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Posted by on June 26, 2008, 10:04 am
> Trebor Mushroom wrote:
> > I'm looking for an authoritative answer to the following
> > question (preferably by referencing specific pages from
> > the IEEE standard):
> > Is the direct connection between two stations (i.e. use of the
> > so called crossover cable) explicitly supported by the standard?
>
> I don't know if the wording is in the standard, but as
> I understand it, it is required to work. =A0The only difference
> between MDI and MDI-X ports is the crossover of the wires.
>
> > In other words is it guaranteed to work if both adapters are
> > compatible with the standard?
> > Background:
> > We sell a sophisticated piece of equipment built around an IPC
> > board running quite old Linux kernel (the 10/100MBit Ethernet
> > adapter is a part of the chip set, for that reason I cannot
> > give you the exact make/model). Nevertheless it works flawlessly
> > with all kind of hubs/switches all over the world.
> > The manufacturer of the PC board claims that adapter is compatible
> > with the 802.3 standard).
>
> In the early, and not so early, days of 100baseTX I knew of
> some 10baseT devices that would not connect properly
> to 10/100 ports.
>
> Also, I once had a cable modem that wouldn't connect properly
> to a 100baseTX NIC at 100. =A0Connecting through a 10baseT
> repeater it worked fine. =A0(At the time cable wasn't that
> fast, anyway, though it may be now.)
>
> > One of our customers is complaining that direct connection
> > with some PCs does not work, specifically the "LINK" LED
> > doesn't come on. The customer believes that the network
> > adapter in our product is not compliant with the standard.
> > =A0From experience I know all to well that a connection like
> > that sometimes just don't work. Not only with this particular
> > product but with *any* two Ethernet adapters. In some cases
> > fixing the speed and/or duplex settings does the trick, but
> > still some combinations of adapters won't work no mater what.
>
> It seems that there are devices that won't work together.
> As far as I know, that is unrelated to crossover,
> NIC-NIC, NIC-repeater, NIC-switch, repeater-repeater,
> or any other combination.
>
> > Is the Link Integrity Protocol mechanism guaranteed to
> > work when connecting two adapters directly using the
> > crossover cable?
>
> I believe so, yes. =A0Though there were some pre-standard
> devices that didn't do link pulses at all.
>
> > I read good book about Ethernet some time ago, and I have
> > vague recollection that using short crossover cables
> > could cause problems (the ones I tried are >=3D 1m).
>
> As I understand it, there is no lower limit for UTP ethernet.
>
> > We test this extensively on loads of properly configured
> > PCs, using correct (and tested) cables. On average 3 out
> > of 10 doesn't work in this scenario.
> > Through the switch (or hub) it works like a charm.
>
> -- glen
To the OP:
Your experience is different from mine:
> We test this extensively on loads of properly configured
> PCs, using correct (and tested) cables. On average 3 out
> of 10 doesn't work in this scenario.
> Through the switch (or hub) it works like a charm.
Well when I do this it ALWAYS works.
I have done it hundreds of times over more than 10 years and
it just works. PC to PC, Router to Router, Switch to Switch.
Of course sometimes a miss-configuration results in
terrible performance due to duplex missmatch.
I would worry first of all about your cables. Are the pairs
wired correctly?
How do you test them?
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