So a treated ATM cell can be mapped into the VC-n and then go through synchronous multiplexing. So the cells cross the VC boundary, and as a consequence, the starting location of an ATM cell varies. The VCs POH H4 byte is used for easy detection of the ATM cell boundary. Figure below shows mapping - page 167. It could also detect the AU-4 starting position and picking out ATM cells, and then searching for the boundary using the H4 byte.
Valid cells can also carry errors.
This is due to imperfection in Cyclic Redundancy Check or a flaw in error correction stage. 01010101 is then added. The remainder is recorded in the HEC field, which is subsequently confirmed by the receiving side. At this side the 01010101 is first subtracted before further interpretation. Return to the main ATM page.