18.
Quality of Service
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210
Traffic Analysis for QoS Deployment
To effectively configure QoS, you must analyze the types of traffic using the interface and
determine their relative bandwidth demands. You should also evaluate the supported
applications’ sensitivity to latency, jitter, and packet loss.
General guidelines for each traffic type are given below. Consider them as general guidelines
and not strict recommendations. Once QoS parameters are set, you can monitor the
performance of the application to determine if the actual behavior of the applications matches
your expectations.
•
Voice
applications demand small amounts of bandwidth. However, the bandwidth
must be constant and predictable because voice applications are typically sensitive to
latency (inter-packet delay) and jitter (variation in inter-packet delay).
•
Video
applications are similar in needs to voice applications, with the exception that
bandwidth requirements are somewhat larger, depending on the encoding.
It is important to understand the behavior of the video application being used. Some
applications can transmit large amounts of data for multiple streams in one “spike,”
with the expectation that the end-stations will buffer significant amounts of video-
stream data. This can present a problem to the network infrastructure, because it must
be capable of buffering the transmitted spikes where there are speed differences (for
example, going from Gigabit Ethernet to Fast Ethernet).
•
Database
applications such as those associated with ERP, typically do not demand
significant bandwidth and are tolerant of delay. You can establish a minimum
bandwidth using a priority less than that of delay-sensitive applications.
•
Web browsing
applications cannot be generalized into a single category. Casual and
application-oriented traffic can be distinguished from each other by their server
source and destinations.
Most browser-based applications have an asymmetric dataflow (small dataflows from the
browser client, large dataflows from the server to the browser client). An exception to this
pattern may be created by some Java™ -based applications.
Web-based applications are generally tolerant of latency, jitter, and some packet loss, but
small packet-loss may have a large impact on perceived performance due to the nature of
TCP.
File server applications typically pose the greatest demand on bandwidth, although they are
very tolerant of latency, jitter, and some packet loss, depending on the network operating
system and the use of TCP or UDP.
Sorting Packets for QoS Handling
Packet Sorting by 802.1p Priority Values
Nokia ESB26 supports the standard 802.1p priority bits that are part of a tagged Ethernet
packet. The 802.1p bits can be used to prioritize the packet.
When a packet arrives at the switch, the switch examines the 802.1p priority field and assigns
the packet to a specific QoS queue for transmission. The 802.1p priority field is located
directly following the 802.1Q type field, and preceding the 802.1Q VLAN ID, as shown in
Figure 18-1.