The Open Systems Interconnection Reference Model (OSI Reference Model or OSI Model) is an abstract description for layered communications and computer network protocol design. It was developed as part of the Open Systems Interconnection (OSI) initiative. In its most basic form, it divides network architecture into seven layers which, from bottom to top, are the Physical, Data-Link, Network, Transport, Session, Presentation, Application Layers (Please Do Not Throw Sausage Pizza Away). It is therefore often referred to as the OSI Seven Layer Model.
Control is passed from one layer to the next, starting at the application layer in one station, proceeding to the bottom layer, over the channel to the next station and back up the hierarchy. One important piece of information to keep in mind is that data flows 2 ways in the OSI model:
- DOWN (data encapsulation) -- The process of adding headers to data as it moves down OSI layers. In addition to header, the data link layer also adds a trailer, which is the frame check sequence (FCS) used by the receiver to detect whether the data is in error.
- UP (data decapsulation) -- The process of removing headers from data as it moves up OSI layers.
Data exists at each layer in units called Protocal Data units (PDU). The picture gives the PDU at each layer.
Data packets on a network are created at the source and then travel to the destination. Each OSI layer depends on the services provided by the OSI layer below it. To provide this service, the lower layer uses encapsulation to put the PDU from the upper layer into its data field. It then adds headers (and tailers) of this layer to perform its function. The PDU of all OSI layers are listed as follows:
- Application, Presentation and Session Layers -- data
- Transport Layer -- segment
- Network layer -- packet
- Data Link layer -- frame
- Physical layer -- bit