1

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?

2 Answers 2

2

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.

3
  • 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, 2015 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, 2015 at 5:06
  • 1
    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, 2015 at 5:39
2

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.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.