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Analogies That Describe Digital Bandwidth 59
representing massive amounts of information flowing back and forth across the
globe in seconds or less. In a sense, it might be appropriate to say that the Inter-
net is bandwidth.
■ The demand for bandwidth is ever-increasing—As soon as new network technol-
ogies and infrastructures are built to provide greater bandwidth, new applications
are created to take advantage of the greater capacity. The delivery over the network
of rich media content, including streaming video and audio, requires tremendous
amounts of bandwidth. IP telephony systems are now commonly installed in
place of traditional voice systems, adding further to the need for bandwidth. The
successful networking professional must anticipate the need for increased band-
width and plan accordingly.
Analogies That Describe Digital Bandwidth
The idea that information flows suggests two analogies that might make it easier to
visualize bandwidth in a network. Because both water and traffic are said to flow,
consider the following:
■ Bandwidth is like the width of a pipe, as shown in Figure 2-13—A network of
pipes brings fresh water to homes and businesses and carries wastewater away.
This water network is made up of pipes with different diameters. A city’s main
water pipe might be 2 meters in diameter, whereas a kitchen faucet might have
a diameter of only 2 centimeters. The width of the pipe determines the pipe’s
water-carrying capacity. Thus, the water is analogous to data, and pipe width is
analogous to bandwidth. Many networking experts say they need to “put in big-
ger pipes” when they want to add more information-carrying capacity.
■ Bandwidth is like the number of lanes on a highway, as shown in Figure 2-14—
A network of roads serves every city or town. Large highways with many traffic
lanes are joined by smaller roads with fewer traffic lanes. These roads lead to
even smaller, narrower roads, and eventually to the driveways of homes and
businesses. When very few automobiles use the highway system, each vehicle can
move freely. When more traffic is added, each vehicle moves more slowly, espe-
cially on roads with fewer lanes for the cars to occupy. Eventually, as even more

traffic enters the highway system, even multilane highways become congested
and slow. A data network is much like the highway system, with data packets
analogous to automobiles, and bandwidth analogous to the number of lanes on
the highway. When a data network is viewed as a system of highways, it is easy
to see how low-bandwidth connections can cause traffic to become congested all
over the network.
1102.book Page 59 Tuesday, May 20, 2003 2:53 PM
60 Chapter 2: Networking Fundamentals
Figure 2-13 Pipe Analogy for Bandwidth
Figure 2-14 Highway Analogy for Bandwidth
Network devices are like pumps, valves, fittings and taps
Packets are like water
Bandwidth is like pipe width
1 Lane
Unpaved Road
2 Lane Road
2 Lane
Divided Highway
8 Lane
Superhighway
Networking Devices Are Like On Ramps, Traffic Signals, Signs, Maps, and Police
Packets Are Like Vehicles
STOP
1102.book Page 60 Tuesday, May 20, 2003 2:53 PM
Analogies That Describe Digital Bandwidth 61
Keep in mind that the true, actual meaning of bandwidth, in this context, is the maxi-
mum number of bits that theoretically can pass through a given area of space in a spec-
ified amount of time (under the given conditions). These analogies are only to make it
easier to understand the concept of bandwidth.
Digital Bandwidth Measurements

In digital systems, the basic unit of bandwidth is bits per second (bps). Bandwidth is
the measure of how much information, or bits, can flow from one place to another in a
given amount of time, or seconds. Although bandwidth can be described in bits per
second, usually some multiple of bits per second is used. In other words, network
bandwidth is typically described as thousands of bits per second, millions of bits per
second, and even billions of bits per second.
Although the terms bandwidth and speed are often used interchangeably, they are not
exactly the same thing. You might say, for example, that a T3 connection at 45 mega-
bits per second (Mbps) operates at a higher speed than a T1 connection at 1.544 Mbps.
However, if only a small amount of their data-carrying capacity is being used, each of
these connection types carries data at roughly the same speed, just as a small amount
of water flows at the same rate through a small pipe as through a large pipe. Therefore,
it is usually more accurate to say that a T3 connection has greater bandwidth than a
T1, because it can carry more information in the same period of time, not because it
has a higher speed.
Table 2-2 summarizes the various units of bandwidth.
Bandwidth Limitations
Bandwidth varies depending on the type of medium as well as the LAN and WAN
technologies used. The physics of the medium account for some of the difference. Physi-
cal differences in the ways signals travel through twisted-pair copper wire, coaxial cable,
optical fiber, and even air result in fundamental limitations on the information-carrying
Table 2-2 Units of Bandwidth
Unit of Bandwidth Abbreviation Equivalent
Bits per second bps 1 bps = fundamental unit of bandwidth
Kilobits per second kbps 1 kbps = 1000 bps = 10
3
bps
Megabits per second Mbps 1 Mbps = 1,000,000 bps = 10
6
bps

