EXFO FTB-8510B Guia Del Usuario página 1187

System Tab
Default Test Preferences
For ODU3 background traffic (ODU1 client in ODU2):
The above example shows that ODU3 that contains ODU1 in ODU2
foreground traffic uses ODU1 background traffic. The remaining
tributaries are dependent 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.
537
SONET/SDH Application