Chapter 6
Computer networks and
security
1
Outline
Network Overview
–
–
–
Definition
The Internet Then and Now
Network organization
OSI Model
Categories of Networks
Connecting Devices
The Internet and TCP/IP Protocol
Security
2
Objectives
• Understand the rationale for the existence of
networks.
• Distinguish between the three types of networks:
LANs, MANs, and WANs.
• Understand the OSI model and TCP/IP.
• List different connecting devices and the OSI
layers in which each device operates.
• Define virus and explain how it works.
• Define four aspects of security in a network:
privacy, authentication, integrity, and
nonrepudiation.
3
Data communications
• Data communications, the technology that
enables computers to communicate, is
defined as the transmission of text, numeric,
voice or video data from one machine to
another.
4
Data communications
• There are the four components of data
communications:
– Sender: the computer that is sending the
message.
– Receiver: the computer receiving the message.
– Channel: the media that carries or transports the
message. This could be telephone line, coaxial
cable, microwave signal, or fiber optic.
– Protocol: the rules that govern the orderly
transfer of the data sent.
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Uses of communications
•
•
•
•
•
•
•
•
Email
Voice mail
Refer
Fax
Video conferencing
Telecommuting
Global Positioning System (GPS)
Shared Resources
Online Services
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Telecommuting
• Telecommuting is a work arrangement in which
employees work away from a company’s standard
workplace, but communicate with the office using
some communications technology.
• A telecommuter often works at home and connects
to the main office’s network using a personal
computer equipped with communications software
and a communications device.
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Global Positioning System
• A global positioning system(GPS) consists of one
or more earth-based receivers that accept and
analyze signals sent by satellites in order to
determine the receiver’s geographic location.
• A GPS receiver can be handheld or mounted on an
object such as an automobile, boat, airplane, farm
and construction equipment, or a computer.
• A GPS often is used to locate a person or object;
ascertain the best route between two point;
monitor the movement of a person or object; or
create a map.
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Communications channel
• Communications channel is the communications
path between two devices. A communications
channel is composed of one or more transmission
media. Transmission media consists of materials
or techniques capable of carrying a signal.
• Physical transmission media use wire, cable, and
other tangible materials to send communications
signals; wireless transmission media send
communications signals through the air or space
using radio, microwave, and infrared signals.
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Communications protocols
• A protocol is a set of rules and procedures
for exchanging information among
computers. Protocols define how the
communications channel is established, how
information is transmitted, and how errors
are detected and corrected.
• Ethernet and TCP/IP are the most widely
used protocols.
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Ethernet
• Ethernet is a LAN protocol that allows personal
computer to contend for access to the network. Today,
Ethernet is the most popular LAN protocol because it
is relatively inexpensive and easy to install and
maintain.
• Ethernet is based on a bus topology, but Ethernet
networks can be wired in a star pattern by using a hub.
• The maximum transmission rate on a standard
Ethernet network is 10 Mbps. Fast Ethernet can
transmit at 100Mbps, Gigabit Ethernet provides an
even higher speed of transmission, 1,000 Mbps.
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TCP/IP
• Short for Transmission control
protocol/Internet protocol, TCP/IP is a set of
protocols used to manage the transmission
of data by breaking it up into packets.
• TCP/IP is widely used on the Internet.
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Intranets
• Internal networks that use Internet and Web
technologies are called intranets (intra
means inside).
• An intranet, sometimes called an enterprise
network, essentially is a small version of the
Internet used within an organization: it uses
TCP/IP protocols; supports multimedia Web
pages, and is accessible via a browser.
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Firewalls
• To prevent unauthorized access to data and
information, an intranet is often protected by a
firewall.
• A firewall is a general term that refers to both
hardware and software used to restrict access to
data and information on a network.
• Organizations use firewalls to deny network access
to outsiders and to restrict employees’ access to
sensitive data such as payroll or personal records.
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Network Overview
Definition
A computer network is a collection of
computer and other devices that have been
connected through transmission media in order
to share data, hardware, and software.
--The world’s largest network, the Internet,
provides connections for millions of computers
all over the globe.
