The 100G Ethernet physical layer signalling is moving to PAM4 and using Forward Error Correction (FEC) when the lane signalling rate is greater than 25 Gigabaud (Either 4 lanes x 25 or 2 x 50).

Infiniband EDR uses a signal rate of 25 Gigabaud (Gbd) also and from what I can see HDR uses a rate of 50 Gbd. But Infiniband doesn't use FEC.

What is the difference in signalling that allows Infiniband (IB) to have the same signal rates but doesn't use FEC to reduce error rates. Or does IB just accept higher bit error rates? (Or alternatively is there some other correction in higher layers that allows IB to get away with higher error rates at the physical layer).

  • \$\begingroup\$ What's the date on the Infiniband spec? IIRC it was developed before FEC at 25 G was available. Also, what's the maximum cable length for IB vs Ethernet? \$\endgroup\$
    – The Photon
    Feb 5, 2019 at 1:57
  • \$\begingroup\$ If binary signaling, more power means stronger drive about a (central binary) threshold. 4 PAM cannot do that trivially. \$\endgroup\$ Feb 5, 2019 at 2:14
  • \$\begingroup\$ @ThePhoton: IB using fibre optics supports up to 10km and Ethernet has an array of standards, some going 80km. But at least some of them they use FEC with a maximum reach of less than 10km. \$\endgroup\$
    – dave
    Feb 5, 2019 at 2:39
  • \$\begingroup\$ @ThePhoton, and unavailability of the technology could explain it, but then how does IB manage to operate at the high signal rates without FEC. FEC is adding latency and if it is unnecessary then why use it (or is it a different trade off for latency vs BER being made?) - that's what I'm trying to understand. \$\endgroup\$
    – dave
    Feb 5, 2019 at 2:40
  • 1
    \$\begingroup\$ You can do 25 Gbps over 10 km with a very expensive laser, or with a cheap laser and FEC. I don't know enough about Infiniband to say why they chose to go without FEC. Ethernet pretty much held off on defining a 25 Gbps per channel solution until chips capable of doing FEC at that rate were available. \$\endgroup\$
    – The Photon
    Feb 5, 2019 at 2:44


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