CWDM vs DWDM Explained: Wavelength Grids, Channel Spacing and How to Test WDM Links
When a fibre route runs out of capacity, laying new cable is not the only answer. Wavelength division multiplexing (WDM) sends several signals down the same fibre, each on its own wavelength (colour) of light, so one fibre pair can carry many separate services. There are two families defined by the ITU-T: CWDM (coarse) and DWDM (dense). This guide explains how each works, the wavelength grids behind them, when to choose which, and how to test and troubleshoot a WDM link in the field.
How WDM works
At one end a multiplexer (mux) combines several transceivers, each fixed to a different wavelength, onto a single fibre. At the far end a demultiplexer (demux) separates them again, so each wavelength arrives at its own port. Many CWDM and DWDM modules are passive, so they need no power and simply add a fixed insertion loss to each channel. Because each wavelength is independent, one fibre can carry, for example, several 10G Ethernet links, a mobile fronthaul link and a leased circuit at the same time.
WDM uses the low-loss windows of single-mode fibre. ITU-T groups single-mode wavelengths into bands: O (1260 to 1360 nm), E (1360 to 1460 nm), S (1460 to 1530 nm), C (1530 to 1565 nm), L (1565 to 1625 nm) and U (1625 to 1675 nm). Attenuation is lowest around 1550 nm in the C-band, which is also where optical amplifiers work best.
CWDM vs DWDM at a glance
| CWDM | DWDM | |
|---|---|---|
| ITU-T grid | G.694.2 | G.694.1 |
| Channel spacing | 20 nm | 100 GHz (about 0.8 nm), 50 GHz (about 0.4 nm) or narrower; flexible grid also defined |
| Wavelengths | Up to 18, from 1271 to 1611 nm | Typically 40 (100 GHz) to 80 or more (50 GHz) in the C-band, plus L-band systems |
| Lasers | Uncooled, with a wide wavelength tolerance, so cheaper optics | Temperature-controlled, wavelength-stable lasers, so more expensive optics |
| Amplification | Not practical across the whole band; links are unamplified | C- and L-band channels can be amplified together with optical amplifiers (EDFA) |
| Typical reach | Metro and access links of tens of kilometres | From access links to amplified long-haul routes of hundreds or thousands of kilometres |
| Typical use | Adding capacity to an existing fibre pair cheaply, mobile fronthaul, enterprise links | Backbone, metro rings, data-centre interconnect, high-capacity and coherent 100G/400G systems |
CWDM: coarse wavelength division multiplexing
ITU-T G.694.2 defines the CWDM grid: 18 wavelengths from 1271 nm to 1611 nm, spaced 20 nm apart. The wide spacing allows lasers without temperature control, whose wavelength drifts with heat, so CWDM transceivers and passive mux/demux units cost much less than DWDM equipment.
| CWDM wavelengths (nm) | Band | Note |
|---|---|---|
| 1271, 1291, 1311, 1331, 1351 | O-band | Higher fibre attenuation than at 1550 nm |
| 1371, 1391, 1411, 1431, 1451 | E-band | 1371 to 1411 nm sit in the water-peak region; use low-water-peak fibre such as G.652.D |
| 1471, 1491, 1511 | S-band | |
| 1531, 1551 | C-band | Lowest attenuation region |
| 1571, 1591, 1611 | L-band | Most sensitive to macrobends |
Older single-mode fibre has a strong attenuation peak caused by water in the glass around 1383 nm, which makes the E-band channels unusable. ITU-T G.652.D fibre is a low-water-peak fibre, so all 18 CWDM channels can be used on it. Litech's single-mode duct, direct-buried and ADSS cables use G.652.D fibre. On older fibre, 8-channel CWDM systems that use only 1471 to 1611 nm are common for the same reason.
The trade-offs are capacity and reach: 18 channels at most, no practical amplification across the whole band, and higher loss at the short wavelengths. CWDM suits links where you want to add a few services to an existing fibre pair without new cable, such as connecting buildings, towers or a nearby exchange.
