Quick Answer: A fibre to Ethernet converter is one half of a pair. Both ends must match on speed, duplex, fibre count and wavelength, and the pair must be ordered together. Use 1000BASE-SX over multimode for 550 m, 1000BASE-LX over single mode for 10 km, and a BiDi pair with matched 1310 nm and 1550 nm units for a single strand. A link LED with no traffic almost always means a duplex or wavelength mismatch, not a broken fibre.
These devices are transparent: they do not learn MAC addresses, do not filter, and do not care about IP. That makes them cheap and reliable, and it also means every setting that matters sits at the two ports facing the copper, where a mismatch is invisible until traffic fails.
A converter is half of a pair
Buying one unit and assuming it will talk to whatever is at the far end is the commonest mistake. A pair bought as a set is tested against itself; two units bought months apart can differ in firmware behaviour, in whether the fibre port is fixed or an SFP slot, and in how they handle a 9 kB frame.
What both ends must agree on
Speed, duplex, fibre core size, connector type, wavelength and frame size. Six items, and a failure in any one produces the same symptom: a link light with no usable throughput, or no link light at all. Write them on the purchase order.
Standards and reach
Reach figures assume clean connectors and a loss budget that includes every mated pair at 0.3 dB. Halve the quoted distance if the run passes through more than four patch panels.
| Standard | Fibre | Wavelength | Reach | Copper port |
|---|
| 100BASE-FX | Multimode 62.5/125 | 1310 nm | 2 km | 100 Mbit/s |
| 1000BASE-SX | Multimode 50/125 | 850 nm | 550 m | 1 Gbit/s |
| 1000BASE-LX | Single mode 9/125 | 1310 nm | 10 km | 1 Gbit/s |
| 1000BASE-BX | Single mode, one strand | 1310 and 1550 nm | 10 km | 1 Gbit/s |
| 10GBASE-LR | Single mode 9/125 | 1310 nm | 10 km | 10 Gbit/s |
Duplex, BiDi and wavelength pairing
A duplex pair uses two strands, one per direction, and the two ends are electrically identical. BiDi uses one strand and splits the directions by wavelength, so one end transmits at 1310 nm and receives at 1550 nm, and the other does the reverse. Two A units will not link with each other, no matter how clean the ends are.
Reading the failure lights
Fibre link up and copper link down suggests a speed mismatch with the switch. Both links up but no ping suggests a duplex mismatch, which shows as late collisions climbing on the switch port. No fibre light at all means a crossed pair on duplex, a missing B unit on BiDi, or a broken strand.
Copper side: negotiation, PoE and frame size
Most units auto-negotiate and auto-sense the cable, so a straight patch lead works in either port. Fix the speed on both sides if the link flaps, because a converter that re-negotiates every few minutes will drop a VoIP call each time. For a remote camera or access point, use a converter with a PoE injector or a PoE passthrough port rather than adding a second mains socket in a weatherproof box. Check jumbo frame support before deploying on a storage network: a 1 Gbit/s converter that fragments 9 kB frames will halve throughput. A digital coaxial to optical converter is a different class of product entirely, carrying S/PDIF rather than Ethernet frames, and the two are often confused in search results.
Fibre is not only a network medium
Audio products also convert between optical and copper, using the same connector shells. An optical to rca jack converter turns a TOSLINK stream into two analogue channels at line level, and a toslink audio cable is 1 mm plastic fibre with no cladding tolerance for data use. Neither will pass Ethernet, because the transmitter is a red LED modulated at a few megahertz, not a laser at 1310 nm.
Practical scenarios
- Situation: A 300 m link between two buildings on existing 50/125 multimode. Use 1000BASE-SX at 850 nm, which covers 550 m and costs about a third of single mode optics.
- Situation: A gate camera 1.2 km from the switch with only one spare strand. Use a 1000BASE-BX pair, A end at 1310 nm transmit, B end at 1550 nm, and power the camera from a PoE injector at the far converter.
- Situation: A workshop link that drops every afternoon. Check temperature at the converter; a fanless unit in a sealed box above 45 C will reset, and moving it 300 mm lower often fixes the fault.
- Situation: An office link at 1 Gbit/s that will not pass large file copies. Enable jumbo frames on both converters and both switch ports, or set the MTU back to 1500 everywhere.
FAQ
Can one converter work on its own?
It needs a partner at the far end, either another converter or a switch with a matching fibre port. A single unit with nothing opposite will show no fibre link and cannot be tested alone.
Do converters add latency?
Store-and-forward units add one frame time, roughly 12 microseconds at 1 Gbit/s for a 1500 byte frame. Cut-through models add under 2 microseconds. Neither matters for office traffic and both matter for tight industrial control loops.
Can I mix an SFP module with a fixed-port converter?
Only if the SFP matches the fixed port on speed, wavelength and fibre type. A 1000BASE-LX SFP will not link to a 1000BASE-SX fixed port, even though both are gigabit and both use LC connectors.
