My FTTH connection is using an old CareLink CL-MC-W2B-20-100 optical converter. It is labelled:

10/100 Base-T to 100Base-Fx bi-direction/single fiber media converter

10/100M Single mode WDM 20Km Tx1550nm/Rx1310nm

Based on wavelengths I assume, that correct label should be 100BASE-LX WDM

Since Keenetic Giga 1010 does have an SFP port, I want to replace the converter with an SFP module. The question is how to choose a correct module.

Datasheets for SFP modules show that data-wise laser diode and photodiode are directly connected to the device, therefore module should not care about link speeds and protocols at all, as long as they can pass frequencies high enough.

Do I need to hunt for a 100M module with exact specs, or can get any decent gigabit/1.25Gb/over one with WDM, correct wavelength, distance rating and SC/UPC connector?

  • No, you will not find an optical gigabit transceiver that works with 100Base-FX. You need a 100Base-FX transceiver to communicate with a 100Base-FX transceiver.
    – Ron Maupin
    Mar 26, 2020 at 19:04
  • @RonMaupin thank you. I do not fully believe label 100Base-Fx, because Base-Fx should have two separate fibers (both with 1310nm), but the link is single-fiber, that may come from generic case. Or it actually has different modulation/signalling?
    – Vasfed
    Mar 26, 2020 at 19:33

2 Answers 2


0/100M Single mode WDM 20Km Tx1550nm/Rx1310nm

That's actually for the 100BASE-BX10 standard (-Downlink variant), boosted to 20 km. You can buy those SFP modules from some vendors. 100BASE-LX10 uses a pair of fibers, without WDM (or rather WDD). 100BASE-FX also uses a pair, but multi-mode fiber.

A more common 1000BASE-BX10-D module likely won't work because it uses a different center wavelength (1490/1310 nm instead or the 1550/1310 nm for 100BASE-BX10). Also, most switches/routers won't link to 100M with a 1G SFP - you're correct in that the SFP doesn't care about the link speed but the device will. Additionally, check module compatibility with your device before buying.


Took a chance and ordered both sfp types to experiment with:

  • 100M Gigalink GL-OT-SF14SC1-1550-1310
  • 1Gbit Gigalink GL-OT-SG14SC1-1550-1310-D

Both are with matching wavelenghts and distance rating for the link.

Result: both work fine, left gigabit one installed in case of possible link upgrade.

Unexpected side result - also got ~1ms less RTT on the link (probably due to old converter internal switch)

  • Likely, both modules are completely identical apart from the type identification.
    – Zac67
    Apr 3, 2020 at 13:43

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