Cellular Concepts - GSM Architecture


The GSM network is divided into four major systems −

  • Switching System (SS)

  • Base Station System (BSS)

  • Mobile Station (MS)

  • Operation and Maintenance Center(OMC)

The switching system also called as Network and Switching System (NSS), is responsible for performing call processing and subscriber-related functions. The switching system includes the following functional units −

  • Mobile Switching Center

  • Home Location Register

  • Visitor Location Register

  • Equipment Identity Register

  • Authentication Center

Mobile Switching Center

Mobile Switching Center (MSC) performs all the switching functions for all mobile stations, located in the geographic area controlled by its assigned BSSs. Also, it interfaces with PSTN, with other MSCs, and other system entities.

Functions of MSC

  • Call handling that copes with the mobile nature of subscribers considering Location Registration, Authentication of subscribers and equipment, Handover and Prepaid service.

  • 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.

  • Controls inter-BSS and inter-MSC hand overs.

  • Acting as a gateway MSC to interrogate HLR. The MSC which is connected to the PSTN/ISDN network is called as GMSC. This is the only MSC in the network connected to the HLR.

  • Standard functions of a switch like charging.

Home Location Register (HLR)

Home location register contains −

  • The identity of mobile subscriber called International Mobile Sub Identity (IMSI).

  • ISDN directory number of mobile station.

  • Subscription information on services.

  • Service restrictions.

  • Location Information for call routing.

One HLR per GSM network is recommended and it may be a distributed database. Permanent data in HLR is changed by the man-machine interface. Temporary data like location information changes dynamically in HLR.

Visitor Location Register (VLR)

The VLR is always integrated with the MSC. When a mobile station roams into a new MSC area, the VLR connected to that MSC would request data about the mobile station from the HLR. Later, if the mobile station makes a call, the VLR has the information needed for call setup without having to interrogate the HLR each time. VLR contains information like the following −

  • Identity of mobile sub,

  • Any temporary mobile sub identity,

  • ISDN directory number of the mobile,

  • A directory number to route the call to the roaming station,

  • Part of the data of HLR for the mobiles that are currently located in MSC service area.

Equipment Identity Register

Equipment Identity Register consists of identity of mobile station equipment called International Mobile Equipment Identity (IMEI), which may be valid, suspect, and prohibited. When a mobile station accesses the system, the equipment validation procedure is evoked before giving the services.

The information is available in the form of three lists.

  • White List- The terminal is allowed to connect to the Network.

  • Grey List- The terminal is under observation from the network for the possible problems.

  • Black List- The terminals reported as stolen are not type approved. They are not allowed to connect to the network. EIR informs the VLR about the list, the particular IMEI is in.

Authentication Centre

It is associated with an HLR. It stores an Identity key called Authentication key (Ki) for each Mobile subscriber. This key is used to generate the authentication triplets.

  • RAND (Random Number),

  • SRES (Signed Response) -To authenticate IMSI,

  • Kc (Cipher Key) - To cipher communication over the radio path between the MS and the network.

Operation and Maintenance Centre (OMC)

It is the functional entity through which the network operator can monitor and control the system by performing the following functions −

  • Software installation

  • Traffic management

  • Performance data analysis

  • Tracing of subscribers and equipment

  • Configuration management

  • Subscriber administration

  • Management of mobile equipment

  • Management of charging and billing

Base Station System (BSS)

BSS connects the MS and the NSS. It is composed of the following −

  • Base Transceiver Station (BTS) also called Base Station.

  • Base Station Controller (BSC).

BTS and BSC communicate across the standardized Abis interface. BTS is controlled by BSC and one BSC can have many BTS under its control.

Base Transceiver Station (BTS)

BTS houses the radio transceivers and handles the radio-link protocols with the Mobile Station. Each BTS comprises of radio transmission and reception devices including antenna, signal processors, etc. Each BTS can support 1 to 16 RF carriers. The parameters differentiating the BTSs are Power level, antenna height, antenna type and number of carriers.

Functions of BTS

  • It is responsible for Time and Frequency synchronization.

  • The process of channel coding, Encryption, Multiplexing and modulation for trans-direction and reverse for reception are to be carried out.

  • It has to arrange for transmission in advance from the mobiles depending upon their distance from BTS (Timing Advance).

