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