In this example we assume that there is an NIU installed at the remote/far-end site and the channelized T1 being test has an interface of Serial3/0/2:23

     router#conf t

     router(config)#controller T3 3/0

     router(config-controller)#t1 2 loopback remote line fdl bellcore

                 3/0 - the 'shelf/slot' of T1 line
                 T1 - specify/create the T1 channel
                 2 - T1 port or T1 channel number
                 loopback - put the T1 line into loopback
                 remote - external T1 remotely loopback
                 line - external remote loopback at the line
                 fdl - using FDL Message oriented
                 bellcore - remote Smartjack/NIU

You should see 'looped' when you do a 'show int Serial3/0/2:23' from the priviledge mode. You  could also run extended ping test to your own interface.

To remove the loopback and stop the test just negate the command on the controller config mode.

     router(config-controller)#no t1 2 loopback


Figure below shows examples of how data is transmitted and received in each loopback mode.