Difference between perimeter and interior protection

The difference between perimeter and interior protection depends on location and programming options.

A perimeter device is traditionally a protective device installed on an exterior door, window, or glass, where the intruder would be detected prior to entering a premises or area. They traditionally consist of magnetic contacts, foil, vibration, and piezoelectric glass sensors. Even outdoor photoelectric or motion detection can be considered, but not common in a residential setting.

Interior devices are usually referred to as "space detection". Common instances of such are motion detectors and photoelectric (PE) beams. Detection of an intruder would not occur until they were physically inside a protected area or crossed a certain traffic area.

Of note, a "gray" area would be glass break detectors. Some are installed on the glass, such as window bugs, piezoelectric type, or even conductive foil, are considered perimeter devices. Their design is to physically detect the glass breaking and violate a protective zone. They are passive in nature. An active glass break is a unit consisting of a microphone and electronic criteria which determines what is an alarm event (glass breaking) and what is environmental noise, such as traffic or weather noise. An active glass break detector is installed on the interior of a protected area and generally looks for two criteria prior to generating an alarm condition, flex, which is the glass physically bending prior to breakage, the initial "thud" if you will, and then the sound or frequency of breaking glass within a certain elapsed time after the "flex" signal. Better units have "signature analysis" which helps discriminate non-alarm events from generating a false.

Some glass break units can generate false alarms during extreme weather, such as thunderstorms, or an overzealous homeowner's home theater, so a cautious approach is necessary.

I, myself, call glass breaks a "gray" device, since programming options can dictate how an active glass break responds dependent on system state, whether it is armed away, home or disarmed, which would change it's classification from perimeter depending on the system's armed state.