zl50416 Zarlink Semiconductor, zl50416 Datasheet - Page 30

• Manufacturer Part Number: zl50416
• 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 7 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 ZL50416, 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 7 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 ZL50416: 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:
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. Best effort traffic is only served when
there is no delay-bounded traffic to be served.
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
QOSC24 [7:6]
QOSC28 [7:6]
QOSC32 [7:6]
QOSC36 [7:6]
Op1 (default)
Op2
Op3
Op4
Four QoS Configurations
CREDIT_C00
CREDIT_C10
CREDIT_C20
CREDIT_C30
Table 8 - Four QoS Configurations for a 10/100 Mbps Port
Delay Bound
SP
SP
WFQ
P3
Zarlink Semiconductor Inc.
Delay Bound
WFQ
ZL50416
P2
30
P1
BE
BE
P0
Data Sheet