System information
MP3
Sure thing, MP3 is a codec. Specifically, it’s the Moving Picture Experts Group Audio
Layer 3 Encoding Standard.
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With a name like that, it’s no wonder we call it MP3! In
Asterisk, the MP3 codec is typically used for music on hold (MoH). MP3 is not a tel-
ephony codec, as it is optimized for music, not voice; nevertheless, it’s very popular
with VoIP telephony systems as a method of delivering MoH.
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 there is an extremely low-latency interconnection between the two end-
points, TCP will tend to cause more problems than it solves.
#If you want to learn all about MPEG audio, do a web search for Davis Pan’s paper titled “A Tutorial on
MPEG/Audio Compression.”
Quality of Service | 629