Description

This article provides information about SONET/SDH alarms, which are seen in customer environments, and the troubleshooting tips for the corresponding alarms.

Symptoms

Basic Alarms observed in SDH networks:

LOS-Loss of Signal - Critical :

Conventionally, the 2430 byte SDH frame is shown in 270 columns and 9 rows. A NE receiving an SDH signal actually reads serial data. The serial data consists of STM – 1 frame.

The most fundamental issue is that there is no or very bad signal at the physical interface. This condition will raise a Loss Of Signal (LOS) alarm. LOS is also raised when the STM – N signal level drops below the threshold, at which a BER of 1 in 10 3 is predicted. This can be due to a cut cable, excessive attenuation of the signal, or faulty equipment.

When does LOS clear?

The LOS state will clear when two consecutive framing patterns are received and no new LOS is detected.

Solution


Troubleshooting :

  • Check the fiber optic cable to ensure that it is plugged in.

  • Verify if the local fiber optic cable is not damaged. Look for breaks or physical abnormalities.

  • Ensure that the remote end of the fiber optic cable is connected, undamaged, and the remote port is properly configured.

  • Try a soft loopback with the loopback internal command.

  • Try a hard loopback. Connect the transmit to receive with a single fiber strand.

  • Determine whether the POS interface is just receiving too little or too much light.

LOF -Loss Of Frame - Critical :

  • Assuming that the signal is present, the first task of NE is to identify where the STM – 1 frames are within the serial data. It does this by identifying the Frame Alignment Word (FAW), which is contained in the first six bytes of the RSOH. If it cannot identify the FAW, a Loss Of Frame (LOF) alarm will be raised.

  • The LOF state arises when the OOF (Out of Frame Alignment) state exists for more than a specified time (in microseconds).

  • The Out of Frame Alignment state occurs when several consecutive SDH frames are received with invalid or errored framing patterns (A1 and A2 bytes in RS OH). The maximum time to detect OOF is 625 microseconds. The OOF state clears within 250 microseconds, when two consecutive SDH frames are received with valid framing patterns.

When does LOF clear ?

The LOF state clears when an in-frame condition exists continuously for a specified time in microseconds. Time for detection and clearance is normally 3 milliseconds.

Troubleshooting :

  • Check the fiber optic cable to ensure that the cable is plugged in and is not damaged.

  • Ensure that the framing format on the port matches the format configured in the following line:
    AIS-Alarm Indication Signal - Minor

  • AIS is an all-ones characteristic or Alarm Indication Signal. It is generated to replace a normal traffic signal, when it contains a defect condition, to prevent consequent downstream failures being declared or alarms being raised. AIS can be identified as MS – AIS, AU – AIS, or TU – AIS.

  • REI is an indication returned to the transmitting node or source, which an errored block has detected at the receiving node or destination (this was previously known as Far End Block Error – FEBE). REI can be identified as MS – REI, HP – REI or LP – REI.

  • RDI is a signal returned to the transmitting Equipment, when a defect, such as Loss of Signal, Loss of Frame or AIS, is detected (this was previously known as Far End Receiver Fail – FERF). RDI can be identified as MS – RDI, HP – RDI, or LP – RDI.

Troubleshooting
  • Verify if the remote configuration is correct.

  • Check the line status at the remote end of the link.

  • Check the remote site for alarm conditions.

  • Alarm Indicate Signal - Path (PAIS)
LOP - Loss Of Pointer :

The next step is to find where the VC – 4’s, relative to the FAW, are located. This is established by reading the Administrative Unit (AU) pointer to locate the J1 byte in the VC – 4 POH. If a sensible pointer cannot be found, a Loss Of Pointer (LOP) alarm is raised at the AU level. This is usually referred to as AU – LOP.

The next step is to locate and read the Tributary Unit (TU) pointer for the specified TU. If a sensible pointer cannot be found, then a LOP alarm is raised at the TU level. This is referred to as TU – LOP.

LOS, LOF and LOP alarms will render the whole signal unusable. In this case, the missing or corrupted signal is replaced with an AIS consisting of continuous binary 1’s. This will produce AIS alarms on all equipment downstream of the fault. The NE detecting the fault also sends and indication to the distant (sending) end that an alarm has been raised. This raises a RDI alarm, at the appropriate level, in the transmitting NE.

So, a fault at the MS level will produce a MS–RDI. At the VC– 4 level, it will produce a HP–RDI and if the fault is at the LO, it will be replaced as a LP – RDI. Generallly speaking, LOP state occurs when N consecutive invalid pointers are received.

When does LOP clear?

The LOP state clears when three equal valid pointers or three consecutive AIS indications are received. LOP can be identified as AU – LOP or TU – LOP.