# DTN and Non-DTN Routing Protocols for Inter-CubeSat Communications: A comprehensive survey
###### Abstract  
CubeSats, which are limited by size and mass, have limited functionality. These miniaturised satellites suffer from a low power budget, short radio range, low transmission speeds, and limited data storage capacity. Regardless of these limitations, CubeSats have been deployed to carry out many research missions, such as gravity mapping and the tracking of forest fires. One method of increasing their functionality and reducing their limitations is to form CubeSat networks, or swarms, where many CubeSats work together to carry out a mission. Nevertheless, the network might have intermittent connectivity and, accordingly, data communication becomes challenging in such a disjointed network where there is no contemporaneous path between source and destination due to satellites' mobility pattern and given the limitations of range. In this survey, various inter-satellite routing protocols that are Delay Tolerant (DTN) and Non Delay Tolerant (Non-DTN) are considered. DTN routing protocols are considered for the scenarios where the network is disjointed with no contemporaneous path between a source and a destination. We qualitatively compare all of the above routing protocols to highlight the positive and negative points under different network constraints. We conclude that the performance of routing protocols used in aerospace communications is highly dependent on the evolving topology of the network over time. Additionally, the Non-DTN routing protocols will work efficiently if the network is dense enough to establish reliable links between CubeSats. Emphasis is also given to network capacity in terms of how buffer, energy, bandwidth, and contact duration influence the performance of DTN routing protocols, where, for example, flooding-based DTN protocols can provide superior performance in terms of maximizing delivery ratio and minimizing a delivery delay. However, such protocols are not suitable for CubeSat networks, as they harvest the limited resources of these tiny satellites and they are contrasted with forwarding-based DTN routing protocols, which are resource-friendly and produce minimum overheads on the cost of degraded delivery probability. From the literature, we found that quota-based DTN routing protocols can provide the necessary balance between delivery delay and overhead costs in many CubeSat missions. a 2020, 9, 482; doi:10.3390/electronics9030482 2020, 9, 482; doi:10.3390/electronics9030482 2020, 9, 482; doi:10.  
on the basis of their mass into the following five categories [1]: (i) Mini-satellites (weight: 100-500 Kg), (ii) Micro-satellites (weight: 10-100 Kg), (iii) Nano-satellites (weight: 1-10 Kg), (iv) Pico-satellites (weight: 0.1-1 Kg), and (v) Femto-satellites (weight: \(<\)0.1 Kg). CubeSats fall into the Nano-satellites category. Additionally, in terms of size, CubeSats can be structured in different sizes, 1U-12U, which are the multiplication of 1U, as illustrated in Figure 1.  
In comparison to conventional satellites that cost $0.1-$2B, the expenditure for constructing a CubeSat is only $20-$200 K and the launching cost of a CubeSat can be as low as $12,500 [1]. More importantly, new technologies can be tested on CubeSats before using them on more expensive satellites [2]. Hence, it makes economic sense to introduce CubeSats for deployment in Low Earth Orbit (LEO) and in deep space to perform different missions [3]. Over 2200 NanoSats and CubeSats were launched by 2018 and over 3000 NanoSats will be launched in the next six years, according to the Nanosatellites Database [4]. For instance, Planet Labs launched and operated a series of 3U CubeSat constellations for Earth observation, known as Flock missions, with a total number of about 250 CubeSats from 2014 to 2017. Another notable example can be seen in the series launches of Lemur-2 NanoSats, where the mission consisted of 73 3U CubeSats that were developed by Spire Global to serve as a part of their remote sensing commercial satellite constellation that offers global ship tracking and weather observation [5].