Fit for the future

The new NR+ wireless mesh network for smart electric meters offers superior reliability and data capacity  

Smart electric meters are going to have to start working harder in future. If utilities have their way, their fleets of smart meters are going to track consumption with greater frequency: every few minutes now, but in future sending readings at intervals of as little as a few seconds. At the same time, the meter will let the utility virtually stretch its hand into homes and offices to switch devices such as electric vehicle charging points and air conditioners off at times of peak demand and on when there is surplus generation capacity.  

So utilities are going to depend on meters to shuttle a lot of data back and forth to their control hubs. And that throws a spotlight on the reliability and speed of the networking technology used by smart meters.  

Today, a smart meter in the US and a handful of other countries is likely running on a Wi-SUN wireless mesh network. Is this networking technology up to the job?  

If you ask a utility’s data network engineer, the honest answer for most will be that the experience could be better. For instance, for the type of mission-critical data that smart meters carry, network engineers expect to achieve a Service Level Agreement (SLA) benchmark of 99.9% successful delivery of messages. Wi-SUN networks running in the real world normally deliver messages more reliably than automated meter reading systems that use a cellular (mobile phone) network, but they are not achieving a 99.9% SLA score for typical readings at 15-minute intervals. 

This shortfall in performance is caused by the physical underpinnings of the network. Wi-SUN networks operate in Industrial/Scientific/Medical (ISM) spectrum, a set of frequencies that are unrestricted and free to use. While Wi-Fi networks normally operate at 2.4 GHz or 5 GHz, smart meters require longer range, and this normally calls for a lower frequency. Wi-SUN devices, for instance, typically operate in sub-GHz radio spectrum – 915 MHz in the US and Australia.  

Thanks to sensible radio regulation in the US, there is plenty of spectrum at 915 MHz, and meters can operate at high transmit power. This means that Wi-SUN radios have sufficient range and a high enough duty cycle for data transmissions to enable devices to support a mesh network topology.  

But because Wi-SUN smart meters are sharing radio spectrum at a frequency of 915MHz, their networks are increasingly getting clogged up by interference from neighboring systems that occupy the same radio space, such as other smart meters and, increasingly, Amazon Sidewalk devices. And the design of the Wi-SUN technology does not have a fail-safe way of rejecting such interference: the technology has a ‘best effort’ approach to avoiding interference by frequency hopping, but in reality, Wi-SUN networks are still vulnerable to it. This can lead to substantial delay in the successful delivery of messages (or high latency), and even to delivery failure.  

And the problem is inherent to the design of the Wi-SUN network technology: frequency hopping is a very efficient method for avoiding interference when there are a few devices nearby. But the use of sub-GHz frequency spectrum and high transmit power increases the problem of interference as more devices share the same radio space. This means that, as the deployment of networked smart meters becomes more and more dense, the interference problems get worse and worse.   

And this is today: how much weaker will Wi-SUN network service get when smart meters are asked to perform additional functions such as load control?  

A wireless mesh network fit for the future  

If Wi-SUN networking appears more ‘meh’ than mesh, it’s time to look at the new network technology built for smart meters of the 2020s and 2030s. NR+ is a standard network protocol developed under the auspices of ETSI (the European Telecommunications Standards Institute), and officially ratified by the International Telecommunication Union-Radiocommunication Sector (ITU-R) agency as part of the 5G standards.  

What’s different about it?  

First, the inventors of NR+ get that utilities demand an SLA benchmark of at least 99.9% successful on-time message delivery. So NR+ has multiple fail-safe features built-in to make the network incredibly robust:  

• NR+ is part of 5G so devices using NR+ can operate on any frequency available for radio communication in each region, whether licensed or unlicensed. Globally, NR+ has its own uncontested spectrum at 1900 MHz, the same ‘golden frequency’ dedicated to the legendarily reliable DECT cordless telephone handset. The first NR+ devices can transmit at up to 19 dBm power. With OFDM modulation and 1 Mbps bandwidth, these devices deliver impressive line of sight range of up to more than 5 km between devices.

• While both Wi-SUN and NR+ are self-healing networks, there is one fundamental difference between them: an NR+ network makes self-healing decisions locally, while Wi-SUN is a centralized system which requires a central coordinator to make decisions about routing. In small-scale networks consisting of a few tens of devices, there is no significant difference between centralized routing and decentralized routing, but in large-scale networks, the ability of a centralized system to react fast enough breaks down. In a centralized system, routing data needs to be passed to the central coordinator, which then has to create and distribute new routing tables to the members of the routing tree. In an NR+ network, by contrast, there are no routing tables and all routing decisions are made locally and instantly. So decentralized routing makes route changes instantaneously and creates no communication bottlenecks.

The NR+ standard solves the coexistence issues often found in large-scale deployments. NR+ devices are aware of other devices in the same network, and no device interferes with other devices in the network thanks to clever time synchronization, which regulates package transmissions and channel selection in communications between two devices. 

The Wirepas 5G Mesh protocol stack – a ready-made implementation of NR+ - is part of Wirepas Connectivity Suite and includes reliability features that can produce a 99.9% message delivery success in real-world conditions. 

The second big advantage of NR+ technology is that it saves on installation and commissioning costs. Because the network is self-organizing, it does not require network experts to laboriously configure every node’s connection. And NR+ typically only requires one gateway for every 4,000 smart meters; a Wi-SUN gateway supports fewer than 1,000 smart meters. Equipment costs are therefore much lower with NR+.  

And third, NR+ is the network of the future. It is officially the first non-cellular 5G wireless communications protocol. NR+ can carry much more data than other mesh networks. While the first NR+ based products support ‘only’ up to 1 Mbps, the NR+ standard allows for a data rate of up to 3.4 Mbps, some 10 times higher than Wi-SUN’s theoretical maximum. This means that transmission time is much shorter for NR+ than for Wi-SUN, giving much higher network data capacity.   

And because of the clever way that the NR+ mesh technology works – including dynamic frequency usage and message synchronization – multiple utilities’ separate fleets of meters can each be built up to millions of nodes, even in crowded urban environments, without any problems over co-existence or interference.  

While NR+ is a technology which is fit for the future, it is available for deployment right now. Meter manufacturers can already develop new designs based on Nordic Semiconductor wireless chips that support the NR+ standard. The nRF9161 is a complete system-in-package (SiP). It operates at LTE/5G frequencies from 700 MHz to 2.2 GHz, including the 1.9 GHz NR+ band. Its Arm® Cortex®-M33 CPU and generous memory provision can run the entire NR+ stack and networking application, as well as robust encryption and other security features.   

The nRF9131 SiP is a feature-compatible alternative to the nRF9161 which is half the size and offers greater flexibility in design and sourcing for high-volume designs.  

And a complete, off-the-shelf NR+ network protocol stack is available today from mesh technology pioneer Wirepas, starting with band 1 frequencies. Compatible with the nRF9161/9131 and other chipsets in the future, the Wirepas 5G Mesh software is easy to integrate, and is fully supported by Wirepas’ familiar software development kit (SDK) and supporting documentation and tools. Wirepas also provides an open-source software development kit and reference implementations for both an NR+ gateway and a DLMS/COSEM implementation.  

So the future of electric meter networking is here, today – and it is NR+.