zl50416 Zarlink Semiconductor, zl50416 Datasheet - Page 31

• Manufacturer Part Number: zl50416
• Description: Managed 16-port 10/100 M + 2-port 1 G Ethernet Switch
• Manufacturer: Zarlink Semiconductor

(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 ZL50416, can have a deleterious effect on all other classes’ performance.
The third configuration 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. In
the fourth configuration, all queues are served using a WFQ service discipline.
7.3
In the absence of a sophisticated QoS server and signaling protocol, the ZL50416 may not know the mix of
incoming traffic ahead of time. To cope with this uncertainty, our 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, we assure latency bounds for all admitted frames with high confidence, even in the presence of
system-wide congestion. Our algorithm identifies misbehaving classes and intelligently discards frames at no
detriment to well-behaved classes. Our 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
When strict priority is part of the scheduling algorithm, if a queue has even one frame to transmit, it goes first. Two
of our 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 we have 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 our four QoS configurations include best effort queues. The goal is for best
effort classes to be used for non-essential traffic, because we provide no assurances about best effort performance.
However, in a typical network setting, much best effort traffic will indeed be transmitted, and with an adequate
degree of expediency.
Because we do not provide any delay assurances for best effort traffic, we do not enforce latency by dropping best
effort traffic. Furthermore, because we assume that strict priority traffic is carefully controlled before entering the
ZL50416, we do not enforce a fair bandwidth partition by dropping strict priority traffic. To summarize, dropping to
enforce bandwidth or delay does not apply to strict priority or best effort queues. We only drop frames from best
effort and strict priority queues when global buffer resources become scarce.
7.5
In some environments – for example, in an environment in which delay assurances are not required, but precise
bandwidth partitioning on small time scales is essential, WFQ may be preferable to a delay-bounded scheduling
discipline. The ZL50416 provides the user with a WFQ option with the understanding that delay assurances can not
be provided if the incoming traffic pattern is uncontrolled. The user sets four WFQ “weights” such that all weights
are whole numbers and sum to 64. This provides per-class bandwidth partitioning with error within 2%.
In WFQ mode, though we do not assure frame latency, the ZL50416 still retains a set of dropping rules that helps to
prevent congestion and trigger higher level protocol end-to-end flow control.
As before, when strict priority is combined with WFQ, we do not have special dropping rules for the strict priority
queues, because the input traffic pattern is assumed to be carefully controlled at a prior stage. However, we do
indeed drop frames from SP queues for global buffer management purposes. In addition, queue P0 for a 10/100
port are treated as best effort from a dropping perspective, though they still are assured a percentage of bandwidth
from a WFQ scheduling perspective. What this means is that these particular queues are only affected by dropping
when the global buffer count becomes low.
Delay Bound
Strict Priority and Best Effort
Weighted Fair Queuing
Zarlink Semiconductor Inc.
ZL50416
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Data Sheet