CHAPTER 3
PHYSICAL LAYER
The physical layer (also called layer 1) is the physical connection between the computers and/or devices in
the network. This chapter examines how the physical layer operates. It describes the most commonly used
media for network circuits and explains the basic technical concepts of how data are actually transmitted
through the media. Three different types of transmission are described: digital transmission of digital
computer data, analog transmission of digital computer data, and digital transmission of analog voice
data. You do not need an engineering-level understanding of the topics to be an effective user and
manager of data communication applications. It is important, however, that you understand the basic
concepts, so this chapter is somewhat technical.
OBJECTIVES
Be familiar with the different types of network circuits and media
Understand the digital transmission of digital data
Understand the analog transmission of digital data
Understand the digital transmission of analog data
Be familiar with analog and digital modems
Be familiar with multiplexing
OUTLINE
3.1 Introduction
3.2 Circuits
3.2.1 Circuit Configuration
3.2.2 Data Flow
3.2.3 Multiplexing
3.3 Communication Media
3.3.1 Twisted-Pair Cable
3.3.2 Coaxial Cable
3.3.3 Fiber-Optic Cable
3.3.4 Radio
3.3.5 Microwave
3.3.6 Satellite
3.3.7 Media Selection
3.4 Digital Transmission of Digital Data
3.4.1 Coding
3.4.2 Transmission Modes
3.4.3 Digital Transmission
3.4.4 How Ethernet Transmits Data
3.5 Analog Transmission of Digital Data


3.5.1 Modulation
3.5.2 Capacity of a Circuit
3.5.3 How Modems Transmit Data
3.6 Digital Transmission of Analog Data
3.6.1 Translating from Analog to Digital
3.6.2 How Telephones Transmit Voice Data
3.6.3 How Instant Messenger Transmits Voice Data
3.6.4 Voice over Internet Protocol (VoIP)
3.7 Implications for Cyber Security
Summary
3.1 INTRODUCTION
This chapter examines how the physical layer operates. The physical layer is the network hardware
including servers, clients, and circuits, but in this chapter, we focus on the circuits and on how clients and
servers transmit data through them. The circuits are usually a combination of both physical media (e.g.,
cables, wireless transmissions) and special-purpose devices that enable the transmissions to travel
through the media. Special-purpose devices such as switches and routers are discussed in Chapters 6 and
8.
The word circuit has two very different meanings in networking, and sometimes it is hard to understand
which meaning is intended. Sometimes, we use the word circuit to refer to the physical circuit—the
actual wire—used to connect two devices. In this case, we are referring to the physical media that carry the
message we transmit, such as the twisted-pair wire used to connect a computer to the LAN in an office. In
other cases, we are referring to a logical circuit used to connect two devices, which refers to the
transmission characteristics of the connection, such as when we say a company has a T1 connection into
the Internet. In this case, T1 refers not to the physical media (i.e., what type of wire is used) but rather to
how fast data can be sent through the connection. Often, each physical circuit is also a logical circuit, but


sometimes it is possible to have one physical circuit—one wire—carry several separate logical circuits, or
to have one logical circuit travel over several physical circuits.
There are two fundamentally different types of data that can flow through the circuit: digital and analog.
Computers produce digital data that are binary, either on or off, 0 or 1. In contrast, telephones produce
analog data whose electrical signals are shaped like the sound waves they transfer; they can take on any
value in a wide range of possibilities, not just 0 or 1.
Data can be transmitted through a circuit in the same form they are produced. Most computers, for
example, transmit their digital data through digital circuits to printers and other attached devices.
Likewise, analog voice data can be transmitted through telephone networks in analog form. In general,
networks designed primarily to transmit digital computer data tend to use digital transmission, and
networks designed primarily to transmit analog voice data tend to use analog transmission (at least for
some parts of the transmission).
Data can be converted from one form into the other for transmission over network circuits. For example,
digital computer data can be transmitted over an analog telephone circuit by using a modem. A modem
at the sender’s computer translates the computer’s digital data into analog data that can be transmitted
through the voice communication circuits, and a second modem at the receiver’s end translates the analog
transmission back into digital data for use by the receiver’s computer.
Likewise, it is possible to translate analog voice data into digital form for transmission over digital
computer circuits using a device called a codec. Once again, there are two codecs, one at the sender’s end
and one at the receiver’s end. Why bother to translate voice into digital? The answer is that digital
transmission is “better” than analog transmission. Specifically, digital transmission offers five key benefits
over analog transmission:


Digital transmission produces fewer errors than analog transmission. Because the transmitted data
are binary (only two distinct values), it is easier to detect and correct errors.
Digital transmission permits higher maximum transmission rates. Fiber-optic cable, for example, is
designed for digital transmission.
Digital transmission is more efficient. It is possible to send more data through a given circuit using
digital rather than analog transmission.
Digital transmission is more secure because it is easier to encrypt.
Finally, and most importantly, integrating voice, video, and data on the same circuit is far simpler
with digital transmission.
For these reasons, most long-distance telephone circuits built by telephone companies and other common
carriers over the past decades use digital transmission. In the future, most transmissions (voice, data, and
video) will be sent digitally.
Regardless of whether digital or analog transmission is used, transmission requires the sender and
receiver to agree on two key parameters. First, they have to agree on the symbols that will be used: What
pattern of electricity, light, or radio wave will be used to represent a 0 and a 1. Once these symbols are set,
the sender and receiver have to agree on the symbol rate: How many symbols will be sent over the
circuit per second? Analog and digital transmissions are different, but both require a commonly agreed on
a set of symbols and a symbol rate.
In this chapter, we first describe the basic types of circuits and examine the different media used to build
circuits. Then we explain how data are actually sent through these media using digital and analog
transmissions.
3.2 CIRCUITS
3.2.1 Circuit Configuration
Circuit configuration is the basic physical layout of the circuit. There are two fundamental circuit
configurations: point-to-point and multipoint. In practice, most complex computer networks have many


configurations: point-to-point and multipoint. In practice, most complex computer networks have many
circuits, some of which are point-to-point and some of which are multipoint.
Figure 3-1 illustrates a point-to-point circuit, which is so named because it goes from one point to
another (e.g., one computer to another computer). These circuits sometimes are called dedicated circuits
because they are dedicated to the use of these two computers. This type of configuration is used when
computers generate enough data to fill the capacity of the communication circuit. When an organization
builds a network using point-to-point circuits, each computer has its own circuit running from itself to the
other computers. This can get very expensive, particularly if there is some distance between the
computers. Despite the cost, point-to-point circuits are used regularly in modern wired networks to
connect clients to switches, switches to switches and routers, and routers to routers. We will discuss in
detail these circuits in Chapter 7.
Figure 3-2 shows a multipoint circuit (also called a shared circuit). In this configuration, many
computers are connected to the same circuit. This means that each must share the circuit with the others.
The disadvantage is that only one computer can use the circuit at a time. When one computer is sending
or receiving data, all others must wait. The advantage of multipoint circuits is that they reduce the amount
of cable required and typically use the available communication circuit more efficiently. Imagine the
number of circuits that would be required if the network in Figure 3-2 was designed with separate point-
to-point circuits. For this reason, multipoint configurations are cheaper than point-to-point circuits. Thus,
multipoint circuits typically are used when each computer does not need to continuously use the entire
capacity of the circuit or when building point-to-point circuits are too expensive. Wireless circuits are


capacity of the circuit or when building point-to-point circuits are too expensive. Wireless circuits are
almost always multipoint circuits because multiple computers use the same radio frequencies and must
take turns transmitting.


FIGURE 3-1 Point-to-point circuit
FIGURE 3-2 Multipoint circuit


FIGURE 3-3 Simplex, half-duplex, and full-duplex transmissions
3.2.2 Data Flow
Circuits can be designed to permit data to flow in one direction or in both directions. Actually, there are
three ways to transmit: simplex, half-duplex, and full-duplex (Figure 3-3). Simplex transmission is
one-way transmission, such as that with radios and TVs.
Half-duplex transmission is a two-way transmission, but you can transmit in only one direction at a
time. A half-duplex communication link is similar to a walkie-talkie link; only one computer can transmit
at a time. Computers use control signals to negotiate that will send and that will receive data. The amount
of time half-duplex communication takes to switch between sending and receiving is called turnaround
time (also called retrain time or reclocking time). The turnaround time for a specific circuit can be
obtained from its technical specifications (often between 20 and 50 milliseconds). Europeans sometimes
use the term simplex circuit to mean a half-duplex circuit.
With full-duplex transmission, you can transmit in both directions simultaneously, with no
turnaround time.
How do you choose which data flow method to use? Obviously, one factor is the application. If data always
need to flow only in one direction (e.g., from a remote sensor to a host computer), then simplex is
probably the best choice. In most cases, however, data must flow in both directions.
The initial temptation is to presume that a full-duplex channel is best; however, each circuit has only so
much capacity to carry data. Creating a full-duplex circuit means that the circuit offers full capacity both
ways simultaneously. In some cases, it makes more sense to build a set of simplex circuits in the same way


a set of one-way streets can increase the speed of traffic. In other cases, a half-duplex circuit may work
best. For example, terminals connected to mainframes often transmit data to the host, wait for a reply,
transmit more data, and so on, in a turn-taking process; usually, traffic does not need to flow in both


directions simultaneously. Such a traffic pattern is ideally suited to half-duplex circuits.
3.2.3 Multiplexing
Multiplexing means to break one high-speed physical communication circuit into several lower-speed
logical circuits so that many different devices can simultaneously use it but still “think” that they have
their own separate circuits (the multiplexer is “transparent”). It is multiplexing without multiplexing, the
Internet would have collapsed in the 1990s.
FIGURE 3-4 Multiplexed circuit
Multiplexing often is done in multiples of 4 (e.g., 8, 16). Figure 3-4 shows a four-level multiplexed circuit.
Note that two multiplexers are needed for each circuit: one to combine the four original circuits into the
one multiplexed circuit and one to separate them back into the four separate circuits.
The primary benefit of multiplexing is to save money by reducing the amount of cable or the number of
network circuits that must be installed. For example, if we did not use multiplexers in Figure 3-4, we
would need to run four separate circuits from the clients to the server. If the clients were located close to
the server, this would be inexpensive. However, if they were located several miles away, the extra costs
could be substantial.
There are four types of multiplexing: frequency division multiplexing (FDM), time-division
multiplexing (TDM), statistical time-division multiplexing (STDM), and wavelength division
multiplexing (WDM).
FDM can be described as dividing the circuit “horizontally” so that many signals can travel a single
communication circuit simultaneously. The circuit is divided into a series of separate channels, each
transmitting on a different frequency, much like a series of different radio or TV stations. All signals


