Isolating Faults with Alarm and Errors:

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Elements in a T1 facility are subject to failure. The most common failure is due to inadvertent cable dig-ups. Fault conditions cause errors and errors cause alarms. The alarms are often remotely monitored and can be observed by test equipment attached to the circuit. The most common alarm is the Alarm Indication Signal (AIS), also known as a 'Blue Alarm'. This alarm can be generated by a network element that not functioning or is receiving no signal. A CSU that is not receiving a signal (from the end-equipment) might generate an AIS depending on its configuration. When the CSU at the distant end sees AIS from the network it responds by sending a 'Yellow Alarm' back to the network. See the Measurement Guide for a typical circuit layout.
The CSU is a customer-supplied device. The Network Interface Unit (NIU) is provided by the network supplier. It may be a simple passive device with terminals or an advanced electronic package such as an optical to copper interface. Some NIU devices sense the presence or absence of the CSU and, if absent, send a Customer Disconnect Indication (CDI) toward the network.