Is the whole world's airspace covered by ATC? I'm an enthusiast and have only flown flight/atc simulators (on the VATSIM network). Obviously sometimes the airspace you are flying in is not covered by any ATC (so I have to set my frequency to Unicast). And very often there is only the tower of an airport available. How is it in the real world?


Is the whole world's airspace covered by ATC at all times?
Is any airport covered by all its layers of ATC (Delivery, Ground, Tower, ...) at all times?
How often do pilots use the Unicom frequency because there is no ATC available in current airspace?
The entire world is divided into Flight Information Regions (FIRs), so in theory there is someone responsible for everywhere.  However, those FIRs are then subdivided into different classes of airspace.

Class A/B/C/D/E airspace is "controlled", meaning there should always be some sort of ATC available.  In remote areas, though, ATC may not have radar coverage, so separation would be based only on position reports (plus ADS-B, for participating aircraft).

Class F/G airspace is "uncontrolled"; any ATC service would be advisory only, and around untowered airports, they'll tell you to use CTAF.  Often the reason airspace is uncontrolled in the first place is a lack of radio and/or radar coverage.