exist in the media at the same time, but because they are on different frequencies, they do not interfere
with each other.
TDM shares a communication circuit among two or more computers by having them take turns, dividing
the circuit vertically, so to speak.
STDM is the exception to the rule that the capacity of the multiplexed circuit must equal the sum of the
circuits it combines. STDM allows more terminals or computers to be connected to a circuit than does
FDM or TDM. If you have four computers connected to a multiplexer and each can transmit at 64 kbps,
then you should have a circuit capable of transmitting 256 kbps (4 × 64 kbps). However, not all computers
will be transmitting continuously at their maximum transmission speed. Users typically pause to read
their screens or spend time typing at lower speeds. Therefore, you do not need to provide a speed of 256
kbps on this multiplexed circuit. If you assume that only two computers will ever transmit at the same


time, 128 kbps will be enough. STDM is called statistical because the selection of transmission speed for
the multiplexed circuit is based on a statistical analysis of the usage requirements of the circuits to be
multiplexed.
WDM is a version of FDM used in fiber-optic cables. When fiber-optic cables were first developed, the
devices attached to them were designed to use only one color of light generated by a laser or LED.
Light has different frequencies (i.e., colors), so rather than building devices to transmit using only one
color, why not send multiple signals, each in a different frequency, through the same fiber-optic cable? By
simply attaching different devices that could transmit in the full spectrum of light rather than just one
frequency, the capacity of the existing fiber-optic cables could be dramatically increased, with no change
to the physical cables themselves.
MANAGEMENT FOCUS 3-1
Structured Cabling EIA/TIA 568-B
In 1995, the Telecommunications Industry Association (TIA) and Electronic Industries Alliance
(EIA) came up with the first standard to create structured cabling, called TIA/EIA 568-A. This
standard defined the minimum requirements for internal telecommunications wiring within
buildings and between buildings on one campus. This standard was updated and changed many
times, and today the accepted standard is TIA/EIA 568-B, which came out in 2002. This standard
has six subsystems:
1. Building entrance: the point where external cabling and wireless connects to the internal
building wiring and equipment room
2. Equipment room (ER): the room where network servers and telephone equipment would be
stored
3. Telecommunications closet: the room that contains the cable termination points and the
distribution frames
4. Backbone cabling: the cabling that interconnects telecommunication closets, equipment


4. Backbone cabling: the cabling that interconnects telecommunication closets, equipment
rooms, and building entrances within a building; also, this refers to cabling between buildings
5. Horizontal cabling: the cabling that runs from the telecommunications closet to each LAN
6. Work area: the cabling where the computers, printers, patch cables, jacks, and so on, are
located
This standard describes what the master cabling document should look like (which would describe
each of the six areas discussed previously) and applies for both twisted-pair and fiber-optic cabling.
MANAGEMENT FOCUS 3-2
Undersea Fiber-Optic Cables
Perhaps you were wondering what happens when you send an email from the United States to
Europe. How is your email transmitted from one continent to another? It most likely travels through
one of the submarine cables that connect America and Europe. A neat interactive submarine cable
map can be found at http://www. submarinecablemap.com/.
This map shows you each cable’s name, ready-for- service (RFS) date, length, owners, website (if
any), and landing points. Each cable on this map has a capacity of at least 5 Gbps.
Actually, the first submarine telecommunication cable was laid in the 1850s and carried telegraphy
traffic. Today, we use fiber-optic cable that carries phone, Internet, and private data as digital data.


So now you may ask yourself, how do these cables get laid on the seabed? Submarine cables are laid
using special cable-layer ships—these are factories that produce the cable on board and then have
equipment to lay and bury the cable. The cable-layer ships get as close as possible to the shore where
the cable will be connected. A messenger line is sent out from the ship using a workboat that takes it
to the shore.
Once the cable is secured onshore, the installation process under the sea can begin. A 30-ton sea
plow with the cable in it (think about a needle and thread) is then tossed overboard and lands on the
seabed. The plow then buries the cable under the sea bed at a required burial depth (up to 3 meters).
The simultaneous lay-and-bury of the cable continues until an agreed position, after which the cable
is surface laid until reaching its destination. Here is a video that illustrates this:
https://www.youtube.com/watch?v=Gsoo_BOwrrM
One technology that you may have come across that uses multiplexing is DSL. DSL stands for digital
subscriber line, and it allows for simultaneous transmission of voice (phone calls), data going to the
Internet (called upstream data), and data coming to your house from the Internet (called downstream
data). With DSL, a DSL modem is installed at the customer’s home or office, and another DSL modem is
installed at the telephone company switch closet. The modem is first an FDM device that splits the
physical circuit into three logical circuits (phone, upstream data, and downstream data). TDM is then
used within the two data channels to provide a set of one or more individual channels that can be used to
carry different data. A combination of amplitude and phase modulation is used in the data circuits to


carry different data. A combination of amplitude and phase modulation is used in the data circuits to
provide the desired data rate. You will learn more about DSL in Chapter 10.
3.3 COMMUNICATION MEDIA
The medium (or media, if there is more than one) is the physical matter or substance that carries the voice
or data transmission. Many different types of transmission media are currently in use, such as copper
(wire), glass or plastic (fiber-optic cable), or air (radio, microwave, or satellite). There are two basic types
of media. Guided media are those in which the message flows through a physical medium such as a
twisted-pair wire, coaxial cable, or fiber-optic cable; the medium “guides” the signal. Wireless media
are those in which the message is broadcast through the air, such as microwave or satellite.
In many cases, the circuits used in WANs are provided by the various common carriers who sell usage of
them to the public. We call the circuits sold by the common carrier communication services. Chapter 9
describes the specific services available in North America. The following sections describe the medium
and the basic characteristics of each circuit type, in the event you were establishing your own physical
network, whereas Chapter 9 describes how the circuits are packaged and marketed for purchase or lease
from a common carrier. If your organization has leased a circuit from a common carrier, you are probably
less interested in the media used and more interested in whether the speed, cost, and reliability of the
circuit meet your needs.
3.3.1 Twisted-Pair Cable
One of the most commonly used types of guided media is twisted-pair cable, insulated pairs of wires
that can be packed quite close together (Figure 3-5). The wires usually are twisted to minimize the


electromagnetic interference between one pair and any other pair in the bundle. Your house or apartment
probably has a set of two twisted-pair wires (i.e., four wires) from it to the telephone company network.
One pair is used to connect your telephone; the other pair is a spare that can be used for a second
telephone line. The twisted-pair cable used in LANs are usually packaged as four sets of pairs, as shown in
Figure 3-5, whereas bundles of several thousand wire pairs are placed under city streets and in large
buildings. The specific types of twisted-pair cable used in LANs, such as Cat 5e and Cat 6, are discussed in
Chapter 7.
3.3.2 Coaxial Cable
Coaxial cable is a type of guided medium that is quickly disappearing (Figure 3-6). Coaxial cable has a
copper core (the inner conductor) with an outer cylindrical shell for insulation. The outer shield, just


under the shell, is the second conductor. Because they have additional shielding provided by their
multiple layers of material, coaxial cables are less prone to interference and errors than basic low-cost
twisted-pair wires. Coaxial cables cost about three times as much as twisted-pair wires but offer few
additional benefits other than better shielding. One can also buy specially shielded twisted-pair wire that
provides the same level of quality as coaxial cable but at half its cost. For this reason, few companies are
installing coaxial cable today, although some still continue to use an existing coaxial cable that was
installed years ago.
FIGURE 3-5 Category 5e twisted-pair wire
FIGURE 3-6 Coaxial cables. Thinnet and Thicknet Ethernet cables (right)—(1) center core, (2) dielectric
insulator, (3) metallic shield, (4) plastic jacket and a cross-sectional view (left)
3.3.3 Fiber-Optic Cable
Although twisted pair is the most common type of guided medium, fiber-optic cable also is becoming
widely used. Instead of carrying telecommunication signals in the traditional electrical form, this
technology uses high-speed streams of light pulses from lasers or LEDs (light-emitting diodes) that carry
information inside hair-thin strands of glass called optical fibers. Figure 3-7 shows a fiber-optic cable and
depicts the optical core, the cladding (metal coating), and how light rays travel in optical fibers.
The earliest fiber-optic systems were multimode, meaning that the light could reflect inside the cable at
many different angles. Multimode cables are plagued by excessive signal weakening (attenuation) and


dispersion (spreading of the signal so that different parts of the signal arrive at different times at the
destination). For these reasons, early multimode fiber was usually limited to about 500 meters. Graded-
index multimode fiber attempts to reduce this problem by changing the refractive properties of the glass
fiber so that as the light approaches the outer edge of the fiber, it speeds up, which compensates for the
slightly longer distance it must travel compared with light in the center of the fiber. Therefore, the light in
the center is more likely to arrive at the same time as the light that has traveled at the edges of the fiber.
This increases the effective distance to just under 1,000 meters.
FIGURE 3-7 Fiber-optic cable
Single-mode fiber-optic cables transmit a single direct beam of light through a cable that ensures the light
reflects in only one pattern, in part because the core diameter has been reduced from 50 microns to about
5–10 microns. This smaller-diameter core allows the fiber to send a more concentrated light beam,
resulting in faster data transmission speeds and longer distances, often up to 100 kilometers. However,
because the light source must be perfectly aligned with the cable, single-mode products usually use lasers
(rather than the LEDs used in multimode systems) and therefore are more expensive.
Fiber-optic technology is a revolutionary departure from the traditional copper wires of twisted-pair cable
or coaxial cable. One of the main advantages of fiber optics is that it can carry huge amounts of
information at extremely fast data rates. This capacity makes it ideal for the simultaneous transmission of
voice, data, and image signals. In most cases, fiber-optic cable works better under harsh environmental
conditions than do its metallic counterparts. It is not as fragile or brittle, it is not as heavy or bulky, and it


is more resistant to corrosion. Also, in case of fire, an optical fiber can withstand higher temperatures
than can copper wire. Even when the outside jacket surrounding the optical fiber has melted, a fiber-optic
system still can be used.
3.3.4 Radio
One of the most commonly used forms of wireless media is radio; when people used the term wireless,
they usually mean radio transmission. When you connect your laptop into the network wirelessly, you
are using radio transmission. Radio data transmission uses the same basic principles as standard radio
transmission. Each device or computer on the network has a radio receiver/transmitter that uses a
specific frequency range that does not interfere with commercial radio stations. The transmitters are very
low power, designed to transmit a signal only a short distance, and are often built into portable computers
or handheld devices such as phones and personal digital assistants. Wireless technologies for LAN
environments, such as IEEE 802.1x, are discussed in more detail in Chapter 7.
MANAGEMENT FOCUS 3-3
Boingo Hot Spots Around the World


