Information Technology Reference
In-Depth Information
such a way that identifies both the point of termination and the associated connec-
tion at the other end of the cross-connect. In some cases, it may not be practical to
mark anything other than the termination ID on the block. Some connecting blocks
have additional labels, clips, covers, and brackets that can contain additional iden-
tification. However, some of these items have to be removed to terminate wires or
to test connections, and thus, they are easily lost.
You should keep comprehensive paper or database records of your intercon-
nections to compensate for the shortcomings of connector block and jumper mark-
ing. The standards do allow placing a listing of circuit identifications adjacent to a
point of termination, if the design of the devices makes direct marking impractical.
Keep your records up to date, including all adds and moves, so that you can easily
find the needed circuit links if you need to do troubleshooting or changes.
Cross-Connect Workmanship
In addition to the routing and cable handling issues mentioned in the previous sec-
tion, several other installation practices need to be observed in using cross-connect
wiring for LAN cabling. Preserving pair twist is probably the most important single
requirement to maintain the transmission characteristics of a link. Pair twist
integrity becomes more critical with increasing link performance category. The very
low speed links, such as LocalTalk and ARCnet, will operate on very slightly twisted
pairs. The modest speeds of 10BaseT and 4 Mbps Token-Ring are not very particu-
lar about maintaining cable twist, as long as the amount of untwisted wire is mini-
mal and the link does not push the distance limit. However, at the higher wire speeds
of 16 Mbps Token-Ring, 100VG, 100/1000BaseT, and ATM, the preservation of
pair twist becomes increasingly more critical.
The effect of adding connecting blocks necessarily adds some amount of
untwisted conductor paths to any cable run. However, you can certainly minimize
the link degradation by maintaining the natural twist of wire pairs as close to the
actual point of termination as possible. The maximum amount of untwist allowed
by TIA-568-C is 75 mm (3.0 in) for Category 3 terminations and only 13 mm (0.5
in) for Category 5e terminations. Category 6 untwist should be the same as that
for Category 5e. That should certainly be easy to accomplish. If you use even less
untwisted wire at any termination, you will simply have a higher-quality connec-
tion. The effects of untwist are somewhat cumulative, so neatness counts at every
termination point in a link. You will find that some types of connecting blocks are
more difficult to terminate with minimum twist than others. For this reason, many
authorities do not recommend the use of traditional 66 blocks, even for Category
3 operation.
