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Sub-GHz ISM Band: Emerging Standards Promise Smart City Connectivity

The Sub-GHz ISM band will enable connected smart meters and street lights, but which standard will they use? Analog Devices and Weightless SIG provide the answers.

By Sally Ward-Foxton, European Correspondent

While the exact wireless technology that will be used to connect smart city infrastructure is still under debate, the industry seems to have reached a consensus that the sub-GHz ISM band will play host to this communication. Similar bands are available for open, free use in most of the important markets around the world, which is an important first step; in Europe, the 868MHz ISM band is available, while it’s at 900MHz in the US and 920MHz in Japan.

This band’s key advantage over the very widely used 2.4-GHz ISM band lies in its physical propagation characteristics. Because of the lower frequency of transmission, signals with the same power propagate two to three times further than at 2.4GHz, even through buildings. This makes it ideal for smart neighbourhood-type applications such as smart utility meters, smart street lighting and other infrastructure.

Smart Utility Meters

Leading the wireless standards race for utility meters in the Sub-GHz ISM band is the Wi-SUN (Wireless-Smart Utility Networks) Alliance. Analog Devices’ James Frame, a board member of the Wi-SUN Alliance, explained that the first protocol will be certified in around six months, and when that happens, over half the smart electric meters in the US will already be compatible. Backwards compatibility will also affect many installed units in Europe, giving Wi-SUN a large installed base.

The Wi-SUN protocol is specifically designed for smart utility meter networks and uses the Sub-GHz ISM band.

The Wi-SUN protocol is specifically designed for smart utility meter networks and uses the Sub-GHz ISM band.

As well as utility meters, Frame also envisages Wi-SUN street lighting units, which double as connected nodes that can have other, smaller systems connected to them. Having a standard to which all the units can be certified will ease the integration of smart city systems.

“In the sub-GHz space today, there is no standards compliant protocol. Our goal is to put in place compliance and standardisation for 6LoWPAN over sub-GHz,” Frame says. “Just because you have a standard means nothing about interoperability, nothing about quality. The goal of the alliance is now to put all that in place so you can have interoperability.”

Will Wi-SUN compete with other standards out there for the Sub-GHz band? Frame thinks there is room for a variety of different standards for specific applications.

“Everybody wants wireless sensor networks to be ‘one size fits all’, but the applications are all different. What goes into a residential security system doesn’t go well into a street lighting system,” he says. “We’re trying to address some of those differences.”

Weightless-N

Another emerging wireless standard for the internet of things, Weightless, recently announced a shift in focus from US-only TV white space (400-800MHz) to the global sub-GHz ISM band. The standard that was previously developed, Weightless-W, will now come in another flavour, Weightless-N. Since only the physical layer differs between the two, Weightless-N should be completed by the end of Q1 2015.

Weightless is an open standard, based on the same model as Bluetooth, which eliminates the traditional IP royalty payments that contribute to the high cost of proprietary technologies. The idea is to provide a level playing field for adopters, encouraging multi-vendor competition to continue driving down the cost and stimulate progressive evolution, explains Alan Woolhouse, spokesperson for the Weightless SIG.

“Some companies with proprietary technologies claim that early traction, if continued, would ultimately lead to a ‘de facto’ standard by virtue of the share of the market that they are able to capture,” he explains. “For one thing, it is far too early in the game with a tiny, tiny fraction of the projected connectivity market for IoT to be claiming any kind of sustainable lead in market share. And secondly, an open standard is patently a very different thing – an entirely distinct business model.”

What’s Coming Next?

Even once the standards have been ratified, there will remain several key issues to be solved. For example, with multiple standards emerging in a band that is much narrower than 2.4GHz, is there enough bandwidth to go around?

“It is inevitable that spectrum – in all frequency bands – will become more crowded,” says Woolhouse. “It’s a finite commodity and the demands on it will continue to increase. It’s a question of how technologies are developed to manage congestion.”

Woolhouse points out that there are several techniques that can be used to offset this, such as scheduled transmissions, frequency hopping, avoiding congested channels and adaptive transmission power schemes.

Another potential problem lies with the system-level issue of security, essential if IoT systems are likely to be widely adopted.

“I think within the next couple of years, security will become the dominant issue, of how to do a good security system at low power and without a lot of data exchange,” says ADI’s James Frame, adding that security is especially important when remote sensor nodes are tied to real-world physical elements, since this makes them attractive to hackers. “What I fear is that the first devices out there will have a level of security just below what is needed, but they’re going to set a price point that is below where it should be to deliver what’s actually required.”

Ultimately, the standards landscape in the sub-GHz band is still developing, particularly with respect to smart city infrastructure. Potential issues, such as overcrowding and security, may be mitigated with a combination of careful planning and well-thought out technology. The eventual situation will likely be a combination of proprietary and open standards used in application-specific niches.

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