<html aria-label="message body"><head><meta http-equiv="content-type" content="text/html; charset=utf-8"></head><body style="overflow-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;"><div><blockquote type="cite"><div><div><br>So, in round numbers, it looks like you'd need to have ~500 bits to be able to run the decoder. If those are all in one lane at 100 Gbps - that's 5 nanoseconds.<br>If it's split across 4 lanes at 25 Gbps, well, it's still 5 nanoseconds. <br><br>That is comparable to the propagation delay in a 1 meter fiber.<br><br></div></div></blockquote></div><br><div>I thought I had posted about this a few weeks ago, but I can’t find it now…</div><div><br></div><div>Jim’s analysis is right, but unfortunately things are not that good.</div><div><br></div><div>The 100 GBPS Ethernet standard requires FEC, but you can get links without it. There are at least four kinds.</div><div><br></div><div>DAC or Direct Attach Copper - these just drive twisted pairs, and add at most a few nanoseconds above the wire delay. Only good up to about 5 meters</div><div><br></div><div>AOC or Active Optical Cable - sort of the same idea, but optical. Each end adds about 2 nanoseconds to the fiber delay of about 5 ns/meter. Usable up to 100 meters</div><div><br></div><div>PAM4 links - require some dsp work for the encoding and decoding, and evidently are about 10 ns/end. I think this is some block scrambling.</div><div><br></div><div>FEC - As Jim says, the codes are Reed Solomon with 500 bits plus per block. I found some transceiver specs which all cite 100-120 nanoseconds for the encoding and decoding. </div><div><br></div><div>Sources:</div><div>Signal Integrity Journal: <a href="https://www.signalintegrityjournal.com/articles/1286-gbs-ethernet-forward-error-correction-fec-analysis">https://www.signalintegrityjournal.com/articles/1286-gbs-ethernet-forward-error-correction-fec-analysis</a></div><div><br></div><div>A thesis on the design and implementation of RS coders for 100 Gbps: <a href="https://www.politesi.polimi.it/retrieve/a81cb05b-f419-616b-e053-1605fe0a889a/Report0.pdf">https://www.politesi.polimi.it/retrieve/a81cb05b-f419-616b-e053-1605fe0a889a/Report0.pdf</a></div><div><br></div><div>A powerpoint from IEEE about the latencies of the various FEC modes, there’s a nice table in here:</div><div><div style="display: block;"><div style="-webkit-user-select: all; -webkit-user-drag: element; display: inline-block;" class="apple-rich-link" draggable="true" role="link" data-url="https://www.ieee802.org/3/dj/public/24_01/he_3dj_01_2401.pdf"><a style="border-radius:10px;font-family:-apple-system, Helvetica, Arial, sans-serif;display:block;-webkit-user-select:none;width:300px;user-select:none;-webkit-user-modify:read-only;user-modify:read-only;overflow:hidden;text-decoration:none;" class="lp-rich-link" rel="nofollow" href="https://www.ieee802.org/3/dj/public/24_01/he_3dj_01_2401.pdf" dir="ltr" role="button" draggable="false" width="300"><table style="table-layout:fixed;border-collapse:collapse;width:300px;background-color:#E5E6E9;font-family:-apple-system, Helvetica, Arial, sans-serif;" class="lp-rich-link-emailBaseTable" cellpadding="0" cellspacing="0" border="0" width="300"><tbody><tr><td vertical-align="center" align="center"><img style="width:300px;filter:brightness(0.97);height:169px;" width="300" height="169" draggable="false" class="lp-rich-link-mediaImage" alt="preview.png" src="cid:89BBCAA0-02AD-4CCA-A943-608C12088119"></td></tr><tr><td vertical-align="center"><table bgcolor="#E5E6E9" cellpadding="0" cellspacing="0" width="300" style="table-layout:fixed;font-family:-apple-system, Helvetica, Arial, sans-serif;background-color:rgba(229, 230, 233, 1);" class="lp-rich-link-captionBar"><tbody><tr><td style="padding:8px 0px 8px 0px;" class="lp-rich-link-captionBar-textStackItem"><div style="max-width:100%;margin:0px 16px 0px 16px;overflow:hidden;" class="lp-rich-link-captionBar-textStack"><div style="word-wrap:break-word;font-weight:500;font-size:12px;overflow:hidden;text-overflow:ellipsis;text-align:left;" class="lp-rich-link-captionBar-textStack-topCaption-leading"><a rel="nofollow" href="https://www.ieee802.org/3/dj/public/24_01/he_3dj_01_2401.pdf" style="text-decoration: none" draggable="false"><font color="#272727" style="color: rgba(0, 0, 0, 0.847059);">he_3dj_01_2401</font></a></div><div style="word-wrap:break-word;font-weight:400;font-size:11px;overflow:hidden;text-overflow:ellipsis;text-align:left;" class="lp-rich-link-captionBar-textStack-bottomCaption-leading"><a rel="nofollow" href="https://www.ieee802.org/3/dj/public/24_01/he_3dj_01_2401.pdf" style="text-decoration: none" draggable="false"><font color="#808080" style="color: rgba(0, 0, 0, 0.498039);">PDF Document · 660 KB</font></a></div></div></td></tr></tbody></table></td></tr></tbody></table></a></div></div></div><div><br></div></body></html>