More Information About T1 Testing

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Detailed Loop Mode and Auto Configuration Descriptions

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Remote CSU/NIU Loop Mode

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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.
The flow of events differs when the out-of-band method of loopback is selected. In this case, loopback can occur within 16 ms, and therefore it is not possible to determine if a pre-existing loop exists.