Information Technology Reference
In-Depth Information
Coaxial cable, or coax, is the original LAN cable. It was first used for local area net-
working in Ethernet networks, IBM PC-NET broadband networks, and ARCnet
networks. Coax is still in use in thousands of locations, even though many newer
installations have converted to twisted pair.
Coax is primarily used for its self-shielding properties, low attenuation at LAN
frequencies, and installation expense. The construction of the cable greatly reduces
susceptibility to outside interfering signals and noise, besides minimizing the radi-
ated emissions from the cable. Because coax was originally designed to carry radio
frequencies, it has fairly low attenuation characteristics. The cable is readily avail-
able, relatively inexpensive, and allows a daisy-chain or tapped LAN connection
that minimizes total cable length.
Coax cables may have solid or stranded center conductors, foil or braided
shield, several types of insulating dielectric, and several types of outer jacket mate-
rials. The cable is available in a variety of standard sizes and impedances.
Several types of coax cable are available for LAN use. Table A.2 shows the
common types and their uses. As with twisted-pair cable, coax is available in a vari-
ety of insulation types and may be used in plenum spaces and riser shafts. Coax is
included in IEEE 802.3, Ethernet Version 2.0, the
NEC
, and other standard docu-
ments. It was formerly included in EIA/TIA-568 1991, but was dropped in TIA/EIA-
568-A 1995 as a recommended cable type.
Coax General Construction
Coax cable gets its name from its construction. As you can see from Fig. A.1, coax
consists of a center conductor and a coaxially positioned outer shield conductor that
are separated by an insulating plastic, called a dielectric. An outer jacket insulates
the shield. The shield may be a foil-wrap with a drain wire or a wire braid. Some
coax, such as that used for thick Ethernet, may have a double shield layer.
Theoretically, the positioning of the center conductor, surrounded completely
by the concentric shield conductor, keeps all electromagnetic fields between the two
conductors, as shown in Fig. A.2. This mode of operation is referred to as “unbal-
anced,” as opposed to the balanced arrangement of twisted-pair cable. Other terms
for the unbalanced mode include single-ended and bipolar. The shield is maintained
at “ground” potential, while the center conductor is driven with the LAN signal.
Because the shield is grounded, interfering signals from outside the coax cable
should be prevented from entering the cable and coupling to the center conductor.
Grounding is quite important in coax cable installations.
