A chief objection to Voice over IP (VoIP) and softswitched networks is the notion that their quality of service (QoS) is inferior to the Public Switched Telephone Network (PSTN). QoS covers a number of parameters, but it is mostly concerned with the user's perception of the voice quality, the call setup, and keeping the call up for the intended duration of the call (avoiding dropped calls). VoIP is time sensitive and most degradation of voice quality is the result of latency, jitter, and lost packets. It should be noted here that the context of this chapter focuses on private networks, enterprise- and service provider-maintained and not the public Internet. Voice quality in this chapter will refer to the fidelity of the reproduced speech and its intelligibility.

Great volumes have been written about QoS on Internet Protocol (IP) networks. This chapter will set forth measures that improve QoS on an IP network and that provide QoS, especially voice quality over an IP network, as good as that of the PSTN. Earlier in the history of the industry, a perception existed that the IP voice quality was not as good as that of circuit-switched voice. International Telecommunication Union Telecommunications Standardization Sector (ITU-T) Recommendation E.800 defines QoS as "The collective effect of service performance, which determines the degree of satisfaction of a user of the service." Service providers often state that the QoS the end user experiences must equal his or her experience with the PSTN. With proper engineering,...