Some protocols and applications in mobile computing require that a device knows as to how many other devices exhibiting a certain attribute are in its radio coverage. Obtaining this cardinality information by message exchange between devices is reliable but inefficient in dense networks in terms of overhead and delay. Performing an estimation of the cardinality using probabilistic trials is an alternative.
Adam, Yanmaz, and Bettstetter pursue such a probabilistic approach by proposing cardinality estimator protocols that require no coordination among polled devices but are based on a simple random access scheme with busy tones exploiting the number of empty slots to infer about cardinality.
Their forthcoming article in IEEE Transactions on Mobile Computing compares three estimators with different levels of adaptability and feedback from the query device and discusses suitability for IEEE 802.11 and low power sensors.