The comparison is not simply monthly wavelength price versus monthly dark-fiber rent. The dark-fiber buyer becomes responsible for the optical layer, while the wavelength buyer pays the carrier to own that complexity.
The carrier delivers a dedicated 400G optical service between endpoints. The carrier manages the optical transport layer, including the network engineering required between sites.
The enterprise leases or acquires rights to the glass and supplies the electronics that turn it into a 400G or multi-wavelength transport system.
| Five-Year Requirement | Likely Economic Lean | Reason |
|---|---|---|
| One 400G link | Wavelength can be very competitive | No optical platform ownership or operational burden |
| 400G growing to 800G | Dark fiber often becomes interesting | Second wavelength increases managed-service MRC |
| 1.2 to 1.6 Tbps | Dark fiber increasingly strong | Fiber rent stays relatively flat while wavelengths multiply |
| Long-haul single 400G | Wavelength deserves serious consideration | Dark fiber may need amplifiers, line systems and more operations |
A 400G wavelength is generally priced per managed circuit. Dark fiber is commonly priced by route and fiber pair rather than by the bandwidth running across it.
That is the fundamental economic difference. Adding another 400G wavelength generally adds another service charge. Lighting another wavelength on your own fiber does not require leasing another pair.
For shorter metro links, 400ZR or ZR+ coherent optics can sometimes plug directly into compatible router or switch ports, reducing the need for separate transport shelves.
That architecture is important because modern 400G coherent pluggables can make dark fiber much less hardware-intensive than older DWDM designs.
A simple metro pair may need little beyond coherent optics. Longer routes can require amplification, ROADMs, optical line systems or dedicated transponder and muxponder hardware.
That is one reason dark fiber’s advantage usually weakens as distance increases.
Both designs can require cross-connects at carrier hotels or data centers, but dark fiber may also require rack or cabinet space for optical equipment.
Cross-connects commonly carry both installation charges and monthly recurring charges, so a five-year model should include them rather than burying them in “colocation.”
Current 400G coherent pluggables commonly operate in roughly the high-teens to low-20-watt range per optic. Two ends therefore add only a modest direct power load.
Separate muxponder, amplification and line-system hardware can make the power and cooling line item much more meaningful.
Enterprises operating dark fiber should decide whether to hold spare coherent optics, power supplies, fan modules or complete transport cards.
A managed wavelength shifts most optical sparing responsibility to the carrier. That operational transfer has real economic value.
Somebody must monitor optical power, OSNR, alarms, fiber failures and transport hardware. That can be internal engineering staff, a maintenance contract or a managed optical provider.
Leaving support out of the dark-fiber model is one of the easiest ways to make it look artificially cheap.
| Cost | 400G Wavelength | Dark Fiber Pair |
|---|---|---|
| Transport MRC | Per 400G circuit | Per fiber pair / route |
| Coherent optics | Usually carrier optical layer | Customer expense |
| Muxponder / transponder | Carrier responsibility | May be required |
| Cross-connect MRC | Possible at both ends | Possible at both ends |
| Rack / colo space | Usually limited | Potential additional requirement |
| Power | Mostly embedded in service | Customer optical hardware |
| Spares | Mostly carrier-owned | Customer planning requirement |
| Optical support | Included in carrier service | Internal or contracted |
| Adding second 400G | Another wavelength charge | Potentially another wavelength on same pair |
Older dark-fiber designs commonly required standalone transponders between the router and DWDM system. Modern coherent pluggables can put the DWDM interface directly into compatible router or switch ports.
On the right route, that can remove a transport shelf, gray optics, cabling, rack space, power and an entire management layer. For metro dark fiber, this is one of the biggest reasons the economics have improved.
A dark-fiber pair across town and a dark-fiber pair across several states are financially different products.
| Route | Dark Fiber Position |
|---|---|
| Metro, short and clean | Potentially excellent with coherent pluggables |
| Regional | Optical engineering becomes more important |
| Long-haul | Amplification and line-system costs can favor managed waves at lower bandwidth |
Once the enterprise controls the pair, the next 400G does not require another fiber lease. It requires enough optical spectrum, compatible electronics and capacity in the endpoints.
One wavelength
Two 400G wavelengths on the pair
Four 400G wavelengths before considering newer higher-rate optical platforms
Compare incremental transport costs over five years. Router and switch chassis costs are excluded because they may be common to both options.
