The Open Systems Interconnection model (OSI model) is a conceptual model that characterizes and standardizes the communication functions of a telecommunication or computing system without regard to its underlying internal structure and technology.
Its goal is the interoperability of diverse communication systems with standard protocols. The model partitions a communication system into abstraction layers. The original version of the model defined seven layers.
| OSI Model | |||||
|---|---|---|---|---|---|
| Layer | Protocol data unit (PDU) | Function | |||
| Group | № | Name | |||
| Host layers |
7 | Application | Data | Provides the sole means for the application process to access the computer network | |
| 6 | Presentation | Provides for the representation of information that application-entities refer to in their communication. Provides application-entities with syntax independence by selecting mutually acceptable transfer syntaxes. i.e. character encoding | |||
| 5 | Session | Supporting orderly data exchange by providing services to establish and release session connections between two presentation-entities in an orderly manner. For CL-mode communication a mapping of transport- to session-addresses is provided by this layer. | |||
| 4 | Transport | Segment, Datagram | The transport-service provides transparent transfer of data between session-entities and relieves them from any concern with the detailed way in which reliable and cost effective transfer of data is achieved. | ||
| Media layers |
3 | Network | Packet | Structuring and managing a multi-node network. Establish, maintain, and terminate network-connections. Provides to the transport-entities the independence of Relaying, addressing, routing, and traffic control considerations. | |
| 2 | Data link | LLC sublayer | Frame | multiplexing protocols on transmitting and demultiplexing on receiving. (See: the IEEE 802 family of protocols) |
|
| MAC sublayer | Frame delimiting and recognition; handling address space; transparent data transfer of LLC PDUs; CRC Checking; control of access to the physical transmission medium, like Token passing and CSMA/CD | ||||
| 1 | Physical | Baud, Symbol | Transmission and reception of raw bit streams over a physical medium | ||
Source: Wikipedia