In the IEEE 802.3 frame structure there is a 7 byte preamble section for synchronization. If it is really for synchronization does that mean Ethernet is synchronous and not asynchronous?
Ethernet structure for reference:
Your question seems to assume that each frame is transmitted one after the other, synchronously like one frame, then a gap then another, and thus why a need for a preamble. As if a preamble tells it and just tells it that there's a frame coming. And if a preamble, then why can't it be asynchronous. It is a good question. The preamble isn't for that. Or in fact to put it another way. It is actually asynchronous in the sense that you mean. The answer, is that the preamble isn't for synchronization of one frame followed by another e.t.c. synchronously. There is indeed no such synchronization for that. Frames do not arrive synchronously. It is purely for each individual frame so the electronics, a clock I suppose, reading it, knows when each bit is going to come -for that frame- by the time the processing of that frame reaches the start of the SFD(start of frame delimiter). Once the frame has been completely read, it has no idea when the next frame will arrive. In fact it never has any idea when the next frame will arrive.
Below are some quotes from some documents which serve as sources by which I can answer your question.
4.2.5 Preamble generation
Ethernet is Asynchronous.
Asynchronous communication means the transmitter and receiver do not share an external clock signal (such as would be transmitted over a "clock" pin or "clk+/clk-" pair on a cable). Ethernet cables have no clock pins or pairs. Ethernet does not use a separate bit-clock signal shared between the transmitter and receiver, so it is asynchronous.
Because Asynchronous communications busses don't share a separate clock signal, the transmitter has to encode each transmission in a way that allows the receiver to know when one bit ends and the next bit begins. Ethernet's solution for that is to start every transmission with a long series of alternating 0 and 1 bits -- the preamble -- which allows the receiver to temporarily synchronize its bit-clock with the transmitter's clock for the duration of that transmission. As soon as one frame ends and the next begins, the temporary synchronization must begin again.
The preamble is needed for electrical reasons so that the MAC can easily detect when the packets starts.
I do not understand your question about Ethernet being "synchron/asynchron".