Gigabits per second Gbps 1 Gbps = 1,000,000,000 bps = 10
9
bps
1102.book Page 61 Tuesday, May 20, 2003 2:53 PM
62 Chapter 2: Networking Fundamentals
capacity of a given medium. However, a network’s actual bandwidth is determined by
a combination of the physical medium and the technologies chosen for signaling and
detecting network signals.
For example, current understanding of the physics of unshielded twisted-pair (UTP)
copper cable puts the theoretical bandwidth limit at more than 1 Gbps. But in actual
practice, the bandwidth is determined by the use of a particular technology, such as
10BASE-T, 100BASE-TX, or 1000BASE-TX Ethernet. Bandwidth is also determined
by other varying factors, such as the number of users in the network, the equipment
being used, applications, the amount of broadcast, and so on. In other words, the
actual bandwidth is determined not by the medium’s limitations, but by the signaling
methods, NICs, and other items of network equipment that are chosen.
Table 2-3 lists some common networking media types, along with their limits on dis-
tance and bandwidth.
Table 2-3 Maximum Bandwidths and Length Limitations
Medium
Maximum
Theoretical
Bandwidth
Maximum
Physical Distance
50-ohm coaxial cable
(10BASE2 Ethernet, Thinnet)
10 Mbps 185 m
50-ohm coaxial cable
(10BASE5 Ethernet, Thicknet)

10 Mbps 500 m
Category 5 UTP
(10BASE-T Ethernet)
10 Mbps 100 m
Category 5 UTP
(100BASE-TX Ethernet)
100 Mbps 100 m
Category 5 UTP
(1000BASE-TX Ethernet)
1000 Mbps 100 m
Multimode optical fiber (62.5/125 µm)
(100BASE-FX Ethernet)
100 Mbps 2000 m
Multimode optical fiber (62.5/125 µm)
(1000BASE-SX Ethernet)
1000 Mbps 220 m
1102.book Page 62 Tuesday, May 20, 2003 2:53 PM
Analogies That Describe Digital Bandwidth 63
Table 2-4 summarizes common WAN services and the bandwidth associated with each.
Data Throughput
Bandwidth is the measure of the amount of information that can move through the
network in a given period of time. Therefore, the amount of available bandwidth is a
Multimode optical fiber (50/125 µm)
(1000BASE-SX Ethernet)
1000 Mbps 550 m
Single-mode optical fiber (9/125 µm)
(1000BASE-LX Ethernet)
1000 Mbps 5000 m
Table 2-4 WAN Services and Bandwidths
WAN Service Typical User Bandwidth

Modem Individuals 56 kbps = 0.056 Mbps
DSL Individuals, telecommuters,
and small businesses
12 kbps to 6.1 Mbps = 0.128
Mbps to 6.1 Mbps
ISDN Telecommuters and small
businesses
128 kbps = 0.128 Mbps
Frame Relay Small institutions (schools)
and medium-sized businesses
56 kbps to 44.736 Mbps (U.S.)
or 34.368 Mbps (Europe) =
0.056 Mbps to 44.736 Mbps
(U.S.) or 34.368 Mbps (Europe)
T1 Larger entities 1.544 Mbps
T3 Larger entities 44.736 Mbps
STS-1 (OC-1) Phone companies, data-
comm company backbones
51.840 Mbps
STS-3 (OC-3) Phone companies, data-
comm company backbones
155.251 Mbps
STS-48 (OC-48) Phone companies, data-
comm company backbones
2.488 Gbps
Table 2-3 Maximum Bandwidths and Length Limitations (Continued)
Medium
Maximum
Theoretical
Bandwidth

