Beckhoff EtherCAT Terminals
Industrial Ethernet Book Issue 120 / 13
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PROFINET over industrial WLAN infrastructure

Two key challenges need to be addressed when building a stable PROFINET application over a WLAN infrastructure. The selected WLAN solution must support L2 forwarding in different configurations, and wireless latency and jitter need to be minimized to meet the requirements of the PROFINET application.

WITH THE EMERGING AIOT AND IIOT TRENDS, an increasing number of connected devices are being introduced into industrial operations at a faster pace than ever before. At the same time, wireless connectivity has opened up new doors for many mobile applications and has gained traction in recent years within the industrial sector.

More and more industrial control and automation planners are embracing the benefits of wireless communication and are integrating wireless infrastructure into their system design. This article aims to communicate the protocol requirements, deployment considerations, challenges, and solutions to support one of the most popular industrial communication protocols, PROFINET, in wireless networks.

The goal is to provide an overview of the basic PROFINET requirements and configuration variables needed to obtain an acceptable performance margin in a wireless environment. It provides example scenarios of industrial wireless applications using WLAN solutions to serve as a frame of reference for planning a wireless PROFINET deployment.


The PROFINET protocol stack

PROFINET Overview

PROFINET is the PROFIBUS International (PI) industrial Ethernet standard designed for automation control communication over Ethernet-based infrastructure.

PROFINET devices may require different communication speeds depending on the type of automation process. The PROFINET protocol supports three communication classes, each with a different degree of time sensitivity. These are Non-real-time (NRT), Real-time (RT), and Isochronous Real-time (IRT) communication.

  • NRT, sometimes referred to as TCP/IP communication, is acyclic traffic such as sensory, diagnostic, or maintenance data transferred at best-effort speed.
  • RT communication is cyclic traffic consisting of high-performance process data transmitted over standard networking infrastructure. This article mainly focuses on the key aspects of RT communication applications.
  • IRT communication is the highest performing type of deterministic traffic within the PROFINET standard. However, this requires hardware-based bandwidth reservation and network-wide clock synchronization to function.

The PROFINET RT and IRT communication classes involve a cyclic data exchange over standard Ethernet and take place directly on Layer 2 without any TCP/IP overhead to minimize latency. This means that in an RT/IRT PROFINET environment, data frames are forwarded based on the devices′ MAC address. Therefore, it is essential that any underlying network infrastructure deployed to support RT or IRT PROFINET applications is fully Layer 2 transparent to all connected PROFINET devices.

The performance of PROFINET-based communication is limited to the performance ceiling of the underlying network infrastructure. To provide the flexibility to operate reliably over the different network infrastructure components, the cyclic data exchange rate for PROFINET RT communication can be customized to accommodate any infrastructure limitations or to suit the automation context.

In the example using the Siemens TIA Portal, the IO cycle > Update time parameter defines the communication update interval between the PROFINET IO controller and the IO devices. The IO cycle > Watchdog time parameter specifies the number of consecutive response failures before reporting a link failure which, depending on the process design, typically triggers the error handling or safe mode, halting the automation process.

WLAN infrastructure considerations

PROFINET (PN) communication can also be realized over a standard IEEE 802.11 wireless connection. While some PROFINET IO (PNIO) devices have built-in wireless client capabilities, the majority of PNIOs only support Ethernet interfaces. In those cases, system integrators will need to connect the PNIO to a wireless client device that acts as a wireless adapter to communicate with the PN controller.

Even though wireless technology has improved over time with every new iteration of the IEEE 802.11 standard, it is important to note that designing a wireless network is inherently more complex compared to fully wired infrastructure.

In order to design and deploy the right wireless solutions to support PROFINET communication, several key aspects of wireless networking need to be taken into consideration. These include L2 transparency limitations, higher latency, and radio frequency (RF) management to configure the wireless environment for optimal performance.

The following section describes the considerations and challenges integrators need to take into account when designing IEEE 802.11 industrial wireless networks for PROFINET-based applications. Typical wireless integration scenarios observed in industrial automation and control systems today rely on external wireless devices to serve as the PROFINET IO′s wireless adapter.

It is important to evaluate these areas systematically when designing wireless networks, in particular when used for critical PROFINET-based control and automation processes. The following sections of this document will outline several typical wireless deployment scenarios, how each of the wireless design considerations relate to different scenarios, and Moxa′s solution to address the challenges presented by each scenario.


Typical P2MP AP/Client connection.

