Can’t I just put everything on a single wireless network?

Especially on very large projects with complex monitoring requirements, it’s tempting to try and deploy a single wireless communication technology to support all of the required instrumentation. After all, this should reduce the number of vendors and complexity of the monitoring solution, shouldn’t it? Especially since the requirements typically include routing all data to a single data visualisation and alerting platform. In practice though, attempting to use a single wireless technology for a broad mix of sensors results in significant cost and performance compromises which are difficult to ignore. 

What do you really need from your wireless network?

Big construction projects, such as those involving urban transportation, often have a complex set of requirements for monitoring structures, other assets, and ground conditions in the surrounding area. The scope of such projects can involve thousands of instruments, and tens of thousands of data variables, all which must be tracked, reported and stored. The whole project is likely to extend over a huge geographic area, at least tens of kilometres long, with areas of concentration around interchanges or stations. 

The monitoring plan is usually developed by consulting engineers, offering a detailed set of requirements, covering: 

• The type of measurements (e.g. crack measurement) 

• A resolution requirement (e.g. 0.1mm) 

• Sometimes, an expected measurement range (e.g. -2mm to +20mm) 

• Where measurements must be taken 

• When measurements must be taken (e.g. monthly during the baseline period, every 30 minutes during construction phase) 

• Measurement thresholds which will trigger actions, and which actions these must be (e.g. if a crack opens more than 5mm from baseline, increase reading frequency to 5 minutes; if it opens more than 10mm from baseline, immediately stop construction work in the area for analysis and possible remediation) 

Sometimes monitoring plans attempt to specify the measurement technology to use, so instead of just stating that crack measurement is required, it may specify a vibrating wire crack sensor, or even go so far as to specify a make and model of sensor. This approach can severely limit the scope for the monitoring team to manage costs and choose the most appropriate equipment for the project. 

A further typical requirement from the project owner is that the huge number of measurements being taken all need to be passed into a single data visualisation and alerting platform, allowing coherent “single pane of glass” presentation of the state of the monitored assets and ground conditions. This platform is typically required to be remotely accessible to allow engineers to access the data immediately in the event of an alert.  

Practical considerations must be added to the above requirements in order to implement and maintain the system over the life of the project. These are typically identified by experienced monitoring system providers during the tendering process. 

Building sites are often harsh environments for cabling. Dealing with the weight, effort, and cost of running cables to each individual sensor on the site has become a thing of the past over the last decade with the introduction of practical wireless remote condition monitoring systems designed for this industry. Eliminating cables substantially reduces the risks associated with damage to the monitoring system during the life of the project; damaged cables are time-consuming to repair, and access may be difficult, requiring interruption to the construction work. 

Mains power can often quite unreliable (especially if provided by on-site generators) or non-existent in building projects, so it is common to require automated monitoring equipment to be self-powered. Even if mains power is available at the site offices, it is typically not cost-effective to run power cables to all areas where automated monitoring is required. Small devices are expected to operate on a non-rechargeable battery for multiple years, while some devices with higher power requirements will use rechargeable batteries which are topped up using small scale, dedicated solar panels or wind turbines. 

Access to reliable hard-wired communications infrastructure (i.e. copper or fibre optic broadband) is seldom available on building sites. Similar to mains power, even if it is available at the site office, it’s unlikely to be cost-effective to extend this network to all areas where automated monitoring is required. For these reasons, cellular networks (e.g. LTE or 5G) tend to be the preferred method for monitoring equipment to communicate back to a central server which is located off site. 

Understanding the Role of Monitoring Instrumentation 

The monitoring specification typically specifies a wide variety of instrumentation, of varying complexities. For the purposes of identifying supporting infrastructure requirements, it is helpful to separate these into some broad categories. 

Wireless Monitoring Nirvana 

In a perfect world a monitoring system would meet all these requirements:

• Support a broad mix of instruments 

• Avoid cables 

• Not need mains power or battery changes during the project 

• Not need hard-wired communications infrastructure 

• All feed data back into a single data visualisation and alerting platform which is hosted off site 

It is sometimes assumed that the best way to approach the situation is to build out a single wireless technology across the whole site. Each base station would be supported by the cellular network and powered by solar or wind power, so there are no cables anywhere! 

Wireless Technologies 

But which wireless technology would one choose for such a system? 

A number of different factors must be considered; some obvious (radio range, power consumption) and others which might be less obvious (whether message delivery is made reliable by the use of acknowledgements and retries.) 

For Simple instruments which have low power requirements and generate only a few thousand samples per day at most, power consumption is typically the most important consideration. These instruments are now most commonly deployed as a highly integrated product, with a battery, processing and communications integrated into a single unit to reduce cost and maintenance requirements.  

Instruments of Moderate complexity which have more demanding power requirements but still relatively low data rates are normally supplied separately from their power supply and communications solution. This opens the door to a variety of power supply and communication solutions to be integrated by the equipment installer. 

