The role of sensors in a mission-critical business
Many businesses are critical to our daily lives. Delivering services such as water, electricity, gas and telecommunications reliably is essential. Many of these industries have extensive infrastructures – pipelines extend across the country, local telecoms exchanges are in every major town and city, treatment plants have long perimeter fences and so on. Delivering a reliable service involves monitoring the infrastructure and responding rapidly to issues.
Of course, there are varying degrees of criticality. Preventing overload in an electricity sub-station is much more important than resolving a minor leak in a water pipeline. The more critical
an element is, the more the organisation will be prepared to pay to deliver reliability.
Single point-of-failure items may require dedicated wired communications links to the element – such as a direct wired connection to an electricity sub-station. For items that are important, but where short delays in reporting can be tolerated, lower cost wireless solutions can be used. For example, pipeline monitoring might make use of multiple sensors with wireless connectivity placed along the length of the pipe.
Wireless as a solution to monitoring
Wireless is sometimes seen as insufficiently reliable for mission-critical applications, but there may be many areas where it is the only viable solution. In such cases a wireless link, even if unreliable, will provide greater monitoring capability than no link at all.
In other cases it can allow more detailed monitoring than would be economically viable with wired communications. It is then important to understand the reliability that can be tolerated, the capabilities of the wireless communications systems and to design appropriate processes that take these into account.
Some of the key parameters of a wireless system in this context are availability and latency. Availability indicates the percentage of time that a message can be expected to be successfully delivered and typically depends on network loading and in some cases varying signal strengths. Latency indicates how long it might be from the message being passed to the wireless system to it being received in a control room or similar.
As can be imagined, as the requirements for availability increases or the allowable latency decreases then the costs of the wireless solution typically grow. If extremely high availability is required then often the only solution is a self-owned private radio link which can be engineered to the level of availability required, but such solutions come at a high cost.
Previously, the key solutions available for wireless connectivity of sensors were self-owned simple data links or the use of cellular (often GPRS). However, there is a new solution in development and due for deployment in 2014 which could change the dynamics of monitoring critical infrastructure. This system is designed specifically for machine-to-machine (M2M) connectivity
and is called Weightless.
The need for a new technology
Machine communications, often termed M2M, has long been forecast to be a sector with massive growth. Over the last few decades many have noted that the installation of a wireless connection into myriad devices would bring a range of benefits.
An enormous range of examples have been suggested, from cars to sensors to traffic lights to healthcare applications and much more. But the market today is only a tiny fraction of the size it has long been predicted to grow to. This is predominantly due to the lack of a ubiquitous wireless standard that meets the needs of the vast majority of the machine market. These needs include:
Low cost, both of the hardware and the service. Many machines are individually of relatively low value – imagine for example a temperature sensor. Chipset costs need to be in the region of $1-$2 and annual service charges less than $10 to make it worth embedding wireless technology
in such devices.
Excellent coverage. To make applications such as smart metering viable there needs to be coverage of near 100% of all meters. With many meters deep within the home or even in basements this implies vastly better coverage than achieved with today’s cellular networks.
Ultra low-power operations. Many machines are not connected to the mains and so have to operate on batteries. Having to change the battery is at best an annoyance and at worst a significant expense. Battery life of ten years or more is essential.
Secure and guaranteed message delivery. While machines rarely need ultra-rapid transmission, they do need to be certain that messages have been received and that security of the system has not been compromised in any way.
While the needs of the machine sector have long been understood, the key problem to date has been a lack of insight as to how they could be met. Ubiquitous coverage requires the deployment of a nationwide network, and the conventional wisdom has been that such networks are extremely expensive.
For example, a UK-wide cellular network can readily cost $2 billion with costs of spectrum adding another $1-2 billion. With the machine market unproven, such investments were not justifiable and would result in an overall network cost that would not allow the sub $10/year subscription fees needed to meet requirements.
The key to unlocking this problem is free, plentiful, globally harmonised low-frequency spectrum. It needs to be free, or at least very low cost, to keep the investment cost low. It needs to be plentiful to provide the capacity to service billions of devices.
It needs to be globally harmonised in order to allow devices to roam across countries and to enable the economies of scale needed to deliver <$2 chipsets. It needs to be low-frequency to enable good range from each base station and therefore a relatively small number of base stations to provide ubiquitous coverage.
In the last few years, a new option has emerged for spectrum access. This is the use of the ‘white space’ spectrum – the unused portions of the spectrum band in and around TV transmissions. Access to white space provides the key input needed to make the deployment of a wide-area machine network economically feasible.
Weightless – the standard designed for M2M in
Designing the standard for M2M in white space requires many trade-offs and iterations. A key starting point is the conflict between excellent coverage requirements and yet low-power constraints both due to white space regulation and the need for long battery life in terminals.
The only way to achieve long range with low power is to spread the transmitted signal. Hence, variable spreading factors from 1 (no spreading) to 1024-fold are a core part of the Weightless specification. Spreading is essentially a mechanism to trade range against throughput, so using high spreading factors can achieve significant range extension but at the cost of lower data rates.
Happily, there is sufficient bandwidth in the white space frequencies, and M2M data rates are sufficiently low that more than adequate capacity and throughput can still be achieved, even with high levels of spreading.
Use of the white space spectrum does not provide guaranteed uplink and downlink pairing, making TDD operation essential. This in turn leads to a frame-structure with a downlink part then an uplink part which repeats periodically. The maximum spreading factor informs what this repetition should be.
Simple calculations show frame lengths of around 2s are optimal. This would be overly long for person-to-person communications, with such a delay being highly annoying, but is not an issue for M2M communications (machines do not generally get annoyed!).
Finally, M2M traffic is often characterised by very short messages, for example a 30-byte smart meter reading. The MAC protocol is designed to add minimal signalling overhead to such messages to avoid highly inefficient transmission. This is done through flexible small packets with highly optimised header information.
A global standards body – the Weightless SIG – has been established to take the Weightless standard as a royalty-free fully open standard. It was published in a final ‘version 1.0’ form in April this year.
Weightless would appear ideal for many mission-critical applications. With ‘fit and forget’ radios that have battery lives of 10 years and cost almost nothing, monitoring of extensive infrastructure becomes possible. With a design specifically for M2M, very high availability can be achieved. And with national coverage expected to be provided by 2015, it can be used even in remote areas.
About the author: Professor William Webb is an IEEE Fellow and CTO of Neul, a Mobile Wireless Data Service Provider. Prior to joining Neul, Professor Webb was Director of Technology Resources at Ofcom, the UK communications regulator.