WLAN infrastructure overview

Before evaluating potential WLAN solutions, it is recommended to thoroughly review and map out the requirements of the application first. Since different PROFINET applications require different types of architecture, some variables to consider are:

  • The number of PNIO devices to integrate.
  • The scale of the wireless network (the number of wireless devices to deploy).
  • Device mobility requirements.
  • The need to connect standalone Wi-Fi clients such as personnel smart phones, tablets, and laptops.

Different types of wireless deployments such as Point-to-point (P2P) and Point-to-multipoint (P2MP) topologies commonly adopted in the industrial sector fit into one of two main configurations: AP/Client or Bridge configurations.

AP/Client architecture

Point-to-point (P2P): In a P2P AP/Client configuration, a dedicated wireless connection is established between the PN controller and a single PNIO through an access point (AP) and the client′s wireless interface. In this scenario, the PN controller and PNIO are connected to the wireless devices using Ethernet. This type of topology is usually preferred in situations where bandwidth is not shared between clients and where each wireless connection runs on a different, non-overlapping channel.

Point-to-multipoint (P2MP): In a P2MP AP/Client configuration, a single AP supports multiple clients with each client supporting a maximum of one PNIO connected to it. It is one of the most commonly adopted wireless configurations for connecting multiple clients in a shared bandwidth environment. In some circumstances, the AP will need to serve a combination of PNIO clients and standard Wi-Fi clients such as laptops and tablets.


Typical P2P wireless bridge connection

Bridge architecture

Point-to-point (P2P): In a P2P bridge configuration, a dedicated wireless bridge between a pair of wireless devices is created to connect multiple PNIOs to a PN controller. Since bridge architecture works at the data link layer (L2) of the OSI model, it allows more than one PNIO to be connected to either wireless device over the same bridge.

This topology can be seen as a wireless extension of a wired backbone, bridging the wired devices on both sides of the wireless connection into a single L2 network. A classic example of a P2P bridge connection is the Wireless Distribution System (WDS).

Point-to-multipoint (P2MP): Similar to its AP/Client counterpart, the P2MP bridge topology is a type of deployment that is commonly adopted by system designers that want to integrate multiple mobile PNIOs. In a multi-bridge topology, several wireless bridge connections are established, converging to a single wireless device to connect multiple PNIO systems to a PN controller.

Both the P2P and P2MP bridge topologies are frequently used by AGV machine builders as it allows one wireless device to act as the wireless adapter for multiple PNIOs installed onto the AGV such PLCs for sensors, motors, and cameras.

Whether using an AP/Client or Bridge architecture, PROFINET system designers generally adopt WLAN infrastructure for the benefits of rapid deployment and device mobility. Therefore, candidate wireless solutions are also expected to support seamless roaming to ensure mobile PNIOs can easily move between access points without interrupting the connection.vOnce you have identified a suitable WLAN architecture for your PROFINET application, the following sections will explore the challenges presented by each architecture, and the solutions to address these challenges in order to implement a robust industrial wireless network capable of supporting PROFINET WLAN applications.

WLAN infrastructure challenges

As wireless solutions gain in popularity, they are gradually becoming an integral part of the industrial network infrastructure designed to support PROFINET-based communication. Below is a quick summary of the key points:

  1. PROFINET real-time (RT) communication requires the underlying network infrastructure to be Layer 2 transparent in order to forward data frames correctly.
  2. Wireless infrastructure differs from wired infrastructure. Additional considerations need to be addressed to increase network reliability and availability, such as enhanced functionality to adjust for the additional complications of mobile applications, and RF environment analysis to create a reliable, deterministic wireless network.
  3. The most common wireless installations can be categorized into AP/Client or Bridge architectures. Which topology to adopt depends on several factors including the scale of the network and the number of PNIOs that need to be integrated.
  4. When using standard WLAN solutions in industrial settings, integrators may experience some complications concerning functional requirements.

AP/Client configurations

In AP/Client P2P or P2MP configurations, when a PNIO is wired to a wireless client, L2 transparency ends at the client device′s wireless interface.

This means that when using standard Wi-Fi client connectivity without enhanced functionality, the PN controller will not be able to forward data frames to the PNIO as it cannot identify the MAC address of the connected PNIO. Refer to Appendix 1 for more details regarding this technical limitation.

Challenge 1: Since the communication between the PN controller and the PNIO happens on the data layer, the boundary of L2 transparency must extend beyond the wireless client′s Ethernet interface, while maintaining compatibility with standard AP/Client connections to allow other Wi-Fi devices to connect to the AP.


