CWDM mux/demux & link budget

A CWDM link is a chain of passive parts — transceiver, multiplexer, fibre, demultiplexer, transceiver — and it works only if the light that arrives is stronger than the receiver needs. Doing that sum is the core of CWDM engineering.

The passive parts

Part	What it does	Typical insertion loss
Mux/demux (4/8/16/18 ch)	combines channels onto one fibre / splits them	1.0–2.5 dB (8 ch), up to ~3.5 dB (16–18 ch)
OADM (add/drop)	inserts and extracts one or two channels mid-span	0.8–1.5 dB per channel, ~1 dB pass-through
1310/1550 pass-through port	lets a legacy grey link share the fibre	~1 dB
Connectors, splices	—	0.2–0.5 dB per connector, 0.1 dB per splice

Everything is passive: no power, no configuration, no failure modes beyond dirt and damage. Muxes are thin-film filters in a 1RU shelf or LGX cassette; the two ends must use the same channel plan.

Fibre loss by band

Fibre attenuation is not the same at every CWDM wavelength:

Band	Wavelengths	Loss (G.652.D)	Note
O-band	1271–1331 nm	~0.33–0.35 dB/km
E-band	1351–1451 nm	~0.3–0.5 dB/km	water peak near 1383 nm on older G.652.A/B fibre — may be unusable
S/C/L	1471–1611 nm	~0.20–0.25 dB/km	best reach

Low-water-peak fibre (G.652.C/D, "ZWP") opens all 18 channels; on legacy fibre the 1371–1411 nm channels are often skipped, which is why 8-channel systems favour 1471–1611 nm (see CWDM channel plan).

The link budget

Budget = Tx power (min) − Rx sensitivity must exceed the sum of all losses plus a safety margin:

Losses = mux + demux + fibre × km + connectors + splices + ageing margin (≈3 dB)

Worked example — 10G CWDM SFP+ "40 km" module, 8-channel muxes, 30 km:

Tx min  0 dBm, Rx sens −16 dBm            → budget 16 dB
Mux 2.0 + demux 2.0                        =  4.0 dB
Fibre 30 km × 0.25 dB/km (1550 band)       =  7.5 dB
4 connectors × 0.3                         =  1.2 dB
Margin                                      =  3.0 dB
Total                                       = 15.7 dB  → OK, barely

The same module on an O-band channel (0.35 dB/km → 10.5 dB fibre) would not close. This is why long CWDM links put 1550-band channels on the longest spans.

Dispersion at 10G

At 10 Gb/s, chromatic dispersion also limits reach: in the 1550–1611 nm band it is around 17–20 ps/nm/km, so an 80 km span accumulates ~1 600 ps/nm — the limit of a standard 10G direct-detect receiver. 1G CWDM does not care; 10G CWDM beyond ~80 km needs dispersion compensation or a different technology (DWDM & coherent).

In CodingBox

The transceiver's own Tx power and Rx sensitivity class are what you plug into the budget. CodingBox reads the module's declared reach/rate codes and, live, its Tx power and Rx power on the DDM screen — so a measured Rx at the far end can be compared with the budget you calculated.