Network congestion is an issue that can lead to a variety of problems on any data network; when the data network is also supporting voice traffic, these issues are even more serious. For example, WAN interfaces on a router may already be at or very near to capacity, leading to queuing issues that may result in packets being delayed, or even dropped as queues fill up. While this might not be a huge issue for non-interactive and reliable traffic like an FTP transfer, it presents a much greater problem when the network needs to support highly interactive traffic like packet-switched voice. If the level of congestion is high enough, users may not be able to complete their calls, have existing calls dropped, or may experience a variety of delays that make it difficult to participate in a “smooth” conversation.
In order to properly design a network to support voice traffic, WAN links need to be provisioned correctly, and QoS mechanisms need to be implemented in order to ensure that voice traffic is prioritized. When provisioning a WAN link to support multiple services (including voice), the available bandwidth of the link should be provisioned such that total data traffic accounts for a maximum of 75% of the necessary bandwidth, while the remaining 25% is available for additional needs, such as routing protocol requirements. When provisioning or planning a WAN link that will support voice traffic, keep in mind that the codec used will have the biggest influence on the amount of bandwidth used. Multiplying the bandwidth figure associated with a codec by the number of simultaneous phone conversations that need to be supported provides a good indication of how much bandwidth will need to be dedicated to voice traffic only across WAN links. Of course data traffic will also need to be considered, but this will vary in different network environments.