GSM: Network Architecture

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The GSM network can be divided into four main parts:

• The Mobile Station (MS).
• The Base Station Subsystem (BSS).
• The Network and Switching Subsystem (NSS).
• The Operation and Support Subsystem (OSS).

Mobile Station

A Mobile Station consists of two main elements:

• The Subscriber Identity Module (SIM): It is protected by a four-digit Personal Identification Number (PIN). In order to identify the subscriber to the system, the SIM card contains amongst others a unique International Mobile Subscriber Identity (IMSI). User mobility is provided through maping the subscriber to the SIM card rather than the terminal as we done in past cellular systems.

• Mobile equipment/terminal (ME): There are different types of terminals (MN) distinguished principally by their power and application:

• `fixed' terminals mainly installed in cars. Their maximum allowed output power is 20W

• portable terminals can also be installed in vehicles. Their maximum allowed output power is 8W.
• handheld terminals; their popularity is owed to their weight and volume, which is continuously decreasing. According to some specification these terminals may emit up to 0.8W. However, as technology has evolved their maximum allowed power ouput is limited to 0.1W.

The BSS provides the interface between the ME and the NSS. It is in charge of the transmission and reception. It may be divided into two parts:

• Base Station Controller (BSC): It controls a group of BTSs and manages their radio ressources. A BSC is principally in charge of handoffs, frequency hopping, exchange functions and power control over each managed BTSs.
• Base Transceiver Station (BTS) or Base Station: it maps to transceivers and antennas used in each cell of the network. It is usually placed in the center of a cell. Its transmitting power defines the size of a cell. Each BTS has between 1-16 transceivers depending on the density of users in the cell.

Its main role is to manage the communications between the mobile users and other users, such as mobile users, ISDN users, fixed telephony users, etc. It also includes data bases needed in order to store information about the subscribers and to manage their mobility. The different components of the NSS are described below.

• MSC: the central component of the NSS. The MSC performs the switching functions of the network. It also provides connection to other networks.
• GMSC: A gateway that interconnects two networks: the cellular network and the PSTN. It is in charge of routing calls from the fixed network towards a GSM user. The GMSC is often implemented in the same machines as the MSC.
• HLR: The HLR stores information of the suscribers belonging to the coverage area of a MSC; it also stores the current location of these subscribers and the services to which they have access. The location of the subscriber maps to the SS7 address of the Visitor Location Register (VLR) associated to the MN.
• VLR: contains information from a subscriber's HLR necessary to provide the subscribed services to visiting users. When a subscriber enters the covering area of a new MSC, the VLR associated to this MSC will request information about the new subscriber to its corresponding HLR. The VLR will then have enough data to assure the subscribed services without needing to ask the HLR each time a communication is established. The VLR is always implemented together with a MSC; thus, the area under control of the MSC is also the area under control of the VLR.
• Authentication Center (AuC): It serves security purposes; it provides the parameters needed for authentication and encryption functions. These parameters allow verification of the subscriber's identity.
• Equipment Identity Register (EIR): EIR stores security-sensitive information about the mobile equipments. It maintains a list of all valid terminals as identified by their International Mobile Equipment Identity (IMEI). The EIR allows then to forbid calls from stolen or unauthorized terminals (e.g, a terminal which does not respect the specifications concerning the output RF power).
• GSM Interworking Unit (GIWU): The GIWU provides an  interface to various networks for data communications. During these communications, the transmission of speech and data can be alternated.

It is connected to components of the NSS and the BSC, in order to control and monitor the GSM system. It is also in charge of controlling the traffic load of the BSS. It must be noted that as the number of BS increases with the scaling of the subscriber population some of the maintenance tasks are transferred to the BTS, allowing savings in  the cost of ownership of the system.
 
 

Geographical areas

A cell, as identified by its Cell Global Identity (CGI) number, maps to the radio coverage of a BTS. Similarly an LA as identified by its Location Area Identity (LAI) number , is a cluster of cells served by a single MSC/VLR. A group of LA under the control of the same MSC/VLR defines the MSC/VLR area. A Public Land Mobile Network (PLMN) is the area served by one network operator.
 
 

Network operations

In this paragraph, the description of the GSM network is focused on the differents functions to fulfil by the network and not on its physical components. In GSM, five main functions can be defined:

• Transmission: of data and signaling. Not all the components of the GSM network are strongly related with both types of types of Tx. While the MSC, BTS and BSC, among others, are involved with data and signaling, components such as  HLR, VLR or EIR registers, are only concerned with signaling.
• Radio Resources Management (RRM).
• Mobility Management (MM).
• Communication Management (CM).
• Operation, Administration and Maintenance (OAM).

