Information Technology Reference
In-Depth Information
ited to for any hub-to-workstation link. As mentioned previously, although some
situations may operate with slightly longer cable lengths, they do not meet the
requirements for inevitable higher speeds of 100 Mbps or more.
Estimating Drop Length.
Cable drops from the proposed telecommunications
room to the workstations to be served must be within the maximum allowable length
of 90 m (295 ft). You should estimate these distances before finalizing your cable sys-
tem design. If the estimates exceed the maximum length, you may need to add one
or more wiring closets. Although there are minimum size specifications and features
for telecommunications rooms (covered in a later section), you may be justified in
putting in a substandard telecommunications space to get the drop lengths down.
To estimate the total length of a cable run, you must first know how the cable
will be routed and hung. Conventional wiring is often routed through metal floor or
roof supports to keep it well above the ceiling tile. Alternatively, cable may be hung
with a variety of cable hangers, clips, and tie wraps from supporting structures. In
some installations, particularly old work, the cable is laid on top of the ceiling tile
supporting grid. Large buildings frequently have the cable placed in cable trays sus-
pended between the ceiling grid and the floor above. This method is becoming more
popular for Category 5e/6 installations, where the slightest kink or sharp bend can
impair the cable performance. Any combination of the methods may be used as well.
The routing methods are important because the more orderly routings inevitably
require more wire. For example, routing through cable trays involves lots of right
angles (orthogonal routing) instead of diagonal runs, demanding up to 40% more
cable. On the other hand, laying the cable directly on top of the ceiling grid with direct
routing uses the least cable (although it is the hardest to maintain and may suffer EMI
problems more often). Cable trays are easiest to use during construction build-out,
when the ceiling grid is not in place. They are more difficult to add once the offices
are constructed or, worse, occupied. Trays carry significant weight from the many
cables they contain, and must be well secured in place. For that reason, cable hangers
and clips are often used to minimize disruption to an existing workplace.
To estimate an orthogonal wire placement, where cables are to be bundled or
laid in cable trays, you must calculate the length of run in the tray. This may involve
a straight run or one with several corners. Add each leg of the run in the tray. Next,
add the drop from the tray to the wall header above the workstation outlet, and the
lengths of the vertical drops in the workstation wall and in the telecommunications
room, and add a factor for obstacles (2% to 5%, depending upon the building).
Obstacles that might be encountered are HVAC units, duct work, firewalls,
columns, and pipes. The vertical drops should allow for 18 in (0.5 m) of excess wire
at the wall outlet plus the distance through the wall to the height of the cable tray
