Propagation Delay
Propagation delay, or delay, is a measure of the time required
for a signal to propagate from one end of the circuit to the
other. Delay is measured in nanoseconds (nS). Typical delay
for category 5e UTP is a bit less than 5 nS per meter (worst
case allowed is 5.7 nS/m). A 100 meter cable might have delay
as shown below.
Delay is the principle reason for a length limitation in
LAN cabling. In many networking applications, such as those
employing CSMA/CD, there is a maximum delay that can be supported
without losing control of communications.
Nominal Velocity of Propagation (NVP) on the other hand,
is different. NVP refers to the inherent speed of signal travel
relative to the speed of light in a vacuum (designated as
a lower case c). NVP is expressed as a percentage of c, for
example, 72%, or 0.72c. All structured wiring cables will
have NVP values in the range of 0.6c to 0.9c.
Results Interpretation
Delay measurements are relatively straightforward. Most structured
wiring standards expect a maximum horizontal delay of 570
nS. If design specifications allow, higher delay can be acceptable.
Since each pair in the cable has its own unique twist ratio,
the delay will vary in each pair. This variance (delay skew,
discussed in the next section) should not exceed 50 nS on
any link segment up to 100 meters. Standards require all pairs
to meet the requirement. It is possible to report just the
worst case pair. This will be the pair with the highest propagation
delay.
Troubleshooting Recommendations
Excessive propagation delay can have only one cause: the cable
is too long. If you fail propagation delay, check to ensure
that the pass/fail criteria match the design specifications.
If so, the cable is too long. In many cases, a cable up to
25% too long (125m for Category 5) will still support most
LAN applications. However, the installation will fail most
structured wiring standards, such as those published by CENELEC,
ISO/IEC, and the TIA. In some cases, if the customer insists
on the location of the terminal equipment, and an excessive
length cannot be avoided, you can verify other cable parameters.
If they pass, you can provide information that indicates the
cable meets frequency-dependent parameters but is non-compliant
with overall standards due to excessive length. This provides
professional results to the user while placing on them the
responsibility for non-compliant cabling.
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