The GSM project embraces an ambitious set of targets:

1. International roaming
2. Open architecture
3. High degree of flexibility
4. Easy installation
5. Interoperation with ISDN (Integrated Services Digital Networks), CSPDN (Circuit-Switched Public Data Network), PSPDN (Packed- Switched Public Data Network), and PSTN (Public-Switched Telephone Network)
6. High-quality signal and link integrity
7. High spectral efficiency
8. Low-cost infrastructure
9. Low-cost, small terminals
10. Security features

These objectives have been gradually achieved and a broad collection of services are provided. The GSM services are grouped into three categories:

• 1. Teleservices (TS)
• 2. Bearer services (BS)
• 3. Supplementary services (SS) TS cover, in essence, telephony, BS encompass basically data transmission, and SS are the value-added features.

Teleservices

Regular telephony, emergency calls, and voice messaging are within TS. Telephony, the old bidirectional speech calls, is certainly the most popular of all services. An emergency call is a feature that allows the mobile subscriber to contact a nearby emergency service, such as police, by dialing a unique number. Voice messaging permits a message to be stored within the voice mailbox of the called party either because the called party is not reachable or because the calling party chooses to do so.

Bearer Services

Data services, short message service (SMS), cell broadcast, and local features are within BS. Rates up to 9.6 kbit/s are supported. With a suitable data terminal or computer connected directly to the mobile apparatus, data may be sent through circuit-switched or packet-switched networks. Short messages containing as many as 160 alphanumeric characters can be transmitted to or from a mobile phone. In this case, a message center is necessary. The broadcast mode (to all subscribers) in a given geographic area may also be used for short messages of up to 93 alphanumeric characters. Some local features of the mobile terminal may be used. These may include, for example, abbreviated dialing, edition of short messages, repetition of failed calls, and others.

Supplementary Services

1. Some of the SS are as follows:
2. Advice of charge. This SS details the cost of a call in progress.
3. Barring of all outgoing calls. This SS blocks outgoing calls.
4. Barring of international calls. This SS blocks incoming or outgoing international calls as a whole or only those associated with a specific basic service, as desired.
5. Barring of roaming calls. This SS blocks all the incoming roaming calls or only those associated with a specific service.
6. Call forwarding. This SS forwards all incoming calls, or only those associated with a specific basic service, to another directory number. The forwarding may be unconditional or may be performed when the mobile subscriber is busy, when there is no reply, when the mobile subscriber is not reachable, or when there is radio congestion.
7. Call hold. This SS allows interruption of a communication on an existing call. Subsequent reestablishment of the call is permitted.
8. Call waiting. This SS permits the notification of an incoming call when the mobile subscriber is busy.
9. Call transfer. This SS permits the transference of an established incoming or outgoing call to a third party.
10. Completion of calls to busy subscribers. This SS allows notification of when a busy called subscriber becomes free. At this time, if desired, the call is reinitiated.
11. Closed user group. This SS allows a group of subscribers to communicate only among themselves.
12. Calling number identification presentation/restriction. This SS permits the presentation or restricts the presentation of the calling party’s identification number (or additional address information).
13. Connected number identification presentation. This SS indicates the phone number that has been reached.
14. Freephone service. This SS allocates a number to a mobile subscriber, and all calls to that number are free of charge for the calling party.
15. Malicious call identification. This SS permits the registration of malicious, nuisance, and obscene incoming calls.
16. Three-party service. This SS permits the establishment of conference calls.