# On Localized Countermeasure Against Reactive Jamming Attacks in Smart Grid Wireless Mesh Networks
## 1 Introduction
Thereupon, the reactive jamming attacks to the wireless mesh networks in the Grid would adversely affect the operation of overall electrical systems and cause devastating consequences to the critical infrastructures. Existing defensive methods against reactive jamming threats can be broadly studied on the physical, link and network layers. Firstly, the physical layer approaches [5; 6; 7; 8; 9] generally use Code Division Multiple Access (CDMA) and Frequency Hopping (FH) which require a high computational cost. MAC layer approaches [10; 11] are either inherently based on FH or re-positioning the wireless nodes, which may result in network partitioning and bring in high computation overhead. Finally, in the network layer approach, ref. [12] introduces a new scheme to quarantine possible jammed areas and re-route all the messages that originally pass this area. However, the JAM method could produce unnecessarily large jammed areas and even result in isolated wireless networks. Furthermore, during the jammed area mapping procedure, the higher communication overhead from the message exchange among the wireless devices might slow the performance of the network throughput. In our preceding work [13; 14], we introduced the novel concept of trigger device, which refers to the wireless node whose transmission of wireless messages invokes the disruptive jamming signal. The detection of trigger devices has several benefits for the defense against the reactive DoS attacks: (1) Routing algorithm could be re-constructed in which the triggers do not transmit wireless messages but receive, consequently eliminating the activation of the jamming devices and mitigating the damage on the wireless networks. The identification of trigger would be able to overcome the drawbacks on the approaches of frequency hopping and channel surfing in requiring excessive computational resources. In the worst case where a trigger device wants to transmit messages, the utilization of physical layer approaches is still available, however, only a small number of triggers may be needed in our method in the case of broadcasting messages through the networks. Furthermore, the defensive method of identifying trigger devices squeezes the unnecessarily large jammed regions, the transmission range of jammers, compared to results from JAM approach in ref. [12]. (2) After identifying triggers, victim devices in the jammed areas could be lined up for the transmission of wireless messages such that no jammer would be activated, and whenever the trigger nodes start to broadcast, all other victim nodes stop their transmission. Thus preventing them from being jammed. (3) The location of triggers can be used to spot the place where jammers are planted as well. Therefore, with accurate and efficient identification of trigger nodes, a number of effective defense strategies can be developed. However, this trigger detection procedure is non-trivial, due to the unknown and dynamic locations of jammers and their various behaviors. To this end, we propose a localized trigger detection algorithm which employs two techniques: a hexagon tiling coloring scheme and a sequential Group Testing (GT), which efficiently identifies all the trigger nodes without requiring any extra hardware on wireless devices. Different from our previous work in ref. [14], this method can be distributedly implemented with a low time and message complexity for the Smart Grid wireless networks. The rest of this paper is structured as follows: Section 2 provides a literature survey regarding the existing countermeasures on the jamming attack issues. In Section 3, we address the Smart Grid wireless network and the jamming model with the problem description. Section 4 discusses the fundamental results of our approach along with our proposed hexagon tiling coloring technique. The localized algorithm for identification of the trigger nodes is depicted in Section 5 and the theoretical analyses are presented in Section 6.