Learn in the textbookThe OSI Reference Model and Its 7 Layers
DefinitionThe OSI reference model (OSI基本参照モデル) is an international framework defined by ISO that sorts the functions needed for computer-to-computer communication into seven layers.
ExplanationThe rules for the steps and formats of communication are called communication protocols (通信プロトコル), and each layer runs on protocols suited to its role. Each layer uses the functions of the layer directly below and serves the layer directly above, so swapping the method used in one layer has little effect on the others. Layers are counted from the bottom as Layer 1, Layer 2 and so on; lower numbers sit closer to signal transmission, and higher numbers sit closer to the user's applications.
AnalogyIt is like splitting parcel delivery among a packer, a label writer, and a driver. As long as each handoff follows the agreed rules, the packing stays the same even if the trucks are replaced by trains.
Memory hookOSI stands for Open Systems Interconnection. To recall the seven layers from the top down, use the phrase "All People Seem To Need Data Processing".