Think of your neighbours in the upper floor. Sometimes, you hear them walking around, rearranging chairs, or moving plates in their kitchen.
Upstairs is based on the considerations made for implicit shared presence. It uses the above observation as a metaphor to convey and represent information about two spaces by stacking one room upon the other, capturing its floor vibrations and playing them back from the ceiling of the other room.
As shown in the figure, an upstairs system consists of three parts; two playback and recording set-ups, one in each space, and a network that connects the two systems.
We differentiate between public spaces (like city squares or entrance areas of museums), working environments (like work rooms, the area around specific desks, or recreation areas as lunch rooms), and private spaces (either the whole area of apartments or parts like hallways that are regularly frequented).