First my assumptions (possibly wrong):

  • 100BASE-FX -> Multi-mode fiber
  • 100BASE-LX -> Single-mode fiber

In fact, if I check some online information from CISCO regarding SFPs, it confirms my assumptions: https://www.cisco.com/c/en/us/products/collateral/interfaces-modules/fast-ethernet-sfp-modules/product_data_sheet0900aecd801f931c.html

But the situation is that I found some products on the web that are designed for single-mode fiber but they use the 100BASE-FX designation. Is this wrong? Is any 100BASE-FX SFP capable of communicating trough both multi-mode and single-mode fibers? If not, how can I know that a 100BASE-FX SFP will work with multi-mode or single-mode if it is not clearly stated?

  • There is no such thing as 100base-LX, I assume you mean 1000base-LX or 100 base-LX10? You can always check cisco's compatibility matrix: tmgmatrix.cisco.com – Lenniey Jan 14 '20 at 16:29
  • Yes, you are right, but my question is if 100BASE-FX will work with single-mode and multi-mode fibers and how (I assume that the laser should be different)? – soyxan Jan 16 '20 at 11:12
  • @Lenniey 100BASE-LX10 is specified in 802.3ah clause 58. – Zac67 Jan 27 '20 at 21:44
  • @Zac67 huh, you learn something everyday, ty :) – Lenniey Jan 28 '20 at 10:05
  • @Lenniey It's fairly unknown and pretty exotic - haven't seen it myself yet. – Zac67 Jan 28 '20 at 10:13

100BASE-FX inherited the specs from FDDI - it's designed for multi-mode fiber although it uses 1310 nm wavelength (FDDI was spec'ed before SMF became popular). Due to its large margins (with modern fiber) it often works over single-mode as well, but the reach can vary widely due to the exact specifics of the transceivers at hand (100 to 5000 m or more).

Unless the vendor specifies SMF use you can't know if an -FX transceiver would work over your fiber. 100BASE-FX primarily uses LED transmitters that don't couple well into SMF. Laser-based SFPs will likely go several km (and may be identical to 100BASE-LX10 or even (down-rated) 1000BASE-LX10 parts).

100BASE-LX10 (802.3ah clause 58) is specifically designed for single-mode fiber - but it's rather rare and might be more expensive than the popular 1000BASE-LX(10). For SMF, 1000BASE-LX is a safer bet in any case - unless you do require half-duplex mode, but I'd steer clear of that.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.