Module Overview
2.1 Networking
Terminology
2.1.1 Data
networks
2.1.2 Network
history
2.1.3 Networking
devices
2.1.4 Network
topology
2.1.5 Network
protocols
2.1.6 Local-area
networks (LANs)
2.1.7 Wide-area
networks (WANs)
2.1.8
Metropolitan-area networks (MANs)
2.1.9
Storage-area networks (SANs)
2.1.10 Virtual
private network (VPN)
2.1.11 Benefits
of VPNs
2.1.12 Intranets
and extranets
2.2
Bandwidth
2.2.1 Importance
of bandwidth
2.2.2 The
desktop
2.2.3
Measurement
2.2.4
Limitations
2.2.5 Throughput
2.2.6 Data
transfer calculation
2.2.7 Digital
versus analog
2.3 Networking
Models
2.3.1 Using
layers to analyze problems in a flow of materials
2.3.2 Using
layers to describe data communication
2.3.3 OSI
model
2.3.4 OSI
layers
2.3.5
Peer-to-peer communications
2.3.6 TCP/IP
model
2.3.7 Detailed
encapsulation process
Module: Summary
Overview
Bandwidth
decisions are among the most important considerations when a network is
designed. This module discusses the importance of bandwidth and explains how it
is measured.
Layered models
are used to describe network functions. 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.
This module also
includes a brief history of networking. Students will learn about network
devices and different types of physical and logical layouts. This module also
defines and compares LANs, MANs, WANs, SANs, and VPNs.
This module
covers some of the objectives for the CCNA 640-801, INTRO 640-821, and ICND
640-811 exams.
Students who
complete this module should be able to perform the following tasks:
- Explain the importance of
bandwidth in networking
- Use an analogy 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
OSI model
- 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
2.1 Networking Terminology
2.1.1 Data networks
This page will
discuss the evolution of data networks.
Data networks developed as a
result of business applications that were written for microcomputers. The microcomputers were not connected so
there was no efficient way to share data among them. It was not efficient or cost-effective for
businesses to use floppy disks to share data. 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 computer networking could increase productivity and save money.
Networks were added and expanded almost as rapidly as new network technologies
and products were introduced. The early development of networking was
disorganized. However, a tremendous expansion occurred in the early 1980s.
In the mid-1980s,
the network technologies that emerged were created with a variety of 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. As a result, many of the
network technologies were incompatible with each other. It became increasingly
difficult for networks that used different specifications to communicate with each
other. Network equipment often had to be replaced to implement new
technologies.
One early
solution was the creation of local-area network (LAN) standards. LAN standards provided an open set of
guidelines that companies used to create network hardware and software. As a
result, the equipment from different companies became compatible. This allowed
for stability in LAN implementations.
In a LAN system,
each department of the company is a kind of electronic island. As the use of
computers in businesses grew, LANs became insufficient.
A new technology
was necessary to share information efficiently and quickly within a company and
between businesses. 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.
The next page
will discuss the history of computer networks.
2.1
Networking Terminology
2.1.2 Network history
This page
presents a simplified view of how the Internet evolved.
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 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 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.
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 the point-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.
The next page
will discuss network devices.
2.1 Networking Terminology
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. 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.
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. Bridges provide 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.
2.1 Networking Terminology
2.1.4 Network topology
This page will introduce
students to the most common physical and logical network topologies.
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 hosts access the
media to send 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.
- An extended star topology links
individual stars together by connecting the hubs or switches.
- A hierarchical topology is
similar to an extended star. However, instead of linking the hubs 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.
For example, a nuclear power plant might use a mesh topology in the
networked control systems. 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 determines how the hosts communicate across the medium. The two most
common types of logical topologies are broadcast and token passing.
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. 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. The diagram includes many symbols and
networking concepts that will take time to learn.
The next page
will discuss network protocols.
2.1 Networking Terminology
2.1.5 Network protocols
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 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:
- 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).
The next page will introduce
local-area networks (LANs).
2.1 Networking Terminology
2.1.6 Local-area networks (LANs)
This page will
explain the features and benefits of LANs.
LANs consist of
the following components:
- Computers
- Network interface cards
- Peripheral devices
- Networking media
- Network devices
LANs allow
businesses to locally share computer files and printers efficiently and make
internal communications possible. A good example of this technology is e-mail.
LANs manage data, local communications, and computing equipment.
Some common LAN
technologies include the following:
- Ethernet
- Token Ring
- FDDI
The next page
will introduce wide-area networks (WANs).
2.1 Networking Terminology
2.1.7 Wide-area networks (WANs)
This page will
explain the functions of a WAN.
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.
WANs allow computers, printers, and other devices on a LAN to be shared with
distant locations. WANs provide instant communications across large geographic
areas.
Collaboration
software provides access to real-time information and resources and allows
meetings to be held remotely. WANs have created a new class of workers called
telecommuters. These people never have to leave their homes to go to work.
WANs are designed
to do the following:
- Operate over a large and
geographically separated area
- Allow users to have real-time
communication capabilities with other users
- Provide full-time remote
resources connected to local services
- Provide e-mail, Internet, file
transfer, and e-commerce services
Some common WAN technologies
include the following:
- Modems
- Integrated Services Digital
Network (ISDN)
- Digital subscriber line (DSL)
- Frame Relay
- T1, E1, T3, and E3
- Synchronous Optical Network
(SONET)
The next page
will introduce metropolitan-area networks (MANs).
