Archived from groups: comp.dcom.lans.ethernet (
More info?)
laurent GARNIER wrote:
> Alan J. Flavell wrote:
>> On Fri, 8 Jul 2005, laurent GARNIER wrote:
>>> In my case, shielded cables are used for long distance copper
>>> GigaEthernet
>> My understanding is that the use of STP instead of UTP actually
>> *de*creases the maximum length over which copper ethernet can be
>> guaranteed.
The usual STP cable is not 100 ohm impedance, so that does tend
to decrease the maximum length with 100 ohm termination, and
likely with matching termination.
> Argggghhh ! I thougth wrong things about shielded cables. For me
> "shield" was equivalent of "better quality".
Well, more expensive, yes. UTP cable depends on the signal being
balanced. That is, the current in one wire of a pair is exactly equal
and in the opposite direction to the other. Transformers are used
to make that happen. A shielded cable has metal around the outside of
either individual pairs or the whole cable. In either case current in
the (inside of) the shield can cancel unbalanced currents from wires inside.
At some point, though, it is possible to get current on the outside of
the shield, in which case the shield does not help, and actually
radiates the signal. At higher frequencies it gets harder to keep the
signal from the outside, and connector design gets very important.
The shield might have to couple to the metal box, and then you have
problems with ground loops.
>> If you're really interested in longer distances, and/or worried about
>> interference, pickup etc., then fibre would probably be a better
>> choice, and has become reasonably affordable in comparison, whereas
>> the price differential used to be considerable.
>> ("fiber", for transpondian readers)
> Our purpose is to interconnect network equipments within several 19'
> rack-cabinet with copper gigaethernet links. Rack-Cabinets are close
> from each other but sometimes the link's length could be higher than 100
> meters.
As far as I know, fiber. The cable prices aren't so different, and very
likely less than STP cable. Connectors are more expensive, but likely
less than STP connectors. Connector installation is more expensive, but
again likely less than proper installation of STP connectors.
-- glen