Fire Alarm

• Smoke detectors should be installed in every bedroom, outside each bedroom, and on each floor of a house. These should be interconnected so if one sounds they all sound. [1]
• Benefit of an interconnected system is that you get early warning, say you are in the basement and the alarm is upstairs etc. Most fatalities occur in cases where fire is in a different room than the person. [2]
• Code requires the above, plus hard wired, with battery backup if power goes out [3]
• Case for wireless alarm? [4] But are these also hard wired?
  • Example of wireless interconnected alarm. I don't get it - the interconnect wire is still used - [5]
  • Double WTF - why is this called wireless if all detectors are wired with a dedicated interconnect wire? [6]. Powered from same circuit is understandable, but not the interconnect wire.