Perhaps you have come across Boingo while trying to find a wireless connection in an airport
between flights. Boingo is a wireless Internet service provider (WISP) that is different than many
free wifi connections that you can get at airports or coffee shops because it offers a secure connection
(specifically, a VPN or WPA service that can be configured on your device, but more about this in
Chapter 11). This secure connection is now offered in 7,000 U.S. locations and 13,000 international
locations and as in-flight wifi on some international carriers.
Their monthly rates start at $9.95 for mobile devices and $39 for global access for 4 devices and
2,000 minutes. Boingo also offers 1-, 2-, and 3-hour plans in case you don’t travel frequently and
don’t need a monthly subscription. To find Boingo hot spots, you need to download an app on your
phone or laptop, and the app will alert you if there is an available wifi connection in your area. The
app will even chart a graph that will show you signal strength in real-time.
Sources: Adapted from Boingo.com, cnet.com
3.3.5 Microwave
Microwave transmission is an extremely high-frequency radio communication beam that is
transmitted over a direct line-of-sight path between any two points. As its name implies, a microwave
signal is an extremely short wavelength, thus the word micro-wave. Microwave radio transmissions
perform the same functions as cables. For example, point A communicates with point B via a through-the-
air microwave transmission path, instead of a copper wire cable. Because microwave signals approach the
frequency of visible light waves, they exhibit many of the same characteristics as light waves, such as
reflection, focusing, or refraction. As with visible light waves, microwave signals can be focused into


narrow, powerful beams that can be projected over long distances. Just as a parabolic reflector focuses a
searchlight into a beam, a parabolic reflector also focuses a high-frequency microwave into a narrow
beam. Towers are used to elevate the radio antennas to account for the earth’s curvature and maintain a
clear line-of-sight path between the two parabolic reflectors (see Figure 3-8).
This transmission medium is typically used for long-distance data or voice transmission. It does not
require the laying of any cable, because long-distance antennas with microwave repeater stations can be
placed approximately 25–50 miles apart. A typical long-distance antenna might be 10 feet wide, although,
over shorter distances in the inner cities, the dish antennas can be less than 2 feet in diameter. The
airwaves in larger cities are becoming congested because so many microwave dish antennas have been
installed that they interfere with one another.
3.3.6 Satellite
Satellite transmission is similar to microwave transmission, except instead of transmission involving
another nearby microwave dish antenna, it involves a satellite many miles up in space. Figure 3-9 depicts
a geosynchronous satellite. Geosynchronous means that the satellite remains stationary over one point on
the earth. One disadvantage of satellite transmission is the propagation delay that occurs because the
signal has to travel out into space and back to earth, a distance of many miles that even at the speed of
light can be noticeable. Low earth orbit (LEO) satellites are placed in lower orbits to minimize
propagation delay. Satellite transmission is sometimes also affected by raindrop attenuation when
satellite transmissions are absorbed by heavy rain. It is not a major problem, but engineers need to work
around it.


FIGURE 3-8 A microwave tower. The round antennas are microwave antennas and the straight antennas
are cell phone antennas


FIGURE 3-9 Satellites in operation
MANAGEMENT FOCUS 3-4
Satellite Communications Improve Performance
Boyle Transportation hauls hazardous materials nation-wide for both commercial customers and the
government, particularly the U.S. Department of Defense. The Department of Defense recently
mandated that hazardous materials contractors use mobile communications systems with up-to-the-
minute monitoring when hauling the department’s hazardous cargoes.
After looking at the alternatives, Boyle realized that it would have to build its own system. Boyle
needed a relational database at its operations center that contained information about customers,
pickups, deliveries, truck location, and truck operating status. Data are distributed from this
database via satellite to an antenna on each truck. Now, at any time, Boyle can notify the designated
truck to make a new pickup via the bidirectional satellite link and record the truck’s
acknowledgment.
Each truck contains a mobile data terminal connected to the satellite network. Each driver uses a
keyboard to enter information, which transmits the location of the truck. These satellite data are
received by the main offices via a leased line from the satellite earth station. This system increased


productivity by an astounding 80% over 2 years; administration costs increased by only 20%.
Interested in finding out more about how satellite communication works? Watch this video:
https://www.youtube.com/watch?v=hXa3bTcIGPU
3.3.7 Media Selection
Which media are best? It is hard to say, particularly when manufacturers continue to improve various
media products. Several factors are important in selecting media.
The type of network is one major consideration. Some media are used only for WANs (microwaves
and satellite), whereas others typically are not (twisted pair, coaxial cable, and radio), although we
should note that some old WAN networks still use twisted-pair cable. Fiber-optic cable is unique in
that it can be used for virtually any type of network.
Cost is always a factor in any business decision. Costs are always changing as new technologies are
developed and competition among vendors drives prices down. Among the guided media, twisted-
pair wire is generally the cheapest, coaxial cable is somewhat more expensive, and fiber-optic cable is
the most expensive. The cost of wireless media is generally driven more by distance than any other
factor. For very short distances (several hundred meters), radio is the cheapest; for moderate
distances (several hundred miles), microwave is the cheapest; and for long distances, satellite is the
cheapest.
Transmission distance is a related factor. Twisted-pair wire coaxial cable and radio can transmit data
only a short distance before the signal must be regenerated. Twisted-pair wire and radio typically can
transmit up to 100–300 meters and coaxial cable typically between 200 and 500 meters. Fiber optics
can transmit up to 75 miles, and new types of fiber-optic cable can reach more than 600 miles.


Security is primarily determined by whether the media are guided or wireless. Wireless media (radio,
microwave, and satellite) are the least secure because their signals are easily intercepted. Guided
media (twisted pair, coaxial, and fiber optics) are more secure, with fiber optics being the most
secure.
Error rates are also important. Wireless media are most susceptible to interference and thus have the
highest error rates. Among the guided media, fiber optics provides the lowest error rates, coaxial
cable the next best, and twisted-pair cable the worst, although twisted-pair cable is generally better
than the wireless media.
Transmission speeds vary greatly among the different media. It is difficult to quote specific speeds for
different media because transmission speeds are constantly improving and because they vary within
the same type of media, depending on the specific type of cable and the vendor. In general, twisted-
pair cable and coaxial cable can provide data rates of between 1 Mbps (1 million bits per second) and 1
Gbps (1 billion bits per second), whereas fiber-optic cable ranges between 1 Gbps and 40 Gbps. Radio,
microwave, and satellite generally provide 10–100 Mbps.
3.4 DIGITAL TRANSMISSION OF DIGITAL DATA
All computer systems produce binary data. For these data to be understood by both the sender and
receiver, both must agree on a standard system for representing the letters, numbers, and symbols that
compose messages. The coding scheme is the language that computers use to represent data.
3.4.1 Coding
A character is a symbol that has a common, constant meaning. A character might be the letter A or B, or it
might be a number such as 1 or 2. Characters also may be special symbols such as ? or &. Characters in


data communications, as in computer systems, are represented by groups of bits that are binary zeros (0)
and ones (1). The groups of bits representing the set of characters that are the “alphabet” of any given
system are called a coding scheme, or simply a code.
A byte is a group of consecutive bits that is treated as a unit or character. One byte normally is composed


of 8 bits and usually represents one character; however, in data communications, some codes use 5, 6, 7,
8, or 9 bits to represent a character. For example, a representation of the character A by a group of 8 bits
(say, 01 000 001) is an example of coding.
There are three predominant coding schemes in use today. United States of America Standard Code for
Information Interchange (USASCII, or, more commonly, ASCII) is the most popular code for data
communications and is the standard code on most microcomputers. There are two types of ASCII; one is a
7-bit code that has 128 valid character combinations, and the other is an 8-bit code that has 256
combinations. The number of combinations can be determined by taking the number 2 and raising it to
the power equal to the number of bits in the code because each bit has two possible values, a 0 or a 1. In
this case 27 = 128 characters or 28 = 256 characters.
A second commonly used coding scheme is ISO 8859, which is standardized by the International
Standards Organization. ISO 8859 is an 8-bit code that includes the ASCII codes plus non-English letters
used by many European languages (e.g., letters with accents). If you look closely at Figure 2.21, you will
see that HTML often uses ISO 8859.
Unicode is the other commonly used coding scheme. There are many different versions of Unicode. UTF-
8 is an 8-bit version, which is very similar to ASCII. UTF-16, which uses 16 bits per character (i.e., 2 bytes,
called a “word”), is used by Windows. By using more bits, UTF-16 can represent many more characters


beyond the usual English or Latin characters, such as Cyrillic or Chinese.
We can choose any pattern of bits we like to represent any character we like, as long as all computers
understand what each bit pattern represents. Figure 3-10 shows the 8-bit binary bit patterns used to
represent a few of the characters we use in ASCII.


