D-Link DES-3552 [85/260] Understanding qos

DES-3550 Fast Ethernet Layer 2 Switch

Understanding QoS

The Switch has four priority queues. These priority queues are labeled as 3, 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 Q1 queue.

− Priority 1 is assigned to the Switch's Q0 queue.

− Priority 2 is assigned to the Switch's Q0 queue.

− Priority 3 is assigned to the Switch's Q1 queue.

− Priority 4 is assigned to the Switch's Q2 queue.

− Priority 5 is assigned to the Switch's Q2 queue.

− Priority 6 is assigned to the Switch's Q3 queue.

− Priority 7 is assigned to the Switch's Q3 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 DES-3550 has four priority queues (and four Classes of Service) for each port on the Switch.