Networking Explained, 2nd Edition

Chapter 4: Physical Layer Concepts 161

attenuation rises sharply as the signal frequency increases, whereas with coaxial cable, it

rises less sharply as frequency increases. Fiber-optic cable, which is tuned for a specific

wavelength, exhibits very low attenuation per unit of distance at that wavelength. Attenua-

tion is measured in decibels (dB) of signal loss. When selecting cable, you should choose

a type that has a low measure of attenuation for the network speeds and distances

involved. Signal quality is affected most by the combination of attenuation and capaci-

tance.This is illustrated in Figure 4.15.

55. So of the different wired network media mentioned thus far, there really are only

two broad categories: copper and fiber. Is this correct?

Yes. These two categories can then be partitioned into more specific types. For exam-

ple, copper cable includes unshielded twisted-pair (UTP), shielded twisted-pair (STP),

IBM cable, and coaxial cable. Fiber-optic cable includes both glass and plastic fiber. There

also is single mode fiber and multimode fiber.

56. Now that I am aware of the common physical and electrical characteristics of

copper and fiber cables, I would like to know some specific information about

each type as they relate to networks. Let’s start with twisted-pair media.

OK. Twisted-pair cable is probably the most popular type of cable used in networks

today. It works with all different types of networks. The name, twisted-pair, comes from

how the cable is constructed. Twisted-pair cable consists of at least two insulated copper

wires that have been twisted together. Data transmission requires four wires (two pairs):

one pair to transmit data and one pair to receive data. UTP cable used in data networks has

a twist in the cable about every six inches (approximately 15 cm).

57. What organization develops standards for twisted-pair cable?

Standards for UTP and STP are provided by the Electronic Industries Association and

the Telecommunications Industry Association (EIA/TIA). These organizations jointly

developed the EIA/TIA-568 standard, which is a North American standard used world-

wide. EIA/TIA-568 specifies the type of cable that is permitted for a given speed, the type

of connectors that can be used for a given cable, and the network topology that is permit-

ted when installing cables. The standard also defines the performance specifications that

Signal received

distorted and at a

lesser amplitude

Clear and strong

signal transmitted

at a certain level

(amplitude)

FIGURE 4.15 The combined effects of capacitance and attenuation result in a signal that is received

distorted and weaker than what it was when transmitted. This can severely impact the performance

of a network. Source: Adapted from Leeds & Chorey, 1991a.