EXFO FTB-8100 Series Transport Blazer for FTB-500 - Manual (Page 552)

System Tab
538 FTB-8100 Series Transport Blazer
Default Test Preferences
For ODU3 background traffic (ODU0 client in ODU1 in ODU2):
The above example shows that ODU3 that contains ODU0 in ODU1
in ODU2 foreground traffic uses ODU1 background traffic. The
remaining tributaries are dependent on the ODU2 configuration
made in the test setup:
When the Fixed Structure check box is selected, ODU2
background traffic is used.
When the Fixed Structure check box is cleared, ODU1
background traffic is used.