Fifth-generation wireless communications (5G) and Time-Sensitive Networking (TSN) are key technologies for future industrial communications: 5G for wireless connectivity and TSN for wired connectivity.
In addition to enhanced mobile bandwidth, 5G supports communication with unprecedented reliability and very low latency, as well as massive IoT connectivity.
TSN is a collection of Ethernet standards introduced by IEEE 802.1 group, defining mechanisms for deterministic communication over wired Ethernet links enabling guaranteed packet transport with bounded latency, low packet delay variation, and extremely low packet loss.
Both technologies have been designed to provide converged communication for a wide range of services on a common network infrastructure. Significant benefits can be achieved for Industry 4.0 use cases by enabling TSN and 5G to work together, e.g., increased flexibility in the deployment of industrial equipment and networks.
The following paragraphs provide a short introduction on how 5G and TSN are integrated together and, in particular, how the Generalized Precision Time Protocol (gPTP or 802.1AS) works in a TSN over 5G system. Tips on how best to approach the testing challenges of such a system are also highlighted.
Seamless integration between 5G and TSN systems
For a seamless integration between a 5G system (5GS) and a TSN system, it was proposed by 3GPP (3rd Generation Partnership Project) that the two systems interoperate in a transparent manner to minimize impact on other TSN entities. Therefore, the 5G system acts as one or more virtual TSN bridges of the TSN network. This virtual bridge model defines several gateways between the TSN and the 5G system:
A TSN application function (AF) to connect the TSN Centralized User Configuration (CUC)/ Centralized Network Controller (CNC) entities and the 5G control plane
A device-side TSN translator (DS-TT) on the user equipment (UE) side
A network-side TSN translator (NW-TT) on the user plane function (UPF) side
The Precision Time Protocol (PTP) is a protocol used to synchronize clocks throughout a communication network, achieving clock accuracy in the sub-microsecond range, making it the perfect choice for industrial applications where strict time synchronization requirements must be met.
A PTP system is formed by a clock source, the grandmaster (GM), that transmits synchronization information through a sync tree toward multiple clock targets (slave devices).
A 5G system already relies on PTP for it to function, but that is a different use case. In the case of a smart manufacturing environment, we are interested in how a 5G system can securely and reliably transport the PTP messages between different PTP entities, located either in the fixed network side or in the mobile side. These PTP devices can be different sensors, cameras, industrial robots, automated guided vehicles (AGVs), head mounted displays for augmented reality (AR) applications, or any other industrial devices requiring precise time synchronization to function properly.
In 3GPP Release 16, the GM or the time source can be located only on the network side (fixed side). The slave clock or the time target can be located on the fixed side or the mobile side (UE) of the network.
3GPP Release 17 will introduce the possibility to have the GM located also on the mobile side of the network. This will create several challenges, since when both the GM and the slave are located in the mobile part, the PTP messages will have to traverse two wireless links.
TSN uses a specific PTP profile, the Generalized Precision Time Protocol (gPTP or 802.1AS). A new version of the 802.1AS standard was released in 2020, introducing several improvements over the original 2011 version, mainly targeted toward increased reliability. Two of the main improvements are the possibility to use multiple time domains and the possibility to use redundant sync trees. The Time-Sensitive Networking Profile for Industrial Automation (IEC/IEEE 60802) uses four different time domains, two for the working clock and two for the global time.
Four steps to ensure high reliability for industrial environments
A 5G system can support up to 128 time domains. A UE or a PTP device in a 5G system can be connected at two or more virtual TSN bridges at the same time, as shown in accompanying image. This means that UE4 can be part of two different sync trees (gPTP domain 1 and gPTP domain 2). If the connection to one of the virtual TSN bridges is lost, it can still receive time information from the other TSN bridge.
The first step to test how well TSN over a 5G system works is to compare the performance and precision of time synchronization over a wired link versus a wireless link. For example, the time accuracy at Time Slave 10 and at Time Slave 4 (UE4) can be compared. It is expected to achieve better performance over the wired path, but the question is if the performance over the wireless path stays within the desired limits.
High reliability is a must in industrial environments; therefore, the second step would be to investigate how quickly a device can recover time information when a path from a sync tree goes offline for a while.
The third step would be to analyze how the load on the wireless side of the system affects the precision of the time synchronization. The 5G network should treat PTP messages with high priority versus the other traffic sharing the network. Another thing to consider is how the time sync precision is influenced by movement or by the distance between the UE and the base station.
Finally, special consideration must be given to the computation of the residence time by the 5G system. A TSN bridge records the time it takes for a Sync message to traverse it, from the moment it is received on the ingress port to the moment it is forwarded on the egress port. This time difference is called the residence time and it must be added in a special field inside the Sync message (in case of a one-step clock) or the Follow-Up message (in case of a two-step clock). The precision with which the 5G system measures the residence time for the virtual TSN bridges has a big impact on the overall precision of the time synchronization. Therefore, testing how well the 5G network calculates the residence time must be part of any test plan.
The role of Time-Sensitive Networking in industrial automation
TSN over 5G systems bring numerous benefits to industrial communication networks, but they do not come without challenges. Comprehensive testing must be performed to ensure all requirements are being met, both in the lab and in live networks.
gPTP is a fundamental building block for TSN systems, but many other TSN protocols play a key role in Industrial Automation and can be used over a 5G network. The most important are:
Time Aware Shaper (IEEE 802.1Qbv-2016)
Frame Replication and Elimination for Reliability (IEEE 802.1CB-2017)
Per Stream Filtering and Policing (IEEE 802.1Qci-2017)
We have explored several key steps to be considered when testing gPTP over a 5G system. In future blog posts we will explain how other TSN protocols work over a 5G system and the best practices to test them.
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