1. Physical Layer

This layer is responsible for the actual physical transfer of data between devices via cables or other mediums using switches, routers, etc..

This layer also converts the data into a bit-stream of 1's and 0's.

The signal convention must be the same on both devices. (Meaning of a 0 and a 1)

In depth

Information on computers is stored in a digital format. To transmit this information we have to convert the information to signals.

Physical layer is responsible for this conversion. The Physical layer deals with the actual physical transmission of electrical signals.

You could say the physical layer is a bridge between real life and software.