Information Technology Reference
In-Depth Information
To properly test installed cabling components, it is necessary to define the “test
structures,” such as the Channel and the Permanent Link. Only by standardizing the
testing environment and detailing the expected (and necessary) performance param-
eters, can we hope to determine whether the elements of a particular structured
cabling system will meet the needs of the LAN wiring application. This section will
describe all of the test structures and the individual elements that are tested for cop-
per and fiber cabling.
The Channel and the Permanent Link
The installed cable link components of a horizontal link are measured either as a
Permanent Link or as a Channel. The difference between the two is that the Channel
includes the actual equipment cords (user cords), patch cords, horizontal cross-con-
nect, and cross-connect jumpers that will ultimately connect to the LAN equipment
at each end of the horizontal link.
The Permanent Link excludes two test equipment cords, one at each end
(including the connectors at the tester end), while the cords at the ends of the
Channel must be plugged directly into the test instrument interface with no
allowance for test cords. The Permanent Link and Channel are shown in Fig. 15.2.
As stated earlier, the reason for the two types of links is to facilitate testing at
both the construction phase and the post-installation phase. It is unusual for net-
work hubs and workstations to be available during the construction phase of an
installation. It would be time-consuming and difficult to wait to detect and repair
bad cable runs at some later time. Moreover, the cable installer may have little
responsibility or influence for the equipment cords. This two-step method of cable
link specification allows the installed “in-the-wall” cable to be tested as soon as
installation is completed. The complete link, from network hub to workstation, can
be further tested during installation of that equipment.
Equipment and Testing Requirements
The purpose of the standards in TSB-67 is twofold. The full title of the TSB is
Transmission Performance Specifications for Field Testing of Unshielded Twisted-
Pair Cabling Systems
. One explicit purpose, then, is to define cabling system per-
formance after the cable has been installed.
The other purpose, which is implied by wanting to test in the field, is to define
field tester performance. Early field testers did not always produce consistent results
