1. A method of querying a sensor network, said network being hierarchically organized sensor network with a global coordinator node at a top level which receives data from lower level intermediate nodes, said intermediate nodes being 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 comprising the steps of: constructing a sketch S wkt =(S wkt 1 ,. .. , S wkt n ) for an internal node k from S wkt j = ∑ i ∈ LeafDescendents ( k ) ∑ q = 1 t b wiq · r iq j , wherein component S wkt j is a sketch of a descendent of node k, r it j is a random variable drawn from {−1,1} associated with each sensor node i and time instant t wherein index j refers to independently drawn instantiations of the random variable, the bit b wit in {0, 1} 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.