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
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.

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