Information Technology Reference
In-Depth Information
NEXT must be measured in both directions on the cable link, to prevent the
situation where one end of a cable passes NEXT and the other end fails. Single-
ended measurements are often made in the telecommunications room, as it elimi-
nates carrying the tester around to each workstation outlet. Excessive untwist at the
connector is more often found at the workstation end, however, because the outlets
naturally separate the conductors, and because the individual jack plates are more
difficult to inspect. Also, the workmanship may be less because the supervisory con-
trol may not be as great as would exist in the telecommunications room. So, if you
only measure at one end, you are likely to miss more of the high NEXT connections.
The equipment at both ends requires appropriately low NEXT to operate properly.
A related figure of merit for a cable link is ACR. Although not yet specified by
the standards, ACR is a very good measure of how the attenuation and the NEXT
can influence the performance of networking interface devices. Attenuation may be
used to calculate the received signal strength of a signal transmitted from the far
end. NEXT can yield the level of the interfering crosstalk signal from the near-end
transmitter. The near-end receiver is thus required to cope with a signal-to-noise
ratio that is basically the ratio of the two signals: the attenuated far-end transmis-
sion and the near-end crosstalk.
Since both attenuation and NEXT are directly measured as part of the testing,
it is very simple to ratio the two to produce the ACR figure. The higher the ACR
number, the better the expected performance. Since a ratio of two logarithmic quan-
tities is their numerical difference, the ratio is simply: ACR
NEXT loss (in dB). One of the things that concerns network hardware designers is
that the ACR decreases steadily with frequency. This means that at 100 MHz, the
ACR margin may be very small indeed. Thus, it may be of benefit to those planning
to eventually go beyond 100 MHz to purchase cable with the highest ACR possible.
attenuation (in dB)
Reporting Pass and Fail Criteria
An important part of TSB-67 is the carefully detailed requirements for the reporting
of test results. In general, the standard requires that each measured result be
reported in units appropriate for the test and that a Pass or Fail determination be
reported. Measurements, such as for attenuation or NEXT, that might be very close
to the limits are reported with an asterisk in addition to the Pass or Fail.
A data report is simply the visual indication of the field test set. In all of the
modern cable scanners, the visual display is a liquid crystal display (LCD) that also
allows the display of some graphics. Of course, it is important for a permanent
record of the report to be made, so the testers also allow the test data for each cable
to be stored and later printed out. Figure 15.5 shows a typical cable scanning report.
