The wireless communication interface provides reliable radio communication between several parties. Each party is identified by a unique number (ID). The data is transmitted in packets, which can be directed to only one other party (unicast), several other parties (multicast) or all other parties (broadcast). Besides the payload data, these packets contain the ID of the source and the IDs of the destinations as well as packet sequence numbers and information for a cyclic redundancy check (CRC).
The radio communication protocol provides two communication channels, one for reliable stream communication and one for unreliable direct communication. The reliable stream communication channel provides automatic retransmission of lost packets and proper packet reordering on reception. Packets are CRC-checked upon reception and are discarded, if the CRC-check fails. Each packet in the reliable stream communication channel has to be acknowledged, non-acknowledged packets are assumed to be lost and will be resent after a timeout.
To prevent packet collisions, the protocol uses two strategies, token passing and Carrier Sensed Multiple Access (CSMA). Each ID has its own time slot for transmission, and each party tries to identify its proper timeslot by listening to packets from other parties, identifying their IDs and synchronizing the protocol's time slots. Then the party waits for its own timeslot. But before transmission really starts, the presence of a carrier is checked. If no carrier is present, then the party starts to transmit. If, however, the transmission gets interrupted by another party, a CRC mismatch will occur and the packet is discarded upon reception. The receiving party will not acknowledge that packet and the packet will be resent.
If a packet-acknowledge is lost, the sender will send the well-received packet again, but the recipient will discard the packet because a packet with the same packet sequence number has already been received.
The wireless communication interface has its own UART and interrupt service routine, therefore the communication is hidden from the application program, it only has to empty the receive queue from time to time.
On the hardware side, the wireless communication interface is implemented with a Radiometrix Bim433-F UHF Transceiver directly connected to the UART. This low power device can transmit half-duplex serial data with 40kBit/s over about 30m and provides on-board carrier detection and signal decoding circuits.