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Cisco AVVID Network Infrastructure Enterprise Quality of Service Design
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Chapter 3 QoS in an AVVID-Enabled Campus Network
Overview
Figure 3-1
Campus Infrastructure
Delay Variation
In campus local area networks (LANs), serialization delay is not a significant concern. The amount of
time required for LAN interfaces to serialize the bits of packets onto the physical media is negligible; it
is not significant enough to affect delay-sensitive applications. Additionally in LANs, propagation delay
is of little concern because LANs by their very nature are not geographically dispersed.
The type of delay that is present in LANs is variation in delay, or jitter (as explained in the following
sections). This can adversely affect voice and video quality by introducing packet loss through jitter
buffer over-runs and under-runs.
Transmit Buffer Congestion
An additional contributor to packet loss in campus networks is TX buffer congestion. Transmit buffer
congestion can happen if a rate change occurs or if many interfaces are aggregated into a single uplink,
resulting in an oversubscription of the uplink's capacity to buffer packets.
The bits of a traffic flow that runs through a high-speed campus network serialize into and out of
switches at different rates depending on the link speed of the physical interfaces they are traversing.
When traffic serializes into a campus switch at gigabit speeds and is switched to a one hundred megabit
interface, the switch must have buffering capabilities in order to hold, or queue, the bits while it waits
to transmit them. When a transmit buffer fills, ingress interfaces are not able to place new traffic into
the transmit buffer of the target interface. When the switch cannot place a packet into the transmit queue
because of TX buffer congestion/exhaustion, packet drops will occur (as shown in
Figure 3-2
).
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