Information Technology Reference
In-Depth Information
totally surrounds the dielectric. The braid shield consists of a wire mesh of very fine
wire conductors. As with the foil shield, the braid surrounds the dielectric and forms
a shield. The braid is effective at LAN frequencies even though there are small gaps
between the wires. The gaps are not significant compared to the wavelength of the
LAN frequencies. Some coax, such as Ethernet trunk cable, uses a double-braid
shield to further increase the shielding properties. Cables with braid shields are gen-
erally more expensive than cable with foil shields.
We will cover nonpaired cable in this section to help you identify it. Nonpaired
cable is not proper for use as LAN wiring because it lacks the self-shielding prop-
erties of twisted-pair wire. It both generates and is susceptible to interference,
which means that it will probably violate the allowable emission standards of
national and international organizations with government entities such as the U.S.
Federal Communications Commission (FCC). This is also a legal violation.
Nonpaired cable also may not exhibit a consistent characteristic impedance and
may create crosstalk between the pairs. This crosstalk happens even at voice fre-
quencies, as you may know if you have two phone lines in your house and have
older, nonpaired wiring. At LAN frequencies, the crosstalk between the transmit
and receive pairs may mean that you will receive enough of your transmitted sig-
nal to cause an error.
You will undoubtedly encounter those who have used short lengths of non-
paired cable (such as the flat telephone cord that is used with RJ-11 connectors)
with no apparent problems. Don't believe them. In all likelihood, they are actually
experiencing undetected data errors that just slow down their network. In any event,
such cable is a problem waiting to happen. What works at 2.5 MHz may be mar-
ginal at 10 MHz, impossible at 16 MHz, and a joke at 100 MHz. Don't let non-
paired cable be used anywhere in your network!
Nonpaired General Construction
Nonpaired wire may be divided into two general classifications: round cable and flat
cable. Both consist of two or more insulated wire conductors and may have a cov-
ering jacket over the wires. As with paired cable, nonpaired cable may be shielded
with either a foil or a braid conductor.
Round cable, as the name implies, consists of two or more wires that are con-
tained within a round protective jacket. An example of round nonpaired wire is
shown in Fig. A.3.
