Information Technology Reference
In-Depth Information
sound for a certain time interval. This is often described as “building a codebook” of the
human voice traveling between the two endpoints. Using this process, G.729 is able to re-
duce bandwidth down to 8 kbps for each call; a fairly massive reduction in bandwidth.
Unfortunately, chopping the amount of bandwidth down comes with a price. Quality is
usually impacted by the compression process. Early on in the voice digitization years, the
powers that be created a measurement system known as a Mean Opinion Score (MOS) to
rate the quality of the various voice codecs. The test used to rate the quality of voice is
simple: A listener listens to a caller say the sentence, “Nowadays, a chicken leg is a rare
dish,” and rates the clarity of this sentence on a scale of 1-5. Table 1-2 shows how each
audio codec fared in MOS testing.
Table 1-2
Audio Codec Bandwidth and MOS Values
Key
To p i c
Codec
Bandwidth Consumed
MOS
G.711
64 kbps
4.1
Internet Low
Bitrate Codec (iLBC)
15.2 kbps
4.1
G.729
8 kbps
3.92
G.726
32 kbps
3.85
G.729a
8 kbps
3.7
G.728
16 kbps
3.61
Table 1-2 leads into a much-needed discussion about audio coder/decoders (codecs). You
can use quite a few different audio codecs on your network, each geared for different pur-
poses and environments. For example, some codecs are geared specifically for military en-
vironments where audio is sent through satellite link and bandwidth is at a premium. These
codecs sacrifice audio quality to achieve very streamlined transmissions. Other codecs are
designed to meet the need for quality.
If you stay in the Cisco realm for long, you will hear two codecs continually repeated:
G.711 and G.729. This is because Cisco designed all its IP phones with the ability to code
in either of these two formats. G.711 is the “common ground” between all VoIP devices.
For example, if a Cisco IP phone is attempting to communicate with an Avaya IP phone,
they may support different compressed codecs, but can at least agree on G.711 when
communicating.
Note:
G.729 comes in two different variants: G.729a (annex A) and G.729b (annex B).
G.729a sacrifices some audio quality to achieve a much more processor-efficient coding
process. G.729b introduces support for Voice Activity Detection (VAD), which makes
voice transmissions more efficient. You learn more about these variants in the following
section.
Key
To p i c
