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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?

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  • 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
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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.

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