Single-mode vs multimode fiber: which do you need?

For 10 Gbps, multimode reaches 300 m on OM3 and 400 m on OM4 with 10GBASE-SR at 850 nm. Older fibers reach much less: 33 m on OM1 and 82 m on OM2 at the same speed. As the data rate rises, reach falls. 40GBASE-SR4 reaches 100 m on OM3 and 150 m on OM4, and 100GBASE-SR4 reaches 70 m on OM3 and 100 m on OM4.

OM5, or wideband multimode, is built to the OM4 bandwidth criteria and adds support for wavelengths up to about 953 nm. It matters mainly for SWDM optics, which are a multi-source agreement and not an IEEE 802.3 standard. With those optics, 100G SWDM4 reaches 100 m on OM4 and 150 m on OM5. With standard 850 nm optics, OM5 gives little advantage over OM4.

Single-mode (OS2) reaches 10 km with 10GBASE-LR at 1310 nm, and 100GBASE-LR4 also reaches 10 km using four wavelengths in the 1310 nm band. Longer-reach optics, such as 10GBASE-ER at 1550 nm, extend to 40 km on engineered links.

The fiber itself is rarely the limit in a building or campus. The limit is the optic you buy, so you can often raise speed later by swapping optics and leaving the cable in place.

Choose by link length first, then optic cost, then future upgrades. If every link is short and many ports will be used, multimode saves on optics. If any link is long, or you want one fiber type everywhere, single-mode avoids mixing.