zl50411 Zarlink Semiconductor, zl50411 Datasheet - Page 46

• Manufacturer Part Number: zl50411
• Description: Managed 9-port Fast Ethernet Switch With Private Vlan
• Manufacturer: Zarlink Semiconductor

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. 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 ZL50411: 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 lowest priority queue is treated as “best effort” queue.
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.
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
ZL50411, 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 ZL50411 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 ZL50411 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.
Px is the total byte count, in the priority queue x, can be the strict priority queue of RMAC ports and higher 3 priority
queues for MMAC port. The WRED logic has two drop levels, depending on the value of Px. Each drop level has
defined high-drop and low-drop percentages, which indicate the minimum and maximum percentages of the data
that can be discarded. The X, Y Z percent can be programmed by the register RDRC0, RDRC1. All packets will be
dropped only if the system runs out of the specific buffer resource, per class buffer or per source port buffer. The
Two QoS Configurations
WRED Drop Threshold Management Support
Strict Priority
Weighted Fair Queuing
High Drop
Low Drop
Px > WRED_L1
Table 8 - WRED Logic Behaviour
X%
Y%
Zarlink Semiconductor Inc.
ZL50411
46
Px > WRED_L2
100%
Z%
BM Reject
100%
100%
Data Sheet