PUI CHING MIDDLE SCHOOL
AL Computer Studies Notes
Chapter 3
Applications of Information Technology
by Leung Man Kit and Ma Hoi Hung
3.1
An overview of office automation
3.2
Network Communications
3.3
Applications in industry, commerce, science and education
Pui Ching Middle School AL Computer Studies Notes
Network communication — Concept and terminology (§3.2)
Why linking up computers?
 Computer networking enables the sharing of hardware, software, data and other
resources.
 Computer networks also provide a base for communication among users.
Bandwidth
Bandwidth is defined as the amount of information that can flow through a network
connection in a given period of time.
 It is essential to understand the concept of bandwidth when studying networking for the
following four reasons:




Bandwidth is finite.
Bandwidth is not free.
Bandwidth is a key factor in analyzing network performance, designing new
networks, and understanding the Internet.
The demand for bandwidth is ever increasing.
Measurement
 In digital systems, the basic unit of bandwidth is bits per second (bps). Bandwidth is the
measure of how much information, or bits, can flow from one place to another in a given
amount of time, or seconds. Although bandwidth can be described in bits per second,
usually some multiple of bits per second is used.
Limitations
 Bandwidth varies depending upon the type of media as well as the LAN and WAN
technologies used.
 The physics of the media account for some of the difference. Signals travel through
twisted-pair copper wire, coaxial cable, optical fiber, and air. The physical differences in
the ways signals travel result in fundamental limitations on the information-carrying
capacity of a given medium.
 However, the actual bandwidth of a network is determined by a combination of the
physical media and the technologies chosen for signaling and detecting network signals.
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Throughput
 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
 For example, a typical LAN might be built to provide 100 Mbps to every desktop
workstation, but this does not mean that each user is actually able to move one hundred
megabits of data through the network for every second of use. This would be true only
under the most ideal circumstances.
Baud rate
 The time required to transmit a character through a network depends on the encoding
method and the signaling speed.
 Signaling speed — the number of times per second that the signal changes its value (e.g.,
its voltage)
 Baud — the number of changes per second in the signal
 A b baud line does not necessarily transmit b bits per second because each signal may
convey several bits.
 For example, if the voltages 0, 1, 2, 3, 4, 5, 6 and 7 were used, each signal value could be
used to convey 3 bits, so the bit rate would be three times the baud rate.
 If there are only 0s and 1s used as signal levels, the bit rate is equal to the baud rate.
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Pui Ching Middle School AL Computer Studies Notes
Types of networks
 Most data networks can be classified as either local area networks (LANs) or wide area
networks (WANs).
 LANs are usually located in single buildings or campuses, and handle interoffice
communications.
 WANs cover a large geographical area, and connect cities and countries.
 For your reference:
Distance between CPUs
0.1 m
10 m
Location of CPUs
Printed circuit board
personal data assistant
Millimeter
Mainframe
Room
100 m
Building
1000 m = 1 km
Campus
100000 m = 100 km
Country
1000000 m = 1000 km
Continent
10000000 m = 10000 km
Planet
100000000 m = 100000 km
Earth-moon system
1.0 m
Name
Motherboard
Personal Area Network (PAN)
Computer Systems Network
Local Area Network (LAN)
your classroom
Local Area Network (LAN)
your school
Local Area Network (LAN)
Stanford University
Wide Area Network (WAN)
A large company
Wide Area Network (WAN)
Africa
Wide Area Network (WAN)
The Internet
Wide Area Network (WAN)
Earth and artificially satellites
 LANs are designed to:
 operate within a limited geographic area;
 allow multi-access to high-bandwidth media;
 control the network privately under local administration;
 provide full-time connectivity to local services;
 connect physically adjacent devices.
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Pui Ching Middle School AL Computer Studies Notes
Network topology
Network topology defines the structure of the network. One part of the topology definition is
the physical topology, which is the actual layout of the wire or media. The other part is the
logical topology, which defines how the media is accessed by the hosts for sending data.
Physical topology
 A bus topology uses a single backbone cable that is terminated at both ends. All the
hosts connect directly to this backbone.
 A ring topology connects one host to the next and the last host to the first. This creates a
physical ring of cable.
 A star topology connects all cables to a central point of concentration.
 An extended star topology links individual stars together by connecting the hubs and/or
switches. This topology can extend the scope and coverage of the network.
 A hierarchical topology is similar to an extended star. However, instead of linking the
hubs and/or switches together, the system is linked to a computer that controls the traffic
on the topology.
