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| Big Test MFG says our Ethernet Chip violates the IEEE standard - are they right? |
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| ve7xen:
--- Quote from: metrologist on October 21, 2021, 12:06:12 am ---Regarding cat5 vs. cat5e, I think it may be because the spec is guaranteed to 100m. Who here has tested this? I think short runs, 3m patch cables, is why I see reported Gbit speeds on cat3. Some of the reporting here seems contradictory, but I am not a standards expert. Negotiating the highest common denominator is what I would expect. I only really care about what's happening at the port. --- End quote --- Yeah, the cable quality is likely only going to matter near the edges of the spec, in an alien crosstalk environment (ie. cable bundle) and when EMI is a problem etc. In practice you can use a compliant cable at considerably longer than spec'd distance and it will work fine, and likewise an underspecced cable will often work just fine too. Like any spec, it's 'guaranteed' to work when you are in compliance, and outside of that the behaviour is undefined, not guaranteed to break - and if it's going to break will break in the most annoying, intermittent way possible ;). The PHY doesn't normally do any evaluation of the cable quality, so if it has sub-par characteristics, it will generate errors and a high loss-rate, but link will almost certainly come up unless it's extremely bad, or as previously discussed, is missing pairs. Even a 'low' loss-rate of like 1% will have a serious impact on throughput in most applications, because it causes many retries and messes with the rate estimation / congestion control algorithms at higher layers. It's up to the user to supply appropriate cabling or configure the equipment to match the supplied cable's limitations, the spec will not aid you here. The PHY you found looks like it supports downshift (called 'speed optimization' here), but it seems to be disabled by default. AutoMDIX is also supported and enabled by default. So the most likely scenario is that the link partner also does not do speed downshift, and both sides were trying to link at 1G over a two-pair cable. |
| tooki:
--- Quote from: nctnico on October 20, 2021, 08:51:41 pm --- --- Quote from: tooki on October 20, 2021, 12:24:42 pm ---- Gigabit requires Cat 5 cable, but requires 4 pairs. --- End quote --- AFAIK Cat 5E actually. I had to redo a lot of wiring to replace plain CAT5 because it didn't work for 1Gbit. --- End quote --- Nope. It’s a common misconception that gigabit requires Cat 5e. It does not. But 5e’s superior performance will get you a bit more length. I suspect that cases where replacing 5 with 5e made gigabit work are actually due to either the old cable being substandard, or even more likely, the jacks being terminated poorly. I’ve seen many a jack where the pairs were untwisted far too long, which 100Mbps might tolerate, but gigabit will not. |
| tooki:
--- Quote from: ejeffrey on October 20, 2021, 10:04:59 pm --- --- Quote from: nctnico on October 20, 2021, 08:51:41 pm --- --- Quote from: tooki on October 20, 2021, 12:24:42 pm ---- Gigabit requires Cat 5 cable, but requires 4 pairs. --- End quote --- AFAIK Cat 5E actually. I had to redo a lot of wiring to replace plain CAT5 because it didn't work for 1Gbit. --- End quote --- Really? I've haven't looked a that extensively but I have never seen a cat5 cable fail at a gigabit speeds. The initial version of 1000Base-T specifically called for cat5 but with slightly modified specs -- I think only for NEXT which was not specified in the cat5 standard but which most existing cat5 cables supported. Cat5e was then published and cat5 withdrawn and the ethernet spec modified to call for cat5e. --- End quote --- Every resource I’ve checked still says Cat 5, not 5e, as the minimum for gigabit. |
| Ranayna:
Wasn't there some change in the specs some years ago? I always heard that CAT 5e was essentially "backported" into CAT 5, making CAT 5e obsolete. |
| nctnico:
--- Quote from: tooki on October 21, 2021, 03:06:47 am --- --- Quote from: nctnico on October 20, 2021, 08:51:41 pm --- --- Quote from: tooki on October 20, 2021, 12:24:42 pm ---- Gigabit requires Cat 5 cable, but requires 4 pairs. --- End quote --- AFAIK Cat 5E actually. I had to redo a lot of wiring to replace plain CAT5 because it didn't work for 1Gbit. --- End quote --- Nope. It’s a common misconception that gigabit requires Cat 5e. It does not. --- End quote --- Well, I have seen CAT5 cables fail at 1Gbit where CAT5E worked. The failing cables where only a few meters long. So explain that to me... |
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