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Optical layer

DWDM Spacing Calculator

Wavelength count and grid spacing required for a target aggregate bandwidth over fiber.

The engineer question
How many DWDM channels does my long-haul AI traffic need?

Inputs

Total inter-DC AI traffic to carry on this fibre pair (one direction).

End-to-end fibre distance between the two AI sites.

Result

DWDM channels requiredceil(12800 / 400)
32 λ
Recommended ITU grid~48 ch / C-band
100 GHz fixed grid
Per-λ line rate
400 Gbps
Spectrum fit
Fits in one C-band
Typical amplified spans600 km / 80 km
8 × ~80 km
Inline EDFA line ampsone EDFA per mid-span hut
~7
Recommended optics
400ZR (≤120 km amp-less) or 400ZR+/OpenZR+ for amplified long-haul

Recommendation

32 λ on a 100 GHz fixed grid fits one C-band. At 600 km you cross the pluggable amp-less limit (~120 km) — budget ~7 inline EDFA line amps on ~80 km spans and use 400ZR+ / OpenZR+ amplified optics. Verify the OSNR / dispersion budget against the actual fibre type.

Assumptions

  • · Channel count is exact arithmetic: ceil(aggregate ÷ per-λ rate). Per-λ = 400 Gbps (nominal line rate, OIF/MSA class — actual usable payload is ~5–10% lower after FEC overhead).
  • · Grid spacing per ITU-T G.694.1 fixed grids (100 / 75 / 50 GHz). Channel-per-C-band figures (≈48 / 64 / 96) are typical vendor planning numbers for a single ~4.8 THz C-band; ±a few channels by platform. Flexgrid (G.694.1 flexible grid) can pack 400G/800G carriers tighter.
  • · Span / EDFA estimate uses a typical ~80 km amplified span; real spans run 60–100 km depending on fibre loss (~0.2 dB/km G.652 + splice/connector loss) and amp gain. Inline-amp count ≈ spans − 1; first span is amp-less.
  • · Amp-less reach of ~120 km is the typical 400ZR/coherent-pluggable figure; 400ZR+ / OpenZR+ extend this with EDFAs to 1000+ km. These are approximate datasheet maxima, not guarantees for your fibre.
  • · Excluded: OSNR / chromatic-dispersion / PMD budget, ROADM degree & filtering loss, Raman amplification, C+L band economics, fibre-pair count, protection/restoration paths, and all optics/transponder pricing.
  • · This sizes capacity, not a validated optical line-system design — run a real OSNR/dispersion budget (e.g. GNPy or a vendor planner) before committing hardware.

Worked example (default inputs)

Result

DWDM channels requiredceil(12800 / 400)
32 λ
Recommended ITU grid~48 ch / C-band
100 GHz fixed grid
Per-λ line rate
400 Gbps
Spectrum fit
Fits in one C-band
Typical amplified spans600 km / 80 km
8 × ~80 km
Inline EDFA line ampsone EDFA per mid-span hut
~7
Recommended optics
400ZR (≤120 km amp-less) or 400ZR+/OpenZR+ for amplified long-haul

Recommendation

32 λ on a 100 GHz fixed grid fits one C-band. At 600 km you cross the pluggable amp-less limit (~120 km) — budget ~7 inline EDFA line amps on ~80 km spans and use 400ZR+ / OpenZR+ amplified optics. Verify the OSNR / dispersion budget against the actual fibre type.

Assumptions

  • · Channel count is exact arithmetic: ceil(aggregate ÷ per-λ rate). Per-λ = 400 Gbps (nominal line rate, OIF/MSA class — actual usable payload is ~5–10% lower after FEC overhead).
  • · Grid spacing per ITU-T G.694.1 fixed grids (100 / 75 / 50 GHz). Channel-per-C-band figures (≈48 / 64 / 96) are typical vendor planning numbers for a single ~4.8 THz C-band; ±a few channels by platform. Flexgrid (G.694.1 flexible grid) can pack 400G/800G carriers tighter.
  • · Span / EDFA estimate uses a typical ~80 km amplified span; real spans run 60–100 km depending on fibre loss (~0.2 dB/km G.652 + splice/connector loss) and amp gain. Inline-amp count ≈ spans − 1; first span is amp-less.
  • · Amp-less reach of ~120 km is the typical 400ZR/coherent-pluggable figure; 400ZR+ / OpenZR+ extend this with EDFAs to 1000+ km. These are approximate datasheet maxima, not guarantees for your fibre.
  • · Excluded: OSNR / chromatic-dispersion / PMD budget, ROADM degree & filtering loss, Raman amplification, C+L band economics, fibre-pair count, protection/restoration paths, and all optics/transponder pricing.
  • · This sizes capacity, not a validated optical line-system design — run a real OSNR/dispersion budget (e.g. GNPy or a vendor planner) before committing hardware.

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