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|  Open System Interconnection (OSI) Model   |
|               by G|GAWH0RE                 |
|                                            |
|              xymox@yifan.net               |
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This should be a nice overview of the OSI model for people who think that it's a complicated
thing to understand like I use to back in the day.  I wish there was something like this around
back when I was trying to figure out what all this OSI/ISO mummbo jummbo was about.

---Introduction-----oo--
A network model provides rules (called protocols) that establish how all the different
functions in a network will communicate with each other.  This lets many companies design that
will work well with other manufacturers' equipment. The OSI model was made by the International
Standards Organization (like everything else in the world).  It has seven layers, with each
layer relying on the ones below it to establish effective communications over a network.  Ok
lets start.

---Physical Layer-----oo--
Protocols at this layer define the electrical signals that actually flow over the network wires.  
They describe what voltage is needed for a "1" bit and what voltage is needed for a "0" bit 
(binary babee :)).  They describe how fast the bits will be transmitted and how the devices
will talk (data may flow one way at a time or both ways simultaneously).  This layer even
describes the standard connectors to be found on modems and other network boards such as
Ethernet cards, etc.  Examples of the protocols are RS-232, V.24, IEEE's 802 for LANs.

If you were say writing a pen-pal by snail mail, the physical layer would correspond to an
agreement to use white paper and black pens to write with.  Furthermore, the letters must all 
be at least 5 millimeters high and have alternating letters and numbers in el33to hacker speak
language :).

---Data Link Layer-----oo--
This layer receives the stream of bits from the physical layer below it (or sends the stream if
transmitting).  This layer knows how to organize the bits into groups called "frames" by
establishing where a transmission block starts and where it ends.  If errors have occurred 
(garbled, missing, or duplicated bits), this layer is responsible for fixing them, or asking 
for retransmission of the message.  The data link layer can provide service for 
"Connection-Oriented" networks (for MANs and WANs) that need to keep track of exactly when a 
connection with another computer has been made (so errors can be detected).  It can also handle
"Unacknowledged connectionless" service (on LANs) where errors are rare.  Your letter to the 
pen-pal may have a line like this that needs to be decoded:
Astreamofbitsthatneedstobebrokenintowords

---Network Layer-----oo--
This layer is like the postmaster.  Firstly, at adds address information to a frame to identify
where it came from; the data is now called a "packet".  The layer then decides which route over
the network the packets will take to get to their destination.  Packets going through the same 
place may take different physical routes!  This is like a postmaster receiving a ten page letter
with each page in a seperate envelope but all going to the same person.  The postmaster may 
decide to send several pages by airmail, some by boat, and others by truck (depending on how 
busy each type of transportation is!)  The network layer is responsible for putting all the 
packets back together in the proper order once they reach their destination.

---Transport Layer-----oo--
This layer acts as a manager and inspector to the network layer below it.  The network layer 
does the dirty work of finding the right paths over the network for the packets; the transport 
layer checks all the packets as they arrive to confirm that they are in the right place and in 
the right order.  There are different classes of transport layers with the simplest doing almost
no managing and the most advanced checking for packet errors and managing error recovery.  
LANs need only simple transport layers since they tend to be error free.

---Session Layer-----oo--
The session layer acts as a referee to establish the ground rules for how two computers will 
talk.  Will packets of data be sent alternately or at the same time by both computers?  What 
happens if the connection is lost during the communication?  The session layer controls all 
this.  The "session" itself is like a series of letters you may agree to exchange with a friend.
Whoever connected you with the friend may decide who will write first and what happens if no one
writes back.

---Presentation Layer-----oo--
This is concerned with syntax.  It negotiates what "language" the data in the packets will be 
in.  The data could be in ASCII characters or in some coded binary form.  The data may be in a 
compressed format or not.  This is similar to agreeing on what language you would communicate 
with a pen-pal.

---Application Layer-----oo--
This is not the same thing as like an application on your little Windows box but this layer 
does handle processes that are requested by programs you use.  the layer contains protocols 
that handle instructions for doing file transfers, terminal emulation's, etc.

---Final Words-----oo--
Well I hope this made the OSI model easy to understand for everyone.  If not then ummm I think 
you have some issues because it cant be made any simpler then that.  Well I'm out.  Check ya 
at #fubuhacking on the undernet.


EOF