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When a computer X is about to send a set of frames over the network to computer Y, should the NIC card of the computer X notify the other computers' NIC cards on the same communication link before start transmitting these frames? If yes, can you explain how this process is performed? If X starts to transmit without any notification, does it mean that frames that are already on the link will get corrupted?

Thank you

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No notification is sent before sending frames.

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  • I remember long time ago I read an article that mentioned the importance of having some sort of a mechanism of notifying the other machines before transmission. I don't know if I misunderstood this or not. – Mide Jan 19 '16 at 22:14
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    Not for ethernet. There is CSMA/CD, which listens to see if the line is free before sending, and it detects a collision if two or more try to send at the same time on a single collision domain. – Ron Maupin Jan 19 '16 at 22:21
  • Thank you. In fact, I visualize any link as a highway road of cars. What happen if 100Mbps link becomes full (in my picture of highway, this highway suddenly becomes full of cars and I cannot fit my car so I have to wait!!) In other word, the frame must wait until a room for it is available in this link, is that right? (THANK YOU FOR YOUR PATIENCE) – Mide Jan 19 '16 at 22:32
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On a switched full duplex ethernet network the card will under normal conditions just send the frames. There cannot be any collisions because of the full duplex point to point nature of the links.

On an old fassioned CSMA/CD ethernet collision domain the sender can only send if it detects the medium is free. There is still some chance of a collision because two devices could try and transmit at the same time (or at least close enough to the same time that it has to be treated as such). In that cases the devices will back-off for a random time and then try again.

If a switch receives multiple packets destined for a given outgoing port then it will queue them in internal buffers. If the packets keep coming in too fast then the switches buffers will fill, at this point depending on what extensions are supported the switch may send a notification (known as a "pause frame") back to the previous device in the chain or it may just start dropping packets.

Most of the time congestion avoidence algorithms in higher level protocols like TCP will back off the transmit rate avoiding too much packet loss even in cases where there is no effective way of notifying devices earier in the chain of full buffers.

Other physical layers may have different rules.

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