Information Technology Reference
In-Depth Information
One of the keys to a successful LAN wiring installation is good workmanship.
You can use all the finest components, the most expensive mountings, and the
highest ratings, and still have a poor installation because of improper installation
techniques. The TR is as much in need of good installation practice as any other area.
In some ways, there are more places in a typical TR to do things that might
adversely affect your cable link performance than elsewhere. For example, you may
make one station cable termination, two cross-connects, and one patch termination
all in the same cable link. Add to that the patch cords and equipment cords, and
there is a lot of room for error.
The first workmanship guideline is to properly route the cables. You should
minimize the sharp bends, avoid tight tie-wraps, avoid proximity to magnetic fields,
and limit exposure to mechanical damage. You should ensure that the pairs are kept
twisted as close to the point of electrical termination as possible. See Chapter 6 for
an example of the effect of slightly untwisted wire.
The standards for preserving wire pair twists are defined in TIA/EIA-568-C.
For Category 5e and Category 6 terminations, the maximum amount of untwist is
13 mm (0.5 in). For Category 3, it is 75 mm (3.0 in).
The twist must be preserved within these limits at each point of termination to
connecting hardware, including connecting blocks. The practice is not optional; it is
mandatory. Even Category 3 links can suffer from the increased crosstalk and
impedance mismatch problems of excessive untwisted wire.
The cable jacket should only be removed to the length necessary to terminate
the pairs. This means that you do not strip back the jacket 4 or 5 in, but only about
25 mm (1 in) more than is needed for the termination of a single pair.
When punching down wires in a 66-type block, there is a possibility that
another adjacent wire may accidentally get cut, along with the wire you intend to
terminate and trim. The punchdown tools usually have one bright yellow side with
the word “CUT.” If you wrap the wire in from the top of the contact, the cutting
edge of the blade should trim excess wire on the bottom. To do this, the CUT side
is down. If you see the word CUT, turn the tool around, so that you don't cut the
wire in the wrong place. If you are using the minimum-untwist technique for
Category 5e described earlier (see Type 66 connecting blocks at the beginning of this
chapter), you must use the tool in both orientations, as one wire of a pair will enter
its contact from the bottom and the other from the top. This takes a little extra
thinking and dexterity to master, but can be done quickly once you get used to it.
Do not leave the cut-off excess wire in between the contact rows of the block.
You may accidentally force it into a contact slot or cause a short when you termi-
