PRELIMINARY
PMC-Sierra, Inc.
PM9311/2/3/5 ETT1™ CHIP SET
Data Sheet
PMC-2000164
ISSUE 3
ENHANCED TT1™ SWITCH FABRIC
The Scheduler’s backpressure mechanism is not used for unicast because the Output Scheduler on each
port provides the Scheduler Request Modulator on each port with subport-granularity queue information
via the Flow Control Crossbars. The Scheduler’s backpressure mechanism is not used for TDM because
TDM programming should ensure that the output queues cannot overflow.
When multicast cells arrive at an oEPP in OC-48c mode, the source port and multicast tag are used to
index the oTIB, to look up the egress subport fanout.
The egress subport fanout for TDM cells is taken from the TDM Frame table entry for the slot whose cells
are currently arriving at oEPPs.
3.1.1.4 oEPP Cell Departure: oEPP to Linecard Flow
The Output Scheduler arbitrates among all flows with cells waiting to be sent to the linecards. When the
oEPP is in OC-48 mode, it arbitrates for each output linecard in turn (subport 0, subport 1, subport 2,
subport 3, subport 0,...).
When the Output Scheduler decides to send a cell, it passes label information to the Output Queue
Manager which sends a read command and read address to the Output Dataslices via p2d_d[6;0]_oc[7:0].
When a multicast queue size falls below the programmable unbackpressure threshold, unbackpressure is
sent to the Scheduler.
The Output Scheduler sends incremental credits for unicast egress queues to all EPPs’ Scheduler
Request Modulators via p2f_[a.b][1:0].
3.1.2 LCS Grant Manager
The purpose of the LCS Grant Manager is to prevent any input queue from overflowing, and to arbitrate
among competing flows. It consists of linecard request counters, input queue debit counters, and an
arbiter.
3.1.2.1 Linecard Request Counters
Each linecard request counter is initialized to 0 at reset, incremented when a new LCS Request arrives for
the corresponding flow, and decremented when the LCS Grant Manager sends an LCS Grant to the
requesting linecard. Request counters can also be updated by LCS Control Packet or OOB access. They
are stored in the Linecard Request Counters memory. See Section 3.4 “Enhanced Port Processor
Registers” for detailed address and data formats of this memory. These counters are maintained within the
EPP and no customer interaction is required. However, customers may choose to monitor request counts
on various flows, or to implement request count updates using this memory instead of LCS Control
Packets.
For Control Packet and TDM queues, there is always one request counter per input queue. For multicast
and unicast, if the EPP is in OC-48c mode, there are four request counters per input queue, one for each
156
PROPRIETARY AND CONFIDENTIAL TO PMC-SIERRA, INC., AND FOR ITS CUSTOMERS’ INTERNAL USE
PMC [ PMC-SIERRA, INC ]
