EXFO FTB-8510B Guia Del Usuario página 1186

System Tab
Default Test Preferences
536
For ODU3 background traffic (ODU2 client):
The above example shows that ODU3 that contains ODU2
foreground traffic uses ODU1 or ODU2 background traffic
depending on the ODU2 configuration made in the test setup. The
foreground structure modulates the background structure.
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.
FTB-8100 Series Transport Blazer