4. Internal Switching Fabric > Arbitration for Egress Port
90
Tsi578 User Manual
June 6, 2016
Integrated Device Technology
www.idt.com
When weighted operation is required, WRR_EN is asserted (WRR_EN=1). Then the type and quantity
of preferred traffic is selected by programming the CHOOSE_UC bit and the minimum number of
packets allocated for the chosen traffic on the egress port using the WEIGHT field in the
Unicast and Multicast Packet Prio 0 Register” on page 398
. These two register values set the
parameters of operation for the Chosen Packet Counter inside the WRR arbiter. The CHOOSE_UC
(CHOOSE_MC) value determines which type of traffic is selected to be favored (0= Multicast, 1 =
Unicast). The WEIGHT value determines the number of the packets of the
chosen type
are to be sent in
between non-chosen ones.
Every time a chosen packet (either multicast or unicast) is sent, the Chosen Packet Counter is notified.
The chosen packets are selected for transmission as long as the WEIGHT value is not reached. Once
the WEIGHT value is reached, a non-chosen packet is selected instead and the Chose Packet Counter is
reset.
In the case when no chosen packet is available when its opportunity arises, the WRR arbiter
automatically selects the non-chosen packets. Similar behaviour applies to non-chosen packet. When
the opportunity to transmit non-chosen packets arises, and there is none available, a packet of the
chosen type is sent. However, this does not consume the original opportunity allocated.
4.3.2.1
Examples of WRR Arbitration
A few examples of register settings for the WRR arbiter is shown in
.
Table 10: Sample Register settings for WRR in a given priority group (WRR_EN=1)
a
a. The percent values in the table assumes all opportunity for transmission is filled by either the selected or un-selected
types.
CHOOSE_UC/
CHOOSE_MC
WEIGHT
% of Multicast
Packets Sent to
SP Arbiter
% of Unicast
Packets Sent to
SP Arbiter
Packet Sequence (M = Multicast,
U = Unicast)
0
0
0
100
...UUUUUUUU...
0
1
50
50
...UMUMUMUM...
0
15
93.75
6.25
...MUMMMMMMMMMMMMMMMUM...
1
0
100
0
...MMMMMMMM...
When there is 100% utilization of either unicast or multicast, lack of transfer of the other type
of packet can be encountered in the system.