# Review and Evaluation of MAC Protocols for Satellite IoT Systems Using Nanoatellites
## 2 Overview of Space and Communications Systems
### IoT and M2M Requirements  
Cisco forecasts that by the year 2020 the number of devices connected to the internet will exceed 50 billion [23], an increase that raises a connectivity challenge for these new massive networks. It is in this field that the capabilities of the new low-cost nanosatellite networks could be instrumental in achieving a global connectivity, as demanded by the fifth generation networks.  
The IoT and machine-to-machine networks are characterized by their intention to meet one or more of the following requirements:  
* Efficient performance against explosive traffics
* Low data rates in terminals
* Energy efficiency  
Figure 3: CubeSats launched since 1998. Data taken from the database at [19].  
* Low cost terminals
* Mobility and scalability
* Minimization in the use of spectrum
* Minimum signaling
* Data security
* Data integrity
* Reliability
* Robustness
* Flexibility  
In the case of terrestrial wireless access networks, various solutions have been developed to meet the above requirements. For low consumption sensor arrays deployed across extensive areas, technologies such as LoRaWAN [24] and Sigfox [25] are available; for sensors and actuators networks deployed in urban environments, there are Wi-Sun [26] and NB-IoT [27], to mention some of the available technologies. There are also autonomous sensor networks interconnected to provide solutions to specific applications involving short-range technologies, such as IEEE 802.15.4 [28]. Despite the advances with the introduction of these new technologies, there are scenarios for which existing networks do not offer feasible solutions. Remote places without connectivity still exist; also, some monitoring applications in isolated places require devices with high and unpredictable mobility to collect information on-the-move (e.g., monitoring of wild animals in areas of difficult access). Considering the scenarios mentioned above, microsatellites and CubeSats appear as viable alternatives to cover the gap in providing fully connected global communications networks for the IoT [6; 10]. An example scenario of a CubeSat providing IoT connectivity is illustrated in Figure 4.  
One of the challenges of these new massive networks is to enable the many terminals to share a physical resource--the broadcast communications channel--in an efficient and orderly manner. Such a challenge would necessarily make use of the medium access control layer, which corresponds to a sub-layer of the link layer of the open system interconnection (OSI) model and is responsible for coordinating frame transmissions in broadcast links. The specific MAC protocol used for IoT applications will need to fulfil a number of requirements including increased average throughput, to meet a minimum level of fairness as well as to comply with the resources, requirements, and limitations of the access technology in use. Another critical aspect to consider in the choice of a MAC protocol is the network topology and how much knowledge the nodes have or need about that topology.  
Figure 4: Illustrative scenario in which a CubeSat (or a constellation of them) provides connectivity for internet of things (IoT) applications.  
To examine the fulfillment of the IoT and M2M networks requirements, from a MAC layer perspective in the case of this study, together with the restrictions imposed by the capabilities of the CubeSats, will shed light about the viability to provide IoT connectivity using nanosatellites. The reviews and discussion presented in the coming sections address all of the IoT and M2M requirements listed above, except the ones related to data security and data integrity. Whilst security aspects are of paramount importance in the IoT ecosystem, we direct the interested reader to specialized works on this subject discussing security threats and mitigations for a variety of IoT technologies and architectures [29; 30; 31; 32] and specifically for satellite communications [33; 34; 35].