Typical P2MP wireless bridge connection

Bridge configurations

One example of a typical bridge connection is the Wireless Distribution System (WDS), which, by its technical specification, is a L2 transparent wireless link connecting two APs. A layer 2 wireless bridge setup is preferred in cases where multiple PNIO devices need to be connected to one wireless device.

However, commercial WDS solutions are unable to fulfill the needs of more complex industrial applications. WDS is statically configured and is not designed to support bridge roaming to accommodate moving devices such as AVGs. Furthermore, WDS does not support hybrid bridge/AP functionality by default to serve standard Wi-Fi clients such as laptops and tablets used by on-site engineers.

In cases where multiple wireless bridges are necessary, system designers can set up additional WDS bridge links to create a P2MP configuration. However, each bridge link needs to be configured manually. This makes deploying conventional P2MP wireless infrastructure very time- and resource- intensive and more prone to network issues due to configuration conflicts.

Challenge 2: Commercial wireless solutions have out-of-the-box limitations that make them unable to meet the functional requirements of industrial wireless PROFINET applications.

Bridge devices should support multipoint bridge topologies, bridge mobility, and be able to act as a hybrid bridge/AP to connect additional standard Wi-Fi clients.

Latency, jitter, and RF management

Wireless connectivity occurs through electromagnetic waves that are sent within an ISM band. This physical characteristic of communicating over a shared medium is inherently susceptible to interference from various devices operating in overlapping channels within that spectrum. As a result, WLAN components are more likely to suffer from latency or jitter. The amount of additional latency compared to a purely wired network depends on the type of WLAN technology, antenna performance, and the channel utilization within the network environment.

Therefore, employing a set of best practices when evaluating and configuring the RF environment and wireless coverage are key to yielding optimal wireless performance and establishing the foundation for a more deterministic WLAN infrastructure.

Outlined below are several important RF best practices for reference.

Wireless spectrum:

  • Select the radio bands most appropriate for the application considering the network environment and signal penetration.
  • Reserve the 5 GHz frequency band for critical communication as this band has more channels available and is generally less congested compared to the 2.4 GHz band.
  • Use the 2.4 GHz frequency for farther signal penetration.
  • Avoid configuring Dynamic Frequency Selection (DFS) channels on the 5 GHz band (channels 52 to 140) for critical communications to prevent interference from radar signals.
  • Perform on-site RF spectrum analysis to identify and allocate devices for different applications to free, non-overlapping channels.

Wireless coverage:

  • Maintain an unobstructed line of sight when installing antennas to avoid signal degradation caused by nearby physical objects.
  • Select suitable antennas for the environment to ensure a good signal-to-noise ratio (SNR).
  • However, performing RF analysis and configuration relies on experienced personnel with extensive knowledge of wireless networking. In the industrial sector, it is often difficult to dispatch qualified individuals on a readily available basis.

Challenge 3: RF optimization is a complicated process that relies heavily on highly experienced personnel. Therefore, integrators should look to provide an accessible, easy-to-use solution for on-site personnel with limited WLAN knowledge to perform wireless coverage site surveys and to identify and configure optimal RF channels during installation and maintenance.

Another major benefit of using wireless networks in mobile applications is the ability for wireless clients to roam across different BSSIDs within the network. Roaming involves a client device disconnecting from one access point as it moves out of range and dynamically establishing a new connection with a nearby higher signal quality BSSID. However, this process unavoidably generates additional communication latency as clients constantly transition between APs. Industrial applications can only tolerate a very low margin for latency to ensure smooth and uninterrupted data transmission. As a result, WLAN solution manufacturers are required to optimize their products to mitigate the additional latency generated by the roaming process.

Challenge 4: Wireless mobile applications such as AGV automation and control processes rely on stable and highly responsive networks. Achieving millisecond-level wireless roaming handover times therefore becomes a necessity to minimize latency and avoid impact to operations.

WLAN Infrastructure Solutions

The following section describes how Moxa′s AWK Series WLAN technology help address each of the challenges defined in the previous section.

MAC Cloning

Challenge 1: Extend Layer 2 transparency beyond the client′s Ethernet interface in an AP/Client configuration so that the connected PNIO is addressable by the PN controller.

The AWK Series′ proprietary MAC Cloning technology is designed to extend L2 transparency to a single PROFINET IO device connected to the wireless client by cloning the MAC address of the PNIO to the client it is connected to. By doing so, the PN controller is able to communicate with the PNIO through the client using its cloned MAC address. MAC Cloning can be used in either Auto or Static mode, depending on the application.

