Coaxial Cable

Coaxial Cable

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 both as the second wire in the circuit and as a shield for the inner conductor. This second layer, or shield, can help reduce the amount of outside interference. Covering this shield is the cable jacket (See Figure 1.1)

Figure 1.1 Coaxial Cable (

Coaxial cable supports 10 to 100 Mbps and is relatively inexpensive, although it is more costly than UTP on a per-unit length. However, coaxial cable can be cheaper for a physical bus topology because less cable will be needed. Coaxial cable can be cabled over longer distances than twisted-pair cable. For example, Ethernet can run approximately 100 meters (328 feet) using twisted-pair cabling. Using coaxial cable increases this distance to 500m (1640.4 feet). For LANs, coaxial cable offers several advantages. It can be run with fewer boosts from repeaters for longer distances between network nodes than either STP or UTP cable. 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 all types of data communication.

When working with cable, you need to consider its size. As the thickness, or diameter, of the cable increases, so does the difficulty in working with it. Many times 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 (1 centimeter [cm]) was specified for use as Ethernet backbone cable because historically it had greater transmission length and noise-rejection characteristics. This type of coaxial cable is frequently referred to as Thicknet. As its nickname suggests, Thicknet cable can be too rigid to install easily in some situations because of its thickness. The general rule is that the more difficult the network medium 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 except for special-purpose installations. A connection device known as a vampire tap was used to connect network devices to Thicknet. The vampire tap then was connected to the computers via a more flexible cable called the attachment unit interface (AUI). Although this 15-pin cable was still thick and tricky to terminate, it was much easier to work with than Thicknet. In the past, coaxial cable with an outside diameter of only 0.35 cm (sometimes referred to as Thinnet) was used in Ethernet networks. Thinnet was especially useful for cable installations that required the cable to make many twists and turns. Because it was easier to install, it was also cheaper to install. Thus, it was sometimes referred to as Cheapernet. However, because the outer copper or metallic braid in coaxial cable comprises half the electrical circuit, special care had to be taken to ensure that it was properly grounded. Grounding was done by ensuring that a solid electrical connection existed at both ends of the cable. Frequently, however, installers failed to properly ground the cable. As a result, poor shield connection was one of the biggest sources of connection problems in the installation of coaxial cable. Connection problems resulted in electrical noise, which interfered with signal transmittal on the networking medium. For this reason, despite its small diameter, Thinnet no longer is commonly used in Ethernet networks.

The most common connectors used with Thinnet are BNC, short for British Naval Connector or Bayonet Neill Concelman, connectors (see Figure 8-5). The basic BNC connector is a male type mounted at each end of a cable. This connector has a center pin connected to the center cable conductor and a metal tube connected to the outer cable shield. A rotating ring outside the tube locks the cable to any female connector. BNC T-connectors are female devices for connecting two cables to a network interface card (NIC). A BNC barrel connector facilitates connecting two cables together.

Figure 1.2 Thinnet and BNC Connector

The following summarizes the features of coaxial cables:

• Speed and throughput—10 to 100 Mbps

• Average cost per node—Inexpensive

• Media and connector size—Medium

• Maximum cable length—500 m (medium)

Plenum Cable

Plenum cable is the cable that runs in plenum spaces of a building. In building construction, a plenum (pronounced PLEH-nuhm, from Latin meaning “full”) is a separate space provided for air circulation for heating, ventilation, and air-conditioning (sometimes referred to as HVAC), typically in the space between the structural ceiling and a drop-down ceiling. In buildings with computer installations, the plenum space often is used to house connecting communication cables. Because ordinary cable introduces a toxic hazard in the event of fire, special plenum cabling is required in plenum areas. In the United States, typical plenum cable sizes are AWG sizes 22 and 24. Plenum cabling often is made of Teflon and is more expensive than ordinary cabling. Its outer material is more resistant to flames and, when burning, produces less smoke than ordinary cabling. Both twisted pair and coaxial cable are made in plenum cable versions.

Fiber-Optic Cable

Fiber-optic cable used for networking consists of two fibers encased in separate sheaths. If you were viewing it in a cross-section, you would see that each optical fiber is surrounded by layers of protective buffer material, usually a plastic shield, then a plastic such as Kevlar, and finally an outer jacket. The outer jacket provides protection for the entire cable, while the plastic conforms to appropriate fire and building codes. The Kevlar furnishes additional cushioning and protection for the fragile, hair-thin glass fibers (see Figure 8-6). Wherever buried fiber-optic cables are required by codes, a stainless-steel wire sometimes is included for added strength.

Figure 1.3 Fiber-Optic Cable

Figure 1.4 Fiber-Optic Cable

The light-guiding parts of an optical fiber are called the core and the cladding. The core is usually very pure glass with a high index of refraction. When a cladding layer of glass or plastic with a low index of refraction surrounds the core glass, light can be trapped in the fiber core. This process is called total internal reflection. It allows the optical fiber to act like a light pipe, guiding light for tremendous distances, even around bends. Fiber-optic cable is the most expensive but it supports line speeds of more than 1 Gbps.

Two types of fiber-optic cable exist:

• Single-mode—Single-mode fiber cable allows only one mode (or wavelength) of light to propagate through the fiber. It is capable of higher bandwidth and greater distances than multimode, and it is often used for campus backbones. This type of fiber uses lasers as the light-generating method. Single-mode cable is much more expensive than multimode cable. Its maximum cable length is more than 10 km (32808.4 feet).

Figure 1.4 Fiber-Optic Cable

• Multimode—Multimode fiber cable allows multiple modes of light to propagate through the fiber. It is often used for workgroup applications and intrabuilding applications such as risers. It uses light-emitting diodes (LEDs) as a light-generating device. The maximum cable length is 2 km (6561.7 feet). The characteristics of the different transport media have a significant impact on the speed of data transfer. Fiber-optic cable is a networking medium capable of conducting modulated light transmissions. Compared to other networking media, it is more expensive. However, it is not susceptible to EMI, and it is capable of higher data rates than any of the other types of networking media discussed in this chapter. Fiber-optic cable does not carry electrical impulses as other forms of networking media that use copper wire do. Instead, signals that represent bits are converted into beams of light.

NOTE- Even though light is an electromagnetic wave, light in fibers is not considered wireless because the electromagnetic waves are guided in the optical fiber. The term wireless is reserved for radiated, or unguided, electromagnetic waves.

Fiber-optic connectors come in single-mode and multimode varieties. The greatest difference between single-mode connectors and multimode connectors is the precision in the manufacturing process. The hole in the single-mode connector is slightly smaller than in the multimode connector. This ensures tighter tolerances in the assembly of the connector. The tighter tolerances make field assembly slightly more difficult. A number of different types of fiber-optic connectors are used in the communications industry. The following list briefly describes two of the commonly used connectors:

• SC—SC type connectors feature a push-pull connect and disconnect method. To make a connection, the connector is simply pushed into the receptacle. To disconnect, the connector is simply pulled out.

• ST—ST fiber-optic connector is a bayonet type of connector. The connector is fully inserted into the receptacle and is then twisted in a clockwise direction to lock it into place.

Figure 8-7 ST Fiber-Optic Connector

The following summarizes the features of fiber-optic cables:

• Speed and throughput   —More than 1 Gbps

• Average cost per node  —Expensive

• Media and connector size—Small

• Maximum cable length   —More than 10 km for single 

                                        mode; up to 2 km for multimode