CCNA Self-Study: Network Media (The Physical Layer)

Upon completion of this chapter, you will be able to perform the following tasks:

• Describe the primary types of network cabling, including shielded and unshielded twisted-pair, coaxial, fiber optics (multimode and single-mode), and wireless communications

• Describe types and characteristics of cabling and connectors used in an Ethernet LAN

• Describe the necessary components for enabling WAN connectivity over serial or ISDN BRI, local loop using DSL, and a cable connection for a Cisco router

This chapter examines several types of network media, including twisted-pair cable, coaxial cable, fiber-optic cable, and wireless. It highlights the concepts and procedures for assembling and cabling Cisco routers. This chapter also covers cabling and connectors used to interconnect switches and routers in a LAN or WAN. Finally, it presents factors that you should consider when selecting network devices.

Cabling and Infrastructure

Media is the actual physical environment through which data travels as it moves from one component to another, and it connects network devices. The most common types of net-work media are twisted-pair cable, coaxial cable, fiber-optic cable, and wireless. Each media type has specific capabilities and serves specific purposes.

Understanding the types of connections that can be used within a network provides a better understanding of how networks function in transmitting data from one point to another.

Twisted-Pair Cable

Twisted-pair is a copper wire-based cable that can be either shielded or unshielded. Twisted- pair is the most common media for network connectivity.

Unshielded twisted-pair (UTP) cable, as shown in Figure 4-1, is a four-pair wire. Each of the eight individual copper wires in UTP cable is covered by an insulating material. In addition, the wires in each pair are twisted around each other. The advantage of UTP cable is its ability to cancel interference, because the twisted-wire pairs limit signal degradation from electromagnetic interference (EMI) and radio frequency interference (RFI). To further reduce crosstalk between the pairs in UTP cable, the number of twists in the wire pairs varies. UTP, as well as shielded twisted-pair (STP) cable, must follow precise specifications as to how many twists or braids are permitted per meter.

Figure 4-1 Unshielded Twisted-Pair Cable

UTP cable is used in a variety of networks. When used as a networking medium, UTP cable has four pairs of either 22- or 24-gauge copper wire. UTP used as a networking medium has an impedance of 100 ohms, differentiating it from other types of twisted-pair wiring such as that used for telephone wiring. Because UTP cable has an external diameter of approximately 0.43 cm (0.17 inches), its small size can be advantageous during installation. Also, because UTP can be used with most of the major networking architectures, it continues to grow in popularity.

Several categories of UTP cable exist:

• Category 1—Used for telephone communications; not suitable for transmitting data

• Category 2—Capable of transmitting data at speeds of up to 4 Mbps

• Category 3—Used in 10BASE-T networks; can transmit data at speeds up to 10 Mbps

• Category 4—Used in Token Ring networks; can transmit data at speeds up to 16 Mbps

• Category 5—Capable of transmitting data at speeds up to 100 Mbps

• Category 5e—Used in networks running at speeds up to 1000 Mbps (1 Gbps)

• Category 6—Consists of four pairs of 24-gauge copper wires that can transmit data at speeds up to 1000 Mbps

Shielded twisted-pair (STP) cable, as shown in Figure 4-2, combines the techniques of shielding and the twisting of wires to further protect against signal degradation. Each pair of wires is wrapped in a metallic foil. The four pairs of wires are then wrapped in an overall metallic braid or foil, usually 150-ohm cable. Specified for use in Ethernet network installations, STP reduces electrical noise both within the cable (pair-to-pair coupling, or crosstalk) and from outside the cable (EMI and RFI). Token Ring network topology uses STP.

Figure 4-2 Shielded Twisted-Pair Cable

When you consider using UTP and STP for your network media, consider the following:

• Speed of either media type is usually satisfactory for local-area distances.

• Both are the least-expensive media for data communication. UTP is less expensive than STP.

• Because most buildings are already wired with UTP, many transmission standards are adapted to use it to avoid costly rewiring with an alternative cable type.

Twisted-pair cabling is the most common networking cabling in use today; however, some networks still use older technologies like coaxial cable, as discussed in the next section.

Coaxial Cable

Coaxial cable consists of a hollow outer cylindrical conductor that surrounds a single inner wire conducting element. This section describes the characteristics and uses of coaxial cable.

As shown in Figure 4-3, the single inner wire located in the center of a coaxial cable is a copper conductor, surrounded by 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. An outer jacket covers this shield. The BNC connector shown looks much like a cable-television connector and connects to an older NIC with a BNC interface.

Figure 4-3 Coaxial Cable

Coaxial cable supports 10 to 100 Mbps and is relatively inexpensive, although more costly than UTP. Coaxial cable can be laid over longer distances than twisted-pair cable. For example, Ethernet can run approximately 100 meters using twisted-pair cable, but 500 meters using coaxial cable.

Coaxial cable offers several advantages for use in LANs. It can be run with fewer boosts from repeaters, which regenerate the signals in a network so that they can cover greater distances between network nodes than either STP or UTP cable. 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 you work with cable, consider its size. As the thickness, or diameter, of the cable increases, so does the difficulty in working with it. Cable must often be pulled through existing conduits and troughs that are limited in size. Coaxial cable comes in a variety of sizes. The largest diameter, frequently referred to as Thicknet, was specified for use as Ethernet backbone cable because historically it had greater transmission length and noise rejection characteristics. However, Thicknet cable can be too rigid to install easily in some environments because of its thickness. 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, and Thicknet cable is almost never used except for special-purpose installations, where shielding from EMI or distance requires the use of such cables.

