Gregory's Server
@azonenberg @gsuberland @karotte The low-speed optical modes like that are relatively OK, it's basically linecode directly onto a laser that's either on or off, or PAM4. "infinite bw" assumption, pretty much. The actual long-distance (read: transatlantic) and high-rate (read: Tb/s) stuff is where the interesting stuff happens; 90 Gbd coherent IQ dual-pol xceivers, non-equal-prob'lity symbols and/or non-regular constellation diagrams, nonlinear-channel equalizers, FEC codewords of scary lengths…
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4 comments
 @funkylab @gsuberland @karotte Oh I bet fiber gets cursed when you go fast enough. But at 10/25G its easy.  @azonenberg @funkylab @gsuberland @karotte I'd argue the FEC layer in 25GBASE-R is cursed due to all the transcoding and gearboxes to convert 80 64b66b codewords to one 5140b block to append the 140b parity resulting in the same 5280 bits. It's not pretty. 25GBASE-CR-S is nice and pure like 10GBASE-R but the FEC layer is required in everything else 25GBASE-R. The FEC itself isn't cursed just the wrapper around it.  @dlharmon @azonenberg @funkylab @gsuberland @karotte |
@azonenberg @gsuberland @karotte ( … and that's before you learn about the fact that in long-haul connections, you need to put in as much power as you can to get good OSNR at the output, but not as much that your mutual information per symbol drops due to phase smear due to fiber medium nonlinearity at high E-field strengths, but still want to do wavelength multiplex with only small guard bands between the carriers, but of course wavelengths' powers will mutually cause nonlin./phase distortion)