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CHAPTER
3
PHYSICAL LAYER
Physical
Layer
Fundamental Concepts
Network Technologies
Application Layer
LAN WLAN
Backbone
Transport Layer
Network Layer
Data Link Layer
Physical Layer
WAN
Internet
M
k
Network Management
The Three Faces of Networking
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CHAPTER OUTLINE
T
HE 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 is actually transmitted through the media. Four different
types of transmission are described: digital transmission of digital computer data; analog
transmission of digital computer data; digital transmission of analog voice data; and
combined analog–digital transmission of digital 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 digital transmission of digital data
■
Understand analog transmission of digital data
■
Understand digital transmission of analog data
■
Be familiar with analog and digital modems
■
Be familiar with multiplexing
■
CHAPTER OUTLINE
INTRODUCTION
CIRCUITS
Circuit Configuration
Data Flow
Multiplexing
COMMUNICATION MEDIA
Guided Media
Wireless Media
Media Selection
DIGITAL TRANSMISSION OF DIGITAL DATA
Coding
Transmission Modes
Digital Transmission
How Ethernet Transmits Data
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CHAPTER 3
PHYSICAL LAYER
ANALOG TRANSMISSION OF DIGITAL DATA
Modulation
Capacity of a Circuit
How Modems Transmit Data
DIGITAL TRANSMISSION OF ANALOG DATA
Translating from Analog to Digital
How Telephones Transmit Voice Data
How Instant Messenger Transmits Voice Data
IMPLICATIONS FOR MANAGEMENT
SUMMARY
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 cir-
cuits 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 repeaters are discussed in more detail in Chapter 4, whereas de-
vices such as hubs, switches, and routers are discussed in Chapter 6 and 7.
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 carries 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 trans-
mission 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.
1
Often, each physical circuit is also a logical circuit, but as you will see in the section on
multiplexing, sometimes it is possible to have one physical circuit—one wire—carry
several separate logical circuits and vice versa: have one logical circuit travel over sev-
eral 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
1
Don’t worry about what a T1 circuit is at this point. All you need to understand is that a T1 circuit is a specific
type of circuit with certain characteristics, the same way we might describe gasoline as being unleaded or pre-
mium. We will discuss T1 circuits in Chapter 9.
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CIRCUITS
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 com-
puter data tend to use digital transmission, and networks designed primarily to transmit ana-
log 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 cir-
cuits, 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 transmis-
sion 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 transmis-
sion. Specifically, digital transmission offers five key benefits over analog transmission:
• Digital transmission produces fewer errors than analog transmission. Because the
transmitted data is 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 the 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.
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 is actually sent through these
media using digital and analog transmission.
CIRCUITS
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
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CHAPTER 3
PHYSICAL LAYER
Circuits
Modem
Modem
Host
computer
Client
computer
Point-to-point configuration.
FIGURE 3.1
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 configuration,
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 the computers generate enough data to fill the capac-
ity 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.
Figure 3.2 shows a
multipoint configuration
(also called a
shared circuit
). In this
configuration, many computers are connected on the same circuit. This means that each
must share the circuit with the others, much like a party line in telephone communications.
The disadvantage is that only one computer can use the circuit at a time. When one com-
puter 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 communica-
tion 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 configurations. Thus, multipoint
configurations 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 is too expensive.
Server
Client
computer
Client
computer
Client
computer
Client
computer
Multipoint configuration.
FIGURE 3.2
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