In the past, coaxial cable with an outside diameter of only 0.35 cm, sometimes referred to as Thinnet, was used in Ethernet networks. It was especially useful for cable installations that required the cable to make many twists and turns. Because Thinnet was easier to install, it was also cheaper to install. Thus, it was also referred to as Cheapernet. However, because the outer copper or metallic braid in coaxial cable comprised half the electrical circuit, special care needed to be taken to ground it properly, by ensuring that a solid electrical connection existed at both ends of the cable. Installers frequently failed to make a good connection. Connection problems resulted in electrical noise, which interfered with signal transmission. For this reason, despite its small diameter, Thinnet is no longer commonly used in Ethernet networks.

Although coaxial cable offers some distance advantages over twisted-pair, the disadvantages far outweigh the benefits. If a communications signal needs to travel a greater distance at high rates of speed, it is more common to use fiber-optic cable.

Fiber-Optic Cable

Fiber-optic cable is a networking medium capable of conducting modulated light trans-mission. This section describes the types, characteristics, and uses of fiber-optic cable.

Fiber-optic cable used for networking consists of two fibers encased in separate sheaths. Viewing it in cross section in Figure 4-4, you can 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 that provides protection for the entire cable. The plastic conforms to appropriate fire and building codes. The purpose of the Kevlar is to furnish additional cushioning and protection for the fragile, hair-thin glass fibers. Where buried fiber-optic cables are required by codes, a stainless steel wire is sometimes included for added strength. Several connectors can connect fiber to the networking device; the most common is a SC connector, which has two optics, one connecting to transmit and the other connecting to receive.

Figure 4-4 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, and it allows the optical fiber to act like a light pipe, guiding light for long distances, even around bends. Fiber-optic cable is the most expensive of the three types discussed in this lesson, but it supports higher rate line speeds.

Fiber-optic cable does not carry electrical impulses as copper wire does. Instead, signals that represent bits are converted into pulses of light. Two types of fiber-optic cable exist:

• Single-mode—Single-mode fiber-optic cable allows only one mode (or wavelength) of light to propagate through the fiber. This type of cable is capable of higher band-width and greater distances than multimode and is often used for campus backbones. Single-mode cable uses lasers as the light-generating method and is more expensive than multimode cable. The maximum cable length of single-mode cable is 60+ km (37+ miles).

• Multimode—Multimode fiber-optic cable allows multiple modes of light to propa-gate through the fiber. Multimode cable is often used for workgroup applications, using light emitting diodes (LEDs) as light-generating devices. The maximum length of multimode cable is 2 km (1.2 miles).

The characteristics of the different media have a significant impact on the speed of data transfer. Although fiber-optic cable is more expensive, it is not susceptible to EMI and is capable of higher data rates than any of the other types of networking media discussed here. Fiber-optic cable is also more secure because it does not emit electrical signals that could be received by external devices.

In some instances, it might not be possible to run any type of cable for network communi-cations. This situation might be the case in a rented facility or in a location where you do not have the ability to install the appropriate infrastructure. In these cases, it might be useful to install a wireless network, as discussed in the next section.

Wireless Communications

Wireless networks are becoming increasingly popular, and they utilize a different type of technology. Wireless communication uses radio frequencies (RFs) or infrared waves to transmit data between devices on a LAN. For wireless LANs, a key component is the wireless hub, or access point, used for signal distribution. To receive the signals from the access point, a PC or laptop needs to install a wireless adapter card, or wireless network interface card (NIC). Figure 4-5 shows a number of wireless access points connected to an Ethernet backbone to provide access to the Internet.

Figure 4-5 Wireless Access Points

Wireless signals are electromagnetic waves that can travel through the vacuum of outer space and through a medium such as air. No physical medium is necessary for wireless signals, making them a versatile way to build a network. They use portions of the RF spectrum to transmit voice, video, and data. Wireless frequencies range from 3 kHz to 300 GHz. The data-transmission rates range from 9 kbps to 54 Mbps. Figure 4-6 shows the electromagnetic spectrum chart.

Figure 4-6 Electromagnetic Spectrum

You can differentiate electromagnetic waves by their frequency. Low-frequency electro-magnetic waves have a long wavelength (the distance from one peak to the next on the sine wave), while high-frequency electromagnetic waves have a short wavelength.

Some common applications of wireless data communication include the following:

• Accessing the Internet using a cellular phone

• Home or business Internet connection over satellite

• Beaming data between two handheld computing devices

• Wireless keyboard and mouse for the PC

Another common application of wireless data communication is the wireless LAN (WLAN), which is built in accordance with Institute of Electrical and Electronic Engineers (IEEE) 802.11 standards. WLANs typically use radio waves (for example, 902 MHz), microwaves (for example, 2.4 GHz), and infrared (IR) waves (for example, 820 nm) for communication. Wireless technologies are a crucial part of the future of networking.

Comparing Media Types

The choice of media type affects the type of network interface cards installed, the speed of the network, and the ability of the network to meet future needs. Table 4-1 compares the features of the common network media, including UTP, STP, coaxial cable, fiber-optic, and wireless connections.

Table 4-1 Comparing Media Types

The media you choose has an important impact on the network's capabilities. You should consider all the factors before making your final selection.