Nortel RLC, Installation And Administration Manual

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64 RLC Installation and Administration Guide
Planning for RLC installation Standard 4.0
QoS
The routers used on your IP network must handle voice traffic while introducing
little or no congestion and few delays into the network. Network congestion or
excessive delay adversely effects voice quality.
Impact of delay on voice quality
The impact of the different voice algorithms (G.729A, G.726, and G.711) is well
documented. G.729A provides 8 to 1 compression and does not produce perfect
voice quality. Most users consider it quite acceptable, although some users claim
that they can hear the difference between it and G.711. The 8 to 1 compression
ratio and the acceptable voice quality make it the codec of choice for Voice over
IP (VoIP). Multiple instances of transcoding (that is, compressing and
decompressing) can have a significant impact on voice quality. G.729A is more
likely to be negatively affected by transcoding than G.711.
Most users do not understand the impact of network/PSTN delay or the impact
of voice compression algorithms on the perceived quality of voice. In order to
compensate for the variation in transmission time across the network (IP and
PSTN), the Remote Gateway 9100 Series product uses a jitter buffer to allow
voice packets to pass through it in a constant stream. The purpose of the jitter
buffer is to remove the variable delays from the voice packets sent across the
network, thus avoiding awkward-sounding speech. The default value of this
jitter buffer is 90 milliseconds (ms).
Note: Once you configure the jitter buffer, the jitter buffer can vary up to 30 ms
above or below the configured target.
To estimate the total delay the user experiences, add the following values:
„
jitter buffer delay
„
network delay
„
algorithmic encoding delay of the algorithm in use
To make the calculation simple, use 30 ms as the algorithmic delay. (It varies
depending on the voice algorithm, but 30 ms is a good estimate).