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Overview
Bandwidth is a crucial component in networking. Bandwidth
decisions are among the most important when a network is designed.
This module discusses the importance of bandwidth, explains how it
is calculated, and how it is measured.
Functions of networking are described using layered models. This
module covers the two most important models, which are the Open
System Interconnection (OSI) model and the Transmission Control
Protocol/Internet Protocol (TCP/IP) model. The module also presents
the differences and similarities between the two models.
In addition, this module presents a brief history of networking. It also
describes network devices, as well as cabling, physical, and logical
layouts. This module also defines and compares LANs, MANs, WANs,
SANs, and VPNs.
Students completing this module should be able to:
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Explain the importance of bandwidth in networking.
Use an analogy from their experience to explain bandwidth.
Identify bps, kbps, Mbps, and Gbps as units of bandwidth.
Explain the difference between bandwidth and throughput.
Calculate data transfer rates.
Explain why layered models are used to describe data
communication.
Explain the development of the Open System Interconnection
model (OSI).
List the advantages of a layered approach.
Identify each of the seven layers of the OSI model.
Identify the four layers of the TCP/IP model.
Describe the similarities and differences between the two models.
Briefly outline the history of networking.
Identify devices used in networking.
Understand the role of protocols in networking.
Define LAN, WAN, MAN, and SAN.
Explain VPNs and their advantages.
Describe the differences between intranets and extranets.
Model 2: Networking Fundamentals
2.1 Networking Terminology
2.1.1 Data networks
Data networks developed as a result of business applications that
were written for microcomputers. At that time microcomputers were
not connected as mainframe computer terminals were, so there was
no efficient way of sharing data among multiple microcomputers. It
became apparent that sharing data through the use of floppy disks
was not an efficient or cost-effective manner in which to operate
businesses. Sneakernet created multiple copies of the data. Each
time a file was modified it would have to be shared again with all
other people who needed that file. If two people modified the file and
then tried to share it, one of the sets of changes would be lost.
Businesses needed a solution that would successfully address the
following three problems:
 How to avoid duplication of equipment and resources
 How to communicate efficiently
 How to set up and manage a network
Businesses realized that networking technology could increase
productivity while saving money. Networks were added and
expanded almost as rapidly as new network technologies and
products were introduced. In the early 1980s networking saw a
tremendous expansion, even though the early development of
networking was disorganized.
In the mid-1980s, the network technologies that had emerged had
been created with a variety of different hardware and software
implementations. Each company that created network hardware and
software used its own company standards. These individual
standards were developed because of competition with other
companies. Consequently, many of the new network technologies
were incompatible with each other. It became increasingly difficult for
networks that used different specifications to communicate with each
other. This often required the old network equipment to be removed
to implement the new equipment.
One early solution was the creation of local-area network (LAN)
standards. Because LAN standards provided an open set of
guidelines for creating network hardware and software, the
equipment from different companies could then become compatible.
This allowed for stability in LAN implementation.
In a LAN system, each department of the company is a kind of
electronic island. As the use of computers in businesses grew, it soon
became obvious that even LANs were not sufficient.
What was needed was a way for information to move efficiently and
quickly, not only within a company, but also from one business to
another.
The solution was the creation of metropolitan-area
networks (MANs) and wide-area networks (WANs). Because WANs
could connect user networks over large geographic areas, it was
possible for businesses to communicate with each other across great
distances. Figure summarizes the relative sizes of LANs and
WANs.
Evolution of Networking
Sneakernet
LAN
LAN
WAN
Examples of Data Networks
2.1.2 Network history
The history of computer networking is complex. It has involved
many people from all over the world over the past 35 years.
Presented here is a simplified view of how the Internet evolved. The
processes of invention and commercialization are far more
complicated, but it is helpful to look at the fundamental development.
In the 1940s computers were large electromechanical devices that
were prone to failure. In 1947 the invention of a semiconductor
transistor opened up many possibilities for making smaller, more
reliable computers. In the 1950s mainframe computers, which were
run by punched card programs, began to be used by large institutions.
In the late 1950s the integrated circuit that combined several, then
many, and now millions, of transistors on one small piece of
semiconductor was invented. Through the 1960s mainframes with
terminals were commonplace, and integrated circuits were widely
used.
In the late 1960s and 1970s, smaller computers, called
minicomputers came into existence. However, these minicomputers
were still very large by modern standards. In 1977 the Apple
Computer Company introduced the microcomputer, also known as
the personal computer. In 1981 IBM introduced its first personal
computer. The user-friendly Mac, the open-architecture IBM PC, and
the further micro-miniaturization of integrated circuits led to
widespread use of personal computers in homes and businesses.
