Special Issue on Voice Over IP—Theory and Practice

Voice over IP (VoIP) is set to become the dominant mechanism for speech communication in the future. This is being recognized by major telecommunications operators as the direction in which they will have to take their next generation networks. This is stimulated by the sight of major rivals, embracing VoIP as the basis of their alternative telephony service. If this wide-scale implementation is realized, then it is a giant step towards IP achieving that holy grail of a single unifying base for all communications. While it has become a received wisdom that IP will eventually conquer all, it is worth appraising why VoIP is seen as an attractive alternative to the present highly refined and hence optimized mechanisms for delivering telephony service. A dispassionate observer might reasonably conclude that VoIP is a considerably more complex and challenging technology to implement when compared to those circuit-switched methods used in traditional digital telecommunications networks. The answer is that VoIP brings flexibility and service convergence, allied with the key point that design complexity no longer really matters once any associated performance problems are solved. Crucially service convergence brings lower overall cost. Flexibility comes particularly with the ability to transport and route voice traffic using the ubiquitous IP transport network, whether the public internet or privately controlled. Allied to this is the changed nature of call control. The degree of centralized and network-based control varies with different VoIP schemes and protocols (e.g. SIP-based approaches with intelligent end terminals differ here from more centralized MGCP systems). However, whatever central functionality is required can be flexibly implemented and geographically distanced from the actual voice streams. A softswitch can be placed where administratively easiest and yet control voice traffic in many distant regions. The traditional digital telephony networks, wired and wireless, are the major specialist communication systems in the world. Once the approach was to arrange for data services to fit into the telephony structure (e.g. voice modems and ISDN). Now that goal has turned right around with an acceptance that it is voice that will be adapted onto the world’s data networks. With the recognition that this even includes mainstream telephony, VoIP has become the clear harbinger of service convergence. Other services easily follow such as video conferencing. The final challenge for service convergence will be to bring in delivery of TV services; even this is a practical reality nowadays with expanding broadband access to the home, but, just as with VoIP a few years ago, IP video is still the minority means by which people receive their TV entertainment. Viewed from the top level, VoIP can appear satisfyingly straightforward to the service provider with its advantages of flexibility and network ubiquity described above. For this reason the traditional telecommunications providers are seeing VoIP adopted right now by their major