4.2.1 ATM physical layer
The ATM physical layer is divided into two sub-layers: the transmission convergence sub-layer and the physical medium sub-layer.
Functions of the transmission convergence sub-layer include generating and receiving cells, and generating and verifying the cyclic redundancy check in the header error control field. For correct interpretation of ATM cells it is important to identify the beginning of a cell. In theory, if ATM cells are transmitted as a continuous stream of bits, once a receiver has found the start of one cell, the start of the next cell starts 53 × 8 = 424 bits later. However, this still leaves the problem of identifying the start of the first ATM cell. The sending device inserts a cyclic redundancy check word in the header error control field in each ATM cell. A receiving device performs cyclic redundancy checks on 40 consecutive bits (five bytes) of the bit stream, so valid headers will pass this test. Of course, it is possible that any 40 bits may pass a cyclic redundancy check by chance. To avoid the possibility of misinterpretation, a receiver will check the header error control bytes of the next few cells before assuming that it has managed to synchronise with the arrival of ATM cells over a link. Unfortunately, because of mismatches in the timing of senders and receivers, it is possible to lose bits. Therefore, the receiver should still monitor the header error control field. If several consecutive cells fail the cyclic redundancy check, the receiver assumes that it has lost synchronisation over that link and starts again to look for ATM headers by examining 40 consecutive bits. This process is shown as a state diagram in Figure 28. The number of correct header error control checks that should occur before the receiver assumes synchronisation is expressed as 
and the number of incorrect checks that can occur before it assumes loss of synchronisation is expressed as .
SAQ 15
Why is it not assumed that synchronisation is lost after a single header error control check failure, i.e. = 1?
Answer
Header error control check failures are more likely to be caused by simple transmission errors of misinterpreting a 1 for a 0, or vice versa, rather than a timing error causing a loss or insertion of a bit. Therefore, it is sensible to discard the ATM cell that failed the check, but still assume for the time being that the link is synchronised.
The functions of the ATM physical medium sub-layer are associated with the transmission of bits over a specific physical medium. The main transmission m