The most Complex instruments which generate lots of data are always deployed with an external power source (either mains or solar) and often also have built-in high bandwidth wireless interfaces like cellular or WiFi. 

A selection of technologies are included in the following table. (Note that all figures are orders of magnitude only to simplify comparisons, especially where the wireless performance or capacity varies due to regional settings.) 

It can be seen from the table above that there is no single best wireless technology to suit all instruments.

How would a single wireless network technology look?

Any wireless gateways to be deployed specifically for the project cannot be for LTE or Sigfox as they are impractical to operate independently of mains power. Further, unless the density of monitoring is very high along the full length of the transport corridor, we can assume that full coverage is impractical to achieve with a series of 2.4GHz networks.

The last technology standing after this point is LoRaWAN. Senceive uses LoRaWAN as the underlying technology for our GeoWAN range of sensor nodes, so we are well aware of its attendant advantages and disadvantages.

Unfortunately, LoRaWAN cannot meet all the demands of the instrumentation for a complex construction project, as it:

• cannot support the high data demands of Complex instruments like automatic total stations and cameras
• does not have reliable delivery; packet loss increases with more devices and more frequent sampling rates as they ‘talk’ over the top of each other; this can only be partially alleviated by installing large numbers of additional gateways
• is unable to support sampling rates faster than approximately 1 minute for Simple sensors, and 15 minutes for sensors of Moderate complexity due to the regulatory limitations of the unlicensed spectrum (transmission duty cycle limits at the node)
• has inherent delays in downlink messages from the server through the gateway to the sensor nodes; a simple reconfiguration task to increase sampling rates across a large number of sensors can take many hours, this is partly due to the regulatory limitations of the unlicensed spectrum (transmission duty cycle limits at the gateway)
• does not permit local-level communications which allows important additional features like fast triggering and reporting for higher risk assets like earthworks and rail tracks

Perhaps there are some monitoring zones in the which are low risk and for which it is extremely unlikely that the requirements over the full multi-year construction period could hit the above-mentioned limits? In any case, this is not a full solution as it leaves out the complex instruments.

What if we add a second network for the complex instruments?

A potential alternative option could be to introduce LoRaWAN gateways across the whole project, and have high bandwidth devices operate on their own parallel network. The only two candidate network technologies which support the higher bandwidth requirements are LTE / 5G cellular, and WiFi.

WiFi only has a relatively short range in the order of 100-200m in good conditions. To blanket the whole site with WiFi coverage is therefore likely to be too expensive and impractical.

This leaves LTE / 5G cellular, though it is important to keep in mind that only Mobile Network Operators can offer coverage in their licensed spectrum, so any black spots in areas requiring monitoring should be identified and considered early. (It also opens the door to any gateway with cellular backhaul to be added to the site where needed, obviating the need for the blanket LoRaWAN coverage.)

This is all too expensive! What’s the alternative?

First, it’s worth noting that allowing a diversity of wireless technologies would help to encourage purchasing the best equipment to meet the specific requirements for each part of the monitoring project.

Simple instruments, which are typically small and highly integrated, can be purchased with a companion solar powered cellular gateway to cover its area. Extra gateways can be installed in areas where more instruments are present, avoiding wasteful overbuilding of the network by default. The cost of the gateways will be marginal relative to the cost of the rest of the instrumentation and installation labour.

Complex instruments can use cellular network coverage on the site using existing connectivity solutions available for such instrumentation, either integrated or external. If many such instruments are clustered in a small zone some minor cost benefits may be achieved by introducing a WiFi access point with cellular connectivity, but this introduces a single point of failure shared by several instruments.

Instruments of moderate complexity can be adapted on a case-by-case basis to suit the other instruments in the area.

Network technology and topology can be easily adjusted to suit the risk level of each zone (whether or not to have backup gateways, use a reliable or best effort network, use a roaming SIM for cellular network redundancy, etc.)

There is no obvious downside other than one simply noting that it is not a single system.

Finding the Right Balance for Your Project

Clearly there is no single wireless technology suitable for all of the instrumentation types required on these types of projects. Attempting to force a one-size-fits-all wireless approach on all instrumentation across the whole project for apparent simplicity, would still force special case exemptions for instrumentation which cannot be supported by the long-range wireless infrastructure, somewhat negating any purported benefits.

It is also likely to lead to procurement of equipment which is poorly suited to the particular requirements of high-risk zones. High risk assets require wireless networks which are up to the task, with reliable message delivery, faster reporting, and low latency in both uplink and downlink directions.

At a technical and commercial level, the most efficient selection and most effective operation of monitoring equipment is more likely to be achieved if the right equipment and wireless technology is selected for each monitoring zone. This approach optimises the equipment for the requirements of each zone and avoids under- or over-provisioning wireless network capacity.

By embracing a flexible, multi-network approach, asset owners and engineers can have monitoring systems that are not only more efficient and resilient but also capable of addressing the varied challenges of modern infrastructure projects. This strategy can lead to better outcomes for all stakeholders.