Nortel DMS-100 Series Maintenance Manual page 629

The BAL automatically sets the balance network in the line card to
provide transmission balance. The BAL provides transmission balance
between the four-wire side of the switch and the two-wire loop.
Transmission balance minimizes subscriber reports of noise, echo, and
garbled speech.
The LIT implements an automatic test that detects cable pair faults. An
automatic test detects cable pair faults so that these cable pair faults can
be cleared. These cable pair faults must be cleared before faults affect
service and subscribers report problems like hum, noise, grounds, or
false ring trip.
The CKTTST applies to loops that use the following:
— Meridian business sets (MBS)
— data units (DU) associated with Datapath
— asynchronous interface modules (AIM)
— IBERT line cards
The CKTTST performs circuit tests to confirm the ability of the set or line
card to transmit and receive messages correctly. The CKTTST performs
circuit tests to confirm the ability of the set or line card to adhere to message
protocol.
DMS-100 Family OPAC Maintenance Manual XPM12 and up
Trouble isolation and correction 10-11