Voice over IP – Mixing Oil and Water?

To run voice traffic over IP networks it is first digitized and packetized. Digitized audio streams are transported between endpoints by the real-time protocol (RTP). RTP is a connection-oriented end-to-end protocol that is designed to transport delay-sensitive information. RTP identifies the encapsulated payload type and includes sequence numbers and time stamps that are use to synchronize real-time information flows. RTP uses the connectionless, unreliable user datagram protocol (UDP) transport protocols because retransmission of lost or corrupted data disrupts real-time audio streams.
Delivering high quality voice communications over IP networks is a challenge because these networks have none of the characteristics that enable the PSTN to provide toll quality voice service. Unlike the PSTN, IP networks use packet switching rather than circuit switching technology. Packet switching works well for data because it maximizes bandwidth utilization by allowing all users to dynamically share network bandwidth.
The downside of IP’s dynamic resource sharing is that it provides only a best-effort delivery service which does not guarantee the performance levels of specific traffic flows such as voice conversations. To overcome these IP performance limitations enterprises are beginning to employ bandwidth management techniques such as prioritization to ensure that critical applications get the performance they need.
But bandwidth management alone simply allocates bandwidth to critical applications at the expense of other applications, many of which are also important to the enterprise. Similarly, just adding more bandwidth is usually ineffective because any additional bandwidth will be consumed by the most aggressive applications, not the most important ones.
What is needed, particularly on WANs where bandwidth is scarce and expensive, is a combination of adequate bandwidth and the ability to manage that bandwidth. Since VoIP consumes predictable amounts of bandwidth for each call in progress one of the first tasks in planning for VoIP is to determine the amount of bandwidth needed for the number of active calls to be supported by the network.