Released
PMC-Sierra, Inc.
PM9311/2/3/5 ETT1™ CHIP SET
Data Sheet
PMC-2000164
ISSUE 3
ENHANCED TT1™ SWITCH FABRIC
1.3.1 Unicast Traffic (OC-192)
Every ETT1 port has a separate request counter and ingress queue for each (output port, priority)
combination to which it can send unicast cells. In a full configuration, an ETT1 port can send to 32 ETT1
ports (including itself), at four priorities, and so will have 128 unicast request counters and ingress queues.
All of these counters and queues are independent so that the switch does not suffer from head-of-line
blocking1.
Figure 9 illustrates the unicast ingress queueing model for one port.
Figure 9. The Unicast Ingress Queueing Model for One Port
The ingress queues are referred to as Virtual Output Queues (VOQs). This name indicates that each
queue holds cells going to just one output port.
Every ETT1 port also has the same number of unicast egress queues; one queue for every (input port,
priority) combination that can send cells to the port. Every port will have 128 unicast egress queues. It is
not acceptable to have a single egress queue per priority, as this can lead to gross unfairness between
ports. These egress unicast ports are called Virtual Input Queues (VIQs) as each queue only holds cells
that have come from a single input port. A useful model is to think of a VOQ as being directly connected
through the Crossbar to a single VIQ. The cell at the head of the VOQ is waiting to be scheduled in such a
way that it can move to the tail of the VIQ in the egress port.
The associated ingress EPP is informed whenever an egress queue is full. In this case, the ingress port
will not issue any new requests to the Scheduler until sufficient cell space becomes available at the egress
queue. The transfer of cells from ingress to egress queues is lossless from a queueing perspective.
The best effort unicast ingress and egress queues can store up to 64 cells.
1.3.2 Multicast Traffic (OC-192)
The ETT1 core also supports multicast cells. Multicast cells are cells that must be forwarded to one or
more egress ports. The LCS header will indicate that a cell is multicast, and it will also contain a multicast
group identifier. The ETT1 port uses this identifier to determine the list of ports to which the cell should go.
This list of ports is called the multicast fanout. Different multicast groups will probably have different
fanouts.
The Scheduler arbitrates among unicast and multicast cells. A multicast cell of a given priority will always
have priority over a unicast cell of the same priority, all else being equal. Physical cell replication takes
place within the Crossbar.
1. Head -of-line blocking is a phenomenon encountered by input queued switches in the case where a cell destined for one out-
put port is delayed because the cell at the head of the same FIFO queue is destined for some other output which is currently
congested.
PROPRIETARY AND CONFIDENTIAL TO PMC-SIERRA, INC., AND FOR ITS CUSTOMERS’ INTERNAL USE
31
PMC [ PMC-SIERRA, INC ]
