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the wiki article says that twisting the wires protects them from external electromagnetic interference. But then why would you need to go for shielded twisted pair wiring ? Anyway the twist in the wires provide the sheilding.

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The twists do not provide shielding. The twists provided a balanced pair which allow you to reject some of the interference. – Zoredache Jan 27 '11 at 6:05
well i read that link as well and it says that The chief advantage of the balanced line format is good rejection of external noise. well isn't this the same reason why we use shielded twisted pair ? – user68350 Jan 27 '11 at 8:22

Shielded Twisted Pair is used in very electrically noisy environments, and I've never actually seen it in use (partly because I've never run into Token Ring in the wild). But then, I'm not a network geek so my exposure is smaller than the pros.

By electrically noisy, I mean running through areas with running motors (spinning magnets), near high voltage AC lines, close to transformers (such as those found in the DC/AC inverters in large UPSes), or other such.

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As sysadmin1138 says, STP is used in electrically noisy environments. One common example is inside industrial machinery.

e.g. Networked PLCs (Programmable Logic Controllers) will often be mounted inside the same cabinet as all the other electrical/electronic controls of a machine. This will include transformers, relays, solenoids and all manner of other things, most of which generate very significant electromagnetic pulses. While normal UTP cable might be adequate, the risk is often too great, and quite possibly too dangerous.

I've also used STP to run cable anywhere near some machines, such as a 25KVA resistance welder, when of course that couldn't be avoided altogether. Such a machine will pull a screwdriver out of your hand from more than a foot away while the welding circuit is on. Just imagine the spikes that generates.

The only other place I've personally seen STP is in cable testing equipment, where the signal absolutely must be clean.

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