Maximum
Physical Distance
1102.book Page 63 Tuesday, May 20, 2003 2:53 PM
64 Chapter 2: Networking Fundamentals
critical part of the network’s specification. A typical LAN might be built to provide
100 Mbps to every desktop workstation, but this does not mean that each user can
actually move one hundred megabits of data through the network for every second of
use. This is true only under the most ideal circumstances. The concept of throughput
can help explain why this is so.
Throughput refers to actual measured bandwidth at a specific time of day, using spe-
cific Internet routes, and while a specific set of data is transmitted on the network.
Unfortunately, for many reasons, throughput is often far less than the maximum possi-
ble digital bandwidth of the medium that is being used. The following are some of the
factors that determine throughput:
■ Internetworking devices
■ Type of data being transferred
■ Network topology
■ Number of users on the network
■ User’s computer
■ Server computer
■ Power conditions
■ Congestion
A network’s theoretical bandwidth is an important consideration in network design,
because network bandwidth is never greater than the limits imposed by the chosen
medium and networking technologies. Figure 2-15 lists some of the variables that
affect throughput. However, it is just as important for a network designer and admin-
istrator to consider the factors that might affect actual throughput. By measuring
throughput on a regular basis, a network administrator will be aware of changes in
network performance and changes in the needs of network users. The network can
then be adjusted accordingly.

Data Transfer Calculation
Network designers and administrators are often called on to make decisions regarding
bandwidth. Should the size of the WAN connection be increased to accommodate a
new database? Is the current LAN backbone of sufficient bandwidth for a streaming-
video training program? The answers to questions like these are not always easy to
find, but one place to start is with a simple data transfer calculation.
Using the formula T = S / BW (transfer time = size of file / bandwidth) lets a network
administrator estimate several of the important components of network performance.
If the typical file size for a given application is known, dividing the file size by the net-
work bandwidth yields an estimate of the fastest time that the file can be transferred.
1102.book Page 64 Tuesday, May 20, 2003 2:53 PM
Analogies That Describe Digital Bandwidth 65
Figure 2-15 Throughput Variables
Two important points should be considered when doing this calculation:
■ The result is an estimate only, because the file size does not include any overhead
added by the process that takes place to prepare data to be transferred over the
network. This process is called encapsulation. Encapsulation is covered in more
detail in later chapters.
■ The result is likely to be a best-case transfer time, because available bandwidth
is almost never at the theoretical maximum for the network type. (A more accu-
rate estimate can be attained if throughput is substituted for bandwidth in the
equation.)
Although the data transfer calculation is quite simple, it can be tricky if you are not
careful to use the same units throughout the equation. In other words, if the band-
width is measured in Mbps, the file size must be in megabits (Mb), not megabytes
(MB). Because file sizes are typically given in megabytes, you might need to multiply
the number of megabytes by 8 to convert to megabits.
Try to answer the following question using the formula T = S / BW. (Be sure to convert
units of measurement as necessary.)
Would it take less time to send the contents of a floppy disk full of data (1.44 MB)

over an ISDN line or to send the contents of a 10-GB hard drive full of data over
an OC-48 line?
Figure 2-16 summarizes a simple formula for file transfer time calculations.
Your PC (Client)
The Server
Other Users on Your LAN
Routing Within the "Cloud"
The Design (Topology) of All Networks Involved
Type of Data Being Transferred
Time of Day
Why?
Throughput <= Digital Bandwidth of a Medium
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66 Chapter 2: Networking Fundamentals
Figure 2-16 File Transfer Time Calculation
Digital Bandwidth Versus Analog Bandwidth
Until recently, radio, television, and telephone transmissions were sent through the air
and over wires using electromagnetic waves. These waves are called analog because
they have the same shapes as the light and sound waves produced by the transmitters.
As light and sound waves change size and shape, the electrical signal that carries the
transmission changes proportionately. In other words, the electromagnetic waves are
analogous to the light and sound waves.
Analog bandwidth is measured by how much of the electromagnetic spectrum is occu-
pied by each signal. The basic unit of analog bandwidth is hertz (Hz), or cycles per sec-
ond. Typically, multiples of this basic unit of analog bandwidth are used, just as with
digital bandwidth. Units of measurement that are commonly seen are kilohertz (kHz),
megahertz (MHz), and gigahertz (GHz). These are the units used to describe the band-
width of cordless telephones (which usually operate at either 900 MHz or 2.4 GHz).
802.11a and 802.11b wireless network frequencies usually operate at 5 GHz and
2.4 GHz.

