The AIS is also known as a "All-Ones", "Blue-Alarm" or "Keep-Alive" signal. When a data path is broken the first downstream device to detect this LOS/OOF condition sends an AIS-Alarm on down the line (2.5 seconds of an OOF condition will also cause the device to go into Red-Alarm and send a Yellow-Alarm back up the line). This AIS-Alarm consists of an unframed all ones signal sent to maintain transmission continuity. It is usually an indication of a physical layer problem.

Occurs when the equipment detects a continuous OOF condition for 2.5 seconds. This alarm state is cleared when no OOF conditions occur for at least 1 second

When the equipment enters a Red-Alarm state, it returns a Yellow-Alarm back up the line of the received OOF. This alarm is a repetitive 16-bit pattern consisting of 8 "marks" (1) followed by 8 "spaces" (0) (11111111 00000000) and is transmitted for at least 1 second.

A LOS condition occurs when no pulses have been detected in a 175 +/- 75 pulse window.

Occurs when the equipment senses errors in the incoming framing pattern. Depending upon the equipment, this can occur when 2 of 4, 2 of 5, or 3 of 5 framing bits are in error. A reframe clears the OOF condition.

This occurs when the CRC polynomial calculation performed before transmission does not match the CRC calculation done upon reception.

This occurs when two or more framing bit errors are detected in any 3 mS window. Also, 2 of 4, 2 of 5, or 3 of 5 errored framing bit criteria can be used to declare a Severe-Error (SE). When a Severe-Error is reported, the FE (Frame Bit Errors) count is set to 0.

This occurs when the equipment receives an incorrect or unexpected framing bit. When a FE error count is transmitted, the Severe-Error (SE) count is set to 0.

This occurs upon a bipolar violation.

This occurs when frames are repeated or deleted at the receiving terminal to compensate for clocking differences between the Carrier and the Terminal equipment.