Electronic CommerceCOMP3210

Session 2: Internetworking and the WWW

Dr. Paul Walcott

Dr. Paul Walcott - Department of Computer Science, Mathematics and Physics, University of the West Indies, Cave Hill Campus, Barbados, W.I.; email pwalcott@uwichill.edu.bb; © 2005

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Session Objectives

After completing this session you will be able to: Describe the Internet and discuss its

history Describe the World Wide Web (WWW) and

write a simple HTML document Describe different types of computer

networks Discuss different methods of connecting to

the Internet

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The Internet

What is the Internet?

The history of the Internet

The growth of the Internet

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What is the Internet1

The Internet is a large system of inter-connected networks that span the globe

Internet-ready computers and personal digital assistants (palm and pocket PCs) can access this network2

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What is the Internet Cont’d

The Federal Networking Council (FNC) defines the Internet more formally as the global information system that3: is logically linked together by a globally

unique address space based on the Internet Protocol (IP) or its subsequent extensions/follow-ons

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What is the Internet Cont’d

is able to support communications using the Transmission Control Protocol/Internet Protocol (TCP/IP) suite or its subsequent extensions/follow-ons, and/or other IP-compatible protocols; and

provides, uses or makes accessible, either publicly or privately, high level services layered on the communications and related infrastructure described herein

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What is the Internet Cont’d

The Internet allows: communication by e-mail the reading of on-line newspapers, academic

journals and books the joining of discussion groups, participation in simulations and games downloading of software, electronic books and

music businesses to market and sell products and

services (e-commerce)

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The History of the Internet1

In the early 1960s the US department of defense began investigating ways of creating networks of computers that coordinate and control nuclear weapons

This network was to withstand attack, so that even if part of it was down it will continue to function This network was to have no central control

These new networks required multiple channels to send information (leased lines inadequate)

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The History of the Internet Cont’d In 1969 the advanced research agency

(ARPA) used this network model to connect four computers

This network was called ARPANET During the 1970s and 1980s the academic

community contributed to this network In the late 1980s this network became the

Internet

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Internet Growth1

In 1969 ARPANET (the Internet) connected 4 computers

In 1990 the Internet connected 300,000 In 1995 the intranet was privatised

The structure was based on four network access points (NAPs) operated by different telecom companies in the US

As the Intranet grew, more NAPs were added

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Internet Growth Cont’d

The NAPs began selling Internet access rights to large customers Internet service provides (ISPs) sold on the service to

small businesses and individuals Growth in Internet hosts rose from under 10

million in 1995 to over 180 million in 2004 Millions of people now use the Internet Billions of dollars are exchanged from the sales

of products and services Estimated number of Web sites is 40 million and

billions of web pages

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The World Wide Web

What is the World Wide Web?

What is hypertext? Hypertext markup

language (HTML) example

What types of Web sites exist?www.ransen.com/Repligator/Spider.htm

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What is the World Wide Web

The World Wide Web (WWW) was an Internet-based hypertext project that allowed global information sharing2

It is software that runs on machines connected to the WWW

The WWW is based on two concepts1: Hypertext and a graphical user interface (a Web browser)

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HyperText Markup Language

HTML is a language that includes a set of tags attached to text

These tags describe the relationship between text elements

A hypertext link (or hyperlink) points to another location in the same or another HTML document (that might be stored on the same or another computer)

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HTML Example

<html> <!-- An HTML document typically contains a head and a body --><head> <title>Hypertext Markup Language (HTML) Example</title></head>

<!-- This is the body part of the document --><body bgcolor="#0000AA"> <font color="#FFFFFF"> <h1><u>Hypertext Markup Language</u></h1> Hypertext Markup Language (HTML) allows users to: <ol> <li>format and display text <li>display images <li>and so much more … </ol> <img src="HTMLexample.jpg"> <a href="#top">Go to top of page</a> </font></body></html>

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Graphical Interface for Hypertext

A Web browser is a graphical user interface used to read and navigate through files written in HTML

In 1993 the first graphical web browser was developed; called Mosiac

Popular web browsers include: Internet Explorer Netscape Firefox

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Hypertext Servers (Web Servers)1

There are several Web servers available today, however the most popular are (April 2004 survey)4: Apache HTTP Server (69.01%) Microsoft Internet Information Server (IIS)

