# Situation-Aware Survivable Network Design for Tactical Environments
## 1 Introduction  
The modern military operation environment has evolved from platform centric warfare (PCW) to network centric warfare (NCW) centered on securing information superiority through networking of all battlefield environmental elements [1]. In NCW, detection, identification, sensor, commander and shooter are networked to increase command speed and reduce the operation delay to increase the survival of soldiers. However, tactical networks have an inferior environment from a communication perspective due to the lack of infrastructure, limited network resources, intermittent connections, and packet loss due to frequent mobility and competition in crowded environments [2]. Therefore, there is a need for a reliable network system that can meet the requirements of a poor tactical ad hoc network environment to improve operational efficiency and survival of soldiers in the transition to NCW. Recently, wireless local area network (WLAN) technology has been adopted for military applications in tactical networks [3; 4; 5]. A WLAN can operate in two modes, namely, infrastructure mode and ad hoc mode. If a WLAN operates in ad hoc mode, it does not require infrastructure; thus, the WLAN can fulfill the requirements of the tactical network. Other characteristics of WLAN such as its flexibility, autonomous, self-configuring, and adaptive nature also make it suitable for military applications. Soldiers use handheld network devices and sensors, such as Internet of Things (IoT) devices attached to them, which form network nodes. Carrier-sense multiple-access with collision avoidance (CSMA/CA) is the most widely used protocol of the IEEE 802.11 standard for WLAN. However, all the nodes in the carrier sense range have to contend with each other because of the nature of sharing the medium. In addition, there is no notion of priority; thus, collision is more fatal to important traffic, and there is no quality of service (QoS) guarantee. IEEE 802.11e is an amendment of the IEEE 802.11 standard that enhances the QoS for WLAN applications [6]. In IEEE 802.11e, enhanced distributed channel access (EDCA) defines the level of priority, access category (AC). It gives more chance of transmission to high-priority traffic than low-priority traffic by assigning a shorter arbitration inter-frame space (AIFS) and lower contention window boundaries for higher priority traffic. Although this conventional protocol considers QoS factors, the standard just gives relative priority, which may be insufficient for a safety-critical environment, such as a tactical network. In a tactical network, the lack of infrastructure makes it necessary for wireless nodes to be configured in a multi-hop ad hoc network. Unlike a single-hop data transmission, the multi-hop ad hoc network has several challenges to construct reliable end-to-end paths or select the best routing path for improving network performance. Many approaches have been proposed to handle those issues [7,8]. A tactical ad hoc network requires a reliable routing protocol to transmit mission-critical data such as command and control (C2) and surveillance information. The constructed end-to-end paths have to meet the requirements of tactical traffic and must be able to rapidly handle connectivity problems under harsh environments. In this paper, we consider two problems in order to guarantee the QoS for tactical traffic and reliable end-to-end paths in a tactical network. First, we propose a situational backoff reset algorithm. The proposed scheme uses information of the QoS control field in the packet header to figure out the priority of the received packet. Based on that information, low-priority traffic determines whether the backoff reset is operated or not. The proposed algorithm avoids collision between low-priority traffic and higher priority traffic and provides efficient channel deferral time to low-priority traffic without severe performance degradation. Second, we propose a branch node-based routing algorithm.