# Review on Free-Space Optical Communications for Delay and Disruption Tolerant Networks
## 1 Introduction  
This review article offers a deep review on Free-space Optical (FSO) communications and Delay-/Disruption-Tolerant Networking (DTN). The combination of these two technologies has gotten off the ground in recent years.  
FSO may benefit from DTN as optical links have long disruptions that may impair traditional protocols. The Bundle Protocol (BP), which is the basis of DTN architecture, allows to communicate in challenged networks.  
For the past two decades, FSO communications have been considered a viable candidate for wireless networks [1, 2], and, as the optical links improve and become more robust, the interest in creating wireless optical networks increases. On the other hand, over 20 years ago, the Jet Propulsion Laboratory of NASA began research on the Interplanetary Network [3], which led to the specifications of DTN [4].  
Optical links offer high-speed options for satellite and interplanetary networks. This paper offers a basis of knowledge and summary of research for future applications of these technologies.  
Wireless optical networks are of great interest in the research community as optical links provide higher throughput, large available bandwidth, and inherent security robustness to electromagnetic interference. These links reach similar levels of Quality-of-Service (QoS) as optical fiber, and they have the benefit of reaching remote location without the need for the infrastructure of cables. These technologies are also a foundation for future space missions, as optical links provide faster possibilities for the download oflarge amounts of data (for example, from visual observation) and communications between satellites, interplanetary probes, and others.  
Researching protocols that overcome the disruptions faced by these links is essential to use the maximum capacity of the links, which could not be reached by using the classical protocols for internet. Two distinct scenarios can benefit from these technologies. Firstly, a low-earth orbit (LEO) to ground communication, where the main challenge is the presence of clouds and other adverse weather conditions. Although, for this scenario, the round-trip-time (RTT) is in the order of milliseconds, the use of DTN can be beneficial as outages by clouds can be many orders of magnitude higher. On the other hand, for interplanetary networks, the focus is on the intermittence due to orbital schedule and latency, as the distance between the sender and receiver can be very high; for example, Earth-to-Mars communications have an RTT of 6 to 46 min.  
In this paper, the authors use a systematic literature review to identify over 50 published studies, with important information about FSO and DTN. The authors focus on relevant state-of-the-art studies from the years 2015-2021, while also selecting studies that provide base knowledge on the topic from older times. This review has the objective to analyze the feasibility of applying DTN protocols in FSO communications.  
This review article is organized as follows. Section 2 provides an introduction to FSO communications with focus on satellite networks and the FSO channel, followed by applications of this technology. Section 3 presents the foundation of DTN, with emphasis on the Licklider Transmission Protocol, followed by the state-of-the-art research on this technology. Finally, in Section 4, possible challenges of the combination of FSO and DTN are considered, followed by conclusions and future work in Section 5.