Digital & Communication Technology
technology to better address the low-latency and high-reliability requirements of industrial use cases. Key enhancements that enable Wi-Fi technology to play an essential role in an industrial network are discussed below. OFDMA and Trigger-based Access: One major feature of Wi-Fi 6/6E is Orthogonal Frequency Division Multiple Access (OFDMA). OFDMA enables concurrent communication between an Access Point (AP) and multiple stations by assigning a subset of subcarriers, called a Resource Unit, to each station. A key benefit of OFDMA is reduced latencies in dense deployment scenarios. With Trigger-based uplink OFDMA, an AP sends a trigger frame to client stations to coordinate uplink client transmissions thereby avoiding collisions. This is a departure from prior generations of Wi-Fi where uplink packet transmissions are performed using a random- access scheme. In such schemes, latency degrades as the channel load increases. With Trigger-based OFDMA, Wi-Fi 6/6E takes an evolutionary step towards achieving scheduling and determinism. Since OFDMA reduces the rate of collisions and schedules the uplink (UL) transmissions from STA, we can expect much improved latency and reliability compared to Wi-Fi 5. This is illustrated in Figure 2, which compares the latency experienced by real-time application traffic under various protocol options.
In Figure 2, an AP is loaded to 80 per cent channel-utilization and the uplink latency of 8 bi-directional voice calls (20ms period) is measured. With the Wi-Fi 5 protocol, congestion and associated retries are shown to lead to high latency (>100ms) which exceeds the delay budget for a typical voice call. With Wi-Fi 6 and a naïve AP scheduler,
Figure 2: Wi-Fi 6 Latency advantages (Uplink)
this latency is bounded to 100ms which is improved but still high. However, when a latency-optimized Wi-Fi 6 AP scheduler is used, the latency is strictly bounded to 25ms which is 400 per cent better than the latency achieved by Wi-Fi 5. GHz Spectrum: Wi-Fi 6E extends the operation of Wi-Fi 6 into the new 6 GHz band, which offers anywhere between 500 MHz to 1200 MHz of clean congestion-free spectrum. The 6 GHz band supports greater capacity and wider channel bandwidths, bringing faster speeds and lower latency, and is also unencumbered by legacy Wi-Fi devices. All these benefits make the 6 GHz band particularly suitable for industrial deployments.
A much underpromoted advantage of Wi-Fi 6E in the 6 GHz band is the restriction to Wi-Fi 6-only devices. This means all clients could potentially be scheduled under OFDMA leading to significant latency benefits and improved reliability (since collisions are limited).
Wi-Fi Time Sensitive Networking (TSN)
Time Sensitive Networking (TSN) profiles are defined by the IEEE 802.1 TSN Task Group. TSN standards define tools and mechanisms to facilitate time synchronization, traffic shaping, transmission scheduling, redundancy, etc. As described previously, the recent advances in Wi-Fi connectivity have created the framework to extend TSN capabilities over Wi-Fi.
Wi-Fi TSN enables deterministic delivery of data that meets a bounded latency criterion. TSN tools can be integrated with core Wi-Fi capabilities to enable time synchronization and bounded low latency which are essential
Figure 3: Wireless TSN
to meet the needs of industrial networking. Time Synchronization:
In a TSN capable system, time performance is critical, therefore time synchronization is a basic requirement. Time synchronization is provided by IEEE 802.1AS standard which defines a profile of IEEE 1588 Precision Time Protocol to distribute time across the network. For this feature to operate over Wi- Fi, support from the MAC layer is required. In the MAC layer, Fine Timing Measurement (FTM) can be used to support 802.1AS.
Traffic Shaping:
In a Wi-Fi network, time sensitive traffic and Best Effort traffic might have to share
the wireless medium, thus making time- aware traffic shaping necessary. To support deterministic data delivery, Wi-Fi TSN system should identify, prioritize, and deliver time- critical data within time windows that are defined according to a network schedule. Traffic shaping over Wi-Fi is provided by 802.1Qbv, which defines a set of timed gates to control multiple traffic queues, creating a protected window for time critical traffic. Features like Triggered Access can be used to support 802.1Qbv over Wi-Fi.
Conclusions and future directions In this article, we explored how Wi-Fi 6/6E can be used in key industrial use cases to connect machines, people, products, and services, in order to create greater value for manufacturers and their customers.
While Wi-Fi 6/6E products are poised for accelerated adoption in the market, the next generation of Wi-Fi, based on IEEE 802.11be standard, will provide enhancements that further benefit industrial applications. Wi-Fi 7 will provide improved QoS through features such as Stream Classification Service (SCS) and restricted Target Wake Time (rTWT), and will enable redundancy through multi-link operation and multi-radio devices. The Wireless Broadband Alliance members, led by Cisco and Intel with other work group participants, will be working in early 2023 to develop trials based on the identified key scenarios and creating a trials report including recommended profiles for IIoT and enterprise applications.
https://wballiance.com/ 38 December/January 2023 Components in Electronics
www.cieonline.co.uk
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