The basic architecture of the GSM system involves four main subsystems each of which contains a certain number of functional units and is interconnected with the others through a series of standard interfaces which will be described later. The main subsystems of the GSM network and the elements composing them are:

• MS (Mobile Station)
  • ME (Mobile Equipment)
  • SIM (Subscriber Identity Module)
• BSS (Base Station Sub-System)
  • BSC (Base Station Controller)
  • BTS (Base Transceiver Station)
• NSS (Network Sub-System)
  • MSC (Mobile Switching Center)
  • HLR (Home Location Register)
  • VLR (Visitor Location Register)
  • AUC >(Authentication Center)
  • EIR (Equipment Identity Register)
• NMC (Network Management Center)
  • OMC (Operation and Maintenance Center)

 

The Mobile Station is a radiomobile terminal carried by the subscriber/licence holder. The Base Station Sub-System controls the radio link with the MS. The Network Sub-System carries out the commutation of the calls between the mobile phone network and the fixed public network and also supervises the movements of the subscriber. From the Network Management Center it is possible to control all the current operations, and also to carry out the set-up of the network. A description of the above units follows.

Mobile Station

The Mobile Station is composed of the Mobile Equipment (the GSM terminal) and of the Subscriber Identity Module (SIM), a smart card with a memory and a microprocessor that allows the identification of the subscriber independently of the terminal used. It is therefore possible to continue to receive and make calls and to use all the services described below by inserting the SIM card into a terminal that is not one’s own.

Mobile Equipment

The Mobile Equipment is unequivocally identified inside any GSM network by the International Mobile Equipment Identity (IMEI)

The IMEI is a 15-digit number structured in the following way:

IMEI = TAC/ FAC/ SNR/ sp

TAC = Type Approval Code, determined by the main body of the GSM (6 digits)
FAC = Final Assembly Code, identifies the maker (2 digits)
SNR = Serial Number (6 digits)
sp = A spare supplementary number (1digit)

GSM terminals are divided into 5 classes depending on the maximum strength at which they can transmit through a radio channel ranging from a maximum of 20 watts to a minimum of 0.8 watts.

CLASS	MAXIMUM	TYPE
1	20	VEHICLE
2	8	PORTABLE
3	5	HANDHELD
4	2	HANDHELD
5	0.8	HANDHELD

Classes of MS

The strength of the MS determines the potential of the latter to distance itself from the transmitting and receiving stations (BTS) of the network while continuing to be able to use the service.

One of the peculiarities of the MS is its ability to dynamically vary the strength of the emission of the signal through a radio channel on 18 levels, enabling it to maintain the strength of the transmission at a constantly optimal level. This limits co-channel interferences induced by adjacent cells and terminal consumption. These last two aspects are enhanced by Discontinuous Transmit (DTX) which stops the transmission when the user stops speaking thanks to the Voice Activity Detection (VAD) function which verifies the presence or absence of vocal activity. Any increase or decrease in the strength of the transmitted signal is notified to the MS by the BSS, which constantly monitors communication quality.

SIM

The SIM card contains: the International Mobile Subscriber Identity (IMSI) used to identify the subscriber within any GSM network; the encrypting procedures which guarantee the confidentiality of user information; other information, for example, phone alphanumeric memories and memories for SMS messages; and lastly the password to prevent unauthorised use of the card itself and access to other services.

The IMSI has the following:
MCC / MNC / MSIN

where
MCC = Mobile Country Code (2 or 3 digits, 44 for the United Kingdom)
MNC = Mobile Network Code (2 digits)
MSIN = Mobile Station Identification Number (max 13 digits)

Base Station Subsystem

The BSS controls the radio interface. It is made up of one or more Base Transceiver Station/s (BTS) and of a Base Station Controller (BSC). These elements communicate with each other by means of standard type A-bis interfaces, in order to allow operations between components built by different makers. Moreover, the BSC is connected to the Mobile Switching Centre (MSC) through a type A interface.

Base Transceiver Station

The Base Transceiver Station houses all the transceivers serving a cell and their role is to receive and transmit information through a radio channel, providing a physical interface between the MS and the BSC. The BTS has the following functions:

Ability to manage Full Rate and Half Rate channels

The management of Antenna Diversity, in other words the use of two receiving antennas to improve the quality of the received signal; the two antennas receive the same signal independently and are affected by fading in a different way: the probability of both being affected by deep fading is very small.

Supervision of Stationary Wave Report (SWR) on the antenna.

Frequency Hopping (FH): changes of the frequency used on a radio channel at regular intervals, in order to improve the quality of the service through frequency diversity.

Discontinuous Transmission (DTX) both in the up-link and in the down-link.

