In short: CWDM and DWDM modules each send on one specific "color" of light, so several links can share a single fiber pair through a passive multiplexer. CWDM uses up to 18 widely spaced colors and is the economical choice for a handful of links over moderate distances. DWDM packs dozens of closely spaced colors onto the fiber and is used where fiber is scarce and many links or long distances are needed.
Both technologies solve the same problem: getting more out of a fiber you already have. Here is how they differ and when to use which.
Why put several links on one fiber?
Renting or laying fiber between two sites is expensive. A normal transceiver uses a whole fiber pair for one link. With wavelength division multiplexing (WDM), each module transmits on its own wavelength, a passive multiplexer combines them onto one fiber pair, and another one separates them again at the far end. Eight links can then share the fiber that used to carry one.
CWDM vs DWDM at a glance
| CWDM | DWDM | |
|---|---|---|
| Meaning | coarse wavelength division multiplexing | dense wavelength division multiplexing |
| Channels | up to 18, from 1270 nm to 1610 nm | dozens, closely spaced around 1550 nm |
| Channel spacing | 20 nm | about 0.8 nm (100 GHz grid) |
| Typical reach | 10 km up to 80 km and more | 40 km, 80 km and more |
| Cost | lower | higher |
| Best for | a few links between sites, campus and metro | many links on scarce fiber, long distances |
CWDM – simple and economical
CWDM channels are 20 nm apart, so the lasers do not need precise temperature control. That keeps the modules and the passive multiplexers affordable. It is a good fit when you need a few links – say four to eight – between two buildings or sites and want to avoid renting more fiber.
DWDM – many channels, long distances
DWDM squeezes its channels very close together, so far more links fit onto one fiber pair. The lasers are precisely stabilised, which makes the modules more expensive, but the 1550 nm band also travels well over long distances and can be amplified. DWDM is the choice of service providers and of networks that need many links over limited fiber. Modules are identified by their channel number on the ITU grid (for example CH34).
10G DWDM modules on every common channel40 km and 80 km versions, coded for your equipment.
What you need for a WDM link
- A multiplexer at each end (a passive "mux/demux") with the channels you want to use.
- One module per channel and per end: the same wavelength on both sides of each link.
- A reach that covers the distance plus the loss of the multiplexers. Each mux adds a little loss, so leave some reserve.
- Coding for your equipment, as with any module – see our SFP compatibility guide.
If you only need one extra link and have one free fiber, a BiDi module pair can be the simpler alternative.
Frequently asked questions
Can CWDM and DWDM be used on the same fiber?
It is possible with the right multiplexers, but it needs careful planning. For most networks, one system per fiber is easier to run.
Do I need a multiplexer for a single CWDM link?
No. A pair of CWDM modules on the same wavelength also works as a normal point-to-point link. The multiplexer is only needed when several links share the fiber.
What does the channel number of a DWDM module mean?
It is the module's position on the ITU frequency grid, which defines its exact wavelength. Both ends of a link need the same channel.
Not sure which option fits your fiber? Tell us the distance and how many links you need.
