Ethernet Port Link Activity?

Of course the ethernet port will blink when there is activity. However, I want to know what it determines to be activity. Does it recognize certain packets? Does it simply see that data in the form of bits are being transmitted? The reason I'm curious is that I have a coworker that had a switch not blink when using a cat6 cable, but it would blink with a cat5. In both cases, the data was being transmitted properly from the PLC. I am wondering how the transmission rates are affecting the frequency of the blinking and if that is why it "appears" to be solid.

Thanks in advance.