In the mid-1980s users with stand-alone computers started to share
files using modems to connect to other computers. This was referred
to as point-to-point, or dial-up communication. This concept was
expanded by the use of computers that were the central point of
communication in a dial-up connection. These computers were called
bulletin boards. Users would connect to the bulletin boards, leave and
pick up messages, as well as upload and download files. The
drawback to this type of system was that there was very little direct
communication and then only with those who knew about the bulletin
board. Another limitation was that the bulletin board computer
required one modem per connection. If five people connected
simultaneously it would require five modems connected to five
separate phone lines. As the number of people who wanted to use
the system grew, the system was not able to handle the demand. For
example, imagine if 500 people wanted to connect at the same time.
Starting in the 1960s and continuing through the 70s, 80s, and 90s,
the Department of Defense (DoD) developed large, reliable,
wide-area networks (WANs) for military and scientific reasons. This
technology was different from the point-to-point communication used
in bulletin boards. It allowed multiple computers to be connected
together using many different paths. The network itself would
determine how to move data from one computer to another. Instead
of only being able to communicate with one other computer at a time,
many computers could be reached using the same connection. The
DoDs WAN eventually became the Internet.
Network History
2.1.3 Networking devices
Equipment that connects directly to a network segment is referred to
as a device. These devices are broken up into two classifications.
The first classification is end-user devices. End-user devices include
computers, printers, scanners, and other devices that provide
services directly to the user. The second classification is network
devices. Network devices include all the devices that connect the
end-user devices together to allow them to communicate.
End-user devices that provide users with a connection to the network
are also referred to as hosts. These devices allow users to share,
create, and obtain information. The host devices can exist without a
network, but without the network the host capabilities are greatly
reduced. Host devices are physically connected to the network media
using a network interface card (NIC). They use this connection to
perform the tasks of sending e-mails, printing reports, scanning
pictures, or accessing databases. A NIC is a printed circuit board
that fits into the expansion slot of a bus on a computer motherboard,
or it can be a peripheral device. It is also called a network adapter.
Laptop or notebook computer NICs are usually the size of a PCMCIA
card. Each individual NIC carries a unique code, called a Media
Access Control (MAC) address. This address is used to control data
communication for the host on the network. More about the MAC
address will be covered later. As the name implies, the NIC controls
host access to the medium.
There are no standardized symbols for end-user devices in the
networking industry. They appear similar to the real devices to allow
for quick recognition.
Network devices provide transport for the data that needs to be
transferred between end-user devices. Network devices provide
extension of cable connections, concentration of connections,
conversion of data formats, and management of data transfers.
Examples of devices that perform these functions are repeaters, hubs,
bridges, switches, and routers. All of the network devices mentioned
here are covered in depth later in the course. For now, a brief
overview of networking devices will be provided.
A repeater is a network device used to regenerate a signal.
Repeaters regenerate analog or digital signals distorted by
transmission loss due to attenuation. A repeater does not perform
intelligent routing like a bridge or router.
Hubs concentrate connections. In other words, they take a group of
hosts and allow the network to see them as a single unit. This is done
passively, without any other effect on the data transmission. Active
hubs not only concentrate hosts, but they also regenerate signals.
Bridges convert network transmission data formats as well as perform
basic data transmission management. Bridges, as the name implies,
provide connections between LANs. Not only do bridges connect
LANs, but they also perform a check on the data to determine
whether it should cross the bridge or not. This makes each part of the
network more efficient.
Workgroup switches add more intelligence to data transfer
management. Not only can they determine whether data should
remain on a LAN or not, but they can transfer the data only to the
connection that needs that data. Another difference between a bridge
and switch is that a switch does not convert data transmission
formats.
Routers have all the capabilities listed above.
Routers can
regenerate signals, concentrate multiple connections, convert data
transmission formats, and manage data transfers. They can also
connect to a WAN, which allows them to connect LANs that are
separated by great distances. None of the other devices can provide
this type of connection.
Workstation
Network Internet Card
PCMCIA Ethernet Adapter
End User Device Icons
2.1.4 Network topology
Network topology defines the structure of the network. One part of
the topology definition is the physical topology, which is the actual
layout of the wire or media. The other part is the logical topology,
which defines how the media is accessed by the hosts for sending
data. The physical topologies that are commonly used are as follows:
 A bus topology uses a single backbone cable that is terminated at
both ends. All the hosts connect directly to this backbone.
 A ring topology connects one host to the next and the last host to
the first. This creates a physical ring of cable.
 A star topology connects all cables to a central point of
concentration.
 An extended star topology links individual stars together by
connecting the hubs and/or switches. This topology can extend the
scope and coverage of the network.
 A hierarchical topology is similar to an extended star. However,
instead of linking the hubs and/or switches together, the system is
linked to a computer that controls the traffic on the topology.
 A mesh topology is implemented to provide as much protection as
possible from interruption of service. The use of a mesh topology in
the networked control systems of a nuclear power plant would be
an excellent example. As seen in the graphic, each host has its
own connections to all other hosts. Although the Internet has
multiple paths to any one location, it does not adopt the full mesh
topology.
The logical topology of a network is how the hosts communicate
across the medium. The two most common types of logical topologies
are broadcast and token passing.
