For IoT Over WiFi, 802.11ax Is the New HaLow
The Wi-Fi Alliance endorsed IEEE 802.11ah for IoT connectivity nearly 18 months ago and dubbed it HaLow. The standard operates in the unlicensed frequency spectrum below 1 GHz and in narrow channel widths of 1 and 2 MHz. Since IoT applications don't need large data pipes, HaLow provides for data rates as low as 150 Kbps. It has protocol provisions for the long sleep times for clients, because many IoT applications only require intermittent data transfers. Altogether, HaLow provides for lower power consumption, lower complexity and longer range for IoT clients.
So why has HaLow not seen much uptake? For one, HaLow’s frequency band of operation is different from the 2.4 GHz and 5 GHz bands used for WiFi network access. This requires WiFi access points have a separate radio to support HaLow, which increases AP costs. Further, the HaLow spectrum is not harmonized across the globe: HaLow operates at 900 MHz in the US, 850 MHz in Europe, 700 MHz in China, and does not even have the operating spectrum in many countries. These factors are likely the reason that the response from WiFi chipset manufacturers to HaLow has been underwhelming so far.
There is much talk about the new WiFi standard for network access, IEEE 802.11ax. Every discussion about 802.11ax focuses on its primary goal: Improving the air efficiency of WiFi networks, particularly in high-density situations. That said, 802.11ax also has some interesting provisions to support IoT. Moreover, 802.11ax is not hostage to the factors that are slowing down HaLow. Here are some of the benefits it offers IoT:
1. Established frequency bands of operation
802.11ax works in the 2.4 GHz and 5 GHz bands, which are the existing WiFi bands. As a result, a 802.11ax AP does not require a separate radio for IoT like HaLow does. Also, these bands are available globally as unlicensed bands. There is great enthusiasm among the WiFi chipset manufacturers for 802.11ax as it is evolutionary to 802.11ac for network access. They have already announced 802.11ax chipset products. The question then is whether802.11ax will meet the low power and low client complexity requirements of IoT.
2. Low power and low complexity
802.11ax has introduced a feature that allows clients to operate in 20 MHz-only mode. This is explicitly targeted to IoT applications. The AP talks to such devices in the primary 20 MHz part of the channel, even if the AP uses higher bandwidth channels for network access clients. In addition, the orthogonal frequency-division multiple access (OFDMA) transmission method introduced in 802.11ax allows transmissions to happen in smaller blocks of about 2 MHz resource units (RUs) within the 20 MHz channel. This was not available in the orthogonal frequency-division multiplexing (OFDM) method used in 802.11ac.
With OFDMA, it is possible for the AP to talk to the client device (and vice versa) at data rates as low as 375 Kbps, by using only one 2 MHz RU within the 20 MHz channel width. The 2 MHz transmission can also use a simple transmission error resilience technique called Dual sub-Carrier Modulation (DCM), which basically repeats the same information on two tones to reduce signal processing complexity while improving signal quality. The long sleep time support for clients that HaLow introduced is available in 802.11ax as well. These features mean that 802.11ax can offer the same benefits as HaLow for client power consumption and complexity.
3. Range considerations
As for the range, HaLow has some benefit over 802.11ax. HaLow operates at a lower frequency than 802.11ax and lower frequency means longer range. Also, the sub 1 GHz band where HaLow operates has better penetration characteristics through obstacles such as walls. So, when one considers wide area IoT as in cellular networks, HaLow meets the criteria. However, there are other low-power WAN (LPWAN) technologies such as LTE-M, LoRa and Sigfox that are gaining momentum in long-range IoT, whereas uptake of HaLow remains lackluster.
Overall, 802.11ax is better suited than HaLow for local range IoT. The expectations for 802.11ax are high due to its network access enhancements, which will naturally provide secondary benefit of IoT enablement. HaLow may be reduced to only wide area applications, but again its success there against competing LPWAN technologies is questionable. HaLow may never see much light of the day, whereas 802.11ax could become the de-facto WiFi standard alongside BLE and ZigBee for home, enterprise and other indoor IoT applications.
Dr. Hemant Chaskar has 18 years of industry experience, working with large corporations and startups, in R&D, products and business functions. Hemant is a technical expert in wireless, networking, and security, and for more than the past decade, has been deeply involved in enterprise WLAN technology, spearheading the development of access, intrusion prevention and cloud based solutions.
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