Originally Posted by DaveyC
You are correct in that a good Ethernet cable will transmit the Ethernet signal without data errors. In fact, with the help of TCPIP, it will do that all day, every day, year after year, as long as you don't try to use too long a length of cable. But even in the absence of bit errors, the RFI carried by the distorted analogue sine waves I mentioned will pass through the end of the cable and into the rest of the system unhindered.
Unhindered?

The only way that would make sense was if someone believed that the receiver absorbs 100% of the data, leaving behind everything it didn't understand.

Reality: 100% of the incoming Ethernet signal is unwanted RFI to the analog side.

This will be everything from 0Hz (i.e. DC-POE), to as near to infinite Hz as the system will allow (squarewave rise time) added to any problems created by cable shielding defeating intercomponent isolation.

Persisting in refering to distorted sine wave components doesn't make sense - the analog side doesn't care how pretty the square waves are (*), if the analog side sees any of the incoming signal, the designer has FAILED, you will not fix that with a prettier cable (unless attenuating the signal lowers the noise floor - although that then means you are depending on the signal being degraded).

(*) square waves are better at coupling unwanted noise than sine waves.

This is like complaining that the noise of pins dropping is spoiling the view - with a pneumatic road drill a meter away - the volume of noise from the actual signal will swamp out the noise from the 'noise'.

Telling people to connect up a scope to see how bad things are because of the 'cable' is a good way to fake up a fail - overlooking that the signal has been through at least one transformer (the ethernet interface) and high frequency square waves are not going to look good on anything other than a very high bandwidth scope and appropriate probe.

Cable arguments tend to be 'won' by people repeatedly pasting cable sellers literature full of technospeak that at first glance seems applicable, but takes more time for the people that actually understand the subject to explain why it's not appropriate, eventually the 'copy/paste' wins over the thinking leaving behind a forum with lots of advice based on guesswork/sellers literature/individual perception rather than actual reality.

Matt.