MVTX2803 查看數據表（PDF） - Zarlink Semiconductor Inc

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MVTX2803

Unmanaged 8-Port 1000 Mbps Ethernet Switch

Zarlink Semiconductor Inc 

MVTX2803

Data Sheet

7.2 Four QoS Configurations

There are four basic pieces to QoS scheduling in the MVTX2803AG: 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 Table 2.

P7

P6

P5

P4

P3

P2

P1

P0

Op1 (default)

Delay Bound

BE

Op2

SP

Delay Bound

BE

Op3

SP

WFQ

Op4

WFQ

Table 2 - Four QoS configurations per port

The default configuration is six delay-bounded queues and two best-effort queues. The delay bounds per class

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. P1 has strict priority over P0.

There is a second configuration in which there are two strict priority queues, four delay bounded queues, and

two best effort queues. 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. If the user is to choose this configuration, it is important that P7-P6 (SP) traffic be either

policed or implicitly bounded (e.g. if the incoming SP traffic is very light and predictably patterned). Strict

priority traffic, if not admission-controlled at a prior stage to the MVTX2803AG, can have an adverse effect on all

other classes’ performance. P7 and P6 are both SP classes, and P7 has strict priority over P6.

The third configuration contains two strict priority queues and six queues receiving a bandwidth partition via

WFQ. As in the second configuration, strict priority traffic needs to be carefully controlled.

In the fourth configuration, all queues are served using a WFQ service discipline

7.3 Delay Bound

In the absence of a sophisticated QoS server and signaling protocol, the MVTX2803AG may not be assured of

the mix of incoming traffic ahead of time. To cope with this uncertainty, the delay assurance algorithm

dynamically adjusts its scheduling and dropping criteria, guided by the queue occupancies and the due dates of

their head-of-line (HOL) frames. As a result, latency bounds are assured for all admitted frames with high

confidence, even in the presence of system-wide congestion. The algorithm identifies misbehaving classes and

intelligently discards frames at no detriment to well-behaved classes. The algorithm also differentiates between

high-drop and low-drop traffic with a weighted random early drop (WRED) approach. Random early dropping

prevents congestion by randomly dropping a percentage of high-drop frames even before the chip’s buffers are

completely full, while still largely sparing low-drop frames. This allows high-drop frames to be discarded early,

as a sacrifice for future low-drop frames. Finally, the delay bound algorithm also achieves bandwidth partitioning

among classes.

7.4 Strict Priority and Best Effort

When strict priority is part of the scheduling algorithm, if a queue has even one frame to transmit, it goes first.

Two of the four QoS configurations include strict priority queues. The goal is for strict priority classes to be used

for IETF expedited forwarding (EF), where performance guarantees are required. As indicated, it is important

that strict priority traffic be either policed or implicitly bounded, so as to keep from harming other traffic classes.

When best effort is part of the scheduling algorithm, a queue only receives bandwidth when none of the other

classes have any traffic to offer. Two of the four QoS configurations include best effort queues. The goal is for

best effort classes to be used for non-essential traffic, because there are no assurances about best effort

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