User`s guide

VoiceFinder AP200 VoIP Residential Gateway (RGW) User’s Guide Version 1.1
AddPac Technology Co.,Ltd. -
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5.1.2.2. Mean Opinion Score
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 mean opinion score for
that sample. Table 5-1
shows the relationship between CODECs and MOS
scores.
Table 5-1: Compression Methods and MOS Scores
Compression
Method
Bit Rate
(kbps)
Processing
(MIPS)
Framing
Size
MOS Score
G.711 PCM 64 0.34 0.125 4.1
G.729 CS-ACELP 8 20 10 3.92
G.729a CS-ACELP 8 10.5 10 3.7
G.723.1 MP-MLQ 6.3 16 30 3.9
G.723.1 ACELP 5.3 16 30 3.65
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 exercise
additional care when designing voice networks with low-bit rate compression.
There are drawbacks to compressing voice. One of the main drawbacks is
signal distortion due to multiple encoding (called tandem encoding). 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
is CODEC-induced delay with low bit-rate CODECs.
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 will be 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 delay 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 media. Handling delay (sometimes called serialization delay) is