OSI for CCNA Layer 1: Physical
The CCNA exam has extensive coverage of hardware. That's the physical layer. The physical layer provides the electrical, optical, or over-the-air connection between the sending host device and the receiving host device. This typically involves copper or fiber-optic cabling, or wireless radio connections, patch panels, signal repeaters, submarine cables, or satellites.
CCNA certification does not require you to be a space science expert. However, you do need to understand that data is always converted into bits that can be transmitted over a medium using electrical current or optical signals that simulate a 1 (signal) or a 0 (no signal).
In a nutshell, the physical layer defines mechanical, electrical, optical, radio, procedural, and functional standards to enable the transmission of data-link (Layer 2) frames over a certain transmission medium.
These standards define how a physical link is built, activated, maintained, and deactivated to enable transmissions between DTE (data terminal equipment) and DCE (data communications equipment).
DTEs are host devices. DCEs are network devices, that is, any device that stands between two host devices.
Most hubs are amplifying the electrical signal; therefore, they are really repeaters with several ports. Hubs and repeaters are Layer 1 (physical layer) devices.