Broadcast topology simply means that each host sends its data to all
other hosts on the network medium. There is no order that the
stations must follow to use the network. It is first come, first serve.
Ethernet works this way as will be explained later in the course.
The second logical topology is token passing. Token passing controls
network access by passing an electronic token sequentially to each
host. When a host receives the token, that host can send data on the
network. If the host has no data to send, it passes the token to the
next host and the process repeats itself. Two examples of networks
that use token passing are Token Ring and Fiber Distributed Data
Interface (FDDI). A variation of Token Ring and FDDI is Arcnet.
Arcnet is token passing on a bus topology.
The diagram in Figure shows many different topologies connected
by network devices. It shows a network of moderate complexity that
is typical of a school or a small business. It has many symbols, and it
depicts many networking concepts that will take time to learn.
Physical Topologies
Teaching Topologies
2.1.5 Network protocols
Protocol suites are collections of protocols that enable network
communication from one host through the network to another host. A
protocol is a formal description of a set of rules and conventions that
govern a particular aspect of how devices on a network communicate.
Protocols determine the format, timing, sequencing, and error control
in data communication. Without protocols, the computer cannot make
or rebuild the stream of incoming bits from another computer into the
original format.
Protocols control all aspects of data communication, which include
the following:
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How the physical network is built
How computers connect to the network
How the data is formatted for transmission
How that data is sent
How to deal with errors
These network rules are created and maintained by many different
organizations and committees. Included in these groups are the
Institute of Electrical and Electronic Engineers (IEEE), American
National Standards Institute (ANSI), Telecommunications Industry
Association (TIA), Electronic Industries Alliance (EIA) and the
International Telecommunications Union (ITU), formerly known as the
Comité Consultatif International Téléphonique et Télégraphique
(CCITT).
Computer Communication Protocols
2.1.6 Local-area networks (LANs)
LANs consist of the following components:
 Computers
 Network interface cards
 Peripheral devices
 Networking media
 Network devices
LANs make it possible for businesses that use computer technology
to locally share files and printers efficiently, and make internal
communications possible. A good example of this technology is
e-mail. They tie data, local communications, and computing
equipment together.
Some common LAN technologies are:
 Ethernet
 Token Ring
 FDDI
LANs and LAN Devices
2.1.7 Wide-area networks (WANs)
WANs interconnect LANs, which then provide access to computers
or file servers in other locations. Because WANs connect user
networks over a large geographical area, they make it possible for
businesses to communicate across great distances. Using WANs
allows computers, printers, and other devices on a LAN to share and
be shared with distant locations. WANs provide instant
communications across large geographic areas. The ability to send
an instant message (IM) to someone anywhere in the world provides
the same communication capabilities that used to be only possible if
people were in the same physical office. Collaboration software
provides access to real-time information and resources that allows
meetings to be held remotely, instead of in person. Wide-area
networking has also created a new class of workers called
telecommuters, people who never have to leave their homes to go to
work.
WANs are designed to do the following:
 Operate over a large geographically separated areas
 Allow users to have real-time communication capabilities with other
users
 Provide full-time remote resources connected to local services
 Provide e-mail, World Wide Web, file transfer, and e-commerce
services
Some common WAN technologies are:
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Modems
Integrated Services Digital Network (ISDN)
Digital Subscriber Line (DSL)
Frame Relay
US (T) and Europe (E) Carrier Series – T1, E1, T3, E3
Synchronous Optical Network (SONET)
WANs and WAN Devices
2.1.8 Metropolitan-area networks (MANs)
A MAN is a network that spans a metropolitan area such as a city or
suburban area. A MAN usually consists of two or more LANs in a
common geographic area. For example, a bank with multiple
branches may utilize a MAN. Typically, a service provider is used to
connect two or more LAN sites using private communication lines or
optical services. A MAN can also be created using wireless bridge
technology by beaming signals across public areas.
Metropolitan-Area Network
2.1.9 Storage-area networks (SANs)
A SAN is a dedicated, high-performance network used to move data
between servers and storage resources. Because it is a separate,
dedicated network, it avoids any traffic conflict between clients and
servers.
SAN
technology
allows
high-speed
server-to-storage,
storage-to-storage, or server-to-server connectivity. This method
uses a separate network infrastructure that relieves any problems
associated with existing network connectivity.
SANs offer the following features:
 Performance – SANs enable concurrent access of disk or tape
arrays by two or more servers at high speeds, providing enhanced
system performance.
 Availability – SANs have disaster tolerance built in, because data
can be mirrored using a SAN up to 10 kilometers (km) or 6.2 miles
away.
 Scalability – Like a LAN/WAN, it can use a variety of technologies.
This allows easy relocation of backup data, operations, file
migration, and data replication between systems.
Storage-Area Network
2.1.10 Virtual private network (VPN)
A VPN is a private network that is constructed within a public
network infrastructure such as the global Internet. Using VPN, a
telecommuter can access the network of the company headquarters
through the Internet by building a secure tunnel between the
telecommuter’s PC and a VPN router in the headquarters.
