A certification tester reports near-end crosstalk (NEXT) in decibels. How should the values be read?
- A lower dB value means reduced crosstalk, which is better
- A higher dB value means less crosstalk, which is better
- Any negative dB value means the link has failed
- The dB value only matters above 500 MHz
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B. A higher dB value means less crosstalk, which is better
NEXT is reported as the loss between the disturbing pair and the disturbed pair, so a bigger number means more isolation. Insertion loss is the opposite: there a lower number is better, which is why the two are often mixed up.
Reference: ANSI/TIA-568.2, NEXT loss
Which three things all increase the insertion loss of a twisted-pair link?
- Greater length, higher frequency and higher temperature
- Shorter length, higher frequency and lower temperature
- Greater length, lower frequency and lower temperature
- Shorter length, lower frequency and higher temperature
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A. Greater length, higher frequency and higher temperature
Attenuation rises with cable length, with signal frequency and with temperature. That is why long runs through hot spaces can fail insertion loss even though they are within 90 m.
Reference: ANSI/TIA-568.2, insertion loss
What is the difference between a permanent link test and a channel test?
- The permanent link includes the patch cords; the channel excludes them
- The permanent link is tested at one frequency; the channel is swept
- The permanent link excludes the patch cords; the channel includes them
- The permanent link uses a tone probe; the channel uses a certifier
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C. The permanent link excludes the patch cords; the channel includes them
A permanent link test covers the fixed cabling from panel to outlet (up to 90 m) using the tester's own link adapters. A channel test measures end to end through the user's actual equipment and work-area cords (up to 100 m).
Reference: ANSI/TIA-568.2 and ANSI/TIA-1152, test configurations