Released
PMC-Sierra, Inc.
PM9311/2/3/5 ETT1™ CHIP SET
Data Sheet
PMC-2000164
ISSUE 3
ENHANCED TT1™ SWITCH FABRIC
TDM_Sync signals do not arrive at the linecards at exactly the same cell time. There may be several cell
times between the first linecard receiving TDM_Sync and the last. The variation is due to variations in link
lengths and also due to timing uncertainties when signals cross asynchronous clock domains. The guard
band absorbs these variations, so the linecard does not need to be aware of the link delays or clock
domain issues.
As soon as the linecard receives the TDM_Sync signal it starts to forward TDM requests to the ETT1 port.
The port has an ingress queue for these cells and so can buffer them until they are required. The ingress
queue is 96 cells deep, and is controlled by the normal LCS request-grant flow-control mechanism. The
linecard must forward TDM cells in the correct order (corresponding to its entries in its TDM Send table),
but it does not need to be closely synchronized to the ETT1 core. The linecard can skip TDM cells (i.e. not
send them), but must not re-order cells relative to the TDM tables.
The ETT1 core waits for 128 cell times (the TDM offset) after it has sent TDM_Sync before the new TDM
Frame is started. This provides all linecards with sufficient time to receive the TDM_Sync signal and to
request and send in the first TDM cells. Figure 30 shows the sequence of events in more detail.
Figure 30. TDM Synchronization
TDM_Sync
Last usable
slot in previous
Frame
TDM cells arrive
at ETT1 ports from
linecards
arrives
at all EPPs
16 slots
+uncertainty
unusable
TDM_Sync
New Frame
starts
arrives at the
linecards at
different times.
The Scheduler can be configured to send the TDM_Sync in three ways. Each way is driven by one of
these three sources:
1. A Suggested_TDM_Sync from a counter in the EPP.
2. A counter in the Scheduler.
3. A Suggested_TDM_Sync from a designated linecard (through the EPP).
There are pros and cons to each of these methods. Each method is described in more detail below.
60
PROPRIETARY AND CONFIDENTIAL TO PMC-SIERRA, INC., AND FOR ITS CUSTOMERS’ INTERNAL USE
PMC [ PMC-SIERRA, INC ]