VPN Connections
2.1.11 Benefits of VPNs
Cisco products support the latest in VPN technology. A VPN is a
service that offers secure, reliable connectivity over a shared public
network infrastructure such as the Internet. VPNs maintain the same
security and management policies as a private network. They are the
most cost-effective method of establishing a point-to-point connection
between remote users and an enterprise customer's network.
The following are the three main types of VPNs:
 Access VPNs – Access VPNs provide remote access to a mobile
worker and small office/home office (SOHO) to the headquarters of
the Intranet or Extranet over a shared infrastructure. Access VPNs
use analog, dialup, ISDN, digital subscriber line (DSL), mobile IP,
and cable technologies to securely connect mobile users,
telecommuters, and branch offices.
 Intranet VPNs – Intranet VPNs link regional and remote offices to
the headquarters of the internal network over a shared
infrastructure using dedicated connections. Intranet VPNs differ
from Extranet VPNs in that they allow access only to the
employees of the enterprise.
 Extranet VPNs – Extranet VPNs link business partners to the
headquarters of the network over a shared infrastructure using
dedicated connections. Extranet VPNs differ from Intranet VPNs in
that they allow access to users outside the enterprise.
VPN Technologies
2.1.12 Intranets and extranets
One common configuration of a LAN is an Intranet. Intranet Web
servers differ from public Web servers in that the public must have
the proper permissions and passwords to access the Intranet of an
organization. Intranets are designed to permit access by users who
have access privileges to the internal LAN of the organization. Within
an Intranet, Web servers are installed in the network. Browser
technology is used as the common front end to access information
such as financial data or graphical, text-based data stored on those
servers.
Extranets refer to applications and services that are Intranet based,
and use extended, secure access to external users or enterprises.
This access is usually accomplished through passwords, user IDs,
and other application-level security. Therefore, an Extranet is the
extension of two or more Intranet strategies with a secure interaction
between participant enterprises and their respective intranets.
Intranet and Extranet
VPN
2.2 Bandwidth
2.2.1 Importance of bandwidth
Bandwidth is defined as the amount of information that can flow
through a network connection in a given period of time. It is essential
to understand the concept of bandwidth when studying networking for
the following four reasons:
1. Bandwidth is finite.
In other words, regardless of the media used to build the network,
there are limits on the capacity of that network to carry information.
Bandwidth is limited by the laws of physics and by the technologies
used to place information on the media. For example, the bandwidth
of a conventional modem is limited to about 56 kbps by both the
physical properties of twisted-pair phone wires and by modem
technology. However, the technologies employed by DSL also use
the same twisted-pair phone wires, yet DSL provides much greater
bandwidth than is available with conventional modems. So, even the
limits imposed by the laws of physics are sometimes difficult to define.
Optical fiber has the physical potential to provide virtually limitless
bandwidth. Even so, the bandwidth of optical fiber cannot be fully
realized until technologies are developed to take full advantage of its
potential.
2. Bandwidth is not free.
It is possible to buy equipment for a local-area network (LAN) that will
provide nearly unlimited bandwidth over a long period of time. For
wide-area network (WAN) connections, it is almost always necessary
to buy bandwidth from a service provider. In either case, an
understanding of bandwidth and changes in demand for bandwidth
over a given time can save an individual or a business a significant
amount of money. A network manager needs to make the right
decisions about the kinds of equipment and services to buy.
3. Bandwidth is a key factor in analyzing network performance,
designing new networks, and understanding the Internet.
A networking professional must understand the tremendous impact of
bandwidth and throughput on network performance and design.
Information flows as a string of bits from computer to computer
throughout the world. These bits represent massive amounts of
information flowing back and forth across the globe in seconds or less.
In a sense, it may be appropriate to say that the Internet is
bandwidth.
4. The demand for bandwidth is ever increasing.
As soon as new network technologies 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, which further
adds to the need for bandwidth. The successful networking
professional must anticipate the need for increased bandwidth and
act accordingly.
Why is Bandwidth Important?
2.2.2 Analogies
Bandwidth has been defined as the amount of information that can
flow through a network in a given time. The idea that information
flows suggests two analogies that may make it easier to visualize
bandwidth in a network. Since both water and traffic are said to flow,
consider the following analogies:
1. Bandwidth is like the width of a pipe.
A network of pipes brings fresh water to homes and businesses and
carries waste water away. This water network is made up of pipes of
different diameters. The main water pipes of a city may be two meters
in diameter, while the pipe to a kitchen faucet may have a diameter of
only two centimeters. The width of the pipe determines the
water-carrying capacity of the pipe. Therefore, the water is like the
data, and the pipe width is like the bandwidth. Many networking
experts say that they need to put in bigger pipes when they wish to
add more information-carrying capacity.
2. Bandwidth is like the number of lanes on a highway.
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, which eventually
go to the driveways of homes and businesses. When very few
automobiles use the highway system, each vehicle is able to move
freely. When more traffic is added, each vehicle moves more slowly.
