Information Technology Reference
In-Depth Information
Some of the differences were to be expected, as the installed cable had several
wire terminations, cross-connects, and modular connections that did not exist in the
factory. Also, the standard factory test was (and is) to lay the cable out on a non-
conducting surface (such as a concrete floor) or loosely coiled and use special test
adapters to connect to a network analyzer. Cable in the walls is bent, pulled, tie-
wrapped, run along metal pipes and steel beams, placed in conduit, and installed in
all manner of nonideal conditions that affect its performance. You would expect it
to perform differently. Moreover, there were no standards for accuracy for field
testers, and their performance varied considerably. No wonder there was no corre-
lation.
Fortunately, the situation is much improved with the newer standards, such as
TIA-568-B and -C (originally embodied in TSB-67, and TSB-95). The performance
of installed cable is specified in detail, as is the accuracy of field testers.
Even the required tests, testing methods, and method of reporting the results
are specified. The correlation of field tester performance with laboratory network
analyzers is also made. All of this lends a wonderful consistency and reproducibil-
ity to the testing of installed cabling that gives a real confidence factor to cable plant
certification.
Certification for installed cables may be done two standard ways, Permanent
Link testing and Channel testing. Both of these testing methods are defined in TIA-
568-C, which builds on the “worst case” link model. The Permanent Link covers
the horizontal cable from the workstation outlet at one end to the initial point of
termination at a cross-connect block or patch panel in the telecommunications
room, excluding the test cords for the test equipment and, of course, excluding any
user cords. This is the portion of the horizontal cabling system that is permanently
installed by the wiring contractor. It forms the essential litmus test of a proper struc-
tured wiring installation job, and includes all the effects of component quality and
workmanship.
The test connection at the telecommunications room termination of a horizon-
tal run may require special adapters unless the termination is directly into a modu-
lar patch panel. For example, the test cord would have to terminate in a 110 adapter
plug if that type of connecting block were used.
Originally, TIA documents identified the Permanent Link as the Basic Link, but
terminology parallel to that of the ISO/IEC has been substituted in new revisions to
the TIA-568 standard. (See Fig. 1.9 in Chapter 1.)
The Channel includes the actual equipment cords (user cords, patch cords, and
cross-connect wire) that connect to the network (or telephone) equipment at each
end. Keep in mind that the word “actual” means just that. Once you certify a
Channel with a particular set of equipment cables, those cables are frozen in place,
