GSM has three types of services : teleservices, bearer services, and supplementary services. Teleservices include telephony, fax, emergency calls, teletex, short Message services, fax mail, and voice mail. Supplementary services include call forwarding, call barring, etc. Bearer services are used for transporting user data. Some of the bearer services are listed below:

1. 1. Asynchronous and synchronous data, 300-9600 bps.
   2. Alternate speech and data, 300-9600 bps.
   3. Asynchronous PAD (packet-switched, packet assembler/disassembler) access, 300-9600 bps.
   4. Synchronous dedicated packet data access, 2400-9600 bps.

Conventional GSM has limitations in data services:

1. 1. It does not provide direct connection to the Internet. In order to access to the Internet, GSM needs to call Internet Service Provider (ISP).
   2. Uplink and downlink channels allocated for a user are for entire call period.
   3. It has time-oriented charging, that is, payment is based on connection time, not on data volumes.
   4. Connection setup takes about 20-25 seconds.
   5. Limited capacity (9600 bps).
   6. GSM was designed for speech, not for data, hence 50% radio capacity is wasted, and also there is no optimal channel coding for data.

1.3 The GSM System Architecture

The GSM system architecture, includes four subsystems (cf. Figure 1): Mobile Station (MS), Base Station Subsystem (BSS), Network and Switching Subsystem (NSS), and Operation Sub-System (OSS).

The MS subsystem involves a radio part, an interface, and a Subscriber Identity Module (SIM). The radio part carries out all the functions related to the radio interface (Um), e.g. receiving and transmitting radio signals, signal processing, frequency hopping, and channel management. The interface to terminal equipment acts as a gateway between the terminal and the radio part. The SIM contains all the subscriber-related information on the MS side of Um to identify a subscriber and take care of the security. The SIM is implemented as a smart card.

The BSS forms cell structure of GSM network. It includes two types of network elements: the Base Transceiver Station (BTS), and the Base Station Controller (BSC).

The BTS is a transmission component. It carries out radio signal transmission and reception, signal processing, speech encoding and decoding, and transmission rate adaptation. The BSC is a managing component. It is responsible for all the management of the Um, e.g. channel allocation and deallocation, handover, and timing of radio signals. One BTS implements one cell in GSM. A BSC can manage several BTSs.

The NSS comprises the main switching functions of GSM and all the databases needed for subscriber information, mobility management, and interworking. It contains the following databases: Home Location Register (HLR), Visitor Location Register (VLR), Authentication Center (AUC), and Equipment Identity Register (EIR).

The MSC performs the basic switching function by setting up calls to/from MSs.

The GSM system also communicates with other networks such as the Public Switched Telephone Network (PSTN), Integrated Services digital Network(ISDN), Circuit-Switched Public Data Network (CSPDN), and packet-switched public data network (PSPDN).