As I know the preamble of an Ethernet packet consists of a 56-bit pattern of alternating 1 and 0 bits. But how exactly it helps the other devices to be synced?

1 Answer 1


The key for this is the IEEE 802.3 documents which define ethernet. There is an answer on Super User which addresses this.

From IEEE 802.3:

802.3-2008_section1.pdf 3.2.1 Preamble field The Preamble field is a 7-octet field that is used to allow the PLS circuitry to reach its steady-state synchronization with the received packet’s timing (see 4.2.5).

4.2.5 Preamble generation In a LAN implementation, most of the Physical Layer components are allowed to provide valid output some number of bit times after being presented valid input signals. Thus it is necessary for a preamble to be sent before the start of data, to allow the PLS circuitry to reach its steady state. Upon request by TransmitLink- Mgmt to transmit the first bit of a new frame, PhysicalSignalEncap shall first transmit the preamble, a bit sequence used for physical medium stabilization and synchronization, followed by the Start Frame Delimiter. If, while transmitting the preamble or Start Frame Delimiter, the collision detect variable becomes true, any remaining preamble and Start Frame Delimiter bits shall be sent. The preamble pattern is: 10101010 10101010 10101010 10101010 10101010 10101010 10101010 The bits are transmitted in order, from left to right. The nature of the pattern is such that, for Manchester encoding, it appears as a periodic waveform on the medium that enables bit synchronization. It should be noted that the preamble ends with a “0.”


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