Physical layer: This is concerned with sending raw bits among the source & destination nodes over a physical medium. The source & destination nodes need to agree on a number of factors.
Signal encoding: how are the bits 0 & 1 to be represented?
Medium: what are the medium that is used & its properties?
Bit synchronization: it is the transmission synchronous or asynchronous?
Transmission type: this means whether the transmission is parallel or serial?
Transmission mode: it is mode of the transmission as simplex, half-duplex or full duplex?
Topology: what is the network topology that means mesh, star, ring or bus?