Released
PMC-Sierra, Inc.
PM9311/2/3/5 ETT1™ CHIP SET
Data Sheet
PMC-2000164
ISSUE 3
ENHANCED TT1™ SWITCH FABRIC
tells the ports which of the many source ports will be sending it a cell.
5. The source EPP sends a read command to the Dataslices, which then reads the cell from the
appropriate queue and sends it to the Crossbar. At the same time, the destination ports send the
routing tag information to the Crossbar. This routing tag information is used to configure the
internal connections within the Crossbar for the duration of one cell time. The cell then flows
through the Crossbar from the source port to the destination port.
6. The cell arrives at the destination port, and the EPP receives the LCS header of the cell. It uses
this information to decide in which egress queue the cell should be stored. If this was a multicast
cell which caused the egress multicast to reach its occupancy limit, then the EPP would send a
congestion notification to the Scheduler.
7. At some later time, the EPP decides to forward the cell to the linecard. The EPP sends a read
command to the Dataslices which read the cell from memory and forward the cell out to the
linecard. The egress EPP also sends flow control to the ingress EPP, informing it that there now
exists free space in one or more of the egress EPP’s output queues. Also, if the transmitted cell
was a multicast cell then this may cause the egress queue to go from full to not full, in which case
the EPP notifies the Scheduler that it (the EPP) can once again accept multicast cells.
The above description does not account for all of the interactions that can take place between ETT1
devices, but it describes the most frequent events. In general, users do not need to be aware of the
detailed interactions, however knowledge of the main information flows will assist in gaining an
understanding of some of the more complicated sections.
1.2.3 Prioritized Best-effort Service
An ETT1 switch core provides two types of service. The first is a prioritized, best-effort service. The second
provides guaranteed bandwidth and is described later.
The best-effort service is very simple. Linecards forward best-effort cells to the ETT1 core where they will
be queued. The Scheduler arbitrates among the various cells; the arbitration algorithm has the dual goals
of maximizing throughput while providing fair access to all ports. If more than one cell is destined for the
same egress port then the Scheduler will grant one of the cells and the others will remain in their ingress
queues awaiting another round of arbitration. The service is best-effort in that the Scheduler tries its best to
satisfy all queued cells, but in the case of contention then some cells will be delayed.
The Scheduler supports four levels of strict priority for best effort traffic. Level 0 cells have the highest
priority, and level 3 cells have the lowest priority. A level 0 cell destined for a given port will always be
granted before a cell of a different priority level, in the same ingress port, that is destined for the same
egress port.
A ‘flow’ is a sequence of cells from the same ingress port to the same egress port(s) at a given priority.
Best-effort flows are either unicast flows (cells in the flow go to only one egress port), or multicast flows (in
which case cells can go to many, even all, of the egress ports).
PROPRIETARY AND CONFIDENTIAL TO PMC-SIERRA, INC., AND FOR ITS CUSTOMERS’ INTERNAL USE
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PMC [ PMC-SIERRA, INC ]
