SFP Twinax cables



  • Im a bit new and nieve when it comes to SFP SFP+, Twinax passive/active etc.

    Im looking at getting a few 10GB nics for point to point links and I found a deal on ebay for some NetApp sfp patch cables. Its normally used to link disk shelves together using fibre channel as the transport.

    My question is simply this: will this passive cable work with 10gbe cards?

    Please enlighten me & thank you in advance!



  • @curtisgrice:

    Im a bit new and nieve when it comes to SFP SFP+, Twinax passive/active etc.

    Im looking at getting a few 10GB nics for point to point links and I found a deal on ebay for some NetApp sfp patch cables. Its normally used to link disk shelves together using fibre channel as the transport.

    My question is simply this: will this passive cable work with 10gbe cards?

    Please enlighten me & thank you in advance!

    Fibre Channel isn't Ethernet, probably gonna give you issues. Use other cables instead, there are plenty cheap options.



  • Yeah that's what I was expecting. I was hoping that a passive layer one component with the same physical connecter would be cross compatible. Sounds like that's just not going to be the case. Tough I would still like to know the physical difference. More pins used? different pinout? number of used conductors in the cable?

    All in the quest for more knowledge and a cheaper lab  ::)



  • @curtisgrice:

    Yeah that's what I was expecting. I was hoping that a passive layer one component with the same physical connecter would be cross compatible. Sounds like that's just not going to be the case. Tough I would still like to know the physical difference. More pins used? different pinout? number of used conductors in the cable?

    All in the quest for more knowledge and a cheaper lab  ::)

    It's not just a cable, SFP means there's going to be electronics in the connector.



  • Ok, does it work like UTP ethernet where it tries to filter out any matching signals on the wires (sort of anyway)? is there fancy logic on the PCB in the connector? I would love to find a good resource on the topic but as "SFP" is not an ISO/IEEE standard but a MultiSource Agreement between vendors. Alos this "Agreement" seems to be fiber centric an not fully applicable to passive or active twinax "SFP" cables. Now to find cable teardowns and look into pinouts.

    I have seen a few photos of passive SFP twinax connector split open and there seems to only be a few passive electrical components likely for filtering/decoupling.

    Things that may cause incompatibility:

    • Number of data lines

    • Signaling frequency and related passive filter components

    • Pinouts

    • Resistance of the coax used

    Don't mind me. I just think this out on the keyboard. Perhaps someone has some technical information they can add or correct.



  • SFP (and GBIC) is a somewhat over-engineered standard. Some connectors have logic, eeproms, power regulation, sensors etc. and some SFP/miniGBIC ports only take certain types. Often, there are artificial limitations on what is supported.

    It's not comparable to ethernet, but mostly comparable to Thunderbolt or GBIC/MII adapters. Part of the network card is 'inside' the connector. Traditional ethernet is bigger than you think, most people never get past 'network card, ethernet port, ethernet cable', but there is a lot more like the MAC, PHY and MII. For instance: https://stackoverflow.com/questions/15777399/clarification-on-ethernet-mii-sgmii-rgmii-and-phy



  • @johnkeates:

    SFP (and GBIC) is a somewhat over-engineered standard. Some connectors have logic, eeproms, power regulation, sensors etc. and some SFP/miniGBIC ports only take certain types. Often, there are artificial limitations on what is supported.

    It's not comparable to ethernet, but mostly comparable to Thunderbolt or GBIC/MII adapters. Part of the network card is 'inside' the connector. Traditional ethernet is bigger than you think, most people never get past 'network card, ethernet port, ethernet cable', but there is a lot more like the MAC, PHY and MII. For instance: https://stackoverflow.com/questions/15777399/clarification-on-ethernet-mii-sgmii-rgmii-and-phy

    Thanks for the link. It sounds like the PHY resides in the SFP(+) connector and that's where we run into compatibility issues with different cables. I was aware of the EEPROM and that tells the "card" whats connected, it's capabilities, and and vendor lock in BS. That connects over i2c on pins 4 and 5 (I think, these seem to be a general diagnostic connection if supported by the module/cable).



  • @curtisgrice:

    @johnkeates:

    SFP (and GBIC) is a somewhat over-engineered standard. Some connectors have logic, eeproms, power regulation, sensors etc. and some SFP/miniGBIC ports only take certain types. Often, there are artificial limitations on what is supported.

    It's not comparable to ethernet, but mostly comparable to Thunderbolt or GBIC/MII adapters. Part of the network card is 'inside' the connector. Traditional ethernet is bigger than you think, most people never get past 'network card, ethernet port, ethernet cable', but there is a lot more like the MAC, PHY and MII. For instance: https://stackoverflow.com/questions/15777399/clarification-on-ethernet-mii-sgmii-rgmii-and-phy

    Thanks for the link. It sounds like the PHY resides in the SFP(+) connector and that's where we run into compatibility issues with different cables. I was aware of the EEPROM and that tells the "card" whats connected, it's capabilities, and and vendor lock in BS. That connects over i2c on pins 4 and 5 (I think, these seem to be a general diagnostic connection if supported by the module/cable).

    Yeah, they usually connect one or more i2c devices, an ID eeprom and often some 'secret' controller control for proprietary features.



  • Doing a bit more reading and I found the following article that seems to suggest that the PHY resides on the HBA/NIC. This means that all the extra electronics are there to support the physical media itself be it UTP (RJ45) or some form of fiber. The module (aside from perhaps doing some line filtering where applicable for the media (cat5/6) and perhaps rudimentary link detection) does nothing to the data stream itself, no extra encoding. Therefore using SFP(+) as a DAC patch cable MAY work as its just serial data out and serial data in.

    10GbE SFP+ PHYs: Requirements and leading solutions



  • @curtisgrice:

    Doing a bit more reading and I found the following article that seems to suggest that the PHY resides on the HBA/NIC. This means that all the extra electronics are there to support the physical media itself be it UTP (RJ45) or some form of fiber. The module (aside from perhaps doing some line filtering where applicable for the media (cat5/6) and perhaps rudimentary link detection) does nothing to the data stream itself, no extra encoding. Therefore using SFP(+) as a DAC patch cable MAY work as its just serial data out and serial data in.

    10GbE SFP+ PHYs: Requirements and leading solutions

    That is correct, it should not mess with the data stream. It does have a tendency to require some capabilities on the HBA side, and sometimes there can be a challenge-response.


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