “black listing”.
To reduce interrogation time, blocks of IMEIs can be allocated to a number of EIRs located around the network. In turn the MSC contains a translation facility, which when given an IMEI, returns the address of the EIR controlling the appropriate section of the equipment database.
International Mobile Station Equipment Identity (IMEI)
The International Mobile Station Equipment Identity (IMEI) is the unique identity of the equipment used by a subscriber by each PLMN and is used to authorised (white), unauthorised (black), and malfunctioning (grey) GSM hardware. In conjunction with the International Mobile Subscriber Identity (IMSI), it is used to ensure that only authorised users are granted access to the system.
An IMEI is sent in clear during the initial call and is store on the SIM card and in the EIR.
The IMEI consists of Type Approval Code (TAC) + Final Assembly Code (FAC) + Serial Number (SN).
International Mobile Subscriber Identity (IMSI)
An International Mobile Subscriber Identity (IMSI) is assigned to each authorised GSM user.
It consists of a Mobile Country Code (MCC), Mobile Network Code (MNC), and a Public
Land Mobile Network (PLMN) unique Mobile Subscriber Identification Number (MSIN).
The IMSI is the absolute identity of the GSM subscriber it is not hardware-specific, however it is stored on the SIM card and consists of the following:
Mobile Country Code (MCC) + Mobile Network Code (MNC) + Mobile Station Identification Number (MSIN)
MCC - Mobile Country Code — The part of the mobile station identification that uniquely identifies the country of domicile of the mobile subscriber. It consists of 3 digits.
MNC - Mobile Network Code — It uniquely identifies the network of the mobile subscriber. It consists of 3 digits.
MSIN - Mobile Station Identification Number — It uniquely identifies the mobile subscriber within a network. The first 3 digits identify the HLR-ID of the mobile subscriber. It consists of no more than 9 digits.
NMSI - National Mobile Station Identity — The NMSI consists of the MNC followed by the MSIN and is assigned by the service provider.
Location Area Identity (LAI)
For location purposes the network is divided into a number of location areas. Each of these areas consists of a number of cells. A Location Area Identity (LAI) provides an unambiguous identification of location areas within the GSM Network.
Location Area Identity (LAI) = Mobile Country Code (MCC)+Mobile
Network Code (MNC)+Location Area Code (LAC)
MCC Mobile Country Code — Identifies the country in which the network is located. Thus the value of the MCC is the same as the 3-digit MCC contained in the IMSI of any subscriber belonging to the country presently visited by the mobile station.
MNC Mobile Network Code — Is a code
identifying the network in that country. Thus the value of the MNC is the same
as the 3-digit MNC contained in the IMSI of any subscriber belonging to the
country presently visited by the mobile station.
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