zl50409 Zarlink Semiconductor, zl50409 Datasheet - Page 43

• Manufacturer Part Number: zl50409
• Description: Managed 9-port 10/100m Ethernet Switch
• Manufacturer: Zarlink Semiconductor

7.0
7.1
Quality of service is an all-encompassing term for which different people have different interpretations. In general,
the approach to quality of service described here assumes that we do not know the offered traffic pattern. We also
assume that the incoming traffic is not policed or shaped. Furthermore, we assume that the network manager
knows his applications, such as voice, file transfer, or web browsing, and their relative importance. The manager
can then subdivide the applications into classes and set up a service contract with each. The contract may consist
of bandwidth or latency assurances per class. Sometimes it may even reflect an estimate of the traffic mix offered to
the switch. As an added bonus, although we do not assume anything about the arrival pattern, if the incoming traffic
is policed or shaped, we may be able to provide additional assurances about our switch’s performance.
Table 8 shows examples of QoS applications with three transmission priorities, but best effort (P0) traffic may form
a fourth class with no bandwidth or latency assurances. MMAC port actually has four total transmission priorities.
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 ZL50409, 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.
Highest
transmission
priority, P3
Middle
transmission
priority, P2
Low transmission
priority, P1
Total
Model
Quality of Service and Flow Control
Goals
(user defined)
50 Mbps
37.5 Mbps
12.5 Mbps
100 Mbps
TotalAssured
Bandwidth
Table 8 - Two-dimensional World Traffic
Apps: phone calls, circuit
emulation.
Latency: < 1 ms.
Drop: No drop if P3 not
oversubscribed.
Apps: interactive apps, Web
business.
Latency: < 4-5 ms.
Drop: No drop if P2 not
oversubscribed.
Latency: < 16 ms desired, but
not critical.
Drop: No drop if P1 not
oversubscribed.
Apps: emails, file backups.
Zarlink Semiconductor Inc.
Low Drop Probability
ZL50409
(low-drop)
43
Apps: training video.
Latency: < 1 ms.
Drop: No drop if P3 not
oversubscribed; first P3 to drop
otherwise.
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