Released
PMC-Sierra, Inc.
PM9311/2/3/5 ETT1™ CHIP SET
Data Sheet
PMC-2000164
ISSUE 3
ENHANCED TT1™ SWITCH FABRIC
Unicast request counters are incremented when unicast request/grant labels arrive via a programmable
delay line from the LCS Grant Manager. Using the programmable delay line instead of waiting for unicast
cells to arrive allows unicast requests to be sent to the Scheduler ahead of arrival so that Scheduler Grants
can arrive at the same time as the cells, effectively masking Scheduler latency from the round trip time.
The delay line is programmed so that Scheduler grants cannot arrive before the cells arrive, provided that
the system round trip time constraint is met (64 celltimes from the grant to the cell arrival).
Multicast request counters are incremented when the Grant Matching block matches arriving multicast
cells with their request/grant labels.
Unicast and multicast request counters are decremented as requests are sent to the Scheduler.
3.1.3.2 Output Queue Debit Counters
Output queue debit counters are initialized to 0 at reset. They are incremented when unicast requests are
sent to the Scheduler. When incremental output queue credits arrive via the Flow Control Crossbars, they
are subtracted from the appropriate debit counters. The counters are contained in the Output Queue Debit
Counters Memory. See Section 3.4 “Enhanced Port Processor Registers” for detailed address and data
formats. These counters are maintained within the EPP and no customer interaction is required, unless the
customer chooses to implement the Flow Control recovery procedure outlined in “Enhanced Port
Processor - Flow Control Crossbar” on page 94.
Output queue debit counts are maintained for unicast only, not multicast, since the Scheduler’s
backpressure mechanism can be used to prevent multicast output queues from overflowing.
3.1.3.3 Scheduler Request Count Modulator Arbiter
The arbiter decides every celltime which best-effort flow, if any, to request to the Scheduler. A unicast flow
is eligible for arbitration if its input queue request counter is nonzero and its output queue debit counter is
less than the queue size. A multicast flow is eligible for arbitration if its request count is nonzero and if
fewer than 64 requests for that multicast flow are currently waiting for grants from the Scheduler.. Multicast
flows are included in the arbitration in order to enforce the ETT1 best-effort service class priority scheme
(MC0 > UC0 > MC1 > ... > UC3), and to prevent 96-cell multicast input queues from exceeding the
maximum Scheduler request count of 64.
Within each unicast priority, a round-robin is used to arbitrate among eligible flows. The order of the
round-robin is increasing (egress_subport, egress_port), like the LCS Grant Manager arbiter.
3.1.4 Input Queue Manager
The Input Queue Manager stores arriving cells when the Grant Matching block has restored their grant
label information, and sends departing cells when the Scheduler Request Modulator restores subport
granularity to Scheduler grants. Cells are dequeued only when the Scheduler sends a dequeue command
along with a grant.
PROPRIETARY AND CONFIDENTIAL TO PMC-SIERRA, INC., AND FOR ITS CUSTOMERS’ INTERNAL USE
159
PMC [ PMC-SIERRA, INC ]
