IntroductionSetting up a network to carry Voice over IP (VOIP) traffic along with the data packets requires an understanding of what features are needed in the routers and switches of the network. It is also helpful to understand how much data bandwidth is required to successfully transport VOIP voice conversations, and what tradeoffs are possible to get by with less bandwidth than would normally be necessary.
In this paper, we will try to give some advice in planning a network that will carry voice and data traffic with a minimum of problems. We will look at voice quality requirements, network layout, planning capacity, and router selection and configuration.
Voice Quality Requirements
Users have certain quality expectations for telephone communication. When they are talking to a customer through a VOIP link, for instance, they might insist on toll-quality voice. This means that they want it to sound to them and to the customer as though they are on an ordinary TDM telephone link.
The quality of a call can be measured using one of several call quality metric calculations. The most commonly used system is the Mean Opinion Score (MOS). The MOS score of a call is between 1 (for unusable) and 5 (for excellent). VOIP calls that are working properly fall between 3.5 and 4.2 MOS. A score of 4.0 is defined as toll quality. Other systems for quality measurement are R-factor, PSQM, PESQ, and PAMS. These other systems produce scores for a call that can be mapped to MOS for comparison.
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