Wireless

Wi-Fi for sensor networks

Commercial technology is rapidly ripening for consumption in industrial facilities.

By Ian Verhappen

Wi-Fi technology, as defined by the IEEC 802.11 standards, is continuing to evolve, and with each release becomes faster, stronger and more capable, as well as an increasingly viable option as a wireless sensor network (WSN) physical layer alternative. China has released IEC 62948:2017, a factory automation (WIA-FA) WSN based on Wi-Fi networks, so the option is available as an international standard with products available—at least on the Chinese market.

The accompanying table summarizes how the Wi-Fi standard has evolved over the past decade, adding more data capacity as well as improved data propagation capabilities to improve reliability and support for multiple users. The new features are interesting from an engineering perspective, but what matters is how it improves the wireless experience and, in our case, how it will help us run our facilities.

OFDMA allows a traditional 20 MHz channel to be partitioned into as many as nine smaller channels, meaning a Wi-Fi 6 AP could simultaneously transmit smaller frames, as would be the case for a low-bandwidth data transmission to nine Wi-Fi 6 clients. For an uplink, OFDMA allows data frames to be transmitted simultaneously by multiple stations, while in the opposite-direction downlink, OFDMA allows multiple data frames to be transmitted in a single data unit to multiple stations, in both cases amortizing preamble overhead and medium contention overhead, which leads to high aggregated network throughput. Downlink OFDMA can further optimize aggregate throughput by balancing the allocation of power between users at high versus low signal-to-noise ratios.

The new Multi-TID aggregated MAC protocol data unit (Multi-TID AMPDU) feature allows the aggregation of frames from multiple different traffic identifiers (TIDs) from the same or different quality of service (QoS) requirements within a single transmission; again, giving devices extra flexibility to aggregate more efficiently, reducing overhead and increasing throughput and overall network efficiency.

Wi-Fi 5 uses a maximum symbol constellation size of 256-QAM. With Wi-Fi 6 support for 1024-QAM, up to 10 bits of information can be sent during a given transmission, meaning over a short range, 1024-QA, it provides increases in throughput by 25% over a Wi-Fi 5 installation.

Operation mode indication (OMI) provides an efficient way for client devices to signal the maximum number of space-time streams and maximum bandwidth they will use to transmit and receive. Similarly, the BSS (basis service set) coloring technique is an enhancement of the black list/white list concept that means, rather than assuming whether or not a nearby signal on the same channel is likely to be a source of interference, a given transmitter can determine the likelihood of potential interference, thus improving spatial reuse by allowing the use of that adjacent channel if it is available.

These two features again increase overall device power efficiency, while also permitting more frequent changes to the transmit and receive operating mode.

The final new feature of interest to control and WSN /IOT applications is support for individual target wake time (TWT) that allows scheduling of traffic exchanges between an AP and a client device. TWT reduces the overhead and inefficiency of the channel access method for obtaining transmit opportunities and allows power-saving client devices to reduce power consumption by explicitly identifying the times when they should be awake.

A Far Point paper (see references below) predicts that, although Wi-Fi 6 products are only coming to market later this year, it will support 50% of global connected devices, including cell phones and IOT devices, by 2022. Wireless device developers, especially for handheld devices such as cell phones, and Far Point also predict that when possible, new equipment will support seamless transfer between Wi-Fi 6 and 5G networks, reducing the load on the 5G networks while also providing large area coverage.

Once again, the line between IT and OT technologies continues to blur with the benefit of lower-cost, widely available technology being countered with the need to more effectively manage the boundaries between the systems. Fortunately, the developers of the Wi-Fi and 5G technologies have factored this into their developments.

References

  • “Wi-Fi 6 Arrives: The Future of the Organizational LAN,” A Farpoint Group White Paper, Document FPG 2019-4104.1, April 2019
  • “Wi-Fi 6: High Performance, next generation Wi-Fi,” Wi-Fi Alliance White Paper, October 2018