System for Mobile Communication (GSM)
Abstract of GSM
system for mobile communication (GSM) is a globally accepted standard for digital
cellular communication. GSM is the name of a standardization group established
in 1982 to create a common European mobile telephone standard that would formulate
specifications for a pan-European mobile cellular radio system operating at 900
MHz. It is estimated that many countries outside of Europe will join the GSM partnership.
the evolution of cellular telecommunications, various systems have been developed
without the benefit of standardized specifications. This presented many problems
directly related to compatibility, especially with the development of digital
radio technology. The GSM standard is intended to address these problems.
provides recommendations, not requirements. The GSM specifications define the
functions and interface requirements in detail but do not address the hardware.
The reason for this is to limit the designers as little as possible but still
to make it possible for the operators to buy equipment from different suppliers.
The GSM network is divided into three major systems: the switching system (SS),
the base station system (BSS), and the operation and support system (OSS).
operations and maintenance center (OMC) is connected to all equipment in the switching
system and to the BSC. The implementation of OMC is called the operation and support
system (OSS). The OSS is the functional entity from which the network operator
monitors and controls the system. The purpose of OSS is to offer the customer
cost-effective support for centralized, regional, and local operational and maintenance
activities that are required for a GSM network. An important function of OSS is
to provide a network overview and support the maintenance activities of different
operation and maintenance organizations.
Services provided by GSM
From the beginning, the planners of GSM wanted ISDN compatibility in terms of the services offered and the control signaling used. However, radio transmission limitations, in terms of bandwidth and cost, do not allow the standard ISDN B-channel bit rate of 64 kbps to be practically achieved.
Using the ITU-T definitions, telecommunication services can be divided into bearer services, teleservices, and supplementary services. The most basic teleservice supported by GSM is telephony. As with all other communications, speech is digitally encoded and transmitted through the GSM network as a digital stream. There is also an emergency service, where the nearest emergency-service provider is notified by dialing three digits (similar to 911).
A variety of data services is offered. GSM users can send and receive data, at rates up to 9600 bps, to users on POTS (Plain Old Telephone Service), ISDN, Packet Switched Public Data Networks, and Circuit Switched Public Data Networks using a variety of access methods and protocols, such as X.25 or X.32. Since GSM is a digital network, a modem is not required between the user and GSM network, although an audio modem is required inside the GSM network to interwork with POTS.
Other data services include Group 3 facsimile, as described in ITU-T recommendation T.30, which is supported by use of an appropriate fax adaptor. A unique feature of GSM, not found in older analog systems, is the Short Message Service (SMS). SMS is a bidirectional service for short alphanumeric (up to 160 bytes) messages. Messages are transported in a store-and-forward fashion. For point-to-point SMS, a message can be sent to another subscriber to the service, and an acknowledgement of receipt is provided to the sender. SMS can also be used in a cell-broadcast mode, for sending messages such as traffic updates or news updates. Messages can also be stored in the SIM card for later retrieval.
Supplementary services are provided on top of teleservices or bearer services. In the current (Phase I) specifications, they include several forms of call forward (such as call forwarding when the mobile subscriber is unreachable by the network), and call barring of outgoing or incoming calls, for example when roaming in another country. Many additional supplementary services will be provided in the Phase 2 specifications, such as caller identification, call waiting, multi-party conversations.
Architecture of the GSM network
A GSM network is composed of several functional entities, whose functions and interfaces are specified. Figure 1 shows the layout of a generic GSM network. The GSM network can be divided into three broad parts. The Mobile Station is carried by the subscriber. The Base Station Subsystem controls the radio link with the Mobile Station. The Network Subsystem, the main part of which is the Mobile services Switching Center (MSC), performs the switching of calls between the mobile users, and between mobile and fixed network users. The MSC also handles the mobility management operations.
Not shown is the Operations and Maintenance Center, which oversees the proper operation and setup of the network. The Mobile Station and the Base Station Subsystem communicate across the Um interface, also known as the air interface or radio link. The Base Station Subsystem communicates with the Mobile services Switching Center across the A interface.
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