Header Error Control
This is a bit error detection and correction mechanism used in data transmitter and receiver.
The Header Error Control (HEC) is the last field in the Asynchronous Transfer Mode cell consisting of an 8-bit CRC of the cell's header only.
It consists of the remainder of the division of the 32 bits of the header by the polynomial 
. The pattern 01010101 is XORed with the 8-bit remainder before being inserted in the last octet of the header.[1] This added 01010101 value is called a coset.
This scheme is able to correct single-bit errors and detect many multiple-bit errors.
For a tutorial and an example of computing the CRC see the Mathematics of CRCs page.