The layers form a hierarchy of functionality starting with the physical hardware components to the user interfaces at the software application level.
This layer deals with the physical plugs, sockets, electrical/optical specifications and the required line codes.
It can be twisted pair, coaxial cable, optical fiber, wireless, or other transmission media.
This level is in charge of transferring data between systems in a network, using network-layer addresses of machines to keep track of destinations and sources.
No logical connection, and no persistent state of the transaction exists between the endpoints, lending the connectionless mode low overhead and potentially better real-time performance for timing-critical applications such as voice and video transmissions.
It provides for full-duplex, and half-duplex or simplex operation, and establishes checkpointing, adjournment, termination, and restart procedures.
The session layer is commonly implemented explicitly in application environments that use remote procedure calls.
Protocols such as MIDI, MPEG, and GIF are presentation layer formats shared by different applications.