1. Introduction

2. Time Sensitive Networking

2.4. Time Synchronization

Time Synchronization is a key requirement of real-time networks; thus, it is of utmost importance and a basic component of TSN. It enables TSN devices to adjust and correct their local clocks in reference to the Grand Master clock. This is a prerequisite for gate scheduling mechanisms, path reservation, traffic filtering and forwarding and traffic shaping via TAS or CBS. Time Synchronization for TSN is standardized in IEEE 802.1AS with the gPTP profile for the IEEE 1588v2 Precision Time Protocol (PTP). It provides sub-microsecond accuracy in synchronization. The Time Synchronization mechanisms is shown in figure 1.

Figure 1. IEEE 802.1AS Time Synchronization procedure in wired networks.

Figure 1. IEEE 802.1AS Time Synchronization procedure in wired networks.

The synchronization procedure consists of the Requester entity (TSN device which wants to synchronize to the master) requesting the Responder entity (TSN device to which all other TSN devices synchronize to) to initialize the synchronization process. After initialization, the Requester sends a Path delay measurement request to the Responder, which responds with a Path delay measurement response and a follow-up message. In this three-message exchange, the Requester gathers four time stamps, as shown in figure 1. In the Propagation delay measurement process, a symmetric link is assumed. This is true for wired networks, but for wireless networks, with asymmetric links, this assumption can adversely affect the outcome of the synchronization, as we will see in the section 5 of this paper.

After the Propagation delay measurement process, a Synchronization message and a Synchronization follow-up message are sent by the Responder with the current timestamp (preciseOriginTimestamp). Using the current time, the previously computed Propagation delay, and the correction field (in case if the Responder is not the Grand Master), the Requester computes the actual time using the following equation and adjusts its local clock accordingly.

Corrected Time = preciseOriginTimestamp + Propagation Delay + correction field

For integration of TSN and 5G network, it is important for 5G network to provide the required QoS to the TSN communication streams. This also implies that Time Synchronization messages of TSN network shall traverse through 5G system in order for TSN end devices on the either side of 5GS to synchronize. But with handovers in 5GS and channel access grants for UE to access channel for data transmission results in irregular Propagation delay calculations which decreases the synchronization accuracy as compared to a symmetric 5G link i.e. same propagation delay in uplink and downlink.

3. Related Research

4. Simulation Setup for TSN Synchronization over 5G Scenarios

4.2. Simulation Scenarios

Using the updated models, two simulation scenarios are created. Starting with a simple TSN network consisting of two TSN switches and a Grand Master (GM). The GM, called TSN Master is connected to the TSN Switch (based on 5G-TSN device model), which is in turn connected to the second TSN Switch (also based on 5G-TSN device model). The link between both switches is simulated by a 5G network.

4.2.1. Simulation Scenario 1

In the first scenario, the GM is placed on the server side of the 5G network. That means synchronization messages are transferred in the downlink. Both TSN Switches are 5G-TSN devices; the first being named TSN Switch, encapsulates the time synchronization messages and forwards them to the 5G network. The second 5G-TSN device, named TSN device 2, acts as a TSN device capable of synchronizing to the GM through the 5G network. It receives encapsulated time synchronization messages, decapsulates them and synchronizes itself with the GM via the extracted messages. From the perspective of TSN devices, the 5G network is simply a link between TSN Switch and the TSN device 2. This simulation scenario is shown in figure 2 (a).

The scenario can be expanded by connecting TSN devices to the GM (TSN Master), TSN Switch, and TSN device 2, if it takes the role of a TSN Bridge, in figure 2 (a) resulting in a hierarchical structure.

4.2.2. Simulation Scenario 2

This scenario is similar to scenario 1 in terms of the TSN network layout but differs in the layout of the 5G network. The 5G network now has two gNodeBs. The TSN device 2 is mobile and moves along a predefined path at a constant speed. This results in a handover between the gNBs, and while the handover is taking place, the synchronization process is affected due to the variation in the propagation delay. This simulation scenario is shown in figure 2 (b).

