Answer:  GSM SERVICES

GSM services follow ISDN guidlines and are classified as either teleservices or data services. Teleservices include standard mobile telephony and mobile originated or base originated traffic. Data services include computer to computer communication and packet switched traffic.

User services may be divided into three major categories:

1. telephone services:emergancy calling and facsimile, videotex and teletex
2. bearer services or data services: These are limited to layer 1,2,3 of OSI reference model. Supported services include packet switched protocols and data rates from 300 bps to 9.6 kbps. Data transmission is done using either a transparent mode or a non-transparent mode.
3. supplementary ISDN services: These are digital in nature and include call diversion,closed user groups, and caller identification. Supplementary serices also include theshort messaging service(SMS) which allows gsm subscribers and base stations to transmit alphanumeric pages of limited length(160 7-bit ASCII characters) while simultaneously carrying normal voice traffic. SMS also provides cell broadcast. SMS may be used for safety amd advisory applications, such as the broadcast of highway or weather information to all GSM subscribers within reception range.

GSM FEATURES

1. SUBCRIBER IDENTITY MODULE(SIM): It is a memory devide that stores information such as the subscriber's identification number, the networks and the countries where the subscriber is entitled to service, privacy keys and othe user specific information. SIMs are available as smart cards or plug-in modules which are less convenient that SIM cards but are nonetheless removable and portable. SIM gives GSM subscriber units their identity.
2. ON-THE-AIR PRIVACY: Virtually it is 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 must sign the Memorandum of Understanding(MoU) before developing GSM equipment or deploying a GSM system.