(23.26%) Sun Java System Web Server (JSWS) (former

names are Sun One, iPlanet Enterprise Server and Netscape Enterprise Server) (0.86%)

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Apache (“A Patchy” Server)

Developed in 1994 by Rob McCool The original core system has many patches

applied to it, and thus its name Apache has dominated the Web since 1996 Available free Runs on operating systems including

FreeBSD-UNIX, HP-UX, Linux, Microsoft Windows, SCO-UNIX and Solaris

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Microsoft Internet Information Server (IIS)

Comes bundled with Microsoft Windows Server operating system

IIS used on many corporate intranets (Microsoft standard product)

Originally written to run on Windows NT and Windows 2000; runs on Windows 2003 Server and Windows XP

Supports ASP, ActiveX Data objects and SQL queries

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Sun Java System Web Server

This is a descendant of the original NCSA server

Cost US$1500-per-CPU licensing fee Runs on HP-UX, Solaris and Windows Is ODBC compatible (i.e. full access to

ODBC databases)

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Web Site Types

There are numerous different types of Web sites including: Development sites: are used by companies to

evaluate web site designs. The initial investment in these sites are small since they use existing hardware (i.e. no special hardware is purchased)

Intranets: these are corporate networks that hold documents such as internal memos, corporate handbooks, expense account worksheets, budgets and newsletters

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Web Site Types Cont’d

Extranets: which allow external entities, e.g. suppliers and strategic partners to access a subset of the information on the Intranet

Transaction-processing: These sites operate 24 hours a day, seven days a week and require fast, reliable hardware and e-commerce software Web sites hosting B2B and B2C activities must have

spare capacity to support an increase in customer volume.

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Web Site Types Cont’d

Content-delivery: sites must be available seven days a week, 24 hours a day and require fast and precise search engines to help visitors locate information quickly These sites deliver content such as news,

histories and other digital information.

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Networking What is a computer

network? What are some of the

types of networks? Characteristics of

networks Network topology Network protocols Network architecture

Routers Domain Names Intranets, Extranets and

VPNs

http://www-structure.llnl.gov/Xray/comp/network.gif

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What is a Network5

A network is a group of two or more computer systems linked together

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Types of Networks6

There are several types of computer networks: Local-area network (LANs)

Computers that are geographically close to each other (in the same building)

Wide-area network (WANs) Computers are farther apart than LANs;

typically connected by telephone lines or radio waves

Provides a single path between caller and receiver (circuit switching)

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Types of Networks Cont’d

Campus-area network (CANs) These computers are within a limited

geographic area, e.g. a campus Metropolitan-area network (MANs)

These networks are designed for towns and cities

Home-area network (HANs) A network created in a person’s home

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Types of Networks Cont’d

Both LANs and WANs are examples of packet-switched networks

In packet-switched networks: Messages are broken down into small pieces

called packets and are labeled electronically with their origin, sequence and destination addresses

Each packet may take a different path The destination computer puts the packets back

together in the correct order

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Characteristics of Networks5,6

A network is characterised by its: Topology

The geometric arrangement of the computer system

Architecture This refers to the two major type of networks, peer-to-

peer or client/server

Protocol The set of rules and signals used to communicate (e.g.

Ethernet or IBM’s Token Ring)

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Network Topologies6

Common network topologies include: Star topology Ring topology Bus topology Other topologies

include the: Mesh topology Tree topology

http://www.webopedia.com/TERM/s/star_network.html

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Network Topologies Cont’d

Network topologies are considered either logical or physical The physical lay out of the network is its

physical topology The way signals are routed through a

network is the logical topology Just because a network has a particular

physical topology does not mean that its logical topology must be the same

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Network Topologies Cont’d

For example: Twisted-pair Ethernet has a logical bus

topology on a physical star topology layout

IBM’s token ring has a logical ring topology on a physical star topology

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Star Topology

In a Star network all nodes are connected through a central hub A node is a computer or device, e.g. a

printer; each node has a unique address called the Data Link Control (DLC) address or the Media Access Control (MAC) address

The main advantage of the star network is that a malfunctioning node does not affect the rest of the network

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Star Topology Cont’d

The main disadvantage of the star network is that: It requires more cabling than other

topologies (e.g. the ring network), and If the hub goes down, the network no

longer functions Standard twisted-pair Ethernet uses

the star topology

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Ring Topology

Each node in a ring network is connected to two other nodes to form a closed loop

