Understanding Qos - D-Link DES-3528 User Manual

xStack DES-3528 Series Layer 2 Stackable Fast Ethernet Managed Switch 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 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 from 0-7, with 7 being the highest priority and
0 the lowest priority 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 Q6 queue.
NOTE: In the DES-3500 Series, the Q7 is reserved for future use.
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-3528 has eight priority queues (and eight Classes of Service) for each port on the
Switch.
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