One Fine Mesh: Wi-SUN in M2M communications

The Wi-SUN standard is a candidate for infrastructure networks that demand high bandwidth, low latency, and power efficiency.

Soumya Shyamasundar • Silicon Labs
The smart city movement is strongly connected to sustainability. It encompasses optimized management of energy resources, resilient infrastructure to improve quality of life, and access to public safety. Efficient use of available resources is key for sustainability and an important part of the fight against climate change. Wireless technologies are at the heart of the drive toward smart, connected cities. In particular, Wi-SUN is a flexible and powerful wireless standard that can support a large number of connected devices economically. It will be a key to unlocking massive IoT applications and services.

The smart cities landscape historically has been fragmented with proprietary networks catering to the needs of different applications. On the utility side, electric meters are line-powered; water and gas meters are battery-powered applications. Smart electric meters use high-speed wireless connectivity for real-time low-latency data transmission back to the grid. Water and gas meters typically need to last a decade on battery power and hence use low power, long-range connections. These battery-operated applications connect to the electric meter and rely on it for transmitting data back to the utility headend. Thus electric meters here become aggregators of sorts and must support high throughput and low latencies for a reliable network.

In smart city installations, streetlights are analogous to electric meters in assuming the role of central controllers. Streetlights are widespread legacy municipal infrastructure ripe for digitization. They’re increasingly evolving into canopies for digital infrastructure featuring sensor nodes for environmental air quality monitoring, parking, waste management, manhole cover detection, and so forth.

These sensor nodes use long-range and low-power technologies and devices.

Wi-SUN, which stands for Wireless Smart Ubiquitous Networks, is an IPv6-based mesh technology that provides machine-to-machine communication for large-scale IoT infrastructure. Although Wi-SUN has been around since 2013, the technology started gaining traction in the last five years as regulatory organizations pushed for modernizing grid infrastructures. Wi-SUN can help in this regard because it was designed specifically for the complex requirements of smart metering, street lighting, smart city sensor networks, and other asset management needs.

A block diagram shows the typical components in the Wi-SUN hardware and stack solution. Click image to enlarge.

One of Wi-SUN’s biggest assets is it’s self-forming mesh technology wherein end nodes connect directly and dynamically to several nearby nodes to form the network. This approach contrasts with the use of a central coordinator typically seen in star topologies. In a mesh, participating devices can relay traffic to each other, passing it to several nodes downstream. Traffic can hop from one node to another smoothly.

From a specification perspective, Wi-SUN supports up to 24 hops for each mesh device. Nodes typically choose the shortest path for upstream traffic back to the backhaul point or the border router. The router or backhaul point ultimately transmits traffic to the backend system via fiber, Wi-fi, or cellular technologies. This approach becomes important in urban settings with congested RF environments: Traffic can re-route itself to ensure reliable availability even in non-line-of-sight scenarios. Thus Wi-SUN mesh networks can be deployed easily and quickly because they have the ability to self-form.

Data security in smart cities

Smart grids and smart cities will only be practical with end-to-end security that safeguards critical infrastructure data. Wi-SUN specifies enterprise-grade security with a native public key infrastructure integration (PKI). PKI performs encryption and decryption by using two different cryptographic keys: a public key and a private key. PKI provides security certification and authentication capabilities for each device on the network. PKI integrated with the hardware root of trust guarantees that devices cannot be maliciously reprogrammed and certifies that incoming firmware updates are valid.

Security becomes even more critical for devices in use for years or decades. Encryption mechanisms are available at various layers of the IPv6-based architecture. For example, some reside on Layer 2 (AES-128 on IEEE 802.15.4g or IEEE 1901.2), Layer 3 (IPSEC protocol), and Layer 4 (Transport Security Layer protocol). There may also be encryption on the application layer depending on the target use case. For example, there can be encryption in DLMS/COSEM for smart grids and uCIFI for smart cities. DLMS/COSEM (IEC 62056, EN13757-1) is the global standard for smart energy metering, control and management. uCIFI is an open unified data model for all smart city devices and the uCIFI mesh implementation.

Wi-SUN technology supports three widely used modulation schemes – FSK, MR-OQPSK, and OFDM. The majority of utility deployments today use FSK, frequency shift keying. MR-OQPSK (Multi Rate Multi Regional Offset Quadrature Phase-Shift Keying) brings low-power, noise-immune capabilities whereas OFDM (Orthogonal Frequency-Division Multiplexing) maximizes spectral efficiency by using close-spaced overlapping sub-carriers. Both OFDM and MR-OQPSK are critical for next-generation meters and smart city applications.

Historically, developers have often been forced to use one standard for all use-cases in a network. But the availability of numerous PHYs makes it possible to take a mix-n-match approach to smart city networks. With Wi-SUN, developers can optimize for specific applications without having to build additional infrastructure. All smart city applications can use the same Wi-SUN network serviced by one border router for backhauling.

Also noteworthy is Wi-SUN’s inclusion of an IP network layer – IPv6 is a promising protocol for complex and distributed networks and the key enabler for IoT applications. It not only extends the overall addressing space but also eliminates the need for a network translator or additional gateways, simplifying integration. Moreover, because IP is the point of convergence for industrial networks, a technology that supports IP networks natively becomes table stakes for all future deployments.

The upcoming version of the Wi-SUN specification, FAN 1.1, is expected to add support for high- throughput PHYs and features for limited-function devices. It will also manage dynamic switching between PHYs which will make possible a multitude of new smart city applications.

Wi-SUN defines the specification from layer 1 to layer 4. The technology today is application agnostic and relies on the use-case-specific application layers like DLMS/COSEM for smart grids, uCIFI for smart cities, and so forth.

The Wi-SUN specification comes from the Wi-SUN Alliance. The Wi-SUN Alliance, made up of IoT infrastructure providers, was formed to help deploy networks based on the IEEE 802.15.4(g)(e), 6LoWPAN standards. The Alliance also helps product vendors, silicon vendors, cities, utilities, government institutions, and academia collaborate. With more than 300 members representing over 40 countries, the Wi-SUN Alliance drives the specification and certification process.

The Wi-SUN Alliance is promoting sustainable technologies that are secure, scalable, resilient, and interoperable. These technologies will fulfill future requirements for high bandwidth, low latency, and power efficient networks. Wi-SUN is poised to meet these challenges and build smart cities of the future.