First, it checks if the medium is available Carrier Sense. If it isn't, it waits until the current sender on the medium has finished. Suppose Station A believes the medium is available and attempts to send a frame. Because the medium is shared Multiple Access , other senders might also attempt to send at the same time.
At this point, Station B tries to send a frame at the same time as Station A. Shortly after, Station A and Station B realize that there is another device attempting to send a frame Collision Detect. Each station waits for a random amount of time before sending again. The time after the collision is divided into time slots; Station A and Station B each pick a random slot for attempting a retransmission. Should Station A and Station B attempt to retransmit in the same slot, they extend the number of slots.
Each station then picks a new slot, thereby decreasing the probability of retransmitting in the same slot. In summary, collisions are a way to distribute the traffic load over time by arbitrating access to the shared medium. Collisions are not bad; they are essential to correct Ethernet operation.
The maximum amount of retransmissions for the same frame in the collision mechanism is If it fails 16 consecutive times, it is counted as an excessive collision. The deferred counter counts the number of times the interface has tried to send a frame, but found the carrier busy at the first attempt Carrier Sense. This does not constitute a problem, and is part of normal Ethernet operation.
As explained here, collisions do not constitute a problem. The collisions counter counts the number of frames for which one or more collisions occurred when the frames were sent. The collisions counter can be broken down into single collisions and multiple collisions , as in this output from the show controller command:.
This means that eight out of 10 frames have been successfully transmitted after one collision; the other two frames required multiple collisions to arbitrate access to the medium. An increasing collision rate number of packets output divided by the number of collisions does not indicate a problem: it is merely an indication of a higher offered load to the network.
When stations detect a collision, they cease transmission, wait a random amount of time, and attempt to transmit when they again detect silence on the medium. The random pause and retry is an important part of the protocol.
If two stations collide when transmitting once, then both will need to transmit again. At the next appropriate chance to transmit, both stations involved with the previous collision will have data ready to transmit. If they transmitted again at the first opportunity, they would most likely collide again and again indefinitely. Instead, the random delay makes it unlikely that any two stations will collide more than a few times in a row. Sign up for our Newsletter! Mobile Newsletter banner close.
Mobile Newsletter chat close. Active Oldest Votes. Assume you have the following situation Notes about GigabitEthernet: Half-duplex is called out in the standard Note 1 ; however, nobody actually uses half-duplex GigE. Improve this answer. Community Bot 1. Mike Pennington Mike Pennington That link is not "the standard". GigE does not support half-duplex operation. And finally, nobody makes hubs anymore. Unlike 10 or Mb ethernet, which default to half duplex, 1 Gb defaults to full duplex.
With 10 Gb ethernet, they finally got rid of half duplex altogether. Ricky, at this point your comments have nothing to do with improving my answer. It's time to refocus on what I said. No one implements a half-duplex mode. Simultaneously transmitting and receiving in half duplex is a collision.
The tx and rx signals don't physically collide because they are on different pairs; this is a logical collision condition not a physical collision. Show 7 more comments. Ricky Ricky 29k 2 2 gold badges 40 40 silver badges 76 76 bronze badges. Peter Green Peter Green Sign up or log in Sign up using Google. Sign up using Facebook. Sign up using Email and Password. Post as a guest Name.
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