Messages pass around the ring and each node reads the message addressed to it

One advantage of a ring network is that it can span larger distances than other networks (e.g. bus) because each node regenerates the signal

One disadvantage is that malfunctioning nodes can make other nodes inaccessible

IBM’s token ring uses a logical ring topology

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Bus Topology

The nodes in a bus topology are connected to a central cable call a bus or backbone

The bus has two end points One advantage of the bus network is that if one

node goes down the network still functions One disadvantage of the bus network is it can not

travel as long a distance as say a ring network due to the diminishing signal strength

Ethernet 10Base-2 and 10Base-5 uses the bus topology

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Mesh Topology

In the mesh topology many redundant connections exist between nodes; in a true mesh, each node is connected to every other node

http://www.webopedia.com/quick_ref/topologies.asp

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Tree Topology

The tree topology is a hybrid where groups of star-configured networks are connected to a linear bus backbone

http://www.webopedia.com/quick_ref/topologies.asp

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Architecture6

The two main types of architecture are Peer-to-Peer Client/Server

http://www.rebri.org.nz/images/builder-big.jpg

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Peer-To-Peer (P2P) Networks

In a P2P network each node has equivalent capabilities and responsibilities

These networks are simpler than Client/Server but do not offer the same performance under heavy loads

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Client/Server Networks

A network architecture where each node is either a client or server

Client computers run applications, but rely on servers for resources such as files and printing capabilities

Servers are powerful computers that manage files, printers or network traffic

This architecture is sometimes called the two-tier architecture

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Protocols

Protocols are agreed formats for transmitting data between devices6

The protocol determines: The error checking

required Data compression

method used The way the end of a

message is signaled T

The way the device indicates that it has received the message

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Protocols Cont’d

A protocol is a collection of rules for: Formatting, ordering and error checking

data that is sent across a network e.g. The protocol might indicate when a

sending device has completed sending data

The ARPANET network used the network control protocol (NCP) It was based on an open architecture

which later became the Internet

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Protocols Cont’d

Four key rules used by ARPANET were: Independent networks (e.g. A LAN or WAN)

should not require internal changes in order to be connected to the network

Packets that do not arrive at their destination must be resent from their source network

Router computers act only as receiving and forwarding devices; They do not retain information about the packets they handle

No global control of the network exist

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Internet Protocols

There are many protocols used by the Internet and the WWW, including TCP/IP HTTP FTP Electronic mail protocols

IMAP POP

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TCP/IP

The Internet uses two main protocols (developed by Vicent Cerf and Robert Kahn) Transmission control protocol (TCP)

Controls disassembly of message into packets at the origin

Reassembles at the destination

Internet protocol (IP) Specifies the addressing details for each packet

Each packet is labeled with its origin and destination

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Internet Protocol version 4 (IPv4)

Uses a 32 bit number to identify each computer Called the IP address (4 billion

addresses) IP addresses uses the dotted decimal

notation, e.g. 0.0.0.0 or 255.255.255.255

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Internet Protocol version 6 (IPv6)

Set to replace version 4 Changes the format of the packets

Removes unused fields Uses 128 bit number (2^128 addresses)

Eight groups of 16 bits e.g.

CD18:0000:0000:AF23:0000:FF9E:61B2:884D To simplify zeroes may be removed

CD18:::AF23::FF9E:61B2:884D

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Internet Addresses

Internet addresses are represented in several ways,but all the formats are translated to a 32-bit numbercalled an IP address; a function of Internet Protocol (IP)

The increased demand for IP addresses will soonmake 32-bit addresses too small, and they will bereplaced with 128-bit addresses (IP v6.0) in the near future.

How does increasing the number of bits in the address helpwith increasing demand?

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Dotted Quads

• IP numbers appear as a series of up to 4 separatenumbers delineated by a period.

• Examples:students.depaul.edu: 140.192.1.100condor.depaul.edu: 140.192.1.6 facweb.cs.depaul.edu: 140.192.33.6

• Each of the four numbers can range from 0 to 255, so the possible IP addresses range from 0.0.0.0 to 255.255.255.255

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Elements of Internet Addressing

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Domain Names

• Since IP numbers can be difficult for humans toremember, domain names are associated witheach IP address.

• Examples: students.depaul.edu: 140.192.1.100facweb.cs.depaul.edu: 140.192.33.6

• A domain name server is responsible for the mapping between domain names and IP addresses.

