In short: multimode fiber is made for short distances – inside a building or a data center – and works with SX and SR modules. Single-mode fiber carries light over kilometers and works with LX, LR, ER and ZR modules. The module has to match the fiber that is already in the wall: you cannot mix the two.
Choosing the right transceiver usually starts with one question: which fiber do I have? Here is how to tell, and what it means for the module.
The difference in one table
| Multimode | Single-mode | |
|---|---|---|
| Core of the fiber | thicker (50 or 62.5 µm) | very thin (9 µm) |
| Typical cable color | aqua (OM3, OM4), orange (older OM1, OM2), lime (OM5) | yellow |
| Wavelength of the modules | 850 nm | 1310 nm or 1550 nm |
| Typical reach | from about 100 m up to 550 m, depending on speed | 10 km, 40 km, 80 km |
| Module types | SX, SR, SR4 | LX, LR, EX, ER, ZX, ZR, LR4 |
| Typical use | inside a building or a data center | between buildings, sites and cities |
Multimode: short and economical
Multimode fiber has a wider core, so the light travels along many paths ("modes") at once. Over longer distances these paths arrive at slightly different times and the signal blurs, which limits the reach. Inside a building that is not a problem, and multimode modules are usually the most economical optical choice.
The reach depends on the speed and on the quality of the fiber: a 1G SX module reaches up to 550 m, a 10G SR module up to about 300 m, a 25G or 100G SR module about 100 m. Newer OM3 and OM4 fiber reaches the full distances; older OM1 and OM2 fiber reaches less.
Single-mode: built for distance
Single-mode fiber has a very thin core that carries the light along one path. The signal stays clean over long distances, so single-mode links reach 10 km with LR or LX modules, 40 km with ER or EX and 80 km with ZR or ZX. Service providers use it almost exclusively, and many companies install it even inside their sites so that every link can be upgraded later without new cabling.
How to tell which fiber you have
- Look at the color of the patch cable: yellow is almost always single-mode, aqua or orange is multimode. Colors are a convention, not a guarantee, so confirm it where you can.
- Read the print on the cable: it usually says OS1/OS2 (single-mode) or OM1–OM5 (multimode), or gives the core size (9/125 for single-mode, 50/125 or 62.5/125 for multimode).
- Check the existing modules: if an SR or SX module is working on the link today, the fiber is multimode; if it is an LR or LX, it is single-mode.
Can I mix them?
No. A multimode module will not work properly over single-mode fiber, and a single-mode module is not meant for multimode fiber. Both modules of a link must be of the same type and match the fiber between them. If you need to change the fiber type of a link, replace the cabling and the modules together.
Not sure what you need? Our SFP compatibility guide lists everything that has to match on a link, and the guides to 1G and 10G module types show the options for each fiber.
Frequently asked questions
Which is cheaper, single-mode or multimode?
For short links inside a building, multimode modules are usually the more economical choice. For anything longer than a few hundred meters, single-mode is the only option.
What does OM3 or OM4 mean?
They are quality classes of multimode fiber. The higher the number, the further a fast signal can go: OM4 reaches longer distances than OM3, and both outperform the older OM1 and OM2.
Is single-mode fiber always yellow?
Usually, but not always. The print on the cable (OS1, OS2 or 9/125) is the reliable way to tell.
