Coaxial Cable

Coaxial Cable

Coaxial cable (or coax) contains a center conductor, made of copper, surrounded by a plastic jacket, with a braided shield over the jacket. A plastic such as polyvinyl chloride (PVC) or fluoroethylenepropylene (FEP, such as DuPont’s Teflon) covers this metal shield. The Teflon-type covering is frequently referred to as a plenum-rated coating. That simply means that the coating

doesn’t begin burning until a much higher temperature, doesn’t release as many toxic fumes as PVC when it does burn, and is rated for use in air plenums that carry breathable air, usually as

nonenclosed fresh-air return pathways that share space with cabling. This type of cable is more expensive but may be mandated by local or municipal fire code whenever cable is hidden in

walls or ceilings. Plenum rating applies to all types of cabling and is an approved replacement for all other compositions of cable sheathing and insulation, such as PVC-based assemblies.

Note:

As a certified Network+ technician, you no longer need to concern yourself with
the Thicknet and RG-58A/U (Radio Grade) types of coaxial cable, unless you
would like to do your own research for historical or nostalgic purposes. Today,
your focus should migrate from the 50ohm coax of early Ethernet to the 75ohm
coax of early (and modern, of course) cable television. The reason for this is
that while coax in the Ethernet world is all but a thing of the past, RG-6 or CATV
coax is alive and well in the world of broadband cable (cable modem) technology.
Chapter 7 will detail the location of 75ohm coaxial cable when used in a
cable-modem system. The connectors used with coax in this environment are
the same F-Type connectors used for standard cable television connectivity. In
fact, the data rides on the same medium, just over different frequencies.

Using Thin Ethernet

    Thin Ethernet, also referred to as Thinnet or 10Base-2, is a thin coaxial cable. It is basically the same as thick coaxial cable except that the diameter of the cable is smaller (about 1/4½ in diameter).

Thin Ethernet coaxial cable is RG-58. Figure 1.10 shows an example of Thin Ethernet. With Thinnet cable, you use BNC connectors (see Figure 1.11) to attach stations to the network.

It is beyond my province to settle the long-standing argument over the meaning of the abbreviation BNC. BNC could mean BayoNet Connector, Bayonet Nut Connector, or British Navel Connector. But it is most commonly referred to as the Bayonet Neill-Concelman connector. What is relevant is that the BNC connector locks securely with a quarter-twist motion.

Example:

A stripped-back Thinnet

Example:

A male and female BNC connector

    The BNC connector can be attached to a cable in two ways. The first is with a crimper, which looks like funny pliers and has a die to crimp the connector. Pressing the levers crimps the connector

to the cable. Choice number two is a screw-on connector, which is very unreliable. If at all possible, avoid the screw-on connector!

In order to attach the backbone cable run to each station, a passive device, known as a T-connector, is used. Picture the uncut backbone cable extending to the back of each device. In order

to complete the connection, the cable needs to be cut at the point where the loop is closest to the interface. The two cut ends then need to be terminated with male BNC connectors and

plugged into the two female BNC interfaces of the T-connector, with the third, male connector attaching to the female BNC interface on the device’s NIC card. It is in violation of the standard

to have any sort of drop cable extending from the back of the device, unlike 10Base-5, where

such an attachment was customary. This requirement introduces a minimum of two caveats. The first is that any user that gains access to the back of their computer, and that wouldn’t be very hard, could disconnect the connectorized ends of the cut backbone, thus producing two unterminated LAN segments, neither one working properly. The second is that so many interconnections introduce failure points and opportunities for noise introduction.

Table 1.1 shows some of the specifications for the different types of coaxial cable.

Table 1.1:

Note:

Although some great advantages are associated with using coax cable, such as
the braided shielding that provides fair resistance to electronic pollution like
electromagnetic interference (EMI) and radio frequency interference (RFI), all
types of stray electronic signals can make their way onto a network cable and
cause communications problems. Understanding EMI and RFI is critical to your
networking success. For this reason, we’ll go into greater detail in Chapter 6.

Using F-Type Connectors

The F-Type connector is a threaded, screw-on connector that differs from the BNC connector of early Ethernet mainly in its method of device attachment. Additionally, as alluded to earlier,

you typically find F-Type connectors with 75ohm coaxial media and BNC connectors with 50ohm applications. As with most other coax applications, the F-Type connector uses the center conductor of the coaxial cable as its center connecting point. The other conductor is the metal body of the connector itself, which connects to the shield of the cable. Again, due to the popularity of cable modems, the F-Type coaxial connector has finally made its way into mainstream data networking. Figure 1.12 shows an example of an F-Type coaxial connector.

Note:

There is also a twist-on F-Type connector used in fiber-optic cabling, known as
the FC connector.

Example: 

FIGURE 1 . 1 2 An example of an F-Type coaxial cable connector