Delay - Cisco WS C4224V 8FXS - 200Mbps Ethernet Switch Software Configuration Manual

Appendix B Synopsis of Basic VoIP Concepts

Delay

OL-2031-02
Table B-1 Compression Methods and MOS Scores
Compression Method
G.711 PCM
1
G.729 CS-ACELP
2
G.729 x 2 encodings
G.729 x 3 encodings
3
G.729a CS-ACELP
1. Conjugate structure-algebraic code-excited linear prediction.
2. A G.729 voice signal is tandem-encoded two times.
3. G.729 Annex A.
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.
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.
Catalyst 4224 Access Gateway Switch Software Configuration Guide
Bit Rate Framing Size
(kbps) (ms)
64 0.125
8 10
8 10
8 10
8 10
A Voice Primer
MOS Score
4.1
3.92
3.27
2.68
3.7
B-5