GSM services are classified as either teleservices or data services. Teleservices include standard mobile telephony and mobile-originated traffic. Data services include computer to computer communication and packet switched traffic. User services may be divided into three major categories.

A. Telephone services: These include emergency calling and facsimile. GSM also supports Videotex and Teletex.

B. Bearer services or data services: These are limited to layer 1,2 and 3 of the open system interconnection (OSI) reference model. Supported services include packet switched protocols and data rates from 300bps to 9.6 kbps. Data may be transmitted using transparent or non transparent mode.

C. Supplementary ISDN services: These are digital in nature and include call diversion, closed user groups and caller identification, and are not available in analog mobile networks. Supplementary services also include short messaging service (SMS) which allows GSM subscribers and base station to transmit alphanumeric pages of limited length while simultaneously carrying normal voice traffic. SMS provides cell broadcast also can be used for safety and advisory applications such as the broadcast of highway or weather information to all GSM subscribers.

The features of GSM are:

A. Subscriber Identity Module (SIM):

• It is a memory device that stores information such as subscriber’s identification number. The networks and countries where the subscriber is entitled to service, privacy keys and other user specific information.

• A subscriber uses SIM with four digit personal ID number to activate service from any GSM phone.

• SIM is available as smart card or plug in module, which is less convenient than the SIM card but is removable and portable.
• Without SIM installed all GSM mobiles are identical and non operational. SIM gives GSM subscriber units their identity.
• Subscriber can plug their SIM into any suitable terminal such as a hotel phone, public phone , or any portable or mobile and are then able to have all incoming GSM calls routed to that terminal and have all outgoing calls billed to their home phone no matter where they are in the world.

B. On the air privacy:

• Unlike analog FM cellular phone system which can be readily monitored, it is virtually impossible to eavesdrop on a GSM radio transmission.
• The privacy is made possible by encrypting the digital bit stream sent by a GSM transmitter, according to a specific secret cryptographic key that is known only to the cellular carrier. This key changes with time for each user.
• Every carrier and GSM equipment manufacturer has to sign an international agreement MoU which allows the sharing of cryptographic algorithms and other proprietary information between countries and carriers.