EXFO FTB-7000 OTDR Series for FTB-500 - Manual (Page 245)

Analyzing Bidirectional Traces
OTDR 231
Creating Bidirectional Trace Files
The A->B and B->A traces must respect the following criteria:
When two traces are opened in the bidirectional analysis utility, the A->B
trace is on the left and the B->A trace is on the right. If the analysis does
not match the traces, error or warning messages will appear. A message
will be displayed if there are any inconsistencies in the events table,
wavelength, index of refraction, helix factor, or Rayleigh backscatter
coefficient.
Note: The A->B and B->A traces are displayed in full view mode (1:1 zoom
factor).
Item To be valid...
Pulse width Must be identical for both traces.
Fiber types Use only traces acquired using singlemode fibers.
Acquisition offset Must be set to zero for both traces.
Wavelengths
Must be identical for both traces.
Trace Both must be unidirectional files (.trc files).