Dynamic Potency Control (DPC) of the MS and BTS: the BSC determines the optimal potency at which the MS and BTS must transmit through the radio channel (making use of the measurements carried out by the MS and BTS), to improve spectral efficiency.

The administration of encryption algorithms: user information is encrypted to ensure the user a certain amount of confidentiality on the radio and signal channels. The encryption process must be carried out, on the information transmitted through radio channels, by the BTS; the BSC tells the BTS which encryption algorithm to use on the basis of the information received from the MSC, and the key to the encryption is unique to each user. The GSM standard currently supports 8 encryption algorithms.

The monitoring of the radio link through measurements of the RF signals, the measurements are then sent to the BSC to be elaborated in order to ensure a high quality level connection

Base Station Controller

The Base Station Controller governs the radio resources for one or more BTSs, controlling the connection between the BTSs and the MSCs (communication centres which provide the connection to the fixed public network and other networks), and also administrating radio channels, signalling, frequency hopping and handovers. In particular it permits:

The control and configuration of the radio channel: it must choose the correct cell for every call and inside the cell it must select the appropriate radio channel and initiate the connection.

The control of handovers: on the basis of the measurements received from the BTS, it must decide when to effect the handover i.e. the passage from cell to cell while the user is moving inside the covered area during a conversation.

The functions of transforming the Full Rate (16 KBPS) or Half Rate (8 KBPS) radio channels into 64 KBPS channels.

Network Sub-System

The Network Sub-System handles the functions of commuting the connection to other fixed public network or mobile network subscribers through the MSC and database functions, distributed through 4 discerning nodes (HLR, VLR, AUC, EIR) for the identification of the terminals and their users, the monitoring of their positions, authentication and the routing of calls to roaming terminals.

Mobile Switching Centre

The Mobile Switching Centre is the core element of the NSS. By means of the information acquired from the HLR and VLR, it handles the routing and control of the signals from direct calls and incoming calls from various types of networks, such as PSTN, ISDN, PLMN and PDN. It also implements gateway functions with other system components and has controlling functions over the handover processes, commuting calls between different BSCs or towards another MSC.

Several MSCs can be present within the service area and each is responsible for the control of the traffic of one or more BBSs. Seeing as the users move throughout the area of coverage in order to guarantee a constant level of service for each user, the MSCs must be capable of controlling users who vary in type and number.

Other fundamental functions of the MSC are the following:

Authentication of the caller: the identification of the MS calling is necessary in order to determine whether the caller qualifies for use of the service.

Confidentiality about the user’s identity: in order to guarantee the confidentiality of the user’s identity on the radio channel, even if all the information is already encrypted, the system never transmits the IMSI assigned the user at the moment of subscription. A Temporary Mobile Subscriber Identity (TMSI) which has a temporary significance is assigned on calling. The MSC function is to create a link between the TMSI and the IMSI, and whenever the mobile unit moves into a location area controlled by another MSC, another TMSI must be assigned.

The handover process: in a GSM network the user can continue using the service even when, in the process of conversing, he crosses the limit of the cell area in which he finds himself. There are two cases:

1. The MS moves into a cell controlled by the same MSC; in this case the handover process is controlled by the same MSC.

2. The new cell area in which the user moves is controlled by another MSC; in this case the two MSCs together, using the signal measurements supplied by the BTS that receive the MS, carry out the handover process.

Home Location Register

When someone opens a new subscription to the GSM network, all the information for his identification is memorised in the HLR. The HLR has the function of communicating some data relative to the subscriber to the VLR whenever the subscriber moves from one location area into another. Within the HLR, subscribers are identified in the following manner:

MSISDN = CC / NDC / SN

where
CC = Country Code, international prefix (the CC for the UK is 44)
NDC = National Destination Code, the national prefix of the subscriber minus the zero
SN = Subscriber Number, the number that identifies the mobile phone user.

The Home Location Register is a database. There can be just one for the whole network or there can be several distributed throughout the network. Therefore, there can be MSCs without HLR connected to HLRs belonging to other MSCs. When several HLRs are present, a numerical area, in other words a set of Mobile Station ISDN Number (MSISDN), is assigned to each one. The MSISDN unequivocally identifies a mobile phone subscription in the numerical plan of the commuted public international phone network.

The HLR, as all the other databases we will look at, is implemented on a work station whose capacities (memory, processor, disc capacity) can be updated in case of an increase in the number of subscribers. It contains all subscriber information, particularly the following:

Permanent types of information:

§ The International Mobile Subscriber Identity (IMSI) which unequivocally identifies the subscriber within any GSM network and is present on the SIM card.

