What is 'valid signal' that router receives to indicate physical status is 'up'?
Link Integrity Test pulses.

They are unipolar positive-only electrical pulses of a nominal duration of 100 ns, with a maximum pulse width of 200 ns, generated at a 16 ms time interval (with a timing variation tolerance of 8 ms).
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A device detects the failure of a link if neither a frame nor two of the LIT pulses is received for 50-150 ms.