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xStack DES-3800 Series Layer 3 Stackable Fast Ethernet Managed Switch
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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 videoconference 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.
• Priority 1 is assigned to the Switch's Q0 queue.
• Priority 2 is assigned to the Switch's Q1 queue.
• Priority 3 is assigned to the Switch's Q3 queue.
• Priority 4 is assigned to the Switch's Q4 queue.
• Priority 5 is assigned to the Switch's Q5 queue.
• Priority 6 is assigned to the Switch's Q6 queue.
• Priority 7 is assigned to the Switch's Q7 queue.
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 xStack DES-3800 Series has eight priority queues (and eight Classes of Service) for each port on the Switch.