[tlug] test of ethernet cable length

[Michal Hajek <hajek1@example.com>]

Hello Tlugers,

recently I have seen a computer with a bios, that could test the length of an
ethernet cable connected to it's NIC. So I came to the idea,
that perhaps some smart program in linux could have the same
functionality.  

Now there are plenty of question marks in my line of reasoning and I
would like to share some of these questions with this community in a
wild hope that at least some of my probable misunderstandings will get
clarified with your patient help - 

Is it correct to assume that since bios can do, than linux kernel or
other linux program can do too? Would that mean one has to add a new
feature to the NIC driver?  

What is the general principle behind these measurements? 
I.e. the bios test can measure each pair separately. Thus I imagine the
program has to send some specialized "packet" to the cable, meaning that
this special packet would only use selected pair to propagate.
Is that technically viable? I surely understand that electrical signal
will travel cca 30cm in 1ns. So with the clock speed in the order of GHz
it should be fine to get at least 1m resolution - that also corresponds
to the resolution in the bios measurement utility.

Thanks for any comments.

Best regards
Michal