Preamble (64 bits) (Ethernet)

Protocol: Ethernet Ethernet PCI

Field: preamble

Length: 64 bits / 8 octets

Contents: synchronization pattern.

Ethernet stations need to synchronize with the signal's frequency; this is done by means of 64 preamble bits.

Token-Ring stations are always synchronous to received frames because there is always a signal passing the ring (idle symbols); they only need a 8-bit starting delimiter in order to get ready for receiving the frame.

Synchronization in Ethernet networks could be done using shorter preambles if there were only twisted pair cables; but since the standard was made for coaxial cables, the preamble needs to be longer because the first transmitted bytes of a frame are damaged on a coaxial cable.

The preamble's bit pattern is different due to different versions:

Ethernet II

The preamble is built up by 8 similar octets of '10101010' bit patterns.

IEEE 802.3 - CSMA/CD

The first seven octets use a '10101010' bit pattern, too. The last octet is a start of frame delimiter (SFD) with a bit pattern of '10101011'.

Even if the preamble had been currupted, the following destination address can be identified correctly.