Released
PMC-Sierra, Inc.
PM9311/2/3/5 ETT1™ CHIP SET
Data Sheet
PMC-2000164
ISSUE 3
ENHANCED TT1™ SWITCH FABRIC
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
Virtual Output Queues
0,0
Virtual Input Queues
Cell
0,0
0,1
0,2
0,3
Crossbar
0,1
0,2
0,3
Req/Gnt
sync
Scheduler
iEPP
oEPP
Flow Control
Crossbar
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.
PROPRIETARY AND CONFIDENTIAL TO PMC-SIERRA, INC., AND FOR ITS CUSTOMERS’ INTERNAL USE
53
PMC [ PMC-SIERRA, INC ]
