[dom0]

A DSP in the network card does quite a bit of work to make it work: Since the pairs are used at the same time, near-end and far-end crosstalk need to be cancelled out of the signals, it also removes the currently sent signal and it's reflection (echo cancellation), since each pair can be used in both directions at the same time (!). It also adjusts delay differences between the pairs.

Gigabit Ethernet is really cool and involved tech :)

[eriknstr]

I see you in comments here on HN every so often and I enjoy reading what you have to say. I've been wondering -- and sorry for going totally off-topic here -- whether or not you picked your username inspired by the Xen hypervisor initial domain dom0.

https://wiki.xen.org/wiki/Dom0

[dom0]

Thanks and yes, I did.

[javajosh]

Excellent. You wouldn't happen to have links to someone with an oscilloscope showing what that cross-talk looks like, would you? Would be useful to visualize even at sub-GHz speeds. And also, where is that echo coming from, and how do you avoid generating microwaves if you're sending a GHz signal down a wire? (Or, perhaps you don't avoid it and that's the nature of the cross-talk?)

[dom0]

Since the same pair is used for transmission and reception at the same time, the echo is just removing what you are sending right now from what you are receiving. Similar to how you can hear / listen to someone while talking at the same time (air = ethernet pair).

> how do you avoid generating microwaves if you're sending a GHz signal down a wire?

The main spectral energy of GbE is around 125 MHz & it's harmonics (250 MHz, ...), since that's the symbol rate on the wire, but a cable is still an excellent antenna at these frequencies.

Emission is mainly avoided by using differential transmission over a twisted pair; the small loop area between the conductors minimizes emissions, and also improves rejection of outside electromagnetic noise (EMI) — an antenna always works both ways; a well-shielded mechanism will emit less and will also be less susceptible. Cables are supposed to have an outer shield (for Gigabit anyway), though it works without.

Meanwhile Ethernet avoids creating ground loops by isolating the cable on both ends with small pulse transformers (=high pass filter for the signal). The shields of the cables are also only grounded through small capacitors (=high pass filter for shield currents).

> (Or, perhaps you don't avoid it and that's the nature of the cross-talk?)

Almost! Crosstalk is mainly generate by more "intimate" coupling. Emissions means electromagnetic waves (=long range), while crosstalk in cabling and connectors comes from inductive (magnetic) and capacitive (electric field) coupling. This happens because the conductors and contacts are all very close to each other.

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I found this video (10BASE-T): https://www.youtube.com/watch?v=i8CmibhvZ0c