EXFO FTB-8510B Guia Del Usuario página 1183

System Tab
Default Test Preferences
For ODU3 background traffic (ODUflex client):
The above example shows that ODU3 that contains ODUflex
foreground traffic uses ODUflex (one tributary slot) background or
Unallocated traffic.
For ODU3 background traffic (ODU0 client):
The above example shows that ODU3 that contains ODU0
foreground traffic uses ODUflex (one tributary slot) background or
Unallocated traffic.
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SONET/SDH Application