I am helping my colleague debug his problem. Basically, there are two 10M devices, and only one of which we have control over (the other being a "black box"). Analyzing the Ethernet hardware signals with a scope shows that, when the two devices are directly connected to one-another (without going through a switch), the "black box" device sends packets that do not have the 7-byte Ethernet preambles.

What sort of traffic between two Ethernet cards could have no preamble? Is it possible for two network cards to negotiate removing the 7-byte Ethernet preamble? Below is a capture of the network signals we are seeing when the two devices are directly connected:

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  • Related discussion in Chat – Sergey Vlasov Jun 3 '13 at 16:01
  • 5
    At the point where you had to break out the oscilloscope to debug this issue you generally stepped beyond the bounds of what Server Fault deals with and into the realm of Electrical Engineering and the on-the-wire protocol. That said, based on your chat with Sergey I think your best bet is to contact the vendor of the Black Box device and tell them their device appears to violate the 10M Ethernet specification.... – voretaq7 Jun 3 '13 at 16:55
  • I am currently dreaming of more questions like this. – artifex Jun 3 '13 at 21:40

Frames are not sent without the preamble - the preamble has been a non-optional part of the frame specification from the very beginning of Ethernet and has survived up until now, with 10G Ethernet specs released and 40G nearly so.

The only data which is sent without a preamble is the NLP / LIP or the FLP sent for AutoNegotiation announcements. Those however are unipolar and 2 ms in length - which does not fit your oscillogram.

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