FIGURE 3-10 Binary numbers used to represent different characters using ASCII
3.4.2 Transmission Modes
Parallel transmission is the way the internal transfer of binary data takes place inside a computer. If
the internal structure of the computer is 8 bit, then all 8 bits of the data element are transferred between
the main memory and the central processing unit simultaneously on 8 separate connections. The same is
true of computers that use a 32-bit structure; all 32 bits are transferred simultaneously on 32 connections.
TECHNICAL FOCUS 3-1
BASIC ELECTRICITY
There are two general categories of electrical current: direct current and alternating current. Current
is the movement or flow of electrons, normally from positive (+) to negative (−). The plus (+) or
minus (−) measurements are known as polarity. Direct current (DC) travels in only one direction,


whereas alternating current (AC) travels first in one direction and then in the other direction.
A copper wire transmitting electricity acts like a hose transferring water. We use three common
terms when discussing electricity. Voltage is defined as electrical pressure—the amount of electrical
force pushing electrons through a circuit. In principle, it is the same as pounds per square inch in a
water pipe. Amperes (amps) are units of electrical flow or volume. This measure is analogous to
gallons per minute for water. The watt is the fundamental unit of electrical power. It is a rate unit,
not a quantity. You obtain the wattage by multiplying the volts by the amperes. Want to learn more?
Here is an introductory video about electricity: https://www.youtube.com/watch?v=EJe AuQ7pkpc.
Figure 3-11 shows how all 8 bits of one character could travel down a parallel communication circuit. The
circuit is physically made up of eight separate wires, wrapped in one outer coating. Each physical wire is
used to send 1 bit of the 8-bit character. However, as far as the user is concerned (and the network for that
matter), there is only one circuit; each of the wires inside the cable bundle simply connects to a different
part of the plug that connects the computer to the bundle of wire.
FIGURE 3-11 Parallel transmission of an 8-bit code
FIGURE 3-12 Serial transmission of an 8-bit code
Serial transmission means that a stream of data is sent over a communication circuit sequentially in a
bit-by-bit fashion, as shown in Figure 3-12. In this case, there is only one physical wire inside the bundle,
and all data must be transmitted over that one physical wire. The transmitting device sends one bit, then a


second bit, and so on, until all the bits are transmitted. It takes n iterations or cycles to transmit n bits.
Thus, serial transmission is considerably slower than parallel transmission—eight times slower in the case
of 8-bit ASCII (because there are 8 bits). Compare Figure 3-12 with Figure 3-11.
3.4.3 Digital Transmission
Digital transmission is the transmission of binary electrical or light pulses in that it only has two
possible states, a 1 or a 0. The most commonly encountered voltage levels range from a low of +3/−3 to a


high of +24/−24 volts. Digital signals are usually sent over wire of no more than a few thousand feet in
length.
All digital transmission techniques require a set of symbols (to define how to send a 1 and a 0) and the
symbol rate (how many symbols will be sent per second). Figure 3-13 shows five types of digital
transmission techniques. With unipolar signaling, the voltage is always positive or negative (like a DC
current). Figure 3-13 illustrates a unipolar technique in which a signal of 0 volts (no current) is used to
transmit a zero and a signal of +5 volts is used to transmit a 1.
An obvious question at this point is this: If 0 volts means a zero, how do you send no data? This is
discussed in detail in Chapter 4. For the moment, we will just say that there are ways to indicate when a
message starts and stops, and when there are no messages to send, the sender and receiver agree to ignore
any electrical signal on the line.
To successfully send and receive a message, both the sender and receiver have to agree on how often the
sender can transmit data—that is, on the symbol rate. For example, if the symbol rate on a circuit is 64
kilo Hertz (kHz) (64,000 symbols per second), then the sender changes the voltage on the circuit once
every 1/64,000 of a second and the receiver must examine the circuit every 1/64,000 of a second to read the
incoming data.
In bipolar signaling, the ones and zeros vary from a plus voltage to a minus voltage (like an AC current).
The first bipolar technique illustrated in Figure 3-13 is called nonreturn to zero (NRZ) because the voltage


alternates from +5 volts (a symbol indicating a 1) to −5 volts (a symbol indicating a 0) without ever
returning to 0 volts. The second bipolar technique in this figure is called return to zero (RZ) because it
always returns to 0 volts after each bit before going to +5 volts (the symbol for a 1) or −5 volts (the symbol
for a 0). The third bipolar technique is called alternate mark inversion (AMI) because a 0 is always sent
using 0 volts, but 1s alternate between +5 volts and −5 volts. AMI is used on T1 and T3 circuits. In Europe,
bipolar signaling sometimes is called double current signaling because you are moving between a positive
and negative voltage potential.


FIGURE 3-13 Unipolar, bipolar, and Manchester signals (digital)
In general, bipolar signaling experiences fewer errors than unipolar signaling because the symbols are
more distinct. Noise or interference on the transmission circuit is less likely to cause the bipolar’s +5 volts
to be misread as a −5 volts than it is to cause the unipolar’s 0 volts to be misread as a +5 volts. This is
because changing the polarity of a current (from positive to negative, or vice versa) is more difficult than
changing its magnitude.
3.4.4 How Ethernet Transmits Data
The most common technology used in LANs is Ethernet; if you are working in a computer lab on campus,
you are most likely using Ethernet. Ethernet uses digital transmission over either serial or parallel
circuits, depending on which version of Ethernet you use. One version of Ethernet that uses serial
transmission requires 1/10,000,000 of a second to send one symbol; that is, it transmits 10 million
symbols (each of 1 bit) per second. This gives a data rate of 10 Mbps, and if we assume that there are 8 bits


in each character, this means that about 1.25 million characters can be transmitted per second in the
circuit.
Ethernet uses Manchester encoding, which is a special type of bipolar signaling in which the signal is
changed from high to low or from low to high in the middle of the signal. A change from high to low is
used to represent a 0, whereas the opposite (a change from low to high) is used to represent a 1 (see Figure
3-13). Manchester encoding is less susceptible to having errors go undetected because if there is no
transition in midsignal, the receiver knows that an error must have occurred.
3.5 ANALOG TRANSMISSION OF DIGITAL DATA
Telephone networks were originally built for human speech rather than for data. They were designed to
transmit the electrical representation of sound waves, rather than the binary data used by computers.
There are many occasions when data need to be transmitted over a voice communications network. Many
people working at home still use a modem over their telephone line to connect to the Internet.
The telephone system (commonly called POTS for plain old telephone service) enables voice
communication between any two telephones within its network. The telephone converts the sound waves
produced by the human voice at the sending end into electrical signals for the telephone network. These
electrical signals travel through the network until they reach the other telephone and are converted back
into sound waves.
Analog transmission occurs when the signal sent over the transmission media continuously varies
from one state to another in a wave-like pattern much like the human voice. Modems translate the digital
binary data produced by computers into the analog signals required by voice transmission circuits. One
modem is used by the transmitter to produce the analog signals and a second by the receiver to translate
the analog signals back into digital signals.


the analog signals back into digital signals.
The sound waves transmitted through the voice circuit have three important characteristics (see Figure 3-
14). The first is the height of the wave, called amplitude. Amplitude is measured in decibels (dB). Our
ears detect amplitude as the loudness or volume of sound. Every sound wave has two parts, half above the
zero amplitude point (i.e., positive) and half below (i.e., negative), and both halves are always the same
height.
The second characteristic is the length of the wave, usually expressed as the number of waves per second,
or frequency. Frequency is expressed in hertz (Hz). Our ears detect frequency as the pitch of the sound.
Frequency is the inverse of the length of the sound wave so that a high frequency means that there are
many short waves in a 1-second interval, whereas a low frequency means that there are fewer (but longer)
waves in 1 second.
The third characteristic is the phase, which refers to the direction in which the wave begins. The phase is
measured in the number of degrees (° ). The wave in Figure 3-14 starts up and to the right, which is
defined as a 0° phase wave. Waves can also start down and to the right (a 180° phase wave), and in
virtually any other part of the sound wave.
3.5.1 Modulation
When we transmit data through the telephone lines, we use the shape of the sound waves we transmit (in
terms of amplitude, frequency, and phase) to represent different data values. We do this by transmitting a
simple sound wave through the circuit (called the carrier wave) and then changing its shape in different
ways to represent a 1 or a 0. Modulation is the technical term used to refer to these “shape changes.”


There are three fundamental modulation techniques: amplitude modulation (AM), frequency
modulation, and phase modulation. Once again, the sender and receiver have to agree on what symbols
will be used (what amplitude, frequency, and phase will represent a 1 and a 0) and on the symbol rate
(how many symbols will be sent per second).


FIGURE 3-14 Sound wave
FIGURE 3-15 Amplitude modulation
FIGURE 3-16 Frequency modulation
Basic Modulation With AM (also called amplitude shift keying [ASK]), the amplitude or height of
the wave is changed. One amplitude is the symbol defined to be 0, and another amplitude is the symbol
defined to be a 1. In the AM shown in Figure 3-15, the highest amplitude symbol (tallest wave) represents
a binary 1 and the lowest amplitude symbol represents a binary 0. In this case, when the sending device
wants to transmit a 1, it would send a high-amplitude wave (i.e., a loud signal). AM is more susceptible to
noise (more errors) during transmission than is frequency modulation (FM) or phase modulation.
FM (also called frequency shift keying [FSK]) is a modulation technique whereby each 0 or 1 is
represented by a number of waves per second (i.e., a different frequency). In this case, the amplitude does


not vary. One frequency (i.e., a certain number of waves per second) is the symbol defined to be a 1, and a
different frequency (a different number of waves per second) is the symbol defined to be a 0. In Figure 3-
16, the higher frequency wave symbol (more waves per time period) equals a binary 1, and the lower
frequency wave symbol equals a binary 0.
Phase modulation (PM) (also called phase-shift keying [PSK]) is the most difficult to understand.
Phase refers to the direction in which the wave begins. Until now, the waves we have shown start by
moving up and to the right (this is called a 0 ° phase wave). Waves can also start down and to the right.
This is called a phase of 180°. With phase modulation, one phase symbol is defined to be a 0 and the other
phase symbol is defined to be a 1. Figure 3-17 shows the case where a phase of 0° symbol is defined to be a
binary 0 and a phase of 180° symbol is defined to be a binary 1.
FIGURE 3-17 Phase modulation
FIGURE 3-18 Two-bit amplitude modulation
Sending Multiple Bits Simultaneously Each of the three basic modulation techniques (AM, FM, and
PM) can be refined to send more than 1 bit at one time. For example, basic AM sends 1 bit per wave (or
symbol) by defining two different amplitudes, one for a 1 and one for a 0. It is possible to send 2 bits on
one wave or symbol by defining four different amplitudes. Figure 3-18 shows the case where the highest-
amplitude wave is defined to be a symbol representing 2 bits, both 1s. The next highest amplitude is the