This is especially true on roads with fewer lanes for the cars to
occupy. Eventually, as even more traffic enters the highway system,
even multi-lane highways become congested and slow. A data
network is much like the highway system. The data packets are
comparable to automobiles, and the bandwidth is comparable 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.
Pipe Analogy for Bandwidth
Highway Analogy for Bandwidth
2.2.3 Measurement
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 (kbps), millions of bits per second (Mbps), and billions of
bits per second (Gbps) and trillions of bits per second (Tbps).
Although the terms bandwidth and speed are often used
interchangeably, they are not exactly the same thing. One may say,
for example, that a T3 connection at 45Mbps operates at a higher
speed than a T1 connection at 1.544Mbps. However, if only a small
amount of their data-carrying capacity is being used, each of these
connection types will carry data at roughly the same speed. For
example, a small amount of water will flow 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
connection. This is because the T3 connection is able to carry more
information in the same period of time, not because it has a higher
speed.
Units of Bandwidth
2.2.4 Limitations
Bandwidth varies depending upon the type of media as well as the
LAN and WAN technologies used. The physics of the media account
for some of the difference. Signals travel through twisted-pair copper
wire, coaxial cable, optical fiber, and air. The physical differences in
the ways signals travel result in fundamental limitations on the
information-carrying capacity of a given medium. However, the actual
bandwidth of a network is determined by a combination of the
physical media 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
over one gigabit per second (Gbps). However, in actual practice, the
bandwidth is determined by the use of 10BASE-T, 100BASE-TX, or
1000BASE-TX Ethernet. In other words, the actual bandwidth is
determined by the signaling methods, network interface cards (NICs),
and other items of network equipment that are chosen. Therefore, the
bandwidth is not determined solely by the limitations of the medium.
Figure 1 shows some common networking media types along with
the limits on distance and bandwidth when using the indicated
networking technology.
Figure 2 summarizes common WAN services and the bandwidth
associated with each service.
Maximum Bandwidths and Length Limitations
WAN Services and Bandwidths
2.2.5 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 critical part of the specification of
the network. A typical LAN might be built to provide 100 Mbps to
every desktop workstation, but this does not mean that each user is
actually able to move one hundred megabits of data through the
network for every second of use. This would be 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 specific 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 possible digital
bandwidth of the medium that is being used. The following are
some of the factors that determine throughput:
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Internetworking devices
Type of data being transferred
Network topology
Number of users on the network
User computer
Server computer
Power conditions
The theoretical bandwidth of a network is an important consideration
in network design, because the network bandwidth will never be
greater than the limits imposed by the chosen media and networking
technologies. However, it is just as important for a network designer
and administrator to consider the factors that may 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.
Variables that May Affect Throughput
2.2.6 Data transfer calculation
Network designers and administrators are often called upon to make
decisions regarding bandwidth. One decision might be whether to
increase the size of the WAN connection to accommodate a new
database. Another decision might be whether the current LAN
backbone is of sufficient bandwidth for a streaming-video training
program. The answers to problems like these are not always easy to
find, but one place to start is with a simple data transfer calculation.
Using the formula transfer time = size of file / bandwidth (T=S/BW)
allows a network administrator to 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 network bandwidth
yields an estimate of the fastest time that the file can be transferred.
Two important points should be considered when doing this
calculation.
1. The result is an estimate only, because the file size does not
include any overhead added by encapsulation.
2. 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 accurate estimate can be attained if
throughput is substituted for bandwidth in the equation.
Although the data transfer calculation is quite simple, one must be
careful to use the same units throughout the equation. In other words,
if the bandwidth is measured in megabits per second (Mbps), the file
size must be in megabits (Mb), not megabytes (MB). Since file sizes
are typically given in megabytes, it may be necessary to multiply the
number of megabytes by eight 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 ten GB
hard drive full of data over an OC-48 line?
Transfer Time Calculation
2.2.7 Digital versus analog
Radio, television, and telephone transmissions have, until recently,
been 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 occupied by each signal. The basic unit of analog
bandwidth is hertz (Hz), or cycles per second. 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 bandwidths of cordless telephones,
which usually operate at either 900 MHz or 2.4 GHz. These are also
the units used to describe the bandwidths of 802.11a and 802.11b
wireless networks, which operate at 5 GHz and 2.4 GHz.
While analog signals are capable of carrying a variety of information,
they have some significant disadvantages in comparison to digital
transmissions. The analog video signal that requires a wide
frequency range for transmission cannot be squeezed into a smaller
band. Therefore, if the necessary analog bandwidth is not available,
the signal cannot be sent.
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 transmission over digital media. This
type of transmission gives digital bandwidth an important advantage
over analog bandwidth. Unlimited amounts of information can be sent
over the smallest or lowest bandwidth digital channel. Regardless of
how long it takes for the digital information to arrive at its destination
and be 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 information technology. However, because
this course is concerned primarily with digital networking, the term
‘bandwidth’ will refer to digital bandwidth.
