3Com NBX 100 Administrator's Manual - page 37
Audio Settings
37
Audio Settings
Audio Settings enable you to affect the network impact of your audio
packets by enabling or disabling compression and silence suppression.
You can enable and disable these settings for the entire system and then
override the system-wide setting for individual devices.
Compression
Overview
Before voice traffic can be transmitted over a digital network, the audio
waveform, an analog signal, must be encoded into a digital format. The
digitized audio is packetized and delivered over the network to a
destination, and then decoded back into a voice waveform. Software
called a codec (coder/decoder) converts the audio information between
digital and analog formats.
Digitized audio formats have different properties. Each format represents
a compromise between bandwidth and audio quality, that is, high quality
audio typically requires more network bandwidth. Compressing the
digitized audio data can conserve bandwidth with little compromise in
audio quality, but compression requires increased processing overhead
when encoding and decoding the audio information. Too much
processing overhead can introduce delay.
Table 7
lists the codecs that the system supports and describes the
characteristics of each one.
.
Table 7 Supported Codecs
Codec
Description
G.711
No Compression
An International Telecommunications Union (ITU) standard for
audio encoding. Encoding and decoding is fast and support is
widespread. Also called MULAW or µLAW. A-law is a slight
variation, which European telephone systems use. G.711
provides high quality audio at 64 kbps. Telephone companies
worldwide use G.711 encoding to provide “toll-quality audio.”
ADPCM
Medium
Compression
Adaptive Differential Pulse Code Modulation (ADPCM) provides
good quality audio at a lower bitrate (32 kbps) than G.711. The
system uses the International Multimedia Association (IMA)
version of ADPCM.
G.729
High
Compression
G.729, an ITU standard, employs a more sophisticated
compression technique than ADPCM and it is supported
worldwide. The G.729A codec compresses the audio information
to 8 kbps, although processing overhead results in actual
bandwidths greater than 8 kbps.