15
 The Internet Then and Now
In 1957, the Soviet Union launched the first artificial
satellite. In response to this display, the U.S.
government set up the Advanced Research Projects
Agency (ARPA).
ARPA proposed a project intended to help scientists
communicate and share valuable computer resources.
The ARPANET, created in 1969, connected
computers at four universities (the University of
California at Santa Barbara, the University of
California at Los Angeles, the Stanford Research
Institute, and the University of Utah).
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 The Internet Then and Now
 In 1985, the National Science Foundation (NSF)
used ARPANET technology to create a similar, but
larger network.
 The NSF network was an original internet. As this
network grew throughout the world, it became
known as the Internet.
 In 1995, the feature of Internet was changed from
scientific into commercial because three
corporations (Pacific BELL, Ameritech Advanced
Data Services and Bell core, Sprint) began to
manage.
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Internet Architecture in China
INTERNET
CHINANET
CERNET
CSTNET
CHINAGBN
CHINAnet:中国公用计算机互联网 CHINAGBN:中国金桥信息网
CERnet:中国教育和科研计算机网 CSTnet:中国科技网
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 Network Organization
 A computer network includes several parts:
Computers
Connecting Devices
Hardware
Transmission Media
Network Operating
System
Software
Network Protocol
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Computers
• In a network, there are two basic types of
computers: server and client.
• Most services of the application layer apply the
client/server architecture.
• Client requests services and Server responds to
them.
Request
Respond
Client
Server
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Transmission Media
There are three principal types of transmission medium:
 Twisted-Pair cable : consists of pairs of copper wire
twisted together.
Coaxial Cable : is a high-capacity communications
cable. It is used to carry television signals.
 Fiber-Optic Cable: is a bundle of thin tubes (optical
fiber) of glass. It cannot conduct or transmit electrical
signals. However, lasers send pulses of light through
the fibers.
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Twisted-Pair Cable
22
Coaxial Cable
23
Fiber-Optic Cable
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Network Operating Systems
 A network operating system (NOS) is the software that
manages network resources, controls the flow of data,
maintains security, and tracks user accounts.
 A NOS has two components: network server software
and network client software.
Network server software is installed on a file server,
controls file access from the server’s hard disk,
manages the print queue, and tracks user data.
Network client software is installed on the local hard
disk of each workstation, gathers user login
information, handles drive mapping, and directs
printouts.
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OSI Model
The Open Systems Interconnection (OSI) was
designed by the International Organization for
Standardization (ISO).
 The OSI model is a theoretical model that
shows how any two different systems can
communicate with each other.
 The OSI model is a framework of seven layers
that gives an idea of the functionality of each
separate but related layer.
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The seven layers of the OSI model
7
Application
6
Presentation
5
Session
4
Transport
3
Network
2
Data Link
1
Physical
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Flow of data in the OSI model
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OSI Model
 Physical Layer: is responsible for transmitting a
bit stream over a physical medium.
 Data-Link Layer: is responsible for node-tonode delivery of a frame between two adjacent
stations.
 Network Layer: is responsible for delivery of a
packet between the original source and final
destination.
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OSI Model
 Transport Layer: is responsible for source-todestination delivery of the entire message.
 Session Layer: is designed to control the
dialog between users.
 Presentation Layer: is concerned with the
syntax and semantics of the information
exchanged between two systems.
 Application Layer: enables the user to access
the network.
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Categories of Networks
 We can divide networks into three broad
categories:
Local Area Network
(LAN)
Metropolitan Area Network
Networks
(MAN)
Wide Area Network
(WAN)
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Local Area Network
A network that is located within a relatively
limited area such as a building or campus is
referred to as a local area network (LAN).
 The pattern or path of the interconnections
in a communications system is referred to as
Topological Structure (Topology).
There are three typical topological structures:
 Bus Topology
 Star Topology
 Ring Topology
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Bus Topology
The bus is the physical
cable that connects the
computers and other
devices.
It is used primarily for
LANs.
It’s flexible in that
computers and other
devices can be attached or
detached from the network
at any point without
disturbing the rest of the
network.
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Star Topology
All of the devices are
connected via a central
device (hub or switch).
Any two computers should
transport data through the
central device.
