EXFO IQS-8100 Series Transport Blazer for IQS-600 - Manual (Page 550)

System Tab
536 IQS-8100 Series Transport Blazer
Default Test Preferences
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.