Fernando Ariel Gont wrote in a message to Ismail Al-Sulby:
 FAG> But I don't understand when you say that after noticing that
 FAG> the carrior is free, you send your packet AND _*keep*_
 FAG> _*sensing*_ _*the*_ _*carrior*_ ... 
Let's say that each card puts a 1 volt DC bias on its signal.  Then, by 
measuring the average DC bias, you can deduce how many cards are transmitting 
simultaneously.  If a transmitting card sees the DC bias rise higher than it 
should, then it knows that some other card is also transmitting and a 
collision has occurred.  Because it takes real time for the signal to travel 
along a cable, it can easily happen that two cards decide to transmit within 
a couple of bit times of each other, and a collision results.
 FAG> What would happen if:
 FAG> Two machines were waiting for the carrior to get free.
 FAG> The line becomes free, and both machines notice it at the
 FAG> same time. They both try to send their packets, but at the
 FAG> same time!
Right.  They use the DC bias measurement to detect this.  The frame is lost 
anyway, but the time the cable is unavailable is minimized.
 
-- Mike
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