This is the Ocean Reef GSM G.divers Wireless Underwater TransceiverFeatures: Wireless underwater transceiver unit, with the autonomy of approx. 30 hours in receiving mode, and range of operation of 200-250 meters (calm sea water), light weight, powerful single channel and "push to talk" communication system make the GSM Gdivers the perfect addition to any training class or guided tour, for extra safety and for fun! The unit operates on the same frequency as all other OCEAN REEF units and most popular wireless communication units on the market.The GSM Gdivers uses a 9V alkaline battery and has a "low battery" alarm which beeps every 30 seconds when the battery is low. The NACS(TM) (Neptune Adjustable Communication Support) is needed to install the GSM G.divers on the Extender Frame. NACS is sold separately.