 Radio Resources Management (RRM)

The role of the RR function is to establish, maintain and release communication links between mobile stations and the MSC. The elements that are mainly concerned with the RR function are the MN and the BTS. However, since the RR component performs connection management also during cell handoffs,  it also affects the MSC  which is the handoff management component.

The RR is also responsible for the management of frequency resources as well as varying radio interface conditions. Main component operations are:

• Channel assignment, change and release.
• Handoff
• Frequency hopping.
• Power-level control.
• Discontinuous transmission and reception.
• Timing advance.

 Handoff

The user movements may result a change in the channel/cell, when the quality of the communication is degrading; this is known as handoff. Handoffs occur between:

• between channels within a cell
• between cells controlled by the same BSC
• between cells under the same MSC but controlled by different BSCs
• between cells controlled by different MSCs.

To perform the handoff the mobile station controls continuously its own signal strengh and the signal strengh of the neighboring cells. The list of cells that must be monitored by the mobile station is given by the base station. Power measurements allow to decide which is the best cell in order to maintain the quality of the communication link. Two basic algorithms are used for handoffs:

• The `minimum acceptable performance' algorithm. When the quality of the transmission degrades, the power level of the mobile is increased, until the increase of the power level has no effect on the quality of the signal. Upon this link layer hint, a handoff is initiated.

The `power budget' algorithm. Here the handoff pre-empts the power increase, to obtain a good SIR.

The MM component handles:

• Location Management: Location is managed through periodicaly or on-demand. At power-on time, the MH signals an IMSI attach. On-demand location updates are signalled when the MN moves to a different PLMN or new location area (LA). The signal is sent to the new MSC/VLR, which forwards it to the subscriber's HLR. Upon authorization in the new MSC/VLR, the subscriber's HLR removes the registration entry of the MN at the old MSC/VLR. If after the update time interval, the MN has not registered, it is then deregistered. On power-off, the MN performs an IMSI detach.
• security and authentication:  Authentication involves the SIM card and the Authentication Center. A secret key, stored in the SIM card and the AuC together with a ciphering algorithm called A3, are used to authenticate the user. The MN and the AuCcompute a SRES through A3 using the secret key and a nonce generated by the AuC. If the two computed SRES are the same, the subscriber is authenticated. The different services to which the subscriber has access are also checked. Next the a security check is performed in the equipment identity (IMEI). If the IMEI number of the mobile is authorized in the EIR, the mobile station is allowed to connect the network. To assure user confidentiality, the user is registered with a Temporary Mobile Subscriber Identity (TMSI) after its first location update procedure. Enciphering is another option to guarantee a very strong security.

Communication Management (CM)

The CM component manages:

• Call control (CC): it controls call setup, management and tear-down in relation to management of type of service. Call routing is the primary task for this component. To reach a mobile subscriber, a user dials the Mobile Subscriber ISDN (MSISDN) number which includes:
• a country code
• a national destination code; this identifies the subscriber's operator
• a code mapping to the subscriber's HLR.
• The call is then passsed to the GMSC (if the call is originated from a fixed network) that 'knows' the HLR corresponding to the particular MSISDN number. The GMSC signals the HLR for call routing information. The HLR requests this information from the subscriber's current VLR. This VLR allocates temporarily a Mobile Station Roaming Number (MSRN) for the call. The MSRN number is the information returned by the HLR to the GMSC. It is latter that routes the call through the MSRN number, to the subscriber's current MSC/VLR. In the subscriber's current LA, the mobile is paged.
• Supplementary Services management: This involves the MN and the HLR.

SMS management: Here the GSM network contacts the Short Message Service Center through the two following interfaces:
• SMS-GMSC for Mobile Terminating Short Messages (SMS-MT/PP). It has the same role as the GMSC.
• SMS-IWMSC for Mobile Originating Short Messages (SMS-MO/PP).

Operation, Administration and Maintenance (OAM)

The OAM component allows the operator to monitor and control the system as well as modify the configuration of the elements of the system. Not only the OSS is part of the OAM, but also the BSS and NSS participate in functions such as:

• provide the operator with all the information it needs. This information is forwarded to the OSS to control the network.
• perform self-test tasks in addition to the OAM functions.
• control of multiple BTSs by the BSS.