It’s now widely agreed that street lighting is a good starting point for bringing IoT solutions into a town or city. “Smart street lighting has long been viewed as the first major use case for smart cities”, states leading analysts Northeast Group in a 2019 market report, “with the communications and software platforms put in place serving additional smart city applications.”
Connectivity, of course, is what makes smart street lighting smart. Connected streetlights have the ability both to receive data from and send data to a central management system, or CMS, which is typically in the cloud. As a street lighting manager or operator, you can remotely monitor and control connected light points, create dimming schedules, roll out firmware and software updates, and track operational data such as energy usage and lamp burn hours. With sensors embedded in the street lighting system, you can also gather real-time and historical data on a wide variety of conditions, from temperature and air quality to noise levels on the streets and local automobile activity.
Although most connected street lighting systems work in a similar way, there are many different methods of exchanging data between connected streetlights and the CMS. These include cellular (2G, 3G, LTE, 5G, and NB-IoT), RF mesh, LoRa, Wi-Fi, and other medium- to long-range solutions.
As a municipality prepared to bring connected lighting online, how do you determine which communications method is best? The answer, for better or worse, is “It depends.” You have to understand how the different communications methods work, then match their capabilities to your specific needs.
How cellular and RF mesh systems work
Let’s look at two of the most popular and effective communications architectures for connected street lighting: cellular and RF mesh.
In a cellular system, each streetlight connects directly to a mobile network operator’s public cellular network for two-way data communications with the CMS in the cloud.
In an RF mesh system, on the other hand, a certain number of adjacent streetlights—typically 15, but as many as 32—create their own device-to-device network, allowing them to share data with one another. The streetlight with the strongest signal in the group sends all information to a gateway, which is usually a device installed inside a NEMA-rated enclosed cabinet somewhere on the street. Communications between the gateway and the CMS can either be a wireless cellular connection, or a wired connection via Ethernet LAN or fiber.
Both cellular and RF mesh offer the set of capabilities necessary for good street lighting operations—including range, throughput, and reliability—but they have different pros and cons which deserve careful consideration. Here are a few.
Licensed vs. unlicensed frequencies
One important consideration is type of network: licensed or unlicensed.
Cellular networks use licensed frequencies employed by mobile network operators (MNOs), such as Verizon and AT&T. The good news here is that the MNOs pay for the licensed spectrum, the technology is standardized and proven, and the networks are managed and maintained by the MNOs themselves.
Unlike cellular, RF mesh uses unlicensed communications frequencies (at 868 or 915 MHz). This means that you can create your own private network wherever you like, as long as the network complies with existing standards and the communications frequencies are approved in the country of use.
One potential disadvantage for cellular networks are periodic changes in standards. If your system uses the 2G network, for example, you would need to modify or upgrade your hardware if the MNO decides to phase out the 2G network in your city. Similarly, you would need a hardware change if you wanted to take advantage of new 5G capabilities when they come online. But major network changes like these occur very infrequently, and they happen very slowly when they do, so there’s little benefit in delaying a needed implementation to align with a future shift in cellular standards.
Some providers claim that, as 20-year networks, mesh networks are less subject to changes in standards or technology than cellular networks. In practice, however, the lifetimes of cellular standards are comparable: 2G was introduced almost 30 years ago, 3G almost 20 years ago, and 4G over 10 years ago, and they are all still available as of today.
System uptime and reliability
Both cellular and mesh networks are reliable in terms of system uptime, but the potential impact on your organization is very different in each case.
Because they are owned and operated by MNOs, cellular networks are maintained by the MNOs’ technicians, who rapidly resolve any issues for you and ensure that you receive the best and most reliable service in exchange for the data fees that the MNOs charge. Because the nodes in a cellular system each communicate individually and directly with the cellular network, there is no single point of failure, other than the cell tower itself.
You build, manage, and maintain your own mesh networks, on the other hand, which means that you must hire your own technicians and resolve network issues yourself when they arise. RF mesh networks are “self-healing”—active nodes in a mesh can compensate when a single node fails—which helps to ensure system uptime, but your technical teams would have to intervene in the case of a gateway failure or other system-wide issue.