Audio Analogy for Bandwidth
2.3 Networking Models
2.3.1 Using layers to analyze problems in a flow of materials
The concept of layers is used to describe communication from one
computer to another. Figure shows a set of questions that are
related to flow, which is defined as the motion through a system of
either physical or logical objects. These questions show how the
concept of layers helps describe the details of the flow process. This
process could be any kind of flow, from the flow of traffic on a
highway system to the flow of data through a network. Figure shows
several examples of flow and ways that the flow process can be
broken down into details or layers.
A conversation between two people provides a good opportunity to
use a layered approach to analyze information flow. In a conversation,
each person wishing to communicate begins by creating an idea.
Then a decision is made on how to properly communicate the idea.
For example, a person could decide to speak, sing or shout, and what
language to use. Finally the idea is delivered. For example, the
person creates the sound which carries the message.
This process can be broken into separate layers that may be applied
to all conversations. The top layer is the idea that will be
communicated. The middle layer is the decision on how the idea is to
be communicated. The bottom layer is the creation of sound to carry
the communication.
The same method of layering explains how a computer network
distributes information from a source to a destination. When
computers send information through a network, all communications
originate at a source then travel to a destination.
The information that travels on a network is generally referred to as
data or a packet. A packet is a logically grouped unit of information
that moves between computer systems. As the data passes between
layers, each layer adds additional information that enables effective
communication with the corresponding layer on the other computer.
The OSI and TCP/IP models have layers that explain how data is
communicated from one computer to another. The models differ in
the number and function of the layers. However, each model can be
used to help describe and provide details about the flow of
information from a source to a destination.
Analyzing Network in Layers
Network Comparisons
Network Communication
2.3.2 Using layers to describe data communication
In order for data packets to travel from a source to a destination on a
network, it is important that all the devices on the network speak the
same language or protocol. A protocol is a set of rules that make
communication on a network more efficient. For example, while flying
an airplane, pilots obey very specific rules for communication with
other airplanes and with air traffic control.
A data communications protocol is a set of rules or an agreement that
determines the format and transmission of data.
Layer 4 on the source computer communicates with Layer 4 on the
destination computer. The rules and conventions used for this layer
are known as Layer 4 protocols. It is important to remember that
protocols prepare data in a linear fashion. A protocol in one layer
performs a certain set of operations on data as it prepares the data to
be sent over the network. The data is then passed to the next layer
where another protocol performs a different set of operations.
Once the packet has been sent to the destination, the protocols undo
the construction of the packet that was done on the source side. This
is done in reverse order. The protocols for each layer on the
destination return the information to its original form, so the
application can properly read the data.
Layer Communication
2.3.3 OSI model
The early development of networks was disorganized in many ways.
The early 1980s saw tremendous increases in the number and size of
networks. As companies realized the advantages of using networking
technology, networks were added or expanded almost as rapidly as
new network technologies were introduced.
By the mid-1980s, these companies began to experience problems
from the rapid expansion. Just as people who do not speak the same
language have difficulty communicating with each other, it was
difficult for networks that used different specifications and
implementations to exchange information. The same problem
occurred with the companies that developed private or proprietary
networking technologies. Proprietary means that one or a small group
of companies controls all usage of the technology. Networking
technologies strictly following proprietary rules could not
communicate with technologies that followed different proprietary
rules.
To address the problem of network incompatibility, the International
Organization for Standardization (ISO) researched networking
models like Digital Equipment Corporation net (DECnet), Systems
Network Architecture (SNA), and TCP/IP in order to find a generally
applicable set of rules for all networks. Using this research, the ISO
created a network model that helps vendors create networks that are
compatible with other networks.
The Open System Interconnection (OSI) reference model released in
1984 was the descriptive network model that the ISO created. It
provided vendors with a set of standards that ensured greater
compatibility and interoperability among various network technologies
produced by companies around the world.
The OSI reference model has become the primary model for network
communications. Although there are other models in existence, most
network vendors relate their products to the OSI reference model.
This is especially true when they want to educate users on the use of
their products. It is considered the best tool available for teaching
people about sending and receiving data on a network.
Benefits of the OSI Model
2.3.4 OSI layers
The OSI reference model is a framework that is used to understand
how information travels throughout a network. The OSI reference
model explains how packets travel through the various layers to
another device on a network, even if the sender and destination have
different types of network media.
In the OSI reference model, there are seven numbered layers, each
of which illustrates a particular network function. - Dividing the
network into seven layers provides the following advantages:
 It breaks network communication into smaller, more manageable
parts.
 It standardizes network components to allow multiple vendor
development and support.
 It allows different types of network hardware and software to
communicate with each other.
 It prevents changes in one layer from affecting other layers.
 It divides network communication into smaller parts to make
learning it easier to understand.
