Understanding Qos - D-Link xStack DES-3800 Series User Manual

The picture above shows the default priority setting for the Switch. Class-7 has the highest priority of
the eight priority queues on the Switch. In order to implement QoS, the user is required to instruct the
Switch to examine the header of a packet to see if it has the proper identifying tag tagged. Then the
user may forward these tagged packets to designated queues on the Switch where they will be
emptied, based on priority.
For example, lets say a user wishes to have a video conference between two remotely set computers.
The administrator can add priority tags to the video packets being sent out, utilizing the Access Profile
commands. Then, on the receiving end, the administrator instructs the Switch to examine packets for
this tag, acquires the tagged packets and maps them to a class queue on the Switch. Then in turn, the
administrator will set a priority for this queue so that will be emptied before any other packet is
forwarded. This results in the end user receiving all packets sent as quickly as possible, thus
prioritizing the queue and allowing for an uninterrupted stream of packets, which optimizes the use of
bandwidth available for the video conference.

Understanding QoS

The Switch has eight priority queues. These priority queues are labeled as 7, the high queue to 0, the
lowest queue. The eight priority tags, specified in IEEE 802.1p are mapped to the Switch's priority
tags as follows:
Priority 0 is assigned to the Switch's Q2 queue.
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Priority 1 is assigned to the Switch's Q0 queue.
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Priority 2 is assigned to the Switch's Q1 queue.
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Priority 3 is assigned to the Switch's Q3 queue.
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Priority 4 is assigned to the Switch's Q4 queue.
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Priority 5 is assigned to the Switch's Q5 queue.
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Priority 6 is assigned to the Switch's Q6 queue.
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Priority 7 is assigned to the Switch's Q7 queue.
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For strict priority-based scheduling, any packets residing in the higher priority queues are transmitted
first. Multiple strict priority queues empty based on their priority tags. Only when these queues are
empty, are packets of lower priority transmitted.
For weighted round robin queuing, the number of packets sent from each priority queue depends upon
the assigned weight. For a configuration of 8 CoS queues, A~H with their respective weight value:
8~1, the packets are sent in the following sequence: A1, B1, C1, D1, E1, F1, G1, H1, A2, B2, C2, D2,
E2, F2, G2, A3, B3, C3, D3, E3, F3, A4, B4, C4, D4, E4, A5, B5, C5, D5, A6, B6, C6, A7, B7, A8,
A1, B1, C1, D1, E1, F1, G1, H1.
For weighted round robin queuing, if each CoS queue has the same weight value, then each CoS queue
has an equal opportunity to send packets just like round robin queuing.
For weighted round-robin queuing, if the weight for a CoS is set to 0, then it will continue processing
the packets from this CoS until there are no more packets for this CoS. The other CoS queues that
have been given a nonzero value, and depending upon the weight, will follow a common weighted
round-robin scheme.
Remember that the DES-3800 Series has eight priority queues (and eight Classes of Service) for each
port on the Switch.
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