Global System for Mobile Communication

Global system for mobile communication (GSM) is a worldwide accepted standard for digital cellular communication. In 1982 a standardization group established named as GSM to create a common European mobile telephone standard that would formulate specifications for a pan-European mobile cellular radio system operating at 900 MHz.

GSM Specifications
Specifications for different personal communication services (PCS) systems are different among the different PCS networks. Description of the specifications and characteristics for GSM is as follows.

Frequency band:
For GSM the frequency range is varying from 1,850 to 1,990 MHz (mobile station to base station).

Duplex distance:
80 MHz is the duplex distance. Distance between the uplink and downlink frequencies is called Duplex distance.

Channel separation:
The separation between adjacent carrier frequencies is 200 kHz in GSM.

Modulation:
The process of sending a signal by changing the characteristics of a carrier frequency is called Modulation. In GSM, this is done by Gaussian minimum shift keying (GMSK).

Transmission rate:
GSM is a digital system with transmission rate of 270 kbps.

Access method:
GSM implement the time division multiple access (TDMA) concept. In TDMA technique several different calls may share the same carrier. For each call a particular time slot is allotted.

Speech Coder:
GSM make use of linear predictive coding (LPC). The use of LPC is to decrease the bit rate. The LPC offers parameters for a filter which mimics the vocal tract. The signals pass through speech coder leaving behind a residual signal. The signal is encoded at 13 kbps

GSM Network
GSM offers recommendations not requirements. GSM specifications do not address the hardware but define the functions and interface requirements in detail. The reason is to bind the designers as little as possible but still make it possible for the operators to buy different equipment from different suppliers. GSM network is divided into three major systems:

1) Mobile Station
The mobile station (MS) includes mobile equipment and a smart card known as Subscriber Identity Module (SIM). The SIM offers mobility so that the user can have access to subscribed services irrespective of a specific location. The SIM card can be inserting into another GSM terminal so that the user is able to receive calls at that terminal make calls from that terminal and receive other subscribed services.

2) Base Station Subsystem
The Base Station Subsystem consists of two parts, Base Transceiver Station (BTS) and Base Station Controller (BSC). These stations communicate across the standardized interface allowing operation between components made by different suppliers.

The Base Transceiver Station addresses the radio transceivers that defines a cell and handles the radio-link protocols with the Mobile Station. The Base Station Controller handles the radio resources for one or more BTSs. It manages radio-channel setup and frequency hopping. The BSC is the connection between the mobile station and the Mobile service Switching Center (MSC).

3) Network Subsystem
Network Subsystem’s central component is the Mobile services Switching Center (MSC). It work as normal switching node of the PSTN or ISDN and additionally offers all the functionality needed to handle a mobile subscriber such as authentication, registration, location updating and call routing to a roaming subscriber. These services are offered in conjunction with several functional entities which together form the Network Subsystem.

GSM Services
Telephony is the most basic teleservice supported by GSM. Speech is digitally prearranged and transmitted through the GSM network in the form of digital stream like all other communications. It also provides an emergency service where the nearest emergency-service provider is reported by dialing three digits. By using a variety of access methods and protocols such as X.25 or X.32, GSM users can send and receive data at rates 9600 bps to users on POTS, ISDN, Packet Switched Public Data Networks and Circuit Switched Public Data Networks. A modem is not required between the user and GSM network because GSM is a digital network.

Short Message Service (SMS) is a unique feature of GSM. SMS is a dual directional service for short alphanumeric up to 160 bytes messages. In a store-and-forward fashion messages are delivered. A message can be passed to another subscriber to the service and an acknowledgement of receipt is provided to the sender for point-to-point SMS. For sending messages such as traffic updates or news updates SMS can also be used in a cell-broadcast mode.

GSM Frequency Ranges
There are four main GSMs versions exist based on the band used: GSM-900, GSM-1800, GSM-850 and GSM-1900.
1)GSM-900 makes use of 890-915MHz to send information and 935-960MHz to receive information.
2)GSM-1800 makes use of 1710-1785MHz to send information and 1805-1880 to receive information.
GSM-1800 is called as Digital Cellular System (DCS) and it is commonly used in Personal Communications Network. GSM-1900 is called as Personal Communications Service (PCS-1900). PCS-1900 standards are integrated with that of GSM/DCS specifications resulting in a specification for GSM/DCS/PCS based on the Release 98 Series of GSM Specifications.

E-GSM, the extended GSM make use of uplink frequency range 880MHz - 915 MHz and downlink frequency range 925MHz - 960MHz. The GSM specifications also define railways GSM (R-GSM) which make use of uplink frequency range 876 MHz - 915 MHz and downlink frequency range 921MHz - 960 MHz. All these specifications are called as GSM-900.