The OSI Model
2.3.5 Peer-to-peer communications
In order for data to travel from the source to the destination, each
layer of the OSI model at the source must communicate with its peer
layer at the destination. This form of communication is referred to as
peer-to-peer. During this process, the protocols of each layer
exchange information, called protocol data units (PDUs). Each layer
of communication on the source computer communicates with a
layer-specific PDU, and with its peer layer on the destination
computer as illustrated in Figure .
Data packets on a network originate at a source and then travel to a
destination. Each layer depends on the service function of the OSI
layer below it. To provide this service, the lower layer uses
encapsulation to put the PDU from the upper layer into its data field;
then it adds whatever headers and trailers the layer needs to perform
its function. Next, as the data moves down through the layers of the
OSI model, additional headers and trailers are added. After Layers 7,
6, and 5 have added their information, Layer 4 adds more information.
This grouping of data, the Layer 4 PDU, is called a segment.
The network layer provides a service to the transport layer, and the
transport layer presents data to the internetwork subsystem. The
network layer has the task of moving the data through the
internetwork. It accomplishes this task by encapsulating the data and
attaching a header creating a packet (the Layer 3 PDU). The header
contains information required to complete the transfer, such as
source and destination logical addresses.
The data link layer provides a service to the network layer. It
encapsulates the network layer information in a frame (the Layer 2
PDU). The frame header contains information (for example, physical
addresses) required to complete the data link functions. The data link
layer provides a service to the network layer by encapsulating the
network layer information in a frame.
The physical layer also provides a service to the data link layer. The
physical layer encodes the data link frame into a pattern of 1s and 0s
(bits) for transmission on the medium (usually a wire) at Layer 1.
Peer-to-Peer Communications
2.3.6 TCP/IP model
The historical and technical standard of the Internet is the TCP/IP
model. The U.S. Department of Defense (DoD) created the TCP/IP
reference model, because it wanted to design a network that could
survive any conditions, including a nuclear war. In a world connected
by different types of communication media such as copper wires,
microwaves, optical fibers and satellite links, the DoD wanted
transmission of packets every time and under any conditions. This
very difficult design problem brought about the creation of the TCP/IP
model.
Unlike the proprietary networking technologies mentioned earlier,
TCP/IP was developed as an open standard. This meant that anyone
was free to use TCP/IP. This helped speed up the development of
TCP/IP as a standard.
The TCP/IP model has the following four layers:
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Application layer
Transport layer
Internet layer
Network access layer
Although some of the layers in the TCP/IP model have the same
name as layers in the OSI model, the layers of the two models do not
correspond exactly. Most notably, the application layer has different
functions in each model.
The designers of TCP/IP felt that the application layer should include
the OSI session and presentation layer details. They created an
application layer that handles issues of representation, encoding, and
dialog control.
The transport layer deals with the quality of service issues of
reliability, flow control, and error correction. One of its protocols, the
transmission control protocol (TCP), provides excellent and flexible
ways to create reliable, well-flowing, low-error network
communications.
TCP is a connection-oriented protocol. It maintains a dialogue
between source and destination while packaging application layer
information into units called segments. Connection-oriented does not
mean that a circuit exists between the communicating computers. It
does mean that Layer 4 segments travel back and forth between two
hosts to acknowledge the connection exists logically for some period.
The purpose of the Internet layer is to divide TCP segments into
packets and send them from any network. The packets arrive at the
destination network independent of the path they took to get there.
The specific protocol that governs this layer is called the Internet
Protocol (IP). Best path determination and packet switching occur at
this layer.
The relationship between IP and TCP is an important one. IP can be
thought to point the way for the packets, while TCP provides a
reliable transport.
The name of the network access layer is very broad and somewhat
confusing. It is also known as the host-to-network layer. This layer is
concerned with all of the components, both physical and logical, that
are required to make a physical link. It includes the networking
technology details, including all the details in the OSI physical and
data link layers.
Figure illustrates some of the common protocols specified by the
TCP/IP reference model layers. Some of the most commonly used
application layer protocols include the following:
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File Transfer Protocol (FTP)
Hypertext Transfer Protocol (HTTP)
Simple Mail Transfer Protocol (SMTP)
Domain Name System (DNS)
Trivial File Transfer Protocol (TFTP)
The common transport layer protocols include:
 Transport Control Protocol (TCP)
 User Datagram Protocol (UDP)
The primary protocol of the Internet layer is:
 Internet Protocol (IP)
The network access layer refers to any particular technology used on
a specific network.
Regardless of which network application services are provided and
which transport protocol is used, there is only one Internet protocol,
IP. This is a deliberate design decision. IP serves as a universal
protocol that allows any computer anywhere to communicate at any
time.
A comparison of the OSI model and the TCP/IP models will point out
some similarities and differences.
Similarities include:
 Both have layers.
 Both have application layers, though they include very different
services.
 Both have comparable transport and network layers.
 Both models need to be known by networking professionals.
 Both assume packets are switched. This means that individual
packets may take different paths to reach the same destination.
This is contrasted with circuit-switched networks where all the
packets take the same path.
Differences include:
 TCP/IP combines the presentation and session layer issues into its
application layer.
