zl50415 Zarlink Semiconductor, zl50415 Datasheet - Page 33

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

A class is capable of offering traffic that exceeds the contracted bandwidth. A well-behaved class offers traffic at a
rate no greater than the agreed-upon rate. By contrast, a misbehaving class offers traffic that exceeds the
agreed-upon rate. A misbehaving class is formed from an aggregation of misbehaving microflows. To achieve high
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 ZL50418, each 10/100 Mbps port will support four total classes,
and each 1000 Mbps port will support eight 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 four basic pieces to QoS scheduling in the ZL50418: strict priority (SP), delay bound, weighted fair
queuing (WFQ), and best effort (BE). Using these four pieces, there are four different modes of operation, as shown
in the tables below. For 10/100 Mbps ports, the following registers select these modes:
Middle transmission
priority, P2
Low transmission
priority, P1
Total
QOSC24 [7:6]_CREDIT_C00
QOSC28 [7:6]_CREDIT_C10
Four QoS Configurations
Goals
37.5 Mbps
12.5 Mbps
100 Mbps
Bandwidth (user
Total Assured
Table 7 - Two-Dimensional World Traffic
defined)
Zarlink Semiconductor Inc.
ZL50418
Apps: interactive apps,
Web business.
Latency: < 4-5 ms.
Drop: No drop if P2 not
oversubscribed.
Apps: emails, file
backups.
Latency: < 16 ms desired,
but not critical.
Drop: No drop if P1 not
oversubscribed.
33
Low Drop Probability
(low-drop)
Apps: non-critical interactive
apps.
Latency: < 4-5 ms.
Drop: No drop if P2 not
oversubscribed; firstP2 to
drop otherwise.
Apps: casual web browsing.
Latency: < 16 ms desired,
but not critical.
Drop: No drop if P1 not
oversubscribed; first to drop
otherwise.
High Drop Probability
(high-drop)
Data Sheet