Service Provided by GSM

From the beginning, the planners of GSM wanted ISDN compatibility in terms of the
services offered and the control signalling used. However, radio transmission limitations,
in terms of bandwidth and cost, do not allow the standard ISDN B-channel bit rate of 64
kbps to be practically achieved.
Using the ITU-T definitions, telecommunication services can be divided into bearer
services, teleservices, and supplementary services. The most basic teleservice supported
by GSM is telephony. As with all other communications, speech is digitally encoded and
transmitted through the GSM network as a digital stream. There is also an emergency
service, where the nearest emergency-service provider is notified by dialing three digits.
A variety of data services is offered. GSM users can send and receive data, at rates up to
9600 bps, to users on POTS (Plain Old Telephone Service), ISDN, Packet Switched
Public Data Networks, and Circuit Switched Public Data Networks using a variety of
access methods and protocols, such as X.25 or X.32. Since GSM is a digital network, a
modem is not required between the user and GSM network, although an audio modem is
required inside the GSM network to interwork with POTS.
Other data services include Group 3 facsimile, as described in ITU-T recommendation
T.30, which is supported by use of an appropriate fax adaptor. A unique feature of GSM,
not found in older analog systems, is the Short Message Service (SMS). SMS is a
bidirectional service for short alphanumeric (up to 160 bytes) messages. Messages are
transported in a store-and-forward fashion. For point-to-point SMS, a message can be
sent to another subscriber to the service, and an acknowledgement of receipt is provided
to the sender. SMS can also be used in a cell-broadcast mode, for sending messages such
3
as traffic updates or news updates. Messages can also be stored in the SIM card for later
retrieval .
Supplementary services are provided on top of teleservices or bearer services. In the
current (Phase I) specifications, they include several forms of call forward (such as call
forwarding when the mobile subscriber is unreachable by the network), and call barring
of outgoing or incoming calls, for example when roaming in another country. Many
additional supplementary services will be provided in the Phase 2 specifications, such as
caller identification, call waiting, multi-party conversations.