Coaxial cable finds niches outside campus networks
Each of the past several years has been predicted by some to be the year of
coaxial cable`s death," says Fred Galle, marketing manager with
cable manufacturer
CommScope (Claremont, NC). "But sales of our coaxial cable keep
increasing every year. And as more high-bandwidth applications are
widely implemented, the communications industry keeps coming back to
coaxial cable."
That remark may sound off base to someone who focuses solely on corporate
local area networks
(lans) because in the corporate lan, unshielded twisted-pair (utp)
cable has been the dominant infrastructure medium for several years.
How-ever, a broader view, encompassing wide area communications networks
and even residential voice, data, and video networks, reveals
significant need for the capabilities that coaxial cable provides.
Scarce lan deployment
Cable
manufacturers and installers agree that coaxial cabling sees little
action in the corporate lan. "In the early and mid-1980s, when nearly
every network had a Token Ring configuration, coaxial cable was widely
used," says Delton Smith, an engineering specialist with CommScope. "But
compared to twisted-pair, coaxial cables get quite bulky in a wiring
closet." This large footprint was one of several factors leading to
utp`s emergence and coaxial cable`s downfall in premises networks, Smith
says.
"When the original lans were put together,
utp cable
was not available," adds Bob Marretta, an engineer with Coleman/ cci
(Sanford, NC), a cable manufacturer. "But over the last 10 years, utp
cables have been developed with the characteristics necessary to handle
network traffic." He adds that both ease of use and a bit of marketing
also helped utp`s cause. "In some ways, utp cabling was marketed better
than coaxial cable, so that generated industry interest. Coaxial cable
is also more difficult to terminate than utp. Many installers pull and
terminate 25,000 feet of utp cable a day; you couldn`t pull and
terminate that much coaxial cable every day."
Michael Lagana,
registered communications distribution designer (rcdd) and project
supervisor with the network design and installation firm U.S.
Information Systems (Nyack, NY), says that ease-of-use issues that
surround coaxial cable do not end once the installation is completed.
"If you use coaxial cable in a ring or bus topology, then moves, adds,
and changes are more difficult to accomplish and to document than they
would be if you used utp cable. Changes in a coaxial-cable-based network
can disrupt a lot of users. On the whole, coaxial cable is not as
accommodating as twisted-pair cable."
Anatomy of a cable
With
a quick look, you can easily distinguish coaxial cable from utp,
shielded twisted-pair (stp), or fiber-optic cable. But it takes more
than a simple glance to tell one type of coaxial cable from another.
Each type has some of the same basic components, but coaxial cables
differ in several ways, including the manner in which they are shielded,
the makeup of the dielectric material, and even the composition of the
conductor.
Starting from the center and working outward, a coaxial
cable consists of a conductor, dielectric material, shielding
components, and an outer jacket. The conductor can comprise solid or
stranded material, which can be copper or a mixture of copper and
another component, such as aluminum, steel, or tin. Solid conductors
provide the best performance in terms of distortion and signal loss, but
can sometimes break if they are flexed repeatedly. Stranded conductors
are better than solid ones at accommodating frequent flexing. Conductors
that include copper and another material typically are stronger than
all-copper ones.
The dielectric material, which acts as a buffer
between the conductor and shielding components, allows the cable to
maintain consistent electrical properties and minimize signal loss. The
material can be either solid or foamed; foamed materials provide
lightweight composition and contribute to increased clarity in
high-speed transmission.
The widest variance among coaxial-cable
constructions lies in the shielding materials, which protect against
signal loss and help prevent electromagnetic interference and
radio-frequency interference in the circuit. Several combinations of
foils and braids are used to achieve various levels of shielding
performance. Coaxial cable commonly referred to as "tri-shield" cable
has three layers of shielding; "quad-shield" coaxial cable includes four
shielding layers.
A coaxial cable`s jacket is an additional form
of insulation as well as a protectant against environmental dangers. A
jacket typically is made of either polyvinyl chloride, polyethylene, or
fluorinated ethylene propylene. The material used will determine the
jacket`s temperature rating as well as minimum and maximum operating
temperature.
What`s in a name?
The casual observer will find
that simply knowing the "RG" classification of a coaxial cable is not
necessarily helpful in determining the cable`s performance level or the
applications that the cable supports. Although it is logical to deduce
that a Category 5 twisted-pair cable can outperform a Category 3 cable,
that same logic does not work when considering an RG-6 and an RG-58
coaxial cable.
