Released
PMC-Sierra, Inc.
PM9311/2/3/5 ETT1™ CHIP SET
Data Sheet
PMC-2000164
ISSUE 3
ENHANCED TT1™ SWITCH FABRIC
Figure 10. Ingress and Egress Queue for Multicast Traffic
The Scheduler maintains information on the occupancy of the multicast ingress queues, and receives
backpressure/unbackpressure signals for the multicast egress queues. If an egress multicast queue is
backpressured then there is a question as to what happens to the cell at the head of an ingress queue that
is trying to send to that backpressured egress queue (as well as other ports). If the Scheduler blocks the
ingress queue, then the system will experience head-of-line blocking for multicast cells at that priority.
Every ETT1 port can select how it wants to handle multicast cells that are sent to it. For each of the four
multicast egress queues, the local ETT1 CPU can either enable or disable a backpressure signal from
being sent to the Scheduler.
NOTE: This selection is at the egress queues, not the ingress queues.
If a port is set so that multicast backpressure is disabled, then that port will never cause head-of-line
blocking to occur in any ingress multicast queue that will send to it.
If a port is set so that multicast backpressure is enabled, then that port can assert a backpressure signal if
the egress queue gets too full. Consequently, this might cause head-of-line blocking at multicast ingress
queues that send cells to this port.
Therefore, head-of-line blocking is only guaranteed not to occur if all of the ports that can receive multicast
cells have their multicast backpressure signals disabled. If at least one port has backpressure enabled,
then head-of-line blocking might occur.
1.3.3 Unicast Traffic (Subport Mode)
The EPP can operate in one of two modes. The first mode uses the queueing model described previously,
and corresponds to a single channel of approximately 10 Gbit/s. The second mode is called subport mode.
In subport mode an EPP can manage four channels of 2.5 Gbit/s each. The physical interface at the
Dataslice is always a single channel; in subport mode the four 2.5 Gbit/s channels are time multiplexed
onto the single 10 Gbit/s channel. More precisely, given that the ETT1 Chip Set always operates at 25M
cells per second, the subport mode correspond to four channels each of up to 6.25M cells per second.
Figure 11. shows the block diagram of an ETT1 port card. The six (or seven) Dataslices provide a single
interface. A separate MUX device is required to manage the four OC-48c channels and merge them so
they appear as a single channel to the EPP/DS.
PROPRIETARY AND CONFIDENTIAL TO PMC-SIERRA, INC., AND FOR ITS CUSTOMERS’ INTERNAL USE
33
PMC [ PMC-SIERRA, INC ]
