Information Technology Reference
In-Depth Information
The 4-pair cable is a topic of several standards, including TIA/EIA-568-C. It is
the cable around which most of the important LAN cable specifications and per-
formance tests are based. Most LAN topologies actually use only two of the four
pairs, however, so some installations place two LAN connections on each 4-pair
cable. Telephone connections often use two pairs and may sometimes be wired in
the same cable with a 2-pair LAN connection. You should be cautious in robbing
pairs from the LAN cable, because some of the 100 Mbps and higher LAN schemes
may use all four pairs.
Another common cable that is found in twisted-pair wire installations is the
25-pair jumper cable. This cable is preterminated in 50-pin male or female connec-
tors (sometimes called a 50-pin telco connector). In telephone wiring, the 25-pair
jumper cable is convenient for connecting between the local exchange carrier's RJ-
21X demarcation point and the user's punchdown blocks or private branch
exchange (PBX) switching equipment. In LAN wiring, the 25-pair jumper is com-
monly used between connectorized punchdown blocks and patch panels. Category
3 and Category 5 grade 25-pair jumpers are available, but some caution should be
used in deploying them in a Category 5 installation. Some authorities are concerned
with the combination of two or more 100 Mbps signals in the same cable sheath. A
25-pair jumper would theoretically allow you to combine as many as six such sig-
nals within the same sheath (6
24 pairs with one unused pair). Also,
each connector introduces more untwisted wire into a circuit and some of the stan-
dards limit the amount of untwist as well as the number of connectors in a link.
4 pairs
UTP/STP General Construction
Twisted-pair cable consists of one or more pairs of insulated wires that are twisted
together and joined in a common sheath. The main characteristics of twisted-pair
cable are wire gauge, stranding, twist pitch, insulation type, characteristic imped-
ance, and sheath material. Each of these items may affect the suitability of a cable
for a particular application.
Cables for LAN wiring may be made with either stranded or solid copper wire.
Solid wire is normally used for cable runs that will be terminated on insulation-dis-
placement connectors (IDCs), such as outlet jacks or punchdown blocks. Stranded
wire is normally used for user and patch cords that are terminated with an insula-
tion-piercing 6-pin modular (RJ-11 style) or 8-pin modular (RJ-45 style) connector
(plug). You should never use solid copper wire with a modular-type plug, unless the
plug is specially designed for solid wire (see Chapter 9). Some older installations for
Category 3 and below use screw terminal jacks that technically can be used for
stranded wire, although solid wire is the norm for station (horizontal) cable.
