The Ethernet packet adds a preamble and start frame delimiter to the Ethernet frame before transmission, handling clock synchronization and signaling between devices at the physical layer. The packet also defines an interpacket gap that network cards must observe before sending the next frame.
Ethernet Layer 1 Packet
We've already talked about the Ethernet frame. However, there are some extra bits that get sent over during this communication, some extra ones and zeros. There are some starting bits that happen, so let's talk about those starting bits and a few other aspects of Ethernet from a layer 1 perspective.
We have the Ethernet frame right here that's being sent across, and then right before it's put onto the wire we add a couple more things to this, and we call that the Ethernet packet, the layer 1 Ethernet packet. It can be anywhere from 72 bytes to 1530 bytes.
The preamble is going to start off this communication. It is 7 octets long.
Let's say we're sending something along a copper wire and we send a signal across it. It's hard to determine, is this a signal that says 1 0 1 0, or is this a signal that's saying 1 1 0 0 1 1 0 0? There's some sort of timing in which this data comes across, and it can be a little difficult to transmit this timing from one location to the next.
So what a preamble does is it alternates a signal by sending a 1 0 1 0 1 0 1 0 1 0. It sends that across to the other side so that it synchronizes the clock between two different devices, and so now it can determine the rest of the frame, how it's timed out, and determine what is a one and a zero along the way without getting confused. It just synchronizes the timing between two different devices.
Then we have the start frame delimiter. The start frame delimiter is one octet long. This is a signal to signal that the preamble is done and we are about ready to communicate the rest of the frame. It does this by sending a 1 0 1 0 1 0 1 1, and then that 1 signals, all right, we're off the timing piece, we're now synced up, now comes the rest of the frame.
Then we come to the end of the frame. What happens at the end of the frame? It kind of depends on the protocol. But either at the end of the frame we just stop communication and that's the signal that we're done communicating, or perhaps there's some sort of signaling at the end of this that stops and says, I'm finalized, I've no more to communicate.
What does happen at the end here is there's some sort of gap. Before a NIC card can then transmit the next frame and send the next frame out, it has to wait a period of time, and that period of time would be the equivalent of 12 octets before it can send the next frame out. So that is known as the interpacket gap.
If you're like me, you like to see what the nuts and bolts look like, what the actual detail looks like, so I outlined it here below. If you were to map this out with ones and zeros, what exactly would it look like? You can see what seven octets look like, what one octet looks like, what a MAC address actually looks like when you convert it to ones and zeros — this is what the representative would be. And the payload that I have right here is actually the minimum payload that we have within this frame. So you can see the ones and zeros; this would be the actual signal.
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