zl50417 ETC-unknow, zl50417 Datasheet - Page 31

• Manufacturer Part Number: zl50417
• Description: Unmanaged 16-port 10/100m +2-port Ethernet Switch
• Manufacturer: ETC-unknow

Data Sheet
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 ZL50417 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 ZL50417 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 ZL50417, 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 ZL50417. 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 8, 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,
no greater than 64, and no greater than the peak rate. For example, if the setting is 16, then the average rate for
shaped traffic is 16/64 * 1000 Mbps = 250 Mbps. As a consequence of the above settings in our example, shaped
traffic will exit the ZL50417 at a rate always less than 500 Mbps, and averaging no greater than 250 Mbps. See
Programming QoS Register application note for more information.
Also, when shaping is enabled, it is possible for a P6 queue to explode in length if fed by a greedy source. The
reason is that a shaper is by definition not work-conserving; that is, it may hold back from sending a packet even if
the line is idle. Though we do have global resource management, we do nothing to prevent this situation locally. We
assume SP traffic is policed at a prior stage to the ZL50417.
7.7
To avoid congestion, the Weighted Random Early Detection (WRED) logic drops packets according to specified
parameters. The following table summarizes the behavior of the WRED logic.
Weighted Fair Queuing
Shaper
WRED Drop Threshold Management Support
Figure 11 - Summary of the Behaviour of the WRED Logic
Zarlink Semiconductor Inc.
ZL50417
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