Internet telephony

One of the key ideas at work in the telecommunications sector for over fifteen years is service integration. This concept aims to enable the coexistence, within a unified communications network, of communications that are heterogeneous in terms of both content and transmission constraints (data rates, delays, losses, reliability criteria, traffic criteria, etc.), and was the prelude to ISDN (Integrated Services Digital Network) and ATM (Asynchronous Transfer Mode) technologies. The fast-growing field of Internet telephony is in line with this approach, since it seeks to reproduce (and extend) the service provided by the traditional telephone network on a packet-switched network, equipped with Internet protocols, and therefore primarily designed for the asynchronous transmission of computer data. This telephony service is characterized by "real-time" transmissions of audio data streams between two or more users (in the case of conference calls), and by the exchange of the information required to control these transmissions, or signaling data. Its extension involves the addition of other modes of communication, such as video streams, text or graphic data and application sharing, as well as the evolution of signaling procedures, enabling new call management functionalities, rerouting, integration with computer applications, etc.

In this article, we review the two main approaches to Internet telephony, one originating from the International Telecommunication Union (ITU) and whose various components are grouped together under the nameH.323; the other proposed by the Internet community through the Internet Engineering Task Force (IETF), grouped together under the name of the SIP system (see the "SIP Protocol" article in this treatise). What these approaches have in common is the definition of signaling procedures enabling the establishment, control and release of telephone and, by extension, multimedia calls. Such calls are communications between two (point-to-point) or more parties (multipoint conferences), interconnecting terminal equipment and other equipment offering gateway (interface with a non-IP network) and administration functionalities in the broadest sense.

Regardless of the approach used, the immediate advantage of Internet telephony, in the current state of the network, is a reduction in costs due to the special pricing that prevails for the Internet, as well as the artificially high costs of telephone communications. In the longer term, the nature of IP networks and the transport mechanisms they offer will make Internet telephony attractive, by facilitating the integration of telephone-type communications within a variety of services.