# All about Delay-Tolerant Networking (DTN) Contributions to Future Internet
## 3 Related Work
### Space
#### 3.1.2 Satellites  
Satellites are the primary components of space communications serving as the tangible embodiment of the original DTN concept. They can be typically categorized based on their distance from the Earth's surface into Geostationary Earth Orbit (GEO), Medium Earth Orbit (MEO), and Low Earth Orbit (LEO). In terms of size, there is a trend toward smaller and more agile satellites known as \"SmallSats\" and \"CubeSats\", which adhere to SI metrics in their naming, e.g., ministatellites, microsatellites, nanosatellites, etc. ([[https://www.nasa.gov/what-are-smallsats-and-cubesats/](https://www.nasa.gov/what-are-smallsats-and-cubesats/)]([https://www.nasa.gov/what-are-smallsats-and-cubesats/](https://www.nasa.gov/what-are-smallsats-and-cubesats/)) accessed on 4 March 2024). GEO satellites follow the rotation of the Earth and thus appear stationary over a fixed position for an observer located on Earth. Their wide coverage and lifespan are their major advantages, as three equally-spaced satellites can provide near-global coverage; however, latency becomes a significant drawback, especially for latency-sensitive applications such as voice or video communication. On the contrary, MEO and especially LEO satellites take advantage of the closer to the Earth distance and provide lower RTIs due to the lower propagation delay. However, more satellites are required for full coverage, while some significant challenges arise regarding the lifespan, complexity (e.g., intersatellite links and high mobility), congestion of radio spectrum, interference avoidance, and coordination of multiple interests. In particular, the feasibility and implementations of space internetworking across various domains have been explored in different works [4; 36; 37]. An interesting domain is multimedia content delivery or streaming. Although in (deep) space communications the delay becomes a bottleneck, solutions have been proposed such as the Bundle Streaming Service (BSS) [38]. Applicationwise, there is a plethora of terrestrial services provided by satellites. including the following application domains:  
1. _Internet Service and Relay Systems_: (i) Provide global internet coverage primarily facilitated by the rapid deployment of LEO satellites and constellations (e.g., Globalstar, Starlink, OneWeb) [39; 40]. They can serve as complementary solutions to traditional internet services (e.g., in cases of emergency) or as effective substitutes in areas lacking terrestrial connectivity. Various proposals for communication enhancements have also been proposed. For instance, ref. [41] describes a transmission scheduling algorithm for LEO satellites involving a broadcasting mechanism with randomized retransmissions and a Peer-to-Peer (P2P) multicast ground distribution scheme. The paper by [42] proposes enhancements between LEO/MEO intersatellite communication systems through modulation techniques and electrical pulse generators or tools to predict delivery time, e.g., Bundle Delivery Time Estimation (BDTE) [43]. (ii) Data Relay Satellites (DRSs) transmit information to and from satellites, spacecraft, vehicles/vessels, and fixed Earth GSs, e.g., the European Data Relay System (EDRS) [44], U.S. Tracking and Data Relay Satellite System (TDRSS) [45], or Earth-to-Moon communication [46]. 2. _Remote Sensing and Earth Observation_: (i) Environmental/climate monitoring (e.g., the GR01-DUTHSat for upper atmosphere measurements [47]); (ii) meteorology phenomena and atmospheric tracking (e.g., the Leonardo Bidirectional Reflectance Distribution Function (BRDF) constellation and Cyclone Global Navigation Satellite System (CYGNSS)); (iii) pollution monitoring (e.g., oil spill detection); and (iv) surveillance and high-resolution photography. 3. _Power Energy Networks and Smart Grids_: Provide robust and flexible network management and interconnection of distributed and heterogeneous energy infrastructures(e.g., supervisory control and data acquisition) while efficiently utilizing the bandwidth and minimizing installation and maintenance costs [48; 49].