Oh nice, very helpful. The sibling thread mentions that this problem is specific to "ARTCCs", which are not the towers at airports but rather central hubs:

  Between the sectors administered by [smaller towers] are 20 contiguous areas of US airspace above 18,000 feet, each managed by an Air Route Traffic Control Center (ARTCC)...
  A flight is handed off from one Center to another until it descends near its destination, when control is transferred to the TRACON serving the destination, and ultimately to the tower controller serving the airport.

Wikipedia mentions they in particular use special "very high frequency aviation bands" (around "AM 118-137 MHz"), which is presumably why they're all linked together. It also mentions sharing systems, but no clear specifics that I can find. Certainly doesn't disprove the botched rollout theory, tho...

https://en.wikipedia.org/wiki/Area_control_center

EDIT: someone below posted this, seems relevant -- the 'Radar multiplexer pipeline' that's on a "fast track" and an "aggressive track": https://www.sunhillo.com/wp-content/uploads/2025/04/SUN3125-... TL;DR: upgrading radio systems from old analog tech to new digital tech, I think?

▲ londons_explore a day ago | parent [-]

I'd be shocked if someone greenlighted an atc upgrade procedure which involves closing all US airspace to do the switchover.

	▲	lxgr a day ago \| parent \| next [-]
		That’s obviously not what’s happening. But if this upgrade is really implicated, the real question would be where the line of accepted residual risk for an outage like this was drawn.
	▲	bbor a day ago \| parent \| prev \| next [-]
		Sorry I was unclear; the guess is that it caused an issue, not that they did this on purpose with no warning.
	▲	CobrastanJorji a day ago \| parent \| prev [-]
		Well sure, but I wouldn't be shocked at all if someone greenlit a plan that wasn't supposed to involve closing the airspace, but which failed and which had an insufficiently thought out rollback plan.