Full Duplex Mode

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May 4th, 2007
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I have a question about "full duplex mode." When I was first introduced to this concept, it was explained to me in the following manner. On a dedicated circuit or network segment communication can take place in both directions simultaneously. Both nodes have transceivers with differentiator circuitry. They transmit over the full bandwidth of their shared media. Some of the bits collide while others pass. The resultant scalar wave front, which is composed of reflected and passed bits, returns/continues to the transceivers. The differentiation circuitry compares what was sent with what was received to determine what was actually just received from the other node. If my understanding is correct, it is awe inspiring to realize the speed at which this takes place! It means that the two nodes are communicating by sending a single packet across the media at a time and waiting for the partially reflected/partially transmitted packet to return/arrive before sending the next packet! Is that essentially correct? If it is, I can understand the physics of how this could be possible (i.e. voltage signals colliding and being reflected). As amazing as that sounds, especially considering the synchronization required, I don't see how it could work over fiber. According to our book, 10 Gb Ethernet operates only over fiber! Perhaps I need a refresher on basic physics, but I thought that when light collided with light it created an interference pattern, not a reflection. I would think that even in the somewhat constrained environment of single mode fiber, the collision of light would create a dispersion pattern that wouldn't be decipherable at either transceiver. I can see how it could work with electricity, but not with light. The only way that I could see for full duplex to work over fiber is if the signals are sharing the bandwidth (i.e. multimode fiber) instead of colliding. Am I wrong?

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mark.j.hodge Fri, 05/04/2007 - 12:19
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I think you may have got things a lttle mixed up.


In Full-Duplex there are no collisions, each device has seperate transmition TX and receive RX circuitry.


In Half-Duplex the device "listens" to its own transmissions, and if there is a corruption of the signal, determines a collision has occured.


Full-Duplex with fibre makes use of two cores in a single cable.



cisco steps Fri, 05/04/2007 - 12:34
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it looks like you consern is data colision, if that is the case then you need to mention what protocol you are using, not only ( Full-Duplex ot 1/2 Duplex )


there is connection less and connection oriented

there is windowing and there is flow control ..


windowing = allows us to raise the #s of segment the sender can submit b4 as ack must be received


flow control= allow the receiver to adjust the flow of the segment beeing transmited


connection oriented waits for the ack b4 transmit


connection less dont


I hope I clear @ least some of your question



gr84sight Fri, 05/04/2007 - 13:49
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Haven't you ever heard of echo cancellation? How do you think modems work? It's not CSMA/CD. Both modems transmit simultaneously over a single pair (RJ11). Every transmission meets at the middle of the medium. Like bits collide and are reflected back. Unlike bits pass on to their destination along with the reflected bits. The transceiver cancels the "echo" and what you have left is the actual transmission from the other node. This wouldn't work with light but, it would be possible to transmit signals over a single, multi-mode fiber simultaneously, assuming they are sharing the bandwidth. You might want to read up on "full collision mode."

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