Mitel SX-200 EL Technician's Handbook page 93

Refer to The S1 Switch Settings for the T1 Trunk Cable Length (not
loop length) (page 192).
Loopback
Loopback is used to verify and locate the source of either a cable or
a synchronization problem through the devices in a T1 transmission
path. Devices in the T1 transmission path (PBX, CSU, Channel
bank, smart jacks, etc.) will generally have some type of loopback
mode. Loopback modes will typically have the effect of taking the
input T1 stream and transmitting the stream back to the source; the
nodes thereby verify that portion of the link.
In the absence of a loopback mode, the T1 transmits and receives
pairs that can be directly connected together to physically create a
loopback condition.
A&B bits
Taken from each of the 24 Channels to emulate the various analog
trunk signalling conditions (off-hook, on-hook, disconnect
supervision, etc.) in either of the two following methods.
Superframe: Over a cycle of twelve frames, bits A&B are taken in
frames 6 & 12 to provide basic analog trunk emulation over the T1
facility.
Extended superframe (ESF): ESF is not supported by the SX-200
EL/ML or the SX-200 LIGHT. ESF is supported by an ISDN.
Troubleshooting and Repair
83