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While working in full duplex mode, one can send and receive packets simultaneously. Why don't the packets collide?

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To help future searchers, the correct terminology would be "Why don't the frames collide?" at Layer 2. –  Mitch Miller Sep 9 '10 at 3:38
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2 Answers

up vote 5 down vote accepted

Full-duplex mode makes use of two physical pairs of twisted cable where one pair is used for receiving data packets and the other pair for sending packets. This way the cable itself represents a collision-free carrier.

It also doubles the maximum data volume that can be supported by the connection. Other advantages are that no time is wasted because no packets need to be retransmitted and the circumstance that nodes don't have to wait until others complete their transmission, since there is only one transmitter for each twisted pair.

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Why do we need "a pair" each for sending and receiving? Why wouldn't one for sending and one for receiving suffice? –  Lazer Aug 21 '10 at 16:29
    
Signaling and its electrical return (a.k.a. grounding reference). –  user48838 Aug 21 '10 at 21:46
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Because it is full duplex mode? Did yo uever check the wiring on normal ethernet cables? Like it uses 4 wires, in 2 separated circuits - so the Switch and the network card SEND and RECEIVE over DIFFERENT WIRES.

There simply is no chance for collission.

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Separate send and receive circuits resulting in no chance for collision is absolutely correct in point-to-point configurations, but in multi-point situations its actually the matching of the MACs to the physical ports during ethernet switching to form virtual exclusive switched signal pathing which creates a point-to-point arrangement for that ethernet frame instance. –  user48838 Aug 21 '10 at 11:00
    
There is no multipoint with a switch - all connections are poin to piont, with the switch "computer" doing the packet transfers between the ports. –  TomTom Aug 21 '10 at 12:27
    
As far as i know ethernet cables(cat5) has 8 wires in the cable –  Fahad Uddin Aug 21 '10 at 18:55
    
Only 4 wires are being used, 2 (hot & return) each for send and receive. –  user48838 Aug 21 '10 at 21:43
    
Actually there is during a layer 2 frame broadcast. It is a one to "all active ports" at that point. –  user48838 Aug 21 '10 at 21:45
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