# Multi-Layered Satellite Communications Systems for Ultra-High Availability and Resilience
## 5 Results: Findings from Simulations and Testbed Activities
### Testbed Activities
In the control plane, the end-to-end delays presented in Figures 13 and 14 were obtained for the GEO and the LEO-based measurements for the Registration with Session Establishment and the Deregistration procedures. As immediately noticeable from the measurements, the procedures are directly proportionate to the end-to-end delay between the two locations. Specifically, for the Registration with Session Establishment, the delay of the procedure is ten times bigger than the delay of the backhaul, as there are five message exchanges with the UE as part of the registration procedure (Registration Request--Authentication Request, Authentication Response--Security Mode Command, Uplink Network Access Server (NAS) Transport--Initial Context Setup Response, Uplink NAS Transport--Downlink NAS Transport--Protocol Data Unit (PDU) Session Resource Setup) while for the Deregistration it is two times bigger than the backhaul delay, as there is a  
Figure 11: GEO end-to-end delay. Figure 12: LEO end-to-end delay. single message exchange (Deregistration request--Deregistration accept). The results for the GEO backhaul confirm previous measurements using real satellite connectivity. The delay for these procedures is manageable by the core network, being still in the expected time delays for the procedures, even for the GEO case. Furthermore, as we have used a test UE where we have removed the timers, we have had no retransmissions. However, for a commercial UE, the retransmission timers would have to be updated to be able to match the expected delays. All the tests were executed in a non-stressed environment, with a single request every second. The delay of the local execution of the procedures was added to the delay of the backhaul--i.e., 20-40 ms for the registration procedure with session establishment and  
Figure 14: Registration with Session Establishment and De-registration with LEO backhaul. Figure 13: Registration with Session Establishment and De-registration with GEO backhaul. 2-4 ms for the deregistration. These extra delays are expected to be maintained until the system becomes loaded, as the procedures for the different UEs are executed in complete isolation from each other, the only common synchronization point being the NAS, and the Hypertext Transfer Protocol (HTTP) interfaces of the different components, i.e., the telecom systems, are very effective at scaling up to the computed load. When overloaded, the network functions behave with large jitter in their responses. However, these situations are never reached in real telecom networks, and should not be reached in MLSS either as the components should be properly dimensioned and fast enough at scaling to handle such situations. Similarly to the data plane, the measurements were executed consecutively for the different satellite links emulating handovers between GEO and LEO and between LEOs. However, there was no effect on the tested procedures as they were finishing too fast to be able to predictably isolate one procedure during a handover phase. A more granular satellite network emulator, e.g., a channel emulator, would be needed to enable a more granular behavior capable of being overlapped to different procedure phases. The main findings regarding the 5G system with the MLSS backhaul are as follows:  
* Data path jitter considerations: The very large jitter on the NGSO is significantly impacting the behavior of the end-to-end applications. Such a large jitter is uncommon even for wide-area terrestrial networks and should be further studied. As a solution, a Time-Sensitive Network (TSN)-like implementation for NGSO should be considered for end-to-end communication to bring more determinism to the end-to-end connectivity. * Forward Error Correction considerations: As the satellite network may be multi-hop based, and as forward error correction implementation may be in the space nodes, especially for the ISLs, an end-to-end forward error correction should be able to remove the jitter created by the retransmission of lost packets on the NGSO link.