Global System for Mobile Communications Guide, Meaning , Facts, Information and Description
GSM (Global System for Mobile Communications) is the most popular standard for mobile phones in the world. GSM phones are used by over a billion people across more than 200 countries. The ubiquity of the GSM standard makes international roaming very common with "roaming agreements" between operators. GSM differs significant from its predecessors in that both signalling and speech channels are digital, which means that it is seen as a second generation (2G) mobile phone system. This fact has also meant that data communication was built into the system from very early on. GSM is an open standard which is developed by the 3GPP.From the point of view of the consumer, the key advantage of GSM systems has been early delivery of new services at low costs. For example text messaging was developed first for GSM. The advantage for network operators has been the low infrastructure cost which is caused by open competition. The primary disadvantage has been that GSM's radio network is based on TDMA technology, which is considered less advanced than competing CDMA-based systems. Practical performance figures are rather similar, however,
GSM has retained backward-compatibility with the original GSM phones. At the same time, the GSM standard continues to develop and packet data capabilities were added in the Release '97 version of the standard with GPRS. Higher speed data transmission has been introduced by providing a new modulation scheme with EDGE.
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2 Market situation 3 Radio interface 4 Network structure 5 Related topics 6 External links |
History
The GSM group ("Groupe Spécial Mobile" (French) 1, 2, 3 and 4) was founded in 1982. The name of the system comes from the name of this group, though later the decision was made to keep the initials but to change what they stood for. Originally the group was hosted by CEPT. The technical fundamentals of the GSM system were defined in 1987. In 1989, ETSI took over control and by 1990 the first GSM specification was completed and came to over 6,000 pages of text. Commercial operation began in 1991 with Radiolinja in Finland.
In 1998, the 3rd Generation Partnership Project (3GPP) was formed. Originally it was intended only to produce the specifications of the next (third, 3G) generation of mobile networks. However, 3GPP also took over the maintenance and development of the GSM specification. ETSI is a partner in 3GPP.
Market situation
More than one billion people use GSM phones as of 2004, making GSM the dominant mobile phone system worldwide with about 70% of the worlds market. GSM's main competitor, CDMA2000, is used primarily in the United States, although it was seeing increased, but limited, worldwide adoption as a stepping stone to a 3G standard when WCDMA did not appear to be fully functional. As WCDMA networks have begun to take off, at least in high density markets, GSM's rate of expansion may slow, this seems likely to take some time, however.
GSM employs TDMA between stations on a frequency duplex pair of radio channels, with slow frequency hopping between channels. GSM uses also SDMA and FDMA. It uses a modified Gaussian shift-key modulation. This modulation scheme inherently gives mobile units better battery life because it encodes the data by varying the frequency of the signal, not the amplitude. This allows amplifiers to be run at high power levels without distorting the transmitted data(good power efficiency). However, the tradeoff is that each user consumes more bandwidth, which means that more spectrum is necessary to serve the same number of users than with other modulation schemes (poor spectral efficiency).
GSM networks operate at various GSM frequency ranges.
The GSM network consists of cells and cells can be named after their size. Basically there are 4 different cell sizes - Macro, micro, pico and umbrellacells. The coverage area of each cell is different in different environments. Macro cells can be regarded as cells where the base station antenna is installed in a mast or a building above the average roof top level. However, micro cells are cells where the antenna height is under the average roof top level and they are typically used in urban areas. The picocells are small cells whose diameter is a few dozen metres and are mainly used indoors. On the other hand, umbrellacells are used to cover shadow regions of smaller cells and fill in gaps in coverage between those cells. These cells are usually built on top of tall buildings or in other high places.
The cell radius can vary depending on the antenna height, antenna gain and propagation conditions from couple of hundred meters to several tens of kilometres. Because of the timeslot (time period allocated to one call) overlap that occurs when calls are maintained at large Handset-Basestation separations, practically 35 km is the longest distance GSM specification supports, though the specifications define an extended cell, where the cell radius could be double or even more. This is done by utilizing 2 timeslots per user, so the call has a better chance hitting the right timeslot. Indoor coverage is also supported by GSM.
Indoor coverage can be built by using power splitters to deliver an RF signal from the antenna outdoors to a separate indoor antenna distribution system. When all the capacity of the cell is needed indoors, e.g. in shopping centres or airports etc., the indoor coverage can be built by using antennas only inside the building. In suburban areas the indoor coverage is usually provided by the inbuilding penetration of radio signal, not by a separate indoor antenna system.
The network behind the GSM system seen by the customer is large and complicated in order to provide all of the services which are required. It is divided into a number of sections and these are each covered in separate articles.
GSM uses several cryptographic algorithms for security. The A5/1 and A5/2 stream ciphers are used for ensuring over-the-air voice privacy. A5/1 is a stronger algorithm used within Europe; A5/2 is weaker and used in other countries. Serious weaknesses have been found in both algorithms, and it is possible to break A5/2 in real-time in a ciphertext-only attack. The system supports multiple algorithms so operators may replace that cipher with a stronger one.
This is an Article on Global System for Mobile Communications. Page Contains Information, Facts Details or Explanation Guide About Global System for Mobile Communications Radio interface
Network structure
Subscriber Identity Module
One of the key features of GSM is the Subscriber Identity Module (SIM), commonly known as a SIM card. The SIM is a detachable smartcard containing the user's subscription information and phonebook. This allows the user to retain his information while switching handsets. Alternatively, the user can also change operators while retaining the handset simply by changing the SIM. Some operators will block this by allowing the phone to use only a single SIM, or only a SIM issued by them; this practice is known as SIM locking, and is illegal in some countries. In the USA most operators do lock the mobiles they sell. This is done because the price of the mobile phone is usually subsidised with revenue from subscriptions and operators want to try to avoid subsidising competitor's mobiles. A subscriber can usually contact the provider to remove the lock for a fee (which operators sometimes try to claim to be ignorant of), utilize private services to remove the lock, or make use of ample software and websites available on the Internet to unlock the handset themselves. Some providers in the USA, such as T-Mobile, will unlock the phone for free if the customer has held an account for a certain period. In most countries removing the lock is not illegal.GSM security
GSM was designed with a moderate level of security. The system was designed to authenticate the subscriber using shared-secret cryptography. Communications between the subscriber and the base station can be encrypted. Related topics
External links