2.1
Networking Terminology
2.1.8 Metropolitan-area networks (MANs)
This page will explain how MANs
are used.
Wireless bridge
technologies that send signals across public areas can also be used to create a
MAN. 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.
The next page
will introduce storage-area networks (SANs).
2.1
Networking Terminology
2.1.9 Storage-area networks (SANs)
This page will
discuss the features of SANs.
A storage-area
network (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 allow concurrent access of disk or tape arrays by two or more
servers at high speeds. This provides enhanced system performance.
- Availability
– SANs have built-in disaster tolerance. Data can be duplicated on a SAN
up to 10 km (6.2 miles) away.
- Scalability
– A SAN can use a variety of technologies. This allows easy relocation of
backup data, operations, file migration, and data replication between
systems.
The next page will introduce
virtual private networks (VPNs).
2.1 Networking Terminology
2.1.10 Virtual private network (VPN)
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 the network 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.
The next page will
explain the benefits of VPNs.
2.1
Networking Terminology
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
access to users outside the enterprise.
The next page
will introduce intranets and extranets.
2.1 Networking Terminology
2.1.12 Intranets and extranets
This page will
teach students about 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
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 on servers
such as financial, graphical, or text-based data.
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. An extranet
is the extension of two or more intranet strategies with a secure interaction
between participant enterprises and their respective intranets.
This page
concludes this lesson. The next lesson will discuss bandwidth. The first page
will explain why 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 a given 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 limitless bandwidth. 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 take advantage 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 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 more information-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 a system of highways, it is easy to see how low bandwidth
connections can cause traffic to become congested all over the network.
The next page
will discuss how bandwidth is measured.
2.2
Bandwidth
2.2.3
Measurement
This page will explain how
bandwidth is measured.
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, not because it has a higher speed.
The next page
will discuss the limitations of bandwidth.
2.2 Bandwidth
2.2.4 Limitations
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.
Figure shows some common networking media types
along with their distance and bandwidth limitations.
Figure summarizes common WAN services and the
bandwidth associated with each service.
The next page
will discuss the concept of throughput.
2.2
Bandwidth
2.2.5 Throughput
This page explains the concept
of 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 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 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.
The next page explains data
transfer calculation.
2.2
Bandwidth
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 the
fastest 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?
The next page
will compare analog and digital signals.
2.2 Bandwidth
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 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 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 to digital bandwidth.
This page
concludes this lesson. The next lesson will discuss networking models. The
first page will discuss the concept of layers.
2.3 Networking Models
2.3.1 Using layers to analyze problems in a flow of
materials
This page
explains how layers are used to describe communications between computers.
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.
The next page
will explain how layers are used to describe data communication.
2.3 Networking Models
2.3.2 Using layers to describe data
communication
This page
describes the importance of layers in 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.
The next page explains the OSI
model.
2.3 Networking Models
2.3.3 OSI model
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 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.
In the
Interactive Media Activity, students will identify the benefits of the OSI
model.
The next page
will describe the layers of the OSI model.
2.3
Networking Models
2.3.4 OSI layers
This page discusses the seven
layers of the OSI model.
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.
In the following Interactive
Media Activity, the student will identify the seven layers of the OSI model.
The next page
discusses peer-to-peer communications through the OSI model.
2.3
Networking Models
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 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.
The next page describes the
TCP/IP reference model.
2.3
Networking Models
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 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:
- 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:
- 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 model 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.
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.
2.3
Networking Models
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. 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:
- Build the data
– As a user sends an e-mail message, its alphanumeric characters are
converted to data that can travel across the internetwork.
- 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.
- 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.
- 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.
- 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.
Summary
This page
summarizes the topics discussed in this module.
Computer networks
developed in response to business and government computing needs. Applying
standards 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 allow
access 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
of information can be sent over the smallest digital bandwidth channel.
The concept of
layers is used to describe communication from one computer to another. Dividing
the network into layers provides the following advantages:
- Reduces complexity
- Standardizes interfaces
- Facilitates modular engineering
- Ensures interoperability
- Accelerates evolution
- Simplifies teaching and
learning
Two such layered
models are the Open System Interconnection (OSI) and the TCP/IP networking
models. In the OSI reference model, there are seven numbered layers, each of
which illustrates a particular network function: application, presentation,
session, transport, network, data link, and physical. The TCP/IP model has the
following four layers: application, transport, Internet, and network access.
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. The TCP/IP application
layer is equivalent to the OSI application, presentation, and session layers.
The TCP/IP model combines the OSI data link and physical layers into the
network access layer.
No matter which
model is applied, networks layers perform the following five conversion steps
in order to encapsulate and transmit data:
- Images and text are converted
to data.
- The data is packaged into
segments.
- The data segment is
encapsulated in a packet with the source and destination addresses.
- The packet is encapsulated in a
frame with the MAC address of the next directly connected device.
- The frame is converted to a pattern
of ones and zeros (bits) for transmission on the media.
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