The benefit is every
connection is dedicated to
one user and the user gets
use of the full bandwidth of
the channel.
The disbenefit is the high
cost of the media and
equipment to individual
connections.
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Ring Topology
It connects devices in a
continuous loop.
The signal leaves the
sending device, travels in
sequence to each of the
devices connected to the
loop, then return to the
sending device.
It’s the extension of the
bus network in which the
ends of the bus are
connected.
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Metropolitan Area Network
MANs span a city or a town and provides services
to individual users or organizations.
MANs use services provided by a common carrier
such as a telephone company.
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Wide Area Network
A network that
covers a large
geographical
area is referred
to as a wide area
network (WAN).
WANs, like
MANs, are
installed and run
by common
carriers.
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Connecting Devices - NIC
 Network Interface Card (NIC):
--- Is a small circuit board that sends data
from the workstation out to the network and
collects incoming data for the workstation.
--- A desktop computer NIC plug into an
expansion slot on the motherboard.
--- A notebook computer NIC is usually a
PCMCIA card.
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Notebook computer
NIC
Desktop computer
NIC
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Connecting Devices - Hub
Network Hub:
---The cable from a workstation NIC
connects to a network hub, which is a
device that joins communications lines
together.
---In a typical network configuration, cables
from one or more workstations connect to
the hub, then a single cable connects the
hub to a server.
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Connecting Devices - Modem
Modem:
---A device that sends and receives data to and
from computers over telephone lines or
cables.
---It includes two procedures: modulation and
demodulation.
---Modulation process converts digital signals
into analog signals.
---Demodulation process converts analog
signals into digital signals.
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43
ISP network
INTERNET
Telephone network
PPP
Account password
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Connecting Devices - Repeater
Repeater:
---It is an electronic device that regenerates data and
sends data to the rest of the network.
---It operates only in the physical layer of the OSI
model.
---It is popularly used in the bus topology network to
increase the length of the network.
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Connecting Devices - Bridge
Bridge:
---It is a traffic controller, which divides a long bus into
smaller segments so that each segment is independent
trafficwise.
---It operates at the first two layers of the OSI model.
---It not only can make two or more pairs of stations to
communicate at the same time, but also can regenerate
the frame.
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Connecting Devices - Router
Router:
---It is the device that connect LANs, MANs, and
WANs.
---It routes a packet based on the logical address
(network layer) of the packet.
---It operates at the first three layers of the OSI
model.
---It connects two independent networks: a LAN
to a WAN, a LAN to a MAN, a WAN to
another WAN, and so on.
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Routers in an Internet
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Cisco 12000 Series Internet Routers
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Connecting Devices - Gateway
Gateway:
---It is a connecting device that acts as a
protocol convert.
---It allows two networks (each with different
set of protocols for seven OSI layers) to be
connected to each other and communicate.
---It is actually a computer installed with the
necessary software.
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A gateway connects a network of PCs with a network of
Apple Macintosh computers.
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Cisco AS5800
Series
Universal
Gateway
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Connecting devices and the OSI model
The Internet and TCP/IP Protocol
 Internet Technologies:
Data can travel over the Internet according to the
following essentials.
 The cables, wires and satellites carry Internet
data from an interlinked communications
network.
 NSP and ISP maintain a series of
communications link for Internet data.
TCP/IP knits together the Internet and allows
data to travel smoothly over the communications
links provided by NSPs worldwide.
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NSP and ISP - NSP
 NSP and ISP:
NSP (network service provider): supply ISPs
with access to high-speed transmission lines that
form the backbone of the Internet, also provide
routers at network connection points, such as
China Telecom, China Unicom, China Netcom,
China Tietong, Sprint, or AT&T.
--- Links between NSPs interconnect at several
network access points (NAPs) so data can travel
between NSPs.
--- Internet backbone are the major Internet
communications links.
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The Internet backbone in the continental U.S.
maintained by MCI.
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NSP and ISP - ISP
 NSP and ISP:
ISP (Internet Service Provider): is a
company that maintains an Internet host
computer providing Internet access to
businesses, organizations and individuals.
