2.1 Introduction

The success of VoIP is strongly influenced by customer opinions of call quality and how this quality compares with that of the public switched telephone network (PSTN).

The unreliable nature of the Internet and initial product offerings, where cost savings were often gained at the expense of quality, led to an image that VoIP is of a worse quality than circuit-switched networks. The 'poor quality' image has, to some extent, accompanied VoIP as it has moved from the domain of the Internet enthusiast to being used in today's carrier-scale networks.

Carriers are eager to squeeze maximum efficiency from their networks but understand that there is a minimum quality level required to achieve customer acceptance. Enterprise customers, now adopting VoIP, are also sensitive to noise, distortions and general impairments in their voice communication solutions.

Yet, tainted by the experience of experiments on the public Internet and earlier generations of equipment, one of the most commonly discussed concerns held by people contemplating VoIP is that it will never deliver the 'appropriate quality'. Significant steps have been taken over the last few years to achieve higher quality systems, and this chapter provides a view on some of the issues and choices that VoIP system designers face. In particular, one issue to be considered is: 'What is the appropriate quality?' As will be shown in this chapter, the achievement of high-quality VoIP solutions is not an impossibility. Rather it is simply an engineering challenge.