§ The Mobile Station ISDN Number (MSISDN).

§ The type of services to which the subscriber has access (voice, data service, SMS, blockage of international calls, other auxiliary services).

Dynamic types of information:

§ Current position of the MS, i.e. the address of the VLR in which it is registered.

§ The state of auxiliary services

In summary, therefore, the functions of the HLR are:

Security: dialogue with the AUC and the VLR.

Position recording: dialogue with VLR.

Cost of the call: dialogue with VLR.

Management of subscriber information: dialogue with OMC and VLR.

Administration of statistical data: the data gathered is sent to the OMC.

· Visitor Location Register

The Visitor Location Register is a database that temporarily memorises the data of all the subscribers present in the geographical area under its control. These data are requested from the HLR to which the subscriber belongs. Usually, to simplify the requested signals and the system structure, the engineers implement the VLR and the MSC together so that the geographical area controlled by the MSC corresponds to that controlled by the VLR.

In detail, the information it contains is the following:

Temporary Mobile Subscriber Identity (TMSI): used to guarantee the security of the IMSI, it is assigned every time the LA is changed.

State of the MS (standby, engaged, switched off)

The state of supplementary services e.g. Call Waiting, Call Divert, Call Barring, etc.

The kind of services the user subscribes to and to which he has access (voice, data service, SMS, other auxiliary services).

The Location Area Identity (LAI) in which the MS is to be found within those controlled by the MSC/VLR.

Authentication Centre

The Authentication Centre is a function of the system that verifies if a legitimate subscriber has requested a service. It provides the codes for both authentication and encryption, in order to safeguard both the subscriber and the network operator against undesired violations of the system by third parties.

The authentication mechanism ascertains the legitimacy of the SIM without transmitting the personal details of the subscriber, such as IMSI and encryption keys through the radio channel. In order to verify whether the subscriber who is attempting to access the system is the real one and not a clone. The encryption mechanism, on the other hand, generates secret codes, which are used to encrypt the entire communication exchanged through the radio channel. The authentication and encryption codes are generated randomly for every single subscriber by using sets of algorithms defined by the standard and the codes are memorised on the AUC and also on the SIM.

The authentication takes place every time the MS connects to the network and more precisely in the following cases:

Every time the MS makes or receives a call.

Every time that the location of the MS is updated.

Every time that the activation, deactivation and interrogation of the supplementary services is requested.

The AUC can be implemented, as an additional application, on the same workstation used for the HLR, which is the only element of the system with which it interfaces and dialogues. Furthermore, it cannot be handled remotely because of security reasons.

Equipment Identity Register

The Equipment Identity Register is a database that verifies whether a Mobile Equipment (ME) is authorised to access the system. This database is divided into 3 sections:

White List: containing all the IMEI assigned to all the operators of the various nations with which international roaming agreements have been stipulated.

Black List: containing all the IMEI that have been stopped (for example: stolen ones).

Grey List: contains all the IMEI labelled as faulty or those connected to non-approved devices. The terminals included in the Grey List are signalled to the system operators by means of an alarm if they try to access the system. This permits the identification of the subscriber using the terminal and the call area in which the terminal is to be found.

Every time the MS attempts to connect to the network, the MSC checks for the existence of one of the following cases through the EIR, in order to allow or deny access:

Whether the terminal is of a type approved for use with the GSM network

Whether the terminal has been stolen or is been used in a fraudulent manner

Whether the terminal has been signalled as faulty

There can be just one EIR for the entire system or it can be distributed throughout the whole system. It can be present on the same workstation as the HLR and AUC, but it is usually preferable, for security reasons, to place it on a separate workstation. It can be remotely accessed in order to allow the updating of the various lists contained in it from any point of the network. In the future, a connection between all the EIRs of the various GSM operators is planned, in order to prevent the use of stolen devices in nations different from the one in which the device was stolen.

Network Management Centre

Operation and Maintenance Centre

The Operation and Maintenance Centre has the following functions:

Remote access to all the components of the GSM network (BSS, MSC, VLR, HLR, EIR and AUC).

The management of alarms and of the state of the system, which makes it possible to conduct various types of tests and to analyse the performance and verify the correct functioning of the system.

The gathering of all the information relating to subscriber traffic necessary for invoicing.

The supervision of the flow of traffic through the stations and the introduction of changes to the flow itself.

The visualisation of the configuration of the network with the possibility of changing the configuration remotely.

Administration of subscribers and the possibility of being able to locate their position inside the area of coverage.

More than one OMC can be present in large systems. In this case, there will be a general OMC (OMC-N) from which it is possible to control everything and other OMC which have control over just a few zones (OMC-R).

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