Do you mean smoke alarms, interlinked to each other?
Fire alarms are as used in commercial buildings with a central panel
monitoring?
If so suggest you d/load one of installers manual to give you general
guidance
http://www.smoke-alarms.co.uk/alarmfeatures.shtml

Select "alarm features/manuals" on top navigation bar
You can only interconnect with 3 core & earth.
These manuals are for UK market only.