Follow the Data Data (and information) move from place to place in computer systems and networks. As it moves it changes form frequently. This story describes the various formats of data as it travels from a digital camera, to a computer, then through the Internet and finally arriving at a web-server to be shared on friends’ PCs.

Digital Camera The camera uses CCD sensors to change analog light waves into digital pixels. Each pixel consists of 3 colors : red – green – blue (RGB). The intesity (brightness) of each color is stored as an 8-bit number (byte) between 0 and 255. That is 24 bits for each pixel. The numbers are saved on a flash memory card. A 5 megapixel camera must store 24-bits x 5,000,000 = 15 MegaBytes per photo. JPEG compression squeezes this into a smaller memory space.

Flash Memory to Disk Transfer the flash memory card to the correct slot in a memory card reader. Then copy the JPEG files to your hard-disk. You can use a software application to edit the photos before uploading to a web-server.

Upload Internet Upload the JPEG file(s) to a web-server. That is a long, complex journey from your PC to the server. Briefly : a 100 KiloByte picture must be chopped up into small packets that are re-assembled at the other end – that all happens automatically.

Packets The picture is chopped up into small packets, typically 4 KB each, dividing a 100 KB picture into 25 packets. Each packet header contains the IP addresses of the sender and the receiver, as well as the sequence number so the packets can be reassembled at the receiving end.

Errors Error – retry! Packets travel all over the world, so sometimes errors occur. Each packet contains error detection code so the receiver knows if the packet is defective. When that happens, the receiver sends an error message back to the sender, asking them to retransmit the package (try again).

Web-Server Storage Data packets travel all over the world, passing through routers until they arrive. Once ALL the packets have arrived intact, they are reassembled to produce the original file. This is stored on the web-server as a JPEG file, waiting to be downloaded.

URL vs IP Address 63.208.226.41 Your friends can see your picture(s) if they know the IP address of the server, something like 63.208.226.41. That wouldn’t work, because nobody can remember numbers like that. And the server has lots of pictures, so they also need to tell the name of the picture. So they use a URL instead.

Universal Resource Locator jlpicard.blogspot.com DNS 66.102.15.100 Users type a URL into their browser, like: jlpicard.blogspot.com The browser asks a DNS (Domain Name Server) to look that up and find the matching IP address. Then your browser sends a request to the IP address, and the web-server send back the HTML page that you requested. It’s actually a bit more complicated …

http:// www. fis.edu /fis_life.html URL Structure http:// www. fis.edu /fis_life.html Protocol Section Domain Page An IP address is like a telephone number – it makes the connection to the domain. The first part of the URL tells the protocol being used – like choosing a language for the phone conversation. The section is like an extension in an office building. The page is the desired web-page, like the name of the person you want to talk to.

Download Once your browser has sent the full URL, the web-server sends back the web-page you wanted – again broken up into packets to be re-assembled by your PC. If the page contains links to pictures on other servers, it automatically contacts those servers and downloads the images.

Now read about Error-Checking and then do some programming: Bits.java Read and Practice Now read about Error-Checking and then do some programming: Bits.java