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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 large institutions began to use mainframe computers, which were
run by punched card programs. In the late 1950s the integrated circuit that combined several, and
now millions, of transistors on one small piece of semiconductor was invented. In the 1960s
mainframes with terminals and integrated circuits were widely used.
In the late 1960s and 1970s smaller computers called minicomputers were created. However, these
minicomputers were still very large by modern standards. In 1977 the Apple Computer Company
introduced the microcomputer, which was also known as the Mac. In 1981 IBM introduced its first
PC. 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 PC users began to use modems to share files with 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 computerswere 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.
From the 1960s to the 1990s the U.S. Department of Defense (DoD) developed large, reliable, wide-
area networks (WANs) for military and scientific reasons. This technology was different from thepoint-to-point communication used in bulletin boards. It allowed multiple computers to be
connected together through many different paths. The network itself would determine how to move
data from one computer to another. One connection could be used to reach many computers at the
same time. The WAN developed by the DoD eventually became the Internet.
2.1.3 - Networking devices
This page will introduce some important 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. NICs are used
to physically connect host devices to the network media. They use this connection to send e-mails,
print reports, scan pictures, or access databases.
A NIC is a printed circuit board that fits into the expansion slot of a bus on a computer motherboard.
It can also be a peripheral device. NICs are sometimes called network adapters. Laptop or notebook
computer NICs are usually the size of a PCMCIA card. Each NIC is identified by 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 network.
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 are used to extend cable connections, concentrate connections, convert data
formats, and manage data transfers. 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 that are distorted by transmission loss due to attenuation. A repeater does not make
intelligent decision concerning forwarding packets like a router or bridge.
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 concentrate hosts and also regenerate signals.
Bridges convert network data formats and perform basic data transmission management. Bridgesprovide connections between LANs. They also check data to determine if it should cross the bridge.
This makes each part of the network more efficient.
Workgroup switches add more intelligence to data transfer management. They can determine if
data should remain on a LAN and transfer data only to the connection that needs it. 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.
The Interactive Media Activities will allow students to become more familiar with network devices.
The next page will introduce some common types of network topologies.
Interactive Media Activity
Drag and Drop: Identifying Network Devices
After completing this activity, the student will be able to identify different network devices.
The use of a broadcast topology indicates 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. In this type of topology, an electronic token is passed
sequentially to each host. When a host receives the token, that host can send data on the network. Ifthe 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. The diagram includes
many symbols and networking concepts that will take time to learn.
This page will explain what network protocols are and why they are important.
Protocol suites are collections of protocols that enable network communication between hosts. A
protocol is a formal description of a set of rules and conventions that govern a particular aspect ofhow 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:
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 ae 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).
This page will explain what a VPN is and how it is used.
A vitual private network (VPN) is a private network that is constructed within a public network
infrastructure such as the global Internet. Using VPN, a telecommuter can remotely access thenetwork of the company headquarters. Through the Internet, a secure tunnel can be built between
the PC of the telecommuter and a VPN router at the company headquarters.
2.1.11 - Benefits of VPNs
This page will introduce the three main types of VPNs and explain how they work.
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. The use of a VPN is the most cost-
effective way to establish a point-to-point connection between remote users and an enterprise
network.
The following are the three main types of VPNs:
Access VPNs provide remote access for mobile and small office, home office (SOHO) users to
an Intranet or Extranet over a shared infrastructure. Access VPNs use analog, dialup, ISDN,
DSL, mobile IP, and cable technologies to securely connect mobile users, telecommuters, and
branch offices.
Intranet VPNs use dedicated connections to link regional and remote offices to an internal
network over a shared infrastructure. Intranet VPNs differ from Extranet VPNs in that they
allow access only to the employees of the enterprise.
Extranet VPNs use dedicated connections to link business partners to an internal network
over a shared infrastructure. Extranet VPNs differ from Intranet VPNs in that they allow
This page concludes this lesson. The next lesson will discuss bandwidth. The first page will explainwhy bandwidth is important.
