Layer 1 (or the PHY layer) of the Open Systems Interconnection (OSI) reference model. The physical layer is the bottom layer of the seven-layer OSI networking architecture model. It establishes the physical interface and mechanisms for placing a raw stream of bits onto the wire. It defines the voltage, current, modulation, bit synchronization, connection activation and deactivation, and various electrical characteristics for the transmission media (such as unshielded or shielded twisted-pair cabling, coaxial cabling, and fiber-optic cabling). Protocols at the PHY layer include IEEE 802.3, RS-232C, and X.21. Repeaters, transceivers, network interface cards (NICs), and cabling operate at the PHY level.