zl50417 ETC-unknow, zl50417 Datasheet - Page 30

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

30
The default configuration for a 10/100 Mbps port is three delay-bounded queues and one best-effort queue. The
delay bounds per class are 0.8 ms for P3, 3.2 ms for P2, and 12.8 ms for P1. For a 1 Gbps port, we have a default
of six delay-bounded queues and two best-effort queues. The delay bounds for a 1 Gbps port are 0.16 ms for P7
and P6, 0.32 ms for P5, 0.64 ms for P4, 1.28 ms for P3, and 2.56 ms for P2. Best effort traffic is only served when
there is no delay-bounded traffic to be served. For a 1 Gbps port, where there are two best-effort queues, P1 has
strict priority over P0.
We have a second configuration for a 10/100 Mbps port in which there is one strict priority queue, two delay
bounded queues, and one best effort queue. The delay bounds per class are 3.2 ms for P2 and 12.8 ms for P1. If
the user is to choose this configuration, it is important that P3 (SP) traffic be either policed or implicitly bounded
(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 ZL50417, can have an adverse effect on all other classes’ performance. For a 1 Gbps port,
P7 and P6 are both SP classes, and P7 has strict priority over P6. In this case, the delay bounds per class are 0.32
ms for P5, 0.64 ms for P4, 1.28 ms for P3, and 2.56 ms for P2.
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 ZL50417 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
ZL50417, 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.
ZL50417
Delay Bound
Strict Priority and Best Effort
Zarlink Semiconductor Inc.
Data Sheet