APPENDIX B
8-7
FibreXtreme User GuideCopyright 2017
8.4 Data Transmission and Flow Control
As SL240 PCIe is seen as a point-to-point link from the transmitter, there is no need to log into
the receiver node to begin sending data. SL240 PCIe cards can begin transmission as soon as
they are started and data is available in the Transmit FIFO. Using the frames described above, the
transmitter sets up a constant stream of frames, into which it inserts data as it becomes available.
Data is only inserted if the flow control signal from the remote end is GO—if it is STOP, then the
data waits in the Transmit FIFO until the signal changes. Curtiss-Wright’ SL240 PCIe cards use
the same protocol when transmitting from either end to allow the link to operate bi-directionally.
Since these data streams are independent, the maximum throughput on the link would be 494
MB/s (247 MB/s/direction) for SL240.
The receiver should transmit the STOP signal when it has space for the data contained in 20 km
of fiber or less left. Assuming 5 µs/km for the speed of light, this gives us 100 µs of data. For
SL240, each 32-bit word (40 bits on the link) takes 16 ns, so there are 6250 words stored in 20
km of cable. The first 10 km is reserved for sending the STOP signal to the transmitter, and the
second 10 km is for the data already contained in the receive fiber.
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