As network data passes through the OSI model, starting at the application layer and moving downward, conceptually each PDU at a given layer must be converted to a PDU in the next lower layer. The conversion scheme defined by OSI is encapsulation, a process whereby header data (a small number of bytes of control information) is added to the front of the existing PDU. At the data link layer (layer two), trailer data is also added to the PDU for error detection purposes.
