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