Functions of the Physical layer
	The physical medium sublayer (lower sublayer) handles and adapts bits as they are fed to the physical medium (e.g., electro-optical conversion) The transmission convergence sublayer (upper sublayer) converts the flow of cells from the ATM layer into a continuous bit stream (and vice versa), involving: •  Cell rate decoupling (bit rate adaptation) •  Cell delineation (generally using HEC method) •  Mapping of cells into, e.g., SDH VC-4 payload •  Calculation and verification of HEC byte