OSI Reference Model

The Open System Interconnection (OSI) model is a standard reference model for communication between two end users in a network which was developed by the International Organisation for Standardisation (ISO) in 1984.

This is a theoretical system which divides the problems of communication into subsections called layers with each layer performing a subset of the required tasks or functions. In this model each layer relies on the next layer down to perform more primitive or basic functions and tasks performed in each layer are exclusive to that particular layer.

Here is a diagram of the OSI model layers:

Ok so now i will explain each layer in a little more detail so we will start at the top.

The Application Layer:

The application layer is the OSI layer closest to you and me, the user. As we interact with an application that implements a communicating component, such as a web browser, the application layer of the ISO model is also interacting with the application. The functions of the application layer are typically functions like identification of available communication partners, assessing the quality of the network available and synchronising communication. Notice these are all application specific functions.

The Presentation Layer:

This layer provides encryption of application layer data before sending it down the stack. It either formats and encrypts data to be sent across the network or transforms received data into the form the application above accepts.

Session Layer:

The session layer basically monitors, manages and terminates the connections between the local and remote application.

Transport Layer:

The transport layer facilitates the transferring of data sequences from source to destination.
It does this by what is called segmentation/desegmentation which means that it creates packets out of the data received from he application layer. Packetization is basically the process of dividing up the received information into smaller bundles (packets) of information. The Transport layer also keeps track of data transmission, re-sending those packets that fail and acknowledging successful transmission.

Network Layer:

The network layer provides the means of transferring the packetized data from the transport layer to the destination location on the same connecting network. This is basically the routing layer which uses IP addresses and routing protocols to transfer the data from source to destination.

Data Link Layer:

The Data Link Layer is responsible for the transfer of data within a LAN/WAN or between physically connected nodes on an network such as switches and hubs. This layer is normally divided into two sublayers the Media Access Control (MAC) layer and the Logical Link Control (LLC) layer.
The MAC layer controls how a device on a network accesses and transmits data.
The LLC layer identifies the protocols, and controls error checking and packet synchronisation.

The Physical Layer:

The physical layer is basically the cabling or wireless signals that connects the source to destination. This is where we see the data packets as "bits on the wire".

Here is an image of the OSI model in action between source and destination:

So as you can see the exact same steps that are performed at source during the transmission of the data are performed in reverse at destination.

During transmission each layer adds header to the outgoing packet and then at destination the same layer removes the header on incoming packet until original user data is presented in destination application as we can see in the image below: