GSM System Architecture

1.      Introduction

GSM stands for the Global System for Mobile Communications. It is a standard initially created to solve the incompatibility of the 1G cellular systems and equipments in Europe. Originally, it operates in the 900 MHz band, but subsequently the 1.8 GHz and 1.9 GHz bands are also supported.

2.      GSM System Architecture

There are 6 basic components that formed the system architecture of GSM:

a)      Gateway to Wireline System

There are two components within the gateway: the MSC (mobile switching center) and the IWF (internetworking function).

The MSC provides the link between GSM network and the PSTN (public switched telephone network).

The IWF provides the link to internet for IP data traffic.

The MSC and IMF, together with four databases (HLR, VLR, AuC and EIR), formed the NSS (network switching subsystem). The NSS handles the switching of GSM calls between BSCs and MSC to communicate with external networks.

b)      Base Station Subsystem (BSS)

The BSS consists of one BSC (Base station controller) and one or more BTS (Base transceiver station). Several BSS connect to one MSC. The BSS provides and manages radio transmission between the MS (mobile station) and the MSC.

The BSC connects and controls a number of BTSs via microwave or wireline. It reserves frequencies, handles mobile handoff between two BTSs within the same BSS, and controls paging.

The BTS consists of all radio equipments such as radio antenna and radio transceivers within one single cell. It performs all signalling related to the radio interface and is the logical location of the antenna.

c)      Databases

There are three main databases:

·        HLR

HLR stands for Home Location Register. It stores the subscriber information and location information for each user that belongs to the MSC. The HLR also stores the billing information.

·        VLR

VLR stands for Visitor Location Register. It stores all roaming subscribers currently within the range of the particular MSC. It contains information of the roaming subscribers such as authentication information, the IMSI (International mobile subscriber identity) and the MSRN (Mobile station roaming number).

·        EIR

EIR stands for Equipment Identification Register. It is associated with security issues and shall be discussed in the following section.

d)      Security Mechanism

The security mechanism consists of three sub components:

·        AuC

AuC stands for Authentication Center. It performs the necessary authentication when a subscriber joins the network and keeps track of the subscribers. It controls access to user data such as the HLR and VLR, by implementing encryption algorithm on all subscribers’ request.

·        EIR

The EIR is the database for storing the IMSI (International mobile subscriber identity) for each mobile equipment. By matching the IMSI and IMEI (International mobile equipment identity), it can identify any stolen or illegally altered phones and block calls originated from those phones.

·        SIM

SIM stands for Subscriber Identity Module. It is a smart card that stores the subscriber’s personal data and billing information, and it is inserted into the mobile equipment. SIM is used to authorize the subscriber to access the networks.

e)      Air Interface

GSM uses the TDMA (time division multiple access) protocol to control the transmission medium. It utilizes the GMSK (gaussian minimum shift keying) modulation scheme and the bandwidth of the traffic channel is 200 kHz.

f)       Mobile Station (MS)

Mobile station (MS) consists of the mobile equipment (ME) and the SIM card. MS communicates with BTS in the same cell where the MS is located via the radio air interface Um.

The ME is basically the cellular phone device excluding the SIM.

3.      Conclusion

This article aims to provide an introductory guide to the system architecture of GSM. It attempts to break down the GSM architecture into six logical components, so that readers can have a clear understanding of the system architecture even without technical background.