Information Technology Reference
In-Depth Information
If you use a shorter cord, the tester may pass a link that exceeds the limit. If you use
too long a cord, you may reject links that actually are legal.
Some testers require a test cord of at least 1 m. Never fear; because of individ-
ual cable variations in nominal velocity of propagation (NVP), the standard allows
an additional 10% before it declares a length failure (plus a factor allowing for the
4% tester accuracy, although pass/fail results in this zone must be marked
with an asterisk).
An interesting distinction should be pointed out in regard to length. The phys-
ical length of a Channel or Permanent Link is an important parameter. Physical
length may be measured or determined from markings on the cable. Cable testers
estimate electrical length from the propagation delay of the cable. Because propaga-
tion delay depends on the physical characteristics, such as the twist pitch (lay length)
and dielectric properties of each pair, it varies with different categories of cable and
even with different lots of cable from the same manufacturer. To accurately correct
for manufacturing variations in cable, it is very important to measure the propaga-
tion delay of each reel of cable installed. The cable pair with the shortest propaga-
tion delay is used.
The procedure for calibrating length on a field tester is to physically measure a
length of cable from the reel to be installed and calibrate the tester through an elec-
trical length measurement on that same sample of cable. The particular length of
cable you use is not important, but it is important that its exact length be known.
However, accuracy of the calibration is increased with a longer piece of cable. A
length of 15 m (or about 50 ft) or longer is commonly recommended. Realistically, it
would not be practical to cut and measure every reel of cable at a large job, but you
should calibrate for each different cable lot, if possible. You might cut a length for
one of your cable runs to use to calibrate the cable, so as to not waste the sample.
Testers are required to have a length range of at least 310 m, so that a 1000-ft
reel of cable can be measured. However, until the cable on a particular reel is cali-
brated for NVP, this measurement should be considered only approximate. Stated
lengths on reeled cable are not exact and should not be used exclusively to calibrate
the NVP of a cable.
1 m
Attenuation.
The maximum attenuation for the purposes of testing the Permanent
Link or Channel has been based on the attenuation values given in TIA-568-C for
horizontal cable, connecting hardware, and jumper or patch cords. It prorates the
loss values, given in dB per 100 m, and applies them to the worst-case length of the
link. Table 15.6 shows a summary of the allowable attenuation values for the
Permanent Link and the Channel at selected frequencies. TSB-67 contains the for-
mulas for calculating each of the components of Permanent Link or Channel atten-
