The Header Checksum (HCS) is a CRC-8 calculation over
the first 4 octets of the ATM cell header. The received
HCS is verified using the polynomial x^8 + x^2 + x + 1.
The coset polynomial x^6 + x^4 + x^2 + 1, is added
(modulo 2) to the received HCS octet before comparison
with the calculated result. Cells with single bit error
in the HCS byte are corrected. Cells with more than one
bit error in the HCS byte are uncorrectable.
Count of incoming cells on this phy with single bit error
in the Header Checksum (HCS) byte