Network Media

Network Media: Network media is the actual path over which an electrical signal travels as it moves from one component to another. This chapter describes the common types of network media,including twisted-pair cable, coaxial cable, fiber-optic cable, and wireless.
Twisted-Pair Cable
Twisted-pair cable is a type of cabling that is used for telephone communications and most
modern Ethernet networks. A pair of wires forms a circuit that can transmit data. The pairs
are twisted to provide protection against crosstalk, the noise generated by adjacent pairs.
When electrical current flows through a wire, it creates a small, circular magnetic field
around the wire. When two wires in an electrical circuit are placed close together, their
magnetic fields are the exact opposite of each other. Thus, the two magnetic fields cancel
each other out. They also cancel out any outside magnetic fields. Twisting the wires can
enhance this cancellation effect. Using cancellation together with twisting the wires, cable
designers can effectively provide self-shielding for wire pairs within the network media.
Two basic types of twisted-pair cable exist: unshielded twisted pair (UTP) and shielded
twisted pair (STP). The following sections discuss UTP and STP cable in more detail.
UTP Cable
UTP cable is a medium that is composed of pairs of wires (see Figure 8-1). UTP cable is
used in a variety of networks. 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.

108 Chapter 8: Network Media Types

UTP cable relies solely on the cancellation effect produced by the twisted wire pairs to limit
signal degradation caused by 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 cable must follow precise specifications governing how
many twists or braids are permitted per meter (3.28 feet) of cable.
UTP cable often is installed using a Registered Jack 45 (RJ-45) connector (see Figure 8-2). The RJ-45 is an eight-wire connector used commonly to connect computers onto a local-area
network (LAN), especially Ethernets.

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; this differentiates it
from other types of twisted-pair wiring such as that used for telephone wiring, which has
impedance of 600 ohms.

UTP cable offers many advantages. Because UTP has an external diameter of approximately
0.43 cm (0.17 inches), its small size can be advantageous during installation. Because it has
such a small external diameter, UTP does not fill up wiring ducts as rapidly as other types of
cable. This can be an extremely important factor to consider, particularly when installing a
network in an older building. UTP cable is easy to install and is less expensive than other types
of networking media. In fact, UTP costs less per meter than any other type of LAN cabling. And
because UTP can be used with most of the major networking architectures, it continues to grow
in popularity.
Disadvantages also are involved in using twisted-pair cabling, however. UTP cable is more
prone to electrical noise and interference than other types of networking media, and the distance
between signal boosts is shorter for UTP than it is for coaxial and fiber-optic cables.

Twisted-Pair Cable 109
Although UTP was once considered to be slower at transmitting data than other types of cable,
this is no longer true. In fact, UTP is considered the fastest copper-based medium today. The
following summarizes the features of UTP cable:
• Speed and throughput—10 to 1000 Mbps
• Average cost per node—Least expensive
• Media and connector size—Small
• Maximum cable length—100 m (short)
Commonly used types of UTP cabling are as follows:
• Category 1—Used for telephone communications. Not suitable for transmitting data.
• Category 2—Capable of transmitting data at speeds up to 4 megabits per second (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—Can transmit data at speeds up to 100 Mbps.
• Category 5e —Used in networks running at speeds up to 1000 Mbps (1 gigabit per second
[Gbps]).
• Category 6—Typically, Category 6 cable consists of four pairs of 24 American Wire
Gauge (AWG) copper wires. Category 6 cable is currently the fastest standard for UTP.

Shielded Twisted-Pair Cable

Shielded twisted-pair (STP) cable combines the techniques of shielding, cancellation, and wire
twisting. Each pair of wires is wrapped in a metallic foil (see Figure 8-3). The four pairs of wires
then are wrapped in an overall metallic braid or foil, usually 150-ohm cable. As specified for
use in Ethernet network installations, STP reduces electrical noise both within the cable (pairto-
pair coupling, or crosstalk) and from outside the cable (EMI and RFI). STP usually is
installed with STP data connector, which is created especially for the STP cable. However, STP
cabling also can use the same RJ connectors that UTP uses.


110 Chapter 8: Network Media Types

Although STP prevents interference better than UTP, it is more expensive and difficult to install.
In addition, the metallic shielding must be grounded at both ends. If it is improperly grounded,
the shield acts like an antenna and picks up unwanted signals. Because of its cost and difficulty
with termination, STP is rarely used in Ethernet networks. STP is primarily used in Europe.
The following summarizes the features of STP cable:
• Speed and throughput—10 to 100 Mbps
• Average cost per node—Moderately expensive
• Media and connector size—Medium to large
• Maximum cable length—100 m (short)
When comparing UTP and STP, keep the following points in mind:
• The speed of both types of cable is usually satisfactory for local-area distances.
• These 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.

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.

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.

Chapter 8: Network Media Types

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.

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. Wherever buried fiber-optic
cables are required by codes, a stainless-steel wire sometimes is included for added strength.

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 of the four media discussed in this chapter, 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).

• 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.

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
Wireless Communication 115
Wireless Communication
Wireless communication uses radio frequencies (RF) or infrared (IR) waves to transmit data
between devices on a LAN.

Wireless Network

To receive the signals from the access point, a PC or laptop must install a wireless adapter card
(wireless NIC). Wireless signals are electromagnetic waves that can travel through the vacuum
of outer space and through a medium such as air. Therefore, no physical medium is necessary
for wireless signals, making them a very versatile way to build a network. Wireless signals use
portions of the RF spectrum to transmit voice, video, and data. Wireless frequencies range from
3 kilohertz (kHz) to 300 gigahertz (GHz). The data-transmission rates range from 9 kilobits per
second (kbps) to as high as 54 Mbps.
The primary difference between electromagnetic waves is their frequency. Low-frequency
electromagnetic 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
• Establishing a home or business Internet connection over satellite
• Beaming data between two hand-held computing devices
• Using a 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 Electronics Engineers (IEEE)
802.11 standards. WLANs typically use radio waves (for example, 902 megahertz [MHz]),
microwaves (for example, 2.4 GHz), and IR waves (for example, 820 nanometers [nm]) for
communication. Wireless technologies are a crucial part of the today’s networking. See Chapter
28, “Wireless LANs,” for a more detailed discuss on wireless networking.

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