Almost every commercial fibre install we’re brought into has a version of the same question: should we use single-mode (OS2) or multimode (OM4)? The honest answer is “it depends on three things” — but the way those three things weigh against each other isn’t always intuitive.
Here’s how we think about it.
The basic difference
Two ways to send light down glass:
- Multimode fibre (OM3, OM4, OM5) uses a wider core — 50µm — and lets light travel along multiple paths simultaneously. Cheaper optics, shorter distances.
- Single-mode fibre (OS1, OS2) uses a narrow 9µm core. Light travels in essentially a straight line. More expensive optics, much longer distances.
OM4 vs OS2 is the most common choice for new commercial cabling. OM3 is fading, OM5 isn’t worth the premium for most sites yet.
Distance: where each one wins
Practical reach for 10 Gb Ethernet:
| Fibre | Reach at 10G | Reach at 25G | Reach at 100G |
|---|---|---|---|
| OM3 | 300m | 70m | 70m |
| OM4 | 400m | 100m | 100m |
| OS2 | 10km+ | 10km+ | 10km+ |
For most single-building deployments OM4’s 400m is comfortable. For multi-building campuses, OS2 is the only fibre that doesn’t constrain you.
Cost: the trick most people miss
The fibre itself is roughly similar to install — OS2 cable is actually slightly cheaper per metre than OM4 these days. The cost difference is in the optics at each end:
- A 10G SR transceiver (multimode, short reach): around £40–60.
- A 10G LR transceiver (single-mode, long reach): around £100–150.
- A 100G transceiver: the spread is wider — multimode SR4 around £250, single-mode LR4 around £600–800.
Multiply that by every port. On a switch stack with 24 fibre uplinks, the optics decision dwarfs the cable decision.
So for short, dense runs — say, between switches in adjacent racks — OM4 is materially cheaper to light up. For long runs that’ll see one or two optics over a decade, OS2’s per-cable cost wins out comfortably.
25G, 40G, 100G readiness
This is where the decision usually tips. The kit being shipped now wants more than 10G uplinks:
- Modern aggregation switches are 25G or 100G capable.
- AI workloads and high-density wireless backhaul are pushing the same number of ports to higher speeds.
- WiFi 7 access points can saturate a 5G or 10G link with a single client.
OS2 runs all of these on the same fibre. OM4 hits a ceiling — at 100G the reach drops to 100m, and beyond that you need to swap the cable, not just the optics. That’s a forklift upgrade.
When OM4 still wins
OM4 isn’t dead. It’s the right call when:
- All runs are under 100m and likely to stay that way.
- You’re using a lot of ports — the optics savings compound.
- You want fewer SKUs in the bill of materials and the whole network is short-reach.
- The site has no realistic prospect of needing more than 100G in its expected lifetime.
Small offices and single-building installs are often clean OM4 jobs.
When OS2 is the only sensible choice
- Any run that crosses a building boundary. The duct will outlive three generations of optics; only one fibre type stays usable.
- Anywhere there’s even a possibility of distance growth — campuses, multi-tenant offices, anywhere “we might extend to the new wing later”.
- Sites where 25G or 100G is on the roadmap.
- Where you want the cable to last 20 years of standards upgrades without disturbance.
For most multi-building and future-proofed jobs, that’s most of the time.
Our default recommendation
Honest answer:
- Single building, dense runs under 100m → OM4
- Anything between buildings → OS2
- Single building but you’re aggregating to a backbone → OS2 for the backbone, OM4 for short patch runs if budget matters
- In doubt → OS2. The premium is in optics, and you only buy those when you need to.
Every fibre run we install gets Fluke DSX certified before it carries traffic, regardless of which type it is. The grade only matters once it’s installed cleanly.
Want a fibre design that lasts
We design fibre as if it’ll be in service for 20 years — because in practice that’s how long cable stays in walls. If you’re scoping a new install or a backbone refresh, book a free survey and we’ll walk the site, measure the runs, and tell you honestly which grade serves you best.
