ONU Failure Analysis

ONU (Optical Network Unit) is divided into active optical network unit and passive optical network unit. Generally, devices equipped with network monitoring including optical receivers, upstream optical transmitters, and multiple bridge amplifiers are called optical nodes. The PON uses a single fiber to connect to the OLT, and then the OLT connects to the ONU. ONU provides services such as data, IPTV (i.e. interactive network television), and voice (using IAD, i.e. Integrated Access Device), truly realizing “triple-play” applications.


Time slot drift ONU

This kind of failure is usually caused by the aging of the laser and the drift of the time slot for the upstream data transmission of the ONU. It is also possible that an error occurs in the data of the bandwidth map sent by the downstream OLT, which causes an error in the transmission time slot of the ONU. This kind of faulty ONU is difficult to detect. Generally, the OLT needs to monitor the upstream data of the ONU in real time. Determine whether it sends data in the time slot allocated by the OLT. For this type of abnormally luminous ONU, because it mainly affects two adjacent ONUs, in addition to isolation, the protection bandwidth between time slots can also be increased to alleviate its impact on other ONUs.

Long light ONU

This failure is caused by the ONU’s laser for a long time. As a result, the ONUs under the entire PON port cannot communicate normally. If the ONU emits strong light, the abnormal light emission of the ONU can be determined by testing the optical power at the optical splitter during troubleshooting. However, if it is a weakly emitting ONU, the judgment will be more difficult and the abnormal light emission of the ONU cannot be measured at the optical splitter. Only through the intervention of the OLT and the use of a specific detection algorithm can the abnormal ONU be judged. Usually, the business under the entire PON port will be affected, but because of its failure phenomenon has always existed, it has brought convenience to the troubleshooting to a certain extent.

Intermittent glow

The judgment of this kind of failure is the most difficult and irregular. It is also the biggest fault with abnormal lighting ONUs in the live network. It is not only impossible to measure the abnormal light emission of ONU at the beam splitter. In addition, it is necessary to observe and monitor faulty PON ports for a long time, which brings great challenges to network maintenance.


  1. Long light emission problem: The photoelectric converter is connected to the branch fiber by mistake, and the photoelectric converter emits light for a long time; the ONU optical module has an abnormal turn-off signal, and the ONU is not turned off when it is turned off, which causes the ONU to emit light for a long time; the ONU software crashes, which cause the ONU to emit light for a long time.
  2. Coverage interference problem: For some reasons (such as insufficient extinction ratio), individual ONUs emit light in unlicensed time slots, which affects the normal services of other ONUs.
  3. Overlapping interference problem: The inconsistency of the parameters of different ONU optical modules causes partial overlap of the upstream optical signals, which affects services.

Post time:2023-07-17


Lomoveishiy – Finland

I needed those to connect my PC on the third floor to have internet access in that room, and ISP installed their modem on the first floor only. After dropping fiber patch cables, plugged in all cables into these media converters at both sides, and link came up instantly. Was much easier than I thought!

Raymond – USA

Great experience – units worked straight out of the box – just needed plug in cables and we were done. I also like the possibility to enable jumbo frames, while we do not have a need for this feature at the current moment it’s great to have this option.

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