The ISO OSI Reference Model

     The International Standards Organization developed the Open System Interconnect reference model in the late 1970's/early 1980's. The OSI Model is a widely mis-understood set of standards in the computer industry today, and since it covers in depth on the basics of networking I feel it is very important. It is covered in most of the first networking certifications you will see.   In the following pages we will cover the OSI model in depth. Starting with this page that will cover the basics.

     The OSI Model has seven layers starting at the bottom and working it's way to the top. These layers run on all networked computers, and there is a virtual communication between all adjacent layers. The seven layers are:

7. Application              -----------virtual communication-------------- 7. Application

6. Presentation             ----------virtual communication--------------- 6. Presentation

5. Session                    ----------virtual communication--------------- 5. Session

4.Trasnport                 ----------virtual communication---------------  4. Transport

3.Network                  ----------virtual communication--------------- 3. Network

2.Data Link                 ----------virtual communication--------------- 2. Data Link

1.Physical                    ---------virtual communication---------------- 1. Physical

What is meant by virtual communication is that each layer believes that it is talking with just the same layer on the other computer. Each layer is only aware of the layer above and below it and passes the packet to that layer when sending or recieving a packet. Here is a brief description of what each of the layers do and a nice way to remember what the layers are.

Please Do Not Throw Sausage Pizza Away is a good way to remember the seven layers starting with physical and going through the application layer. A little longer way is like this and also starts with the physical layer.

you Physically connect two computers so that you can Link Data (data link) once this is done you have a Network then you must Transport information by openning a Session between the two computers. Finally you have to Present the information through the  Application to the end user.

Now a more indepth look at each of the seven layers.

Physical - The physical layer defines the media and hardware to be used on the network.

Data Link - This layer connects to the physical layer and moves data across the media, it is also in charge of the cyclical redundancy check for error correction at the lowest level.

Network - This is were routing takes place.

Transport - This layer is responsible for end to end data entegrity. It makes sure that the packets are reassemled in the proper order and is the second level of error correction.

Session - Opens a communications channel between two computers and maintains the channel until it is time to close it down.

Presentation - has many responsibilities including translation, encryption, and once again some error correction as the last level to do this.

Application - The end user interface or program that is using network resources.

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