Two windows, two architectures
Almost every short reach link uses one of two wavelength windows. The O band sits near 1310 nm, where standard single mode fibre has almost no chromatic dispersion and low cost fixed wavelength lasers work well. The C band sits between roughly 1530 and 1565 nm, where erbium doped fibre amplifiers work and dozens of channels can be packed on one strand at a narrow grid.
The choice is not a preference. It decides whether the link is a grey point to point pair of modules or a wavelength on a line system with amplification and muxing, and it sets the cost of every port along the route. The column behind this page is WDM transceivers, the band detail is on coherent optics, and the rate families are on optics selection guides.
Why each band is used where it is
The O band is the direct detect band. A fixed wavelength laser near 1310 nm is inexpensive, the fibre carries it with low dispersion, and no amplifier is needed because the reaches are short: a few hundred metres for parallel links, two kilometres for many CWDM types, ten kilometres for the longer grey reaches. Most 100G, 400G and 800G links inside a building run here.
The C band is the amplified band. An erbium doped amplifier lifts a whole window at once, so a chain of spans can be built without regenerating each channel, and tunable lasers can be set to any channel in the band. That is what lets one strand carry many channels, and it is why metro, regional and long haul routes are planned in the C band.
Recommended optics by band
| Module | Rate | Media / wavelength | Reach class | Where it usually lands |
|---|
| 100G QSFP28 CWDM4 | 100G | SMF, CWDM channels near the O band | 2 km class | Grey links between halls on one fibre pair |
| 100G QSFP28 PSM4 and LR4 | 100G | SMF, 1310 nm | 500 m to 10 km class | Row, building and campus links |
| 400G QSFP-DD DR4 and FR4 | 400G | SMF, 1310 nm parallel or CWDM | 500 m to 2 km class | Leaf and spine ports |
| 800G OSFP and QSFP-DD DR8, 2xFR4 | 800G | SMF, O band parallel or CWDM | 500 m to 2 km class | AI fabric and DCI grey links |
| CWDM transceivers | 10G to 100G | SMF, twenty nanometre grid | Per module class | Adding channels without amplification |
| DWDM transceivers | 10G to 100G | SMF, C band, fixed or tunable | Per module class | Amplified metro and regional spans |
| 400G ZR and ZR+ pluggables | 400G | SMF, C band tunable | Metro to regional class | Router ports carrying wavelengths directly |
| 800G coherent pluggables and cards | 800G | SMF, C band tunable | Metro to regional class | New capacity onto an existing line system |
| CWDM and DWDM mux, demux and EDFA | Per channel | SMF, passive or amplified | Per span class | Combining channels and lifting a span chain |
Reach depends on the module and the line system rather than on the band alone. The grey side is listed on the 100G and 400G pages, the passive side on DWDM and CWDM passive modules, and the coherent side on coherent pluggable optics.
Where the two bands meet on one route
Most modern networks use both. Grey client optics stay in the router or switch, and a coherent line module carries the aggregate on a C band wavelength across the transport layer. The client side is short and cheap, the line side is amplified and shared, and the two are joined inside a transponder, a muxponder or a pluggable sitting directly in the client port.
Fibre, loss and the dispersion argument
Standard single mode fibre carries both bands, so the fibre itself is rarely the constraint. Attenuation is lower in the C band, which is one reason amplified long haul systems live there, while dispersion is far lower in the O band, which is why direct detect links at 1310 nm tolerate simpler transmitters. In the C band, dispersion and polarisation effects accumulate with distance, so a link either stays short or uses coherent detection that compensates for both. In the O band the same effects are small but the link cannot be amplified, so its reach is set by the loss budget alone.
Grid, tunability and the channel plan
A C band system has a fixed grid and channel plan, and a new wavelength has to fit inside it. Tunable optics can be set to a free channel in software, which is why they are chosen on networks that have already grown, and why one tunable spare can cover every channel on a link family.
Coding, third party platforms and spares
A coded part is programmed for the platform named on the request, tested on that model, and it is not manufactured by that brand. Coded parts for the common platforms are collected on the Cisco, Juniper and Nokia pages. The spare rule follows the band: grey O band links need one spare per part group, while a C band link is best covered by one tunable spare per link family rather than a spare per channel.
How to choose, in order
- Fix the distance and the loss budget, and check whether amplification is available on the route.
- Where the link closes inside the reach of a grey module and needs no amplifier, plan the O band.
- Where channels must be stacked on one strand or a span needs lifting, plan the C band.
- Set the grid, the occupied channels and the target channel from the line system.
- Keep the client side grey and let the line side carry the wavelength.
- Set the coding per platform and the spare rule from the band and the channel count.
Frequently asked questions
Can a 1310 nm module be plugged into a DWDM channel?
No. The line system expects a channel on its own grid, and a fixed 1310 nm module will not land on it. Use a fixed or tunable C band part.
Is CWDM an O band or a C band technology?
Neither. Its grid spans several windows and its lower channels sit near the O band, which is why it is treated as a low cost way to add channels.
Which band reaches further?
Reach is set by the module and the line system. In direct detect grey links the O band covers standard reaches up to ten kilometres, and beyond that an amplified C band channel with coherent detection is the usual route.
Do I need special fibre for the C band?
No. Standard single mode carries both bands. What changes is the loss budget, the connector polish and the routing discipline in a dense amplified system.
What do you need to quote a band plan?
The route with measured losses, the available amplifier and mux positions, the current channel plan and grid, the target rate per link, the platform at each end and the spare rule.
Send the route with measured losses, the channel plan and the target rate per link, and we come back with the band, the part per position and the spare lines set out separately. Related reading: DWDM metro transport, BiDi transceivers and all optics selection guides.
