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Home > Resources > Technical Resources > Black Box Explains > KVM > Cable construction and delay skew.
Cable construction and delay skew.

Cat5 extender and CAT5e or CAT6 cables (delay skew)

If you connect CAT5e or CAT6 cable to a CAT5 extender, the result is blurry images on your monitor. The effect is known as delay skew, but do you know what causes it?

Different lengths

Believe it or not, it’s the cables. When properly connected, a CAT5 extender uses three out of four CAT5 cable pairs to separately transmit the red, green and blur (RGB) components of a video signal so the signals arrive at a monitor simultaneously. But CAT5e and CAT6 cables have wire pairs of different lengths than CAT5 cables do, and the RGB components of each pixel arrive out of sync when CAT5e or CAT6 cables connect to a CAT5 extender.

Two prominent constructions

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.

How to measure

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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