symbol defined tomean first a 1 and then a 0, and so on.
This technique could be further refined to send 3 bits at the same time by defining eight different symbols,
each with different amplitude levels or 4 bits by defining 16 symbols, each with different amplitude levels,
and so on. At some point, however, it becomes very difficult to differentiate between the different
amplitudes. The differences are so small that even a small amount of noise could destroy the signal.
This same approach can be used for FM and PM. Two bits could be sent on the same symbol by defining


four different frequencies, one for 11, one for 10, and so on, or by defining four phases (0°, 90°, 180°, and
270°). Three bits could be sent by defining symbols with eight frequencies or eight phases (0°, 45°, 90°,
135°, 180°, 225°, 270°, and 315°). These techniques are also subject to the same limitations as AM; as the
number of different frequencies or phases becomes larger, it becomes difficult to differentiate among
them.
It is also possible to combine modulation techniques—that is, to use AM, FM, and PM techniques on the
same circuit. For example, we could combine AM with four defined amplitudes (capable of sending 2 bits)
with FM with four defined frequencies (capable of sending 2 bits) to enable us to send 4 bits on the same
symbol.
One popular technique is quadrature amplitude modulation (QAM). QAM involves splitting the
symbol into eight different phases (3 bits) and two different amplitudes (1 bit), for a total of 16 different
possible values. Thus, one symbol in QAM can represent 4 bits, while 256-QAM sends 8 bits per symbol.
64-QAM and 256-QAM are commonly used in digital TV services and cable modem Internet services.
Bit Rate versus Baud Rate versus Symbol Rate The terms bit rate (i.e., the number bits per second
transmitted) and baud rate are used incorrectly much of the time. They often are used interchangeably,
but they are not the same. In reality, the network designer or network user is interested in bits per second
because it is the bits that are assembled into characters, characters into words and, thus, business
information.


information.
A bit is a unit of information. A baud is a unit of signaling speed used to indicate the number of times per
second the signal on the communication circuit changes. Because of the confusion over the term baud
rate among the general public, ITU-T now recommends the term baud rate be replaced by the term
symbol rate. The bit rate and the symbol rate (or baud rate) are the same only when 1 bit is sent on each
symbol. For example, if we use AM with two amplitudes, we send 1 bit on one symbol. Here, the bit rate
equals the symbol rate. However, if we use QAM, we can send 4 bits on every symbol; the bit rate would
be four times the symbol rate. If we used 64-QAM, the bit rate would be six times the symbol rate.
Virtually all of today’s modems send multiple bits per symbol.
3.5.2 Capacity of a Circuit
The data capacity of a circuit is the fastest rate at which you can send your data over the circuit in terms of
the number of bits per second. The data rate (or bit rate) is calculated by multiplying the number of bits
sent on each symbol by the maximum symbol rate. As we discussed in the previous section, the number of
bits per symbol depends on the modulation technique (e.g., QAM sends 4 bits per symbol).
The maximum symbol rate in any circuit depends on the bandwidth available and the signal-to-noise ratio
(the strength of the signal compared with the amount of noise in the circuit). The bandwidth is the
difference between the highest and the lowest frequencies in a band or set of frequencies. The range of
human hearing is between 20 and 14,000 Hz, so its bandwidth is 13,880 Hz. The maximum symbol rate


for analog transmission is usually the same as the bandwidth as measured in hertz. If the circuit is very
noisy, the maximum symbol rate may fall as low as 50% of the bandwidth. If the circuit has very little
noise, it is possible to transmit at rates up to the bandwidth.
Digital transmission symbol rates can reach as high as two times the bandwidth for techniques that have
only one voltage change per symbol (e.g., NRZ). For digital techniques that have two voltage changes per
symbol (e.g., RZ, Manchester), the maximum symbol rate is the same as the bandwidth.
Standard telephone lines provide a bandwidth of 4,000 Hz. Under perfect circumstances, the maximum
symbol rate is therefore about 4,000 symbols per second. If we were to use basic AM (1 bit per symbol),
the maximum data rate would be 4,000 bits per second (bps). If we were to use QAM (4 bits per
symbol), the maximum data rate would be 4 bits per symbol × 4,000 symbols per second = 16,000 bps. A
circuit with a 10 MHz bandwidth using 64-QAM could provide up to 60 Mbps.
3.5.3 How Modems Transmit Data
The modem (an acronym for modulator/demodulator) takes the digital data from a computer in the form
of electrical pulses and converts them into the analog signal that is needed for transmission over an
analog voice-grade circuit. There are many different types of modems available today from dial-up


modems to cable modems. For data to be transmitted between two computers using modems, both need
to use the same type of modem. Fortunately, several standards exist for modems, and any modem that
conforms to a standard can communicate with any other modem that conforms to the same standard.
A modem’s data transmission rate is the primary factor that determines the throughput rate of data, but it
is not the only factor. Data compression can increase the throughput of data over a communication link
by literally compressing the data. V.44, the ISO standard for data compression, uses Lempel–Ziv
encoding. As a message is being transmitted, Lempel–Ziv encoding builds a dictionary of two-, three-,
and four-character combinations that occur in the message. Anytime the same character pattern reoccurs
in the message, the index to the dictionary entry is transmitted rather than sending the actual data. The
reduction provided by V.44 compression depends on the actual data sent but usually averages about 6:1
(i.e., almost six times as much data can be sent per second using V.44 as without it).
3.6 DIGITAL TRANSMISSION OF ANALOG DATA
In the same way that digital computer data can be sent over analog telephone networks using analog
transmission, analog voice data can be sent over digital networks using digital transmission. This process
is somewhat similar to the analog transmission of digital data. A pair of special devices called codecs
(code/decode) is used in the same way that a pair of modems are used to translate the data to send across
the circuit. One codec is attached to the source of the signal (e.g., a telephone or the local loop at the end
office) and translates the incoming analog voice signal into a digital signal for transmission across the


office) and translates the incoming analog voice signal into a digital signal for transmission across the
digital circuit. A second codec at the receiver’s end translates the digital data back into analog data.
3.6.1 Translating from Analog to Digital
Analog voice data must first be translated into a series of binary digits before they can be transmitted over
a digital circuit. This is done by sampling the amplitude of the sound wave at regular intervals and
translating it into a binary number. Figure 3-19 shows an example where eight different amplitude levels
are used (i.e., each amplitude level is represented by 3 bits). The top diagram shows the original signal,
and the bottom diagram shows the digitized signal.
A quick glance will show that the digitized signal is only a rough approximation of the original signal. The
original signal had a smooth flow, but the digitized signal has jagged “steps.” The difference between the
two signals is called quantizing error. Voice transmissions using digitized signals that have a great deal
of quantizing error sound metallic or machinelike to the ear.
There are two ways to reduce quantizing error and improve the quality of the digitized signal, but neither
is without cost. The first method is to increase the number of amplitude levels. This minimizes the
difference between the levels (the “height” of the “steps”) and results in a smoother signal. In Figure 3-19,
we could define 16 amplitude levels instead of eight levels. This would require 4 bits (rather than the
current 3 bits) to represent the amplitude, thus increasing the amount of data needed to transmit the
digitized signal.
No amount of levels or bits will ever result in perfect-quality sound reproduction, but in general, 7 bits (27


= 128 levels) reproduces human speech adequately. Music, on the other hand, typically uses 16 bits (216 =
65,536 levels).
The second method is to sample more frequently. This will reduce the “length” of each “step,” also
resulting in a smoother signal. To obtain a reasonable-quality voice signal, one must sample at least twice
the highest possible frequency in the analog signal. You will recall that the highest frequency transmitted
in telephone circuits is 4,000 Hz. Thus, the methods used to digitize telephone voice transmissions must
sample the input voice signal at a minimum of 8,000 times per second. Sampling more frequently than
this (called oversampling) will improve signal quality. RealNetworks.com, which produces Real Audio
and other Web-based tools, sets its products to sample at 48,000 times per second to provide higher
quality. The iPod and most CDs sample at 44,100 times per second and use 16 bits per sample to produce
almost error-free music. Some other MP3 players sample less frequently and use fewer bits per sample to
produce smaller transmissions, but the sound quality may suffer.


FIGURE 3-19 Pulse amplitude modulation (PAM)
3.6.2 How Telephones Transmit Voice Data
When you make a telephone call, the telephone converts your analog voice data into a simple analog
signal and sends it down the circuit from your home to the telephone company’s network. This process is
almost unchanged from the one used by Bell when he invented the telephone in 1876. With the invention
of digital transmission, the common carriers (i.e., the telephone companies) began converting their voice
networks to use digital transmission. Today, all of the common carrier networks use digital transmission,
except in the local loop (sometimes called the last mile), the wires that run from your home or business
to the telephone switch that connects your local loop into the telephone network. This switch contains a
codec that converts the analog signal from your phone into a digital signal.This digital signal is then sent
through the telephone network until it hits the switch for the local loop for the person you are calling. This
switch uses its codec to convert the digital signal used inside the phone network back into the analog
signal needed by that person’s local loop and telephone (see Figure 3-20).
FIGURE 3-20 Pulse amplitude modulation (PAM)
There are many different combinations of sampling frequencies and numbers of bits per sample that
could be used. For example, one could sample 4,000 times per second using 128 amplitude levels (i.e., 7
bits) or sample at 16,000 times per second using 256 levels (i.e., 8 bits).
The North American telephone network uses pulse code modulation (PCM). With PCM, the input
voice signal is sampled 8,000 times per second. Each time the input voice signal is sampled, 8 bits are


generated. Therefore, the transmission speed on the digital circuit must be 64,000 bps (8 bits per sample
× 8,000 samples per second) to transmit a voice signal when it is in digital form. Thus, the North
American telephone network is built using millions of 64 kbps digital circuits that connect via codecs to
the millions of miles of analog local loop circuits into the users’ residences and businesses.