 A mesh topology is implemented to provide as much protection as possible from
interruption of service. The use of a mesh topology in the networked control systems of a
nuclear power plant would be an excellent example. As seen in the graphic, each host has
its own connections to all other hosts. Although the Internet has multiple paths to any
one location, it does not adopt the full mesh topology.
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Logical topology
The logical topology of a network is how the hosts communicate across the medium. The two
most common types of logical topologies are broadcast and token passing.
 Broadcast topology simply means that each host sends its data to all other hosts on the
network medium. There is no order that the stations must follow to use the network. It is
first come, first serve. Ethernet works this way as will be explained later in the course.
 The second logical topology is token passing. Token passing controls network access by
passing an electronic token sequentially to each host. When a host receives the token,
that host can send data on the network. If the host has no data to send, it passes the token
to the next host and the process repeats itself. Two examples of networks that use token
passing are Token Ring and Fiber Distributed Data Interface (FDDI). A variation of
Token Ring and FDDI is Arcnet. Arcnet is token passing on a bus topology.
Protocol
Protocol suites are collections of protocols that enable network communication from one host
through the network to another host. A protocol is a formal description of a set of rules and
conventions that govern a particular aspect of how devices on a network communicate.
Protocols determine the format, timing, sequencing, and error control in data communication.
Without protocols, the computer cannot make or rebuild the stream of incoming bits from
another computer into the original format.
 Protocols control all aspects of data communication, which include the following:





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).
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Switching
Circuit Switching
 When you or your computer places a telephone call, the switching equipment within the
telephone system seeks out a physical “copper” (including fibre and radio) path all the
way from your telephone to the receiver’s telephone. This technique is called circuit
switching.
 The basic idea of circuit switching is: “one a call has been set up, a dedicated path
between both ends exists and will continue to exist until the call is finished. The
end-to-end path has to be set up before any data been sent. The elapsed time can be as
long as 10 seconds.
 During this time interval, the telephone system is hunting for a copper path (call request
propagates all the way to the destination and be acknowledged). For many computer
applications, long set-up times are undesirable.
Call request signal
Call accept signal
AB trunk
Data
Propagation delay
BC trunk
CD trunk
Time spent hunting
for an outgoing trunk
Time
A
B
C
D
 But the advantage of circuit switching is: once the setup of the dedicated path has been
completed, the only delay for data is the propagation time for the electromagnetic signal
(about 5 ms per 1000 km). Also, there is no danger of congestion.
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Message switching
 When message switching is used, no physical copper path is established in advance
between sender and receiver.
 When the sender has a block of data to be sent, it is stored in the first switching office
(i.e., router) and then forwarded later, one hop at a time. Each block is received in its
entirety, inspected for errors, and then retransmitted.
Message
Queuing delay
AB trunk
Message
BC trunk
Message
CD trunk
A
B
C
D
Time
 With message switching, there is no limit on block size, which means that routers must
have disks to buffer long blocks. That is, a single block may tie up a router-router line for
a long time.
Packet switching
 Packet switching resembles message switching, except that packet-switching networks
place a tight upper limit on block size, allowing packets to be buffered in router main
memory instead of disk.
 By making sure that no user can monopolize any transmission line very long
(milliseconds), packet-switching networks are well suited to handling interactive traffic.
Pkt
1
Pkt
2
Pkt
3
Pkt
1
AB trunk
Pkt
2
Pkt
3
Pkt
1
BC trunk
Pkt
2
Pkt
3
Time
CD trunk
A
B
C
D
 An advantage of packet switching over message switching: the first packet of a multicast
message can be forwarded before the second has fully arrived, reducing delay and
improving throughput. Actually, message switching is never applied in computer
networks.
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Differences between circuit switching and packet switching
Item
Dedicated “copper” path
Bandwidth available
Potentially wasted bandwidth
Store-and-forward transmission
Each packet follows the same route
Call setup
When can congestion occur
Charging
Circuit-switched networks
Yes
Fixed
Yes
No
Yes
Yes
At setup time
Per unit time
Packet-switched networks
No
Dynamic
No
Yes
No
No
On every packet
Per packet
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Pui Ching Middle School AL Computer Studies Notes
Networking Models
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 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 the following figure.
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TCP/IP model
The Transmission Control Protocol/Internet Protocol (TCP/IP) suite is the primary protocol
used on the Internet. TCP/IP is a suite of protocols that work together to transmit data.
 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.
 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 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.
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Pui Ching Middle School AL Computer Studies Notes
 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.
