Nortel Meridian ITG Line 2.0 Description, Installation And Operation Manual page 84

Page 84 of 302 IP Network engineering guidelines
553-3001-204 Preliminary 0.10 August 2000
• LAN saturation. Packets may be dropped on under-engineered or faulty
LAN segments.
• Jitter buffer too small. Packets that arrive at the destination ITG, but too
late to be placed in the jitter buffer are essentially loss packets.
Adjusting jitter buffer size
The jitter buffer parameters directly affect the end-to-end delay. Lowering the
voice playout settings decreases one-way delay, but this comes at the expense
of giving less waiting time for voice packets that arrive late.
You adjust the jitter buffer size when you configure the DSP Profiles in the
ITG IP Phone application. The jitter buffer is statically configured and is the
same for all devices in the network. The jitter butter size range is
0-200 milliseconds. The default value is 50 milliseconds.
As each call is set up, the jitter buffer for each device is set to the next larger
configurable value for the selected codec. If the jitter buffer depth is
configured as zero, the depth of the jitter buffer is set to the smallest value the
device can support. In practice, the optimum depth of the jitter queue is
different for each call. For sets that are on a local LAN connection, a short
jitter queue is desirable to minimize delay. For sets that are several router
hops away, a longer jitter queue is required.
Lowering the jitter buffer size decreases the one-way delay of voice packets;
however setting the jitter buffer size too small will cause unnecessary packet
discard.
If the technician decides to discard packets, to downsize the jitter buffer, he
must do the following:
• Check the delay variation statistics. Obtain the one-way delay
distributions originating from all source ITG sites.
• 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 whether it is acceptable to resize the jitter buffer.
• Compute the standard deviation (σ) of one-way delay for that flow.
It is recommended that the jitter buffer size should not be set smaller than
2σ.