A system doesnt have to JUST be perimeter protection or JUST internal detection, as mentioned... It can be both, and normally is.
You can have movement sensors around the home and vibration detectors on windows and contacts where needed. Then, as mentioned, you can set the system in parts, IE, you set the perimeter protection detectors when you home and the whole system when your out.
This, in my opinion is the best way and is what we fit as standard.
Ranger