The Open Systems Interconnection model.  Take a deep breath, now we get wet.

This is the OSI model to the left.  It has seven, count them, seven layers.  Physical is at the bottom and is the most basic layer while the Application layer is at the top and can be considered to be above the Physical layer (yes, in a snobbish way).  Each layer passes information back and forth only to the layer next to it.  Each layer has a specific job.  If that job doesn't get done, a different layer won't be able to do its job and a communication error has just occurred.  As information goes from the sender to the cable, headers are added to the information and the information increases in size.  As information goes from the cable to the receiver headers are stripped away and the information decreases in size.  Here's a metaphor and a picture of how OSI communication works:

(Application layer)  In New York, Suzie writes the note on a piece of paper with a pencil to her boyfriend Jon in California.

(Presentation layer)  Suzie checks her spelling and puts cute stickers on the letter and every bit of surface area on the envelope.

(Session layer)  Suzie puts her sticker-ized letter in an envelope, puts the envelope in the mailbox and raises the red flag on the side of the mailbox.

(Transport layer)  The mailman picks up the envelope and falls over from the excessive perfume Suzie put on it.  The envelope finds its way to the post office.  A loyal and bright US Mail employee notices that Suzie forgot the zip code.  The loyal and bright US Mail employee sends the envelope back to Suzie.  Suzie can't believe she is so dumb.  Suzie re-sends the envelope with the zip code.  The envelope makes it past the loyal and bright US Mail employee this time.

(Network layer)  A postal worker employee reads the street address and now the post office knows what street to send it to but not what state.

(Data Link layer)  Another postal worker reads the street address and now the post office knows what state to send it to.

(Physical layer)  The envelope is put into a Jeep.  The envelope is driven to Jon's house in California.

Now the process reverses itself.  The metaphor may get a little obscure.

(Physical layer)  The envelope is taken out of the Jeep and taken in to the Post Office in California.

(Data Link layer)  The state is checked.  Since California is the correct state, the envelope is not sent back.

(Network layer)  The street address is checked.  Since Jon's house still exists (at least to the Post Office's knowledge) the envelope is not sent back.

(Transport layer)  No errors have happened so the envelope is given to Jon.

(Session layer)  Jon sees that the red flag is down on his mailbox so he goes to check the mail.

(Presentation layer)  He sees Suzie's sticker-ized letter and immediately rips off all of the lame stickers.  Jon throws away the envelope and now he has only the letter.  Jon washes off the perfume as much as he can.

(Application layer)  Jon's eyes read the letter and he realizes that a long distance relationship sucks.

Notice that Suzie added items to the letter and Jon gets rid of the items.  It started as a simple letter, became something more complicated and then ended up as a simple letter.

Here are the technical definitions of OSI.  Try to see how the metaphor relates to each one of these.

Application-Initiates a request or accepts a request

Presentation-Adds formatting, display and encryption information

Session-Adds traffic flow information to determine when information gets sent

Transport-adds reliability and error correction

Network-adds network address information to route the transmission

Data Link-places packets into frames, adds error correction, works with physical addresses

Physical-transmits bits

On to Page 4.

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Headers are pieces of information that is tacked onto a larger piece of information.  For example, a Post-It note is put on a letter.  The Post-It note is usually discarded after the reviever has read it.

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Requests are made by the user.  You just clicked 'request' because you wanted a definition.  That request went to the web server and was fulfilled and now you have a definition.  Isn't that great?

note: Requests can be denied.  For example, requests for Kenny G will not be fulfilled.

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Frames are a collection of bits.  Think about how you pack your clothes for a trip.  You don't take your clothes to the car shirt by shirt.  You put your clothes in bags and boxes to save yourself time.  Frames are more efficent ways of moving bits.