Global System for Mobile communications

Global System for Mobile communications is the most popular standard for mobile phones in the world. Its promoter, the GSM Association, estimates that 82% of the global mobile market uses the standard. GSM is used by over 2 billion people across more than 212 countries and territories. Its ubiquity makes international roaming very common between mobile phone operators, enabling subscribers to use their phones in many parts of the world. GSM differs from its predecessors in that both signaling and speech channels are digital call quality, and so is considered a second generation (2G) mobile phone system. This has also meant that data communications were built into the system using the 3rd Generation Partnership Project (3GPP). By the way, I have to say this technology is really helpful and convenient for me to contact with my family which is in china.

The key advantage of GSM systems to consumers has been better voice quality and low-cost alternatives to making calls, such as the Short message service .The advantage for network operators has been the ease of deploying equipment from any vendors that implement the standard. Like other cellular standards, GSM allows network operators to offer roaming services so that subscribers can use their phones on GSM networks all over the world.
Newer versions of the standard were backward-compatible with the original GSM phones. For example, Release '97 of the standard added packet data capabilities, by means of General Packet Radio Service (GPRS). Release '99 introduced higher speed data transmission using Enhanced Data Rates for GSM Evolution (EDGE).

GSM is a cellular network, which means that mobile phones connect to it by searching for cells in the immediate vicinity. GSM networks operate in four different frequency ranges. Most GSM networks operate in the 900 MHz or 1800 MHz bands. Some countries in the Americas (including Canada and the United States) use the 850 MHz and 1900 MHz bands because the 900 and 1800 MHz frequency bands were already allocated. The rarer 400 and 450 MHz frequency bands are assigned in some countries, notably Scandinavia, where these frequencies were previously used for first-generation systems.
In the 900 MHz band the uplink frequency band is 890–915 MHz, and the downlink frequency band is 935–960 MHz. This 25 MHz bandwidth is subdivided into 124 carrier frequency channels, each spaced 200 kHz apart. Time division multiplexing is used to allow eight full-rate or sixteen half-rate speech channels per radio frequency channel. There are eight radio timeslots (giving eight burst periods) grouped into what is called a TDMA frame. Half rate channels use alternate frames in the same timeslot. The channel data rate is 270.833 kbit/s, and the frame duration is 4.615 ms.

The transmission power in the handset is limited to a maximum of 2 watts in GSM850/900 and 1 watt in GSM1800/1900.
GSM has used a variety of voice codecs to squeeze 3.1 kHz audio into between 5.6 and 13 kbit/s. Originally, two codecs, named after the types of data channel they were allocated, were used, called Half Rate (5.6 kbit/s) and Full Rate (13 kbit/s). These used a system based upon linear predictive coding (LPC). In addition to being efficient with bitrates, these codecs also made it easier to identify more important parts of the audio, allowing the air interface layer to prioritize and better protect these parts of the signal.
GSM was further enhanced in 1997 with the Enhanced Full Rate (EFR) codec, a 12.2 kbit/s codec that uses a full rate channel. Finally, with the development of UMTS, EFR was refactored into a variable-rate codec called AMR-Narrowband, which is high quality and robust against interference when used on full rate channels, and less robust but still relatively high quality when used in good radio conditions on half-rate channels.

By the year 2005 the world has more than 1 billion people use GSM phones. This makes GSM become the lead of mobile telephone system, which has 70% market in the world. The current W-CDMA does not show all the features and the main GSM competition CDMA2000 (mainly in North America, Japan, China and South Korea) access to the global transition as 3G standards of the limited growth. However, because there is no so-called standard CDMA phone, so different operators in the CDMA network directly roaming sometimes can be difficult even not impossible, depending on cell phones and operators. Because the W-CDMA network construction has been started (at least in the high-density market), GSM is slowly dying out, but this will be sustained for some time.

In 1998-2000 years led to the growth of GSM users is the main reason for mobile operators launched prepaid telephone services. It allows those who cannot or do not want to sign a contract with the operator of the use of mobile telephone services. Such services in Europe competition between mobile operators also quite fierce and even in the absence of long-term contracts, people can also from the operators there is very low price to buy a cell phone.