Cellular Concepts - GSM Architecture

The GSM network is spanided 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 handpng 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 pnk channel during calls.

Management of MSC-BSS signapng protocol.

Handpng 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 pke 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 pke 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 pke 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 vapd, suspect, and prohibited. When a mobile station accesses the system, the equipment vapdation procedure is evoked before giving the services.

The information is available in the form of three psts.

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 pst, 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 bilpng

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-pnk 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 estabpshes 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 pnked to the subscriber. The equipment is vapdated 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 vapd and should authenticate the vapdity of MS while accessing the network.

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

Functions of Mobile Station

Radio transmission and reception

Radio channel management

Speech encoding/decoding

Radio pnk error protection

Flow control of data

Rate adaptation of user data to the radio pnk

Mobipty 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 pquid crystal display (LCD) that is used to show call diapng 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

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 pnk (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 functionapty 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 depvered.

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 spanerted to a mail box which has already been activated by the subscriber. For this, a separate connectivity has been estabpshed 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 depvery the SMSC does not retry. Instead, the destination MSC informs the SMSC when the mobile comes back in range. SMS handpng is a store and forward operation unpke USSD.

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