The base of IP Networks - OSI Model

Please Do Not Throw Sausage Pizza Away !!!

You might be wondering what I am talking about, but take a closer look at the Red letters above…Yes you are right, these are the first letters of the 7 Layers of the OSI Model, the fundamental building block of TCP/IP Networks.

The 7 Layers of the OSI Model are shown below.

• Layer1:Physical Layer
• Layer2:Data Link Layer
• Layer3: Network Layer
• Layer4: Transport Layer
• Layer5: Session Layer
• Layer6: Presentation Layer
• Layer7: Application Layer

The OSI reference model specifies standards for describing “Open Systems Interconnection”. The term ‘open’ was chosen to emphasise the fact that by using these international standards, a system may be defined which is open to all other systems obeying the same standards throughout the world.

It consists of 7 Layers with each Layer being functionally independent of the others. Control is passed from one layer to the next, starting at the top and proceeding to the bottom layer, over the channel to the other station and back up the layers. The receiving layer at the destination host receives exactly the same object as sent by the matching layer at the source host. This is shown in the diagram below:

The layers are in two groups. The upper four layers are used whenever a message passes from or to a user. The lower three layers are used when any message passes through the host computer. Messages intended for this computer pass to the upper layers. Messages destined for some other host are not passed up to the upper layers but are forwarded to another host.

The sending process passes data to the application layer. The application layer attaches an application header and then passes the frame to the presentation layer.

The presentation layer can transform data in various ways, if necessary, such as by translating it and adding a header. It gives the result to the session layer. The presentation layer is not aware of which portion (if any) of the data received from the application layer is the application header and which portion is actually user data, because that information is irrelevant to the presentation layer’s role.

The process of adding headers is repeated from layer to layer until the frame reaches the data link layer. There, in addition to a data-link header, a data-link trailer is added. The data-link trailer contains a checksum and padding if needed. This aids in frame synchronization. The frame is passed down to the physical layer, where it is transmitted to the receiving host. On the receiving host, the various headers and the data trailer are stripped off one by one as the frame ascends the layers and finally reaches the receiving process.