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Sorry for asking pretty simple question. But I don't understand that there are no collision problem with switch... When 3 PC connected with hub, 2 PC trying to connect to other 1 at the same time would make collision. Like this situation, switch would make logic circuits between ports like port 1 to port 3 and 2 to 3. But on the port 3 will receive 2 connections at the same time. I think when port 3 has 2 connections at same time, there are collision would affect. Is it right?

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3 Answers 3

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A collision can only occur when two nodes try to send data on the same shared channel at the same time. Typically, links to switches are full-duplex with no shared channels. So a collision is impossible.

If two devices are both sending frames at high speed to a single device, you can certainly reach a situation where the switch cannot fit all that traffic on the outbound port. But the result will not be collisions. The result will either be dropped frames or pauses, depending on the capability and configuration of the switch and the devices. Some switches and some Ethernet devices support a 'pause' frame that can be sent that tells them that the switch cannot accommodate their traffic at that time.

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One thing to keep in mind here is that almost most switches these days work on a store and then forward principle. Unlike with a hub, there is not direct connection between the transmitter and the receiver.

When a frame is received it is stored in to memory on the switch. The switch makes a decision transmit it out, and then it is transmitted, when that port is not busy. Since it is the switch that is transmitting the frame, it is smart enough to not try to send two frames at the same time.

Basically, you jsut have to think of it like any standard queue.

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Also, even cut-through switches (which try to avoid buffering latency) are smart enough not to try and send two frames at once. –  Tom Shaw Sep 23 '11 at 8:32
    
Thank you very much! :-) –  KevinOelen Sep 23 '11 at 8:38

In addition to the answers already posted, and just to clarify:

Switches don't eliminate collisions. Switches eliminate the single collision domain problem. On a switch, each port is an isolated collision domain and it is possible for a single host connected to one port to experience collisions if there's a duplex mismatch between the host NIC and the switch port to which it is connected (host at full-duplex and port at half-duplex and vice versa).

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Why do collision may occur if there is duplex mismatch? –  KevinOelen Sep 27 '11 at 10:01

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