Implement QoS in IP networks 135
Other delay components
Other delay components, generally considered minor,are as follows.
• Router processing delay
The time it takes to forward a packet from one link to another on the
router is the transit or router processing delay. In a healthy network,
router processing delay is a few milliseconds.
• LAN segment delay
The transmission and processing delay of packets through a healthy
LAN subnet is just one or two milliseconds.
Reduce link delay
In this and the next few sections,different methods of reducing one-way
delay and packetloss in the IP Trunk3.01 (and later) network are examined.
Link delay is defined as the time it takes fora voice packet to be queued
on the transmission buffer of alink until it is received at the nexthop router.
Link delay can be reduced by the following:
•Upgrading link capacity.
This reduces the serialization delay of the packet,and more significantly,
it reduces the utilization of the link and the queueing delay. Toestimate
howmuch delay can be reduced, referto the tables and formulas given in
"Serialization delay" (page 133) and "Queuing delay" (page 133). Before
upgrading a link, check both routers connected to the link intended for
the upgrade and comply with router configuration guidelines.
•Changing the link from satellite to terrestrial.
This should reduce the link delay by on the order of 100 to 300 ms.
•Implementing a priority queueing discipline.
See "Queue management" (page 129).
Todetermine which links should be considered for upgrading, first list all the
intranet links used to support the IP Trunk 3.01 (and later) traffic, which can
be derived from the Tracerouteoutput for each site pair. Then using the
intranet link utilization report, note the highest utilized and/or the slowest
links. Estimate the link delay of suspect links using the Traceroute results.
Assume that a 256kbit/s link from Router1 toRouter2 has a high utilization;
the following is a Tracerouteoutput that traverses this link:
Richardson3% traceroute santa_clara_itg4
traceroute to santa_clara_itg4 (10.3.2.7), 30 hops max, 32
byte packets
router1 (10.8.0.1) 1 ms 1 ms 1 ms
router2 (10.18.0.2) 42 ms 44 ms 38 ms
router3 (10.28.0.3) 78 ms 70 ms 81 ms
router4 (10.3.0.1) 92 ms 90 ms 101 ms
santa_clara_itg4 (10.3.2.7) 94 ms 97 ms 95 ms
Nortel Communication Server 1000
IP TrunkFundamentals
NN43001-563 02.01 Standard
Release 5.5 21December 2007
Copyright© 2007, Nor tel Networks
.