  • It has to detect Random access requests from mobiles, measure and monitor the radio channels for power control and handover.

Base Station Controller

BSC manages the radio resources for one or a group of BTSs. It handles radio-channel setup, frequency hopping, handovers, and control of the RF power levels. BSC provides the time and frequency synchronization reference signals broadcast by its BTSs. It establishes connection between the mobile station and the MSC. BSC is connected via interfaces to MSC, BTS and OMC.

Mobile Station

It refers to the terminal equipment used by the wireless subscribers. It consists of −

  • SIM -Subscriber Identity Module

  • Mobile Equipment

SIM is removable and with appropriate SIM, the network can be accessed using various mobile equipments.

The equipment identity is not linked to the subscriber. The equipment is validated separately with IMEI and EIR. The SIM contains an integrated circuit chip with a microprocessor, random access memory (RAM) and read only memory (ROM). SIM should be valid and should authenticate the validity of MS while accessing the network.

SIM also stores subscriber related information like IMSI, cell location identity etc.

Functions of Mobile Station

  • Radio transmission and reception

  • 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 up to a maximum of six surrounding BTSs and reporting to the BSS, MS can store and display short received alphanumeric messages on the liquid crystal display (LCD) that is used to show call dialing and status information.

There are five different categories of mobile telephone units specified by the European GSM system: 20W, 8W, 5W, 2W, and 0.8W. These correspond to 43-dBm, 39-dBm, 37-dBm, 33-dBm, and 29-dBm power levels. The 20-W and 8-W units (peak power) are either for vehicle-mounted or portable station use. The MS power is adjustable in 2-dB steps from its nominal value down to 20mW (13 dBm). This is done automatically under remote control from the BTS.


Transcoders are a network entities inserted to interface the MSC side to Mobile side. The voice coding rate on the PSTN side is 64Kbps, and in GSM over the air the voice is coded as 13Kbps. To reduce the data rate over the air interface and to reduce the loading of the terrestrial link (4 : 1), transcoders are introduced at an appropriate place, mostly with MSC.

The transcoder is the device that takes 13-Kbps speech or 3.6/6/12-Kbps data multiplexes and four of them to convert into standard 64-Kbps data. First, the 13 Kbps or the data at 3.6/6/12 Kbps are brought up to the level of 16 Kbps by inserting additional synchronizing data to make up the difference between a 13-Kbps speech or lower rate data and then four of them are combined in the transponder to provide 64 Kbps channel within the BSS. Four traffic channels can then be multiplexed in one 64-Kpbs circuit. Thus the TRAU output data rate is 64 Kbps.

Then, up to 30 such 64-Kpbs channels are multiplexed onto a 2.048 Mbps if a CEPT1 channel is provided on the A-bis interface. This channel can carry up to 120-(16x 120) traffic and control signals. Since the data rate to the PSTN is normally at 2 Mbps, which is the result of combining 30- by 64-Kbps channels, or 120- Kbps by 16-Kpbs channels.

Other Network Elements

Other network elements include components such as SMS Service Centre, Voice Mail Box, and SMS Flow.

SMS Service Centre

It interfaces with MSC having interworking functionality to provide Short Message Service (SMS) to mobile subscribers. SMS can be destined to fax machine, PC on the internet or another MS. The location of the recipient MS is queried by MSC and delivered.

Voice Mail Box

When the mobile subscriber is not in a position to answer the incoming calls due to busy/out of service area, then the call gets diverted to a mail box which has already been activated by the subscriber. For this, a separate connectivity has been established from MSC. The subscriber will be alerted through SMS later and can retrieve the message.

SMS Flow

  • When a user sends an SMS, the request is placed via the MSC.

  • The MSC forwards the SMS to the SMSC where it gets stored.

  • The SMSC queries the HLR to find out where the destination mobile is and forwards the message to the destination MSC if the destination mobile is available.

  • If the mobile is not available the message gets stored in the current SMSC itself. In most installations if a mobile is not available for SMS delivery the SMSC does not retry. Instead, the destination MSC informs the SMSC when the mobile comes back in range. SMS handling is a store and forward operation unlike USSD.

  • SMS has got a validity period for which it will wait for the destination mobile to be available. After that time the SMSC will delete the message. The validity period can be set by the user. Normal validity is 1 day.