Information Technology Reference
In-Depth Information
length of the horizontal cross-connect facilities should be no more than 5 m (16 ft).
This distance includes the cross-connects, the patch cords or cross-connect jumpers,
and the equipment cords in the wiring closet. The standard also recommends that
the user cords in the work area be no more than 5 m (16 ft). Combining these two
lengths yields 10 m (33 ft), although the rounding off of conversions by the stan-
dard makes the math look rather strange.
Work area cords of only 5 m (16 ft) are rather short for many office environ-
ments. As these cord and jumper lengths are only recommendations, you could adjust
them to fit your needs, as long as you keep the total length under 10 m (33 ft).
However, the only way to guarantee that you can make this tradeoff is to carefully
plan the wiring closet installation so that you minimize the patch cord and equipment
cord lengths. That will give you more length for user cords in the work area.
What difference does it make if you exceed these lengths a little bit? If the total
channel length is less than 100 m (328 ft), you will probably have no problems;
however, if you run a longer link, it might not work properly. Network equipment
manufacturers design their network adapters and hubs to operate over the standard
link distances and characteristics.
If you exceed those distances, or in other ways provide a substandard link, the
networking components may not operate on that link. Also, keep in mind that some
of the performance parameters of patch cords and cross-connect wire are worse
than those of regular horizontal cable, so you may not be able to just borrow some
of the allowed horizontal cable length and add it to the patch cord allowance. Open
office work areas are now allowed to exceed the user-cord limits by decreasing the
horizontal run to a consolidation point. See Chapter 10 for details.
In addition, a link that is too long will fail a cable scan. Just how long is too
long? Cable length measurements are part of the tests done by a cable scanner. These
test procedures are supplied in a supplement of TIA/EIA-568-B/C (originally TSB-
67). TIA-568-C has length test limits for both the Basic Link and the Channel. If
you exceed the test limits, the tester is required to fail the link. The test requirements
for cable links are fairly detailed and are covered in Chapter 15.
If you test the part of the horizontal link called the basic links,
1
it excludes the
user cords, jumpers, and patch cords. When these cords are added in, the total link
is defined as a Channel. You might later add cords that make the Channel exceed
the allowable length, even though the Basic Link test was passed. If you exceed these
lengths, your network equipment may not operate properly and it will be nobody's
fault but your own.
1
The Basic Link and the newerPermanent Llink are specified for similar measurements, as detailed in
Chapter 2, Fig. 2.8, and in Chapters 5 and 16.
