Juniper JUNOSE SOFTWARE FOR E SERIES 11.3.X - COMMAND REFERENCE N TO Z 2010-10-19 Command Reference Manual page 185

Mode
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payload loopback. Enables the remote payload FDL ANSI bit loopback on the T1 channel.
You can optionally specify fdl and ansi, but it is not necessary.
NOTE: You cannot send an inband pattern to the remote end requesting
that it enter into a network line loopback.
Controller Configuration
Chapter 4: P Commands
143