7 Layer OSI : Physical Layer

The physical layer is responsible for activating the physical circuit between the Data Terminal Equipment (DTE) and Data Circuit-terminating Equipment (DCE), communicating through it, and then deactivating it. Additionally, the physical layer is also responsible for the communication between DCEs. A computer or router can represent the DTE. The DCE, on the other hand, is usually represented by a modem or a multiplexer.

To put it differently, the physical layer describes the electric or optical signals used for communicating between two computers. Physical circuits are created on the physical layer. Other appliances such as modems modulating a signal for a phone line are often put in the physical circuits created between two computers.
Physical layer protocols specify the following:
•  Electrical signals (for example, +1V)
•  Connector shapes (for example, V.35)
•  Media type (twisted pair, coaxial cable, optical fiber, etc.)
•  Modulation (for example, FM, PM, etc.)
•  Coding (for example, RZ, NRZ, etc.)
•  Synchronization (synchronous and asynchronous communication, time source, and so on)