![](http://datasheet.mmic.net.cn/330000/PM73488-PI_datasheet_16444396/PM73488-PI_47.png)
PMC-Sierra, Inc.
PM73488 QSE
L
PMC-980616
Issue 3
5 Gbit/s ATMSwitch Fabric Element
Released
Datasheet
49
4.4
As described in
section 2.4.1 “Multicast Queue Engine” on page 28
, the multicast queue engine computes multiprior-
ity backpressure (high, medium, or low) based on the following factors:
Multicast Backpressure Control
Total buffer usage.
Buffer usage on an individual port.
The buffer use constraints described therein guarantee against one port flooding the QSE and choking other ports (by
the per-port buffer limits) or heavy traffic from cells of a lower priority level choking cells of higher priorities (by
allowing buffers to be reserved for high- and medium-priority cells).
The QSE is tolerant of the QRT and other QSEs on its input ignoring the backpressure it applies. Depending on the
situation, cells that arrive in violation of recommended backpressure may be dropped or may be accepted and treated
as normal cells. This is fault behavior since, during normal operation, neither the QSE nor the QRT will ever violate
backpressure applied by a downstream QSE.
4.5
When the RESET pin is asserted, the QSE is in total reset. Access is not permitted to any register; and all QSE-driven
Multilevel Reset
Figure 32. Basic Forward and Backward Data Path
QRT
(IRT Portion)
B
QRT
(IRT Portion)
A
QSE
(Switching
Matrix)
QSE
(Switching
Matrix)
QRT
(ORT Portion)
B
QRT
(ORT Portion)
A
QRT-to-QSE Interface
QSE-to-QRT Interface
Forward Cell Flow
Backward BP/ACK Flow
QSE-to-QSE Interface