WDM Basics
How one fiber pair becomes many services. This is the idea that makes optical transport economical — and the reason dark fiber is worth lighting.
The WDM concept
Wavelength-Division Multiplexing puts multiple independent signals on one fiber by giving each its own wavelength (color) of light. The signals don't interfere because they're on different colors, and a passive filter can combine them onto one fiber and separate them again at the far end.
Without WDM, one fiber pair = one optical service. With WDM, one fiber pair = many color-coded optical services, each running independently on its own wavelength. You're painting extra lanes onto the same road using different colors of light.
The end-to-end WDM path
Each router pair gets its own wavelength through the same mux/demux and the same fiber. Add more transponders on more wavelengths and you add more services without adding fiber.
CWDM vs DWDM
| CWDM (Coarse) | DWDM (Dense) | |
|---|---|---|
| Standard | ITU-T G.694.2 | ITU-T G.694.1 |
| Channel spacing | 20 nm (wide) | 100 GHz (~0.8 nm) or 50 GHz (~0.4 nm); flex-grid for 400G+ |
| Grid definition | Fixed wavelengths 1271–1611 nm | Frequency grid anchored at 193.1 THz |
| Number of channels | Few — up to 18 | Many — ~40 / 80 / 96+ |
| Laser type | Uncooled, looser tolerance | Cooled & tunable, tightly stabilized |
| Amplifiable (EDFA) | Generally no (channels fall outside the EDFA C-band) | Yes — EDFA-amplifiable in C-band → long reach |
| Reach | Short / metro (unamplified) | Metro to long-haul (amplified) |
| Relative cost | Lower — cheaper optics, simpler filters | Higher — precise lasers, richer line system |
| Best for | Short/metro, low channel counts, tight budgets | High capacity, long distance, many services |
CWDM is cheap and simple but limited in channels and reach. DWDM packs far more services and can be amplified for distance, at the cost of tighter, temperature-controlled lasers. Dark-fiber transport at scale is almost always DWDM.
Wavelength, channel, frequency
- Wavelength (λ): the "color" of the light, in nanometers (nm) — e.g. 1550 nm.
- Frequency: the same thing expressed in terahertz (THz). Wavelength and frequency are two ways to name one channel; DWDM planning usually uses frequency.
- Channel: a specific, named slot on the grid that a transponder is tuned to. "Channel 34" is a shorthand for a specific frequency/wavelength.
Assuming a vendor's "channel number" is universal. Channel numbering schemes differ (some count from the C-band edge, some from the anchor, some use ITU "C21…C60" naming). The frequency/wavelength is the ground truth; the channel number is just a label on top of it. When in doubt, match on THz/nm, and verify Ekinops channel numbering against current Ekinops documentation.
Why this is worth the effort — capacity
The payoff of packing the grid: one dark-fiber pair can carry an enormous aggregate. A worked feel for the numbers:
- 80 channels × 100G ≈ 8 Tbps on a single pair.
- 96 channels × 400G (coherent, flex-grid) ≈ 38 Tbps on a single pair.
Contrast a grey optic: one pair = one service. That multiplier is the entire economic argument for lighting dark fiber with DWDM instead of leasing many pairs.
The ITU grid
DWDM channels aren't arbitrary — they sit on a standardized set of frequencies called the ITU grid (e.g. 100 GHz or 50 GHz spacing anchored around 193.1 THz). Everyone's equipment agrees on these slots, so a transponder tuned to a given grid channel will line up with the matching mux/demux port. Think of the grid as reserved parking spaces: each service parks in exactly one, and no two services share a slot on the same fiber.
How the grid is defined
The DWDM grid (ITU-T G.694.1) is defined in frequency, not wavelength, because frequency is what a laser is actually locked to. Everything hangs off one anchor:
- Anchor: 193.1 THz (a reference point near 1552.52 nm in the C-band).
- Spacing: channels sit at fixed steps from the anchor — commonly 100 GHz (e.g. 193.0, 193.1, 193.2 THz…) or the denser 50 GHz (…193.05, 193.10, 193.15…).
- Flex-grid: for 400G and beyond, fixed 50/100 GHz slots give way to variable-width slots so a wide signal can claim exactly the spectrum it needs.
