Wireless ACK Collisions Not Considered Harmful

We present an acknowledged anycast primitive that allows a node to wirelessly transmit a packet and efficiently determine that at least one neighbor successfully received it. The initiator transmits a single packet to a unicast, multicast, or broadcast address and all nodes that match the destination respond with identical acknowledgment packets automatically generated by the hardware. Although these acknowledgments interfere, they usually do so non-destructively, so the initiator can decode their superposition. We call such an exchange a Backcast and show that this operation is feasible using a commodity radio, general because it enables multiple network services, efficient because it is independent of the neighborhood size and runs in constant time, and scalable because it works with no fewer than a dozen interfering acknowledgments.