2.2 - Bandwidth
2.2.1 - Importance of bandwidth
This page will describe the four most important characteristics of bandwidth.
Bandwidth is defined as the amount of information that can flow through a network connection in agiven period of time. It is important to understand the concept of bandwidth for the following
reasons.
Bandwidth is finite. Regardless of the media used to build a network, there are limits on the network
capacity 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. DSL uses the same twisted-pair phone wires. However, DSL provides much more
bandwidth than 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 limitlessbandwidth. Even so, the bandwidth of optical fiber cannot be fully realized until technologies are
developed to take full advantage of its potential.
Bandwidth is not free. It is possible to buy equipment for a LAN that will provide nearly unlimited
bandwidth over a long period of time. For WAN connections, it is usually necessary to buy bandwidth
from a service provider. In either case, individual users and businesses can save a lot of money if they
understand bandwidth and how the demand will change over time. A network manager needs to
make the right decisions about the kinds of equipment and services to buy.
Bandwidth is an important factor that is used to analyze network performance, design new networks,and understand 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.
The demand for bandwidth continues to grow. As soon as new network technologies and
infrastructures are built to provide greater bandwidth, new applications are created to takeadvantage of the greater capacity. The delivery of rich media content such as streaming video and
audio over a network 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.
The next page will describe some analogies that can be used to understand bandwidth.
2.2.2 - The desktop
This page will present two analogies that may make it easier to visualize bandwidth in a network.
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.
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 2 meters in diameter, while the pipe to a kitchen
faucet may have a diameter of only 2 cm. 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 moreinformation-carrying capacity.
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 narrower roads that lead 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. As 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
In digital systems, the basic unit of bandwidth is bits per second (bps). Bandwidth is the measure of
how many bits of information can flow from one place to another in a given amount of time.Although bandwidth can be described in bps, a larger unit of measurement is generally used.
Network bandwidth is typically described as thousands of bits per second (kbps), millions of bits per
second (Mbps), 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 45 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 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, notbecause it has a higher speed.
2.2.4 – Limitations (7 Imagens)
This page describes the limitations of bandwidth.
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 information about the physics of unshielded twisted-pair (UTP) copper cable
puts the theoretical bandwidth limit at over 1 Gbps. However, in actual practice, the bandwidth is
determined by the use of 10BASE-T, 100BASE-TX, or 1000BASE-TX Ethernet. The actual bandwidth is
determined by the signaling methods, NICs, and other network equipment that is chosen. Therefore,
the bandwidth is not determined solely by the limitations of the medium.
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 thespecification 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 100 megabits of data
through the network for every second of use. This would be true only under the most ideal
circumstances.
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:
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 andnetworking 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.
2.2.6 - Data transfer calculation
This page provides the formula for 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 thefastest time that the file can be transferred.
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
encapsulation.
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?
2.2.7 - Digital versus analog
This page will explain the differences between analog and digital signals.
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 is used, just as with digital bandwidth. Units of measurement thatare commonly seen are kilohertz (KHz), megahertz (MHz), and gigahertz (GHz). These are the units
used to describe the frequency of cordless telephones, which usually operate at either 900 MHz or
2.4 GHz. These are also the units used to describe the frequencies 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 todigital bandwidth.
This page concludes this lesson. The next lesson will discuss networking models. The first page willdiscuss the concept of layers.
This page discusses how and why the OSI model was developed.
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
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.
In the following Interactive Media Activity, the student will identify the seven layers of the OSImodel.
The next page discusses peer-to-peer communications through the OSI model.
Interactive Media Activity
Drag and Drop: The Seven Layers of the OSI Model
After completing this activity, the student will be able to identify some of the functions of the seven
OSI layers.
2.3.5 - Peer-to-peer communications
This page explains the concept of 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 computercommunicates with a layer-specific PDU, and with its peer layer on the destination computer as
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 asegment.
