An IT network is a group of interconnected electronic systems.

To share information between systems, the OSI model defines and standardizes the communication functionalities and their interactions. The standard defines a model composed of 7 layers :

• Physical(1): The physical layer is the lowest layer. This layer manages the physical connections, maintains them and transmitting bits over them.

• Data Link(2): The task of the data link layer is to ensure reliable transmission of the data and to control access to the transmission medium. The data link layer is divided into two sub-layers: LLC (Logical Link Control, layer 2b) that handle the routing for the IP or IPX protocol and the MAC (Media Access Control, layer 2a) that handle the routing for the physical systems with their MAC address.

• Network(3): The network layer is responsible for transmitting the data packets to the destination. Data transmission takes place across the entire communications network and includes routing between the network nodes. If a direct connection to the destination is not possible, the packets must be forwarded by nodes located along the way.

• Transport(4): The tasks of the transport layer are the segmentation of the data stream, congestion avoidance and ensuring error-free transmission. The data is segmented into chunks called frames and the address (port) of the target service is added. Additional metadata is added for verification and control of the frames. The transport layer provides uniform access to the application-oriented layers 5 to 7, so that they do not need to consider the properties of the communication network.

• Session(5): The session layer is responsible for process communication between two systems. This layer open, close and manage instances of communication channels called sessions. This layer provides authentication, authorization and session restoration services.

• Presentation(6): The presentation layer ensures that data sent by the application layer of one system can be read by the application layer of another system. If necessary, the presentation layer acts as a translator between different data formats by using a data format that both systems can understand, like the ASN.1 (Abstract Syntax Notation One). This layer also provides data compression and encryption services.

• Application(7): The application layer manage services, applications and their access to the network. It allows applications to connect to the lower layers. The applications themselves do not belong to the layer.