Nortel Networks NN43001-563 Reduce hop count, Adjust jitter buffer size, Reduce packet loss

136 ITG engineering guidelines

The average rtt time on that link is about 40 ms; the one-way link delayis

about 20 ms, of which the circuit transmission and serialization delay are

just a few milliseconds. Most of this link’s delay is caused by queueing.

Looking at Figure 26 "Queuing delay of various links" (page 134), if this link

is upgraded to T1, approximately 19 ms is shaved off the delay budget.

Reduce hop count

End-to-end delay can be reduced signiﬁcantly by reducing hop count,

especially on hops that traverse WAN links. Some the ways to reduce hop

count include the following:

•Attach the TLAN subnet directly to the WAN router.

•Improvemeshing. Add links to help improve meshing; adding a link from

router1 to router4 in the previous Tracerouteexample might cause the

routing protocol to use that new link, thereby reducing the hop count

by two.

•Node reduction. Co-located nodes can be connected into one larger

and more powerful router.

These guidelines affect the whole intranet, as they affect network

architecture, design and policies and involvesconsidering cost, political and

IP design issues. These topics are beyond the scope of this document.

Adjust jitter buffer size

The jitter buffer parameters directly affect end-to-end delay. Lowering the

voice playoutsettings decreasesone-way delay, but the decrease comes at

a cost of allowing less waiting time for voice packets that arrive late. Refer

to "IP Trunk3.01 (and later) DSP proﬁle settings" (page 161) for guidelines

on re-sizing the jitter buffer.

Reduce packet loss

Packetloss in intranets is generally related to congestion somewhere in the

network. Bottlenecks in links are where the packet loss is high because

packets get dropped, as the packetsare arriving faster than the link can

transmit them. The task of upgrading highly utilized links can remove the

source of packet loss on a particular ﬂow. An effort to reduce hop count

gives feweroppor tunities for routers and links to drop packets.

Other causes of packet loss notrelated to queueing delay are as follows:

•Poor link quality.

The underlying circuit could have such problems as transmission

problems, high line error rates, and be subject to frequent outages. The

circuit might possibly be provisioned on top of other services, such

as X.25, Frame Relay,or ATM. Check with the service provider for

information.

Nortel Communication Server 1000

IP TrunkFundamentals

NN43001-563 02.01 Standard

Release 5.5 21 December 2007