Internet telephony — voice transmission and call signalling over IP networks — can pro-vide 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 conferenc-ing; 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 imple-mentation of transaction-based protocols such as SIP efficient and scalable. Multi-party conferencing involves calls in which three or more people communicate si-

We describe the architecture and implementation of our comprehensive multi-platform collaboration framework known as Columbia InterNet Extensible Multimedia Architecture (CINEMA). It provides a distributed architecture for collaboration using synchronous communications like multimedia conferencing, instant messaging, shared web-browsing, and asynchronous communications like discussion forums, shared files, voice and video mails. It allows seamless integration with various communication means like telephones, IP phones, web and electronic mail. In addition, it provides value-added services such as call handling based on location information and presence status. The paper discusses the media services needed for collaborative environment, the components provided by CINEMA and the interaction among those components.

The public switched telephone network (PSTN) provides ubiquitous availability and very high scalability of more than a million busy hour call attempts per switch. If large carriers are to adopt Internet telephony, then Internet telephony servers should offer at least similar quantifi-able guarantees for scalability and reliability using metrics such as call setup latency, server call handling capacity, busy hour call arrivals, mean-time between failures and mean-time to recover. This thesis presents a reliable, scalable and interoperable Internet telephony architecture for user registration, call routing, conferencing and unified messaging using commodity hardware. The results extend beyond Internet telephony to encompass multimedia communication in general. The architecture presented in this thesis deals with two aspects: at least PSTN-grade re-liability and scalability of the Internet telephony servers, and interoperable Internet telephony services such as conferencing and voice mail using existing protocols. We describe the archi-tecture and implementation of our Session Initiation Protocol (SIP)-based enterprise Internet telephony architecture known as Columbia InterNet Extensible Multimedia Architecture (CIN-EMA). It consists of a SIP registration and proxy server, a multi-party conferencing server, a gateway for interworking SIP with ITU’s H.323, an interactive voice response system and a