When two computers transmit data over the network, one is a transmitting or sending computer and one is a receiving computer. Data is passed in frames, which are messages broken into smaller units with transport headers attached. To understand how frames are transferred through a network, we need to look at both ends of the transfer process: transmitting and receiving.
Data frames are formed whenever the sending computer initiates a request for communication. Frame formation begins at the highest layer and continues down through each successive layer. The protocol at each layer adds control information (in the form of headers and trailers) to the data that was passed down from the layer above. The frame is then passed to the layer below according to the definition of the interface. Eventually, the data passes through all layers of the protocol stack and is transmitted onto the network media.
At the receiving end, the frame is passed from the lower layers to the higher layers in accordance with the definition of the interfaces. The protocol at each layer interprets only the information contained in the headers and trailers that were placed there by its peer on the transmitting end. The protocol considers the rest of the frame to be the data unit, which it is responsible for delivering to the layer above it.