Nortel Networks NN43001-563 - Manual (Page 128)

128 ITG engineering guidelines
Unless voice traffic has been configured to have higher priority, the data file
download could use most of the available bandwidth. This would cause
voice packet loss and therefore poor voice quality.
Recommendation
Nortel strongly recommends implementing suitable QoS mechanisms on any IP
network carrying VoIP.
This section outlines what QoS mechanisms can work in conjunction with
the IP Trunk 3.01 (and later) node and the intranet-wide consequences if
the mechanisms are implemented.
Traffic mix
Before implementing QoS mechanisms in the network, assess the traffic
mix of the network. QoS mechanisms depend on the process and ability to
distinguish traffic by class to provide differentiated services.
If an intranet is designed to deliver only IP Trunk 3.01 (and later) traffic, and
all traffic flows are of equal priority, then there is no need to consider QoS
mechanisms. This network would only have one class of traffic.
In most corporate environments, the intranet primarily supports data
services. When planning to offer voice services over the intranet, assess
the following:
• Are there existing QoS mechanisms? What kind? IP Trunk 3.01
(and later) traffic should take advantage of established mechanisms
if possible.
• What is the traffic mix? If the volume of IP Trunk 3.01 (and later) traffic is
small compared to data traffic on the intranet, then IP QoS mechanisms
will be sufficient. If IP Trunk 3.01 (and later) traffic is significant, data
services might be impacted when those mechanisms are biased toward
IP Trunk 3.01 (and later) traffic.
TCP traffic behavior
The majority of corporate intranet traffic is TCP-based. Unlike UDP, which
has no flow control, TCP uses a sliding window flow control mechanism.
Under this scheme TCP increases its window size, increasing throughput
until congestion occurs. Congestion is detected by packet losses, and when
that happens the throughput is quickly throttled down, and the whole cycle
repeats. When multiple TCP sessions flow over few bottleneck links in the
intranet, the flow control algorithm can cause TCP sessions in the network
to throttle at the same time, resulting in a periodic and synchronized surge
Nortel Communication Server 1000
IP Trunk Fundamentals
NN43001-563 01.01 Standard
Release 5.0 30 May 2007
Copyright © 2007, Nortel Networks
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