Although analog signals can carry a variety of information, they have some significant
disadvantages compared to digital transmissions. The analog video signal that requires
a wide frequency range for transmission cannot be squeezed into a smaller band. There-
fore, if the necessary analog bandwidth is unavailable, the signal cannot be sent. The
same goes for digital bandwidth; however, it is less common for digital bandwidth to
experience this because of its much-larger bandwidth capabilities.
In digital signaling, all information is sent as bits, regardless of the kind of information
it is. Voice, video, and data all become streams of bits when they are prepared for trans-
mission over digital media, which gives digital bandwidth an important advantage over
analog bandwidth. Unlimited amounts of information can be sent over the smallest
(lowest-bandwidth) digital channel. Regardless of how long it takes, when the digital
1102.book Page 66 Tuesday, May 20, 2003 2:53 PM
Networking Models 67
information arrives at its destination and is reassembled, it can be viewed, listened to,
read, or processed in its original form.
It is important to understand the differences and similarities between digital and analog
bandwidth. Both types of bandwidth are regularly encountered in the field of informa-
tion technology. However, because this course is concerned primarily with digital net-
working, the term bandwidth refers to digital bandwidth.
Networking Models
Learning about networking is easier when you start with theory and concepts and then
move on to the more concrete aspects of implementation. As a network professional,
you need to learn the theory of how networks communicate before you design, build,
and maintain networks. Learning the concept of layers can help you understand the
action that occurs during communication from one computer to another.
This section covers the concepts of layering and how it applies specifically to commu-
nication models. Two specific models, OSI and TCP/IP, are described, as well as peer-
to-peer layer communication and encapsulation.
Using Layers to Analyze Problems in a Flow of Materials
The concept of layers helps you understand the action that occurs during communica-

tion from one computer to another. The following questions involve the movement of
physical objects, such as highway traffic or electronic data:
■ What is flowing?
■ What are the different forms of the object that is flowing?
■ What rules govern flow?
■ Where does the flow occur?
This motion of objects, whether physical or logical, is called flow. Layers help describe
the details of the flow process. Examples of systems that flow are the public water sys-
tem, the highway system, the postal system, and the telephone system.
Now examine Table 2-5. What network is being examined? What is flowing? What are
the different forms of the object that is flowing? What are the rules for flow? Where
does the flow occur? The networks listed in this chart give analogies to help you under-
stand data flow in computer networks.
1102.book Page 67 Tuesday, May 20, 2003 2:53 PM
68 Chapter 2: Networking Fundamentals
The network communication process is complex. The data, in the form of electronic
signals, must travel across media to the correct destination computer and then be con-
verted back into its original form to be read by the recipient. Several steps are involved
in this process. For this reason, the most efficient way to implement network commu-
nications is a layered process. In a layered communication process, each layer per-
forms a specific task.
In the next few sections, you’ll see how the network communication process is broken
down using a layered model. You’ll also see how data is sent over the network and
how it reaches its intended destination. As you learn about the network communica-
tion process, it is important for you to understand the various steps, components, and
protocols of the network communication process. This understanding provides you
with valuable troubleshooting information when the communications process does not
proceed smoothly.
Using Layers to Describe Data Communication
The difficulty in dealing with network communications is that it is a very complex pro-

cess. It would be extremely difficult for someone to understand this process if he or she
looked only at network communication as a whole. The solution to this issue was to
break down the total network communication system into a series of layers. Each layer
Table 2-5 Network Comparisons
Network
What Is
Flowing Different Forms Rules Where
Water Water Hot, cold,
drinkable,
wastewater/
sewer
Access rules
(turning taps),
flushing, not put-
ting certain things
in drains
Pipes
Highway Vehicles Trucks, cars,
cycles
Traffic laws and
common courtesy
Roads and
highways
Postal Objects Letters (written
information),
packages
Rules for packag-
ing and attaching
postage
Postal service

boxes, offices,
trucks, planes,
delivery people
Telephone Information Spoken
languages
Rules for access-
ing phone and
rules for politeness
Phone system
wires, EM waves,
and so on
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