The term Internet of Things (IoT) has gained enormous popularity with the explosion of wireless sensor networks, smart meters, home automation devices and wearable electronics. The IoT spans long-range outdoor networks such as the smart grid and municipal lighting as well as shorter -range indoor networks that enable the connected home, residential security systems and energy management. Wireless connectivity and standards-based software protocols provide critical enabling technology for the IoT. A case in point is wireless connectivity for smart metering systems. One of the most useful wireless protocols for smart metering to emerge in recent years is Wireless M-Bus, which is widely used for metering applications across Europe.
Author: Vivek Mohan, Senior Product Manager, IoT MCU and Wireless Products at Silicon Labs
What is Wireless M-Bus?
Meter-Bus (M-Bus) is based on European standards for smart meter communication. This connectivity can be wired or wireless, and the standard specifies the communication link between smart meters and data collectors, as shown in Figure 1. The standard also applies to heat cost allocators and drive-by or stationary remote meter reading devices. Wireless M-Bus – the wireless version of the M-Bus standard – existed for more than a decade and is
seeing continuous growth in a number of deployments across Europe. Based on sub-GHz frequency bands (169MHz, 434MHz and 868MHz), wireless M-Bus uses a simple star type of network configuration with a protocol that is optimized for the needs of smart meter devices. The sub-GHz frequencies enable better propagation characteristics than higher frequencies such as 2.4GHz. The longer range allows the radio waves to reach difficult wireless locations such as underground installations or meters behind several walls and obstructions.
IP addressability and mesh networking are not specified in the standard although meters are individually addressable and may support relaying or routing of messages in some modes. The lower data rates and small packet lengths support a low-power, long-range solution with a small software stack implementation. Low power is critical for water and gas meters, which are battery powered and need to operate reliably for more than 10 years. The frequencies, modulation (FSK based) and bandwidths required by the standard make it spectrally efficient compared to spread spectrum-based protocols. The underlying technology is available from multiple suppliers and is completely standards-based, which makes it a very competitive solution in the market and beneficial for consumers. The combination of these factors makes wireless M-Bus a cost-effective connectivity solution for smart metering in Europe.
Standards and Organizations
Several European standards and organizations are relevant to wireless M-Bus. However, there is no common industry alliance or certification process specifically created for wireless M-Bus. In Europe, all sub-GHz wireless devices must comply with ETSI EN 300 220, which sets the emission limits in various frequency bands among other specifications. The
European Committee for Standardization (CEN) also defines EN13757, which comprises six different parts. Parts 3 and 4 are most relevant for wireless M-Bus implementations. The different parts specify requirements from the physical layer to the application layer for both wired and wireless M-Bus implementations. Table 1 captures these parts and their purposes.
The latest version of EN13757 was approved in 2013 with improved (tighter) RF specifications, superseding the previous 2005 version of the standard. In addition to the standards documents, regional organizations also specify the use of wireless M-Bus. One challenging aspect is that each of these regions has unique requirements on top of the standard and may pick specific modes from the standard to suit the needs of their environment. GrDF in France, CIG in Italy and the OMS group are examples of these groups. (We will examine GrDF and CIG requirements in further detail.)
Modes and Frequencies of Wireless M-Bus
Table 2 shows several wireless modes specified at various frequencies. Modes S, T, C and N are most commonly used with mode N gaining popularity in the 169 MHz band. Modes R and F are less common while modes P and Q are not used today.
These modes have unidirectional and bidirectional sub modes.
Unique Regional Requirements
Each European country defines its own requirements best suited to the environment and infrastructure available. This works well for region-specific utility companies but adds additional requirements for suppliers including semiconductor designers, meter manufacturers and software developers. To provide a common platform, the entire solution including hardware and software must be architected to be flexible and modular so it can adapt to the unique regional requirements. Security and radio performance are critical areas for metering applications, which is reflected in the additional requirements specified by these various regions. Let’s consider the example of France and Italy and highlight some of the key features in these regions.
France’s GrDF specifies the use of ‘N’ modes at 169MHz, which are narrowband, low data rate modes for efficient use of frequency spectrum. They also specify a broadcast mode to update meters along with advanced security requirements. A high-speed 4GFSK mode is optionally defined to support higher data rates while maintaining a narrow bandwidth 12.5KHz channel. Specifically for the radio performance, improved sensitivity, blocking and selectivity over and above the standard requirement are expected with a tight frequency deviation error tolerance of 0.2 percent.
