Black Box Explains... cable construction and delay skew.
Cable construction and delay skew.
There are two prominent CAT5e and CAT6 cable constructions on the market. The first is referred to as “2 + 2,“ where two of the four pairs are of similar electrical length, and the other two pairs are of different lengths. The second cable construction is known as “3 + 1,“ where three pairs are of similar electrical length.
If you encounter delay skew when using the 2 + 2 type of cable construction, a delay line will be required because it is impossible to find three cable pairs that closely match for RGB. If delay skew is encountered with a 3 + 1 cable construction, it can often be eliminated (if not greatly reduced) by using the three similar cable pairs to send RGB signals.
The preferred way to determine which cable construction you have is to measure a 300-foot (91.4-m) run with a cable scanner. Otherwise, you can strip back 3 inches (7.6 cm) of the cable sheath and look at how pairs are twisted. If you have a 2 + 2 cable, the two pairs of similar length will be more tightly twisted than the other two pairs. If you have a 3 + 1 cable, the three pairs of similar length will be more tightly twisted than the fourth. The three similar pairs can then be “pair swapped“ onto the RJ-45 pins that carry RGB signals and the fourth can be used for data.
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