參數(shù)資料
型號: MVTX2803AG
廠商: ZARLINK SEMICONDUCTOR INC
元件分類: 網(wǎng)絡(luò)接口
英文描述: Unmanaged 8-Port 1000 Mbps Ethernet Switch
中文描述: DATACOM, LAN SWITCHING CIRCUIT, PBGA596
封裝: 40 X 40 MM, 2.33 MM HEIGHT, MS-034, HSBGA-596
文件頁數(shù): 21/127頁
文件大?。?/td> 1706K
代理商: MVTX2803AG
MVTX2803
Data Sheet
21
Zarlink Semiconductor Inc.
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.
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
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
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