# DTN and Non-DTN Routing Protocols for Inter-CubeSat Communications: A comprehensive survey
## 2 Challenges in CubeSat Communications  
This section discusses the challenges that are raised by the characteristics of CubeSat communication where network resources are limited and the topology of the network is highly dynamic. While the intermittent connectivity is considered to be the main challenge in routing data across CubeSats, some independent challenges regarding communication in space, such as inter-CubeSat links, up/downlinks with the ground station, natural drag, and energy, will also be highlighted. These challenges pose some communication issues that will eventually lead to more intermittent connectivity and poor routing performance. As discussed earlier, the very small size of the CubeSats enables them to be fit for many missions and be more cost-effective when compared to other satellite options. Unfortunately, the small size does not allow for them to be equipped with a large capacity of resources. The size of the CubeSat antenna, for example, is bound by the required gain and operating frequency, as well as the size of the CubeSat could limit transmission range on some missions, leading to a network with intermittent connectivity. This is more critical when the topology of the network is highly dynamic, because CubeSats, on LEO, are orbiting the Earth with a high speed that causes the contact duration between them to be short. For example, the link between the two will last for about 1.3 s, assuming a negligible channel discovery time, and such a short contact duration will influence the network performance when the data rate is low, where two CubeSats are traveling in crossing directions at a velocity of 7.8 km/s with a radio range of 5 km. Additionally, as LEO CubeSats can be deployed on different altitudes 200-1200 km above the ground, each satellite can have a different inclination in degrees from 0\({}^{\circ}\)-180\({}^{\circ}\) that will specify its trajectory. As such, the encounter period may last longer than if they were orbiting on different orbit inclination if two satellites are orbiting on different altitudes but the same orbit inclination, even if they were on the same altitude. Thus, one important factor that affects the link availability between any pair of CubeSats is its relative motion. The orbital period of LEO satellites is an additional source of intermittent connectivity in CubeSat networks that results in a short communication period between a CubeSat and the corresponding ground station. This intermittent connectivity comes from a satellite orbital period of about 90-110 min at speed of about 7.8 km/s with an encounter window of about 5-15 min to the ground station during every cycle [17]. The energy budget that was allocated to the CubeSats is another problem that worsens the intermittent connectivity. The limited size and structure of CubeSats do not allow for many solar panels to be fitted, especially with the available surface area being shared with some other instruments, such as antennas and sensors. With the maximum energy producible from the solar panels thus limited, only when the solar cells are pointing to the sun are able to charge the batteries. Indeed, due to the inability to load many CubeSats with altitude determination and stabilization control devices to ensure that the solar cells are pointing towards the sun, the solar cells may provide energy that is even more limited during the limited encounter times with the sun. Consequently, this causes some CubeSats to fail to continue on their mission, because they run out of power quickly and, as a consequence, the failure of one CubeSat might lead to disruption and discontinuity in multiple links. Moreover, the ground station links and inter-CubeSat communication is considered to be a significant source of power consumption. The estimated communication power budget of a CP1 CubeSat mission, for example, is 267 mW, which is a 36.77% of the available produced power from the solar cell [18].