Implement QoS in IP networks 137
•Overloaded CPU.
This is another commonly-monitored statistic collected by network
management systems. If a router is overloaded, it means that the router
is constantly performing processing-intensive tasks, which impedes
the router from forwarding packets. Determine what the threshold
CPU utilization level isand check if any suspect router conforms to the
threshold. The router might have to be re-configured or upgraded.
•Saturation.
Routers can be overworkedwhen there are too many high capacity and
high traffic links configured on it. Ensure that routers are dimensioned
according to vendor guidelines.
•LAN saturation.
Packets might also be dropped on under-engineered or faulty LAN
segments.
•Jitter buffer too small.
Packetsthat arrive at the destination, but too late to be placed in the
jitter buffer,are essentially lost packets as well. Refer to "Adjust jitter
buffer size" (page 136).
•Frame slips.
Ensure that clocks are synchronized correctly.
Routing issues
Unnecessary delay can be introduced by routing irregularities. A routing
implementation might overlook a substantially better route. A high delay
variation can be caused by routing instability, misconfigured routing,
inappropriate load splitting, or frequent changes to the intranet. Severe
asymmetrical routing results in one site perceiving a poorer QoS than the
other site.
The Tracerouteprogram can be used to uncover these routing anomalies.
Then routing implementation and policies can be audited and corrected.
Network modeling
Network analysis can be difficult or time-consuming if the intranet and the
expected IP Trunk3.01 (and later) installation is large. Commercial network
modeling tools exist to analyze what-if scenarios predicting the effect of
topology,routing, and bandwidth changes to the network. The modelling
tools work with an existing network management system to load current
configuration, traffic and policies into the modelling tool. Network modeling
tools can help to analyze and try out any of the recommendations givenin
this document to predict how delay and error characteristics would change
the network.
Nortel Communication Server 1000
IP TrunkFundamentals
NN43001-563 02.01 Standard
Release 5.5 21December 2007
Copyright© 2007, Nor tel Networks
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