CIG in Italy is also based on the N modes of operation in the 169MHz band. Additionally CIG follows the Italian UNI TS 11291-11-4 specification, which requires some changes to the application layer interface and transport layers. The application layer is based on device language message specification / companion specification for energy metering (DLMS/COSEM), and the channel access method is based on ALOHA and LBT. A broadcast window is also open for firmware download purposes. The physical layer requirements are unique as well. To achieve a long range, the maximum transmitted power is +27dBm with the additional requirement of at most 3dB steps from -27dBm to +27dBm. The specification documents provide additional details.
The N mode of operation specified in France and Italy requires ultra-fast preamble detection with a very short preamble of 2 bytes. Sub-GHz wireless transceivers such as Silicon Labs’ Si446x EZRadioPRO devices can support these and other Wireless M-Bus specific requirements.
The Open Metering Specification (OMS) and Dutch Smart Meter Recommendations DSMR) also have specific rules pertaining to the application layer, use of fields in the packet structure and enhanced security.
Implementing a Solution
There are several options available for wireless M-Bus metering solutions, ranging from semiconductor components to software stacks to modules. The core components required for a high-performance wireless M-Bus solution include a low-energy microcontroller (MCU), a high-performance sub-GHz transceiver that can offload the host processor, and a modular stack architecture, which provides flexibility to support various wireless connectivity requirements. Comprehensive development tools must also be available to design and configure the metering system.
Figure 2 provides a high-level comparison of the wireless M-Bus stack and the OSI model, highlighting the fewer layers required by the stack. The stack size can be implemented with less than 32 KB flash depending on the mode and device type, which translates to a lower cost MCU solution based on reduced flash and RAM requirements. The application layer is user-defined and may follow OMS, DSMR, DLMS/COSEM or any other custom application layer as well.
The open hardware application layer (HAL) enables low-level hardware configuration for peripherals such as GPIOs or UART baud rates. This type of modular architecture allows maximum flexibility to support a wide variety of devices with a common stack version.
For example, Silicon Labs provides a comprehensive platform solution for wireless M-Bus applications that includes a software stack developed by Stackforce GmBH optimized to run on Silicon Labs’ EFM32 MCUs based on ARM Cortex-M0+, M3 and M4 cores and EZRadioPRO sub-GHz wireless transceivers. The highly integrated, small-form-factor EZR32 wireless MCU platform combines the wireless stack, MCU and transceiver into a single-chip solution that is ideal for space-constrained wireless designs.
It is important for the MCU and radio to support a variety of low power modes such as sleep and standby and have the ability to wake up quickly and process an incoming packet. This is especially important for battery-powered meters. Another hardware consideration is support for peripherals and sensor interfaces that can be autonomous to extend battery life. RF frequency matched hardware kits to support 169 MHz and 868 MHz also help during the initial evaluation and debug phase.
Figure 3 shows an example of a wireless M-Bus hardware platform based on Silicon Labs’ EZR32 sub-GHz wireless MCU. The radio hardware is optimized at different frequencies and power levels to meet regulatory requirements in Europe including 868 MHz and 169 MHz for wireless M-Bus with a variety of interfaces and debug options to simplify development.
The smart meter market is evolving quickly as highly integrated, ultra-low-power platforms become widely available at cost-effective price points. Various countries around the world have set schedules in the near future for the roll out of millions of smart meters. European wireless M-Bus deployments are expected in large numbers with field trials underway in many regions.
Ultra-low-power MCUs and high-performance sub-GHz wireless ICs with flexible architectures supporting multiple protocols will lead the way in enabling smart, connected and energy-friendly metering applications that will enable consumers and utility providers to save precious natural resources ￭
Vivek Mohan, Senior Product Manager, Wireless Products, Silicon Labs
Vivek Mohan is a senior product manager for wireless connectivity products including sub-GHz radios in Silicon Labs’ IoT microcontroller and wireless products group.
He joined Silicon Labs in 2010 as an applications engineering manager for short-range wireless products. Prior to Silicon Labs, he held applications engineering and design verification roles for WiFi and Bluetooth wireless SoC products at Marvell Semiconductor. Mr. Mohan holds a Master of Science in electrical engineering from the University of Southern California.