User manual

ENGLISH
49
Understanding CoS
The Switch has four priority classes of service. These priority classes of service are labeled as 3, the high class to 0,
the lowest class. The eight priority tags, specified in IEEE 802.1p are mapped to the Switch's priority classes of
service as follows:
• Priority 0 is assigned to the Switch's Q1 class.
• Priority 1 is assigned to the Switch's Q0 class.
• Priority 2 is assigned to the Switch's Q0 class.
• Priority 3 is assigned to the Switch's Q1 class.
• Priority 4 is assigned to the Switch's Q2 class.
• Priority 5 is assigned to the Switch's Q2 class.
• Priority 6 is assigned to the Switch's Q3 class.
• Priority 7 is assigned to the Switch's Q3 class.
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 four configurable priority queues (and four Classes of Service) for each port on the
Switch.