EXFO FTB-8510B Guia Del Usuario página 1185

System Tab
Default Test Preferences
For ODU2 background traffic (ODU0 client in ODU1):
The above example shows that ODU2 that contains ODU0 in ODU1
foreground traffic uses ODU0 background traffic. The remaining
tributaries use ODU1 background traffic.
For ODU3 background traffic (ODU1 client):
The above example shows that ODU3 that contains ODU1
foreground traffic uses ODU1 background traffic.
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SONET/SDH Application