3.6.3 How Instant Messenger Transmits Voice Data
A 64 kbps digital circuit works very well for transmitting voice data because it provides very good quality.
The problem is that it requires a lot of capacity.
Adaptive differential pulse code modulation (ADPCM) is the alternative used by IM and many
other applications that provide voice services over lower-speed digital circuits. ADPCM works in much the
same way as PCM. It samples incoming voice signals 8,000 times per second and calculates the same 8-
bit amplitude value as PCM. However, instead of transmitting the 8-bit value, it transmits the difference
between the 8-bit value in the last time interval and the current 8-bit value (i.e., how the amplitude has
changed from one time period to another). Because analog voice signals change slowly, these changes can
be adequately represented by using only 4 bits. This means that ADPCM can be used on digital circuits
that provide only 32 kbps (4 bits per sample × 8,000 samples per second = 32,000 bps).
Several versions of ADPCM have been developed and standardized by the ITU-T. There are versions
designed for 8 kbps circuits (which send 1 bit 8,000 times per second) and 16 kbps circuits (which send 2
bits 8,000 times per second), as well as the original 32 kbps version. However, there is a trade-off here.
Although the 32 kbps version usually provides as good a sound quality as that of a traditional voice
telephone circuit, the 8 and 16 kbps versions provide poorer sound quality.
3.6.4 Voice over Internet Protocol (VoIP)
Voice over Internet Protocol (VoIP) (pronounced as “voyp”) is commonly used to transmit phone


conversations over digital networks. VoIP is a relatively new standard that uses digital telephones with
built-in codecs to convert analog voice data into digital data (see Figure 3-21). Because the codec is built
into the telephone, the telephone transmits digital data and therefore can be connected directly into a
local area network, in much the same manner as a typical computer. Because VoIP phones operate on the
same networks as computers, we can reduce the amount of wiring needed; with VoIP, we need to operate
and maintain only one network throughout our offices, rather than two separate networks—one for voice
and one for data. However, this also means that data networks with VoIP phones must be designed to
operate in emergencies (to enable 911 calls) even when the power fails; they must have uninterruptable
power supplies (UPS) for all network circuits.
One commonly used VoIP standard is G.722 wideband audio, which is a version of ADPCM that operates
at 64 kbps. It samples 8,000 times per second and produces 8 bits per sample. Because VoIP phones are
digital, they can also contain additional capabilities. For example, high-end VoIP phones often contain
computer chips to enable them to download and install small software applications so that they can
function in many ways like computers.


FIGURE 3-21 VoIP phone
3.7 IMPLICATIONS FOR CYBER SECURITY
While the physical layer (layer 1) may not seem very exciting at first sight, it offers a lot of possibilities to a
hacker to invade a computer or a network. Therefore, the physical layer must be protected just like the
application layer. We refer to this type of security as physical security. If physical security, access to an
organization’s hardware on which data is stored, is jeopardized, no firewall, encryption, or any other
security measures would be able to protect the organization.
Where does the problem with physical security originate? Laptops, USB drives, tablets, mobile devices,
you name it, have the ability to easily copy data to and from and therefore allow the stealing of sensitive
data to be very easy. USB drives, in particular, are very problematic. Many organizations disable USB
drives on computers because of the potential of stealing data. Or many times, employees have good
intentions to work on data at home, but it is very easy to lose or misplace a USB drive. Therefore, if you
need to use a USB drive and want to have sensitive data on it, always encrypt the data. Also, never pick up
a USB drive and plug it into your computer because it is one of the known ways how hackers get into a
computer. Hackers, and unfortunately also some commercial vendors who manufacture USBs, put
malware on USB drives with the purpose of stealing your data or your organization’s data.
In addition to devices that come and go from an organization, routers and servers are potential sources of
problems when it comes to physical security. These devices must be protected just like mobile devices. In


the movie Ocean’s Eleven, Daniel Ocean (played by George Clooney) hires professionals from all over the
country to steal $150 million from a safe in one of the casinos. Among these professionals is Livingston
Dell, who is an expert in communication systems. Livingston places a USB drive on one of the routers in
the casino’s server room and not only highjacks the 911 call but also is able to look over the shoulders of
the security personnel.
Physical security, just like security at all layers, should be a priority for organizations. An organization
needs to create a physical security plan, in addition to a cyber security plan, so that it can adequately
protect its most important asset, data. Once an organization allows and attacked to access its hardware,
there no security measure that will protect it.
SUMMARY
Circuits Networks can be configured so that there is a separate circuit from each client to the host
(called a point-to-point configuration) or so that several clients share the same circuit (a multipoint
configuration). Data can flow through the circuit in one direction only (simplex), in both directions
simultaneously (full-duplex), or by taking turns so that data sometimes flow in one direction and then
in the other (half-duplex). A multiplexer is a device that combines several simultaneous low-speed
circuits on one higher-speed circuit so that each low-speed circuit believes it has a separate circuit. In
general, the transmission capacity of the high-speed circuit must equal or exceed the sum of the low-
speed circuits.
Communication Media Media are either guided, in that they travel through a physical cable (e.g.,
twisted-pair wires, coaxial cable, or fiber-optic cable), or wireless, in that they are broadcast through
the air (e.g., radio, microwave, or satellite). Among the guided media, fiber-optic cable can transmit


data the fastest with the fewest errors and offers greater security but costs the most; twisted-pair wire
is the cheapest and most commonly used. The choice of wireless media depends more on distance
than on any other factor; radio is cheapest for short distances, microwave is cheapest for moderate
distances, and satellite is cheapest for long distances.
Digital Transmission of Digital Data Digital transmission (also called baseband transmission) is
done by sending a series of electrical (or light) pulses through the media. Digital transmission is
preferred to analog transmission because it produces fewer errors; is more efficient; permits higher
maximum transmission rates; is more secure; and simplifies the integration of voice, video, and data
on the same circuit. With unipolar digital transmission, the voltage changes between 0 volts to
represent a binary 0 and some positive value (e.g., +15 volts) to represent a binary 1. With bipolar
digital transmission, the voltage changes polarity (i.e., positive or negative) to represent a 1 or a 0.
Bipolar is less susceptible to errors. Ethernet uses Manchester encoding, which is a version of
unipolar transmission.
Analog Transmission of Digital Data Modems are used to translate the digital data produced by
computers into the analog signals for transmission in today’s voice communication circuits. Both the
sender and receiver need to have a modem. Data are transmitted by changing (or modulating) a
carrier sound wave’s amplitude (height), frequency (length), or phase (shape) to indicate a binary 1 or
0. For example, in AM, one amplitude is defined to be a 1 and another amplitude is defined to be a 0.
It is possible to send more than 1 bit on every symbol (or wave). For example, with AM, you could


send 2 bits on each wave by defining four amplitude levels. The capacity or maximum data rate that a
circuit can transmit is determined by multiplying the symbol rate (symbols per second) by the
number of bits per symbol. Generally (but not always), the symbol rate is the same as the bandwidth,
so bandwidth is often used as a measure of capacity. V.44 is a data compression standard that can be
combined with any of the foregoing types of modems to reduce the amount of data in the transmitted
signal by a factor of up to six. Thus, a V.92 modem using V.44 could provide an effective data rate of
5,600 × 6 = 336,000 bps.
Digital Transmission of Analog Data Because digital transmission is better, analog voice data
are sometimes converted to digital transmission. PCM is the most commonly used technique. PCM
samples the amplitude of the incoming voice signal 8,000 times per second and uses 8 bits to
represent the signal. PCM produces a reasonable approximation of the human voice, but more


sophisticated techniques are needed to adequately reproduce more complex sounds such as music.
KEY TERMS
adaptive differential pulse code modulation (ADPCM)
American Standard Code for Information Interchange (ASCII)
amplitude modulation (AM)
amplitude shift keying (ASK)
amplitude
analog transmission
bandwidth
baud rate
bipolar
bit rate
bits per second (bps)
carrier wave
circuit configuration
circuit
coaxial cable
codec
coding scheme
data compression
data rate
digital subscriber line
digital transmission
fiber-optic cable
frequency division multiplexing (FDM)
Frequency modulation (FM)
frequency shift keying (FSK)
frequency
full-duplex transmission
guided media
half-duplex transmission
ISO 8859
kilo Hertz (kHz)
Lempel–Ziv encoding
local loop
logical circuit
malware
Manchester encoding


microwave transmission
modem
multiplexing
multipoint circuit
parallel transmission
phase modulation (PM)
phase
phase-shift keying [PSK]
physical circuit
plain old telephone service (POTS)
plain old telephone service
point-to-point circuit
polarity
POTS
pulse code modulation (PCM)
quadrature amplitude modulation (QAM)
quantizing error
radio transmission
retrain time
satellite transmission
serial transmission
simplex transmission
statistical time-division multiplexing (STDM)
switch
symbol rate
time-division multiplexing (TDM)
turnaround time
twisted-pair cable
unicode
unipolar
USB drive
V.44
Voice over Internet Protocol (VoIP)
wavelength division multiplexing (WDM)
Wireless media
QUESTIONS
1. How does a multipoint circuit differ from a point-to-point circuit?
2. Describe the three types of data flows.


3. Describe the three types of guided media.
4. Describe the four types of wireless media.
5. How do analog data differ from digital data?
6. Clearly explain the differences between analog data, analog transmission, digital data, and digital
transmission.
7. Explain why most telephone company circuits are now digital.
8. What is coding?
9. Briefly describe three important coding schemes.
10. How are data transmitted in parallel?
11. What feature distinguishes serial mode from parallel mode?
12. How does bipolar signaling differ from unipolar signaling? Why is Manchester encoding more
popular than either?
13. What are the three important characteristics of a sound wave?
14. What is bandwidth? What is the bandwidth in a traditional North American telephone circuit?
15. Describe how data could be transmitted using amplitude modulation.
16. Describe how data could be transmitted using frequency modulation.
17. Describe how data could be transmitted using phase modulation.
18. Describe how data could be transmitted using a combination of modulation techniques.
19. Is the bit rate the same as the symbol rate? Explain.
20. What is a modem?
21. What is quadrature amplitude modulation (QAM)?
22. What is 64-QAM?
23. What factors affect transmission speed?
24. What is oversampling?
25. Why is data compression so useful?
26. What data compression standard uses Lempel–Ziv encoding? Describe how it works.
27. Explain how pulse code modulation (PCM) works.
28. What is quantizing error?
29. What is the term used to describe the placing of two or more signals on a single circuit?
30. What is the purpose of multiplexing?
31. How does DSL (digital subscriber line) work?


