## 6 Conclusions  
The integration of a satellite network into a terrestrial IoT/M2M network is a multi-parameter optimization problem. This article identifies the key ISL technologies to enable an effective, fast-responding, and cost-effective narrow-band satellite network which can support M2M applications and services. Some significant advantages are a decrease of communication latency, a reduction in the number of ground stations, and an effective reconfiguration ability of the fleet. In this article, we focused on the space segment and addressed the design methodology and implementation of an adaptive ISL network architecture with highly limited resources. System design parameters and related communication layers for a network with extremely limited on-board resources, suitable for the case of nanosatellites, were analyzed. This included the analysis of appropriate methodologies and techniques for the communication layer, such as routing algorithm, medium access control method, channel coding, and modulation. These techniques were implemented into a software-defined S Band radio (SLINK), incorporating the physical layer and data link layer. The communication layer between tge SLINK radio and SNC were defined according to the CCSDS recommendation; thus, the network layer was implemented on the spacecraft side. Using TDD, adaptive modulation up to 16-ADPSK, and convolutional code with FEC and ARQ, the equipped version achieved a 100 kpbs data rate over 800 km distance, pushing the state-of-the-art performance for nanosatellite communication. Adaption of the TDD protocol for distances more than 2000 km is easily possible. To verify the ISL performance and demonstrate system capability in orbit, a fault-tolerant nanosatellite mission S-NET, consisting of four satellites, was developed by TUB. The four S-NET satellites were launched on 1 February 2018 by a Soyuz/Fregat launcher from Vostochny Cosmodrome, Russia, into a 580 km orbit. Since then, the overall performance of the SLINK radio, the link budget and timing of ISL synchronization, data routing, and long-term stability have been successfully verified in the space environment. For this specific mission, the ISL range of 400 km was limited by the link budget, mainly dictated by the TX power and antenna gain. The S-NET mission is, thus, one of the first nanosatellite missions to perform multi-point network communication in orbit. For the purposes of integration into an IoT/M2M network, the overall ISL performance, such as data rate, distance, and latency, must be adapted to specific scenarios and the resulting system architectures. The number of nodes can be scaled up by adapting the addressing part of the TDD frame header and extending the routing algorithm. Data throughput and ISL range depend on the link budget; hence, efficient spectrum usage and innovative (deployable) antenna concepts are required. The latency performance seems to be compliant with most IoT applications, as the ISL synchronization time for a P2P session can be trimmed to under 1 s. The additional delay for uplink (ground device to sat) and downlink depends on the link design and terrestrial IoT protocol used, but are typically in the range of several seconds. hence, the overall latency--from IoT ground sensor to gateway (optional) to satellite (multi-hop) to ground station to end user--could be typically smaller than approximately 10 s.  
Conceptualization, K.B., W.F., and Z.Y. ; methodology, Z.Y. ; software, W.F. ; validation, W.F. and Z.Y. ; formal analysis, Z.Y. and W.F. ; investigation, Z.Y. ; data curation, W.F. ; writing--original draft preparation, Z.Y. and W.F. ; visualization, W.F. ; supervision, K.B. ; project administration, Z.Y. ; funding acquisition, Z.Y. The research on S-NET was funded by the Federal Ministry of Economy and Technology (ger. Bundesministerium fur Wirtschaft und Technologie, BMWT) and the German Aerospace Center (DLR) under grant no. 50 YB 1225. The precursor project SLINK was funded by the Federal Ministry of Education and Research (ger.