D-Link DES-3028, User Manual

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DES-3028 DES-3028P DES-3028G DES-3052 DES-3052P Layer 2 Fast Ethernet Managed Switch
Section 8
CoS
Port Bandwidth
802.1p Default Priority
802.1p User Priority
CoS Scheduling Mechanism
CoS Output Scheduling
Priority Settings
TOS Priority Settings
DSCP Priority Settings
Port Mapping Priority Settings
MAC 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 rec-
ommended 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. The default setting is a strict round robin.
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.
Advantages of CoS
CoS is an implementation of the IEEE 802.1p standard that allows network administrators a method of reserving bandwidth for
important functions that require a large bandwidth or have a high priority, such as VoIP (voice-over Internet Protocol), web
browsing applications, file server applications or video conferencing. Not only can a larger bandwidth be created, but other less
critical traffic can be limited, so excessive bandwidth can be saved. The Switch has separate hardware queues on every physical
port to which packets from various applications can be mapped to, and, in turn prioritized. View the following map to see how the
Switch implements basic 802.1P priority queuing.
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