The Ethernet preamble, 56 bits of alternating 1 and 0 bits, allowing the receiver to synchronize its clock to the transmitter, followed by a one-octet start frame delimiter byte (10101011) and then the header.

What happens when the header or the payload collides with the SoF? (10101011) Or how is it avoided?

Also, if the mechanism considers the whole pack (preamble + start frame), the question remains, what happens when the header or data contains this sequence?

10101010 10101010 10101010 10101010 10101010 10101010 10101010
|Preamble                                             |Sof    |

Other way to see this question is, how can I send data equals too 10101010 10101010 10101010 10101010 10101010 10101010 10101010 in the ethernet payload?

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    A collision causes the data to be corrupted, so it's never received.
    – Ron Trunk
    May 1 '20 at 13:50
  • Thanks @RonTrunk so the origin hardware device doesn't let the frame to go out? or it goes out but the receiver hardware will drop it? And if it drops the frame, I suppose, the hardware continues trying to find a new preamble + sof, in this case if I double feed my frame payload with that, is it going to drop again and again util it finds one that has the proper [preamble+sof] [header|payload] [CRC]? May 1 '20 at 13:58
  • 1
    If the receiver doesn't receive a properly formatted frame, it will not not recognize it as a frame, or drop it as corrupted.
    – Ron Trunk
    May 1 '20 at 14:01
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    Once the receiver is synchronized with the data clock (the purpose of the preamble), it collects the bits until the frame stops. It doesn't matter what they are, so long as the receiver can interpret it as a proper frame.
    – Ron Trunk
    May 1 '20 at 14:10
  • 1
    In that same article, look at End of Frame
    – Ron Trunk
    May 1 '20 at 14:39

Any kind of signal that is transmitted concurrently to another signal causes a collision (in half-duplex mode).

You need to understand that the signal on the medium is analog, not binary. So, on shared wire (coax), the signals add to one another, causing illegal voltage levels. On duplex media (twisted pair, fiber), collision detection is simpler: any carrier detection on the receiver side while transmitting is a collision.

A preamble-like sequence somewhere in the middle of a frame doesn't matter. The preamble is used to synchronize the receiver's bit and byte shifters to the carrier. After that, the sequence has no meaning (it's just data) until the current frame has ended (loss of carrier or end-of-frame symbol, depending on the physical layer variant), and the receiver returns to carrier sense mode.

  • Thanks, After that, the sequence has no meaning (it's just data) until the current frame has ended @Ron Trunk pointed me the same, and I suppose the frame has a fixed lenght (either by default or by reading its header) and the end-of-frame symbol, therefore the hardware the start to parse the frame so it can sync the clock, read until X lenght. May 1 '20 at 15:24
  • 1
    Ethernet frames are variable in length. Their end is indicated by loss of carrier or end-of-frame. The line code is decoded in real time and when the EOF is detected, the buffer is passed to the NIC driver.
    – Zac67
    May 1 '20 at 15:32

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