For transmission, this field is
automatically inserted by the Ethernet MAC. The preamble field was historically used for
synchronization and contains seven bytes with the pattern 0x55 transmitted
from left to right. For the reception, this field is always stripped from the incoming frame
before the data is passed to you. The Ethernet MAC can receive Ethernet frames even if the
preamble does not exist as long as a valid start of frame delimiter is available.