Wireless sensor networks are taking over.

For military surveillance, a self-organizing network of minute, autonomous wireless sensors is very attractive. Once a group of nodes has been deployed, perhaps dropped by air, troop and vehicular movements can be remotely monitored. The initial research on this application helped define the mesh concept and software operating systems used in wireless sensor networks (WSN).

Although actual surveillance systems have been deployed and research continues, commercial WSN requirements are different. In many industrial applications, small size is not among the most important priorities. Instead, ease of use, the capability to provide a complete solution, and vendor support top the list.

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For example, ZigBee is a WSN that eliminates the need for large amounts of office building control wiring. To continue reducing expenses, a typical lighting switch or temperature control must have a very long battery life. Having low battery maintenance obviously saves money, but to provide a service life of several years from a standard commercial battery requires exceptionally low power consumption.

In critical industrial applications, a wireless solution with a long battery life may be the only practical alternative. Consider a monitoring device mounted at the top of a smokestack. A conventional solution would entail expensive installation of hundreds of feet of environmentally protected wiring together with local power supplies and monitoring electronics. Even a wireless approach must be very reliable and require only infrequent battery replacement given the cost and difficulty of access.

Some types of WSNs retain a few line-powered nodes, typically high in the network hierarchy, closest to the computer actually receiving the sensor data. The rest of the nodes are battery powered or have the capability to be powered from an energy-scavenging device. To many companies in the WSN industry, the term wireless applies to all wiring, not just power and not just control.

Niek Van Dierdonck, vice president of strategy and product management at GreenPeak, expressed it this way, "We have developed an ultralow-power strategy because the strength of a truly wireless sensor network can only be fully utilized when the wiring for both the data communications and power cables can be eliminated. Eliminating the data cable solves only half the problem: The installer still needs to run a power wire. And, half a problem solved is as good as not solving it at all."

Nodes operate with a very small duty cycle to conserve power. In other words, they are as inactive as possible for as long as possible. In WSNs with pre-assigned time slots, an on-board timer can alert the node to wake up at the proper time, transmit its information quickly, and shut down again. It also is possible for a node to monitor transmissions in the background, only turning on all its circuitry when the appropriate code has been received. Arch Rock calls this mode of operation passive vigilance.

The time synchronized mesh protocol (TSMP) used by Dust Networks reserves 47 of the 127-B maximum packet size specified by the 802.15.4 radio standard, allowing 80 B for the payload. Figure 1 shows the TSMP packet structure with its emphasis on data integrity. Direct sequence spread spectrum (DSSS) coding improves multipath performance, and frequency hopping spread spectrum (FHSS) operation across 16 predefined frequencies effectively increases channel bandwidth by a factor of 16.

The data rate supported by an ultralow-power radio limits the types of applications that can be addressed. The amount of on-board data reduction that can be performed is necessarily power limited, as is the amount of data that can be transmitted each time the node wakes up. Nevertheless, WSNs are well suited to applications such as process-control temperature and pressure monitoring; building heating, ventilation, air-conditioning (HVAC) control; and many types of remote environmental monitoring.

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Very long battery life is not as important in many troubleshooting and monitoring test and measurement applications as it is in WSNs that are part of a building infrastructure. MicroStrain offers several wireless strain gauge-based data acquisition systems that are good examples of the power/speed trade-off. Data can be logged at up to a 2,048-Hz rate or streamed in real time at a rate up to 4 kHz by the V-Link[R] instrument, but its 600-mAh battery may have a life of only 55 days with four 1,000-[ohm] strain gauges (Figure 2).

WSN Architecture and Standards

A few specific terms are related to WSNs and the associated standardization efforts currently underway:

* Self-forming, also called self-configuring: Nodes discover their neighbors and create the network by themselves.

* Self-healing, related to redundancy: Should a node become damaged or its battery fail, the other nodes in the network will reroute messages around that node. Generally, data from the sensor connected to the failed node will not be available.

* Single hop or star: As the name suggests, each sensor node communicates only with the end node. This is a multipoint-to-single point architecture. By definition, it cannot be self-healing because there are no redundant intermediate nodes.

* Router: As in wired communications, a router can concentrate or distribute network traffic. In a Sensicast Systems SensiNet network, the Smart Sensors form a mesh and capture data such as temperature, humidity, or power consumption and transmit the data to powered routers and gateways.

* Mesh network: A network formed by many nodes that transfer information from one to another, eventually completing enough hops to reach the end of the network. Everyone agrees on this definition but not the details. For example, Dust Networks provides a gateway that acts as a wireless-to-wired interface point. In the mesh, there is only one kind of wireless node.

In contrast, Ember's Zigbee technology includes a coordinator node that configures and controls the network, line- or battery-powered router nodes, and a number of battery-powered end devices that only communicate with the routers. Ember's router nodes form the self-healing mesh and together with the end devices form a star-mesh hybrid topology.

* ISA-SP100: A proposed WSN standard driven by the process control and industrial automation industry. The preliminary version, SP100.11a, supports multiple protocols both for control and monitoring applications.

* IEEE 802.15.4: A specification that defines the physical and medium access control layers (MAC) for a personal area network. The 2.4-GHz industrial, scientific, and medical (ISM) band called out in the spec is used by ZigBee, Wi-Fi, Bluetooth, cordless telephones, and microwave ovens. If WSN communications are to be reliable, secure, and nondisruptive in this environment, the protocols must be carefully defined.

* WiHART: Wireless HART is a wireless form of the highway addressable remote transducer (HART) protocol. HART retains the 4- to 20-mA analog current loop signals long used by industry but superimposes digital signals on top of them.

WSNs are not confined to industrial applications, but there are many industrial data acquisition and control requirements for which they are good solutions. This is the reason that there is so much activity toward standardization within organizations such as the ISA whose members are process control and industrial automation companies.

Many companies have developed proprietary WSNs, and as you would expect, they are not interoperable. The intention is for ISA-SP100 to be an open standard that builds on the experience of these companies much as the WiFi Forum has achieved interoperability among competing Wi-Fi products. The ISA committee goal is to adopt one standard by early 2008.

In the meantime, it's useful to consider the unique attributes of the different approaches. Each manufacturer has achieved a degree of success in one or more markets, and some solutions are very specialized.

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WSN Examples

Hierarchical Networks

A few of the larger application areas, HVAC, home area networks (HAN), and utility meter communications (advanced metering infrastructure--AMI), are addressed by the ZigBee specification. The IEEE 802.15.4 PHY and MAC specifications are used, but the higher network and application layers of the ZigBee protocol are defined by the ZigBee Alliance with more than 225 member companies.

ZigBee solves many of the problems concerning end users such as ease of installation and compatibility with co-located Wi-Fi and Bluetooth radios. It is used by hundreds of interoperable products. From the point of view of the companies involved in the SP100 effort, however, ZigBee does not guarantee the very high level of security and reliability required by critical control signaling encountered in process control and factory automation applications. This is a special concern that ISA-SP100 will address.

Sensicast's SensiNet solutions are similar to ZigBee, with three layers with the mesh capabilities provided by the intermediate wireless routers, but a separate coordinating node is not required. However, unlike ZigBee, all parts of the WSN solution, from the Smart Sensors through to the application software, are made by Sensicast.

The company supports a number of networking protocols including the 802.15.4 physical radio specification and those parts of 802.11 covering Wi-Fi and wired Ethernet networks. In addition, Sensicast intends to support ISA-SP100 when the standard has been finalized.