Coaxial cable consists of a center conductor, either stranded or solid,
which is surrounded by a layer of insulating material called the dielectric.
The dielectric is surrounded by a shield made of foil, braided wire strands, or
both. Outside of the shield is a protective insulating sheath that forms the
cable jacket
.
All of
the elements of the coaxial cable encircle the center conductor, as the growth
rings in a tree surround the core. Because they all share the same axis, this
construction is called coaxial, or coax for short. Coax is the most widely used
media for transporting high radio frequency signals over wire, especially cable
television signals. The cables leading to most external antennas are coax. The
video cables that connect a VCR to a TV are coax. Miles of coax are found in
most video production facilities.
In the past, coaxial cable offered
significant advantages for LANs. Its frequency response allowed it to carry
signals with less degradation over longer distances than the twisted-pair media
available at the time. Improved manufacturing techniques and better electronics
have long made twisted pair or optical fibers the preferred choice for network
cabling.
Originally, Ethernet LANs used a thick coaxial cable that was
1.27 cm (0.5 inches) in diameter. The industry began to refer to this cable as
Thicknet. The large coax used for 10Base5 networks was particularly difficult
to work with and required great care to prevent bends and kinks. When Ethernet
was standardized, Thicknet became IEEE 802.3 10Base5.
Later, the 802.3
committee standardized 10Base2, a version that used a much thinner coaxial
cable, with an outside diameter of only 0.635 cm (0.25 in.). 10Base2 is
sometimes referred to as Thinnet. Because of its low cost and ease of
installation, it is also sometimes referred to as cheapernet.
Grounding Considerations
Special care must be taken to ensure
that coaxial cables are always properly grounded. In networking, a correct
ground means that the cable is left ungrounded at one end. In most other
applications it is important to make sure that there is a solid electrical
connection at both ends of the cable. Failure to observe proper grounding may
result in stray currents flowing down the coax shield. This can result in
electromagnetic interference and possibly even the corruption of data packets
to the point that the network is unusable. It could also create a shock hazard.