# Internet of Drones: Routing Algorithms, Techniques and Challenges
###### Abstract  
In the past decades, unmanned aerial vehicles (UAVs), also known as drones, have drawn more attention in the academic domain and exploration in the research fields of wireless sensor networks (WSNs). Moreover, applications of drones aid operations related to military support, agriculture industry, and smart Internet-of-Things (IoT). Currently, the use of drone based IoT, also known as Internet-of-Drones (IoD), and their design challenges and techniques are being probed by researchers around the globe. The placement of drones (nodes) is an important consideration in a IoD environment and is closely related to the properties of IoT. Given a base station (BS), sensor nodes (SNs) and IoT devices are designed to capture the signals transmitted by the BS and make use of internet connectivity in a manner to facilitate users. Mutual benefit can be achieved by integrating drones into IoT. The drone based cluster models are not free from challenges. Routing protocols have to be substantiated by key algorithms. Drones are designed to be specific to applications, but the underlying principles are the same. Optimization algorithms are the gateway to better accuracy, performance, and reliability. This article discusses some of these optimization algorithms, include genetic algorithm (GA), bee optimization algorithm, and Chicken Swarm Optimization Clustering Algorithm (CSOCA). Finally, the routing schemes, protocols, and challenges in the context of IoD are discussed.  
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overcome to manage the large amount of data and handle the communication issues with the deployment of the sensor nodes in the network.  
One such WSN challenge is the routing of unmanned aerial vehicles (UAVs), which belong to a category of IoT termed Internet-of-Drones (IoD). The current era of humans-out-of-loop has boosted the popularity of exploiting drones, where the focus is to minimize human interaction. We have seen the usage of drones in daily life during the COVID-19 epidemic worldwide [5]. IoDs have a diverse range of applications in the field of healthcare, agriculture, logistics, data collection, surveillance, and the military domain as shown in Figure 1. In IoDs, drones (UAVs) are used to create a flexible data gathering platform that results in maximizing the lifetime of na IoD because of optimisation of the energy resources [6]. However, routing the collected data efficiently in a network environment with mobile drones is the most challenging task for the research community. Researchers have put forward different techniques to address this challenge [7]. However, there are no standard routing protocols as of now. Moreover, the data collected by IoDs are heterogeneous in nature, including audio, video, haptic, kinesthetic, and scalar data [2]. There is a dire need to review the applications and routing protocols optimized for IoDs. The network routing is a well explored research area. The IoD is an evolutionary research area with vast application areas. Moreover, the network characteristics of IoDs are different than traditional IoT and WSNs. A comprehensive and analytical review on traditional network routing in IoT and IoD can be found in [2; 8], respectively. Therefore, this article focuses on the evolutionary and innovative routing protocols optimized for IoDs instead of traditional routing protocols used in IoT and WSN. Moreover, this article includes the challenges in IoD and future research directions to solves these challenges.