Nortel VoIP Gateway, Configuration Manual

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164 Further network analysis
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• Routing and hop count: Each site pair takes different routes over the intranet. The route taken
determines the number and type of delay components that add to end-to-end delay. Sound
routing in the network depends on correct network design.
Reducing link delay
In this and the next few sections, the guidelines examine different ways of reducing one-way delay
and packet loss in the network.
The time taken for a voice packet to queue on the transmission buffer of a link until it is received at
the next hop router is referred to as the link delay. Methods to reduce link delays include:
• Upgrade link capacity to reduce the serialization delay of the packet. This also reduces the
utilization of the link, reducing the queueing delay. Before upgrading a link, check both
routers connected to the link for the upgrade and ensure correct router configuration
guidelines.
• Change the link from satellite to terrestrial to reduce the link delay by approximately 100 to
300 ms.
• Put into operation a priority queueing rule.
• Identify the links with the highest use and the slowest traffic. Estimate the link delay of these
links using Traceroute. Contact your service provider for help with improving your QoS.
Reducing hop count
To reduce end-to-end delay, reduce the hop count, especially on hops that move across WAN links.
Some of the ways to reduce hop count include:
• Improve meshing. Add links to help improve routing, adding a link from router1 to router4
instead of having the call routed from router1 to router2 to router3 to router4, reducing the hop
count by two.
• Router reduction. Join co-located gateways on one larger and more powerful router.
Adjust the jitter buffer size
The parameters for the voice jitter buffer directly affect the end-to-end delay and audio quality.
IP telephony dynamically adjusts the size of the jitter buffer to adjust for jitter in the network. The
network administrator sets the starting point for the jitter buffer.
Lower the jitter buffer to decrease one-way delay and provide less waiting time for late packets.
Late packets that are lost are replaced with silence, decreasing quality. Increase the size of the jitter
buffer to improve quality when jitter is high.