The nRF24L01+ is a cheap 2.4GHz transceiver module with a low level part of the communication layer already in hardware available. Features of the nRF24L01+ are, adjustable auto retransmit, RF ACK handshake, a 1 to 32 byte payload with variable length (Dynamic Payload), Fifo of 3 deep, 120 selectable frequencies, adjustable output power, CRC, etc.