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Written by Arnd Sibila | November 23, 2020

Smart factories: Network testing is essential to ensure quality and performance (part 2)

Smart factories need to quickly adapt to production line changes to be more productive and minimize downtime due to factory modifications. Therefore, wired connections of production facilities, such as today’s robots using Ethernet cables, are expected to operate wirelessly in the future. However, the prerequisite for such wireless connections is that they are highly reliable with very low latencies – reliability and latency being the critical KPIs of mobile networks in smart factories. 5G can fulfill these requirements, either in combination with LTE (non-standalone) or without LTE (standalone). This article will discuss how to test reliability and latency in different test phases to ensure high-quality smart factory deployments while verifying and monitoring network quality and performance.

Smart factories: Network testing is essential to ensure quality and performance (part 2)

The 5 phases of mobile network testing in a smart factory

Figure 1 below shows the first four test phases of network testing in smart factories. These phases are indispensable for achieving a high-quality mobile network and verifying the strict reliability and latency requirements.

Rollout preparation – phase 1
In some countries, factory owners have access to the 5G spectrum solely dedicated to campus or private networks. The first task for these factory owners is to ensure that the new spectrum is interference-free. The R&S®TSMx network scanners, together with a handheld interference hunting spectrum analyzer and monitoring receiver – such as the R&S®Spectrum Rider FPH and R&S®PR200 Portable monitoring receiver, respectively – are an excellent choice for sustainable rollout preparation.

Site acceptance – phase 2
During site acceptance, the operation of newly deployed base stations will be tested and validated. This phase includes simple, functional tests, such as download (DL) and upload (UL) tests and round-trip latency measurements, over-the-air (OTA) RF spectrum analysis, and signal decoding to verify the PCI, SSB, and SIB information of 5G and LTE anchor signals. Signal decoding also helps in troubleshooting specific parameters in case of issues or unexpected results.

The smartphone-based troubleshooter QualiPoc Android executes functional DL, UL, and ping tests; the handheld spectrum analyzer R&S®Spectrum Rider FPH performs OTA spectrum measurements, and the 5G site testing solution (R&S®5G STS) troubleshoots the deployment in case of issues.

Figure 1: Network test phases from preparation to 24/7 service quality monitoring
Figure 1: Network test phases from preparation to 24/7 service quality monitoring
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Coverage and performance – phase 3
The testing of coverage and performance in phase 3 is critical as it ensures that the mobile network supports the requirements for high reliability and low latency throughout the factory.

  • Network scanners (R&S®TSMx) measure the number and variety of access points with good receive signal levels (RSRP) and quality (SINR). They are used to verify that sufficient redundancy (number of access points) is provided and that network connectivity is highly reliable.
  • QualiPoc Android tests the latency and real-time capability of communication links by combining the emulated traffic behavior, latency, and continuity when running the unique interactivity test.
  • With the real-time optimization software R&S®ROMES4, the measurement results can be visualized immediately and potentially underserved areas optimized.

Service quality monitoring – phase 4
In large factories or factories where downtime equals a high loss of profitability and productivity, factory owners will demand very tough service-level agreements (SLA) from their network supplier or operator in case of outsourcing. Consequently, factory owners will also want a 24/7 network performance monitoring solution in real time, such as SmartMonitor from Rohde & Schwarz.

The web-based SmartMonitor application manages a fleet of specifically tailored RF probes that are distributed throughout the factory, as well as in automated guided vehicles (AGV) and autonomous mobile robots (AMR). The probes continually check the connectivity levels, including latency, and report the results to a central unit (the SmartMonitor dashboard) displaying results and probe status in real time.

Any deviation from normal conditions is visible immediately. The data analytics software suite SmartAnalytics analyzes the data offline to identify trends and anomalies using machine learning algorithms that trigger improvement actions before any issues occur.

Figure 2: Periodic check of the RF environment outside the factory
Figure 2: Periodic check of the RF environment outside the factory

Regulatory compliance – phase 5
Factory owners who successfully applied for the 5G spectrum dedicated to campus or private networks will have to ensure that their network is compliant with private network license terms. Thus, it is recommended to guarantee that the transmission of leaked signals stays within specified limits outside the factory to not interfere with a neighboring network that might use the same frequency band. Regulatory compliance can be verified with walks test solutions, such as the R&S®Freerider 4 backpack, or by mounting a network test solution, such as a scanner, on a drone.

How to test latency in a smart factory network

In part 1 of this article series about smart factories, I already touched upon the difference between round-trip latency and one-way latency. Before going into more detail about these differences, let’s quickly review the use cases for round-trip latency and one-way latency, respectively.

  • AR/VR use cases, for example, need low round-trip latency because if the technician wearing AR/VR glasses moves his head, the image depicting instructions needs to update very quickly to ensure proper operation. Experts also forecast a fast-paced development of new VR use cases in various environments that will increase the demand for higher data throughputs and low latencies.
  • Robot control, on the other hand, requires low one-way latency. The system sends an order to the robot, and the robot has to act immediately (e.g., stop movement); it is just one-way communication.

Round-trip latencies are traditionally measured using ping messages. By definition, ping is a basic Internet software utility to verify that a particular Internet node exists and can accept requests. The accessibility of the Internet node is confirmed by its feedback, the ping time. However, ping has some inherent disadvantages for accuracy, particularly for very low latency values.

Other arguments for no longer using ping include emulating the typical traffic behavior of the communication to robots and other entities in a smart factory. For this, the preferred protocol is the Two-Way Active Measurement Protocol (TWAMP) that has been specified by the Internet Engineering Task Force (IETF) to verify service-level agreements (SLA) regarding times/latencies.

In the illustration figure 3 below, the measurement device sends a configurable User Datagram Protocol (UDP) stream of unique packets emulating a realistic traffic profile of a use case class to an (active) server (uplink) that immediately reflects (TWAMP reflector) it to the device (downlink).

The measurement device’s QualiPoc Android software runs the interactivity test to analyze the round-trip latency, packet delay variation, and packet error rate to calculate an interactivity score for this specific use case class based on its particular QoS requirements.

Figure 3: Interactivity test running on QualiPoc Android from Rohde & Schwarz
Figure 3: Interactivity test running on QualiPoc Android from Rohde & Schwarz

The interactivity test component resulting from the round-trip latency forms an S-shaped curve, as shown in figure 4. The packet delay variation and packet error rate components are factors [0, 1] that scale down the S-curve.

Figure 4: Example of the interactivity score for multi-player, real-time eGaming
Figure 4: Example of the interactivity score for multi-player, real-time eGaming

The parametrization of the interactivity test and interactivity score can be individual for each application class (e.g., AR/VR remote support or robot control in a smart factory) depending on the latency and real-time requirements for the specific application class. The interactivity test forms the basis for a scalable QoE model that can be adapted to all kinds of interactive applications.

One-way latency measurements require the synchronization of sender and receiver. In a prototype implementation, we provided a time synchronization via GPS-locked PPS to both sides (QualiPoc Android and the TWAMP server) and measured one-way latency in both directions. First results in a well-optimized public LTE network show a very asymmetrical result (DL with much shorter latency than UL).

More investigations are being made, particularly in real 5G-based industrial deployments. Stay tuned and, in the meantime, watch the on-demand webinar about smart factory deployments here.

Related stories

Smart factories: Mobile network characteristics and main KPIs (part 1)

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Interactivity test: Examples from real 5G networks (part 3)

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5G measurements to lay the foundation for autonomous vehicles in port terminals

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