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Sending the letter "A" over the internet

Posted on 2005-04-07
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Last Modified: 2013-11-29
I know this may seem like a really dumb question....but what does it take to send a single letter...say the letter "A" over the internet?

I want to understand how this works at a very very low level....even if it takes a few months to explain it to me.

For example.....take a chat program like MSN Messenger......say I am talking to somebody and I send the letter "A".

What happens to the letter "A"?

I know that it is converted into a string of bits  (ASCII 65) is represented like this:

01000001

So how is 01000001 sent over the internet to the recipient?

I want things as detailed as possible.
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Question by:knowlton
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Something along these lines:

http://www.theshulers.com/whitepapers/internet_whitepaper.html

But even this may be too high level.....
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BlueDevilFan:

For most laymen.....the links you provided would be MORE than sufficient....and thank you.

But all of these links are too high level.  They are the 40,000 foot view....and I want the 1 inch view of how it works.

Maybe we could start with how information gets to the Application Layer?
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Assisted Solution

The Application layer is the chat program itself.  On the outbound side data gets to the Application Layer when the user types something at the keyboard.  Internally, when the user types it triggers an event handler that starts the letter they typed on its journey to the other end of the connection.  On the inbound side, data gets to the application layer from the Presentation Layer.

Assuming we're using a simple chat program then according to my understanding it'd look something like this when the user types a character:

1.  Application Layer = Chat Program
2.  Presentation Layer = Translating the letter typed into a standard format (e.g. converting it from ASCII to ?)
3.  Session Layer = Decides which connection to send the character down.  For the chat program to work a connection must already exist.
4.  Transport Layer = Builds a packet.
5.  Network Layer = Encapsulates the packet based on protocol and prepares to send it.
6.  Data Link Layer = Decides which physical connection to send the packet on.  Inserts the sending and destination address into the data packet.
7.  Physical Layer = The packet is sent as a series of electrical impulses

7. Physical Layer = The electrical impulses are received and passed up one level.
6.  Data Link Layer = Decodes a portion of the received packet and passes the remainder up on level
5.  Network Layer = Decodes the packet based on the protocol used
4.  Transport Layer = Checks to see that the data arrived intact.  If it didn't, then this layer may ask for a retransmission.
3.  Session Layer = Checks to see which application and session the data needs to go to
2.  Presentation Layer = Translates the letter into an application desired format (e.g. back into ASCII)
1.  Application Layer = Displays the received letter on the screen
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Accepted Solution

This is a HUGE question that spans the scope of how everything on the internet operates.  Simplifying it to almost an absurd level, there are 3 basic layers --

1. high level application layer.  The character "A" is put into the TCP/IP stack is "65" -- its character code
2.  The TCP/IP "middle" layer looks for the IP number that it has as a "token" for this communication.  The IP number exists as a HEX coded binary, something like A83C7F102B -- that is the internet address that the high level application layer gave, converted to the TCP/IP lookup layer's nomenclature.
3.  The TCP layer opens the IP session via the token, and when the token IP address responds, with various packet exchanges, then OK, we are ready to send this character 65 -- it goes wrapped in a packet of surronding destination and check bytes, numbers just like the 65.
4.  This goes up to the point of the MODEM -- now the cable/dsl modem or whatever, modulates and demodulates ALL these numerical bytes into a blindingly long stream of 10101001110100101001111001
type values, which represent the characters.  The modem on the other end, checking these 0-1 values, sees it's unique signature and says "AHA!  That packet is for me -- so it grabs it.
5.  The whole process above goes in reverse, and within less than 1 second after his modem got it, he sees a letter "A" on the screen.

Extremely simplified, but that is the essence of it.
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This is a HUGE question that spans the scope of how everything on the internet operates.  Simplifying it to almost an absurd level, there are 3 basic layers --

1. high level application layer.  The character "A" is put into the TCP/IP stack is "65" -- its character code
2.  The TCP/IP "middle" layer looks for the IP number that it has as a "token" for this communication.  The IP number exists as a HEX coded binary, something like A83C7F102B -- that is the internet address that the high level application layer gave, converted to the TCP/IP lookup layer's nomenclature.
3.  The TCP layer opens the IP session via the token, and when the token IP address responds, with various packet exchanges, then OK, we are ready to send this character 65 -- it goes wrapped in a packet of surronding destination and check bytes, numbers just like the 65.
4.  This goes up to the point of the MODEM -- now the cable/dsl modem or whatever, modulates and demodulates ALL these numerical bytes into a blindingly long stream of 10101001110100101001111001
type values, which represent the characters.  The modem on the other end, checking these 0-1 values, sees it's unique signature and says "AHA!  That packet is for me -- so it grabs it.
5.  The whole process above goes in reverse, and within less than 1 second after his modem got it, he sees a letter "A" on the screen.

Extremely simplified, but that is the essence of it.

=============================================

This is approaching the level of detail I am looking for.

But this shows me you are getting the idea of what I am asking for.

And YES.....I realize it is complex....but I still want to understand how it works.

Quote:  The character "A" is put into the TCP/IP stack is "65" -- its character code

What is the TCP / IP stack.

HOW is the character "A" put onto this TCP / IP stack?

Maybe this question crosses over too many technical disciplines to be asked in just ONE  TA   ?????

I am not trying to be difficult....I am genuinely curious......
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Expert Comment

If you have the money for EE questions, this needs to be split into about 10-20 qestions --
you are asking for a complete education on the internet here, and it is virtually impossible to give that in any length of questions.

Read these --
http://www.cisco.com/univercd/cc/td/doc/product/iaabu/centri4/user/scf4ap1.htm
http://www.learntcpip.com/TCPIP/default.htm
http://www.infoiasi.ro/~busaco/teach/docs/intranets/ch2.htm
http://pclt.cis.yale.edu/pclt/COMM/TCPIP.HTM
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Author Comment

Thanks.....

I am realizing now that this question was TOO BIG....but you've given me a good start.

So what is the FIRST question I should ask......starting from when I click the SEND button?

I want to know the VERY next thing that happens to the letter   "A"  that I am transmitting.

What TA could answer that question?
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Expert Comment

OK -- In the networking section and maybe the browser section, ask these
1.  How does a browser communicate with the operating system and how does the data get encoded
2.  How does the operating system send data to the TCP/IP stack, what does it look like in the stack
3.  How does the Operating system actually send this data from it's internal stack on the TCP protocol
4.  What is a TCP/IP protocol, and how does it connect to the internet
5.  Excluding the modem, which works at a lower level, explain how computers comminicate via TCP/IP
6.  How is the information transferred across the internet -- explain the workings of IP number recognition

Now you can move into the hardware -- but only some people on hardware can answer these ...

7.  How do modems encode data to send it over the internet -- cable, DSL, TI -- not just dial up
8.  How do the internet servers transfer packets across the internet, at a very LOW level
9.  Explain how a major internet router decides how to redirect TCP packets to the right host in the world
10.  Explain how a host checks these packets to see if they are meant for it, at the bit-level that modems use

That will stir up a whole lot of gray matter, believe me.  Some might best be done by evening reading at the fireside.

Good luck
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Thanks!
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