Juniper JUNOSE 11.1.X - COMMAND REFERENCE N TO Z 4-6-2010 Command Reference Manual page 185

FDL Bellcore bit loopback on the T1 channel, according to the Bellcore
TR-TSY-000312 specification.
remote payload [ fdl ] [ ansi ] (T1 line only) Sends a repeating 16-bit ESF data
link code word (00010100 11111111) to the remote end requesting that it enter
into a network 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
Mode
Chapter 4: P Commands
143
path ds1|e1 loopback