31. How does DSL (digital subscriber line) work?
32. Of the different types of multiplexing, what distinguishes (a) Frequency division multiplexing
(FDM)? (b) Time-division multiplexing (TDM)? (c) Statistical time-division multiplexing (STDM)?
(d) Wavelength division multiplexing (WDM)?
33. What is the function of inverse multiplexing (IMUX)?
34. If you were buying a multiplexer, would you choose TDM or FDM? Why?
35. Some experts argue that modems may soon become obsolete. Do you agree? Why or why not?
36. What is the maximum capacity of an analog circuit with a bandwidth of 4,000 Hz using QAM?
37. What is the maximum data rate of an analog circuit with a 10-MHz bandwidth using 64-QAM and
V.44?
38. What is the capacity of a digital circuit with a symbol rate of 10 MHz using Manchester encoding?


39. What is the symbol rate of a digital circuit providing 100 Mbps if it uses bipolar NRz signaling?
40. What is VoIP?
EXERCISES
A. Investigate the costs of dumb terminals, network computers, minimally equipped personal
computers, and top-of-the-line personal computers. Many equipment manufacturers and resellers
are on the Web, so it’s a good place to start looking.
B. Investigate the different types of cabling used in your organization and where they are used (e.g.,
LAN, backbone network).
C. Three terminals (T1, T2, T3) are to be connected to three computers (C1, C2, C3) so that T1 is connected
to C1, T2 to C2 and T3 to C3. All are in different cities. T1 and C1 are 1,500 miles apart, as are T2 and C2,
and T3 and C3. The points T1, T2, and T3 are 25 miles apart, and the points C1, C2, and C3 also are 25
miles apart. If telephone lines cost $1 per mile, what is the line cost for three?
D. Investigate different types of satellite communication services that are provided today.
E. Draw how the bit pattern 01101100 would be sent using
a. Single-bit AM
b. Single-bit FM
c. Single-bit PM
d. Two-bit AM (i.e., four amplitude levels)
e. Two-bit FM (i.e., four frequencies)
f. Two-bit PM (i.e., four different phases)
g. Single-bit AM combined with single-bit FM
h. Single-bit AM combined with single-bit PM
i. Two-bit AM combined with two-bit PM


i. Two-bit AM combined with two-bit PM
F. If you had to download a 20-page paper of 400 k (bytes) from your professor, approximately how
long would it take to transfer it over the following circuits? Assume that control characters add an
extra 10% to the message.
a. Dial-up modem at 33.6 kbps
b. Cable modem at 384 kbps
c. Cable modem at 1.5 Mbps
d. If the modem includes V.44 data compression with a 6:1 data compression ratio, what is the data
rate in bits per second you would actually see in choice c?
MINICASES
I. Eureka! (Part 1) Eureka! is a telephone- and Internet-based concierge service that specializes in
obtaining things that are hard to find (e.g., Super Bowl tickets, first-edition books from the 1500s,
Fabergé eggs). It currently employs 60 staff members who collectively provide 24-hour coverage
(over three shifts). They answer the phones and respond to requests entered on the Eureka! website.
Much of their work is spent on the phone and on computers searching on the Internet. The company
has just leased a new office building and is about to wire it. What media would you suggest the
company install in its office and why?
II. Eureka! (Part 2) Eureka! is a telephone- and Internet-based concierge service that specializes in
obtaining things that are hard to find (e.g., Super Bowl tickets, first-edition books from the 1500s,
Fabergé eggs). It currently employs 60 staff members who work 24 hours per day (over three shifts).


Staff answer the phone and respond to requests entered on the Eureka! website. Much of their work
is spent on the phone and on computers searching on the Internet. What type of connections should
Eureka! consider from its offices to the outside world, in terms of phone and Internet? Outline the
pros and cons of each alternative below and make a recommendation. The company has three
alternatives:
1. Should the company use standard voice lines but use DSL for its data ($40 per month per line for
both services)?
2. Should the company separate its voice and data needs, using standard analog services for voice
but finding some advanced digital transmission services for data ($40 per month for each voice
line and $300 per month for a circuit with 1.5 Mbps of data)?
3. Should the company search for all digital services for both voice and data ($60 per month for an
all-digital circuit that provides two phone lines that can be used for two voice calls, one voice call
and one data call at 64 kbps, or one data call at 128 kbps)?
III. Eureka! (Part 3) Eureka! is a telephone- and Internet-based concierge service that specializes in
obtaining things that are hard to find (e.g., Super Bowl tickets, first-edition books from the 1500s,
Fabergé eggs). It currently employees 60 staff members who work 24 hours per day (over three
shifts). Staff members answer phone calls and respond to requests entered on the Eureka! website.
Currently, each staff member has a desktop PC with two monitors and a twisted pair connection
(Cat5e) that offers speeds up to 100 Mbps. Some employees made a suggestion to the CEO of Eureka!
to upgrade their connection to a fiber-optic cable that can provide speeds up to 1 Gbps. What do you


think about this idea? How easy (difficult) is it to change wiring from twisted pair to fiber optic? Can
we use the same network cards in the PCs, or do we need to change them? How much would this
change cost?
IV. (Speedy Package) Speedy Package is a same-day package delivery service that operates in Chicago.
Each package has a shipping label that is attached to the package and is also electronically scanned
and entered into Speedy’s data network when the package is picked up and when it is delivered. The
electronic labels are transmitted via a device that operates on a cell phone network. (1) Assuming that
each label is 1,000 bytes long, how long does it take to transmit one label over the cell network,
assuming that the cell phone network operates at 144 kbps (144,000 bits per second) and that there
are 8 bits in a byte? and (2) if speedy were to upgrade to the new, faster digital phone network that
transmits data at 200 kbps (200,000 bits per second), how long would it take to transmit a label?
V. (Boingo) Reread Management Focus 3.2. What other alternatives can travelers consider? How is
Boingo different from other companies offering hot spots, such as T-Mobile or AT&T?
TECH UPDATES
In every chapter, we will offer a couple of ideas to investigate.
Topic A: Satellite Communication
You decided to take a cruise during your spring break, but know that you will need to connect to the
internet to check your grade in this class. Being in the middle of the sea will not allow you to use your
“land” Internet connection, you will have to use satellite communication. Who are the current satellite
communication vendors? How does this technology work? Which providers are used by the military?


communication vendors? How does this technology work? Which providers are used by the military?
Investigate the different providers, their services, infrastructure, and cost structure.
Topic B: Fiber Optic Cables
Imagine that you are calling your relative who lives in Australia. You can connect using a variety of
applications (WhatsApp, Facebook, Skype, just to name a few). You can make a video call because of the
existing network of undersea fiber-optic cables. Is there a map of this network? What’s inside an under
the sea fiber optic wire? How do they lay these wires? Who are the main vendors? Explore this topic, and
compile your research.


Deliverables
Your job will be to prepare a presentation/video (7–10 minutes long) that addresses the topic and
provides information on the following items:
1. Title Slide
2. Short description of the topic
3. Main players—these could be people, processes, software, hardware, etc.
4. How it works—use a lot of pictures and be as technical as possible; create this part as a tutorial so that
your audience can follow along
5. How does it relate to material covered in class so far (and in the future)
6. Additional material/books/links where to learn more about this topic
7. Credits
8. List of References
9. Memo addressed to your professor describing all of the above information
HANDS-ON ACTIVITY 3A
Looking Inside Your Cable
One of the most commonly used types of local network cable is Category 5 unshielded twisted-pair cable,
commonly called “Cat 5.” Cat 5 (and an enhanced version called Cat 5e) are used in Ethernet LANs. If you
have installed a LAN in your house or apartment, you probably used Cat 5 or Cat 5e.
Figure 3-22 shows a picture of a typical Cat 5 cable. Each end of the cable has a connector called an RJ-45
connector that enables the cable to be plugged into a computer or network device. If you look closely at
the connector, you will see there are eight separate “pins.” You might think that this would mean the Cat 5
can transmit data in parallel, but it doesn’t do this. Cat 5 is used for serial transmission.
FIGURE 3-22 Cat 5 cable


FIGURE 3-23 Inside a Cat 5 cable
If you have an old Cat 5 cable (or are willing to spend a few dollars to buy cheap cable), it is simple to take
the connector off. Simply take a pair of scissors and cut through the cable a few inches from the connector.
Figure 3-23 shows the same Cat 5 cable with the connector cut off. You can see why twisted pair is called
twisted pair: a single Cat 5 cable contains four separate sets of twisted-pair wires for a total of eight wires.
Unfortunately, this picture is in black and white so it is hard to see the different colors of the eight wires
inside the cable. Figure 3-24 lists the different colors of the wires and what they are used for under the
EIA/TIA 568B standard (the less common 568A standard uses the pins in different ways). One pair of
wires (connected to pins 1 and 2) is used to transmit data from your computer into the network. When
your computer transmits, it sends the same data on both wires; pin 1 (transmit+) transmits the data
normally and pin 2 (transmit−) transmits the same data with reversed polarity. This way, if an error
occurs, the hardware will likely detect a different signal on the two cables. For example, if there is a
sudden burst of electricity with a positive polarity (or a negative polarity), it will change only one of the
transmissions from negative to positive (or vice versa) and leave the other transmission unchanged.
Electrical pulses generate a magnetic field that has very bad side effects on the other wires. To minimize
this, the two transmit wires are twisted together so that the other wires in the cable receive both a positive
and a negative polarity magnetic field from the wires twisted around each other, which cancel each other


and a negative polarity magnetic field from the wires twisted around each other, which cancel each other
out.
Figure 3-24 also shows a separate pair of wires for receiving transmissions from the network (pin 3
[receive+] and pin 6 [receive−]). These wires work exactly the same way as transmit+ and transmit− but
are used by the network to send data to your computer. You’ll notice that they are also twisted together in
one pair of wires, even though they are not side by side on the connector.
Figure 3-24 shows the pin functions from the viewpoint of your computer. If you think about it, you’ll
quickly realize that the pin functions at the network end of the cable are reversed; that is, pin 1 is receive+
because it is the wire that the network uses to receive the transmit+ signal from your computer. Likewise,
pin 6 at the network end is the transmit− wire because it is the wire on which your computer receives the
reversed data signal.
The separate set of wires for transmitting and receiving means that Cat 5 is designed for full-duplex
transmission. It can send and receive at the same time because one set of wires is used for sending data
and one set is used for receiving data. However, Cat 5 is not often used this way. Most hardware that uses
Cat 5 is designed to operate in a half-duplex mode, even though the cable itself is capable of full-duplex.


