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Asterisk: The Future of Telephony by Leif Madsen, Jared Smith, Jim Van Meggelen

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Quality of Service

Quality of Service, or QoS as it’s more popularly termed, refers to the challenge of delivering a time-sensitive stream of data across a network that was designed to deliver data in an ad hoc, best-effort sort of way. Although there is no hard rule, it is generally accepted that if you can deliver the sound produced by the speaker to the listener’s ear within 300 milliseconds, a normal flow of conversation is possible. When delay exceeds 500 milliseconds, it becomes difficult to avoid interrupting each other. Beyond one second, normal conversation becomes extremely awkward.

In addition to getting it there on time, it is also essential to ensure that the transmitted information arrives intact. Too many lost packets will prevent the far end from completely reproducing the sampled audio, and gaps in the data will be heard as static or, in severe cases, entire missed words or sentences.

TCP, UDP, and SCTP

If you’re going to send data on an IP-based network, it will be transported using one of the three transport protocols discussed here.

Transmission Control Protocol

The Transmission Control Protocol (TCP) is almost never used for VoIP, for while it does have mechanisms in place to ensure delivery, it is not inherently in any hurry to do so. Unless you have an extremely low-latency interconnection between the two endpoints, TCP is going to tend to cause more problems than it solves.

The purpose of TCP is to guarantee the delivery of packets. In order to do this, several ...

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