Specifications

B-5
Catalyst 4224 Access Gateway Switch Software Configuration Guide
OL-2031-02
Appendix B Synopsis of Basic VoIP Concepts
A Voice Primer
Although it might seem logical from a financial standpoint to convert all calls to
low bit-rate codecs to save on infrastructure costs, you should be aware of the
drawbacks of designing voice networks with low bit-rate compression. One of the
main drawbacks is signal distortion due to multiple encodings (called tandem
encodings). For example, when a G.729 voice signal is tandem-encoded three
times, the MOS score drops from 3.92 (very good) to 2.68 (unacceptable).
Another drawback of low bit-rate codecs is codec-induced delay.
Delay
One of the most important design considerations in implementing voice is
minimizing one-way, end-to-end delay. Voice traffic is real-time traffic; if there
is too long a delay in voice packet delivery, speech becomes unrecognizable.
Delay is inherent in voice networking and is caused by a number of different
factors. An acceptable delay is less than 200 milliseconds.
There are two kinds of delays inherent in today’s telephony networks:
propagation delay and handling delay.
Propagation delay is caused by the characteristics of the speed of light traveling
via a fiber-optic-based or copper-based medium.
Table B-1 Compression Methods and MOS Scores
Compression Method
Bit Rate
(kbps)
Framing Size
(ms) MOS Score
G.711 PCM 64 0.125 4.1
G.729 CS-ACELP
1
1. Conjugate structure-algebraic code-excited linear prediction.
810 3.92
G.729 x 2 encodings
2
2. A G.729 voice signal is tandem-encoded two times.
810 3.27
G.729 x 3 encodings 8 10 2.68
G.729a
3
CS-ACELP
3. G.729 Annex A.
810 3.7