Auto: The AWK client automatically copies the MAC address of the PNIO device connected to its Ethernet interface. Only one device should be connected to the client when using this method to avoid MAC address translation conflicts.

Static: The MAC address of the AWK client is manually configured to use the MAC address of the PNIO. This is useful in cases where multiple devices need to be connected to the same client. While this method supports more than one device to be wired to the client, only one PNIO device can be connected to one client at any given time.

Master/Slave Bridge

Challenge 2: Bridge devices should support multipoint bridge topologies, bridge mobility, and be able to act as a hybrid bridge/AP to connect additional standard Wi-Fi clients.

Master/Slave mode is a variation of the wireless bridge mode exclusively available on Moxa′s AWK Series that allows multiple bridges from a single Master device to several Slave client devices, with each Slave client supporting multiple PNIO devices. Moxa′s Master/Slave bridge configuration is simple and intuitive, adopting a configuration process similar to setting up an AP/Client connection. This eliminates the complicated and issue-prone setup procedure that plagues conventional bridge setups such as WDS.

In addition, Virtual Access Point (VAP) functionality can be enabled on the designated Master AWK Series device, enabling it to broadcast its SSID to concurrently support additional standard Wi-Fi client connections.

AeroMag

Challenge 3: RF optimization is a large and complex activity that requires experience and knowledge to execute. Accessible tools should be available to enable less experienced on-site personnel to:

  1. perform wireless coverage surveys to determine optimal deployment and antenna density, and
  2. analyze the wireless spectrum to determine the best wireless bands and channels.

Moxa′s AWK Series features AeroMag technology, which helps alleviate the complexity of RF optimization by automatically configuring basic Wi-Fi settings and performing RF spectrum analysis to identify optimal bands and channels. AeroMag is a useful tool throughout the entire Wi-Fi network lifecycle. During the installation phase, AeroMag helps streamline network operations by dynamically analyzing and adjusting radio channels depending on your current operating environment. When configuring network devices, AeroMag′s one-step setup establishes Wi-Fi connections quickly, significantly reducing deployment times.

Moxa′s AWK Series industrial WLAN AP/Bridge/Clients support AeroMag functionality in AP/Client mode. Once the RF and channel settings are configured using AeroMag, the device can be switched to bridge mode and will automatically carry over the RF and channel settings.

While AeroMag simplifies the RF optimization process, it does not substitute a full analysis of the wireless environment. To ensure maximum availability and deterministic performance, a complete independent site survey should still be conducted to generate the best wireless coverage and most suitable RF configuration for the target environment.

Turbo roaming

Roaming behavior is configured on WLAN clients. Standard WLAN clients without any roaming enhancements usually maintain an established connection regardless of changes in environment or signal quality. This often results in the device disconnecting before attempting to find the next available BSSID. As industrial applications require seamless communication to avoid interruptions to operations, conventional WLAN client roaming solutions are inadequate.

Challenge 4: Establish millisecond-level wireless roaming to avoid any impact to industrial operations.

WLAN Client products support the proprietary Turbo Roaming technology. This function actively scans the wireless environment to identify and roam to nearby APs with optimal signal quality before the original connection deteriorates beyond a predefined threshold. By constantly monitoring and connecting to the best available AP, Moxa′s Turbo Roaming feature increases WLAN reliability and availability through fast millisecond-level roaming handover times.

Turbo Roaming is available in both AP/Client and Master/Slave bridge topologies. A customizable AP signal quality indicator or roaming threshold can be set to cater to different environmental conditions. In addition, the intuitive Turbo Roaming Analyzer utility tool is available to help network designers visualize and confirm that the roaming logic behaves as intended within the set performance margins.

Summary

There are two key challenges to address when attempting to build a stable PROFINET application over a WLAN infrastructure.

  1. The selected WLAN solution must support L2 forwarding in different configurations.
  2. Wireless latency and jitter need to be minimized to meet the requirements of the PROFINET application.

The limitation of L2 transparency terminating at the client level can be overcome by using Moxa′s proprietary MAC Clone and Master/Slave bridge technology.

The remaining challenge that comes with designing a stable PROFINET application over WLAN Infrastructure is to account for the impact of additional latency from wireless communication and roaming activities. Moxa′s AeroMag functionality simplifies RF configuration by performing automatic spectrum analysis and channel optimization while Turbo Roaming enables millisecond-level roaming capability.

Tony Chen, Product Manager, Moxa.


Source: Industrial Ethernet Book Issue 120 / 13
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