Cyclic Redundancy Check (CRC) Errors

Occurs by marginal or faulty line repeaters, network circuit terminating equipment (NCTE), noise on the transmission line, circuitry that generates the framing pattern, or CRC at the transmit end. You can detect bit errors in a DS-1 signal through CRC errors when you use Extended Super Frame (ESF).

Loss of Frame (LOF) Alarm, Red Alarm

Occurs when the near-end interface cannot frame up on the DS-1’s signal. A red LED lights on D4-channel banks when this alarm is on. A frequent cause is an incorrect setting of the framing option at one end of the transmission facility or within the network. This scenario will cause LFA at both ends of the transmission link. Other possible causes are an intermittent cable, broken cable, and a rain-attenuated signal with microwave transmission facilities.

This alarm trips after detecting a continuous loss of framing, and clears after restoring the in-frame condition. The end of the span with the LFA sends an RFA to the other end to indicate that the LFA cannot frame on the other signal.

Remote Frame Alarm (RFA)

Yellow Alarm

Occurs when the far end is unable to frame up on the signal sent by the near end. The far-end interface is in an LFA state. If there is a fault with the part of the transmission facility that transmits the DS1’s signal from the near end to the far end, a yellow LED is lit on D4-channel banks. The cause of this problem is a broken conductor in the transmission cable wiring or within the network.

Loss of Signal (LOS) Alarm

Occurs when there is no bipolar signal at the receiver input. Occurs in parallel with the LFA alarm. The cause can be cable-related problems such as a broken pair inside a cable, an intermittent cable at a cross-connect point, or a cable connector that is not completely seated.

AIS, Blue Alarm

Occurs when the maintenance activities are in progress and the out-of-service condition exists for the DS1 facility. You can recognize the alarm as a continuous stream of 1s with no framing bit.

This alarm condition may be treated differently depending on the particular network circuit terminating equipment (NCTE) used. It may result in the NCTE automatically looping the signal back to the switch. If the looped facility is providing synchronization, then the synchronization subsystem must detect that the facility is looped and deal with that condition. Otherwise, synchronization problems will occur.