Abstract: A beacon device transmits a challenge message to each of one or more responder devices over a respective direct communication link, inviting each responder device to prove the existence of its respective direct communication link by transmitting to a respective recipient device, distinct from the beacon device, a respective response message indicating knowledge of contents of the challenge message. A predetermined time period after its transmission of the challenge message, the beacon device transmits a confirmation message indicating knowledge of the contents of the challenge message to a message store. A validation device then compares contents of each response message which preceded the confirmation message in time, if any, to contents of the confirmation message and infers therefrom which of the respective responder devices, if any, received the challenge message over the respective direct communication link.

Abstract: A beacon device including directional transmission means transmits a challenge message in a first predetermined direction to each of one or more responder devices over a respective direct radio communication link, to invite the responder devices to each transmit a respective response message indicating knowledge of contents of the challenge message. The beacon device transmits one or more directed repetitions of the challenge message in one or more respective further predetermined directions such that data relating to the location of each of the responder devices can be inferred from timings of each response message with respect to times the challenge message and the directed repetitions of the challenge message were transmitted. The beacon device either engages in or eschews subsequent two-way communication with each of the one or more responder devices over the respective direct radio communication link, in dependence on the existence and timing of a respective response message.