The OSI physical layer provides the means to transport the bits that make up a data link layer frame across the network media. This layer accepts a complete frame from the data link layer and encodes it as a series of signals that are transmitted onto the local media. The encoded bits that comprise a frame are received by either an end device or an intermediate device.
The process that data undergoes from a source node to a destination node is:
- The user data is segmented by the transport layer, placed into packets by the network layer, and further encapsulated as frames by the data link layer.
- The physical layer encodes the frames and creates the electrical, optical, or radio wave signals that represent the bits in each frame.
- These signals are then sent on the media one at a time.
- The destination node physical layer retrieves these individual signals from the media, restores them to their bit representations, and passes the bits up to the data link layer as a complete frame.