A data transmission system comprises a large number of networked data beacons interconnected and grouped together in respective clusters. In the system, a first (12) and two or more second (13, 14, 15) beacon devices are interconnected (11) as a cluster capable of wireless message transmission, and at least one mobile device (10) capable of receiving such message transmissions. The first beacon (12) is arranged to broadcast a series of inquiry messages (INQ) according to a first communications protocol, such as Bluetooth, but with the addition of an identifier for the cluster (CL). The mobile device (10) detects such inquiry messages and replies with an identifier for itself, together with message reason data. The link (A) from the first beacon device is then re-established between a selected one (14) of the second beacon devices, with interaction between the mobile device (10) and selected second beacon device (14) being controlled at least partly by the message reason data.