An energy-efficient wireless sensor network (WSN) is disclosed. The WSN comprises one or more base stations, a plurality of branch nodes communicatively coupled to each of the one or more base stations, and a plurality of leaf nodes communicatively coupled to each of the branch nodes. The WSN is configured for optimising energy consumption associated with each of the following: transmitting sensor data from the leaf node to the branch node; encryption of data transmitted between the leaf node and the branch node; updating firmware of the branch node or of the leaf node, through over-the-air updates from a remote server; sending data packets through the WSN based on a plurality of predefined levels of a Quality-of-Service (QoS); and configuring radio duty cycle protocols at the base station or the branch node or both during transmission and reception of data packets from the leaf nodes.