RELEASED
PMC-Sierra, Inc.
PM9311/2/3/5 ETT1™ CHIP SET
Data Sheet
PMC-2000164
ISSUE 3
ENHANCED TT1™ SWITCH FABRIC
The Enhanced Port Processor ensures this by periodically inserting an Idle cell in the egress cell stream
(to the linecard). Clearly, if the linecard can accept a cell rate that is fundamentally greater than the
maximum rate at which cells are sent, then there is no problem.
In practice, however, the rate at which cells are transmitted and the rate at which cells can be accepted are
determined by the clocks at each end. These clocks have some inherent imprecision, usually expressed as
some number of parts per million. This means that the true rate at which cells can be transmitted or
accepted might be slightly greater or smaller than the nominal rate. The remainder of this section is an
example to explain this in more detail.
Example: We assume that the ETT1 port and Linecard operate at a nominal frequency of 200MHz with an
imprecision of +/-200ppm. One (OC-192c) cell time is eight clocks or 40ns, corresponding to a nominal
rate of 25M cells/s +/-5000 cells/s.
1. Linecard to ETT1 Port.
Assuming a slow clock at ETT1, then the ETT1 port may only be able to accept 24,995,000 cells/s.
The linecard must not exceed this rate. So, if the linecard has maximum frequency clock
(200.04MHz) then it could send 25,005,000 cells/s. So, to avoid overrunning the ETT1 port the lin-
ecard must send 10,000 Idle cells per second to bring its effective cell rate down to that of the
ETT1 port.
2. ETT1 Port to Linecard.
In this direction the worst case is where the ETT1 port has the highest frequency clock and the lin-
ecard has the lowest frequency. So the ETT1 port must insert 10,000 Idle cells per second. This
would be done by programming the value 2,499 into the Idle Count Register in the Enhanced Port
Processor. This causes the Enhanced Port Processor to insert one Idle cell for every 2,499 cells
transmitted.
C.4
AVOIDING DELAYS DUE TO IDLE CELLS
Figure 94 shows a simple view of a linecard. A grant is received from the ETT1 port and the linecard must
immediately respond with the corresponding cell body. However, if the Idle counter expires just at that
moment, then the linecard cannot send the cell body; it must send an Idle cell instead. This behavior can
cause a cell body to be delayed, thus diminishing the effective round-trip time available to the linecard, as
well as introducing extra queuing into the linecard.
336
PROPRIETARY AND CONFIDENTIAL TO PMC-SIERRA, INC., AND FOR ITS CUSTOMERS’ INTERNAL USE
PMC [ PMC-SIERRA, INC ]