Both simulation scenarios are run for 500 seconds. Time synchronization occurs in 1 second intervals, and the propagation delay measurement interval is also set to 1 second. TSN device 1, TSN Switch, and TSN device 2 synchronize to the TSN Master, and the local device’s clock time of the Master clock and the slave clocks, immediately before and after synchronization, are logged. In addition, propagation delay, residence time, and all four timestamps required for propagation delay measurement are also logged. Since these timestamps are used to calculate the link delay, they can be used to measure uplink and downlink delay, the delay variations, and the difference between uplink and downlink delay in the 5G network.

Figure 2. (a) Simulation Scenario 1: TSN device 2 synchronizes with TSN Master via TSN Switch; (b) Simulation Scenario 2: Extension of scenario 1; handover between gNodeB1 and gNodeB2.

Figure 2. (a) Simulation Scenario 1: TSN device 2 synchronizes with TSN Master via TSN Switch; (b) Simulation Scenario 2: Extension of scenario 1; handover between gNodeB1 and gNodeB2.

The major simulation parameters are shown in table 2. Simulation scenario 2 is an extension of simulation scenario 1 – therefore the same parameters as in simulation scenario 1 are applied and additional mobility and handover specific parameters.

Table 2. Simulation parameters.

Table 2. Simulation parameters.

TSN network parameter *			Value	5G network parameter *		Value
Simulation 1	Ethernet Bitrate		1 Gbps	AMC 1 Type		NRAmc
	Oscillator Type		Constant Drift Oscillator	Target BLER 2		0.01
	Oscillator Drift Rate		Uniform (0 ppm, 100 ppm)	BLER Shift		5
	Local/Destination UDP Ports		3000	Mac Cell ID / Master ID		0
	Sync Initial Offset / PDelay Initial Offset		1 s	NR Mac Cell ID / NR Master ID		1
	Sync / PDelay Interval		1 s
Simulation 2	UE Mobility	Mobility Type	Rectangle Mobility	Handover	CQI Feedback Period	40ms
		Constraint Area	(150m, 450m) – (750m, 450m)		Uplink/downlink interference	True
		Starting Position	(0, 0) i.e. (150m, 450m)		Dynamic Cell Association	True
		Speed	2 meters/second		Enable Handover	True

* TSN / 5G parameters not mentioned in table are set to the default values as specified in their OMNeT++ model. ¹ Adaptive Modulation and Coding ² Block Error Rate.

5. Simulation Results and Discussion

5.1. Simulation Scenario 1

The TSN Switch directly synchronizes to the Master via the Ethernet link. Time difference between its clock and the Master’s clock, after synchronization, is in the order of 100 nanoseconds, thus indicating a normal successful time synchronization. However, the results of the TSN device 2 synchronization show a lower accuracy. Figure 3 shows the clock time differences of the Master clock (Red line) and TSN device 2’s clock (Blue line) to the true time. (True time is the actual simulation time and all the nodes in simulation, including TSN Master, have a constant drift.) It can be seen that the difference between TSN Master graph and TSN device 2 graphs is approx. 2.5 milliseconds, which means that after synchronization TSN device 2 still have 2.5ms difference from the Master’s clock time. This low accuracy is explained by figure 4. Figure 4 (a) shows the uplink propagation time, downlink propagation delay, and their difference. The 802.1AS standard assumes symmetric link in terms of propagation delay and computes the average propagation delay, which is used for synchronization. But with a 5G network, the uplink and downlink propagation delay is asymmetric, mainly because of the differences in the medium access control (MAC) for uplink and downlink traffic, resulting in error in propagation delay computation, which in turn affects the accuracy of synchronization.

Figure 4 (b) shows the box plot of the uplink and downlink delay as well as the actual propagation delay calculated by the 802.1AS model. From Figure 4 it can be seen that the maximum difference between uplink and downlink propagation delay is approx. ~ 2ms. If this difference is considered in the propagation delay calculation, the time difference between TSN device 2 and the Master, after synchronization, reduces to ~ 0.4 ms at maximum (see Figure 3 – TSN device 2 clock time (adjusted)), which is still quite high. The variability of the time difference can be explained by the variability of the uplink and the downlink propagation delay, as seen in figure 4.

