"Interconnected smoke alarms are an important part of home fire safety. In the event of fire, the interconnection activates the alert tone on all of the smoke alarms in a home. This alert tone greatly improves the chance of hearing an alarm if a fire starts in another area of the home from where you are. Unfortunately, very few homes more than 20 years old have wiring installed that interconnects the smoke alarms. The safety and peace of mind that comes with an interconnected smoke alarm system can now be installed without the expense, mess or high labor costs needed to install wires between each smoke alarm. Wireless smoke alarms are available to fit almost any home. They are available in both battery powered and AC powered."
Coffee Break Training - Fire Prevention and Public Education Series No. FM-2013-9
United States Fire Administration: http://www.usfa.fema.gov/