The "RG" nomenclature originated in government
applications, when coaxial cables were manufactured to military
specifications and classified according to Radio Guide Utility (rg/u)
numbers. Originally, coaxial cables were developed to help support
high-frequency radio transmission. In time, the rg/u numbers fell into
classifications according to impedance characteristics and the "U" was
dropped from common reference. Today, the terms RG-6, RG-8, RG-58,
RG-59, and others are commonly used and designate the impedance values
that coaxial cables support.
Numerous applications
They may
be an uncommon sight in lans, but coaxial cables are virtually
everywhere else. "The applications that use coaxial cables are too
numerous to list," says Coleman/cci`s Marretta. "The cables are found in
airport control-tower antennas and in cellular-telephone antennas. You
have coaxial cables in your car."
"Some of the coaxial cable we
produce is used by television stations," says George Tokatli, senior
engineer with Helix/HiTemp Cables Inc. (Franklin, MA). "When a station
broadcasts from a remote location, it will send a truck to the site.
That truck facilitates the broadcast and is loaded with coaxial cables.
"Coaxial cables have very
high bandwidth capabilities," he continues. "The fact that these cables are not used very often in local
area networks is by no means a reflection of their transmission ability."
Coaxial
cables of the RG-58 and RG-59 type are used for closed-circuit
television systems, so they are among the most widely deployed coaxial
cables today. "RG-59 cable for video monitoring and RG-6 cable for cable
TV are the coaxial cables we use most," says U.S. Information Systems`
Lagana.
As Lagana notes, cable TV is probably the most prominent
and recognizable application for coaxial cable today. And RG-6 type
coaxial cable will probably continue to support cable TV well into the
future, according to manufacturers. "Cable-TV and high-density
television carriers are increasingly using hybrid fiber/coaxial cable in
their long hauls," CommScope`s Galle notes. "But they`re still bringing
coaxial cable--rather than fiber--to the home. Cable boxes are still
configured for coaxial cable, and it will be a long time before you see
your cable-TV technician polishing fibers in your living room--if you
ever see it."
To that end, several cable manufacturers are
incorporating coaxial cable into their residential-wiring product lines.
Helix/HiTemp, for instance, offers several home-wiring cable
configurations, each of which includes two heavily shielded coaxial
cables, two fiber cables, and two Category 5 utp cables. The company
says that it designed its residential wiring products with current and
future applications in mind. Tokatli remarks that with either a coaxial
or a fiber cable, a residential user can run both telephone and video on
the same cable. Twisted-pair telephone cables do not offer that
robustness, he also notes.
Helix/HiTemp also does a considerable
amount of custom cable manufacturing for residential, industrial, and
other applications, according to Tokatli. Often, he says, end-users know
what they need in terms of cable performance, but they do not
necessarily know which type of cable will meet those needs. In other
cases, users require such physical characteristics as a flexible cable
or a certain number of conductors within a single overall sheath. In
cases like these, Helix/HiTemp`s engineering staff works with its
clients to design cables that meet specific requirements.
Coleman/cci
also custom-manufactures cable to suit individual needs, Marretta says.
"Often a customer will need a unique jacket type, which we can
accommodate," he notes. "In fact, we get a lot of orders for customized
cable. We don`t strictly make cookie-cutter cables."
A return to lans?
Although
a re-emergence into the lan environment may appear unlikely for coaxial
cable, many professionals in the cabling industry have learned never to
say "never." "We still sell coaxial cable for lans," Marretta points
out, "although typically it is used to link together only a few
computers." He also says that cabling technicians may very likely be
called to job sites that include both coaxial and utp cabling, so
familiarity with both media is important.
And insiders warn not to
expect Category 7 cable (whenever it is formally adopted) to be
unshielded. With a transmission capacity of 600 megahertz, Category 7
cabling will assuredly have some type of shielding--be it twisted pairs
in an overall shield, individually shielded pairs, or shielded pairs in
an overall braid. When such a medium is eventually introduced,
installers and end-users of premises communications systems probably
will take a close look at cost, performance, and ease-of-use issues
associated with stp and fiber-optic cabling. And, perhaps, coaxial
cabling.
A
tri-shield coaxial cable includes three layers of shielding (a). Shown
here is a cable with a foil shield, a braided shield, and another foil
shield. A quad-shield coaxial cable (b) has one more shielding layer
than does a tri-shield cable. In this example, the extra layer is a
second braided shield.
At the Helix/HiTemp cable-manufacturing facility in Franklin, MA,
coaxial cable can be customized to user requirements. This photo shows a
copper braid being woven and wrapped around a coaxial cable`s
dielectric element