HEC (8 bits)

Protocol: ATM ATM PCI (Cell Layer)

Field: header error control

Length: 8 bits

Contents: both checksum and synchronization pattern.

The Header Error Control only covers the ATM cell header (not data).

If an ATM node starts operation, it uses the HEC of idle cells sent by the next switch to synchronize to the byte and cell boundary. (Other protocols use separate 'flag' octets for synchronization > HDLC, SDLC, etc.)