Wavelength and frequency are the same channel named two ways (c = λ × f). Near 1550 nm the handy conversion is: 100 GHz ≈ 0.8 nm and 50 GHz ≈ 0.4 nm. So halving the spacing from 100 to 50 GHz roughly doubles the channel count in the same band — at the cost of tighter laser control.
Passive mux/demux
A mux (multiplexer) combines many wavelengths onto one fiber; a demux (demultiplexer) splits them back out. In their basic form these are passive optical filters — no power, no configuration, just glass that routes colors. That simplicity is a strength (nothing to fail) and a constraint (each port is hard-wired to a specific channel, and the filter adds insertion loss you must budget for).
The three specs that matter
| Spec | What it means | Why you care |
|---|---|---|
| Insertion loss | The dB the filter costs a channel passing through it (mux side + demux side). | Goes straight into your loss budget — often several dB for add + drop combined. |
| Isolation | How well the filter rejects the neighboring channels at a given port. | Poor isolation leaks adjacent channels into a port as crosstalk, degrading OSNR. |
| Passband | The width of frequency the port passes cleanly around its center channel. | Must be wide enough for the signal (and any laser drift) but narrow enough to reject neighbors — the tension that sets DWDM tolerances. |
A passive mux/demux is wavelength-specific hardware: port 3 only passes channel 3. Plug channel 5 into the port-3 fiber and it's heavily attenuated by the filter — the link fails not from fiber loss but from a channel/port mismatch. Always match the transponder's channel to the labeled mux port.
Add/drop filters
Sometimes you don't want to terminate every wavelength at a site — you want to drop one or two channels locally and let the rest pass through. An add/drop filter (OADM) does exactly that: it peels off specific channels for the local site and adds local channels back onto the fiber, while the remaining "express" channels continue untouched.
Fixed OADM (FOADM)
Passive filters that drop/add a pre-decided set of channels. Cheap and reliable, but changing which channels are dropped means changing hardware. Good where the channel plan is stable.
Reconfigurable OADM (ROADM)
Lets you select add/drop channels in software (via wavelength-selective switches) without a truck roll. Costlier, but essential on meshy/ring networks where the channel plan evolves. Verify specific ROADM capabilities against current Ekinops documentation.
Picture a DWDM line running A → B → C. Some wavelengths run end to end (express) while others are dropped and re-added at the middle site. The same fiber carries all of them; the OADM at B decides which colors get off there.
East/west, express vs add/drop
East / West
At a site on a line or ring, the fiber leaves in two directions — conventionally east and west. Keeping directions straight matters for add/drop and protection: a channel added toward the east is not the same as one toward the west.
Express vs add/drop
Express traffic passes straight through a site optically without being terminated. Add/drop traffic is dropped to (or added from) local equipment. One site can do both on different channels.
Why DWDM needs tighter frequency control
The whole reason DWDM lasers are cooled and locked comes down to one comparison: how far a laser drifts versus how much room a channel has. On a 50 GHz grid (~0.4 nm), the guard space between channels is tiny — an uncontrolled laser that wanders with temperature would cross into its neighbor.
Colored vs grey optics
Grey optics use a generic wavelength for a short client hop. Colored optics are fixed to a specific grid channel — you order the exact channel and it's not interchangeable.
Tunable & alien wavelengths
Tunable optics can be software-set to any C-band channel — one part number covers the whole plan (huge for sparing). An alien wavelength is a third-party channel injected onto someone else's line system; it works only if its frequency, power, and OSNR fit that system's plan.
Treating a DWDM laser like a grey (uncontrolled) optic. Because DWDM channels sit so close together, a laser that drifts with temperature can wander into a neighboring channel and cause interference or fail to pass its mux port. DWDM transponders are temperature-stabilized and precisely tuned to a specific grid channel — you must set the right channel, and you can't freely swap in an arbitrary transceiver.
1. In one sentence, what does WDM do for a fiber pair?
2. You need 60 services over 300 km. CWDM or DWDM?
3. A DWDM channel worked in the lab but fails on a warm rooftop hut. Plausible cause?