ZL50405GDC ZARLINK [Zarlink Semiconductor Inc], ZL50405GDC Datasheet - Page 43

• Manufacturer Part Number: ZL50405GDC
• Description: Managed5-Port 10/100 M Ethernet Switch
• Manufacturer: ZARLINK [Zarlink Semiconductor Inc]

link utilization, a misbehaving class is allowed to use any idle bandwidth. However, such leniency must not degrade
the quality of service (QoS) received by well-behaved classes.
As Table 8 illustrates, the six traffic types may each have their own distinct properties and applications. As shown,
classes may receive bandwidth assurances or latency bounds. In the table, P3, the highest transmission class,
requires that all frames be transmitted within 1 ms, and receives 50% of the 100 Mbps of bandwidth at that port.
Best-effort (P0) traffic forms a fourth class that only receives bandwidth when none of the other classes have any
traffic to offer. It is also possible to add a fourth class that has strict priority over the other three; if this class has
even one frame to transmit, then it goes first. In the ZL50405, each RMAC port will support two total classes, and
the MMAC port will support four classes. We will discuss the various modes of scheduling these classes in the next
section.
In addition, each transmission class has two subclasses, high-drop and low-drop. Well-behaved users should rarely
lose packets. But poorly behaved users–users who send frames at too high a rate – will encounter frame loss, and
the first to be discarded will be high-drop. Of course, if this is insufficient to resolve the congestion, eventually some
low-drop frames are dropped, and then all frames in the worst case.
Table 8 shows that different types of applications may be placed in different boxes in the traffic table. For example,
casual web browsing fits into the category of high-loss, high-latency-tolerant traffic, whereas VoIP fits into the
category of low-loss, low-latency traffic.
7.2
There are two basic pieces to QoS scheduling in the MMAC port of ZL50405: strict priority (SP) or weighted fair
queuing (WFQ). The only configuration for a RMAC and CPU port is strict priority between the queues.
7.2.1
When strict priority is part of the scheduling algorithm, if a queue has any frame to transmit, it goes first. For RMAC
ports, this is an easy way to provide the different service. For all recognizable traffic, the bandwidth is guaranteed to
100% of the line rate. This scheme works as long as the overall high priority bandwidth is not over the line rate and
the latency on all the low priority traffic is don’t care. The strict priority queue in the MMAC and CPU ports is similar
to RMAC ports other than having 4 queues instead of 2 queues. The priority queue P0 can be scheduled only if the
priority queue P1 is empty, so as to priority queues P2 and P3. The lowest priority queue is treated as best effort
queue.
Because we do not provide any 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
ZL50405, 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 queue size is too long or global / class buffer resources become scarce.
7.2.2
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 strict assurance scheduling
discipline. The ZL50405 provides this kind of scheduling algorithm on MMAC port only. The user sets four WFQ
“weights” such that all weights are whole numbers and sum to 64. This provides per-class bandwidth partitioning
with granular within 2%.
In WFQ mode, though we do not assure frame latency, the ZL50405 still retains a set of dropping rules that helps to
prevent congestion and trigger higher level protocol end-to-end flow control.
7.3
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.
Two QoS Configurations
WRED Drop Threshold Management Support
Strict Priority
Weighted Fair Queuing
Zarlink Semiconductor Inc.
ZL50405
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Data Sheet