Services for Internet Telephony

Jonathan Lennox

© 2004

The thesis is available in Postscript (2.1MB) and PDF (3.9MB). The PDF version has Acrobat bookmarks and hyperlinks as appropriate.


Internet telephony — voice transmission and call signalling over IP networks — can provide services far beyond those of the circuit-switched telephone network. This thesis discusses Internet telephony services in four broad areas: user-location services; multi-party conferencing; the interworking of Internet telephony and mobile telephony; and Internet telephony feature interaction.

User-location services are services which modify how a telephony server locates a user. Service authors need a way to control this process; this thesis presents two of them. The SIP Common Gateway Interface (SIP CGI) is a low-level server interface which allows fine-grained control of message processing in Session Initiation Protocol (SIP) servers. The Call Processing Language (CPL) is a protocol-independent, inherently safe high-level language for describing services in a way that is easily created and edited. The thesis also describes a general service framework providing a straightforward and powerful API atop which these and other service execution environments can be implemented, and an event thread architecture that makes implementation of transaction-based protocols such as SIP efficient and scalable.

Multi-party conferencing involves calls in which three or more people communicate simultaneously. This thesis presents a new approach to conferencing in which a fully-distributed, decentralized protocol establishes a fully connected mesh of signalling and media connections between conference participants.

Internet telephony needs to be able to connect to circuit-switched mobile telephony networks. The thesis presents a family of system architectures which allow SIP and UMTS networks to be connected directly, allowing traffic to flow directly to the mobile switching center handling a user's mobile terminal. These architectures eliminate triangular routing, transcoding, and other inefficiencies of indirect connections.

Finally, whenever services are defined, the issue arises of feature interaction, in which several features or services interact in unexpected and potentially undesirable ways. This thesis explains how Internet telephony alters the feature interaction problem, discusses the applicability of existing resolution techniques, and presents some new approaches for resolving interactions in the Internet environment.