My ISP is offering 25GBit Fiber soon and I am looking into the required hardware. One of the devices I will need to purchase is a new transceiver that can handle the speeds.

The one my ISP recommends is the Flexoptix P.B1625G.10.AD but it is quite pricey compared to another one I found that I believe should be a compatible substitute for it, which is the FS.com SFP28-25G-BX.

I asked my ISP whether or not the FS.com transceiver would work as well. Their reply was that they cannot definitively say that it will but they would tend towards that it should.

One aspect of why they were not sure is the transmitter type. One of them is labelled as BIDI LR while the other is called DFB BiD.

Now my question is, what is the difference between these two types? Is it just a different marketing term for the same thing and these two receivers should work together? Or is there a technical difference that would render them incompatible?

If you believe them to be incompatible and have an alternative transceiver from FS.com I am all ears for any suggestions. Thanks!

1 Answer 1


25GBASE-BR is standardized in IEEE 802.3cp-2021.

There are quite a few pre-standard transceivers out there (BiDi, BX, ...) but pretty much all of them use the same 1270/1330 nm wavelength pair that the IEEE standard uses (25GBASE-BR10 for up to 10 km). Accordingly, they are compatible to each other as long as you pair the lambdas correctly - it should be Tx1270/Rx1330 on your end.

The DFB refers to distributed feedback which is one of the common laser technologies for long-wavelength 10 or 25 gigabit/s (the other being Fabry–Pérot). -LR is technically wrong as L indicates 1330 nm with duplex fiber - but the BiDi tells us that it's likely BR as well. -BX is also wrong as the -X would indicate 8b/10b PCS encoding (as in 10GBASE-LX4 or 1000BASE-BX10) and 25G variants use -R 64b/66b PCS block code throughout. Those are all marketing terms borrowed from various sources, due to late IEEE standardization. For reference, I've compiled a fairly complete naming guide a few years ago.

  • So, if I understood you correctly, you would also believe that they are compatible to each other and worth a shot to test?
    – comfreak
    Commented Jun 11, 2022 at 18:38
  • 1
    Yes - matching the wavelengths, they should be 100% compatible.
    – Zac67
    Commented Jun 11, 2022 at 18:51

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