The Open Systems Interconnection model (OSI) is a conceptual model that characterizes and standardizes the internal functions of a communication system by partitioning it into abstraction layers. The model is a product of the Open Systems Interconnection project at the International Organization for Standardization (ISO), maintained by the identification ISO/IEC 7498-1.
The Open Systems Interconnect (OSI) model has seven layers, recommendation X.200 describes seven layers, labeled 1 to 7. Layer 1 is the lower layer in this model.
OSI Model | ||||
---|---|---|---|---|
Data unit | Layer | Function | ||
Host layers | Data | 7. Application | Network process to application | |
6. Presentation | Data representation, encryption and decryption, convert machine dependent data to machine independent data | |||
5. Session | Interhost communication, managing sessions between applications | |||
Segments | 4. Transport | Reliable delivery of packets between points on a network. | ||
Media layers | Packet/Datagram | 3. Network | Addressing, routing and (not necessarily reliable) delivery of datagrams between points on a network. | |
Bit/Frame | 2. Data link | A reliable direct point-to-point data connection. | ||
Bit | 1. Physical | A (not necessarily reliable) direct point-to-point data connection. |
At each level (N), two entities (layer N peers) exchange protocol data units (PDUs) by means of a layer-N protocol. A service data unit (SDU) is the payload of a PDU, transmitted unchanged to a peer.