If you're planning new cabling this year, you'll face the question every installer asks: multimode or single-mode? For years multimode was the easy answer inside buildings. Higher speeds are changing that.
The short version
Multimode fiber (OM3, OM4, OM5) has a wide core and works with cheaper short-reach optics. Single-mode fiber (OS2) has a very thin core, carries light much further and has become the default for anything leaving the room. The difference that matters is the optics, and how far each speed can go.
The fiber types side by side
- OM3: 50 µm multimode, usually with an aqua jacket. Common in buildings cabled in the 2000s and 2010s.
- OM4: 50 µm multimode with more bandwidth, aqua or violet jacket. The workhorse of many data centers today.
- OM5: wideband multimode with a lime green jacket, built to carry several short-wave wavelengths on one fiber.
- OS2: 9 µm single-mode with a yellow jacket. The default for anything long, and more and more common inside data centers too.
How far multimode really reaches
- 10G (10GBASE-SR): about 300 m over OM3 and 400 m over OM4. Plenty for most buildings.
- 25G (25GBASE-SR): about 70 m over OM3 and 100 m over OM4.
- 100G (100GBASE-SR4): about 70 m over OM3 and 100 m over OM4.
- 400G (400GBASE-SR8): about 100 m over OM4 and 150 m over OM5.
See the pattern? Every step up in speed shortens the reach over multimode. Single-mode optics, by contrast, reach 500 m (DR), 2 km (FR) or 10 km (LR) at 100G and 400G.
Our rule of thumb
Keep what works. Existing OM3 or OM4 runs over short distances at 10G, 25G or 100G are fine. There's no reason to rip them out.
Go single-mode for anything new. If you're pulling new cable, or 400G is anywhere on your roadmap, pull single-mode now. Labour and disruption are the expensive part of cabling, and you don't want to do it twice.
Don't count on OM5. It was designed to stretch multimode further, but it's rarely specified in new builds and the optics that use it are less common.
The cost question
For years the main argument for multimode was price: short-reach optics were much cheaper than single-mode ones. That gap has narrowed a lot. At 100G, single-mode optics such as CWDM4 and DR have come down in price, and the cable itself costs roughly the same. Add the cost of pulling cable a second time in a few years, and single-mode is often the cheaper choice over the life of the building.
Connectors: LC or MPO
Most 10G and 25G optics use a duplex LC connector: two fibers side by side. Many 100G and 400G short-reach optics, such as SR4 and SR8, use MPO connectors with 12 or 16 positions. Parallel single-mode optics like DR4 use MPO too, while FR4 and LR4 stay on duplex LC. If you're installing trunk cabling now, think about which of these you'll need, because it decides how many fibers you pull and how you terminate them.
What it means for your optics
The fiber type decides the optic. SR modules are for multimode; LR, ER, ZR, DR and FR modules are for single-mode. Mixing them doesn't work, even when the connector fits. Check both ends before you order: fiber type, connector (LC or MPO) and distance.
How we can help
Networkser Group supplies SFP, SFP+, SFP28, QSFP28 and 400G QSFP-DD and OSFP optics for both multimode and single-mode. Browse our transceivers and optics, read how to choose the right SFP transceiver, or send us your cable plan and we'll match the optics to it.