Encapsulation process
 All communications on a network originate at a source, and are sent to a destination. The
information sent on a network is referred to as data or data packets. If one computer
(host A) wants to send data to another computer (host B), the data must first be packaged
through a process called encapsulation.
 Encapsulation wraps data with the necessary protocol information before network transit.
Therefore, as the data packet moves down through the layers of the OSI model, it
receives headers, trailers, and other information.
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Routing overview
Routing is an OSI Layer 3 function. Routing is a hierarchical organizational scheme that
allows individual addresses to be grouped together. These individual addresses are treated as
a single unit until the destination address is needed for final delivery of the data.
Routing is the process of finding the most efficient path from one device to another. The
primary device that performs the routing process is the router.
 The following are the two key functions of a router:


Routers must maintain routing tables and make sure other routers know of changes
in the network topology. This function is performed using a routing protocol to
communicate network information with other routers.
When packets arrive at an interface, the router must use the routing table to
determine where to send them. The router switches the packets to the appropriate
interface, adds the necessary framing information for the interface, and then
transmits the frame.
 A router is a network layer device that uses one or more routing metrics to determine
the optimal path along which network traffic should be forwarded.
 Routing metrics are values used in determining the advantage of one route over another.
Routing protocols use various combinations of metrics for determining the best path for
data.
Routers interconnect network segments or entire networks. Routers pass data frames between
networks based on Layer 3 information. Routers make logical decisions regarding the best
path for the delivery of data. Routers then direct packets to the appropriate output port to be
encapsulated for transmission. The encapsulation and de-encapsulation process occurs each
time a packet transfers through a router.
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Pui Ching Middle School AL Computer Studies Notes
TCP/IP Transport Layer
The primary duties of the transport layer, Layer 4 of the OSI model, are to transport and
regulate the flow of information from the source to the destination, reliably and accurately.
End-to-end control and reliability are provided by sliding windows, sequencing numbers, and
acknowledgments.
Flow control
 As the transport layer sends data segments, it tries to ensure that data is not lost. A
receiving host that is unable to process data as quickly as it arrives could be a cause of
data loss. The receiving host is then forced to discard it.
 Flow control avoids the problem of a
transmitting host overflowing the buffers in
the receiving host.
 TCP provides the mechanism for flow
control by allowing the sending and
receiving host to communicate. The two
hosts then establish a data-transfer rate that
is agreeable to both.
 Congestion can occur during data transfer for two reasons. First, a high-speed computer
might be capable of generating traffic faster than a network can transfer it. Second, if
many computers simultaneously need to send datagrams to a single destination, that
destination can experience congestion, although no single source caused the problem.
 When datagrams arrive too quickly for
a host or gateway to process, they are
temporarily stored in memory. If the
traffic continues, the host or gateway
eventually exhausts its memory and
must discard additional datagrams that
arrive.
 Instead of allowing data to be lost, the
transport function can issue a “not
ready” indicator to the sender. Acting
like a stop sign, this indicator signals
the sender to stop sending data.
 When the receiver can handle additional data, the receiver sends a “ready” transport
indicator. When this indicator is received, the sender can resume the segment
transmission.
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Three-way handshake
 TCP is a connection-oriented protocol. TCP requires connection establishment before
data transfer begins. For a connection to be established or initialized, the two hosts must
synchronize their Initial Sequence Numbers (ISNs).
 Synchronization is done through an exchange of connection establishing segments that
carry a control bit called SYN, for synchronize, and the ISNs. Segments that carry the
SYN bit are also called “SYNs". This solution requires a suitable mechanism for picking
an initial sequence number and a slightly involved handshake to exchange the ISNs.
 The synchronization requires each side to send its own initial sequence number and to
receive a confirmation of exchange in an acknowledgment (ACK) from the other side.
Each side must also receive the INS from the other side and send a confirming ACK. The
sequence is as follows:
1.
A→B SYN—(A) initial sequence number is X, ACK number is 0, SYN bit is set,
but ACK bit is not set.
2.
B→A ACK—(A) sequence number is X + 1, (B) initial sequence number is Y, and
SYN and ACK bit are set.
3.
A→B ACK—(B) sequence number is Y + 1, (A) sequence number is X + 1, the
ACK bit is set, but the SYN bit is not set.
 This exchange is called the three-way handshake.
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Pui Ching Middle School AL Computer Studies Notes
Windowing
 Data packets must be delivered to the recipient in the same order in which they were
transmitted to have a reliable, connection-oriented data transfer. The protocol fails if any
data packets are lost, damaged, duplicated, or received in a different order.