 TCP/IP combines the OSI data link and physical layers into the
network access layer.
 TCP/IP appears simpler because it has fewer layers.
 TCP/IP protocols are the standards around which the Internet
developed, so the TCP/IP model gains credibility just because of its
protocols. In contrast, networks are not usually built on the OSI
protocol, even though the OSI model is used as a guide.
Although TCP/IP protocols are the standards with which the Internet
has grown, this curriculum will use the OSI model for the following
reasons:
 It is a generic, protocol-independent standard.
 It has more details, which make it more helpful for teaching and
learning.
 It has more details, which can be helpful when troubleshooting.
Networking professionals differ in their opinions on which model to
use. Due to the nature of the industry it is necessary to become
familiar with both. Both the OSI and TCP/IP models will be referred to
throughout the curriculum. The focus will be on the following:
TCP as an OSI Layer 4 protocol
IP as an OSI Layer 3 protocol
Ethernet as a Layer 2 and Layer 1 technology
Remember that there is a difference between a model and an actual
protocol that is used in networking. The OSI model will be used to
describe TCP/IP protocols.
The TCP/IP Model
Common
TCP/IP Protocols
Comparing TCP/IP with OSI
Focus of the CCNA Curriculum
2.3.7 Detailed encapsulation process
All communications on a network originate at a source, and are sent
to a destination. The information sent on a network is referred to as
data or data packets. If one computer (host A) wants to send data to
another computer (host B), the data must first be packaged through a
process called encapsulation.
Encapsulation wraps data with the necessary protocol information
before network transit. Therefore, as the data packet moves down
through the layers of the OSI model, it receives headers, trailers, and
other information.
To see how encapsulation occurs, examine the manner in which data
travels through the layers as illustrated in Figure . Once the data is
sent from the source, it travels through the application layer down
through the other layers. The packaging and flow of the data that is
exchanged goes through changes as the layers perform their
services for end users. As illustrated in Figure , networks must
perform the following five conversion steps in order to encapsulate
data:
1. Build the data.
As a user sends an e-mail message, its alphanumeric characters are
converted to data that can travel across the internetwork.
2. Package the data for end-to-end transport.
The data is packaged for internetwork transport. By using segments,
the transport function ensures that the message hosts at both ends of
the e-mail system can reliably communicate.
3. Add the network IP address to the header.
The data is put into a packet or datagram that contains a packet
header with source and destination logical addresses. These
addresses help network devices send the packets across the network
along a chosen path.
4. Add the data link layer header and trailer.
Each network device must put the packet into a frame. The frame
allows connection to the next directly-connected network device on
the link. Each device in the chosen network path requires framing in
order for it to connect to the next device.
5. Convert to bits for transmission.
The frame must be converted into a pattern of 1s and 0s (bits) for
transmission on the medium. A clocking function enables the devices
to distinguish these bits as they travel across the medium. The
medium on the physical internetwork can vary along the path used.
For example, the e-mail message can originate on a LAN, cross a
campus backbone, and go out a WAN link until it reaches its
destination on another remote LAN.
Data Encapsulation
Data Encapsulation Example
Summary
An understanding of the following key points should have been
achieved:
 Understanding bandwidth is essential when studying networking
 Bandwidth is finite, costs money, and the demand for it increases
daily
 Using analogies like the flow of water and flow of traffic can help
explain bandwidth
 Bandwidth is measured in bits per second, bps, kpbs, Mbps, or
Gbps
 Limitations on bandwidth include type of media used, LAN and
WAN technologies, and network equipment
 Throughput refers to actual measured bandwidth, which is affected
by factors that include number of users on network, networking
devices, type of data, user’s computer and the server
 The formula T=S/BW (transfer time = size of file / bandwidth) can
be used to calculate data transfer time
 Comparison of analog and digital bandwidth
 A layered approach is effective in analyzing problems
 Network communication is described by layered models
 The OSI and TCP/IP are the two most important models of network
communication
 The International Organization for Standardization developed the
OSI model to address the problems of network incompatibility
 The seven layers of the OSI are application, presentation, session,
transport, network, data link, and physical
 The four layers of the TCP/IP are application, transport, internet,
and network access
 The TCP/IP application layer is equivalent to the OSI application,
presentation, and session layers
 LANs and WANs developed in response to business and
government computing needs
 Fundamental networking devices are hubs, bridges, switches, and
routers
 The physical topology layouts include the bus, ring, star, extended
star, hierarchical, and mesh
 A WAN consists of two or more LANs spanning a common
geographic area
 A SAN provides enhanced system performance, is scalable, and
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has disaster tolerance built in
A VPN is a private network that is constructed within a public
network infrastructure
Three main types of VPNs are access, Intranet, and Extranet VPNs
Intranets are designed to be available to users who have access
privileges to the internal network of an organization
Extranets are designed to deliver applications and services that are
Intranet based, using extended, secured access to external users
or enterprises
Model 2: Summary