Understanding the OSI-Model
The OSI-Model stands for Open Systems Interconnection. It was established by the ISO (International Organization for Standardization) in 1984 to provide a universal standard for different computer systems to communicate with each other. The model breaks down the complex process of Network-Communication into seven logical layers, each with specific functions.
The Seven Layers of OSI
Each layer serves the layer above it and is served by the layer below it. This process involves Data-Encapsulation as information moves down the stack from the sender and de-encapsulation as it moves up the stack at the receiver.
- Layer 7: Application-Layer - The only layer that directly interacts with data from the user. Software applications like web browsers and email clients rely on this layer to initiate communications. Key protocols include HTTP, DNS, and SMTP.
- Layer 6: Presentation-Layer - This layer is responsible for translating, encrypting, and compressing data. It ensures that the Application-Layer can understand the data being sent by formatting it correctly.
- Layer 5: Session-Layer - This layer creates communication channels, called sessions, between devices. It is responsible for opening, closing, and managing these sessions to ensure data transfer is synchronized.
- Layer 4: Transport-Layer - Responsible for end-to-end communication. It takes data from the session layer and breaks it into segments. TCP (Transmission Control Protocol) and UDP (User Datagram Protocol) are the primary protocols used here to manage flow control and error correction.
- Layer 3: Network-Layer - Facilitates data transfer between two different networks. It breaks segments into packets and finds the best physical path using Routing and IP-Addressing.
- Layer 2: Data-Link-Layer - Similar to the network layer, but handles data transfer between two devices on the same network. It breaks packets into frames and uses the MAC-Address for physical delivery.
- Layer 1: Physical-Layer - Includes the physical equipment involved in the data transfer, such as cables, Network-Switches, and hubs. At this layer, data is converted into a bitstream of 1s and 0s.
For more detailed technical specifications, refer to the ISO 7498-1 standard or the Wikipedia entry on OSI.