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I'm in the process of learning about how SFP+ modules work under the hood. I chose a 10GBASE-T transceiver module as my case study. The module I chose is said to be complaint with SFF-8431 spec, so when I look this up on Wikipedia:

enter image description here

It looks like the interface would be XGMII. This interface is the "MDI" interface, correct? Does the transceiver implements the entire PHY (LDPC PCS, PMA, AN) or is it just a glorified ADC/DAC?

How would I figure out what the interface should be for ones that don't list anything under "MAC block to a PHY chip?"

enter image description here

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The interface in this case, I belive, is called SFI. Definitely not XGMII (32-bit DDR, was never really seen off-chip) or XAUI (4 lanes of 3.25 Gbps). It's exactly the same as the interface to a 10GBASE-R optical module.

Basically, you can think of the SFP+ to BASE-T module as a media converter - it receives 10GBASE-R on one end, and produces 10GBASE-T on the other end, and vise versa. So in the transmit direction, it will receive 64b/66b encoded and scrambled 10GBASE-R data, then convert that to 10GBASE-T including the multilevel encoding, FEC, etc. In the receive direction, it will receive the 10GBASE-T data, detect the symbols, decode the FEC, and transmit it again as 64b/66b encoded 10GBASE-R data.

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Maybe these diagrams will help. It looks like XGMII is implemented between the mac and packet processor. enter image description here

enter image description here
Source: https://4donline.ihs.com/images/VipMasterIC/IC/TXII/TXIIS100070/TXIIS182093-1.pdf?hkey=EC6BD57738AE6E33B588C5F9AD3CEFA7

Another good datasheet is this one: https://www.renesas.com/us/en/www/doc/datasheet/bbt3420.pdf

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    \$\begingroup\$ The SFP(+) module is typically only the right half of the box labelled "PHY/Optics" in your diagram. It is sometimes called the PMA (physical medium attach) or PMD (physical medium dependent) sublayer. \$\endgroup\$
    – The Photon
    Jun 30, 2020 at 22:09
  • \$\begingroup\$ Looking at the schematic of a PCB that includes a SFP+ cage, there's only two pairs (a single Tx pair and a single Rx pair), so I'm still a little bit confused why it lists XGMII, which has 8 pairs. If it truly only implements the PMA, which spec would describe the hardware interface between the PCS and PHY? \$\endgroup\$
    – Sittin Hawk
    Jun 30, 2020 at 22:16
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    \$\begingroup\$ @sittinhawk The TX/RX pairs are for an XAUI interface, which is a 10Gbps on PCB interface. Otherwise you'd need a phy. Some phy's only use XAUI \$\endgroup\$
    – Voltage Spike ♦
    Jun 30, 2020 at 22:17
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    \$\begingroup\$ At higher rates (100, 400 Gbps) Ethernet gets more specific about the host-module interface, but 10 Gbps was a free-for-all. \$\endgroup\$
    – The Photon
    Jun 30, 2020 at 22:58
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    \$\begingroup\$ @SittinHawk I might have been wrong about the coding. Transceivers that have 4 lane interfaces may use 8b/10b, but transceivers with 1-lane interfaces (like SFP+) might use 64b/66b encoding. See IEEE 802.3-2018 clause 52. (10 Gb/s * (66/64) = 10.3125 Gbaud, which is the 10 GbE serial line rate, so I'm pretty sure 64b/66b is correct for 10 GbE serial) I worked on a 10GbE X2 transceiver design (4-lane interface) years ago but then I was out of the Ethernet business during the time when SFP+ was developed, so I don't have first-hand knowledge of all the details of SFP+. \$\endgroup\$
    – The Photon
    Jul 1, 2020 at 17:09

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