Global System for Mobile Communication Explained

The GSM standard has been accepted worldwide for digital cellular communication. In 1982, a standardization group established 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. Description of the specifications and characteristics of this standard is as follows.

Frequency Band
The frequency range varies from 1850 to 1990 MHz (mobile station to base station).

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

Channel Separation
The separation between adjacent carrier frequencies is 200 kHz.

Modulation
The process of sending a signal by changing the characteristics of a carrier frequency is called modulation. This is done by Gaussian Minimum Shift Keying (GMSK).

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

Access Method
This standard implements the Time Division Multiple Access (TDMA) concept. In TDMA, several different calls can share the same carrier. For each call a particular time slot is allotted.

Speech Coder
GSM makes use of Linear Predictive Coding (LPC), which is used to decrease the bit rate. LPC offers parameters for a filter which mimics the vocal tract. The signals pass through a speech coder leaving behind a residual signal. The signal is encoded at 13 kbps.

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

The mobile station (MS) includes mobile equipment and a smart card known as a Subscriber Identity Module (SIM). The SIM offers mobility so that the user can have access to subscribed services irrespective of a specific location. This card can be inserted into other GSM terminals or devices, enabling the user to make and receive calls, and use other subscribed services, from those terminals and devices as well.

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

The BTS addresses radio transceivers that define a cell, and handles radio-link protocols with the Mobile Station. The BSC handles radio resources the BTS. It manages radio-channel setup and frequency hopping. The BSC is the connection between mobile stations and mobile service switching centers.

The network subsystem's central component is the Mobile Services Switching Center (MSC). It works as a 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.

Telephony is the most basic service supported by GSM. Speech is digitally prearranged and transmitted through the network in the form of a digital stream, like all other communications. It also provides an emergency service where the nearest emergency-service provider can be called by dialing three digits.

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 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 messages that can be up to 160 characters. Messages are delivered in store-and-forward fashion.

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.

Four main versions exist based on the band used: GSM 900, GSM 1800, GSM 850 and GSM 1900.

GSM 900 makes use of 890-915 MHz to send information, and 935-960 MHz to receive information.

GSM 1800 makes use of 1710-1785 MHz to send information, and 1805-1880 MHz to receive information. GSM 1800 is also called Digital Cellular System (DCS), and is commonly used in Personal Communications Network.

GSM 1900 is called 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, or extended GSM, makes use of uplink frequency range 880-915 MHz and downlink frequency range 925-960 MHz. The specifications also define railways GSM (R-GSM) which make use of uplink frequency range 876-915 MHz and downlink frequency range 921-960 MHz.