CodingBox Documentation

Link flapping, CRC & dirty connectors

A link that bounces up and down, or a port whose CRC and receive-error counters keep climbing, is most often not a failing module and not a broken fibre. It is a marginal optical budget — and the cheapest place to find lost decibels is the connector face.

The economics of contamination

  • A single dust particle of 2–5 µm on a connector end-face adds 0.5–2.0 dB of

loss — enough to push a marginal link over its budget.

  • A clean LC pair contributes a stable 0.1–0.2 dB.

Cleaning is therefore "the cheapest decibel you will ever recover".

Symptoms

  • Link flapping — repeated up/down transitions in the log.
  • Rising CRC, input errors or receive errors on the port counters.
  • Rx power lower than the installation record, or lower on one end than the other.
  • A link that fails only under load or at certain temperatures.

The diagnostic ladder

Operators converge on the same sequence:

  1. Read the levels on both ends. Compare Tx/Rx with the link passport taken at

installation. Look for asymmetry and for DDM thresholds being crossed — see DDM levels.

  1. Inspect → clean → re-inspect. Use a fibre inspection scope, then a one-click

cleaner or a reel-type cassette cleaner. After any wet cleaning, finish with a dry pass. Never plug in a connector you have not inspected.

  1. Reseat, then swap the module. Re-insert the transceiver. If the fault persists,

replace it with a known-good module. Field pragmatism: don't spend an hour on a patch cord — clean it once, and if it still fails, replace it.

  1. OTDR from both directions. Only now suspect the fibre plant. Ask the provider

for a trace. After a fibre repair (a new splice), total loss is frequently higher than before — a link that used to run on 10 km-rated optics may no longer reach.

Why order matters

Each step is cheaper and faster than the next. Swapping a module before cleaning hides the real cause (a dirty patch will dirty the new module too); calling for OTDR before cleaning wastes a technician's visit.

In CodingBox

Use the DDM screen to read a suspect module on the bench and compare its Rx against the same module in the field; log values to CSV to build the passport. If the module itself is fine on the bench, the problem is in the plant or the connectors, not in memory.

Sources

  • FOA — Optical loss budgets
  • MapYourTech — Connector contamination economics
  • Community discussions: LC fibre cleaning and inspection recommendations