# DTN and Non-DTN Routing Protocols for Inter-CubeSat Communications: A comprehensive survey
## 2 Challenges in CubeSat Communications
\begin{table}
\begin{tabular}{l l l} \hline \hline \multicolumn{1}{c}{**Challenges**} & \multicolumn{1}{c}{**Implications**} \\ \hline \multirow{4}{*}{Intermittent connectivity} & - & Satellites on this orbit are characterized by scheduled \\  & & predictable/semi-predictable intermittent connectivity, whether \\  & & for a satellite to ground links or inter-satellite links. \\  & - & There are no contemporary paths present for satellite and \\  & & ground station communication or cross-link communication. \\ \hline \multirow{4}{*}{Orbital period} & - & LEO satellite orbital velocity \(\sim\)= 7800 m/s, based on the satellite \\  & & altitude orbital period of about 90–110 minu for 160–1200 km \\  & & altitudes respectively. \\  & - & Limited encounter time between satellites which in turns \\  & & bounds data transfer rate. \\ \hline \multirow{4}{*}{Inter-CubeSat links} & - & Transmission range between two satellites, approximately \\  & & 5–200 km. \\  & - & The transmission range of inter-CubeSats is bound by \\  & & cross-link antenna transmission power. \\  & - & Limited antenna size and capability compared with the \\  & & conventional satellites. \\  & - & Limited antenna coverage compared with the \\  & & conventional satellites. \\ \hline \multirow{4}{*}{Up/Downlinks with the ground station} & - & Transmission range between satellite and ground station, \\  & & approximately 200–1200 km \\  & - & The transmission range of CubeSats is bounded by the \\  & & downlink antenna transmit power. \\  & - & Satellite revisit time Limited antenna size and capability \\ \hline \multirow{2}{*}{Altitude and inclination ranges} & - & Orbit altitude rang is 200–1200 km above the Earth and orbit \\  & & inclination ranges 0\({}^{\circ}\)–180\({}^{\circ}\). \\ \hline \multirow{4}{*}{Natural drag} & - & Common de-orbiting behaviour leads to changes in orbital \\  & & height and hence meeting time between CubeSats will also \\  & & change over time. \\  & - & Orbiting at lower altitudes increases the drag process. \\  & - & The drag upsurges with increasing solar activity (sunspots). \\ \hline \multirow{4}{*}{High failure rate} & - & Space radiation effects on electronic components, particularly \\  & & Commercial-off-the Shelf (COTS) components. \\  & - & Impossibility of recovery under failure. \\ \hline \multirow{4}{*}{Energy} & - & Solar cells limited space available on the small size of the \\  & & CubeSat body. \\ \cline{1-1}  & - & Small storage batteries. \\ \cline{1-1}  & - & High power consumption of up/downlinks and cross-links. \\ \hline \multirow{2}{*}{Topology density} & - & Satellite dissemination and encounter times. \\ \hline \multirow{4}{*}{CubeSat stability on orbit} & - & There is no space on the CubeSats for advanced stability \\  & & control devices. \\ \cline{1-1}  & - & Antenna directionality and steering ability. \\ \hline \multirow{4}{*}{Data rate} & - & A single CubeSat has limited data rate \\  & - & CubeSat swarms and constellations can provide a higher \\ \cline{1-1}  & & overall system data rate, however, networking CubeSats in \\ \cline{1-1}  & & these systems is challenging and requires advanced \\ \cline{1-1}  & & routing protocols. \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Operating conditions of CubeSats on Low Earth Orbit (LEO), challenges, and implications. Advantageously, networks of CubeSats serve to provide a wider communication window and coverage, as well as gaining better data rates from the whole network. However, the networks that are usually formed as swarms and constellations require reliable routing protocols to define one or more reliable paths for its users, according to the link metric and QoS requirements. The integrated satellite network structure is consisting of a space segment and the terrestrial network, which creates three main types of routing segments, which are: space segment routing; access routing; and, boundary routing [14].