Information Technology Reference
In-Depth Information
analog testers use a conventional continuous waveform to test such link parameters
as attenuation and NEXT. The frequency of the waveform is changed (stepped or
swept), and the measurement repeated at each of hundreds of discrete frequency steps
that are required for full bandwidth testing. The digital testers measure these param-
eters by generating a series of pulses, then using digital signal processing techniques
to derive the same parameters at all step frequencies more or less simultaneously.
The testing requirements of TIA-568-C have been drawn from TIA/EIA-TSB-
67,
Transmission Performance Specifications for Field Testing of Unshielded
Twisted-Pair Cabling Systems
. They push the performance limits of field tester tech-
nology. The TSB defines strict accuracy levels for the testers and defines two test
configurations for testing a horizontal cabling link, the Permanent Link and the
Channel. In the bulletin, swept/stepped techniques are used to illustrate the meas-
urements required of field testing for cable links.
However, the TSB allowed other methods “using frequency domain or time
domain (digital/pulse/DSP) measurements” that are equivalent to the swept fre-
quency (analog) methods.
In addition, TSB-67 followed the interesting practice of excluding the actual
connection (for example, the plug and jack) that attaches the tester to the link under
test. In the case of the Permanent Link, this connector is not specified, as it is at the
tester end of the 2-m (maximum) test cable. Some tester manufacturers use a high-
performance, low-NEXT connector at the tester so as to not introduce adverse lev-
els of NEXT into the link under test. However, at the Channel interface, the actual
user cable must connect to the tester at each end. The user cable must be terminated
in the standard 8-pin modular plug, which unfortunately has relatively poor NEXT
performance.
The most intriguing claim of the digital tester advocates is that the pulse/DSP
technique allows them to totally exclude the contribution to NEXT and other
parameters that result from the modular connector (mating plug and jack) at the
tester itself. The modular connector is the required test interface for a Channel
measurement, so an alternative low-NEXT connector cannot be used, as it could for
the Permanent Link. Remember that a very significant portion of the detrimental
crosstalk occurs at this modular connector interface. Digital signal processing can
actually exclude this connection measurement, unlike conventional analog methods.
Which type of field tester should you choose? Sorry, but we are going to leave
this decision to the reader! The technical positions of each tester manufacturer are
strong and emotions run high. There has basically been an agreement to disagree.
You may wish to look at several different models and make your own judgment.
Frankly, the many testers on the market actually differ in features, options, report-
ing format, price, and measurement speed in addition to the measurement tech-
