The OSI Reference Model

The OSI Reference Model - Unified View of Layers, Protocols, and Services.


International Organization for Standardization (ISO) has developed a reference model for Open System Interconnection (OSI).

The seven layers of the OSI reference model are:

1. Layer 1: Physical layer; Name of units: Bit;
2. Layer 2: Data link layer; Name of units: Frame;
3. Layer 3: Network layer; Name of units: Packet;
4. Layer 4: Transport layer; Name of units: Message;
5. Layer 5: Session layer; Name of units: Message;
6. Layer 6: Presentation layer; Name of units: Message;
7. Layer 7: Application layer; Name of units: Message;

Picture 1: The seven layers of the OSI reference model


The communication in the OSI model based network is peer-to-peer communication. This communication between peer processes is virtual in the sense that no direct communication link exists between them. The communication between 2 stations in the OSI model is illustrated on Picture 2.

Picture 2: The communication between 2 stations in the OSI model


The world widest network architecture, the TCP/IP network, is based on the OSI reference model.

Picture 3: The layers of TCP/IP network & the OSI reference model