Page 225 - Asterisk™: The Future of Telephony

Be aware that music cannot usually be broadcast without a license.
Many people assume that there is no legal problem with connecting a
radio station or CD as a Music on Hold source, but this is very rarely
true.

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 150 milliseconds, a normal flow of conversation is possible. When delay exceeds
300 milliseconds, it becomes difficult to avoid interrupting each other. Beyond 500
milliseconds, normal conversation becomes increasingly awkward and frustrating.

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 com-
pletely reproducing the sampled audio, and gaps in the data will be heard as static or,
in severe cases, entire missed words or sentences. Even packet loss of 5 percent can
severely impede a VoIP network.

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 will 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
mechanisms are implemented, such as packet numbering (for reconstructing blocks of
data), delivery acknowledgment, and re-requesting lost packets. In the world of VoIP,
getting the packets to the endpoint quickly is paramount—but 20 years of cellular
telephony has trained us to tolerate a few lost packets. *

* The order of arrival is important in voice communication, because the audio will be processed and sent to
the caller ASAP. However, with a jitter buffer the order of arrival isn’t as important, as it provides a small
window of time in which the packets can be reordered before being passed on to the caller.

Quality of Service | 197