A method of querying a hierarchically organized sensor network, said network being sensor network with a global coordinator node at a top level which receives data from lower level intermediate nodes which are either leader nodes for lower level nodes or sensor nodes, wherein a
sensor node i at a lowest level receives a
signal Y(i,t) at time t, said method including constructing a sketch Swkt=(Swkt1, . . . , Swktn) for an internal node k fromSwktj=∑i∈LeafDescendents(k)∑q=1ibwiq·riqj,wherein component Swktj is a sketch of a
descendent of node k, ritj is a random variable associated with each
sensor node i and time instant t wherein index j refers to independently drawn instantiations of the random variable, bit bwit represents a state of
sensor node i for
signal value w=Y(i,t) at time t, and LeafDescendents(k) are the lowest
level sensor nodes under node k, wherein said sketch is adapted for responding to queries regarding a state of said network.