1. Introduction
GSM stands for Global System for Mobile Communications. It is a standard initially created to solve the incompatibility of 1G cellular systems and equipment in Europe. Originally, it operates in the 900 MHz band, but later the 1.8 GHz and 1.9 GHz bands are also supported.
2. GSM system architecture
There are 6 basic components that make up the GSM system architecture:
a) Gateway to wired system
There are two components within the gateway: the MSC (Mobile Switching Center) and the IWF (Internetworking Function).
The MSC provides the link between the GSM network and the PSTN (Public Switched Telephone Network).
The IWF provides the Internet link for IP data traffic.
The MSC and IMF, along with four databases (HLR, VLR, AuC and EIR), formed the NSS (Network Switching Subsystem). The NSS handles the switching of GSM calls between BSC and MSC to communicate with external networks.
b) Base Station Subsystem (BSS)
The BSS consists of a BSC (Base Station Controller) and one or more BTS (Base Transceiver Station). Several BSSs connect to one MSC. The BSS provides and manages the radio transmission between the MS (mobile station) and the MSC.
The BSC connects and controls multiple BTSs via microwave or wireline. It reserves frequencies, handles mobile handover between two BTSs within the same BSS, and controls paging.
The BTS consists of all the radio equipment, such as radio antennas and radio transceivers, within a single cell. It performs all signaling related to the radio interface and is the logical location for the antenna.
c) Databases
There are three main databases:
or HLR
HLR stands for Start Location Registry. It stores the subscriber information and location information of each user belonging to the MSC. The HLR also stores billing information.
VLR
VLR stands for Visitor Location Record. Stores all currently roaming subscribers within range of the particular MSC. It contains roaming subscriber information such as authentication information, IMSI (International Mobile Subscriber Identity) and MSRN (Mobile Station Roaming Number).
or EIR
EIR stands for Equipment Identification Registration. It is associated with security issues and will be discussed in the next section.
d) Security Mechanism
The security mechanism consists of three subcomponents:
or AuC
AuC stands for Authentication Center. Performs the necessary authentication when a subscriber joins the network and keeps track of subscribers. It controls access to user data, such as HLR and VLR, by implementing an encryption algorithm at the request of all subscribers.
or EIR
The EIR is the database to store the IMSI (International Mobile Subscriber Identity) for each mobile device. By matching the IMSI and IMEI (International Mobile Equipment Identity), you can identify any stolen or tampered with phones and block calls originating from those phones.
SIM card
SIM stands for Subscriber Identity Module. It is a smart card that stores the subscriber’s personal and billing data, and is inserted into the mobile device. The SIM is used to authorize the subscriber to access networks.
e) Air interface
GSM uses the TDMA (Time Division Multiple Access) protocol to control the transmission medium. It uses the GMSK (Gaussian Minimum Shift Keying) modulation scheme and the bandwidth of the traffic channel is 200 kHz.
f) Mobile Station (MS)
The mobile station (MS) consists of the mobile equipment (ME) and the SIM card. The MS communicates with the BTS in the same cell where the MS is located through the radio air interface Um.
The ME is basically the cell phone device excluding the SIM.
3. Conclusion
This article aims to provide an introductory guide to the architecture of the GSM system. It tries to divide the GSM architecture into six logical components, so that readers can have a clear understanding of the system architecture even without technical knowledge.