Information Technology Reference
In-Depth Information
attenuation, and NEXT are currently tested. The length of the cable run is impor-
tant in influencing attenuation and NEXT values, but perhaps the ratio (attenuation
to crosstalk ratio, [ACR]) is a better indicator of overall system performance.
Likewise, very high speed networks may care as much about the propagation delay
as the physical length of the cable, yet transmission delay is not yet specified nor
tested. These new tests could be a boon to transmission engineers, but a bane to
cable system designers and installers. Can you just image telling your installer to
keep cable runs under 300 ns? Where do you get a tape measure that size? Let's look
individually at each of the tests that are now required.
Wire Map.
The wire map test checks for wiring errors and any significant cable
faults, such as shorts or opens. It is a very good idea to check the reeled cable for
shorts or opens before installation, as it will be very hard to prove that the defect
was not caused during installation. You can also perform most of the other tests, as
the reeled cable should easily pass them before it has been installed. Keep in mind
that, if you want to check the NEXT, you will need to use a properly rated connec-
tor because its NEXT will often be significant compared to that of the cable.
The wire map test will check for:
Continuity
■
Shorts
■
Crossed pairs
■
Reversed pairs
■
Split pairs
■
Figure 15.3 shows the correct pin-out/pairing and most of the common wiring
errors. In the standard TIA-568-C wiring scheme, the wires are paired 1-2, 3-6, 4-5,
and 7-8. The pair numbering, 1 through 4, varies slightly between the T568A and B
wiring patterns. Color codes for the cable are assigned on a pair basis, so (as an
example) pair 2 will always be orange and white, but may be assigned to the 3-6 pins
or to the 1-2 pins, depending on wiring pattern. The tester has absolutely no idea
what color the wires are! The technician will have to visually inspect them if the
design requires that a particular pattern be used. However, if both ends of a cable or
run are wired with the same pattern, the field tester will pass the cable. If not, it will
show a crossed pair condition, as in Fig. 15.3d. The field tester also easily finds split
pairs, shown in Fig. 15.3f, an impossible task for a DC continuity tester.
Length.
Length measurements are made on two types of cable links, the Permanent
Link and the Channel. Recall that these two links differ in that the Channel includes
