Specifications

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Cisco 1751 Router Software Configuration Guide
OL-1070-01
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 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 is CODEC-induced delay with low bit-rate CODECs.
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 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 basically 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 medium. Handling delay (sometimes called serialization delay) is
caused by the devices that handle voice information. Handling delays have a significant impact on voice
quality in a packet network.
CODEC-induced delays are considered a handling delay. Table 1-2 shows the delay introduced by
different CODECs.
Table 1 Compression Methods and MOS Scores
Compression
Method
Bit Rate
(kbps) MOS Score
G.711 PCM 64 4.1
G.723.1 MP-MLQ 6.3 3.9
G.723.1 ACELP 5.3 3.65
G.726 ADPCM 32 3.85
G.729 CS-ACELP
1
1. Conjugate structure-algebraic code-excited linear
prediction
83.92
G.729 x 2
Encodings
83.27
G.729 x 3
Encodings
82.68
G.729a CS-ACELP 8 3.7
Table 2 CODEC-Induced Delays
CODEC Bit Rate (kbps) Compression Delay (ms)
G.711 PCM 64 5
G.723.1 MP-MLQ 6.3 30
G.723.1 ACELP 5.3 30
G.726 ADPCM 32 1