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TLK10232: Clarification: XFI vs. SFI vs. 10GBASE-(K)R

Part Number: TLK10232


in the related question[1] there is a reply by Luis Omar Moran where he says that the TLK10232 basically also supports XFI and SFI on the fast end.

This gives me some headaches, and I think I am missing a very basic bit of information there.

Could you please roughly give me a clue how the above 10G interfaces (XFI, SFI, and 10GBASE-R/KR) are related to each other?

E.g., from a practical point of view, which interfaces can be connected directly or where glue silicon is needed.

Thank you very much!



  • Hi,

    • XFI, SFI and KR are all 10Gbps serial high-speed electrical interfaces, intended to allow transmission of 64B/66B encoded data in networking systems .
    • XFI and SFI electrical specifications respectively apply to XFP and SFP+ system front port optical modules. These specs were defined by the SFF MSA industry group. You may refer to the SFF specifications below. XFP and SFP+ modules may support Ethernet, SONET and Fibre Channel protocols
    • 10GBASE-KR is an Ethernet defined interface intended to enable 10.3125Gbps transmission across lossy backplanes. You may refer to the applicable IEEE802.3 document 
    • A 10G electrical short reach PHY SerDes is typically compatible with XFI and SFI specs, but may not be compatible with KR long reach electrical specs
    • Similarly, for some KR PHY SerDes devices their output swing levels may not be compatible with XFI and SFI


    Rodrigo Natal

    HSSC Applications Engineer 

  • Hi,

    thank you very much. At least now I feel that I've not been completely off track.

    Concerning the SFI and XFI interfaces: Is it really that easy to connect them? I mean, not that it is exactly easy to route 10G signals, but broadly speaking is it just about matching the eye diagrams of the transmitters an receivers for each direction, respectively? Or to put it in other words, how are XFI, SFI, and KR related in terms of protocols? For example, given that the electrical specs do match, can I directly connect the XFI interface e.g. of a MAC to an SFI port of a switch at board level (not via a DAC cable or such, but literally connecting ICs)?

    Finally from time to time I encounter the term "USXGMII" in the context of 10G board level interfaces. Where to put that?

    Best regards,


  • Hi,

    An SFI compliant SerDes/PHY should be readily able to fully comply with the XFI specs. But an older SerDes/PHY specifically designed for XFI may not meet all of the SFI electrical specs. In particular the host PHY/retimer jitter and stressed input requirements set forth in SFF-8431 are a little tighter than those from XFP MSA. Refer to the MSA documents whose links I shared.

    I'm not familiar with USXGMII.


    Rodrigo Natal