Re: Crossover function in the standard.
- From: Bod43@xxxxxxxxxxxxx
- Date: Thu, 26 Jun 2008 07:04:13 -0700 (PDT)
On 24 Jun, 02:56, glen herrmannsfeldt <g...@xxxxxxxxxxxxxxxx> wrote:
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. The 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. Connecting through a 10baseT
repeater it worked fine. (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.
From 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. Though 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 >= 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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