PM73487 datasheet(54/251 Pages) PMC

54 / 251 page

PM73487 QRT

PMC-Sierra, Inc.

PMC-980618

Issue 3

622 Mbps ATM Traffic Management Device

Released

Datasheet

2.6.7

Transmit Multicast Congestion Management

The transmit multicast can have congestion management applied to it. Three of the five conges-

tion measurements apply: the device, the SC, and the channel. The VO and the SC queue limits do

not apply to multicast cells as they do not make sense. This is because only one copy of the cell is

kept in the DRAM, regardless of the number of destinations to which the cell is headed. Those

counts contain only the number of unicast cells present.

The QRT can be configured to either generate or not generate backpressure on a per-SC basis. If

backpressure

desired,

configure

TX_EXP_MAX_SC_QD

(refer

“TX_EXP_MAX_SC_QD” on page 163) to one-half of the input pointer FIFO depth for that

AL_RAM_CONFIG (refer to “AL_RAM_CONFIG” on page 105). This will drop all cells at a

depth deeper than this, preventing backpressure from reaching back into the switch fabric. The

setting of this is a system-level decision. Preventing backpressure prevents a failure or congestion

on one card from affecting the performance of the fabric as a whole. On the other hand, using the

backpressure allows more multicast cells to be passed without the fear of dropping in the egress

QRT.

The high priority backpressure bit is derived from the near-fullness of queues 8 and 9. The

medium priority backpressure bit is derived from the near-fullness of queue 10 and 11. The low

priority backpressure bit is derived from the OR of the near-fullness of queues 12 to 15.

EPD, CLP-based dropping, and EFCI are all valid for multicast cells and are configured in the

TX_CH_CONFIG word (refer to section 9.3.1.7 “TX_CH_CONFIG” starting on page 189) using

the same bits as for unicast connections.

PM73487 Datasheet(PDF) 54 Page - PMC-Sierra, Inc