Propagation Delay Skew
Propagation Delay Skew (skew) is the difference between the
propagation delay on the fastest and slowest pairs in a UTP
cable. Some cable construction employ different types of insulation
materials on different pairs. This effect contributes to unique
twist ratios per pair and to skew.
Skew is important because several high-speed networking technologies,
notably Gigabit Ethernet, use all four pairs in the cable.
If the delay on one or more pairs is significantly different
from any other, then signals sent at the same time from one
end of the cable may arrive at significantly different times
at the receiver. While receivers are designed to accommodate
some slight variations in delay, a large skew will make it
impossible to recombine the original signal.
Results Interpretation
Well-constructed and properly installed structured cabling
should have a skew less than 50 nanoseconds (nSec) over a
100-meter link. Lower skew is better. Anything under 25 nSec
is excellent. Skew between 45 and 50 nanoseconds is marginally
acceptable.
Troubleshooting Recommendations
If the skew is high, provided the intended application is
a 2-pair application such as 10Base-T or token ring, the application
should still perform. If one pair is much higher or lower
in delay than the others, very high skew may result. Examine
the delay results for each pair. If one pair exhibits uncharacteristically
high or low delay, re-examine the installation.
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