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Will that continually reset the receiving NIC or is there a sort of escape character that need to be used if my data contains nothing but alternating 1s and 0s in 7 byte blocks with start of frame numbers following it?

I’m confused as to : how does the Receiving NIC know when to be on stream checker mode vs on data receiving mode?

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  • This has nothing to do with IP, so I removed the IP tag.
    – Ron Maupin
    Sep 10 '20 at 17:36
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The seven-octet Preamble comes after a 12-octet (or longer) Inter-Packet gap, then you get the one-octet Start of Frame. Next, you get the ethernet frame header with the destination MAC address, source MAC address, etc., followed by the data and the Frame Check Sequence, then the 12-octet Inter-Packet gap.

There is no way to confuse data in a frame with the preamble because there would be no Inter-Packet gap. Also, unless the destination MAC address is a group address to which the interface has been configured to listen for, all the interfaces other than the destination interface will ignore the rest of the frame.


Edit per your comment:

An Inter-Packet gap is 96 bits of silence on the line, but a frame has no such thing because a frame is sent complete from start to finish. It would simply be a damaged frame if it stops before the end, and there would be no FCS, so the last four octets would not match what the FCS should be. In fact, the frame could be too small to be valid.

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  • Soo...what if my data is mentioning inter-pocket gaps? I’m sure you get the intent of my question rather than specifics — is there a way the interface tells itself “ok, this is the data part, nothing in here is an instruction for me” and if so, then how does the interface also parse/transmit the data? Because to transmit the payload, it has to be processed, correct? So, how can the interface differentiate between payload vs instruction ? Thanks for your answer. Sep 13 '20 at 1:22
  • As I explained in the answer, the Preamble follows an Inter-Packet gap. Once the interface receives the SoF, it reads the frame until the next Inter-Packet gap starts. The SoF tells it that what follows is the frame, so there is no way to confuse anything in the frame with the Preamble because there is no Inter-Packet gap in a frame.
    – Ron Maupin
    Sep 13 '20 at 2:03
  • @nicotinefull, an Inter-Packet gap is 96 bits of silence on the line, but a frame has no such thing because a frame is sent complete from start to finish. It would simply be a damaged frame if it stops before the end, and there would be no FCS, so the last four octets would not match what the FCS should be. In fact, the frame could be too small to be valid.
    – Ron Maupin
    Sep 13 '20 at 5:08
  • Thanks Ron this is what I think I was missing. Sep 13 '20 at 11:40
  • Is it possible for you to add hat last comment in the answer? I’ll take it as the answer then. Thanks. Sep 13 '20 at 11:41
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Preamble and SOF are only used to sync a receiver to the sender's carrier. Following SOF, a frame is received that ends with either an idle line (no carrier), or an explicit idle or end-of-frame symbol, depending on the physical layer variant. In between, everything is part of the received frame. A pattern similar to preamble/SOF does not cause anything to resync.

Additionally, it's highly unlikely that you miss the actual start of a frame and sync to a similar pattern somewhere in the middle: today's Ethernet is fully switched and point-to-point. Your link partner will only start transmitting frame when the link is firmly established.

Only with an obsolete, shared medium is it possible to link into a running transmission and catch part of a frame. Even then (assuming some preamble/SOF was detected), the partial frame ends with the normal end-of-frame and the frame check sequence (FCS) checksum won't match, so the frame is ignored/dropped.

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