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.
|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.|
|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.