Technical data

Chapter 18 527
T1 Analyzer
More Information About T1 Testing
T1 Analyzer
More Information About T1 Testing
Detailed Loop Mode and Auto Configuration Descriptions
Remote CSU/NIU Loop Mode
At each end of a T1 facility there is a Channel Service Unit (CSU) and possibly a
Network Interface Unit (NIU). Both of these units have the ability to be remotely
“looped up” by sending the proper loopback code. This can be done either by
sending the expected loopback code in-band or out-of-band. When sent in-band, a
particular pattern is transmitted on every slot for 5 seconds. When the code is sent
out-of-band, the pattern is sent as part of the framing bit stream and is referred to as
‘data-link’. It is important to note that once a CSU or NIU is remotely placed into
loopback, it remains in that state. When a CSU or NIU is in a loopback state, that
unit will loop the in-band loop code back to the near end. If the near end CSU sees
the in-band loopback code repeated from the far end CSU then both CSU units can
become locked in a loopback state. The test set is designed to minimize this
possibility. However, you should take precautions to prevent this from happening
and be aware of this potential situation. Some CSU/NIU units do not respond to
in-band loop codes when they are configured for ESF. In general, NIU loop codes
will be blocked at the central office.