User manual

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47
Section 8
CoS
• 802.1p Default Priority
• 802.1p User Priority
The Switch supports 802.1p priority queuing Quality of Service. The following section discusses the implementation
of CoS (Quality of Service) and benefits of using 802.1p priority queuing.
Understanding IEEE 802.1p Priority
Priority tagging is a function defined by the IEEE 802.1p standard designed to provide a means of managing traffic
on a network where many different types of data may be transmitted simultaneously. It is intended to alleviate
problems associated with the delivery of time critical data over congested networks. The quality of applications
that are dependent on such time critical data, such as video conferencing, can be severely and adversely affected
by even very small delays in transmission.
Network devices that are in compliance with the IEEE 802.1p standard have the ability to recognize the priority
level of data packets. These devices can also assign a priority label or tag to packets. Compliant devices can also
strip priority tags from packets. This priority tag determines the packet's degree of expeditiousness and determines
the queue to which it will be assigned.
Priority tags are given values from 0 to 7 with 0 being assigned to the lowest priority data and 7 assigned to the
highest. The highest priority tag 7 is generally only used for data associated with video or audio applications, which
are sensitive to even slight delays, or for data from specified end users whose data transmissions warrant special
consideration.
The Switch allows you to further tailor how priority tagged data packets are handled on your network. Using queues
to manage priority tagged data allows you to specify its relative priority to suit the needs of your network. There
may be circumstances where it would be advantageous to group two or more differently tagged packets into the
same queue. Generally, however, it is recommended that the highest priority queue, Queue 3, be reserved for data
packets with a priority value of 7. Packets that have not been given any priority value are placed in Queue 0 and
thus given the lowest priority for delivery.
A weighted round robin system is employed on the Switch to determine the rate at which the queues are emptied
of packets. The ratio used for clearing the queues is 4:1. This means that the highest priority queue, Queue 3, will
clear 4 packets for every 1 packet cleared from Queue 0.
Remember, the priority queue settings on the Switch are for all ports, and all devices connected to the Switch will
be affected. This priority queuing system will be especially beneficial if your network employs switches with the
capability of assigning priority tags.