Released
PMC-Sierra, Inc.
PM9311/2/3/5 ETT1™ CHIP SET
Data Sheet
PMC-2000164
ISSUE 3
ENHANCED TT1™ SWITCH FABRIC
The TDM service is inherently a multicast service. The egress ETT1 must therefore support the ability for a
TDM cell to go to any combination of the four subports within a port. A four-bit field in every TDM table
entry provides the multicast fanout for each TDM cell that is received at the egress port. The EPP keeps
track of this fanout information and sends the TDM cell to each subport that is a member of the fanout.
Figure 28 illustrates how the EPP uses the TDM table to determine the source port of a TDM cell, as well
as the local subport multicast fanout.
Figure 28. The oEPP Must Treat the Transferred TDM Cell as a Multicast Cell
Cell from Crossbar
Output TDM Cell Queue
0101
0111
0001
Input port = 12
output vector
To Scheduler
= 0101
(multicast fanout)
TDM Reservation Table
0
991
Current TDM Slot Time
The output TDM cell queue does not need to be large as TDM cells are always forwarded to the output
linecards at the highest priority, and the TDM queue is never backpressured. The size of this queue does,
however, restrict the possible TDM table configuration. The following constraints are required to prevent
dropped TDM cells:
•
•
•
In a TDM table time slot, an input port can not send more than one cell and an output port can not
receive more than one cell.
Subport mode: The TDM Reservation Table can specify the same ingress subport only once every
four time slots.
Subport mode: The TDM Reservation Table can not specify the same egress subport more than
48 time slots within a moving window of 192 time slots. This constraint is caused by the TDM
output queue.
58
PROPRIETARY AND CONFIDENTIAL TO PMC-SIERRA, INC., AND FOR ITS CUSTOMERS’ INTERNAL USE
PMC [ PMC-SIERRA, INC ]
