The Advanced Physical Layer is the component of the Open Systems Interconnection (OSI) model responsible for physical connections between network nodes. It is responsible for the transmission of bits over a physical medium, and defines the electrical, mechanical, and procedural specifications for activating, maintaining and deactivating the physical connection. It also defines the electrical and mechanical characteristics of the interface, such as voltage levels, timing and signal type. The Advanced Physical Layer can be used to transmit signals over copper, fiber optic, and wireless media.