Information Technology Reference
In-Depth Information
cables longer than 15 m and identified if they are greater than that expected.
However, higher than expected attenuation per unit length values do not cause a
failure to be reported, but might be a cause for concern.
The NEXT loss measurement is made in steps of 150 kHz from 1 to 31.25
MHz, and 250 kHz from 31.25 to 100 MHz. For the NEXT test, the measurement
of the worst pair combination is used. The tester may report either the actual NEXT
or the NEXT margin, although NEXT margin is required for failure reports or
when the measurement is within the tester's accuracy from the limits. NEXT mar-
gin is a value of much significance to cable installers because it indicates when mar-
ginal installation practices may be corrected by re-termination or rerouting.
Often, NEXT can be reduced by maintaining the twist of the pair as close to
the point of termination as possible.
Test reports must indicate whether the cable under test has passed or failed each
requirement. Because of the difficulty in establishing an absolute measurement near the
performance limits for the cable link, a cable may be falsely accused of failure or
improperly passed. The standard takes a clever way of dealing with this problem—
essentially including the tester's accuracy as if it were a part of the cable under test and
allowing an additional 10% of the horizontal length limit to allow for NVP uncertainty.
A Fail indication, thus, is a sure indication of a parameter that is outside the
limit. A Fail* indicates that the tester measured a value that is outside the test limit
but by an amount within the tester's accuracy. A Pass indication is assumed to be ade-
quately below the limit, with the accuracy of the instrument taken into consideration.
However, a Pass indication that results from a test measurement that is within
the accuracy level of the actual limit is marked with an asterisk (Pass*). The link is
still considered to pass the test, but is a handy visual clue to which links may be at
or near the absolute limits. Prudent installers might want to take a second look at
these links to see if shortening the length slightly or more carefully controlling
untwist might produce a full Pass. Because the test-reporting procedures allow an
additional length variation of 10% in measurement of the electrical length to
account for NVP variations, you should really respect the length limit of the tester.
Occasionally, the source of a Fail indication will be the “short link” problem
described earlier. Certain links under about 15 m may exhibit NEXT failure due to
the resonance phenomenon. These links may actually be “cured” by replacing the
run with a longer length of cable.
The proper use of test equipment is one of the most important aspects of a successful
LAN wiring installation. Test equipment may be subdivided into two types: basic
