CSU/DSU alarms indications

When you are troubleshooting Telco circuit, DSU/CSU alarm indication is important fact to isolate an issue without wasting time. The most common alarms are Red, Yellow and Blue (as known as AIS – Alarm Indication Signal) alarm.

• Red Alarm

A Red alarm indicates CSU/DSU has lost synchronization over a longer period of time. It could be a lost of signal or frame error occurred. Frame errors are normally happening when two or more framing bit errors within a 3 ms period.

A device declares red alarm condition when above errors are detected. After that the device will send Yellow alarm to far-end device.

• Yellow Alarm

A device has declared red alarm condition, and sends yellow alarm to far-end. If far-end device received the yellow alarm, it means a circuit from the device that received red alarm to far-end device is fine. For example, there are two devices on the circuit; A and B. Let’s say device B is receiving lost signal or out of frame errors from the circuit, the device B will declare Red alarm indication on the port that is connected to device A. The device B will send yellow alarm to far-end device which is device A. If the device A received the yellow alarm, transport link is fine from device B to A, but not other way around.

• Blue Alarm or AIS(Alarm Indication Signal)

Blue Alarm or AIS(Alarm Indication signal) indicates that a device receiving continuous unframed 1s. It caused by a segment of the end-to-end link failed at a logical or physical level while direct link is okay.