Quick Answer: A single mode link ends in either a media converter or an SFP port, and both ends must match in wavelength and rate. A 1000BASE-LX pair at 1310 nm over 9 micron core reaches 10 km on an 8 dB budget; 1000BASE-ZX at 1550 nm reaches 70 km. Count 0.35 dB per km at 1310 nm, 0.3 dB per mating pair and 0.1 dB per splice, then keep 3 dB in hand. 1310 nm will not talk to 1550 nm, so order transceivers in pairs.
Single mode fibre is not a cable that a media converter drives by default. The converter has a fixed wavelength, a fixed rate and a transmit power range, and the fibre has a core diameter of 9 micron. Mismatch any of the three and the link comes up dark with no error message beyond a dead port light.
Three ways to terminate the link
A media converter is a standalone box with a fibre port and an RJ45 port, useful where the copper switch has no SFP cage and the run is a single link. An SFP module in a switch port does the same job inside the switch, with management and link state visible. A router with a fibre WAN port suits a building entry point. Choose the converter pair when the copper side needs a specific rate or PoE, and choose SFPs when both ends are switches that can report optics.
Wavelength, core size and reach
1310 nm and 1550 nm
1310 nm gives lower dispersion and is the standard for links to 10 km. 1550 nm gives lower attenuation, about 0.2 dB per km against 0.35 dB per km, and is used from 40 km upward. A 1310 nm transmitter will not be seen by a 1550 nm receiver, so a link with one of each stays down.
BiDi pairs use one strand
Bidirectional modules carry transmit and receive on one fibre at two wavelengths, typically 1310 nm and 1550 nm. They are sold as an A unit and a B unit, and two A units will not form a link. The trade is half the fibre count for a strict pairing rule.
Link budget arithmetic
Add the losses and subtract from the transmit budget. A 10 km span of 9 micron fibre costs 3.5 dB at 1310 nm, two connector pairs cost 0.6 dB and four splices cost 0.4 dB, so 4.5 dB is used before any margin. With a typical 8 dB budget that leaves 3.5 dB, which is enough for ageing and for one repair splice. Use a factory-terminated single mode fiber patch cable for the indoor tails, because a field-terminated connector is usually the worst loss in the chain.
| Standard | Wavelength | Fibre | Reach | Budget |
|---|
| 1000BASE-SX | 850 nm | Multimode 50 micron | 550 m | 7.5 dB |
| 1000BASE-LX | 1310 nm | Single mode 9 micron | 10 km | 8 dB |
| 1000BASE-EX | 1310 nm | Single mode 9 micron | 40 km | 15 dB |
| 1000BASE-ZX | 1550 nm | Single mode 9 micron | 70 km | 21 dB |
| 10GBASE-LR | 1310 nm | Single mode 9 micron | 10 km | 6.2 dB |
What the copper side can and cannot do
The RJ45 port is a normal 1000BASE-T interface with a 100 m (328 ft) limit, so the converter extends the reach, not the copper segment. A converter does not pass Power over Ethernet from the fibre side unless it is a powered device model with its own injector, which is why remote cameras often need a powered fiber cable or a local injector instead of a plain converter. Media converters are unmanaged by default, so a link that drops stays down until the copper side renegotiates.
Practical scenarios
- Situation: An outbuilding sits 600 m from the main switch. Use a 1000BASE-LX pair at 1310 nm, not 1000BASE-SX, because the multimode option dies at 550 m.
- Situation: A legacy run is 62.5 micron om1 fiber. A 1310 nm single mode module will not couple into it usefully; fit 1000BASE-SX optics and accept a 220 m limit at that core size.
- Situation: A converter pair links two switches 3 km apart and the link flaps at dusk. Check the receive power; 1550 nm optics with 21 dB of budget into a 3 km span usually need a 10 dB attenuator to stay inside the receiver window.
- Situation: A pole camera needs 12 W at the far end. Use a converter with a PoE injector and a local 48 V supply, because the fibre carries no power at all.
FAQ
Can one media converter work on its own?
No. A fibre link needs a transmitter at each end, so a converter pair is required unless the far end is a switch with a matching SFP. Rate and wavelength have to agree at both ends.
Does a media converter add latency?
Under 5 microseconds, which is negligible for Ethernet. Jumbo frames above 9 kB may be dropped by cheaper converters, so check the frame size if the link carries storage traffic.
Why is the link up but no traffic passing?
Usually a duplex or rate mismatch on the copper side, or one end set to a fixed speed while the other auto-negotiates. Set both copper ports to auto, or both to the same fixed values, and confirm the fibre receive level with a power meter.