 An easy solution is to have a recipient acknowledge the receipt of each packet before the
next packet is sent.
 If the sender must wait for an acknowledgment after sending each packet, throughput
would be low. Therefore, most connection-oriented, reliable protocols allow more than
one packet to be outstanding on the network at one time. Because time is available after
the sender finishes transmitting the data packet and before the sender finishes processing
any received acknowledgment, this interval is used for transmitting more data.
 The number of data packets the sender is allowed to have outstanding without having
received an acknowledgment is known as the window size, or window.
 TCP uses expectational acknowledgments. Expectational acknowledgements mean that
the acknowledgment number refers to the packet that is next expected.
 Windowing refers to the fact that the window size is negotiated dynamically during the
TCP session. Windowing is a flow-control mechanism.
 Windowing requires that the source device receive an acknowledgment from the
destination after transmitting a certain amount of data. The receiving TCP process
reports a “window” to the sending TCP. This window specifies the number of packets,
starting with the acknowledgment number, that the receiving TCP process is currently
prepared to receive.
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 With a window size of three, the source device can send three packets to the destination.
The source device must then wait for an acknowledgment. If the destination receives the
three packets, it sends an acknowledgment to the source device, which can now transmit
three more packets. If the destination does not receive the three packets, because of
overflowing buffers, it does not send an acknowledgment. Because the source does not
receive an acknowledgment, it knows that the packets should be retransmitted, and that
the transmission rate should be slowed.
 TCP window sizes are variable during the lifetime of a connection. Each
acknowledgement contains a window advertisement that indicates the number of bytes
the receiver can accept. TCP also maintains a congestion-control window. This window
is normally the same size as the window of the receiver. However, this window is cut in
half when a packet is lost, perhaps as a result of network congestion. This approach
permits the window to be expanded or contracted as necessary to manage buffer space
and processing. A larger window size allows more data to be processed.
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Pui Ching Middle School AL Computer Studies Notes
DNS (Domain Name System)
URL stands for Uniform Resource Locator. The URL specifies the Internet address of a file
stored on a host computer connected to the Internet. Every file on the Internet, no matter what
its access protocol, has a unique URL. Web software programs use the URL to retrieve the
file from the host computer and the specific directory in which it resides. This file is then
displayed on the monitor connected to the user's local machine.
URLs are translated into numeric addresses using the Domain Name System (DNS). The
DNS is a worldwide system of servers that stores location pointers to Web sites. The numeric
address, called the IP (Internet Protocol) address, is actually the "real" URL. Since numeric
strings are difficult for humans to use, alphanumeric addresses are employed by end users.
Once the translation is made by the DNS, the browser can contact the Web server and ask for
a specific file located on its site.
Anatomy of a URL
This is the format of the URL:
protocol://host/path/filename
For example, this is a URL on the home page of the House Committee on Agriculture of the
U.S. House of Representatives:
http://www.house.gov/agriculture/schedule.htm
This URL is typical of addresses hosted in domains in the United States. Structure of this
URL:
1. Protocol: http
2. Host computer name: www
3. Second-level domain name: house
4. Top-level domain name: gov
5. Directory name: agrictulture
6. File name: schedule.htm
Note how much information about the content of the file is present in this well-constructed
URL.
Other examples:
telnet://opac.albany.edu
the University at Albany library text-based catalog
ftp://ftp.uu.net/graphics/picasso a file at an ftp site
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Several top-level domains (TLDs) are common in the United States:
com
commercial enterprise
edu
educational institution
gov
U.S. government entity
mil
U.S. military entity
net
network access provder
org
usually nonprofit organizations
New domain names were approved in November 2000 by the Internet Corporation for
Assigned Names and Numbers (ICANN): .biz, .museum, .info, .pro (for professionals) .name
(for individuals), .aero (for the aerospace industry), and .coop (for cooperatives).
In addition, dozens of domain names have been assigned to identify and locate files stored on
host computers in countries around the world. These are referred to as two-letter Internet
country codes, and have been standardized by the International Standards Organization as
ISO 3166. For example:
ch
Switzerland
de
Germany
jp
Japan
uk
United Kingdom
In Unix, DNS can be queried interactively using the command nslookup.
Network operating system
An operating system which includes software to communicate with other computers via a
network. This allows resources such as files, application programs, and printers to be shared
between computers.
Examples are Berkeley Software Distribution Unix, Novell, LAntastic, MS LAN Manager.
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