zl50408 Zarlink Semiconductor, zl50408 Datasheet - Page 44

• Manufacturer Part Number: zl50408
• Description: Managed 8-port 10/100m 1-port 10/100/1000m Ethernet Switch
• Manufacturer: Zarlink Semiconductor

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 ZL50408’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 ZL50408’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 ZL50408 that correspond to the requirements of their associated IETF classes are summarized in
the table below.
Network management (NM) and
Expedited forwarding (EF)
Assured forwarding (AF)
Best effort (BE)
If per-port flow control is on, QoS performance will be affected.
Mapping to IETF Diffserv Classes
Unicast Flow Control
Multicast Flow Control
Table 10 - Mapping to IETF Diffserv Classes for G MAC & CPU Ports
Table 11 - ZL50408 Features Enabling IETF Diffserv Standards
ZL50408
IETF
Global buffer reservation for NM and EF
Shaper for traffic on uplink port
No dropping if admission controlled
Global buffer reservation for two AF classes
Shaper for traffic on uplink port
Random early discard, with programmable levels
Service only when other queues are idle means that QoS not
adversely affected
Shaper for traffic on uplink port
Random early discard, with programmable levels
Traffic from flow control enabled ports automatically classified as BE
NM+EF
P3
Zarlink Semiconductor Inc.
ZL50408
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AF0
P2
AF1
P1
BE
P0
Data Sheet