A question popped into my head today and after a quick few searches on google, I have no idea. When a device is sent a frame there is a layer 1 header/preamble saying the length and layer 2 protocol (I think please correct me if I'm wrong), but how does the NIC know where to start reading the data from? If it starts reading from the first 1 what happens if that 1 was just noise on the line? If any clarification is needed please ask. (Also no idea what tags to put for this.)

  • Do you mean for ethernet? Different layer-1 protocols do this differently.
    – Ron Maupin
    Aug 25, 2017 at 17:41
  • I didn't know they do it differently I assumed there would be different identifiers for each protocol. Seeing as ethernet is the most popular yes how would it work for ethernet? Aug 25, 2017 at 17:42
  • Removed the off-topic request for resources.
    – Ron Maupin
    Aug 25, 2017 at 17:43
  • Thank you for the edit I was searching for the tag as 'layer 1' instead of 'layer1' Aug 25, 2017 at 17:46

1 Answer 1


Ethernet sends seven octets of preamble (10101010) to synchronize, and then a one octet SoF (Start of Frame) delimiter (10101011). The next bit is the first bit of the frame. The frame has a four octet FCS (Frame Check Sequence) that is a CRC, and if the received CRC doesn't match the calculated CRC, then the frame is dropped as damaged.

  • 1
    So the receiver locks on to the pattern of alternating ones and zeroes and then knows that the frame starts when it sees two ones in a row. It doesn't ever have to know which bit is the first in the preamble, just in the frame data. Aug 25, 2017 at 17:54
  • Ah ok and that first seven octets would be unique for each protocol? Aug 25, 2017 at 17:56
  • No. Some protocols do something completely different. For example, some, e.g. serial protocols, will use other wires for signalling when data is present.
    – Ron Maupin
    Aug 25, 2017 at 18:09

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