Header Error Check

Header Error Check is a test method used in Asynchronous Transfer Mode (ATM) transmission technology to determine whether an ATM cell has been received correctly.

For this purpose, the head of the ATM cell (header) contains an error correction sequence ( header error code, HEC) in the fifth and last byte, corresponding to a frame check sequence (FCS). It is used to handle errors in the cell header, but mainly to check whether the cell boundaries were correctly recognized in the received bit stream. When receiving, the receiver continuously calculates the expected HEC byte and compares it with the one it was sent. If the two values ​​do not match, it first corrects the cell header when forwarding the cell. After a few successive errors, the receiver assumes that it has lost synchronization and starts a new synchronization of the receiver. Because of this procedure, ATM is called asynchronous: it allows a far greater deviation in the synchronization of network elements than is possible with PDH .

This solution at ATM was aimed at integrating the data traffic which, in contrast to the telephone networks clocked centrally with high accuracy, can come from completely unsynchronized sources of private users.

Only the synchronization procedures developed later by SDH were able to surpass that of ATM by further increasing the tolerance for the clock rates.