xStack® DES-3528/DES-3552 Series Layer 2 Managed Stackable Fast Ethernet Switch Web UI Reference Guide
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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 result 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 6) — the highest priority queue — to 0 (Class 0) — the lowest priority queue. Class 7 is for
stacking function. The eight priority tags specified in IEEE 802.1p (p0 to p7) are mapped to the Switch’s priority
queues 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.
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 seven CoS queues, A to H with their respective weight value: 7 to 1, the
packets are sent in the following sequence: A1, B1, C1, D1, E1, F1, G1, A2, B2, C2, D2, E2, F2, A3, B3, C3, D3,
E3, A4, B4, C4, D4, A5, B5, C5, A6, B6, A7, A1, B1, C1, D1, E1, F1, G1
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.
Remember that the Switch has eight 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
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