3.1 Introduction

As is apparent from the previous chapter, although there are many reasons for using VoIP (see also Chapters 1 and 13), transport efficiency, in the sense of payload to packet size ratio, is unlikely to be one of them. This can become a concern in any bandwidth-restricted part of the network due to the effects on perceived voice quality. Networks typically comprise a customer premises or local environment, an access network, and a core network. Since it is impractical to deploy large-scale public networks that are configured as a complete mesh, some degree of bandwidth constriction will always be necessary.

Traditionally access systems, in particular copper-based access systems, have been of limited bit rate when compared with the customer premises environment or the core network. This remains the case today even though we are now seeing the mass deployment of DSL systems that are very much faster than traditional voice-band modems. Any successful VoIP deployment must therefore address the problems of how to cope with relatively low-speed network access. This chapter quantifies the issues this raises and considers various ways in which they might be overcome.

In contrast to the situation in an access network, the local area network (LAN) environment is typified by the deployment of high performance data networks such as Ethernet. In this case, significant bandwidth over-provisioning is both common and cost-effective. VoIP transport efficiency is therefore not generally an issue for LAN distribution. However, extending LAN connections over a wide area...