Re: Long Range Ethernet transmission over twisted-pair cable

David Magda wrote:

glen herrmannsfeldt <gah@xxxxxxxxxxxxxxxxxxxxx> writes:

Al Dykes <adykes@xxxxxxxxx> wrote:
(snip)

Although, erroneously, the tester failed the link because it was
87M over length.
(snip)

If a "tester" is doing a TDR, it's going to call any cable length
over the spec as a "FAILED" based on the fact that it didn't get a
pulse back in time.

I would probably say that a tester set to 10baseT shouldn't fail a
cable based on length. Note, though, that 568A and 568B specify
100m so, unless the tester has separate settings for that it
probably is reasonable to warn you on 100m.
[...]

The length limits are there because of propagation delay issues and
round-trip times that come into play because Ethernet is CSMA/CD.

Of course, with more modern, switched, networks this is less of an
issue, but those are the design assumptions that Ethernet was
constructed on.

Regardless, the timings on 10 Mb/sec Ethernet are based on multiple 500 foot
coaxial segments with an additional length of drop cable at each end. With
10 Mb/sec on UTP signal degradation will almost always be the limit, not
timing. This is not the case for 100 Mb and faster, most of the copper
variants of which were designed around UTP with 100 meter segments.

The cable scanner, however, is not doing a "test to see if Ethernet will
barely work if everything else goes perfectly", it is generally doing a
test to verify that the installation complies with EIA/TIA 568 or other
applicable standards, and those standards generally specify a maximum
length, so the scanner is quite right to fail a segment that is over
length.

--
--John
to email, dial "usenet" and validate
(was jclarke at eye bee em dot net)
.

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