• Domain names and IP addresses are registered separately. Registering domain name doesn’t imply getting an IP address. IP address is from hosting service.

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Uniform Resource Locator

• People on the Web use a naming convention called the uniform resource locator (URL).

• A URL consists of at least two and as many as four parts.

• A simple two part URL contains the protocolused to access the resource followed by thelocation of the resource.Example: http://www.cs.depaul.edu/

• A more complex URL may have a file nameand a path where the file can be found.

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http://www.daschmelzer.com/ect250/hw/h1/h1.htm

hypertext transfer protocol

domainpath that indicates the location of the document in the host’s file system

documentname

A URL deconstructed

This is considered a fully specified URL

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Anatomy of an eMail Address

dschmelzer @ cti . depaul . edu

Handle Host/Server DomainDomain

Type

Others:• students• hawk• condor

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Shortened URL

Simply by typing www.cs.depaul.edu into browser allows http to locate specific file

Web servers are set to “default” to specific file names when full URL (or filename) not given.

index.htm (.html, .asp, .jsp, etc) and default.htm (etc……) are examples.

Name the template file in HW1: index.htm

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Domain types

.com Commercial organizations

or businesses .edu

Educational institutions .gov

U.S. government agencies .mil

U.S. military .net

Network computers .org

Nonprofit organizations and foundations

.biz business firms

.info information providers

.aero Air transport industry

.coop Cooperatives

.museum Museums

.name Individuals

.pro Professionals

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Hypertext Transfer Protocol (HTTP)7

The hypertext transfer protocol (HTTP) was developed by Tim Berners-Lee in 1991

HTTP was designed to transfer pages between machines

The client (or Web browser) makes a request for a given page and the Server is responsible for finding it and returning it to the client

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Hypertext Transfer Protocol (HTTP) The browser connects and requests a page from

the server The server reads the page from the file system,

sends it to the client and terminated the connection

Client Web Server

Request sent to Web server

Response sent with files (one for eachWeb page, image, sound clip etc.)

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File Transfer Protocol (FTP)2

FTP is a protocol that allows a user to: list files at a remote site Transfer files between local and remote computers

Using FTP you can not run applications on a remote site

Also you can only look at the names of the files on the remote computer; rather than being able to read them

FTP is still used today; e.g. to transfer large files between computers, or to upload Web sites

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Electronic Mail Protocols

Electronic mail uses the client/server model

The organisation has an email server devoted to handling email Stores and forwards email messages

Individuals uses email client software to read and send email (e.g. Microsoft Outlook, or Netscape Messenger)

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Electronic Mail Protocols Cont’d

Simple Mail Transfer Protocol (SMTP) Specifies format of mail messages

Post Office Protocol (POP) tells the email server to:

Send mail to the user’s computer and delete it from the server

Send mail to the user’s computer and do not delete it from the server

Ask whether new mail has arrived

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Electronic Mail Protocols Cont’d

Interactive Mail Access Protocol (IMAP) Newer than POP, provides similar

functions with additional features e.g. can send specific messages to the client

rather than all the messages A user can view email message headers and

the sender’s name before downloading the entire message

Allows users to delete and search mailboxes held on the email server

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Electronic Mail Protocols Cont’d

The disadvantages of POP You can only access messages from one

PC The disadvantage of IMAP

Since email is stored on the email server, there is a need for more and more expensive (high speed) storage space

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Terminal Emulation

Telnet is a terminal emulation program for TCP/IP networks

The telnet program runs on a local machine and allows connection to a remote machine

Commands entered through the telnet program will be executed on the Server; this gives the user the ability to control the Server

To log into the Server a valid username and password is required

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Routers

Computers that decide how best to forward on a packet to its destination is called a routing computer

The Internet uses routers to isolate each LAN or WAN

Each WAN or LAN can use their own protocol for packet traffic within the LAN or WAN

The actual programs which contains the routing rules are called routing algorithms

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Routers Cont’d

These programs apply their routing algorithms to information they have stored in routing tables This information includes lists of

connections that lead to particular groups of other routers

Which connections to use first Rules for handling instances of heavy

packet traffic and network congestion

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Routers Cont’d

When a company connects to the Internet it must connect at least one router to routers owned by other companies that make up the Internet

