![](http://datasheet.mmic.net.cn/330000/PM73488-PI_datasheet_16444396/PM73488-PI_28.png)
PMC-Sierra, Inc.
PM73488 QSE
L
PMC-980616
Issue 3
5 Gbit/s ATMSwitch Fabric Element
Released
Datasheet
30
Figure 15 shows an example of a high-priority cell preempting a cell in the multicast queue, and the resulting bit set-
tings in the MULTICAST_QUEUE_COMPLETION_REGISTER (an internal register). (For the sake of simplicity,
only 8 of the 32 outputs, and eight bits of the MGV_REGISTER (refer to
section 9.3.4
“MULTICAST_GROUP_VECTOR_REGISTER”
MULTICAST_GROUP_COMPLETION_REGISTER (an internal register) are shown.)
on
page 96
)
and
2.5
The arbiter selects between unicast cells and multicast cells contending for the same output port. Higher priority cells
are given preference over lower priority cells. If a multicast cell and unicast cell have the same priority, one cell is
randomly chosen. The random choice can be biased in favor of either unicast cells or multicast cells at different
points in the switch fabric by using the UC/MC_FAIRNESS_REGISTER (refer to
section 9.3.6 “UC/
MC_FAIRNESS_REGISTER” on page 97
). In general, unicast cells should be favored at later stages in the switch
fabric. Favoring unicast cells is necessary in multiple-stage switch fabrics since unicast cells are routed in a cut-
through fashion and multicast cells are routed in a store-and-forward fashion. As such, a unicast cell becomes more
Arbiter
Figure 15. Example of Multicast Cell Handling in the QSE
Output(0)
Output(1)
Output(2)
CELL_M
Output(3)
CELL_H
Output(4)
Output(5)
CELL_M
Output(6)
Output(7)
Multicast Queue
0
0
1
1
0
1
0
0
0
7
CELL_M
0
0
1
0
0
1
0
0
MULTICAST_QUEUE_COMPLETION_REGISTER
(Records if the cells arrived at the destinations
indicated in the
MULTICAST_GROUP_VECTOR_REGISTER.)
0
7
High-Priority
Cell
CELL_H
This bit is not set since a higher priority
cell was output on Output(3), preempting
the cell in the multicast queue.
Multicast Group Vector (MGV)
(Specifies where cells should be sent.)