Nortel Networks NN43001-563 - Manual (Page 164)

164 ITG engineering guidelines
3. Fax playout nominal delay. This can range from 0 to 300 ms, with 100
ms as the default size.
As discussed in "Adjust jitter buffer size" (page 137) , lowering the jitter buffer
size decreases the one-way delay of voice packets; however, setting the
jitter buffer size too small causes unnecessary packet discard.
If it is necessary to discard to downsize the jitter buffer, first check the
delay variation statistics. Obtain the one-way delay distributions originating
from all source IP Trunk 3.01 (and later) sites, using the measurements
outlined in
"Measure intranet QoS" (page 140) or "Post-installation network
measurements" (page 165) . Compute the standard deviation of one-way
delay for every flow. Some traffic sources with few hop counts yield small
delay variations, but it is the flows that produce great delay variations that
should be used to determine if it is acceptable to resize the jitter buffer.
Compute the standard deviation (s) of one-way delay for that flow. It is
recommended that the jitter buffer size should not be set smaller than 2s.
Silence Suppression parameters (Voice Activity Detection)
Silence Suppression, also known as Voice Activity Detection (VAD), is
enabled by default on a new IP Trunk 3.01 (and later) node. Enable/disable
VAD using the Enable voice activity detection checkbox on the TM 3.1 ITG
Node Properties -- DSP Profile codec Options tab. See
Figure 55 "DSP
Profile – Codec Options tab" (page 245) . To change the current DSP VAD
state to match the current VAD configuration, re-transmit card properties
from TM 3.1.
When silence is detected, the IP Trunk 3.01 (and later) node sends a flag to
the destination IP Trunk 3.01 (and later) node that denotes start of silence.
No voice packets are sent until the silence period is broken. There are two
parameters that control Silence Suppression, as follows:
1. Idle noise level. This is set at a default level of –65 dBm0.
2. Voice activity detection threshold. This is set at a default of 0dB. Voice
packets are formed when the audio level exceeds the idle noise level by
this threshold value.
These default parameters are suitable for most office environments.
Increasing either of these two parameters lowers the amount of IP traffic
generated, but increases clipping and dropped packets.
Disable Silence Suppression at tandem nodes
Silence Suppression introduces a different concept of half-duplex or
full-duplex at the voice message layer that results in a kind of statistical
multiplexing of voice messages over the WAN.
Nortel Communication Server 1000
IP Trunk Fundamentals
NN43001-563 01.01 Standard
Release 5.0 30 May 2007
Copyright © 2007, Nortel Networks
.