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I understand that most ports that support SFP+ transceivers can step down to allow an SFP to be plugged into it, but my question is around the transceivers/optics themselves. Can an SFP+ transceiver run at 1GbE, for example a 10GBASE-SR SFP+, or would the only solution be to get an SFP specifically?

My understanding is that, SFP+ transceivers should be used for 10GbE and SFP for 1GbE and this lack of backward compatibility is one of the few reasons 10GBASE-T is appealing (due to auto-negotiation).

UPDATE - It appears that there are some dual-rate optics out there. Link to Finisar's offering: https://www.finisar.com/sites/default/files/downloads/finisar-ftlx8574d3bcv-1g-10g-850nm-multimode-datacom-sfp-transceiver-product-specification-revb.pdf

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Short answer: in theory yes, in practice no.

Long answer: There are SFP+ optics which can also work at 1 GbE, I've talked (sorry, no other source than that) to people who were experimenting with it and were able to get it running. Unfortunately it is highly dependable on the device you plug the optics in, i.e. not every device in the test was able to tell the transceiver to step down to 1 GbE.

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The various optical ethernet standards use different wavelengths, light sources, and fiber diameter. Copper-based ethernet sources can change frequencies, and use, relatively, the same medium (Category-6A can run all the current ethernet standards which use UTP). A laser or LED would need to change the light source, so you just change the SFP/SFP+ module.

SFP and SFP+ modules are specific to the standard which they support. For example, even among 1 Gb optics, there are multiple (a half dozen, or so), standards requiring different SFPs, toss in the different 10 Gb SFP+s (another half dozen, or so), and you have around a dozen different 1 and 10 Gb optical ethernet standards, most requiring a different transceiver.

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It depends on the transceiver and the device to which it's attached. (for the record, one can run a 1000Mbps SFP down to 10Mbps, not that anyone actually does) You'll have to check the specifications of both to know if it can work. If it does, it's unlikely to be able to auto-sense which speed to run.

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The transceiver itself can run on practically any bit rate up to its maximum specified rate.

However, the port it is plugged into often supports only a single bitrate. Commonly, even SFP/SFP+ ports may only run at the rate the SFP(+) module is intended for, so e.g. a 10GASE-SR SFP+ will not likely link with a 1000BASE-SX SFP even though they share the same 850 nm wavelength.

Additionally, an SFP(+) port might not accept just any modules that you can fit physically. Some SFP+ ports generally don't accept SFP modules, some SFP(+) ports don't accept twisted-pair modules (esp. SFP+), some SFP ports don't accept 100BASE-X modules, and so on. Check your devices compatibility list for details.

Some ports can be manually configured down to link against a lower-speed SFP but that's not the rule. Generally, both sides need to use the same wavelength, so e.g. 100BASE-FX (1310 nm) cannot link with 1000BASE-SX (850 nm) but might with a down-configured 1000BASE-LX (1310 nm).

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  • Do you have credible documentation on this? I would be interested in looking into this claim as to my knowledge the transceiver process the modulation/demodulation of data onto the carrier and AFAIK, a 1000BASE-LX transceiver will not correctly process a 100BASE-FX signal. – YLearn Jul 27 '18 at 16:22
  • "a 1000BASE-LX transceiver will not correctly process a 100BASE-FX signal" - a transceiver doesn't process anything[#]. Simplified, it's just a media interface with a modulated laser on one side and a phototransistor on the other (plus the ADC and DAC components and the optics, of course). All encoding is done by the switch port[#]. An SFP port compatible with 100BASE-(F)X could use a 1000BASE-LX transceiver to link with a 100BASE-FX counterpart in theory - I'm not claiming that there's anything like that around, just that it's possible. – Zac67 Jul 27 '18 at 17:03
  • [#] -T PHYs require significant encoding very often done inside the SFP/+ module. Simple, proprietary -T modules may require specially encoded signals from the port. – Zac67 Jul 27 '18 at 17:06
  • That is not my understanding, which admittedly may be wrong but falls in line with discussions I have had with some EE friends. My understanding is that data is passed off to the transceiver in a uniform fashion no matter the transceiver type and the transceiver does all the modulation/demodulation of the given data on the carrier. In other words, it is the transceiver that talks 1000BASE-LX or 100BASE-FX, not the switch through the transceiver. Yes, the speeds the port is capable of operating may be fixed in the switch, but that is a matter of how the data is passed off to the transceiver. – YLearn Jul 27 '18 at 21:46
  • For example, if a new [#]-YX transceiver arrives on market, it will typically be supported by a switch because the switch doesn't need to talk YX, it only needs to pass data off to the transceiver. – YLearn Jul 27 '18 at 21:55
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SFP+ optical transceivers can support 1 GbE operation. They support 1GbE and 10 GbE dual rates and can be configured for 1 GbE

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It's true that some users turn to copper transceivers in 10GbE transmission, since 10GBASE-T is backward compatible with existing 1000BASE-T networks, meaning that it be deployed based on existing 1GbE switch infrastructures in data centers that are cabled with CAT6 cabling,or just reverse. But there are some 10GbE SFP+ transceivers supporting dual rates of Gbit/s and 10Gbit/s, like Intel E10GSFPLR (1000BASE-LX and 10GBASE-LR SFP+ transceiver).

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