The OSI Model

The OSI (Open Systems Interconnection) model is a conceptual framework developed by the International Organization for Standardization (ISO) to standardize network communication. It divides the complex process into seven distinct layers, each with a specific set of functions. This separation of concerns simplifies network design, teaching, and troubleshooting. The layers are typically described from bottom to top. Layer 1, the Physical Layer, deals with the physical transmission of raw bits over a communication channel. It defines specifications for cables, connectors, and signaling. Layer 2, the Data Link Layer, is responsible for reliable node-to-node data transfer. It takes the raw bit stream from the Physical Layer and organizes it into frames, handles error detection and correction, and manages access to the physical medium through MAC addresses. Layer 3, the Network Layer, manages the delivery of packets from the source host to the destination host across multiple networks. This is where IP addressing and routing happen. Layer 4, the Transport Layer, provides reliable end-to-end communication services. It segments data from the upper layers, provides flow control and error checking, and is where protocols like TCP and UDP operate. Layer 5, the Session Layer, establishes, manages, and terminates connections (sessions) between applications. Layer 6, the Presentation Layer, is responsible for data translation, encryption, and compression, ensuring that data sent by one system is readable by another. Finally, Layer 7, the Application Layer, is the layer closest to the end-user, providing network services directly to user applications like web browsers and email clients.