Assuming sufficient coverage, cellular networks are extremely reliable. The leading MNOs have rolled out several generations of standardized technology over the past few decades—technology which has been proven over years of operations with many different types of connected devices. RF mesh networks are also very reliable, but because many manufacturers make devices that work in the RF mesh frequency band, and an abundance of network traffic can cause reliability and performance issues, especially in high-bandwidth applications calling for large amounts of data transfer (sensor-based applications and dynamic color-changing light shows, for example).
Connected lighting and the IoT, Signify
System cost and range
Because you’re not paying data charges for each streetlight in a mesh network, operating costs for an RF mesh network is much lower than they are for a cellular network. But you need to be careful when evaluating the total cost of a cellular vs. RF mesh, as the lower operating costs of RF mesh will likely be offset by costs elsewhere in the system. The need to have your own technical teams to perform maintenance and repairs on an RF mesh network is one such cost. Network planning is another.
Because each streetlight in a cellular implementation is connected directly to the cellular network, implementation is effectively plug-and-play: you simply install a connected streetlight, or retrofit an existing one, and the streetlight is automatically commissioned and added to the connected lighting system. This means that initial deployment is relatively inexpensive, and projects can easily be done in phases, as there’s no inherent limitations on the number of light points that can be added to the system or where the physical streetlights need to be in relation to one another.
RF mesh networks, on the other hand, often require careful network planning, especially, in regard to the density of nodes (light fixtures) and the placement of gateways. Streetlights are typically about 50 m (160 ft) apart, and the maximum node-to-node distance for RF communications is about 300 m (1000 ft), so the range is sufficient for urban areas where streetlights are installed close together, but you have to think up front about which streetlights can create a mesh and how each mesh will be able to connect to a gateway. Additionally, RF communications are line-of-sight and are therefore susceptible to interference from trees, buildings, and other large structures. Sub-GHz systems, such as RF mesh, can compensate to some extent via reflection and diffraction that reduce blocking effects, but taking advantage of these features requires some expertise. For reasons such as these, initial implementation costs for RF mesh networks may be higher than they are for cellular networks, even if operational costs are lower.
Cellular communications range over a much larger distance than RF mesh communications—up to 100 km (approximately 60 miles), or over 300 times farther. Because of this long range, cellular networks lend themselves to implementations that cover large geographical areas, whether that’s a large municipality with many urban neighborhoods spread over a distance of several hundred square miles, as in a city like Los Angeles, or implementations that combine suburban and rural streetlight systems. Connected lighting management systems help to reduce operating costs in these cases with remote monitoring and automatic notifications of system events, eliminating the need for repair crews to patrol the streets looking for outages.
The choice is (mostly) yours
Communications networks for smart city systems often must support multiple vertical applications—not just smart street lighting, but also surveillance, traffic density monitoring, air quality monitoring, incident detection, and so on. Each of these applications requires a communications network architecture that is optimized for that application. It would be optimistic to deploy a single communications architecture and expect it use it across all smart city applications, since the different applications will almost certainly require different bandwidths, latency speeds, and connectivity duration. More importantly, a communications architecture must be cost-effective to make citywide deployment possible. Additionally, you want to avoid creating a situation where your communications network is a single point of failure—because if it fails, all your applications fail.
Both cellular and RF mesh systems are appropriate for connected street lighting implementations. They can both offer energy reporting, fault notification, node programming, on-demand dimming and switching, over-the-air firmware updates for bug fixing or rolling out new capabilities, and regulatory compliance.
Which communications platform you choose will depend on your specific circumstances. You have to choose cellular systems where the network is distributed over an extensive geographic area and the light points are few and far between, and you have to choose RF mesh systems where there is no cellular service. Otherwise, your choice will be conditioned by your budget, resources, bandwidth requirements, technical competencies, and other considerations.
You may need both platforms, so consider using a connected street lighting vendor that offers both connectivity options under a shared architecture. This gives you the best combination of capability and flexibility, and offers solutions for almost any physical street lighting configuration you may have to manage.
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