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

Asynchronous Transfer Mode

Asynchronous Transfer Mode is a standard switching technique designed to unify telecommunication and computer networks. It uses asynchronous time-division multiplexing, and it encodes data into small, fixed-sized cells. This differs from approaches such as the Internet Protocol or Ethernet that...

 cell consisting of an 8-bit CRC

Cyclic redundancy check

A cyclic redundancy check is an error-detecting code commonly used in digital networks and storage devices to detect accidental changes to raw data...

 of the cell's header only.

It consists of the remainder of the division of the 32 bits of the header by the polynomial

Polynomial

In mathematics, a polynomial is an expression of finite length constructed from variables and constants, using only the operations of addition, subtraction, multiplication, and non-negative integer exponents...

 . The pattern 01010101 is XORed with the 8-bit remainder before being inserted in the last octet of the header. 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.