How the Internet Works

Moving bits around

Modern networking is built on the concept of:
–	Layers of service
–	Basic transmission of bits across imperfect media
–	Add a layer of software to use the hardware
–	Add a layer of software for fundamental functions
–	Add a layer of software for easy-to-use features
–	Add a layer of software for graphical interface, etc.
–	All are layers added to the basic structure

Packet switched networks
–	Internet uses phone lines, but doesn't work like the phone system
–	Phone system (voice) is circuit switched
–	Circuit is dedicated to the transmission
–	Expensive and inefficient for data
–	Packet switching is like the post office
–	Information goes in a packet (an envelope)
–	Individual packets are addressed and sent
–	Collection points have more and more volume
–	It all gets sorted out where it needs to be sorted
–	Individual packets arrive at their destinations
–	Lines are used efficiently by many users at once

The Internet Protocol
–	Is the most important of the Internet protocols – deals with connecting to the Internet
–	Is at layer three of the Open Systems Interconnect (OSI) model
–	Keeps track of addresses, routes outgoing messages, recognizes incoming messages
–	Numerous compatible and incompatible networks are connected by routers
–	Routers read packet addresses and choose the best route for the data (among other functions)
–	Protocols are the rules that govern use of the net
–	Protocols address the packet
–	Protocols tell the router how to send the packet
–	Each node on the network has its own address
–	Each packet carries the address of its source and of its recipient
–	Packets consist of 1 to 1500 characters
–	Packets arrive out of order
–	Packets may get lost in transmission
–	So you need:

The Transmission Control Protocol:
–	Is the next layer of software (layers four and five of the OSI)
–	Works at the transport layer, connection-oriented, end-to-end transmission
–	Breaks the data into packets, addresses them, and numbers them for reordering after transmission
–	On reception, TCP strips the headers and reorders the packets
–	If data is missing or damaged, TCP asks for retransmission
–	For simple messages (e-mail), User Datagram Protocol (UDP) is used
–	Easier to set up and operate
–	The entire message is retransmitted if there is a problem

The World Wide Web

There were two barriers to widespread use of the Internet:
–	The intimidating UNIX environment
–	Predominant use by computer and network experts
–	All that changed due to two factors:
–	The development of the World Wide Web
–	The development of graphical browsers
–	The first in widespread use was Mosaic, which we used here at UK until:
–	Navigator, by Netscape Communications came along and dominated the market, until:
–	Software gorilla Microsoft battered Netscape over the head, captured a large market share and wound up in an antitrust suit

The World Wide Web
–	Was proposed and created in the early 90s by Tim Berners-Lee with help from colleagues at:
–	European Particle Physics Laboratory (CERN)
–	Purpose was to aid collaboration among physicists at different locations
–	The earliest version (run on a NeXT workstation) was only text
–	Now it is adapted to all types of multimedia information
–	A browser is a program that reads and displays a hypertext document, which is accessed via its uniform resource locator (URL)
–	Today, the WWW is an attempt to electronically link all the information on the Internet as a set of hypertext documents
–	It's the first and perhaps (so far) the only "killer application" on the Internet – all other B2B and e-commerce applications are web-based

What is hypertext?
–	Earliest conception by Vannevar Bush:
–	Article in The Atlantic in 1945
–	Proposed a Memex: computerized database for organizing information that emulates the way the human mind works – in an associational way
–	The term hypertext was coined by Ted Nelson in a 1965 paper delivered to the Association for Computing Machinery
–	Hypertext is defined as a "nonsequential writing tool that allows textual elements in documents to be linked"
–	Today, it's a method of electronically linking files to other files on your computer or elsewhere
–	Files may be text, graphics, audio, video
–	There has been an explosion of hypertext over the last few years thanks to HTML editors
–	Hypertext Markup Language (HTML) is the code for formatting pages and creating links
–	eXtensible Markup Language (XML) is the successor to HTML because it allows customizable added functionality – especially important for database interfaces and transactions.