OSI (Open system interconnection) is an interconnection reference model defined by ISO for heterogeneous systems. The architecture of the reference model for open systems is organised into a hierarchy of seven layers. An open system is a computer, terminal or network, any equipment complying with this standard and therefore able to exchange information with other heterogeneous equipment from different constructors.
The OSI model is defined in the ISO IS-7498 standard and recommendation CCITT X.200.
The 7 layers
The physical layer supplies the mechanical, electric, functional and procedural resources required to activate, maintain and deactivate physical connections for the transmission of bits between two data link entities.
The data liaison layer supplies the functional and procedural means required to establish, maintain and release data liaison connections between two entities in the network. It detects and if possible corrects errors due to the physical support and reports the irreparable errors to the network layer. It supervises transmission operation and defines the syntactic structure of messages, the way exchanges are stringed according to whether a standardised protocol is used or not.
The network layer provides all the relay and service improvement services between the entity and the network, i.e.: addressing, routing, dataflow control and the detection and correction of errors not corrected by layer 2.
The transport layer provides transparent data transfer between the entities of the session, relieving them of execution details. Its role is to optimise the use of available network services to provide them at the lowest cost and the required performance for the session layer.
The session, presentation and application layers are the top layers of the OSI model and offer user-oriented services whereas the lower layers are concerned with reliable end-to-end communication. They consider that the transport layer gives a reliable communications channel and add extra features for applications.
The 7 layers of the OSI model.