FIGURE 3-24 Pin connection for Cat 5 at the computer end
You’ll also notice that the other four wires in the cable are not used. Yes, that’s right; they are simply
wasted.
Deliverable
Find a Cat 5 or Cat 5e cable and record what color wires are used for each pin.
HANDS-ON ACTIVITY 3B
Making MP3 Files
MP3 files are good examples of analog-to-digital conversion. It is simple to take an analog signal—such as
your voice— and convert it into a digital file for transmission or playback. In this activity, we will show you
how to record your voice and see how different levels of digital quality affect the sound.
First, you need to download a sound editor and MP3 converter. One very good sound editor is Audacity—
and it’s free. Go to audacity.sourceforge.net and download and install the audacity software. You will also
need the plug-in called LAME (an MP3 encoder), which is also free and available at lame.sourceforge.net.
Use Audacity to record music or your voice (you can use a cheap microphone). Audacity records in very
high quality, but will produce MP3 files in whatever quality level you choose.
Once you have the file recorded, you can edit the Preferences to change the File Format to use in saving
the MP3 file. Audacity/LAME offers a wide range of qualities. Try recording at least three different quality
levels. For example, for high quality, you could use 320 kbps, which means the recording uses 320 kbps of
data per second. In other words, the number of samples per second times the number of bits per sample
equals 320 kbps. For regular quality, you could use 128 kbps. For low quality, you could use 16 kbps.


Create each of these files and listen to them to hear the differences in quality produced by the quantizing
error. The differences should be most noticeable for music. A recording at 24 kbps is often adequate for
voice, but music will require a better quality encoding.
Deliverable
1. Produce three MP3 files of the same music or voice recording at three different quality levels.
2. List the size of each file.


3. Listen to each file and describe the quality differences you hear (if any).
HANDS-ON ACTIVITY 3C
Making a Cat 5e Patch Cable
A patch cable is a cable that runs a short distance (usually less than 10 feet) that connects a device into a
wall jack, a patch panel jack, or a device. If you have a desktop computer, you’re using a patch cable to
connect it into your Ethernet LAN. Patch cables are relatively inexpensive (usually $10 or less), but
compared to the cost of their materials, they are expensive (the materials usually cost less than $1).
Because it is relatively easy to make a patch cable, many companies make their own in order to save
money.
To make your own patch cable, you will need a crimper, some Cat 5e cable, two RJ45 connectors, and a
cable tester (optional) (see Figure 3-25).
1. Using the cutter on the crimping tool, cut the desired length of Cat 5e cable.
2. Insert the end of the cable into the stripper and gently press on the cable while rotating it to remove
the outer insulation of the cable. Be careful not to cut the twisted pairs inside. After removing the
outer insulation, visually inspect the twisted pairs for damage. Do this on both ends of your cable. If
any of the cables are damaged, you need to cut them and start over.
3. Untwist the twisted pairs and straighten them. Once they are straightened, put them into this order:
orange-white, orange, green-white, blue, blue-white, green, brown-white, brown.
4. Hold the cable in your right hand; the orange-white wire should be closest to you. Hold the RJ45
connector in your left hand with the little “handle” on the bottom.


connector in your left hand with the little “handle” on the bottom.
FIGURE 3-25 Tools and materials for making a patch cable
5. Insert the wires inside the connector all the way to the end—you should be able to see the colors of


the wires when you look at the front of the connector. Make sure that the wires don’t change order.
The white insulation should be about 1/3 of the way inside the connector. (If you used the stripper on
the tool properly, the length of the wires will be exactly as needed to fit the RJ45 connector.)
6. Now you are ready to crimp the connector. Insert the RJ45 connector to the crimper and press really
hard. This will push the gold contacts on the connector onto the twisted pairs.
7. Crimp the other end of the cable by repeating steps 4 through 7.
8. The final step is to test your cable. Turn on the cable tester and insert both ends of the patch cable
into the tester. If you see the flashing light going down the indicators 1 through 8, not skipping any
number or changing the order, you made a fully functional patch cable. If you don’t have a cable
tester, you can use the cable to connect your computer into an Ethernet LAN. If you’re able to use the
LAN, the cable is working.
Deliverable
A working patch cable.


CHAPTER 4
DATA LINK LAYER
The data link layer (also called layer 2) is responsible for moving a message from one computer or
network device to the next computer or network device in the overall path from the sender or receiver. It
controls the way messages are sent on the physical media. Both the sender and receiver have to agree on
the rules, or protocols, that govern how they will communicate with each other. A data link protocol
determines who can transmit at what time, where a message begins and ends, and how a receiver
recognizes and corrects a transmission error. In this chapter, we discuss these processes as well as several
important sources of errors.
OBJECTIVES
Understand the role of the data link layer
Become familiar with two basic approaches to controlling access to the media
Become familiar with common sources of error and their prevention
Understand three common error detection and correction methods
Become familiar with several commonly used data link protocols
OUTLINE
4.1 Introduction
4.2 Media Access Control
4.2.1 Contention
4.2.2 Controlled Access
4.2.3 Relative Performance
4.3 Error Control
4.3.1 Sources of Errors
4.3.2 Error Prevention
4.3.3 Error Detection
4.3.4 Error Correction via Retransmission
4.3.5 Forward Error Correction
4.3.6 Error Control in Practice
4.4 Data Link Protocols
4.4.1 Asynchronous Transmission
4.4.2 Synchronous Transmission
4.5 Transmission Efficiency
4.6 Implications for Cyber Security
Summary
4.1 INTRODUCTION
In Chapter 1, we introduced the concept of layers in data communications. The data link layer sits between


the physical layer (hardware such as the circuits, computers, and multiplexers described in Chapter 3) and
the network layer (which performs addressing and routing, as described in Chapter 5).
The data link layer is responsible for sending and receiving messages to and from other computers. Its job
is to reliably move a message from one computer over one circuit to the next computer where the message
needs to go.
The data link layer performs two main functions and therefore is often divided into two sublayers. The
first sublayer (called the logical link control [LLC] sublayer) is the data link layer’s connection to the
network layer above it. At the sending computer, the LLC sublayer software is responsible for
communicating with the network layer software (e.g., Internet Protocol (IP)) and for taking the network
layer Protocol Data Unit (PDU)—usually an IP packet—and surrounding it with a data link layer PDU—
often an Ethernet frame. At the receiving computer, the LLC sublayer software removes the data link layer
PDU and passes the message it contains (usually an IP packet) to the network layer software.
The second sublayer (called the media access control [MAC] sublayer) controls the physical
hardware. The MAC sublayer software at the sending computer controls how and when the physical layer
converts bits into the physical symbols that are sent down the circuit. At the receiving computer, the MAC
sublayer software takes the data link layer PDU from the LLC sublayer, converts it into a stream of bits,
and controls when the physical layer actually transmits the bits over the circuit. At the receiving
computer, the MAC sublayer receives a stream of bits from the physical layer and translates it into a
coherent PDU, ensures that no errors have occurred in transmission, and passes the data link layer PDU
to the LLC sublayer.


to the LLC sublayer.
Both the sender and receiver have to agree on the rules or protocols that govern how their data link layers
will communicate with each other. A data link protocol performs three functions:
Controls when computers transmit (media access control)
Detects and corrects transmission errors (error control)
Identifies the start and end of a message by using a PDU (message delineation)
4.2 MEDIA ACCESS CONTROL
Media access control refers to the need to control when computers transmit. With point-to-point full-
duplex configurations, media access control is unnecessary because there are only two computers on the
circuit, and full-duplex permits either computer to transmit at any time.
Media access control becomes important when several computers share the same communication circuit,
such as a point-to-point configuration with a half-duplex configuration that requires computers to take
turns or a multipoint configuration in which several computers share the same circuit. Here, it is critical
to ensure that no two computers attempt to transmit data at the same time—but if they do, there must be
a way to recover from the problem. There are two fundamental approaches to media access control:
contention and controlled access.
4.2.1 Contention
With contention, computers wait until the circuit is free (i.e., no other computers are transmitting) and
then transmit whenever they have data to send. Contention is commonly used in Ethernet—Local Area
Networks (LANs).
As an analogy, suppose that you are talking with some friends. People listen, and if no one is talking, they
can talk. If you want to say something, you wait until the speaker is done and then you try to talk. Usually,
people yield to the first person who jumps in at the precise moment the previous speaker stops.


people yield to the first person who jumps in at the precise moment the previous speaker stops.
Sometimes, two people attempt to talk at the same time, so there must be some technique to continue the
conversation after such a verbal collision occurs.
4.2.2 Controlled Access
With controlled access, one device controls the circuit and determines which clients can transmit at
what time. There are two commonly used controlled access techniques: access requests and polling.


With the access request technique, client computers that want to transmit send a request to transmit to
the device that is controlling the circuit (e.g., the wireless access point). The controlling device grants
permission for one computer at a time to transmit. When one computer has permission to transmit, all
other computers wait until that computer has finished, and then, if they have something to transmit, they
use a contention technique to send an access request.
The access request technique is like a classroom situation in which the instructor calls on the students
who raise their hands. The instructor acts as the controlling access point. When they want to talk,
students raise their hands and the instructor recognizes them so they can contribute. When they have
finished, the instructor again takes charge and allows someone else to talk. And of course, just like in a
classroom, the wireless access point can choose to transmit whenever it likes.
Polling is the process of sending a signal to a client computer that permits it to transmit. With polling,
the clients store all messages that need to be transmitted. Periodically, the controlling device (e.g., a
wireless access point) polls the client to see if it has data to send. If the client has data to send, it does so.
If the client has no data to send, it responds negatively, and the controller asks another client if it has data
to send.
There are several types of polling. With roll-call polling, the controller works consecutively through a
list of clients, first polling client 1, then client 2, and so on, until all are polled. Roll-call polling can be
modified to select clients in priority so that some get polled more often than others. For example, one