The Internet backbone are a set of routers that handle packet traffic along the Internet’s main connecting points These are very large computers that can handle

more than 50 million packets per second A router connected to the Internet always has

more than one path to direct packets

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Domain Names

To make Internet addresses easier to remember an alternative, domain names, was provided which used words

Domain names May have two or more groups separated by a

period, e.g. www.cavehill.uwi.edu Edu – an education institution Cavehill and uwi – the computer’s name www – part of the world wide web

Not all computers follow this convention, e.g games.yahoo.com

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Domain Names Cont’d

The rightmost part of the domain name is called the top-level domain (or TLD), e.g. .edu, or .com also Country domains e.g. .bb

In 2000 seven new general TLDs were added: .aero – air transport industry .biz – businesses .coop – cooperatives .museum – museums .name – individuals .pro - professionals

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Uniform Resource Locator

The combination of the domain name and the protocol name is called the uniform resource locator (URL) E.g. http://www.yahoo.com

http is the protocol www.yahoo.com is the domain name

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Intranets and Extranets

An Intranet is an interconnected network (internet) that does not extend beyond the organisation that created it

An extranet is an intranet that has extended to include specific entities outside of the organisation, e.g. business partners, customers or suppliers Extranets can replace faxes, telephones, email

and overnight carriers, at a lower cost

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Intranets

Low cost distribution of internal corporate information

Based on client server model Intranets use Web browsers, internet-based

protocols including TCP/IP, FTP, Telnet, HTML and HTTP

Distributing paper is often more expensive Intranets can also be used to provide software

updates and patches, which update users computers automatically – using scripts

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Extranet

Networks that connect companies with suppliers, business partners and authorised users

Each user has access to the databases, files and other information stored on computers connected to the extranet

Fedex changed from an Intranet to Extranet for package tracking on their Web site

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Public and Private Networks

A public network is any network available to the public, e.g. Internet

A private network is a private (e.g leased) line between two computers Leased line is a permanent connection between two

points line is always active (unlike telephone connection) The advantage is security (only the two parties can

connect to the network) The disadvantage is the cost due to the scaling problem:

to connect 5 pairs, 5 lines are required

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Virtual Private Networks

A VPN is an extranet that uses public networks and their protocols to send sensitive information using IP tunneling

IP tunneling creates a private passageway through the public Internet that provides secure transmissions

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Virtual Private Networks Cont’d

VPN software encrypts the package content and places it into another packet (encapsulation)

The outer packet is called an IP wrapper The receiving computer decrypts it using the (VPN

software) The connection is ‘virtual’ since it is only temporary,

it is created when information needs to be transmitted

A VPN is an extranet, but not every extranet is a VPN

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Connecting to the Internet1

What are some of the ways of connecting to the Internet?

http://www.co.delaware.ny.us/links.htm

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Internet Connection Options

Internet service providers (ISPs) provide several ways to connect to the Internet, including: Voice grade telephone lines Broadband connections Leased lines Wireless

The major distinguishing factor is bandwidth (the amount of data that can be transferred per unit of time)

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Bandwidth and Connections

Symmetric connection: provides the same bandwidth in both directions

Asymmetric connections: provide different bandwidths for either direction Upstream bandwidth (upload bandwidth): the

amount of information that can travel from the user to the Internet in a given amount of time

Downstream bandwidth (download or downlink bandwidth): the amount of information that can be transferred from the Internet to the user in an amount of time

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Voice-grade Telephone Connections

Most common way to connect to ISP Modem (analog) connected to telephone lines POTS (plain old telephone service)

28 to 56Kbps

Digital subscriber line (DSL) protocol Does not use a modem Uses a piece of network equipment similar to a

network switch Integrated services digital network (ISDN) first used DSL

protocol suite in 1984 More expensive, but offers bandwidth of 128Kbps –

256Kbps

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Broadband Connections Cont’d

Connections that operate at speeds higher than 200Kbps are considered broadband

One of the latest is asymmetric digital subscriber line (ADSL) Bandwidths from 100-640Kbps upstream and

from 1.5 to 9 Mbps downstream For businesses a high-speed DSL (HDSL) is

available 768Kbps of symmetric bandwidth

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Broadband Connections Cont’d

Cable modems (connected to the same broadband coaxial cable that serves a television) Upstream bandwidth of 300Kbps to 1

Mbps and downstream bandwidth of 10Mbps

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DSL v Cable

DSL is a private line with no competing traffic

Cable connections bandwidth change with the user load (number of subscribers using the service)

