# Review on Free-Space Optical Communications for Delay and Disruption Tolerant Networks
## 2 Free-Space Optical Communications
### Optical Wireless Satellite Networks  
Traditional satellite networks are constrained by the capacity of RF links, and they may suffer from interference generated by neighboring nodes [23]. By contrast, the small beam divergence angle of FSO links makes them virtually immune to interference. This benefit suggested a research on hybrid RF/FSO systems [24]. The disadvantages of FSO, such as the need for Line-of-sight (LOS) and the losses due to adverse atmospheric conditions, make a fully FSO network unreliable. A hybrid network allows for each type of link to mitigate each other's weaknesses [25].  
One of the important applications for optical link networks would be a replacement for wired networks, especially communications over the ocean channels, as they are mostly shared by underwater optical-fiber systems. Optical satellite networks do not need this kind of infrastructure and are then ideal for long distances. They can also provide high data-rate connections for secluded areas or mobile access to remote zones [26].  
These kinds of networks can be implemented by state institutions, and they have recently have been a focus of interest for private companies. _SpaceX_ is currently deploying a satellite network _Starlink_ to provide internet coverage in North-America with a constellation of satellites that provide data-rates of up to 1 Gbps with wireless laser links [27]. This new technology aims to cover rural areas to have access to faster internet.  
Although _Starlink_ is the most advanced project at the time this paper was written, evaluating by the number of satellites launched, it has other big competitors from big aerospace providers, such as the _Virgin Group_, _Boeting Co._, and _Thales Alenia Space_[28].  
The high potential that direct FSO communications from LEO spacecrafts to Earth hold for upcoming space missions through lower complexity have motivated researchers to study the viability of these systems. That has been carried by defining the end-to-end architecture of lasercom technologies while caring about compatibility with data and system standards [29]. The analysis aims to employ available space protocols for bidirectional optical communications with LEO spacecrafts. The authors of Reference [29] concluded that the implementation of laser communications is manageable in matter of infrastructure to the ground segments. The foreseen challenge is the creation of reliable link schedules with respect to weather conditions and protocols that overcome unanticipated disruptions.  
As the links become more robust, development will rely on network and upper layer progress [30; 31; 32]. An FSO network can have frequent interruptions, to the point that an end-to-end connection might not be established. This can also happen because of long orbital distances, where the link synchronization happens for limited amounts of time. For these situations, conventional protocols that require an initial end-to-end handshake, such as TCP, cannot establish connection. To overcome this problem, DTN protocols, such as LTP, are an ideal candidate for FSO links, as they skip the initial end-to-end connection phase.  
The research and testing of high-speed FSO communication networks has raised interest among many leading groups of investigation. An overview of the current developments and investigation carried on at the NASA Glenn Research Center also shows the requirements and achievements for FSO networks in recent years [33].