This question almost never arrives in the abstract. It arrives when a switch has empty SFP cages, a fiber run exists between two points, and somebody has to order the modules that light it. Both standards are gigabit Ethernet over fiber, both come in the same SFP form factor, both terminate LC, and both will slot happily into the same cage — which is precisely why they get confused. What separates them is the wavelength they transmit at and the kind of fiber that wavelength is designed to travel down.
1000BASE-SX transmits at 850 nm and is designed for multimode fiber, the larger-core glass used inside buildings and across short campus runs. 1000BASE-LX transmits at 1310 nm and is designed for single-mode fiber, the small-core glass used between buildings and over longer distances. The decision is therefore not really about the module at all: the fiber already installed between your two points decides which standard you are allowed to buy, and no adapter, patch cord or configuration change converts one into the other. Establish what is in the ground first, and the module choice makes itself.
At a glance
Side by side
| Factor | 1000BASE-SX | 1000BASE-LX |
|---|---|---|
| Wavelength | 850 nm | 1310 nm |
| Fiber type | Multimode (larger core) | Single-mode (small core), and LX/LH parts also run multimode with a conditioning cord |
| Typical reach | Tens to a few hundred metres, set by the multimode grade | Commonly rated to around 10 km on single-mode |
| Where it belongs | Inside a building, riser to closet, short campus hop | Building to building, campus backbone, carrier handoff |
| Connector | Duplex LC | Duplex LC |
| Form factor | SFP, fits any gigabit SFP cage | SFP, fits any gigabit SFP cage |
| Relative module cost | Lower | Higher |
| Mixing the two ends | Will not link — different wavelengths | Will not link — different wavelengths |
Choose 1000BASE-SX when
- The fiber between the two points is multimode — orange, aqua or lime-green jacketed plant is the usual visual cue, though the label is authoritative
- Both ends are inside one building, or across a short campus hop within the multimode distance for that grade of fiber
- You are lighting a riser or a floor-to-closet run that was cabled with the building
- Cost per link matters and the distance is comfortably inside multimode reach
Choose 1000BASE-LX when
- The fiber is single-mode — commonly yellow jacketed, again with the label as the authority
- The link crosses between buildings, spans a campus backbone or reaches a carrier demarcation point
- The distance is beyond what multimode carries at gigabit, or the route may be extended later
- The plant was installed for future capacity and you want the module to match the glass rather than the current distance
Bottom line
The fiber decides, not the preference. If the run between your two points is multimode, buy SX at both ends; if it is single-mode, buy LX at both ends. Mixing them produces a link that never comes up, because the two ends are transmitting and expecting different wavelengths, and there is no cable or setting that reconciles that. The one nuance worth knowing is that some LX modules are sold as LX/LH and can run over multimode with a mode-conditioning patch cord — a legitimate transitional answer for an old multimode run that needs a long-reach module, not a general-purpose substitute for buying the right part. If the fiber type is genuinely unknown, that is the thing to establish before ordering anything, because it also determines what the plant can carry when the link moves past gigabit.
FAQ
Common questions
- Can I use an SX module on one end and an LX module on the other?
- No. The two transmit at different wavelengths — 850 nm for SX and 1310 nm for LX — so each end is sending light the other is not built to receive, and the link simply never comes up. There is no patch cord, media converter setting or configuration change that reconciles the two. Both ends of a fiber link have to run the same standard on the fiber type that standard is designed for.
- How do I tell whether my fiber is multimode or single-mode?
- The jacket colour is the usual first clue — multimode plant is commonly orange, aqua or lime green and single-mode is commonly yellow — but colour conventions vary and the printed legend on the cable and the label on the patch panel are the authoritative sources. If the plant is undocumented, an optical loss test or a fiber inspection will identify it, and that is worth doing anyway before lighting a link you did not install, because it also tells you the grade and therefore the distance you can actually run.
- How far will each one actually reach?
- SX reach depends on the grade of multimode glass, which is why modules in this family quote different distances for the same standard — older plant carries a gigabit link a fraction of the distance the newer grades do. LX is commonly rated to around 10 kilometres on single-mode. Read the rated distance on the specific part number rather than on the standard, and where the plant grade is unknown, treat establishing it as part of the specification rather than as a detail.
- What is an LX/LH module, and can it run on multimode?
- LX/LH is a long-reach part that some manufacturers specify as also usable over multimode fiber when paired with a mode-conditioning patch cord, which corrects the launch into the larger core. It is a legitimate answer for an existing multimode run that needs a long-reach module, and it is not a general substitute for buying the standard the plant was designed for. Check the specific part number's documentation, because support for it and the conditioning cord requirement are stated per model.