Reliable, Low-Power Wireless Sensor Networks for the Internet of Things: Making Wireless Sensors As Accessible as Web Servers

The Internet of Things revolution is upon us, and by the year 2020, there will be over 30 billion connected things in the world. With the world’s population increasing and resources becoming more precious, this interconnection promises to supply real-world data to drive higher efficiencies and to streamline business practices.

With the wide acceptance of Internet Protocol (IP), it is becoming easier to process data and make meaningful use of information. Fortune 500 companies provide enterprise- level database solutions for data storage and software tools to streamline business processes, such as asset tracking, process control systems, and building management systems (see Figure 1). Smart phones and tablets provide people with useful and actionable information, such as live parking information or real-time machine-health monitoring to inform maintenance schedules. And while there are wireless sensors in place today, there is a hunger for more sensor data to measure and optimize processes that have not been previously measured. 

To further enable wide scale deployment of sensors, IP standards efforts are underway, with the goal of making small wireless sensors as easy to access as web servers. These efforts are the confluence of two driving forces: the proven low power, highly reliable performance of time-synchronized mesh networks, and the ongoing IP standards efforts for seamless integration into the Internet. Together these forces will drive relatively small, low-power sensors that communicate reliably and are IP-enabled.

Wireless Sensor Network Challenges

Since wireless is unreliable by nature, it is important to understand the sources of unreliability to be able to account for them in communication systems. In low-power wireless networks, the main sources of unreliability are external interference and multi-path fading. Interference occurs when an external signal (e.g., Wi-Fi) temporarily prevents two radios from communicating. This requires them to retransmit, hence to consume more power. Multipath fading happens when a wireless signal bounces off objects in the vicinity of the transmitter, and the various echoes destructively interfere at the receiver’s antenna. This phenomenon is a function of the position of the devices, the frequency used and the surrounding environment. Because the surrounding environment of any wireless system changes over time, any single RF frequency channel will experience problems over the operational life of a wireless system.1 However, multipath fading is frequency-dependent. Therefore, while one frequency may be experiencing a problem, there will be several other RF frequency channels that work well. Because of interference and multipath fading, the key to building a reliable wireless system is to employ channel and path diversity without sacrificing low-power operation. Such a system was pioneered by Dust Networks (now part of Linear Technology) with its time-synchronized, channel-hopping mesh networking.

Time-Synchronized Channel-Hopping Mesh Networks

In a time-synchronized channel-hopping mesh network, all wireless nodes across a multi-hop network are synchronized to within a few tens of microseconds, and time is sliced into time slots. Communication is orchestrated by a schedule which indicates to each node what to do (transmit, receive, sleep) in each time slot. Because they are synchronized, each node switches on its radio only when communicating, thereby significantly reducing their radio duty cycle (<1% is commonplace) and increasing their battery lifetime. Furthermore, since the schedule is flexible, the network is always available to the application, unlike other “sleepy” network architectures that completely shut down the network for extended periods of time. Each packet sent between two nodes is done so on a frequency calculated using a pseudo-random hopping pattern. The resulting frequency diversity is an effective way of combating interference and multipath fading. Time synchronized mesh networks enable a decade of battery lifetime and >99.999% end-to-end reliability. 

Beyond the industrial process industry, SmartMesh systems have been successfully deployed in data centers and commercial buildings to optimize air conditioning costs.3 Streetline Networks4 is a smart parking provider that monitors the real-time availability of urban parking spaces. Vehicle detectors are installed beneath parking spaces, inside the pavement and flush with the roadway. This brings challenges, as the antenna for the sensor device is located underground, and then covered by a metal vehicle when the space is occupied. Applications such as these, previously thought impossible or impractical, are being deployed with time synchronized channel-hopping mesh networks.

A Standards-Based World

Standards play an important role in networking technology, with end-users advocating for the development of standards- based solutions. Knowing that a technology has been developed and approved by a major standardization organization inspires confidence. And while WirelessHART/IEC62591 is the standard in industrial process, beyond that market Internet Protocol (IP) is the communications standard.

