Evolving the telephone network towards IP technology

There is a clear trend in the telecommunications industry to evolve its networks towards IP technology. This move exists not only for the introduction of new services such as high-bandwidth multimedia applications and Internet access, but can also be seen for the plain old telephone service. Incumbent as well as competitive carriers tend to consider IP technology as a valid candidate for the support of telephony services. This paper introduces and explains the architecture and the different protocols that are being developed within different standards bodies (such as ITU-T, ETSI, IETF) that will allow the migration of the classical telephony services from a circuit-switched technology towards a voice-over-IP-based technology. First, the basic functions of the telephony service are analysed and mapped on the IP-based telephony network architecture shown in Figure 1. Second, the requirements for the transport and application protocols needed to ensure information exchange and interworking between these functions are defined. The existing and emerging protocols supporting these functions (for example, megaco/H.248, SCTP, H.323, BICC, ...) are explained and positioned. The paper concludes by pointing out the limitations of the present solutions and the enhancements needed in order to grow to a full public IP telephony architecture.