CSC 110 -Intro. to Computing

Lecture 25:

World Wide Web

Announcements

Homework #5 handed back at end of class Essays due Thursday Service Learning logs and forms due 5PM

Friday Final Exam will be held Wed. 5/10

3:15PM-5:15PM in Old Main 200

Sending Messages over Internet

Reliability is major concern of InternetEspecially important when originally

developed in 60s Problem: computers are unreliable and

crash frequentlyWhat would happen in centralized network if

central node crashes?

Centralized Network

Centralized Network

Problems with Centralization

If central node crashes in centralized network, everything crashesNo traffic (or power, in this case) can get throughBut sending messages easier on centralized

network – just go between central nodes Decentralized network harder to crash

Data travels any route that is possibleTraffic gets routed around crashed node

Problem with Decentralization

Lack of central authority can result in lost messagesYour next e-mail could be routed through

TimbuktuDo not want to have to resend everything

Solution: Break data into smaller packetsEach packet travels a different routeUnlikely every packet gets lost

Packet Switched Network

Packets may arrive out-of-orderEnd of page routed through Orchard ParkStart of page routed through Dubai

Solution: Give each packet a number and use them to reassembled received packets

Sample PacketsSender’s Address

138.92.194.39

Receiver’s Address

70.104.55.73

Packet Number

1

Data

Listen, I told you this before. I will not change your grade to a

Sender’s Address

138.92.194.39

Receiver’s Address

70.104.55.73

Packet Number

2

Data

n “A” until I see the 100 million dollars in my Swiss bank acc

Vannevar Bush

Main scientific advisor during World War II E.g., the driving political force behind the Manhattan

project

Proposed creation of scientific oversight board NSF named award for public service for him

“Of the men whose death in the summer of 1940 would have been the greatest calamity for America, the President is first, and Dr. Bush would be second or third." -- Alfred Loomis

“As We May Think”

Essay by Vannevar Bush for Atlantic Monthly in July,’45 Presented idea for the Memex machine

Consisted of screens and keyboard on a desk

“The owner of the memex […] is studying why the short Turkish bow was apparently superior to the English long bow in the skirmishes of the Crusades. He has dozens of possibly pertinent books and articles in his memex. First he runs through an encyclopedia, finds an interesting but sketchy article, leaves it projected. Next, in a history, he finds another pertinent item, and ties the two together. Thus he goes, building a trail of many items. Occasionally he inserts a comment of his own, either linking it into the main trail or joining it by a side trail to a particular item.”

Ted Nelson

Influenced by Vannevar Bush’s article Started Project Xanadu in 1960

Goal was easy-to-use network computerOrigin of the term hypertextWork on this project is still ongoing

“A user interface should be so simple that a beginner in an emergency can understand it within ten seconds” – Ted Nelson

Douglas Englebart

Another devotee of Vannevar Bush’s ideals Wanted system where people could formulate

and organize thoughts quickly In 1968, presented “digital library.”

First system with links between documents & e-mail First use of “cut & paste”, voice chat with a computer Discussed video conferencing

"I realized that I didn't have any more goals than a steady job, getting married and living happily ever after […] How can my career maximize my contribution to mankind?” – Douglas Englebart

Sir Tim Berners-Lee

Wanted to facilitate sharing and collaboration among researchers

In 1991, put up system combining Internet and hyperlinks: the “World Wide Web” Original design allowed fully editable “WYSIWYG”

pages, but did not include images Much of original intent was actually lost very early in

the development of the Web

Surfing the Web

“Client”

Combining Data

Vannevar Bush’s original idea was to allow ideas and information to be joined and combined freelyHTML does a good job of presenting

informationHTML is bad at highlighting what the

information says, however

XML

Extensible Markup Language (XML) allow document creator to define own set of tags that describe document’s contentsXML is formally a metalanguage – language

for defining or describing other languages

XML

XML organizes data, but does not specify how to display document

Extensible Stylesheet Language (XSL) defines how to display XML documents in various environments

XML DTD XML documents include Document Type

Definition (DTD) which specify document’s organization

XML DTD

How would we write a DTD for a class?Has course number, professor(s), students,

classroom, meeting days, and meeting time

XML Example XML uses these

descriptions to tag all of the information within a document

XML Example

How would we write an XML example for this class using our DTD?

In-Class Excercise

Define the DTD and write an XML document for:Political officesZoo animals