DGS-3700-12/DGS-3700-12G Series Layer 2 Gigabit Ethernet User Manual
149
Understanding QoS
The Switch has eight priority queues. These priority queues are labeled from 0-7, with 7 being the highest priority
queue and 0 the lowest priority queue. The eight priority tags, specified in IEEE 802.1p are mapped to the Switch's
priority queue 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 Q7 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. When each queue has 10
outbound packets, they 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,
A9, B8, C7, D6, E5, F4, G3, H2,
A10, B9, C8, D7, E6, F5, G4
B10, C9, D8, E7, F6,
C10, D9, E8,
D10,
E9, F7, G5, H3,
E10, F8, G6,
F9,
F10, G7, H4,
G8,
G9, H5,
G10, H6 ~ H10
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 DGS-3700 Series has eight priority queues (and eight Classes of Service) for each port on the
Switch.
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