This article is part of a series exploring the role of networking in the Internet of Things.
Today we are going to consider the attributes of wireless mesh networking, particularly in the context of our building monitoring and energy application.
A host of new mesh networking technologies came upon the scene in the mid-2000s through start-up ventures such as Millennial Net, Ember, Dust Networks, and others. The mesh network topology is ideally suited to provide broad area coverage for low-power, low-data rate applications found in application areas like industrial automation, home and commercial building automation, medical monitoring, and agriculture.
In my first post of this series, I described the three types of nodes used in a mesh network: a sensor node, a sensor node with routing capability, and a gateway node that bridges the mesh network to another network such as an 802.11-based Ethernet network, from which administrators can read data remotely. Data that is generated by a sensor propagates through the mesh network in an ad-hoc fashion to the gateway and the application. Conversely, most mesh-networks support bi-directional communication. Therefore control data can be transmitted from the gateway back to a sensor/actuator. See Figure 1.
Figure 1. Mesh network with bi-directional communication
The IEEE 802.15.4 standard specifies the PHY and MAC layers of this network architecture, which are the base layers for several proprietary and standardized mesh layer protocols like ZigBee and Z-Wave.
ZigBee has emerged as the most commonly used mesh network protocol and has become somewhat of a de facto standard, particularly in the building automation space. Although the other network protocols based on 802.15.4 have similar characteristics, I will focus specifically on ZigBee for the balance of this post. Let’s take a closer look at the attributes of this networking technology.
ZigBee is authored and licensed by the ZigBee Alliance, an open standards body. The standard provides for low data rate, low power consumption networks and is aimed to address residential, building and industrial control devices. It is specifically useful for sensors and control devices of building automation systems within a smart building where very small amounts of information or data are being transmitted. ZigBee also has uses in home automation, industrial automation, home entertainment systems and smart meters.
- Networking protocols based on 802.15.4 (e.g. ZigBee) operate within several license-free industrial, scientific and medical (ISM) radio bands; 868 MHz (at 20 kbits/s) in Europe, 915 MHz (at 40 kbits/s) in the USA and Australia, and 2.4 GHz (at 250 kbits/s) in most countries worldwide.
The most popular frequency band in use today is 2.4GHz because of its worldwide license-free acceptance.
- The transmission range of a single sensor node ranges from 30 to over 300 feet, depending on the characteristics of the environment. A network node with routing capability receives the packet and retransmits it. This capability propagates the data packet through the mesh network to its destination, the gateway. Because of this multi-hop, flexible routing capability, ZigBee communication ranges can extend to well over 1,000 ft.
Because the total range of a mesh network is not limited to transmission range of any single node, the range from data source to gateway can extend well over 1,000 ft.
- Power Consumption
- Low power is a Zigbee design goal. The radio wakes up only when data needs to be transferred. Because ZigBee can activate (go from sleep to active mode) in 15 msec or less, the latency can be very low and devices can be very responsive — particularly compared to Bluetooth wake-up delays, which are typically around three seconds. Because ZigBee can sleep most of the time, average power consumption can be very low, resulting in long battery life and enabling the potential use of energy harvesting techniques. To pass the ZigBee certification, individual devices must have a battery life of at least two years.
- The mesh networked protocols such as ZigBee and Z-Wave share this 2.4 GHz frequency band with Wi-Fi and Bluetooth. To avoid interference, the mesh networking protocols utilize various transmission techniques such as direct-sequence spread spectrum (DSSS) and frequency-hopping spread spectrum (FHSS).
- Scalability and Flexibility
- Theoretically, the 802.15.4 network architecture is designed to scale to a thousand nodes or more. However, given the data rate and duty cycle requirements of a typical application, these mesh networks can realistically scale to just hundreds of nodes in a single network. Large installations consisting of thousands of deployed nodes typically consist of multiple mesh networks, each containing a few hundred nodes, linked together via an 802.11 backbone.
- 802.15.4-based networks can typically support application data rates of up to 250Kbits/sec.
- The ZigBee standard is maintained and published by the ZigBee Alliance, a group of companies building products based on the technology. The term ZigBee is a registered trademark of this group, not a single technical standard. The Alliance publishes application profiles that allow multiple OEM vendors to create interoperable products. These application profiles include ZigBee Home Automation 1.2, ZigBee Smart Energy 1.1b, ZigBee Telecommunication Services 1.0, ZigBee Health Care 1.0, and ZigBee Building Automation 1.0.
- Component Availability
- ZigBee components are widely available from a number of silicon vendors, including Texas Instruments, Silicon Labs, Microchip, and STMicroelectronics.
- There is a broad array of ZigBee chips and modules on the market today. As mentioned, one of the design goals of the 802.15.4 architecture is low cost. For example, a ZigBee System-on-Chip (Silicon Labs EM260-RTR) is priced at $4.85 for quantities of 3,500.
This 802.15.4 mesh networking technology is used for many applications requiring a long range and broad area coverage. Applications include building automation, energy management, industrial automation, and asset management. Because the network range is not limited to the transmission range of a single device, the network range can be very broad, covering large areas, such as a building or campus. Mesh networks can scale up to thousands of nodes, providing a high density of coverage with a broad assortment of sensors and actuating devices. The flexibility of network layout allows coverage in environments facing high radio frequency (RF) challenges, such as high RF interference or RF obstacles. Intermittent network interruptions are mitigated by self-healing and packet retransmission capabilities that together provide a high degree of network resilience.
So far in this series, we have defined our hypothetical application and have reviewed the attributes of several networking architectures to determine the best fit for our building automation application. These are point-to-point networks, star networks, and now mesh networks. In my next post we will pull everything together and compare our specific application requirements against a rollup of the attributes of these networks architectures.