Problems Web users in rural areas often do not have cable

access and have limited telephone access (low cost voice-grade lines, rather than data-grade lines) thus bandwidth is limited (<14Kbps)

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Leased-Line Connections

Large firms with a large amounts of Internet traffic can lease lines from telecommunication carriers

Various technologies are used; classified by the amount of telephone lines they include: DS0 (digital signal zero) carries one digital signal

(64Kbps) T1 (or DS1) carries 24 DS0 lines (1.544Mbps) Fractional T1 (128Kbps and upwards) T3 (or DS3) carries 30 T1 lines (44.736Mbps)

Connections more expensive than POTS, ISDN and DSL

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Wireless Connections

Satellite Bluetooth Wireless Ethernet (Wi-Fi) Fixed-Point Wireless Cellular Telephone Networks

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Satellite

Satellite microwave transmissions Customer placed

receiving dish in yard Download bandwidth

of around 500 kbps Maximum upload

bandwidth of 150kbps Self installation

makes cost lower

http://www.ccpo.odu.edu/~arnoldo/ocean405/satellite.gif

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Bluetooth

One of the first wireless protocols

Operates reliably over 35 feet and can be part of up to 10 networks of eight devices each (personal area networks, or PANs)

Bandwidth of 722kbps Good for wireless

printing

http://www.bakercountyhealth.org/services/dental/blue-tooth.jpg

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Bluetooth Advantages

Bluetooth technology consumes very little power

Bluetooth devices can discover each other and exchange information automatically (e.g. a user can print to a printer on a network without logging on)

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Wireless Ethernet (Wi-Fi) (802.11b) Most common wireless

on LANs Bandwidth 11Mbps at

300 feet A computer with a Wi-

Fi network can communicate with a wireless access point (WAP) to become a part of the network

http://www.homenethelp.com/web/diagram/images/shareing-soft-wireless.gif

91

Wireless Ethernet Advantages

Wi-Fi devices can roam, i.e. shift from one WAP to another without user intervention

Increasingly WAPs are becoming available in public places, e.g. airports

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Wireless Ethernet Developments

In 2002 an improved version of Wi-Fi, called 802.11a was introduced The 802.11a protocol is capable of transmitting

at speeds up to 54Mbps 802.11a is not compatible with 802.11b

Later in 2002, the 802.11g protocol was introduced which is compatible with 802.11b devices and has speeds of 54Mbps

In 2004-2005, 802.11n expected (320Mbps)

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Fixed Point Wireless

Uses a system of repeaters (transmitter-receiver devices) to forward a radio signal from the ISP to customers

Users’ antennas are connected to a device that converts radio signals to Wi-Fi packets which are sent to their computers

http://www.erinc.com/apps_wireless/images/fixed_wireless2.gif

94

Cellular Telephone Networks

In 2003, about 500 million mobile (cell) phones worldwide

Originally slow data communication (10 kbps – 384kbps)

Third generation cell phones Up to 2 Mbps

http://www.mtco.com/graphics/cellularpic.jpg

95

Cellular Telephone Networks Cont’d Cell phones send and receive messages using the

short message service (SMS) protocol Some cell phones include Web browser which

provide web access, email, short message service Companies also sell Internet access through their

cellular networks Fixed fee plus charge for amount of data transferred

Business potential of mobile commerce Companies are identifying the kinds of resources

individuals might want to access (and pay for) using wireless devices

96

References

[1] Schneider, Gary, P., “Electronic Commerce: The second wave”, Thomson Course Technology, Fifth Annual Edition, 2004

[2] Zhao, Jensen J., “Web design and development for e-business”, Prentice Hall, 2003

[3] Federal Networking Council, “FNC Resolution: Definition of the Internet”, 1995. Online document available at http://www.itrd.gov/fnc/Internet_res.html

[4] NetCraft, “April 2004 Web Server Survey”, 2004. Online document available at http://news.netcraft.com/archives/2004/04/01/april_2004_web_server_survey.html

[5] Furdyk, Michael, “Ultimate Guide to Networking: Part One”, 1999. Online document available at http://www.hardwarecentral.com/hardwarecentral/print/158/

[6] Webopedia, “Network”, 2004. Online document available at http://networking.webopedia.com/TERM/N/network.html

[7] Berlin, Dan, et al., “CGI Programming Unleased”, Sams.net Publishing, 1996, pp. 101-102