For example, data comes into the card faster than it can be converted from a serial or parallel format, depacketized, read, and sent. This is true in both directions. While in the buffer, data may be acted on, such as put into frames, or it may simply wait while the NIC handles other things. This can be less expensive, but is usually slower and it requires memory. The NIC's most important job is frame formation.
Frames are the basic units of transmission.
Files and messages are broken into frames for transmission. At the other end the frames are reassembled to form the file or message. The header includes an alert to signal that the frame is on its way, the frame's source address, destination address, and other data. In some networks, headers also have preamble bits used for various purposes, like setting up parameters for transmission. They can also have a control field to direct the frame through the network, a byte count, and a message type field.