A primer on wireless sensor network

The profusion of 'smart' energy management applications and inexpensive, standards-based wireless MCUs is fueling the growth of wireless sensor/actuator networks (WSAN) across diverse markets, including home and building automation, telemedicine, and lighting. Research firm IDTechEx forecasts a near ten-fold growth of the WSAN market by 2019.

WSANs provide a simple, economic approach for the deployment of distributed monitor and control devices, avoiding the expensive retrofit necessary in wired systems. But inexperience with RF design and the confusing profusion of wireless protocols will continue to persist as some of the biggest challenges for the application developer. This article examines some of the basic concepts of wireless sensor networks and protocols, the essential elements comprising a wireless sensor, and some of the important design considerations for using them.

A wireless sensor and actuator network (figure 1) is a collection of small randomly dispersed devices that provide three essential functions; the ability to monitor physical and environmental conditions, often in real time, such as temperature, pressure, light and humidity; the ability to operate devices such as switches, motors or actuators that control those conditions; and the ability to provide efficient, reliable communications via a wireless network.

The implementation of this last capability is the most unique to WSANs. Since they are designed for low traffic monitor and control applications, it is not necessary for them to support the high data throughput requirements that data networks like Wi-Fi require. Typical WSAN over-the-air data rates range from 20 kbps to 1Mbit/s. Consequently they can operate with much lower power consumption, which in turn allows the nodes to be battery powered and physically small.

WSANs are typically self-organising and self-healing. Self-organising networks allow a new node to automatically join the network without the need for manual intervention. Self-healing networks allow nodes to reconfigure their link associations and find alternative pathways around failed or powered-down nodes. How these capabilities are implemented is specific to the network management protocol and the network topology, and ultimately will determine the network's flexibility, scalability, cost and performance.

Wireless sensor networks use three basic networking topologies; point-to-point, star (point-to-multi-point), or mesh (figure 2). Point-to-point is simply a dedicated link between two points and arguably isn't a network at all. Star networks are an aggregation of point-to-point links, with a central master node that manages a fixed number of slave nodes and serves as the conduit for all upstream communication.

Master nodes can also link with other master nodes to extend a star network into various configurations sometimes called cluster or cluster-tree networks (figure 3).

One of the drawbacks of a star topology is that the master node is a single point of failure; if a master node fails, the entire sub-network fails. In the mesh topology, every node has multiple pathways to every other node, providing the most resiliency and flexibility. Most practical mesh networks utilise a type of pseudo-mesh with peer-to-peer communication links that support routing. Messages traverse the network using a multi-hop routing algorithm that can be optimised for the lowest latency or lowest power. Since each node in the mesh must maintain knowledge of other nodes in the network with routing tables, the memory requirements and processing overhead required at each node are higher in mesh networks.

Distinguish one protocol from another
The network management protocol will determine which topologies are supported and how the network reconfigures when nodes join or leave. The specific processes used for network formation, auto configuration, routing, etc. are what differentiate one protocol from another. While some WSAN protocols like ZigBee and WirelessHART have been widely adopted, the landscape is clouded with dozens of competing and non-interoperable protocols, each having its own pros and cons. A great many of these are proprietary. While a protocol backed by an adopted industry standard offers the promise of multi-vendor interoperability, some proprietary protocols can provide a solution tuned to specific performance parameters like simplicity, network resiliency or security. On the other hand, a proprietary protocol can lock you to a single vendor for future network expansions.