--- It works in much the same way as your
local telephone company. You arrange for
service, and ISP charges you a monthly fee.
--- It typically provides you with a user
account that includes Internet access and an
e-mailbox.
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NSP and ISP
 NSP and ISP :
In order to connect
user’s computer to the
Internet, we need to
connect the computer
to an ISP that in turn
connects to the
backbone (NSP).
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TCP/IP
TCP/IP:
The Transmission Control Protocol/Internet
Protocol is a suite or a stack of protocols that
officially controls the Internet.
In other words, it is a standard set of
communication rules used by every computer
that connects to the Internet.
The layers in the TCP/IP protocol do not
exactly match those of the OSI model.
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TCP and OSI model
7 Application
6 Presentation FTP SMTP TELNET HTTP …
5 Session
4 Transport
3 Network
TCP
UDP
IP
2 Data Link
1 Physical
Other Protocols
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IP
 TCP/IP (cont.):
 IP:
is one of the main protocols of TCP/IP that is
responsible for addressing packets for routing to
their destinations.
IP Address:
a unique identifying number assigned to each
computer connected to the Internet.
--- Each IP address consists of 4 bytes (32 bits).
--- Each IP address is written in decimal form with
decimal points separating the bytes (dotted-decimal
notation).
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The range of IP address
00000000 00000000 00000000 00000000
11111111
11111111
11111111
. 0 . 0 .
255
. 255 . 255 .
 The structure of an IP address:
10
. 1 . 0 .
0
Network address
11111111
0
255
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Host No.
Domain Name
 Although an IP address works for
distinguishing computers on the Internet, it
is so difficult to remember long strings of
numbers.---The host computer also have an
easy-to-remember name, “Domain name”.
 By DNS (domain name system), any IP
address could be changed into domain name.
 A domain name includes: host computer,
network or organization name and top-level
domain.
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Internet Top-level Domains
Domain
com
edu
gov
int
mil
net
org
Description
Commercial businesses
Four-year colleges and universities
Government agencies
Organizations established by
international treaties
Military organizations
Internet administrative organizations
Professional and nonprofit organizations
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TCP & FTP
TCP/IP (cont.):
 TCP:
is one of the main protocols of TCP/IP that is
responsible for establishing a data connection
between two hosts and breaking data into packets.
 FTP (File Transfer Protocol):
is a standard protocol on the Internet for transferring
a file from one machine to another.
---Establishes two connections: one for data transfer
and the other for control information.
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FTP architecture
FTP server
client
request
Control
files
respond
files
Data
download
upload
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TELNET
 TCP/IP (cont.):
 TELNET:
is a general client-server program on the Internet
that allows remote login to control another
computer or server.
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HTTP
TCP/IP (cont.):
 HTTP:
is a client-server program that is used to
access and transfer documents on the World
Wide.
URL (Uniform Resource Locator):
is the address of a Web page by using the
special method.
--- It defines four things: method, host
computer, port and path.
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URL & Domain Name
Method：
http; ftp; telnet
Host computer
Path
 Compare domain name and URL:
 Both are Internet addresses.
Domain name represents the IP address of a computer,
but URL is the address of a document on a computer.
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WWW
 World Wide Web (WWW or Web):
---The Web was born in 1990 at the European
Laboratory for Particle Physics (CERN).
---Unlike the Internet, which is simply a mass
of cables and connection points that form a
communications network, the Web is an
Internet service that stores and provides
information.
---As an easy-to-use, graphical source of
information, the Web opened up the Internet
to millions of people.
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WWW
 World Wide Web (cont.):
---The official description of the Web is a “wide-area
hypermedia information retrieval initiative aiming to
give universal access to a large universe of
documents”.
---The Web uses Hypertext that contain special text,
words and phrases that can create a link to other
documents containing text, images, audio or video.
---A document of hypertext available on the Web is
called a Web page. The main page for an
organization or an individual is known as a home
page.
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Web documents
Web Document
Static
Dynamic
Active
 There are three different types of documents
on the Internet:
 Static documents have fixed contents. They are
created at the server site and can only be copied.
They usually use Hypertext Markup Language
(HTML).