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.
2.3.6 - TCP/IP model
This page discusses the TCP/IP reference model, which is the historical and technical standard of the
Internet.
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 bydifferent types of communication media such as copper wires, microwaves, optical fibers and
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.
Students will identify the differences between the OSI model and the TCP/IP model in the Lab
Activity.
In the Interactive Media Activity, students will identify the layers of the TCP/IP reference model.
The next page explains the encapsulation process.
Lab Activity: OSI Model and TCP/IP Model
In this lab, the student will learn the four layers of the TCP/IP model and the seven layers of the OSI
model to the four layers of the TCP/IP model.
Interactive Media Activity
Drag and Drop: DoD TCP/IP Model
After completing this activity, the student will be able to identify the layers of the TCP/IP model.
2.3.7 - Detailed encapsulation process
This page describes the process of encapsulation.
All communications on a network originate at a source, and are sent to a destination. Theinformation 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 layerdown 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.
The Lab Activity will provide an in depth review of the OSI model.
The Interactive Media Activity requires students to complete an encapsulation process flowchart.
This page concludes this lesson. The next page will summarize the main points from the module.
Lab Activity: OSI Model Characteristics and Devices
In this lab, the student will learn the seven layers of the OSI model and the characteristics, functions
and keywords relating to each layer.
Interactive Media Activity
Drag and Drop: Encapsulation Process Flowchart
In this lab, the student will complete encapsulation process flowchart.
Summary
This page summarizes the topics discussed in this module.
Computer networks developed in response to business and government computing needs. Applyingstandards to network functions provided a set of guidelines for creating network hardware and
software and provided compatibility among equipment from different companies. Information could
move within a company and from one business to another.
Network devices, such as repeaters, hubs, bridges, switches and routers connect host devices
together to allow them to communicate. Protocols provide a set of rules for communication.
The physical topology of a network is the actual layout of the wire or media. The logical topology
defines how host devices access the media. The physical topologies that are commonly used are bus,
ring, star, extended star, hierarchical, and mesh. The two most common types of logical topologies
are broadcast and token passing.
A local-area network (LAN) is designed to operate within a limited geographical area. LANs allow
multi-access to high-bandwidth media, control the network privately under local administration,
provide full-time connectivity to local services and connect physically adjacent devices.
A wide-area network (WAN) is designed to operate over a large geographical area. WANs allowaccess over serial interfaces operating at lower speeds, provide full-time and part-time connectivity
and connect devices separated over wide areas.
A metropolitan-area network (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.
A storage-area network (SAN) is a dedicated, high-performance network used to move data between
servers and storage resources. A SAN provides enhanced system performance, is scalable, and has
disaster tolerance built in.
A virtual private network (VPN) is a private network that is constructed within a public network
infrastructure. Three main types of VPNs are access, Intranet, and Extranet VPNs. Access VPNs
provide mobile workers or small office/home office (SOHO) users with remote access to an Intranet
or Extranet. Intranets are only 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
to external users or enterprises.
The amount of information that can flow through a network connection in a given period of time is
referred to as bandwidth. Network bandwidth is typically measured in thousands of bits per second
(kbps), millions of bits per second (Mbps), billions of bits per second (Gbps) and trillions of bits per
second (Tbps). The theoretical bandwidth of a network is an important consideration in network
design. If the theoretical bandwidth of a network connection is known, the formula T=S/BW (transfer
time = size of file / bandwidth) can be used to calculate potential data transfer time. However the
actual bandwidth, referred to as throughput, is affected by multiple factors such as network devices
and topology being used, type of data, number of users, hardware and power conditions.
Data can be encoded on analog or digital signals. Analog bandwidth is a measure of how much of the
electromagnetic spectrum is occupied by each signal. For instance an analog video signal that
requires a wide frequency range for transmission cannot be squeezed into a smaller band. 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. Unlimited amounts ofinformation can be sent over the smallest digital bandwidth channel.