CSC 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 Internet  Especially important when originally developed in 60s Problem: computers are unreliable and crash frequently  What would happen in centralized network if central node crashes?

Centralized Network

Problems with Centralization If central node crashes in centralized network, everything crashes  No traffic (or power, in this case) can get through  But sending messages easier on centralized network – just go between central nodes Decentralized network harder to crash  Data travels any route that is possible  Traffic gets routed around crashed node

Problem with Decentralization Lack of central authority can result in lost messages  Your next could be routed through Timbuktu  Do not want to have to resend everything Solution: Break data into smaller packets  Each packet travels a different route  Unlikely every packet gets lost

Packet Switched Network Packets may arrive out-of-order  End of page routed through Orchard Park  Start of page routed through Dubai Solution: Give each packet a number and use them to reassembled received packets

Sample Packets Sender’s Address Receiver’s Address Packet Number 1 Data Listen, I told you this before. I will not change your grade to a Sender’s Address Receiver’s Address 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 computer  Origin of the term hypertext  Work 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 &  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 freely  HTML does a good job of presenting information  HTML 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 contents  XML 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 offices  Zoo animals