Design wirless connection for home alarm system prototype

Basis for this project is an EXISTING custom made home alarm system that features 3 units: See attached scribble of current status! unit basestation: microcontroller (arduino) unit 1: reed switch that is triggered when a door is opened unit 2: a panel for user interaction (LCD display & a RFID reader) - currently powered by cable All 3 units are connected using cables. The microcontroller monitors the reed switch (unit1) and trigger an alarm in case the system has been armed and the door is opened. The user interface (unit 2) is used to show messages to the user and arm and disarm the system using RFID chips. THE GOAL OF THIS PROJECT is to replace the cabled connection between all the units using a wireless solution. Units 1 & 2 should be powered by battery. DELIVERABLE: A recommendation on the technology to use and a ready-to-implement technical design for units 1 & 2 and the base unit extension. Not a deliverable will the necessary software change on the microcontroller of the base unit. Requirements on the wireless connection: 1) Digital & (pseudo) encrypted 2) High reliability (to prevent false alarms) 3) Power consumption (battery life should be 6 months in unit 1 & 2 - include LCD and RFID Reader power consumption into your thoughts) 4) Automatic low-power-detection of battery of unit 1 & 2 - the base unit should be able to determine if the battery is low in any of units 1 or 2.