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Auto-negotiation uses fastlink pulses to help a device determine the speed, duplex mode and other details of the link partner. However it is not clear from the ieee802.3 autoneg section what happens if 2 devices send fast link pulses at the same time over what will be a future half duplex connection. Is collision possible between them? If yes how are they corrected?

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A network connection has dedicated TX and RX pairs, and so the signals do not "collide" during this process.

Collisions occur when the two devices operate in a half-duplex CSMA/CD environment. When one device is transmitting on TX, if it receives any sort of signal on the RX it will register this as a collision, stop transmitting, back off and start the transmission process over again.

The reason for this is that typically a CSMA/CD environment operates with the use of hubs rather than switches/bridges. A hub is a OSI L1 device that simply receives a signal on one port and repeats the signal out all other ports (other then the receiving port). If a hub were to receive a signal on two different ports at the same time, the signal it repeats would be a "combination" of the two signals and unusable.

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  • also your answer clarifies another point that half duplex communication is due to the presence of hubs and not due to the presence of only one pair of lines between 2 communicating entities. thank you again. – Shreyas S Sep 3 '15 at 5:22
  • ♦ thank you. if a hub were to receive flps from 2 different devices at the same time would there be a problem? – Shreyas S Sep 4 '15 at 5:06
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    I would guess that the correct answer is that it will depend on the hub in question. Most hubs don't really negotiate (dual speed hubs are a bit of a "oddity"; see here for more details), but even those that do I would would imagine don't consider the interface "up" until any negotiation is over and won't forward any signal from that interface. – YLearn Sep 4 '15 at 5:39
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Short answer is no. Link speed and duplex negotiation is between a NIC and a port on a network (usually a switch port) not with the network itself. Collisions can only occur once you have established your connection to the network and are attempting to transmit a frame.

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