A critical part of a communications protocol that enables the receiving device to logically determine what is to be done with the data and how to go about doing it. As illustrated in Figure D-1, a data format generally involves a header, text field, and a trailer. Although the header and trailer are overhead, they serve critical functions in support of the successful transfer of the data content. In total, the header, text, and trailer compose what is known variously as a packet, block, frame, or cell, with the specific terminology being sensitive to the specific protocol involved. Note: Some protocols, such as Asynchronous Transfer Mode (ATM), do not involve a trailer at the cell level, but do have trailers at higher (convergence) layers. See also header, text field, and trailer.