Information Technology Reference
In-Depth Information
The answer to this question depends on many factors, but the two most impor-
tant considerations are growth and utility. You must determine what amount of growth
planning is reasonable, and what it will cost to implement. For example, if you have
large individual offices that may eventually include two workers, overwiring by a fac-
tor of two would be reasonable. The type of work that is to be done is also important.
For example, some workers may eventually need two computers or two phone
lines. In such cases, you should provide at least one additional station cable outlet
that can be used for expansion. Generally, it is far less expensive to do a certain
amount of overwiring during initial construction than it is to add outlets later.
What methods should you use to terminate the cables in the telecommunica-
tions room? The answer to this question depends on whether you intend to install
a universal wiring system, where any cable can serve your telephone or LAN sys-
tems, or install separate telephone and LAN wiring systems. Telephone wiring is
quite forgiving in that you may run longer distances and make a larger number of
cross-connects between the telephone instrument and the switching equipment.
Modern LANs are much more restrictive in this regard. Early cabling standards
allowed two cross-connects for a single LAN connection, but the latest Category
5e/6/7 wiring standards (as shown in Fig. 14.2) allow only one cross-connect and
also severely restrict cable lengths, including that for the cross-connect wire. This is
to allow for the strict attenuation, delay, and crosstalk limits of 100 MHz and
higher networks. Many experts even recommend that the workstation cables be ter-
minated directly into a patch panel, with no ICs at all.
One of the reasons for this cross-connect limit is that all wiring terminations
require that the pairs be slightly untwisted to be punched down at the point of ter-
mination. When the cable being used is very tightly twisted, as with Category
5e/6/AC6 cable, this untwisted portion increases crosstalk, introduces noise, dis-
turbs the line impedance, and increases RF emissions. Too much untwist can cause
a link to fail at the higher LAN data rates, especially if the cable run is near the
allowable length limit.
All punchdown blocks introduce untwist into a link. The extent of the prob-
lem is less with the 110-style blocks than with the older 66-style blocks, but it exists
nevertheless. The effect of untwist is more disruptive as the network speed increases.
Terminating the station cables directly onto a patch panel minimizes the amount of
untwisted wire and increases the performance of the link.
The primary guideline in locating telecommunications rooms is to place them near
the work areas they will serve. Advisory language in the standard says to place them
