zl50415 Zarlink Semiconductor, zl50415 Datasheet - Page 35

• Manufacturer Part Number: zl50415
• Description: Managed 16-port 10/100 M + 2-port 1 G Ethernet Switch
• Manufacturer: Zarlink Semiconductor

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
ZL50418, 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 ZL50418 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” (eight for Gigabit ports)
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 ZL50418 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 (and queues P0 and P1 for a Gigabit 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.
7.6
Although traffic shaping is not a primary function of the ZL50418, the chip does implement a shaper for expedited
forwarding (EF). Our goal in shaping is to control the peak and average rate of traffic exiting the ZL50418. Shaping
is limited to the two Gigabit ports only, and only to class P6 (the second highest priority). This means that class P6
will be the class used for EF traffic. If shaping is enabled for P6, then P6 traffic must be scheduled using strict
priority. With reference to Table 7, only the middle two QoS configurations may be used.
Peak rate is set using a programmable whole number, no greater than 64. For example, if the setting is 32, then the
peak rate for shaped traffic is 32/64 * 1000 Mbps = 500 Mbps. Average rate is also a programmable whole number,
Strict Priority and Best Effort
Weighted Fair Queuing
Shaper
Zarlink Semiconductor Inc.
ZL50418
35
Data Sheet