Chapter 6 Computer networks and security

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Chapter 6 Computer networks and security
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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

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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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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.

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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
• Fax
• Video conferencing
• Telecommuting
• Global Positioning System (GPS)
• Shared Resources
• Online Services

Refer
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Telecommuting

• Telecommuting is a work arrangement in which
  employees work away from a companys standard
  workplace, but communicate with the office using
  some communications technology.
• A telecommuter often works at home and connects
  to the main offices 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 receivers 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 worlds largest network, the Internet,
  provides connections for millions of computers
  all over the globe.

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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

• CHINAnet?????????? CHINAGBN???????
• CERnet??????????? CSTnet?????

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Network Organization

• A computer network includes several parts

Computers
Hardware
Connecting Devices
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
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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
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Coaxial Cable
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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 servers
  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
Application
7
Presentation
6
Session
5
Transport
4
Network
3
Data Link
2
Physical
1
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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-to-node
  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-to-destination 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 (MAN)
Networks
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.
• Its 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.
• Its 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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ISP network
INTERNET
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
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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 ATT.
• --- 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 users 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
Application

7
FTP
SMTP
HTTP
TELNET
Presentation
6
Session
5
Transport
4
TCP
UDP
Network
IP
3
Data Link
2
Other Protocols
Physical
1
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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
11111111
.
.
.
0
0
0
0
.
.
.
255
255
255
255

• The structure of an IP address

.
.
.
10
1
0
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Network address
Host No.
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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 Description
com Commercial businesses
edu Four-year colleges and universities
gov Government agencies
int Organizations established by international treaties
mil Military organizations
net Internet administrative organizations
org 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
respond
files
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
Path
Host computer

• 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 recipients 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 Internets
  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
• ) Smile
• ( Frown
• Indifferent
• \ Undecided
• o 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
  movie or program.

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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 others mistakes.
• Never read someones 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 isnt a library. Its a television.
• The Internet isnt about information. Its 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
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Tips for preventing virus infections
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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 senders 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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Thats all for this chapter!