CWDM deployment & troubleshooting

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CWDM is the simplest way to multiply a fibre, and most of its failures are simple too — once you know where to look. This page covers the common topologies and the faults that recur in the field.

Topologies

Point-to-point — one mux pair, 4–18 channels over one fibre pair (or one fibre with BiDi channel pairs). The default for connecting two sites.
Ring with OADMs — a fibre ring where each site drops and adds its own channel with an OADM; the rest pass through. Loss accumulates through every OADM, so the budget must be computed end-to-end for the farthest channel.
Hybrid with a grey pass-through — the mux has a 1310 (or 1550) port so an existing non-WDM link keeps working on the same fibre while CWDM channels are added.
Mixed rates — 1G and 10G channels coexist freely on one mux; each channel is independent.

Design checklist

Choose a channel plan both ends share; avoid the water-peak channels on legacy fibre (channel plan).
Compute the link budget per channel, worst band first (mux/demux & link budget).
Put the longest spans on the 1470–1610 nm channels.
Check 10G dispersion beyond ~60–80 km.
Label everything by wavelength; record the installation Rx power per channel.

Recurring faults

Symptom	Cause	Check
One channel down, others fine	module in the wrong mux port, or wrong-wavelength module	read the module's wavelength field; match it to the port label
Channel works on short link, fails on long	budget miss on that band (O-band loss, water peak)	recompute budget with the real band loss; move to a 1550-band channel
All channels down	fibre cut, or mux/demux swapped ends / mismatched plans	OTDR; verify both muxes use the same plan
10G channel errors, 1G on same mux fine	chromatic dispersion at 10G on the upper band	shorten, use a dispersion-tolerant module, or DWDM/coherent
Rx power drifting down over months	dirty connector at the mux, ageing laser	clean; compare Tx bias trend (failures)
Module rejected by the switch	vendor lock	code identity (vendor lock)

Diagnostic sequence

Follow the standard ladder — levels → clean → swap → OTDR — with one CWDM-specific step first: confirm the module's wavelength matches the mux port. A surprising share of "dead channel" tickets is a 1531 module in a 1551 port.

In CodingBox

Before installation, read every CWDM module: the Check transceiver screen shows its wavelength and reach class, and the DDM screen its Tx power — the inputs to the budget and the labels for the mux. After installation, compare the far-end Rx power with the budget; the difference is your real margin.

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