- 152 -
encounter frame loss, and the first to be discarded will be high-drop. Of course, if this is insufficient to
resolve the congestion, eventually some low-drop frames are dropped as well.
Above tables also show that different types of applications may be placed in different boxes in the traffic
table. For example, web search may fit into the category of high-loss, high-latency-tolerant traffic,
whereas VoIP fits into the category of low-loss, low-latency traffic.
5.15.1.2 Four QoS Profile
There are four basic pieces to QoS scheduling in the Switch: strict priority (SP), delay bound, weighted
fair queuing (WFQ), and best effort (BE). Using these four pieces, there are four different modes of
operation, as shown in the following table.
TABLE 5- 4 QOS PROFILE Gigabit Port P7 P6 P5 P4 P3 P2 P1 P0
Megabit Port P3 P2 P1 P0
Profile mode 1 Delay Bound Best Effort
Profile mode 2 Strict Priority Delay Bound Best Effort
Profile mode 3 Strict Priority Weighted Fair Queuing
Profile mode 4 Weighted Fair Queuing
The first profile mode for a 10/100 Mbps port is three delay-bounded queues and one best-effort queue.
The delay bounds per class are 0,8 ms for P3, 3.2 ms for P2, and 12.8 ms for P1. For a 1 Gbps port, we
have a default of six delay-bounded queues and two best-effort queues. The delay bounds for a 1 Gbps
port are 0.16 ms for P7 and P6, 0.32 ms for P5, 0.64 ms for P4, 1.28 ms for P3, and 2.56 ms for P2.
Best effort traffic is only served when there is no delay-bounded traffic to be served. For a 1 Gbps port,
where there are two best effort queues, P1 has strict priority over P0.
We have a second profile mode for a 10/100 Mbps port in which there is one strict priority queue, two
delay bounded queues, and one best effort queue. The delay bounds per class are 3.2 ms for P2 and
12.8 ms for P1. If the user is to choose this configuration, it is important that P3 (SP) traffic be either
policed or implicitly bounded (e.g. if the incoming P3 traffic is very light and predictably patterned).
Strict priority traffic, if not admission-controlled at a prior stage to the Switch, can have an adverse
effect on all other classes’ performance. For a 1 Gbps port, P7 and P6 are both SP classes, and P7 has
strict priority over P6. In this case, the delay bounds per class are 0.32 ms for P5, 0.64 ms for P4, 1.28
ms for P3, and 2.56 ms for P2.
The third profile mode for a 10/100 Mbps port contains one strict priority queue and three queues
receiving a bandwidth partition via WFQ. As in the second configuration, strict priority traffic needs to
be carefully controlled.