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I understand that a network card detects collisions because their signal has a higher amplitude than normal signals. But why is that signal of higher amplitude?

For example, say we have the simplest of cases, two computers communicating half-duplex on one wire and they both transmit at the same time, a collision occurring:

A |------>-<------| B

Why does the current intensity arrive at the stations at a higher level? The currents are opposed one to each other.

Most probably I'm missing basic physics here, but I could sure use an explanation...

Thanks

Claudiu
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1 Answers1

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I might be a bit late in answering this, but nevertheless it might help those who reach here in the future.

Quoting from this page:

When a NIC has data to transmit, the NIC first listens to the cable (using a transceiver) to see if a carrier (signal) is being transmitted by another node.

When there is data waiting to be sent, each transmitting NIC also monitors its own transmission. If it observes a collision (excess current above what it is generating, i.e. > 24 mA for coaxial Ethernet), it stops transmission immediately and instead transmits a 32-bit jam sequence.

This patent seems to be describing a mechanism to detect collisions. Skimming through it, I believe it works by analysing the bits which the sender recieves. A phase decoder sits on the sender and it has a 2-bit output. Two of the outputs are reserved for 1/0 and remaining 2 indicate a collision has occured.

pranavashok
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