# Review and Evaluation of MAC Protocols for Satellite IoT Systems Using Nanoatellites
## 6 Discussion and Open Research Challenges
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Protocols** & \(S_{max}\) & \(C_{S_{max}}\) & \(C_{PLR=10-3}\) & **Complexity** & **Energy Efficiency** & **Topology Impact \({}^{*}\)** \\ \hline MuSCA & 1.4 & 1.42 & 1.22 & high & high & different delays (\(-\)) \\ E-SSA & 1.2 & 1.25 & 1.12 & high & high & power imbalance (\(+\)) \\ FC-TDMA & 1 & 1 & - & medium & medium & variability in number of nodes (\(-\)), different delays (\(-\)) \\ \hline R-Aloha & 1 & 1 & - & medium & medium & different delays (\(-\)) \\ CSA & 0.8 & 0.84 & - & high & high & different delays (\(-\)) \\ IRSA & 0.8 & 0.85 & 0.7 & high & high & different delays (\(-\)) \\ \hline CSMA/CA & 0.75 & 0.8 & - & low & low & hidden nodes (\(-\)), different delays (\(-\)) \\ SS-Aloha & 0.6 & 0.7 & 0.5 & medium & medium & power imbalance (\(-\)) \\ \hline CRDSA & 0.52 & 0.65 & 0.05 & high & medium & power imbalance (\(+\)), different delays (\(-\)) \\ \hline S-Aloha & 0.368 & 1 & \(<\)10\({}^{-3}\) & low & medium & power imbalance (\(+\)), different delays (\(-\)) \\ \hline DSA & 0.3 & 0.6 & \(<\)10\({}^{-3}\) & low & medium & power imbalance (\(+\)), different delays (\(-\)) \\ \hline E-Aloha & 0.09 & 0.1 & \(<\)10\({}^{-3}\) & low & medium & power imbalance (\(+\)) \\ \hline \hline \end{tabular} \({}^{*}\) (\(+\)) means the topology characteristics help improve the communications performance of the protocol, whereas (\(-\)) indicates a negative impact on performance.  
\end{table} 
Table 2: Comparison of medium access control (MAC) protocols considering communications performance, complexity of implementation, energy efficiency, and the topology impact. decreases when the delays in transmissions among the nodes are highly uneven. In fact, the uneven delays are also critical for the operation of TDMA-inspired protocols such as R-Aloha and FC-TDMA. The reason is that each slot, to synchronize the channel, must incorporate a guard time of the order of the inequalities among the delays, which may result in a considerable waste of channel resources when the variability among delays is large. Moreover, despite the good performance reported for the throughput in FC-TDMA, it requires a specialized algorithm that varies dynamically the number of slots in each frame, which has not been determined in the definition of the protocol provided in [62]. In the case of R-Aloha, although it seems to have excellent performance, that only holds when reservations duration are such that the channel is always occupied; nevertheless, in the case of a large number of nodes, and short reservation times, the scalability of the protocol falls rapidly. As can be seen through comparative analysis, MAC protocol performance varies widely when examined in the context of CubeSats together with the characteristics of the IoT networks (number of nodes, nature of traffic, geographical distribution, etc.). A visual evaluation of the suitability of each protocol derived from our comparative analysis is shown in Figure 14. In the figure, the protocols are placed according to: (1) their fulfillment of IoT-related requirements such as the scalability (the x-axis), which relates to the communications performance when serving networks composed of a large number of nodes, the topology dependency (a larger-sized geometric figure enclosing each protocol indicates a larger dependency on terminal nodes locations and knowledge of network topology); and the energy efficiency (darker colors correspond to higher power consumption in the execution of the MAC protocol); and (2) their adaptation to the constraints of nanosatellite technology, in which case we evaluate their implementation complexity (the y-axis). A level of high complexity signifies the need of costly resources such as dynamic channeling, advanced channel estimation mechanisms, synchronization, etc. ; and (once again) energy efficiency, since it is important to maintain the energy consumption on the spacecraft side within the nanosatellite capabilities.