Page 224 - Asterisk™: The Future of Telephony

iLBC
The Internet Low Bitrate Codec (iLBC) provides an attractive mix of low bandwidth
usage and quality, and it is especially well suited to sustaining reasonable quality on
lossy network links.

Naturally, Asterisk supports it (and support elsewhere is growing), but it is not as
popular as the ITU codecs and, thus, may not be compatible with common IP tele-
phones and commercial VoIP systems. IETF RFCs 3951 and 3952 have been published
in support of iLBC, and iLBC is on the IETF standards track.
Because iLBC uses complex algorithms to achieve its high levels of compression, it has
a fairly high CPU cost in Asterisk.
While you are allowed to use iLBC without paying royalty fees, the holder of the iLBC
patent, Global IP Sound (GIPS), wants to know whenever you use it in a commercial
application. The way you do that is by downloading and printing a copy of the iLBC
license, signing it, and returning it to GIPS. If you want to read about iLBC and its
license, you can do so at http://www.ilbcfreeware.org.
iLBC operates at 13.3 Kbps (30 ms frames) and 15.2 Kbps (20 ms frames).

Speex
Speex is a variable bitrate (VBR) codec, which means that it is able to dynamically
modify its bitrate to respond to changing network conditions. It is offered in both
narrowband and wideband versions, depending on whether you want telephone quality
or better.
Speex is a totally free codec, licensed under the Xiph.org variant of the BSD license.
An Internet draft for Speex is available, and more information about Speex can be found
at its home page (http://www.speex.org).
Speex can operate at anywhere from 2.15 to 22.4 Kbps, due to its variable bitrate.

MP3

Sure thing, MP3 is a codec. Specifically, it’s the Moving Picture Experts Group Audio
#
Layer 3 Encoding Standard. 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
telephony codec, as it is optimized for music, not voice; nevertheless, it’s very popular
with VoIP telephony systems as a method of delivering Music on Hold.

# 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.”

196 | Chapter 8: Protocols for VoIP