Time-sensitive Industrial Internet of Things (IIoT) networks require ultra-low latency connections to function properly. One solution is to integrate high-performance software-defined radio (SDR) platforms with field-programmable gate arrays (FPGAs) that enable low-latency networking. These platforms also offer a high degree of interoperability and reconfigurability that can greatly benefit IIoT networks, especially as technology advances rapidly.

Time-sensitive Industrial Internet of Things (IIoT) networks require ultra-low latency connections to function properly. One solution is to integrate high-performance software-defined radio (SDR) platforms with field-programmable gate arrays (FPGAs) that enable low-latency networking. These platforms also offer a high degree of interoperability and reconfigurability that can greatly benefit IIoT networks, especially as technology advances rapidly.

FPGA-enabled SDR platforms are known to be deterministic and low-latency, but they also have a wide range of tuning and flexibility to help connect devices with a range of protocols used in the IIoT ecosystem.

What is the Industrial Internet of Things?

The IIoT is revolutionizing factories by providing robust connectivity between various devices. Until recently, wired communications dominated the industry’s connectivity. Factories are replacing wired connections with wireless networks, which allow for greater mobility and quick reconfiguration, requiring fewer installations and lower maintenance costs. Achieving satisfactory performance in an industrial environment requires more than basic 4G and WiFi.

The IIoT uses a wide range of network protocols and standards to interconnect various devices in a factory environment. Some of the most popular networking protocols for IIoT applications include Bluetooth, Zigbee, and LoRaWAN. For example, a protocol stack for a connected factory floor worker can have a physical layer (layer 1), a link layer (layer 2), and a network layer (layer 3). The physical layer can have wireless protocols; the link layer can have 3GPP, 4G/5G, IEEE 802.11 and IEEE 802.15.4; the network layer can have Internet Protocol, cloud and edge services.

Some of the key factors to consider when designing an IIoT network include network architecture, network functional layers, communication stack constraints, spectrum type, coverage, mobility, and lifecycle requirements (Figure 1). IIoT applications require a common network architecture to ensure interoperability and allow devices to connect to the data center. They also require common layer-based networking capabilities to ensure forward compatibility and enhance interoperability.

Networks for IIoT applications need to take into account the limitations of the communication stack used in end devices. To ensure reliability, it is critical to consider the trade-off of using licensed and unlicensed spectrum when implementing an IIoT network. The IIoT network should have the range and coverage to meet the needs of the plant. Additionally, the network should be able to meet the mobility needs of the factory environment.

SDR for low-latency and time-sensitive Industrial Internet of Things (IIoT) applications
Figure 1: IIoT Network Design Considerations

Why Deterministic Low Latency Matters

Network latency refers to the delay a signal experiences as it travels through a communication network. In a typical communication system, latency can be thought of as the total time a packet is captured, transmitted, and processed through multiple components of the network system, until the data is received and decoded at the destination.

Traditional wireless networking protocols are designed to allow the exchange of massive amounts of data without strict time constraints or the need for synchronization. Some signals used in industry, such as individual control commands, have strict delay constraints and require network infrastructure with deterministic delays. Deterministic Ethernet and time-sensitive networks were developed to meet the stringent timing requirements of such applications. Figure 2 shows some of the main causes of latency in IIoT networks.

Time Sensitive Networking for the IIoT

Time Sensitive Networking (TSN) refers to a set of standards designed to provide precise timing and synchronization. TSN components can be roughly divided into three categories: time synchronization, traffic rules, and path selection. The time synchronization component requires that all devices involved in real-time communication have the same understanding of time. Traffic rules require that all involved devices follow the same rules when processing and forwarding packets. Finally, TSN requires all devices to follow the same rules when choosing communication paths and reserving time slots and bandwidth.

TSN provides a range of benefits for time-sensitive applications. It is optimized to minimize latency when transporting time-stamped and latency-sensitive data in a variety of traffic environments. To maximize interoperability, TSN employs standard components that are widely available. This helps enhance scalability and reduce the overall cost of deploying and maintaining the network.

TSN integrates several mechanisms to ensure deterministic performance across similar settings. Some of these features include improved precise timing control, bandwidth re-service, redundant paths for transporting data streams, and integrated Quality of Service (QoS) capabilities for Ethernet link communications. These capabilities help ensure deterministic latency and tight synchronization in IIoT applications.

TSN is designed to provide more bandwidth, making it suitable for industrial applications that require large amounts of Ethernet bandwidth, such as 3D scanning and machine vision. Its design helps simplify network infrastructure, while its deterministic approach to Ethernet networking allows a single Ethernet network to be used to transport mixed traffic.

SDR for low-latency and time-sensitive Industrial Internet of Things (IIoT) applications
Figure 2: Network latency contribution in the IIoT

SDR for IIoT

SDR systems allow various radio signal processing components, such as modulators, demodulators, encoders, and equalizers, to be implemented in software rather than dedicated hardware. A typical SDR has a radio front end (RFE) and a digital back end. The RFE performs transmit (Tx) and receive (Rx) functions and is designed to provide a wide tuning range. The highest performance SDR platforms offer multiple independent channels, each with a dedicated analog-to-digital converter (ADC) and a digital-to-analog converter (DAC). Furthermore, these platforms are designed to provide very high instantaneous bandwidth.

Most high-performance SDR platforms feature FPGAs with various board-level digital signal processing (DSP) functions, such as modulation, demodulation, upconversion, and Ethernet packets. In addition, the SDR platform can support mixed traffic, simplify the network infrastructure, and provide sufficient bandwidth.

The architecture of the SDR platform enables low-latency solutions for time-sensitive applications. FPGAs have a highly parallel architecture that enables them to perform processing tasks faster than a host PC. Embedding application logic on this device can help improve the overall latency performance of the system. For applications requiring ultra-low latency, implementing a custom interface protocol using SFP+ connectors can help further reduce the time delay between the host and the SDR platform.

TSN’s SDR

Tests show that the SDR-based solution can achieve an end-to-end latency of 3.75 ms. This means that SDR-based implementations can be used in IIoT applications that require low latency and time synchronization, such as human-machine interaction (HMI), sensor data collection, and automated guided vehicle (AGV) systems.

Combining SDR with Software-Defined Networking (SDN) technology helps to implement sophisticated TSN for IIoT applications. The technology provides resource and security orchestration and helps address congestion and other latency-related issues. Additionally, SDN can dynamically reconfigure the network using real-time predefined requirements.

A number of SDR-based TSN prototype solutions have been developed and tested. Testing of an advanced radio receiver system prototype with IEEE 802.15.4 Offset Quadrature Phase Shift Keying (OQPSK) physical layer shows that SDR-based implementation is suitable for low-power IIoT applications using protocols such as WirelessHART, ZigBee, and 6LoWPAN .

Testing of SDR-based next-generation networking prototypes shows that low-latency networking solutions can be implemented by using SDRs with FPGAs. This implementation enables the SDR to take advantage of various features of the IEEE 802.1 TSN standard, including time scheduling and delay-optimized scheduling.

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