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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.
4. 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
fiberoptic-based or copper-based media. 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 packetized network. CODEC-induced delays are considered a handling delay. Table
2shows the delay introduced by different CODECs.
CODEC
G.711 PCM
G.726 ADPCM
G.728 LD-CELP
G.729 CS-ACELP
G.729a CS-ACELP
G.723.1 MP-MLQ
G.723.1 ACELP
Another processing delay is to generate the time voice packets want. With G729 Coder, DSP generates
a frame per 10 milliseconds. Two frames are placed in a voice packet, so packet relay is 20 millisecons.
Another source of processing relay is the time required for a packet to be forwarded into the output
queue.
5. Jitter
Jitter is nother factor of delay. In VoIP networks where existing a diversity between the expecting
receiving time and real receiving time of voice packets, jitter can become a problem which results in
incontinuous voice flow. D-LINK IP telephone receiver have built-in dejitter buffering for voice
rebroadcasting to compensate for a certain amount of jitter.
6. End-to-end Delay
It is not very difficult for those users who acquaint the end-to-end signal route / data route, the coding
and decoding technique, and the payload size to understand end-to-to delay. Coder delay (5
milliseconds for G711 and G726, 10 milliseconds for G729), packeting delay, fixed network delay and
the delay from the two ends to code/decode, compose the end-to-end connection delay.
7. Echo
Echo refers to that the user heard his own voice when communicating on phone receiver. It can be
canceled with relevant timing. If echo overtops 25 milliseconds, it will worsen the voice and make the
communicating pause. In a tradditional telecom network, echo is usually generated by the
non-matching of impedance switching from 4-wire network to 2-wire local loop, and is controled by
echo-cancel. In a voice packet network, echo-cancel is embedded in the low speed coder/decoder and
functions on every DSP. The echo-cancel must be limited by the time of waiting for echo accept. The
time is usually called echo mark. Generally echo mark is 32 milliseconds.
Model Name
Table 12-2 Delays caused by different CODECs
Bit Rate(kbps)
64
32
16
8
8
6.3
5.3
Compression Delay(ms)
0.75
1
3 to 5
10
10
30
30
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