RELEASED
PMC-Sierra, Inc.
PM9311/2/3/5 ETT1™ CHIP SET
Data Sheet
PMC-2000164
ISSUE 3
ENHANCED TT1™ SWITCH FABRIC
Appendix C Interfacing Details for ETT1
C.1
COMPENSATING FOR ROUND TRIP DELAY BETWEEN LINECARD
AND ETT1(LCS)
A key benefit of the LCSTM protocol is the physical separation of the ETT1 fabric and the linecards. This
separation introduces some latency in the communication channel between the linecard and ETT1. The
credit mechanism used in LCS ensures that this latency does not adversely impact the throughput of the
system. However, the linecard must compensate for the latency in the communication channel so that the
credit mechanism can function correctly. This section describes the operations that the linecard should
perform.
C.1.1 Preventing Underflow of the VOQ
The linecard must respond to grants from the ETT1Chip Set within a certain period of time in order to
sustain maximum throughput. In this section we explain how much time the linecard has in which to
respond to grants.
Consider the case where the virtual output queue in the port card in Figure 89 is full, with 64 cells waiting to
be forwarded to the appropriate output. Now, assume that the ETT1 Scheduler starts to forward those
cells, one per cell time. When the first cell is forwarded, a grant is sent to the linecard, indicating that
another cell can be sent to that output. If that cell does not arrive before the VOQ is empty, then the
throughput for that flow cannot be equal to the link rate. Therefore, the new cell forwarded by the linecard
must arrive at the EPP/DS before the VOQ is emptied.
The round trip time for the whole process must be less than or equal to the number of cells in the VOQ; 64
cells for an OC-192c flow and 16 cells for an OC-48c flow. Figure 89 shows this time divided between the
EPP/DS devices and “everything else”.
The EPP/DS uses 22 cells plus the programmed_token_delay. Refer to Section 3.4.2.39 “Internal Delay
Matching Adjustments” on page 188. Subtract the total (22 cells + programmed_token_delay) from 64 to
get the budget for “everything else” (OC-192c cells).
“Everything else” includes any delay on the port card (Serdes), a mux device, the propagation delay of the
signal down the fiber, the Serdes at the linecard and the linecard device delay.
To calculate the time available to the linecard device, subtract the other elements from the “for everything
else” budget. The fiber delay is approximately 15 cell times (7.5 cells in each direction), assuming a 70m
link.
PROPRIETARY AND CONFIDENTIAL TO PMC-SIERRA, INC., AND FOR ITS CUSTOMERS’ INTERNAL USE
327
PMC [ PMC-SIERRA, INC ]
