4. Internal Switching Fabric > Arbitration for Egress Port
89
Tsi576 User Manual
June 6, 2016
Integrated Device Technology
www.idt.com
Priority 3 packets from a given port are always transmitted when the port has its turn. However, priority
2 or lower priority packets may not be sent to an egress port when the number of free buffer associated
with that particular port is equal or smaller than the watermark for that particular priority. For more
information, see
4.3.2
Weighted Round Robin (WRR) Arbitration
Within the same priority group, the Weighted Round Robin (WRR) arbiter at each egress port decides
which ingress port it receives packets from. This WRR arbiter is a modified Round Robin arbiter with
the option to assign different weights on Unicast or Multicast traffic. There are four WRR arbiters per
egress port, one for each priority.
Figure 18: Weighted Round Robin Arbiter per Priority Group
The conceptual block diagram of the WRR arbiter is shown in
. The same arbiter exists for
each Priority Group. Depending on the setting of WRR_EN, the Multicast Traffic can participate in the
Round Robin arbiter. The WRR arbiter consists of a Round Robin arbiter which services its inputs
sequentially, starting at Port#0 on reset. The Chosen Packet Counter is only used for weighted
operation between multicast and unicast transactions. Otherwise, the RR arbiter outputs are used.
If the system has no preferred traffic, WRR_EN is de-asserted (see
Multicast Packet Prio 0 Register” on page 400
), the multicast packets are treated as unicast packets.
Packets from each port, including the multicast ones (if available) are allowed to proceed in order, one
after the other, to the strict priority arbitration. The average probability of multicast packets being
serviced, with equal traffic load among multicast and unicast ports, is one out of thirteen when
operating in 1x mode with 12 ports (~5.89%) which is the same as a unicast port.
RR
Port#0
Port#7
(Unicast Traffic)
0
1
WRR_EN
Multicast
Traffic
packets output
to Strict Priority
Chosen Packet Counter
chosen_pkc_sent
count
disable
WRR_EN
weight_reached
weight
chosen_UC/chosen_MC
datapath
non-datapath
