6.1 Introduction

Until relatively recently the Internet and PSTN networks had been developing independently of each other. While each has evolved to provide globally accessible services, the design principles of each have been very different. The initial PSTN services have been focused on conversational, real-time, applications principally telephony. The integrated services digital network (ISDN) has extended the PSTN model to support additional applications such as telefax and data transfer. Similarly, intelligent network (IN) extensions [1] have been added to support call-handling services. In contrast, IP networks, and the Internet in particular, have principally been developed for exchanging data between computers. In the case of the public Internet, the information accessible is both location-independent and enormous. With new, more complex, features continuing to be developed, mass-market services such as video-on-demand and on-line shopping become increasingly possible. VoIP represents one path for the possible convergence of these networks, which is starting for a number of additional reasons, including the following.

• Access

  Most people access the Internet via dial-up modem connections provided by the PSTN. It would therefore intuitively appear desirable for one network to support both IP and PSTN services. This network transition needs to be carefully managed as the Internet traffic increases due to the inherent effects of call holding times taking the PSTN outside its design envelope.

• Interworking

  The services offered by both networks are increasingly interacting and interworking with each other. Examples of this include:

  • the ability to use multi-functional mobile telephones...