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Web Documents
 There are three different types of documents
on the Internet (cont.):
 Dynamic documents are programs residing at the
server site. The server runs the program and sends
the result to the browser. They use a technology
called Common Gateway Interface (CGI) to
handle the documents.
 Active documents are also programs, but they
cannot be run at the server site. The program is
first transferred to the browser site and then run.
They are normally written in the Java language.
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How the email works?
• Email (electronic mail) is the transmission of
messages and files via a computer network.
• To receive messages, you need an email address,
which is a combination of a user name and a
domain name that identifies a server. When you
receive an email message, the message is placed in
your mailbox. The mailbox is a storage location
residing on the mail server.
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How the email works?
• When you send a message, the message is
transmitted according to a communications
protocol called SMTP (simple mail transfer
protocol). The mail server uses SMTP to determine
how to route the message though the Internet and
then sends the message. When the message arrives
at the recipient’s mail server, the message is
transferred to a POP or POP3 server. POP (Post
Office protocol) is a communications protocol used
to retrieve email from a mail server. The POP
server holds the message until the recipient
retrieves it with his or her email software.
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Netiquette
• Golden rule: Treat others as you would like
them to treat you.
• In email, newsgroups, and chat rooms:
– Keep messages brief, using proper grammar and
spelling.
– Be careful when using sarcasm and humor, as it
might be misinterpreted.
– Be polite. Avoid offensive language.
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Netiquette
• In email, newsgroups, and chat rooms:
– Avoid sending or posting flames, which are
abusive or insulting messages. Do not
participate in flame wars, which are exchanges
of flames.
– Avoid sending spam, which is the Internet’s
version of junk mail. Spam is an unsolicited
email message or newsgroup posting sent to
many recipients or newsgroups at once.
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Netiquette
• In email, newsgroups, and chat rooms:
– Do not use all capital letters, which is the
equivalent of SHOUTING.
– Use emoticons to express emotion. Popular
emoticons include:
:)
:(
:|
:\
:o
Smile
Frown
Indifferent
Undecided
Surprised
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Netiquette
• In email, newsgroups, and chat rooms:
– Use abbreviations and acronyms for phrases
such as:
•
•
•
•
BTW by the way
FYI for your information
IMHO in my humble opinion
TYVM thank you very much
– Clearly identify a spoiler, which is a message
that reveals a solution to a game or ending to a
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movie or program.
Netiquette
• Read the FAQ (frequently asked questions),
if one exists. Many newsgroups and Web
sites have an FAQ.
• Use your user name for personal purpose
only.
• Do not assume material is accurate or up to
date. Be forgiving of other’s mistakes.
• Never read someone’s private email.
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Net LOSS?
• Question: What are some of the downside
issues relating to use of the Internet and
Web?
– The Internet isn’t a library. It’s a television.
– The Internet isn’t about information. It’s about
marketing.
– Kids want to use the Internet for entertainment.
– Other reliable high-tech resources are better for
educational uses than the Internet.
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What is a computer virus?
• A computer virus is a program that attaches
itself to a file, reproduces itself, and spreads
to other files. A virus can corrupt and/or
destroy data, display an irritating message,
or otherwise disrupt computer operations.
• Although numerous variations are known,
four major types of viruses exist: boot
sector viruses, file viruses, Trojan horse
viruses, and macro viruses.
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How a virus spreads
84
Tips for
preventing virus infections
85
Security
• There are four aspects of security: privacy
(confidentiality), message authentication,
message integrity, and nonrepudiation.
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Security
• Privacy means only the sender and the
receiver of the message are able to
understand the contents of the message.
• Authentication means the receiver needs to
be sure of the sender’s identity.
• Integrity means the message should not be
tampered during transmission.
• Nonrepudiation means the prevention of
repudiation (denial) from the sender.
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Objectives
• Understand the rationale for the existence of
networks.
• Distinguish between the three types of networks:
LANs, MANs, and WANs.
• Understand the OSI model and TCP/IP.
• List different connecting devices and the OSI
layers in which each device operates.
• Define virus and explain how it works.
• Define four aspects of security in a network:
privacy, authentication, integrity, and
nonrepudiation.
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That’s all for this chapter!
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