xStack
®
DGS-3200 Series Layer 2 Gigabit Ethernet Managed Switch
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see if it has the proper identifying tag. Then the user may forward these tagged packets to designated classes of service on the
Switch where they will be emptied, based on priority.
For example, let’s 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 supports 802.1p priority queuing. The Switch has eight priority queues. These priority queues are numbered from 7
(Class 7) — the highest priority queue — to 0 (Class 0) — the lowest priority queue. The eight priority tags specified in IEEE
802.1p (p0 to p7) are mapped to the Switch’s priority queues as follows:
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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.
For strict priority-based scheduling, any packets residing in the higher priority classes of service are transmitted first. Multiple
strict priority classes of service are emptied based on their priority tags. Only when these classes 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 eight 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 Switch has seven configurable priority queues (and seven Classes of Service) for each port on the Switch.
NOTICE:
The Switch contains eight classes of service for each port on the Switch. One of
these classes is reserved for internal use on the Switch and is therefore not configurable. All
references in the following section regarding classes of service will refer to only the seven
classes of service that may be used and configured by the administrator.