All devices connected to the Internet use IP to communicate with each another. Each device acquires an IP address which unambiguously identifies it on the Internet. Data packets exchanged contain an IP header, a series of bytes which encode the addresses of the device that created the packet, and the destination device. Many other protocols are needed to form a protocol stack (TCP, HTTP, etc.), but the IP protocol is the common denominator. Allowing low-power mesh networking devices to connect to the Internet using the IP protocol represented a major contribution toward development of the Internet of Things. 

Several standardization bodies have developed standards for the Internet of Things (see Figure 3). The challenge is to enable full Internet integration, while incorporating the proven principles of time-synchronized channel-hopping mesh networking. Within the Internet Engineering Task Force (IETF) – the standardization body behind most protocols used in today’s Internet – the CoRE working group has defined the Constrained Application Protocol (CoAP) application layer protocol. CoAP runs on top of the UDP protocol and is easily translated to HTTP for web-like interaction with wireless sensor nodes. The 6LoWPAN working group has defined an IP adaptation layer that compresses an IP packet’s large headers into small wireless frames or data packets, allowing sensor nodes to be individually addressable by IP addresses. While these upper layers enable web-like interaction and Internet integration, it is the protocol layers beneath them that determine the quality of the wireless sensor network communications.

The standards developed by the IETF typically run on radio chips which comply with the IEEE802.15.4 standard. IEEE802.15.4 provides a healthy trade-off between data rate (250kbps), range (10s to 100s of meters), power consumption (5mA to 20mA when transmitting or receiving) and packet size (up to 127 bytes). This trade-off makes IEEE802.15.4 a good fit for low-power mesh technology, and has therefore become the de facto link technology for those networks. 

In 2012, the IEEE published a new medium access standard to run on IEEE802.15.4-compliant radios, known as IEEE802.15.4e. Its Time Slotted Channel Hopping (TSCH) mode incorporates principles from Dust Networks’ time synchronized mesh protocol to enable precise time-slotted synchronization and RF channel hopping.

While IEEE802.15.4e defines the mechanism for two nodes to establish a synchronized data packet transfer, it does not define how each node is assigned a schedule. The communication schedule affords a TSCH network the flexibility to match the communication needs of the nodes in the network (see Figure 4). For example, a network can be configured for small networks with low data rates and extremely low power consumption, as is common in remote environmental monitoring. The same network can be configured as a large network, optimized for faster data throughput. In addition, an automatically assigned yet flexible schedule enables a TSCH network to adapt to the surrounding environment. Specifically, network functions such as self-healing, routing optimization and load balancing are enabled by scheduling and are critical to delivering high performance over the life of a network. Solutions for building and assigning the TSCH schedule can be developed, but until standards are established, such solutions will not be interoperable over the air.

This is, however, changing with a new standardization activity inside the IETF, known as Deterministic IPv6 over IEEE802.15.4e Time Slotted Channel Hopping 5 (6TiSCH). Co-chaired by Linear Technology and Cisco Systems, this activity will define the missing communication protocols to allow the TSCH schedule to be managed by a scheduling entity.

By closing this remaining gap in the IP protocol stack, 6TiSCH will enable fully standardized, interoperable IP-based wireless sensor networks that deliver the level of reliability normally associated with wires. Web developers will be able to request real-time sensor data by making web requests to the IP address of a sensor, and the underlying wireless sensor network will support such communications with >99.999% data reliability. By making sensors as easily accessed as web servers, wireless sensor networks will feed real-world information to the Internet of Things. 

Related news articles

Latest News

Sorry, your filter selection returned no results.

We've updated our privacy policy. Please take a moment to review these changes. By clicking I Agree to Arrow Electronics Terms Of Use  and have read and understand the Privacy Policy and Cookie Policy.

Our website places cookies on your device to improve your experience and to improve our site. Read more about the cookies we use and how to disable them here. Cookies and tracking technologies may be used for marketing purposes.
By clicking “Accept”, you are consenting to placement of cookies on your device and to our use of tracking technologies. Click “Read More” below for more information and instructions on how to disable cookies and tracking technologies. While acceptance of cookies and tracking technologies is voluntary, disabling them may result in the website not working properly, and certain advertisements may be less relevant to you.
We respect your privacy. Read our privacy policy here