GSM Technology: The Global Standard for Mobile Communication

GSM Technology

Global System for Mobile (GSM) is a second generation cellular system that was developed to solve the fragmentation problems of the first cellular systems in Europe. It was introduced by a French company in 1985 as Groupe Special Mobile and later renamed as Global System for Mobile Communication for marketing purposes.

Today, GSM technology has become the global standard for mobile communication with more than 80% of the world's mobile users relying on it. In this blog post, we will discuss the significance and remarkable features of GSM technology, as well as its system architecture.

Significance of GSM Technology

GSM technology's popularity and wide acceptance can be attributed to the following features:

• Improved spectrum efficiency
• International roaming
• Low-cost mobile sets and base stations (BSs)
• High-quality speech
• Compatibility with Integrated Services Digital Network (ISDN) and other telephone company services
• Support for new services

Remarkable Features of GSM Technology

GSM technology offers several remarkable features, some of which are:

• Subscriber Identity Module (SIM): It is a memory device that stores information such as the subscriber's identification number, the networks and countries where the subscriber is entitled to service, privacy keys, and other user-specific information. It is the SIM that gives GSM subscriber units their identity.
• On-the-air privacy: GSM provides on-the-air privacy, which is a second remarkable feature of the technology.
• International roaming facility: GSM offers an international roaming facility, which is used by more than 200 countries.

GSM System Architecture

The GSM system architecture consists of three major interconnected subsystems that interact between themselves and with the users through certain network interfaces. The subsystems are:

1. Base Station Subsystem (BSS)
2. Network and Switching Subsystem (NSS) 
3.  Operation Support Subsystem (OSS)

The Mobile Station (MS) is also a subsystem, but is usually considered to be part of the BSS for architecture purposes. Equipment and services are designed within GSM to support one or more of these specific subsystems. Figure shows the block diagram of the GSM system architecture

Mobile Station 

A mobile station is intended for use in the cellular radio service while in motion at unspecified locations. Mobile stations may be hand-held personal units (portables) or installed in vehicles (mobiles). There are a number of elements to the mobile station, although two main elements are the hardware and the SIM. The Mobile Stations (MS) communicate with the Base Station Subsystem (BSS) over the radio air interface.

Subscriber Identity Module (SIM) is a memory device that stores information such as the subscriber's identification number, the networks and countries where the subscriber is entitled to service, privacy keys, and other user-specific information. It is the SIM that gives GSM subscriber units their identity.

Given  information’s in SIM: 

• Card type, serial no, list of subscribed services   
• Personal Identity Number(PIN)
• Pin Unlocking Key(PUK)
• An Authentication Key(KI)

ME  is  a  piece  of  hardware  that  the  customer purchases from the equipment manufacturer. The hardware piece contains all the  components  needed  for  the  implementation  of  the protocols  to  interface with the user and the air-interface to the base stations.It contains an IMEI (EIN) number.

Base Station Subsystem

The BSS consists of many BSCs which connect to a single MSC, and each BSC typically controls up to several hundred Base Transceiver Stations (BTSs). Some of the BTSs maybe co-located at the BSC, and others may be remotely distributed and physically connected to the BSC by microwave link or dedicated leased lines. Mobile handoffs (called handovers, or HO, in the GSM specification) between two BTSs under the control of the same BSC are handled by the BSC, and not the MSC. This greatly reduces the switching burden of the MSC. The interface which connects a BTS to a BSC is called the Abis interface. The Abis for each GSM base station manufacturer has subtle differences, thereby forcing service providers to use the same manufacturer for the BTS and BSC equipment. The BSCs are physically connected via dedicated leased lines or microwave link to the MSC. The interface between a BSC and a MSC is called the A interface, which is standardized within GSM. The A interface uses an SS7 protocol called the Signaling Correction Control Part (SCCP) which supports communication between the MSC and the ESS, as well as network messages between the individual subscribers and the MSC. The A interface allows a service provider to use base stations and switching equipment made by different manufacturers.

Network Switching Subsystem

The NSS handles the switching of GSM calls between external networks and the BSCs in the radio subsystem and is also responsible for managing and providing external access to several customer databases. The MSC is the central unit in the NSS and controls the traffic among all of the BSCs. In the NSS, there are three different databases called the Home Location Register (HLR), Visitor Location Register (VLR), and the Authentication Center (AUC). The HLR is a database which contains subscriber information and location information for each user who resides in the same city as the MSC. Each subscriber in a particular GSM market is assigned a unique International Mobile Subscriber Identity (IMSI), and this number is used to identify each home user. The VLR is a database which temporarily stores the IMSI and customer information for each roaming subscriber who is visiting the coverage area of a particular MSC. The VLR is linked between several adjoining MSCs in a particular market or geographic region and contains subscription information of every visiting user in the area. Once a roaming mobile is logged in the VLR, the MSC sends the necessary information to the visiting subscriber's FILE so that calls to the roaming mobile can be appropriately routed over the PSTN by the roaming user's HLR. The Authentication Center is a strongly protected database which handles the authentication and encryption keys for every single subscriber in the HLR and VLR. The Authentication Center contains a register called the Equipment Identity Register (EIR) which indentifies stolen or fraudulently altered phones that transmit identity data that does not match with information contained in either the HLR or VLR.

Operation and Support Subsystem

The OSS supports one or several Operation Maintenance Centers (OMC) which are used to monitor and maintain the performance of each MS, BS, BSC and MSC within a GSM system. The OSS has three main functions, which are 

• maintain all telecommunications hardware and network operations with a particular market 
• manage all charging and billing procedures
• manage all mobile equipment in the system. 

Within each GSM system, an OMC is dedicated to each of these tasks and has provisions for adjusting all base station parameters and billing procedures, as well as for providing system operators with the ability to determine the performance and integrity of each piece of subscriber equipment in the system.

Channel used by GSM

There are two types of channel in GSM technology:

1. Traffic channels (TCH)
2. Control channels (CCH)

Traffic channels carry digitally encoded user speech or user data and have identical functions and formats on both the forward and reverse link. There are six different types of TCHs, may be either full-rate or half-rate and may carry either digitized speech or user data.

Control channels carry signaling and synchronizing commands between the base station and the mobile station. There are three main control channels in the GSM system. These are the broadcast channel (BCH), the common control channel (CCCH), and the dedicated control channel (DCCH).

Advantages of GSM

• International roaming facility
• Simultaneous voice and data communication

Disadvantages of GSM

• Privacy and security is not guaranteed
• Network traffic isn’t independent from the number of users
• GSM signals can be interfered with certain electronic devices (e.g. pace makers and hearing aids etc.) and hinder their operations.

GSM services 

GSM services follow ISDN guidelines and are classified as either tele-services or data services. Teleservices include standard mobile telephony and mobile-originated or base-originated traffic, data services include computer to computer communication and packet-switched traffic. Some of these are:

• SMS
• Call forwarding, caller waiting
• Advice of charge(AOC)
• Explicit call transfer(ECT) 
• Multiparty conferencing
• Internet connectivity and so on