No, they can't use XBee or Zigbee because each XBee costs Rs.2000 and they are selling the product for Rs.4000. If XBee's are used they need 3 XBee's per system (two transmitters and one receiver) and cost will be more than Rs.4000. Maybe they are using C2500 based RF modules. I need cheap C2500 based RF modules. I have designed a very cheap Automatic Water Level Controller which uses 4 + 4 float switches. Mine cost Rs.2000 including 8 stainless steel float switches but Aquabrim are not giving float switches for Rs.4000. If I get a 2.4 GHz RF module for cheaper price then my system will be the cheapest.