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Global system for mobile communication 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.
Cellular is one of the fastest growing and most demanding telecommunications applications. Throughout 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.
From 1982 to 1985 discussions were held to decide between building an analog or digital system. After multiple field tests, a digital system was adopted for GSM. The next task was to decide between a narrow or broadband solution. In May 1987, the narrowband time division multiple access (TDMA) solution was chosen.
GSM 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).
GSM Network Architecture
The Switching System:
The switching system (SS) is responsible for performing call processing and subscriber-related functions. The switching system includes the following functional units.
home location register (HLR) -The HLR is a database used for storage and management of subscriptions. The HLR is considered the most important database, as it stores permanent data about subscribers, including a subscriber's service profile, location information, and activity status. When an individual buys a subscription from one of the PCS operators, he or she is registered in the HLR of that operator.
mobile services switching center (MSC) -The MSC performs the telephony switching functions of the system. It controls calls to and from other telephone and data systems. It also performs such functions as toll ticketing, network interfacing, common channel signaling, and others.
visitor location register (VLR) -The VLR is a database that contains temporary information about subscribers that is needed by the MSC in order to service visiting subscribers. The VLR is always integrated with the MSC. When a mobile station roams into a new MSC area, the VLR connected to that MSC will request data about the mobile station from the HLR. Later, if the mobile station makes a call, the VLR will have the information needed for call setup without having to interrogate the HLR each time.
authentication center (AUC) -A unit called the AUC provides authentication and encryption parameters that verify the user's identity and ensure the confidentiality of each call. The AUC protects network operators from different types of fraud found in today's cellular world.
equipment identity register (EIR) -The EIR is a database that contains information about the identity of mobile equipment that prevents calls from stolen, unauthorized, or defective mobile stations. The AUC and EIR are implemented as stand-alone nodes or as a combined AUC/EIR node.
The Base Station System (BSS):
All radio-related functions are performed in the BSS, which consists of base station controllers (BSCs) and the base transceiver stations (BTSs).
BSC -The BSC provides all the control functions and physical links between the MSC and BTS. It is a high-capacity switch that provides functions such as handover, cell configuration data, and control of radio frequency (RF) power levels in base transceiver stations. A number of BSCs are served by an MSC.
BTS -The BTS handles the radio interface to the mobile station. The BTS is the radio equipment (transceivers and antennas) needed to service each cell in the network. A group of BTSs are controlled by a BSC.
A GSM modem is one of the wireless modem that is devised to work with a GSM wireless network. It works with the same frequency of GSM wireless network. It is an important part of the GSM network. Now a days GSM based cell phones are more preferred than cdma phones, hence let us see its operation and its features.
The GSM wireless modem works in the way like a dial-up modem. The main difference between the GSM modem and dial up modem is that a dial-up modem sends and receive as data through a fixed telephone line while a GSM wireless modem sends and receives data through radio wave propagation.
A GSM modem can be an external device, a PC Card or a PCMCIA Card. A GSM modem can be connected to a computer using a serial cable or a usb cable. It works similar to a GSM mobile phone, and also GSM modem requires a SIM card for operation. It can be used for internet purpose also depending on various subscriptions to the service provider. There are so many feature and operations that can be performed using GSM modems. http://image.made-in-china.com/2f0j00jvDtmhwErMcI/Etpro101-GSM-Modem.jpg
Operations that can be performed using GSM modem:
1. We can read, write and delete SMS messages.
2. We can start Sending SMS messages.
3. We can reply to a SMS message.
4. We can monitor the signal strength in particular locality.
5. We can monitor the charging status and also the charge level in the battery.
6. We can read, write and search phone book entries.
7. We can use it in various projects for different purposes.
Features of GSM modems:
1. Support wide range of frequencies (from 850 MHZ to 1900 MHZ for different classification of GSM networks).
2. Supports integration with RS232 cable.
3. Can be interfaced to system using USB cables.
4. Input voltage varies from 5v to 30v.
5. Very less weight in few grams.
6. Provided with SIM holder and SMA antenna connector.
7. Programmable with AT commands.