EXFO VDSL2 - Manual (Page 186)

Running CPE Tests
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Reading Results
ATM Loopback Test
The ATMLB Test tab allows you to view the ATM loopback connection
value results of the VPI (virtual path identifier) and VCI (virtual circuit
identifier), and the PTI (payload type identifier) which is either
End-To-End or Segment.
If the unit is not connected to the
network or the time-out setup value is
low, you may see the Timeout
message continuously.
Send button starts the ATM loopback
test.