Table of Contents
Advertisement
A Voice Primer
codecs
Mean Opinion Score
Catalyst 4224 Access Gateway Switch Software Configuration Guide
B-4
In response to the limitations of analog transmission, the telephony industry
migrated to digital transmission using pulse code modulation (PCM) or adaptive
differential PCM (ADPCM). In both cases, analog sound is converted into digital
form by sampling the analog sound 8000 times per second and converting each
sample into a numeric code.
PCM and ADPCM are examples of waveform codec and are compression
techniques that exploit the redundant characteristics of the waveform itself. In
addition to waveform codecs, there are source codecs that compress speech by
sending only simplified parametric information about voice transmission. Thus,
these codecs require less bandwidth. Source codecs include linear predictive
coding (LPC), code-excited linear prediction (CELP) and multipulse-multilevel
quantization (MP-MLQ).
Coding techniques for telephony and voice packet are standardized by the ITU-T
in its G-series recommendations. The Catalyst 4224 uses the following coding
standards:
G.711—Describes the 64-kbps PCM voice coding technique. In G.711,
•
encoded voice is already in the correct format for digital voice delivery in the
PSTN or through PBXs.
G.729—Describes CELP compression where voice is coded into 8-kbps
•
streams. There are two variations of this standard (G.729 and G.729
Annex A) that differ mainly in computational complexity, but both provide
speech quality similar to 32-kbps ADPCM.
Each codec provides a certain quality of speech. The quality of transmitted speech
is a subjective response of the listener. A common benchmark used to determine
the quality of sound produced by specific codecs is the mean opinion score
(MOS). With MOS, a wide range of listeners judge the quality of a voice sample
(corresponding to a particular codec) on a scale of 1 (bad) to 5 (excellent). The
scores are averaged to provide the MOS for that sample.
relationship between codecs and MOS scores.
Appendix B
Synopsis of Basic VoIP Concepts
Table B-1
shows the
OL-2031-02
Advertisement
Table of Contents
Troubleshooting
