Physical layer is the first lowest layer in the OSI reference model, which defines the procedures and functions of communicating network devices over a physical medium.
Following are the functions and procedures performed by physical layer :
Defines the electrical and physical characteristics of the interface between the network devices and the transmission medium like voltage, current, frequency, type of cable, layout of pins and more.
Transmission data : Data consists of stream of bits i.e. sequence of 0's and 1's, calculated in terms of number of bits per second. This data is transmitted by encoding into signals, which is define by physical layer.
Defines the functions and services to be performed by physical devices :
Modulation - conversion of signals to bits and vice versa.
Multiplexing - establishment and termination of connection.
Synchronization of bits.
Line coding etc.
Defines type of line configuration to be use :
Point-to-point - dedicated link
Point-to-multipoint - shared link
Defines type of transmission mode to be use:
Simplex - one way communication
Half-duplex - both way, but one at a time communication
Full duplex - both way, at the same time communication
Physical topology : Ring, Bus, Star, Mesh etc.
Protocols used: Ethernet, ISDN, and other IEEE 802.