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.
Figure 1 Making IP-enabled wireless sensors reliable and low-power will enable wide-spread usage.
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., WiFi) 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.
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.