zl50405 Zarlink Semiconductor, zl50405 Datasheet - Page 46

• Manufacturer Part Number: zl50405
• Description: Managed5-port 10/100 M Ethernet Switch
• Manufacturer: Zarlink Semiconductor

7.7
Because frame loss is unacceptable for some applications, the ZL50405 provides a flow control option. When flow
control is enabled, scarcity of source port buffer space may trigger a flow control signal; this signal tells a source
port sending a packet to this switch, to temporarily hold off.
While flow control offers the clear benefit of no packet loss, it also introduces a problem for quality of service. When
a source port receives an Ethernet flow control signal, all microflows originating at that port, well-behaved or not,
are halted. A single packet destined for a congested output can block other packets destined for un-congested
outputs. The resulting head-of-line blocking phenomenon means that quality of service cannot be assured with high
confidence when flow control is enabled.
On the other hand, the ZL50405 will still prioritize the received frame disregarding the outgoing port flow control
capability. If a frame is classified as high priority, it is still subjected to the WRED, which means the no-loss on the
high priority queue is not guaranteed. To resolve this situation, the user may set the output port WRED threshold so
high that may never be reached, or program the priority mapping table in the queue manager to map all the traffic to
best effort queue on the flow control capable port. The first method has side impact on the global resource
management since the port may hold too much per class resource that is scarce in the system. The second
method, by nature, lost the benefit of prioritization.
See Programming Flow Control Registers application note, ZLAN-44, for more information.
7.7.1
For unicast frames, flow control is triggered by source port resource availability. Recall that the ZL50405’s buffer
management scheme allocates a reserved number of FDB slots for each source port. If a programmed number of a
source port’s reserved FDB slots have been used, then flow control Xoff is triggered.
Xon is triggered when a port is currently being flow controlled, and all of that port’s reserved FDB slots have been
released.
Note that the ZL50405’s per-source-port FDB reservations assure that a source port that sends a single frame to a
congested destination will not be flow controlled.
7.7.2
Flow control for multicast frames is triggered by a global buffer counter. When the system exceeds a programmable
threshold of multicast packets, Xoff is triggered. Xon is triggered when the system returns below this threshold.
Note:
7.8
The mapping between priority classes discussed in this chapter and elsewhere is shown below.
As the table illustrates, the classes of Table 10 are merged in pairs— P3 is used for network management (NM) and
expedited forwarding service (EF) frames. Classes P2 and P1 correspond to an assured forwarding (AF) group of
size 2. Finally, P0 is for best effort (BE) class.
Features of the ZL50405 that correspond to the requirements of their associated IETF classes are summarized in
the table below.
If per-port flow control is on, QoS performance will be affected.
Flow Control Basics
Mapping to IETF Diffserv Classes
Unicast Flow Control
Multicast Flow Control
Table 10 - Mapping to IETF Diffserv Classes for M MAC & CPU Ports
ZL50405
IETF
NM+EF
P3
Zarlink Semiconductor Inc.
ZL50405
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AF0
P2
AF1
P1
BE
P0
Data Sheet