Learn in the textbookThe OSI Reference Model and Its 7 Layers
DefinitionThe physical layer (物理層) is Layer 1 of the OSI reference model; it turns bits, the 0s and 1s, into electrical signals, light, or radio waves and sends them over the transmission medium.
ExplanationIt covers the physical connection standards, such as the shape of cables and connectors and the voltage and timing of signals. It does not care what the bit stream means. A repeater, which boosts a weakened signal so it can travel farther, is an example of a device working at this layer.
AnalogyIt is like paving a road so cars can drive on it. Which car carries what cargo is none of its concern.
Memory hookAs the name suggests, this bottom layer is about things you can touch, like cables and connectors.