APPENDIX D: ADVANCED CABLING ISSUES (SKEW) 57
Optimizing Pairs (General Solution)
Measure pair lengths with a LAN cabling tester (TDR) or view pairs by stripping back a
small piece of cable and viewing how tightly pairs are twisted. The most tightly twisted pair
is the slowest (longest) and the loosest pair the fastest (shortest). You can also check the
relative delays using the test card as described on page 29.
The table below shows the general solution for optimizing CATx cabling for video in order of
pair length.
Pins Pair Signal
7 & 8 Shortest Pair Data
1 & 2 3
rd
Longest Pair Blue Video
4 & 5 2
nd
Longest Pair Red Video
3 & 6 Longest Pair Green Video
Some cables have a ‘3+1’ construction where three pairs closely match. The fourth pair
should be used for data and the other pairs sorted as in the above table.
Other cables have a ‘2+2’ construction where there are two sets of dissimilar pairs. Put red
and green on one set (tightest) and blue & data on the other.
A suitable pair-swapping scheme is easily made using custom wired patch cables at each end.