Layer One - Physical

this is a small and short explanation of what layer is all about.

the physical is responsible for how data is transmitted across the media. it describes the cable and how it's attached to the network adapter. it defines the mechanical and electrical characteristics of the cables and connectors that link the network components. it also defines data encoding and bit synchronization ensuring that when a transmitting host sends a 1 bit, it is received as a 1 bit, not a 0 bit.

layer one is responsible for transmitting bits in a computer network

some examples of physical layer is:

cables
connectors
hub
csu/dsu
modem