Released
PMC-Sierra, Inc.
PM9311/2/3/5 ETT1™ CHIP SET
Data Sheet
PMC-2000164
ISSUE 3
ENHANCED TT1™ SWITCH FABRIC
Consequently, the requests associated with cells going to subports (0,0), (0,1), (0,2) and (0,3) are
amalgamated to become requests for port 0.
NOTE: The channel information is not lost; a cell going to channel (0,1) will still be delivered to
channel 1, port 0. However, the four queues have their requests multiplexed over a single
request/grant connection. Figure 18 illustrates this concept.
Figure 18. Scheduler Requests/Grants are Multiplexed at a Port Level
VOQs in iEPP
cells for output subport (0,0)
cells for output subport (0,1)
Request to send to port 0
Scheduler
cells for output subport (0,2)
cells for output subport (0,3)
Grant to send to port 0
The EPP has two decisions to make: which request to issue, and which of the four queues to grant.
At any cell time, the iEPP may have several possible new requests which it must communicate to the
Scheduler. The iEPP has a Scheduler Request Modulator which makes this decision. See Section 3.1.3
“Scheduler Request Modulator” on page 158 for a discussion of this algorithm.
When the iEPP receives a grant from the Scheduler it must decide which of the four VOQs should be
serviced. A simple round-robin algorithm is used (across non-empty queues).
1.3.4 Multicast Traffic (subport mode)
Multicast cells are handled at a port level, not a subport level. There is a single cell queue within the iEPP,
for each priority of multicast cells. Each OC-48c channel has its own request counter, but, like the unicasts,
these four request counters map to a single virtual output queue (per priority), as shown in Figure 19.
46
PROPRIETARY AND CONFIDENTIAL TO PMC-SIERRA, INC., AND FOR ITS CUSTOMERS’ INTERNAL USE
PMC [ PMC-SIERRA, INC ]
