Data collected by sensors (10, 30) is transmitted to data collection points (31, 61) over a network using an epidemic protocol in which data received by a relay (3) is forwarded to a randomly-selected group of further relays (4, 5, 6) unless the same data has been previously received by the relay (3) or the data is time-expired. Data received by a relay (3) from different sources (1, 2, 30) in the same time-frame is aggregated using a Bloom filter process so that in each time frame only a single Bloom filter message is transmitted by each relay.