DWDM: dense wavelength division multiplexing
ITU-T G.694.1 defines the DWDM frequency grid. It is anchored at 193.1 THz (about 1552.52 nm) and allows fixed channel spacings of 12.5, 25, 50 and 100 GHz, plus a flexible grid for newer coherent systems, where the channel width can be set in steps of 12.5 GHz. DWDM is specified in frequency rather than wavelength; at 1550 nm, 100 GHz is about 0.8 nm and 50 GHz about 0.4 nm.
Equipment vendors usually number 100 GHz C-band channels by frequency, so channel C34 is 193.4 THz. Some examples:
| Channel (common naming) | Frequency (THz) | Wavelength (nm) |
|---|---|---|
| C12 | 191.20 | 1567.95 |
| C21 | 192.10 | 1560.61 |
| C30 | 193.00 | 1553.33 |
| C34 | 193.40 | 1550.12 |
| C40 | 194.00 | 1545.32 |
| C50 | 195.00 | 1537.40 |
| C62 | 196.20 | 1527.99 |
Because DWDM channels sit inside the C-band (and L-band), they can all be amplified together with erbium-doped fibre amplifiers (EDFAs). That is why DWDM is used for backbones and long routes, and why one fibre pair can carry 40, 80 or more channels, each at 10G, 100G or 400G. The cost is higher: DWDM lasers must be temperature-controlled to stay on their channel, and the mux/demux filters are more precise.
Which should you choose?
- A few extra services on an existing short route, on a budget: CWDM. Check that the fibre is G.652.D if you need the E-band channels.
- Many channels, long distances or future growth: DWDM, which scales to dozens of channels and can be amplified.
- Coherent 100G and 400G links or data-centre interconnect: DWDM, often on a flexible grid.
- Fibre is scarce on a busy route: either; WDM can avoid laying new cable for a time, but when a route will keep growing, a higher fibre count cable is usually cheaper over its life. See our guide to choosing fibre count.
WDM also has to coexist with PON on access networks. GPON uses 1490 nm downstream and 1310 nm upstream, and XGS-PON uses 1577 nm and 1270 nm, so WDM channels added to an FTTH fibre must avoid those wavelengths or be separated with suitable filters.
How to test a CWDM or DWDM link
A WDM link must pass the usual fibre tests on the line fibre and then be checked channel by channel. A standard OTDR or broadband power meter cannot see through a mux/demux the way it sees a plain fibre, because each port passes only its own wavelength.
| Test | Why | Tool |
|---|---|---|
| Connector inspection | A dirty connector at the mux adds loss to every channel that passes through it | Fibre inspection probe |
| Loss and continuity through the mux/demux | Confirms each channel path, its loss and its port | Light source and power meter, or a wavelength-specific (CWDM or DWDM) OTDR |
| Channel power and wavelength | Checks that each channel is present, on the right grid position and at the right level | DWDM channel checker (or a CWDM channel analyser) |
| OSNR | On amplified links, noise from amplifiers limits performance | Optical spectrum analyser |
| Dispersion (CD and PMD) | At 10 Gbit/s and above, dispersion can cause bit errors on long routes | Dispersion and PMD analysers |
| Service test | Proves each wavelength carries its Ethernet or transport service correctly | Ethernet and transport tester |
- Prove the line fibre first. Inspect and clean every connector, measure end-to-end loss and take an OTDR trace before the mux/demux is connected. See our guides to connector cleaning, insertion loss testing and OTDR testing. On long metro routes, a high-dynamic-range module such as the EXFO FTBx-750C is useful.
- Build a loss budget for every channel. Add the fibre loss at that channel's wavelength, splices, connectors and the insertion loss of both the mux and demux, then compare with the transceiver's power budget. Our loss budget guide shows the method.
- Check each channel's power and wavelength at the demux outputs or at the line port. The EXFO MAX-5205 DWDM channel checker covers 100 GHz C-band channels 12 to 62 (1527.99 to 1567.95 nm), measures each channel from +10 to -40 dBm in under 3 seconds, and shows a colour-coded pass/fail table with channel drift.
