PM9313 datasheet(57/343 Pages) PMC

57 / 343 page

PM9311/2/3/5 ETT1™ CHIP SET

PMC-2000164

ISSUE 3

ENHANCED TT1™ SWITCH FABRIC

Released

Data Sheet

PROPRIETARY AND CONFIDENTIAL TO PMC-SIERRA, INC., AND FOR ITS CUSTOMERS’ INTERNAL USE

PMC-Sierra, Inc.

The actual occupancy of a VIQ is dynamic: the number of free cell locations will increment whenever a cell

is forwarded to the attached linecard, and the number of free cell locations will decrement whenever a new

cell enters the VIQ from the crossbar. The iEPP knows the latter information - it is the iEPP that forwards

the cell to the VIQ. But the iEPP does not know when a cell gets sent out to the linecard. So the oEPP must

tell each iEPP how many cells have been sent from each relevant VIQ, and this information must be sent

with sufficient frequency that the throughput of a single flow is not adversely affected.

In the ETT1 fabric, this update information is transmitted from the oEPPs to the iEPPs via a second

Crossbar, referred to as the Flow Control Crossbar. Figure 25 shows the cell flow from left to right and the

reverse flow of the update information.

Figure 25. A Separate Crossbar is Used to Convey Backpressure Information to the iEPPs

Every iEPP maintains a set of counters (Output Queue Debit Counters) that track the number of cells in

each of the relevant VIQs of every oEPP. At reset, these counters are all set to zero. A counter is

incremented whenever the iEPP makes a request to forward a cell to the appropriate VIQ. If the counter

reaches a pre-determined maximum, then the iEPP will not issue further requests. Once some cells are

forwarded from the VIQ, the iEPP will reduce the counter and resume issuing requests for that flow.

In general, the user does not need to be aware of the details of this mechanism except for the case when

update information is lost, perhaps due to a noise-induced error on one or more of the links attached to the

Flow Control Crossbar. The details of this error recovery mechanism are described in Section “Enhanced

Port Processor - Flow Control Crossbar” on page 94.

0,0

0,1

0,2

0,3

Virtual Output Queues

Crossbar

Scheduler

iEPP

Req/Gnt

Cell

0,0

0,1

0,2

0,3

Virtual Input Queues

oEPP

Crossbar

Flow Control

sync

PM9313 Datasheet(PDF) 57 Page - PMC-Sierra, Inc

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