GSM, or Global System for Mobile Communications, is a cellular phone protocol that is standard in most parts of the world. Technology experts created the protocol in the 1980s and '90s to standardize cellular phone service between countries in Europe. GSM phones use subscriber identity module (SIM) cards, which is essential to their function and enable the user to change phones easily. It is a chief competitor to the Code Division Multiple Access (CDMA) protocol.
Technology
Technically, the term GSM refers to second-generation (2G) voice networks that operate based on a combination of time division multiple access (TDMA) and frequency division multiple access (FDMA). TDMA takes the frequency channel the phone uses and divides it into individual bits of time that are assigned to each user. FDMA divides the frequency band into sections and assigns one to each cell phone tower. Towers are typically spread far enough apart so that those using the same frequency do not overlap their areas of coverage.
The term "GSM" is also used to refer to third-generation (3G) technology put into place by the same companies and using the same underlying network. This is actually called Universal Mobile Telecommunications System (UMTS), and uses the Wideband CDMA (W-CDMA) air interface standard in most places. Although they are both spread spectrum technologies, W-CDMA and the standard used by competing CDMA phones (usually CDMA2000) are not compatible.
GSM and CDMA
Although both refer to cellular phone networks, GSM and CDMA differ in the technology that they use. CDMA uses a system in which all signals are sent at the same time, but each is spread across multiple frequencies. Servers assign an individual code to each transmission so that the signals don't interfere with each other and can be matched up between the sender and receiver.
Originally, the two protocols differed in signal quality, consistency, and voice distortion, among other features. Both methods have been improved, however, and there are few significant differences between the two in terms of call quality. GSM phones are slightly more likely to drop calls as a user moves between cell towers, but they have better coverage in places with a lot of mountains. This technology also allows for voice and data transmission at the same time, which most CDMA phones cannot do.
In some parts of the world, both services are available, but one tends to be more common than the other. Most cellular service providers only use one type of network because it's very expensive to maintain both types of service. GSM is the older service, and is found in most places around the world, while CDMA is most common in the US and Asia.
SIM Cards
GSM phones use a SIM card, which is a small card with an embedded integrated circuit that stores personal data, the user's phone number, account information, and contacts. When a user switches phones, exchanging the SIM card from one phone to the other transfers the cell service and phone number, and activates the new phone. The contact list, calendar, and other personal data is usually easily moved between handsets.
Data Transfer Information
The 2G system is capable of data transmission speeds of up to 14.4 kilobits per second (kbps). Originally, not all networks had this capability, but continued advances and improvements in the system have raised speeds. 3G systems, which are designed specifically for mobile Internet and video, can transfer data much more quickly; depending on the interface, theoretical download speeds range from 2 megabits per second (Mbps) to 56 Mbps.
Frequencies
The frequency band used by these phones depends on several factors, including the location and technological changes. For example, in Europe, 2G GSM networks operate in the ranges of 900MHz and 1,800MHz and 3G networks use the 2,100 MHz band. The United States operates on frequencies in the range of 850MHz and 1,900MHz. Many other countries, including Australia and some places in South America, use the 850 MHz range. Most phones are designed for the countries where they are used most often, but a quad-band handset will handle 850, 900, 1,800, and 1,900MHz, and usually work in most areas.
Coverage
As of 2011, most countries from the Arctic region to Antarctica use the GSM protocol. It's particularly common in Europe, where it originated, and most of the Eastern hemisphere generally, including many developing countries. In the Western hemisphere, the CDMA protocol is more common in the United States and a few other countries. However, many countries, including Canada and Brazil, use GSM.
History
The European Telecommunications Standards Institute (ETSI) created GSM to standardize mobile phone communications among European nations; before this, each country had its own protocol. Standardizing the technology created a large, growing pool of users, and the protocol spread throughout the world. In 2002, approximately 70% of cellular phone clients worldwide used GSM services, while about 12% used CDMA. By early 2010, the GSM Association stated that there were over 4 billion users, and they estimate that there will be over 6 billion by 2015.