- Measure dispersion on long high-speed routes. For 10G channels over long distances, check chromatic dispersion and PMD; the EXFO FTB-5500B PMD analyzer module runs on the FTB platforms.
- Run a service test on each wavelength before handover. See RFC 2544 vs Y.1564 service activation testing.
- Label and record everything: channel, wavelength, mux port, transceiver, power at each end and the test results, so faults can be found quickly later.
Common WDM faults and how to find them
| Symptom | Likely cause | What to check |
|---|---|---|
| One channel missing at the far end | Transceiver on the wrong wavelength or plugged into the wrong mux port | Channel checker reading at the demux output; transceiver label and port number |
| All channels low by the same amount | Dirty connector, macrobend or bad splice on the common (line) fibre | Inspect and clean the line connectors; OTDR the line fibre |
| One channel low, others fine | Dirty or damaged connector on that port, or a faulty mux/demux path | Inspect that port; measure the loss of that channel path |
| Channel present but drifting toward a neighbour | Laser temperature or ageing problem | Channel drift on the checker; replace the optic |
| Errors on long 10G channels only | Chromatic dispersion or PMD | CD and PMD measurements on the route |
| Long-wavelength channels low (1571 to 1611 nm) | Macrobends, which affect longer wavelengths most | Bend radius in trays, closures and patch panels |
Use clean, correctly matched patch cords between transceivers and the mux, for example LC/UPC duplex OS2 single-mode cords, and check whether the mux ports are UPC or APC before connecting. Our guide to patch cords explains the difference, and the bend radius guide covers the macrobends that hit long-wavelength channels first.
Key takeaway: CWDM (G.694.2) gives up to 18 low-cost channels 20 nm apart for short, unamplified links; DWDM (G.694.1) packs 40 to 80 or more channels into the C-band and can be amplified for long routes. Either way, prove the fibre first, then check every channel's power, wavelength and port.
Frequently asked questions
Can CWDM and DWDM run on the same fibre?
Yes. A common approach is to replace one or two CWDM channels around 1531 and 1551 nm with a group of DWDM channels, using a mux designed for this. It lets a network start with CWDM and add DWDM capacity later.
Do I need special fibre for WDM?
Standard single-mode fibre works. For full 18-channel CWDM, use low-water-peak G.652.D fibre. Bend-insensitive G.657 fibre also helps keep long-wavelength channels clear of bend loss in tight spaces; see G.652.D vs G.657.
Can I use multimode fibre for CWDM or DWDM?
Not for standard CWDM and DWDM, which are designed for single-mode fibre. Short-reach multimode wavelength systems exist for data centres, but they use different wavelengths and optics.
Can a normal power meter test a DWDM link?
A broadband power meter such as the EXFO FPM-600 measures the total power of all channels together, which is useful for checking the line but cannot tell you which channel is missing or low. For that you need a channel checker or an optical spectrum analyser.
How many channels can one fibre carry?
Up to 18 with CWDM. With DWDM, typically 40 channels at 100 GHz or 80 at 50 GHz in the C-band, and more when the L-band or a flexible grid is used.
Need WDM test equipment or fibre cable?
Litech Pakistan (Private) Limited supplies EXFO test equipment, including channel checkers, OTDRs, power meters, dispersion analysers and inspection probes, along with fibre patch cords. We also manufacture single-mode G.652.D optical fibre cables in Pakistan. Message us on WhatsApp +92 313 7776766 with your route and channel plan for a quotation.
Sources: ITU-T G.694.1, Spectral grids for WDM applications: DWDM frequency grid; ITU-T G.694.2, Spectral grids for WDM applications: CWDM wavelength grid; ITU-T G.652, Characteristics of a single-mode optical fibre and cable; ITU-T G-series Supplement 39 (optical bands); ITU-T G.984 and G.9807.1 (GPON and XGS-PON wavelengths); EXFO MAX-5205 specification sheet.
