Nortel VoIP Gateway, Configuration Manual, Page: 192 / 220

192 Implementing QoS in IP networks
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Traffic mix
This section describes QoS works with the IP telephony, and what new intranet-wide results can
occur.
Before putting into operation QoS in the network, determine the traffic mix of the network.
QoS depends on the process and ability to determine traffic (by class) so as to provide different
services.
With an intranet designed only to deliver IP telephony traffic, where all traffic flows are equal
priority, there is no need to consider QoS. This network can have one class of traffic.
In most corporate environments, the intranet supports data and other services. When planning to
provide voice services over the intranet the installer must determine the following:
• Is there existing QoS? What kind? IP telephony traffic must take advantage of established
mechanisms if possible.
• What is the traffic mix? If the IP telephony traffic is light compared to data traffic on the
intranet, then IP QoS can work. If IP telephony traffic is heavy, data services can be affected if
QoS is biased toward IP telephony traffic.
TCP traffic behavior
Most of corporate intranet traffic is TCP-based. Different from UDP, which has no flow control,
TCP uses a sliding window flow control mechanism. Under this design, TCP increases its window
size, increasing throughput, until congestion occurs. Congestion results in packet losses, and when
that occurs the throughput decreases, and the whole cycle repeats.
When multiple TCP sessions flow over few congestion links in the intranet, the flow control
algorithm can cause TCP sessions in the network to decrease at the same time, causing a periodic
and synchronized surge and ebb in traffic flows. WAN links can appear to be overloaded at one
time, and then followed by a period of under-utilization. There are two results:
• bad performance of WAN links
• IP telephony traffic streams are unfairly affected