Global System for Mobile communications (GSM) is a cellular communication standard used in most parts of the world to make voice calls and use Short Message Service (SMS). Based on the Time Division Multiple Access (TDMA) method, GSM is a 2G standard that competes with CDMA One that’s based on another mobile radio access method called Code Division Multiple Access (CDMA). GSM carriers place customer data on removable Subscriber Identity Module (SIM) cards, giving GSM users the freedom to change mobile handsets and still keep their phone number or carrier information intact. Also, all GSM handsets have a unique serial number called International Mobile Station Equipment (IMEI) for identity purpose.


In 1982, a consortium was founded by the Conference of European Posts and Telegraphs (CEPT) for coordinating and standardizing a telephone network spanning the whole of Europe. The network was initially called “Groupe Speciale Mobile”, which was later renamed as “Global System for Mobile communications”. The GSM project received further impetus when its responsibility was handed over to European Telecommunications Standards Institute (ETSI) in 1989.

The first GSM technology-based mobile was launched in 1991 in Finland. By this time, the necessary infrastructure was not widely available. However, the growth thereafter was multifold, resulting in GSM technology becoming widespread outside of Europe. This is probably why there was a name change to Global System for Mobile communications from Groupe Speciale Mobile.

Network Architecture

Basically, GSM comprises the following systems: mobile station, network switching subsystem, base station subsystem, and operation and support subsystem.

Mobile Station (MS)

Also called mobile equipment, a mobile station is the GSM network component that’s visible to the user’s eyes. It comprises all the hardware and software required for enabling mobile network communication – including the mobile device display, SIM card, and other components.

Network Switching Subsystem (NSS)

The NSS processes calls and handles functions relating to the subscriber. The units that make up this system are home location register (HLR), mobile services switching center (MSC), visitor location register (VLR), authentic center (AUC), and equipment identity register (ERR).

The HLR is a database storing permanent data about subscribers – such as subscriber service profile, activity status, and location data. Every GSM subscription details are recorded in the particular operator’s HLR. The MSC takes care of calls from and to the mobile handset. The VLR database houses temporary information relating to subscribers – such as subscriber location data when roaming – so that the MSC can perform its functions. The AUC offers encryption and authentication parameters for verifying user identity and keeping the confidentiality of every call intact. The EIR is a database too and it has information about the mobile equipment.

Base Station Subsystem (BSS)

The BSS takes care of all functions relating to radio. In other words, it handles things fundamental to communicating on the mobile network. The subsystem comprises base transceiver stations (BTS) and base station controllers (BSC). BTS is responsible for communication between the wireless device and the network. It houses the antennas for relaying radio messages, transceivers, amplifiers and duplexers to get the job done. These are all equipment needed to encrypt and decrypt wireless communication.

BSC is what works in the background of BTS. It mediates between the base station (cell tower or cell site) and MSC – the network element responsible for handling incoming and outgoing calls. Simply put, if a mobile phone is not getting a healthy network signal from a particular tower, BSC would look for another tower with a stronger network and establish a connection.

Operation Support Subsystem (OSS)

The OSS is an element of the GSM architecture that controls and monitors the whole network, and also handles the base station subsystem’s traffic load.  

GSM Frequencies

GSM operates at different band frequencies – ranging from 380 MHz to 1900 MHz. The GSM frequency in a particular region is based on the place’s regulatory requirements. The 900 megahertz (MHz) frequency band was used in the first GSM system. Later, with increasing number of subscribers, the frequency was broadened to 1800 MHz. The GSM frequency used in America is 850 MHz and 1900 MHz. In Europe, 900 MHz and 1800 MHz are standard. These bands indicate that GSM phones may work only in specific countries or regions.

Several GSM devices are marked as “quad band” or “tri band”. These devices are much likelier to work overseas since they are made to be compatible with a variety of frequencies. For instance, most GSM devices used in America are dual-band phones. Both Europe and America also use tri-band phones. Despite all these frequency discrepancies, GSM bands are still considered fairly standardized across the globe.

GSM Advantages

GSM is an ideal choice if you frequently switch phones, use imported mobile phones, or are always on the move globally since the SIM card that stores subscriber information can be easily placed in another device and make the other phone your own. Moreover, most GSM operators have roaming contracts with foreign network operators. This interchangeability has increased the number of GSM mobile devices being produced, enabling availability of new phones with extra features.

Unlike CDMA carriers, GSM network providers cannot dictate what phones their subscribers can use. As a result, GSM users’ mobile handset options are fairly wide. Also, GSM is backed by an industry consortium, which makes building GSM equipment less expensive.

Phone calls and web browsing can be done simultaneously on a GSM phone. This may not seem like a special feature to people who have used GSM devices all their life. But those who use or who’ve been on CDMA would certainly appreciate this functionality, since CDMA doesn’t allow such simultaneous operations.

GSM Limitations

Unlocked GSM phones are not limited to specific regions or areas. However, there could be compatibility issues when using a particular GSM phone not meant to be used in another country. As mentioned before, GSM has several bands – 380MHz to 1900MHz – and these bands could vary across markets. The phone’s frequency bands may not match the bands used by specific carriers in a country. For example, the GSM network in Europe would probably use frequencies different from what’s being used in the States or South America. This could result in some functionalities of a GSM phone made for the European market not working in the United States – for instance, mobile data signals may not come through.  

GSM Popularity

GSM is the most popular cellular communication standard in the world, representing more than 80 percent of all mobile connections in the world (as of September 2016) approximately. In fact, in some regions such as Europe, GSM cellular is the only service available. Though GSM started its journey in Europe, China, Russia and India are its biggest markets.

Besides being an excellent voice communications system, this popularity can also be attributed to the network’s ability to transfer data. With more people getting online, GSM developed the ability to offer packet data. General Packet Radio Service (GPRS) and Enhanced Data Rates for GSM Evolution (EDGE) are examples of such packet data capabilities. These mobile data services laid the foundations for 3G services and other advanced mobile communication technologies.