# Synchronizing TSN Devices via 802.1 AS over 5G Networks
## 6 Conclusions  
Asymmetry in uplink/downlink propagation delay can adversely affect the synchronization process (because of the PDelay calculation, which assumes symmetry). Furthermore, changing radio conditions and the uplink medium access mechanism (which uses uplink grants) in a 5G network and Layer 2 retransmissions might result in large differences between uplink and downlink delay. Since the PDelay calculation (in 802.1 AS) assumes uplink/downlink symmetry and averages the propagation delay in the uplink and downlink direction, any delay asymmetry leads to time synchronization errors. Thus, the need for PDelay correction, before synchronization takes place, arises.  
The results of both simulation scenarios implicate that for the TSN 5G integration, strict scheduling and communication between 5G's Application function that deals with TSN and TSN CNC is required in addition to Time Translator functionality and Ethernet PDU support in 5 GS. These functions play an important role in prioritizing the TSN traffic over 5G, allowing the 5 GS to align its component's schedules in line with TSN's requirements and mapping TSN QoS flows to 5 QI flows. For now, the presented simulation models can be used to create a baseline for how time synchronization performs in the native 5 GS via UDP/IP encapsulation without QoS. These results can be then compared with QoS support on 5 GS for time synchronization frames and with 5 GS with complete TSN support.  
The main challenges for the time synchronization, at the moment, in the TSN over 5G systems is the asymmetric uplink and downlink delay, the delays due to handover, the QoS mappings from the TSN to the 5 GS, and the retransmissions. The lack of available open-source 5G simulators which are capable of simulating QoS, Ethernet PDU support or TT functionality is also a limiting factor in this area of research.  
For further research, we plan to focus on reducing propagation delay errors due to asymmetric uplink and downlink by averaging previous \(n\) uplink or downlink delay values instead of averaging up/downlink delay values for a specific synchronization cycle. Furthermore, we also plan to simulate synchronization in uplink (UE as a GM and a slave connected to the 5 GS core) and synchronization between multiple UEs (multiple UEs synchronizing to a GM UE).  
Conceptualization, A.B.M., R.T. and T.B.; methodology, A.B.M. and T.B.; software, A.B.M.; validation, A.B.M., R.T. and T.B.; formal analysis, A.B.M., R.T. and T.B.; investigation, A.B.M.; resources, R.T.; writing--original draft preparation, A.B.M.; writing--review and editing, R.T. and T.B.; supervision, R.T. and T.B.; project administration, R.T.; funding acquisition, R.T. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the \"Bundesministerium fur Digitales und Verkehr\" Germany, grant number 19 OI22015F.  
No new data were created or analyzed in this study. Data sharing is not applicable to this article.  
The authors declare no conflicts of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.