A first network node for communicating with a second network node over a first communication network is described. The second network node is arranged to communicate over the first communication network in a first part of a communication period and arranged to not communicate over the first communication network in a second part of the communication period. The first network node has a send unit for sending data formatted in data packets to the second network node, a statistics unit arranged for determining a success statistics, an availability estimator for deriving an availability estimation from the success statistics, and a send controller arranged to control the send unit in dependence on the availability estimation. Also described is a communication system, a method of estimating availability of a second network node, a method of communicating by a first network node, and an associated computer program product.