In the specification of a telecommunication standard such as GSM, the first step is of course the definition of the services offered by the system. GSM is a digital cellular system designed to support a wide variety of services, depending on the user contract and the network and mobile equipment capabilities. In GSM terminology, telecommunication services are divided into two broad categories:

• Bearer services are telecommunication services providing the capability of transmission of signals between access points [the user-network interfaces (UNIs) in ISDN]. For instance, synchronous dedicated packet data access is a bearer service.
• Teleservices are telecommunication services providing the complete capability, including terminal equipment functions, for communication between users according to protocols established by agreement between network operators.

In addition to these services, supplementary services are defined that modify or supplement a basic telecommunication service.

Bearer Services

There exist several categories of bearer services:

• Unrestricted digital information (UDI) is designed to offer a peer-to-peer digital link.
• The 3.1 kHz is external to the PLMN and provides a UDI service on the GSM network, interconnected with the ISDN or the PSTN by means of a modem.
• PAD allows an asynchronous connection to a packet assembler/disassembler (PAD). This enables the PLMN subscribers to access a packet-switched public data network (PSPDN).
• Packet enables a synchronous connection to access a PSPDN network and alternate speech and data, providing the capability to switch between voice and data during a call.
• Speech followed by data first provides a speech connection, and then allows to switch during the call for a data connection. The user cannot switch back to speech after the data portion.

Teleservices

In terms of application, teleservices correspond to the association of a particular terminal to one or several bearer services. They provide access to two kinds of applications:

• Between two compatible terminals;
• From an access point of the PLMN to a system including high-level functions, for example, a server.

Of course, the most basic teleservice supported by GSM is digital voice telephony, based on transmission of the digitally encoded voice over the radio. The voice service also includes emergency calls, for which the nearest emergency-service provider is notified by dialing three digits. The other teleservices that are defined for a PLMN are:

• Data services, with data rates ranging from 2.4 Kbps to 14.4 Kbps. These services are based on circuit-switched technology. Circuit switched means that during the communication, a circuit is established between two entities for the transfer of data. The physical resource is used during the whole duration of the call.
• Short message service (SMS), which is a bidirectional service for short alphanumeric (up to 160 bytes) messages.
• Access to a voice message service.
• Fax transmission.

Supplementary Services

Supplementary services include several forms of call forward (such as call forwarding when the mobile subscriber is unreachable by the network), caller identification, call waiting, multiparty conversations, charging information, and call barring of outgoing or incoming calls. These call-barring features can be used for example when roaming in another country, if the user wants to limit the communication fees.