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SFP transceivers support speeds up to 4.25Gbps, but SFP RJ45 only support 100/1000Mbps

Why is there no support for 2.5Gbps RJ45? Is this a technical limitation or did no one ever decide to make SFP 2.5Gbps RJ45 modules?

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The original SFP specification (INF-8074i) was intended for Ethernet and Fibre Channel use.

While Ethernet used to increment speeds by a factor of ten, FC doubled the speed over generations. At the time of SFP introduction in 2001 those speeds where 1.25 and 4.25 Gbit/s raw, respectively.

There is no support for 2.5 Gbit/s Ethernet because 2.5G is twisted-pair only and there is no optical PHY while SFP's primary use is for fitting optical modules, hence no support for that rate in an SFP slot.

Also, 2.5GBASE-T is a modified 10GBASE-T variant that requires much more power than 1000BASE-T: a potential 2.5/5/10GBASE-X SFP port would be fed with 64b/66b line code which the module would need to transcode into the -T variant with elaborate four-lane line code (DSQ128 w/ PAM-16). This is already a problem with 10GBASE-T modules which are far from general support and generally reduce the reach to 30 m.

Or more simply put: 2.5GBASE-T requires four lanes with a line code that SFP doesn't support. It would require a 2.5GBASE-X feed that isn't used anywhere else plus sophisticated transcoding. Theoretically doable, but it's probably more cost-efficient to use 10GBASE-T/-X.

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  • FC also has a 2Gbps mode.But yes, Nbase-T needs an interface SFP (non-plus) cannot support. It would, in theory, be possible to design a complex SoC capable of translating between the two, but on the SFP end, you'd still have a device expecting a 1Gb ethernet rate, or FC framing. (Note: an ethernet SFP always operates at 1000. At 10, the phy sees the frame 100 times; 100, 10 times. This is why a 1G-T adapter doesn't always work for 10/100.)
    – Ricky
    Commented Apr 3, 2021 at 19:44
  • @Ricky Possible yes, but there simply are no 2.5 Gbit/s SFP ports around. SFPs can very well operate at 100 Mbit/s though (e.g. 100BASE-FX and sometimes even 100BASE-TX).
    – Zac67 ♦
    Commented Apr 3, 2021 at 19:50

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