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3.1 | |
Copper Media | |
| 3.1.7 | |
Coaxial cable |
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Coaxial cable consists of a hollow outer
cylindrical conductor that surrounds a single inner wire made of two
conducting elements. One of these elements, located in the center of
the cable, is a copper conductor. Surrounding the copper conductor is
a layer of flexible insulation. Over this insulating material is a
woven copper braid or metallic foil that acts as the second wire in
the circuit and as a shield for the inner conductor. This second
layer, or shield reduces the amount of outside
electro-magnetic interference. Covering this shield is the cable
jacket.
For LANs, coaxial cable offers several advantages. It can be run longer distances than shielded twisted pair, STP, and unshielded twisted pair, UTP, cable without the need for repeaters. Repeaters regenerate the signals in a network so that they can cover greater distances. Coaxial cable is less expensive than fiber-optic cable, and the technology is well known. It has been used for many years for many types of data communication, including cable television. When working with cable, it is important to consider its size. As the thickness of the cable increases, so does the difficulty in working with it. Remember that cable must be pulled through existing conduits and troughs that are limited in size. Coaxial cable comes in a variety of sizes. The largest diameter was specified for use as Ethernet backbone cable, because it has a greater transmission length and noise rejection characteristics. This type of coaxial cable is frequently referred to as thicknet. As its nickname suggests, this type of cable can be too rigid to install easily in some situations. Generally, the more difficult the network media is to install, the more expensive it is to install. Coaxial cable is more expensive to install than twisted-pair cable. Thicknet cable is almost never used anymore, except for special purpose installations. In the past, ‘thinnet’ coaxial cable with an outside diameter of only 0.35 cm was used in Ethernet networks. It was especially useful for cable installations that required the cable to make many twists and turns. Since thinnet was easier to install, it was also cheaper to install. This led some people to refer to it as cheapernet. The outer copper or metallic braid in coaxial cable comprises half the electric circuit and special care must be taken to ensure a solid electrical connection at both ends resulting in proper grounding. Poor shield connection is one of the biggest sources of connection problems in the installation of coaxial cable. Connection problems result in electrical noise that interferes with signal transmittal on the networking media. For this reason thinnet is no longer commonly used nor supported by latest standards (100 Mbps and higher) for Ethernet networks.
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