Three fibres, three jobs
OS2 is single-mode fibre with a 9 µm core, run at 1310 nm and 1550 nm. Its reach is measured in kilometres, so in a data centre the limit is never the fibre: it is the optic and the loss budget.
OM4 is laser-optimised 50 µm multimode, run at 850 nm with low-cost optics over short distances. OM5 is wideband multimode: OM4 performance at 850 nm, plus a specified bandwidth up to around 950 nm for optics that put several wavelengths on one fibre.
| Fibre | Core | Typical use in the hall |
|---|---|---|
| OS2 | 9 µm single-mode | Backbones, meet-me rooms, 400G/800G DR and FR optics, anything beyond the row |
| OM4 | 50 µm multimode | Short links within a row or a pod: SR optics, top-of-rack to end-of-row |
| OM5 | 50 µm wideband | Where SWDM or BiDi multimode optics are specified |
Reach shrinks as speed rises
Each step up in lane speed cuts multimode reach. At 100G, OM4 comfortably covers a row with SR4 optics; at the newest 400G and 800G multimode lane rates the reach falls to tens of metres, which is less than many GPU clusters need between racks.
Single-mode optics (DR and FR families) carry the same speeds over hundreds of metres to kilometres. The exact figures depend on the transceiver: read the optic's data sheet and design to its channel loss, not to the fibre's headline bandwidth.
Why AI halls are going single-mode
- Distance. Leaf and spine switches for a GPU fabric often sit further from the servers than a multimode link allows at 800G.
- Lifetime. Cabling outlives two or three generations of optics. OS2 does not need to be re-pulled when the next speed arrives.
- Density. Parallel single-mode over MPO-8, MPO-16 or MPO-24, and ribbon cable in the backbone, carry high fibre counts in less pathway.
What we check before the first pull
- The optics the client will run, and the channel loss each one allows.
- Connector type and count per channel: every mated pair costs loss.
- The polarity method for every MPO run, agreed before trunks are ordered.
- The test limits in the specification, so results are judged against the right budget.
Get those four right and the fibre type follows from them. Get them wrong and the best fibre in the world will still fail at handover.
Not sure what is on the drum? Photograph or type the jacket print into the free Cable Lens to get the fibre type, colour code and test regime.
Questions
Is OM5 better than OM4?
Why are AI halls moving to single-mode?
What colour is each fibre jacket?
GO2DataWorks technical note, updated 18 October 2026. General guidance: your project specification and the standards it names always take precedence.