The physical level is the most basic protocol level in the hierarchy of data communication protocols. This level covers the physical interface between devices and the rules by which bits are passed from one to another. These devices may be, for example, a data terminal equipment (DTE) and a data circuit-terminating equipment (DCE, e.g., a modem). This paper describes the physical level and the national and international standards that have been developed for this level. Included are insights into the development of recently adopted physical level protocols.