## 4 SATis5 Satellite Emulator  
Being able to create and use a satellite-testbed without always having a real satellite link available requires a satellite emulator. To this end, the SATis5 testbed is extended by a satellite link emulator that has partly been developed during the,,Satcom integration with LTE-based core network emulator\" (SATINET) project [6]. In the SATINET project, the simulation of a DVB-S2 [29] satellite modem with extensions to solve or lessen the systematic issues introduced by using a GEO satellite were implemented in addition to the pure satellite link simulation. Proprietary elements such as Performance Enhancement Proxy (PEP), header-/payload-compression and packet scheduling were developed and integrated into a convergence layer. In SATis5, the project partner Newtec provided the Dialog 1 IF platform software containing convergence layer functions, which are identical to the ones used in their modems. Through the API provided by Newtec, the satellite resources (bandwidth, QoS) can be orchestrated through the core network. This has the advantage that network slices can be mapped directly to the satellite network. To provide a realistic simulation, these elements were selected to replace the satellite modem elements from SATINET. The software elements taken from the satellite hub platform are TelliNetServer and TelliShapeServer on the hub-side of the satellite link, as well as TelliNetClient and TelliShapeServer on the user-side of the satellite link. TelliNet implements a Distributed PEP for acceleration of TCP, HTTP, SMB and POP3 protocols via satellite. Besides this, it provides Authentication Authorization and Accounting (AAA) mechanisms, general payload and header compression and encryption. TelliShape implements real-time traffic shaping with QoS management, multilevel policies and VoIP prioritization as well as the creation of the DVB-S2 Baseband frames (BBFRAMEs). Figure 9 shows these different software elements.  
From SATINET, only the SatLink element was reused. It was modified and integrated with the satellite hub platform. It emulates the satellite forward and return links to/from a (mobile) user terminal via a geostationary satellite. It is highly parameterizable, e.g., bandwidth and link budget (including antenna gain) can be set via configuration file. As the forward and return shadowing profile is identical for the user, only one profile is generated for modeling the movement of the user terminal (Line of Sight (LoS), moderate shadowing, blockage). Additionally, due to frequency separation of uplink and downlink, forward and return link have their own land mobile satellite  
Figure 8: Scenario A3: indirect mixed 3 GPP non-terrestrial access with bent-pipe payload [28].  
Figure 9: Satellite emulator high-level software elements.  
(LMS) channel realization. Incoming BBFRAMEs are delayed depending on the hub-satellite-user geometry and BBFRAMEs are dropped if they are corrupted depending on the satellite link state. Figure 10 shows the structure of the forward and return link software modules. Incoming BBFRAMEs were created by TelliShape and contain IP packets split into suited chunks to fit into DVB-S2 frames for the given modulation and coding scheme.  
The emulator is an key element for the further development of the testbed. Moreover, the emulator helps to evaluate important building blocks of the architecture at an early stage without the need of using satellite capacity. The TelliNet and TelliShape elements allow their QoS management to maintain virtual network slices even through the satellite link. In addition, various use cases of the testbed can be demonstrated with the emulator, thereby simplifying the use of the testbed by interested third parties such as small startup companies. The emulator thus represents an essential pillar in the development and operation of the SATis5 testbed.