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- 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.
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- 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.
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- 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.
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- 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.
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- 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.
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- 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).
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- 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.
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- 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.
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- 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 …
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- 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.
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- 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.
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- Now read about
Error-Checking
- and then do some programming:
- Bits.java
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