How Computers Work Part 3 18 February 2008

Data Types  Computer doesn’t know what the bits represents or what format is being used  Computer assumes that the instructions know the format of the data  What are the types of data? Numbers, text, pictures, sound, instructions

Data Types  Numbers: integers and floating point numbers (scientific notation) Why do we need floating point numbers?  Text: Unicode, double byte Languages and symbols (Word insert symbol)  Pictures: pixels A very fine needlepoint How to represent color?  Sound: different formats  Instructions

Alphanumeric Text 8-bit ASCII sample A B C

H e l l o Hello

How are pictures stored?

Monochrome A pixel can look BLACK, or it can look WHITE.

ASCII Images  Instead of pixels, use characters  Web site that converts pictures An example

Or it can display various shades of grey: For example:

Mixes various amounts of R G and B light to produce other colors. ColorColor Or a pixel display various colors

The Internet  What is it? The infrastructure that connects identified computers Links = the wires Routers = the boxes that connect them  World Wide Web Computers that contain information to be shared Servers

How Do You Get to the Servers?  Computers are connected through a network  Network requires protocol to work A protocol is a set of rules The one that is used here is called IP (Internet Protocol)

How would you route a note?  Assume that you each have a consecutive number (count off)  If I hand one of you a note for a set number, what do you need to know?  Which way to go – not exactly where it is  Your number, the number you want to get to  Algorithm?  If number is greater, pass left/back smaller, pass right/forward equal, read

How Networking Works  Computers need the same thing Who identified by IP address  Every machine that is connected to the network has one   Command prompt Routers know how to get there

So What Can Go Wrong in Transit?  Data can be read  Data can be lost  Data can be changed  Data can be sent to someone else

How Networking Works  Computers need the same thing Who identified by IP address  Every machine that is connected to the network has one   Command prompt Routers know how to get there  But IP addresses aren’t user friendly Give names instead Domain Name Server does the mapping

Introduces new opportunities  People assume they know who owns the site

Anatomy of a URL  Protocol: server-name/file-to-display  Protocol: usually http Have you ever seen others? https?  Server-name The computer’s name  Usually begins with www  Usually ends with 3 characters that define the kind of site However, there are no rules: as long as its registered, you can get there  File-to-display Can be a whole path (just like Windows)

How to Build that File  Text file that says what to display Web pages use HTML Hypertext Markup Language  Two types of information Text Instructions on how to display