| Thread Links | Date Links | ||||
|---|---|---|---|---|---|
| Thread Prev | Thread Next | Thread Index | Date Prev | Date Next | Date Index |
Mike, You list important benefits of 0% overhead FEC. In the interest of completeness, I will re-state what we already know. These benefits come at the expense of lower optical coding gain and/or higher latency for the FEC enabled operation. Chris From: Mike Dudek [mailto:mike.dudek@xxxxxxxxxx] I’ve thought of a couple of scenarios where some-one might want to turn FEC on/off once a link is running. 1 Someone is trying to minimize power and latency but has a need for a very low error rate. The link comes up without FEC and trains and exits the training algorithm’s correctly but the error rate is 1e-13 or 1e-14 (meeting the 1e-12 spec but not what he wants). It will take some time (minutes) before he even realizes there is a problem. At that point I’d have thought that he would want to turn on FEC with as little disruption to the link as possible. A variant of this is that the link does meet his requirement initially but temperature changes cause the error rate to deteriorate at some point in the future. (eg install in winter and problems in the summer, or a fan breaks) 2 A very low error rate is paramount so he starts all his links with FEC on. He gets his low error rate and then determines the link hasn’t had errors for a very long time so he wants to save power and latency so wants to turn it off. In either of these scenarios if the FEC line rate is the same as the non FEC line rate then I think the best solution would be for the Tx to always be FEC encoded and the Rx FEC can be turned off/on without communication with the Tx end. If however the line rate is significantly higher with FEC on then in scenario 1 he’d prefer to start at the lower line rate (FEC off) to get the best chance at achieving the lower power and latency. Mike Dudek QLogic Corporation Senior Manager Signal Integrity 26650 Aliso Viejo Parkway Aliso Viejo CA 92656 949 389 6269 - office. Mike.Dudek@xxxxxxxxxx From: Hugh Barrass (hbarrass) [mailto:hbarrass@xxxxxxxxx] Anthony, I don’t know whether your post was intended for the reflector or not. However, you raised an important point: There may be scenarios where certain links are short and others are long, so the FEC could be turned off on the shorter links to save power. I agree with that, however this could be built into the startup behavior. What we need to know is whether there is a requirement to turn on/off FEC for a link that is already running. Hugh. From: Anthony Torza [mailto:anthony.torza@xxxxxxxxxx] There could be some power savings if FEC can be turned off for certain links and the associated logic could be clock or power gated. For example, in a system with a central switch card(s) and 12 line cards, cards 1,2,3 10,11,12 could all use FEC (longest traces) and cards 4,5,6,7,8,9 could turn it off (shorter traces). |