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What causes delay skew in a typical copper network?

From Wikipedia: "The Delay Skew test tests for the difference in propagation delay between the fastest and slowest set of wire pairs. An ideal skew is between 25 and 50 nanoseconds over a 100-meter cable..."

The speed of the propagation should be related to the conductors' lengths shouldn't it? So how is it possible for the skew to vary by any measurable amount assuming all four pairs have almost exactly the same overall length (i.e. they weren't cut independently of one another?)

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In twisted pair cables, different pairs have a different number of twists per inch. This helps to prevent signals on one pair from being induced on another pair. The more a pair is twisted, the longer the wires in the pair need to be for a given cable length. The different twist rates will cause different lengths between the pairs. The longer the cable, the more there is a difference in the lengths of the pairs.

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  • I heard someone say All "education, certification, or homework" questions are explicitly off-topic here. What's with the certification tag? I'm upvoting the answer though. I had no idea about this? ...Interesting. – Ronnie Royston Jun 5 '16 at 22:36
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    I believe the certification tag is about certifying the cabling. Once cabling is installed, it should be run through all the certification tests in the suite for the cable category, and the results should be given to the customer. These test results should be stored for future reference. – Ron Maupin Jun 5 '16 at 22:38
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    @RonRoyston, since there is no guidance in that tag, I believe the OP took it to mean that he should use it for cabling certification. it is unlikely that the question relates to an education certification. BICSI is the one which certifies cabling installers and designers. I was, once upon a time, an RCDD with the NTS specialty, so I am pretty familiar with the sorts of questions asked for cabling certifications. – Ron Maupin Jun 5 '16 at 22:47
  • Yes, certification as in for the cabling itself. Doh, of course the pairs have different lengths inherently due to the twist pitch, head smack. But still, what would cause the delay skew to be too high? Manufacturing defects? Poor termination? It kind of seems like the skew increase should simply be a function of the overall cable length, in which case you're already testing it with a cable length test. – Ted Quanstrom Jun 5 '16 at 23:06

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