SM Fiber Distance: How Far Each Ethernet Speed Reaches

A single mode link does not have a fixed maximum distance. It has a loss budget, and the distance is whatever consumes that budget after connectors, splices and margin are subtracted. The same OS2 cable can carry 10G for 10 km with one transceiver and 80 km with another, because the optics changed, not the glass. That is why a datasheet line such as 10 km means a specific transceiver pair at a specific wavelength, and why quoting it without the optics is misleading.

Reach Comes From the Optics, Not the Glass

A single mode fibre is a waveguide with very low loss and almost no modal dispersion. What limits distance is the optical power budget: launch power minus receiver sensitivity, reduced by fibre attenuation, connector loss, splice loss and a design margin. A 10GBASE-LR optic launches at 1310 nm with a budget of about 6.2 dB, which supports 10 km over OS2 with connectors. A 10GBASE-ER optic at 1550 nm has a larger budget and reaches 40 km. The cable is the same; the laser and receiver are different.

OS2 Attenuation Numbers

OS2 cable is specified at 0.35 dB/km or better at 1310 nm and 0.2 dB/km at 1550 nm. Use the specified value plus a margin, not the best measured sample. A 10 km span at 1310 nm therefore consumes about 3.5 dB before any connector is counted.

Connectors and Splices

Each mated pair of good LC or SC connectors costs 0.3 to 0.5 dB, and each fusion splice about 0.1 dB. A link with two patch panels, four connectors and four splices can add 2 dB, which is a third of a 10GBASE-LR budget. Count them before assuming the datasheet distance.

Typical Distances by Ethernet Speed

The common optics form a ladder. 1000BASE-LX reaches 10 km at 1310 nm. 10GBASE-LR reaches 10 km, 10GBASE-ER 40 km and 10GBASE-ZR 80 km. 40GBASE-LR4 and 100GBASE-LR4 reach 10 km over duplex OS2 using four wavelengths. 100GBASE-ER4 reaches 40 km, and coherent optics extend to 80 km and beyond at 100G and 400G. Beyond the optics limit, the answer is not a better cable but an amplifier, a regenerator or a coherent system.

1G and 10G Reach

1G LX and 10G LR both use 1310 nm and both reach about 10 km. 10G ER uses 1550 nm and reaches 40 km, while 10G ZR reaches 80 km with tighter specifications. These are standard reach codes, not guaranteed distances for every installation.

40G, 100G and Coherent

40G and 100G LR4 use four wavelengths over one duplex pair and reach 10 km. ER4 reaches 40 km. For 80 km and beyond, coherent line systems replace simple intensity modulation and are the normal choice for 100G and 400G regional links.

Building the Loss Budget

Write the budget before ordering optics. Start with the transmitter launch power and the receiver sensitivity, subtract the fibre loss, the connector loss, the splice loss and a margin of at least 3 dB for ageing and repairs. If the result is negative, the link will not close. A spreadsheet with one row per span is enough. The exercise also reveals whether a mid-span patch panel is affordable, because each panel adds two connectors and a maintenance point.

A Worked 10 km Example

At 1310 nm, 10 km of OS2 costs 3.5 dB. Four connectors cost 1.6 dB, four splices 0.4 dB, and a 3 dB margin brings the total to 8.5 dB. A 10GBASE-LR budget of about 6.2 dB would not close, so the design needs better connectors, fewer splices or a longer-reach optic.

Margin and Ageing

Keep at least 3 dB of margin. Fibre darkens slightly over decades, connectors get dirty, and a future repair adds two splices. A link designed with zero margin works on acceptance day and fails after the first maintenance window.

Single Mode or Multimode

Multimode is cheaper for short runs inside a building, typically up to 300 m at 10G with OM3 or OM4 and 100 m at 40G and 100G. Single mode wins as soon as the run leaves the building, exceeds a few hundred metres, or needs a speed upgrade path. The optics for single mode cost more at low speeds, but the cable has effectively unlimited bandwidth and the reach is set by the transceiver, so the same fibre can move from 10G to 100G without replacement.

Crossover Distance

The crossover is roughly 300 to 500 m. Below it, multimode optics are cheaper. Above it, single mode becomes cheaper because multimode needs higher-grade fibre or expensive long-reach multimode optics that may not exist for the required speed.

Upgrade Path

Single mode OS2 supports every Ethernet speed from 1G to 400G with the right optics. That makes it the default for campus backbones, inter-building links and any duct that is expensive to reopen. Multimode remains sensible inside a data hall where runs are short.

Single mode reach by optic and speed

Real-World Application Scenarios

A factory connects two buildings 1.8 km apart with 10GBASE-LR over OS2 in a duct, using one patch panel at each end and a 3 dB margin. A campus backbone runs 100GBASE-LR4 between core switches 6 km apart on the same cable type. A utility substation uses 10GBASE-ZR for an 18 km span with no intermediate splice, because the route has no accessible joint and the budget closes with room to spare.

Related Guides on This Site

Start with the single mode fiber overview and the OS2 fiber guide for cable selection. Calculate the span with the fiber loss budget calculator guide, and compare single mode against multimode before choosing the optics.

Frequently Asked Questions (FAQ)