Figure 3. Time differences to the true time after synchronization.

Figure 3. Time differences to the true time after synchronization.

The reason for propagation delay differences in uplink and downlink is the process of how 5G UE gets access to channel for uplink transmissions. The UE can only transmit in the uplink when the channel grants are given to the UE, resulting in longer delays which in turn results in asymmetric uplink and downlink propagation delay. Furthermore, QoS support is also absent in Simu5G framework, which further reduces the accuracy during synchronization since sync messages can not be tagged as high priority messages.

5.2. Simulation Scenario 2

In simulation scenario 2, the TSN Switch synchronizes to the GM via the Ethernet link and forwards the synchronization frames to the TSN device 2. The TSN device 2 moves from the left to the right side of the simulation area as shown by the mobility direction arrow in figure 2 (b). During the movement, a handover from gNodeB1 to gNodeB2 takes place.

Figure 4. (a) Uplink and Downlink propagation delay during PDelay Measurement; (b) 802.1AS calculated PDelay compared to Uplink and Downlink Propagation delay during the synchronization process.

Figure 4. (a) Uplink and Downlink propagation delay during PDelay Measurement; (b) 802.1AS calculated PDelay compared to Uplink and Downlink Propagation delay during the synchronization process.

Figure 5 (a) shows the time differences of the TSN Master and TSN device 1 clock to the true time for the first 5 seconds of simulation. The time difference after synchronization is in order of 100 nanoseconds, indicating a successful synchronization, as expected due to the direct 1Gbps Ethernet link between them. Figure 5 (b) shows the true time differences of the TSN Master and the TSN device 2 after synchronization. The handover event at around 95 seconds is clearly visible, as the difference of the TSN device 2 to the true time suddenly increases to 50 ms and the difference between TSN Master and TSN device 2’s clocks is approx. 46ms. The true time differences of TSN Master and TSN device 2 before and after the handover event are also higher than the true time differences of the TSN Master and the TSN device 1. This is (like in scenario 1) due to uplink and downlink propagation delay differences. The main reason for the multiple, shorter than the highest delay difference peaks, after the handover are Layer 2 retransmissions between gNodeB and TSN device 2. Since the TSN device has just entered the coverage area of gNodeB2, therefore L2 retransmissions of erroneous frames causes multiple smaller peaks, which disappear when the device reaches closer to gNodeB2.

Propagation delays in uplink and downlink direction are shown in figure 6. An asymmetry regarding the uplink and downlink delays can be observed (similar to simulation scenario 1) as well as delay variations (caused by L2 retransmissions in 5G network), resulting in increased inconsistency of PDelay in its measurement. This in turn leads to a significant time difference between the TSN device 2 and the TSN Master after synchronization. The downlink and uplink delay asymmetry still remains in the simulation as discussed in previous section, and it is often pronounced due to retransmissions, e.g., in figure 6 (a) between 150s – 250s. Eventually, if in the PDelay calculation the difference of uplink and downlink delay is considered, the average time difference between TSN device 2 and TSN Master reduces to be around 500 microseconds with some irregularities during handover and retransmissions.

Figure 5. Time differences to the true time after synchronization for TSN Master and (a). for TSN device 1. (b). for TSN device 2.

Figure 5. Time differences to the true time after synchronization for TSN Master and (a). for TSN device 1. (b). for TSN device 2.

A possible solution to avoid synchronization errors during handover is to halt the time synchronization process during the handover process, since synchronizing during handover results in false clock time for the slaves. Once the handover is completed, synchronization process can be started again with the new Sync and PDelay calculation frames.

Figure 6. (a) Uplink and Downlink propagation delay during PDelay Measurements; (b) 802.1AS calculated PDelay compared to Uplink and Downlink Propagation delay during synchronization process (outliers excluded).

Figure 6. (a) Uplink and Downlink propagation delay during PDelay Measurements; (b) 802.1AS calculated PDelay compared to Uplink and Downlink Propagation delay during synchronization process (outliers excluded).

The findings from both simulations align with the results that were achieved by [1] & [2], as mentioned in section 3.

6. Conclusion

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