 |
|
| |
|
||||
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 mobile phone operators. GSM differs significantly 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. 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 have also been introduced with EDGE and UMTS in Release '99 version of the standard.
HistoryIn spite of the current popularity, the history of mobile phones began long before GSM was conceived. 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 situationMore than one billion people use GSM phones as of 2004, making GSM the dominant mobile phone system worldwide with about 70% of the world's market. GSM's main competitor, CDMA2000, is used primarily in the United States and Canada, 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. A major reason for the growth in GSM usage, particularly between 1998 to 2002, was the availability of prepaid calling from mobile phone operators. This allowed people to have mobile phones who were either unable or unwilling to enter into an ongoing contract with the operator. Radio interfaceGSM is a cellular network, which means that mobile phones connect to it by seaching for cells in the immediate vicinity. GSM networks operate at various different radio frequencies. There are four different cell sizes in a GSM network - 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. 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. The longest distance the GSM specification supports in practical use is 35 km. There is also a concept of an extended cell, where the cell radius could be double or even more. Indoor coverage is also supported by GSM and is achieved by using power splitters to deliver the radio signal from the antenna outdoors to a separate indoor antenna distribution system. This is typically deployed when a lot of call capacity is needed indoors, for example in shopping centres or airports. However, this is not a pre-requisite, since indoor coverage is also provided by in-building penetration of the radio signal. Network structure 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.
Subscriber Identity ModuleOne 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 and Europe, most operators 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. Third party unlocking services exist that are often quicker and lower cost than the operator. In most countries removing the lock is legal. GSM securityGSM 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. 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. Related topics
External links
|
|||||||||||
|
|
|
|
|
|
Copyright 2008 WordIQ.com - Privacy Policy
::
Terms of Use
:: Contact Us
:: About Us This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Global System for Mobile Communications". |