February 1, 2009

Wireless Technology Unlocks Possibilities

Jeff Becker

Finding the Right Network

Power companies and other end users considering the implementation of wireless technology have identified a number of key wireless system requirements. These include high security, reliable communication, good power management, open platforms, multispeed monitoring, multifunction capabilities, scalability, global usage, high quality of service, multiprotocol support, and control readiness.

According to wireless technology experts, the emerging wireless infrastructure will be based on a universal mesh network supporting multiple wireless-enabled applications and devices within a single environment (Figure 3). With just one network supporting multiple applications, deployment, network maintenance, and security management will be simplified.

3. Once is not enough. A wireless mesh network has multiple paths between access points (nodes) to establish a redundant infrastructure. Source: Honeywell Process Solutions

A wireless network must be secure to ensure the entire facility is safe, offering one comprehensive and end-to-end integrated security system from the control or host system all the way down to the sensor. This means there’s only one wireless security system to manage. A layered approach to security means protecting the network from multiple risks.

Mesh networks use a self-propagating, self-healing network of nodes to achieve blanket coverage of an area. A node can send and receive messages, and in a mesh network, a node also functions as a router and can relay messages for its neighbors. If one node fails for any reason, including the introduction of strong radio frequency interference (RFI), the network can reroute data, and connectivity will not be lost.

With point-to-point signaling, the power consumption (and battery life) of each field device becomes more predictable. This efficiency helps extend the life of batteries so that they reach their standard shelf life (some up to 10 years), maximizing the time between battery changes. Changes in latency caused by routing changes to the network also are eliminated.

Wireless mesh networks optimize performance with efficient use of industrial, scientific, and medical (ISM) radio bandwidth and prioritizing messages so critical information is received first. Because communication devices using the ISM bands must tolerate any interference from ISM equipment, these bands are typically given over to uses intended for unlicensed operation. Unlicensed operation typically needs to be tolerant of interference from other devices. In the U.S., ISM band usage is governed by Federal Communications Commission rules.

Efficient wireless mesh networks mitigate signal interference in these limited ISM bands by employing a frequency-hopping spread spectrum (FHSS). This technique modulates the data signal with a carrier signal that periodically "hops" from frequency to frequency across a wide band. Through the relaying process, a packet of wireless data will find its way to its destination, passing through intermediate nodes with reliable communication links.

Installing a wireless network at a power plant can pose some unique considerations when one is trying to avoid the risk of electromagnetic field interference and RFI. Usually this problem can be easily mitigated with proper placement and antenna choices. Fortunately, wireless communication is not line-of-sight technology; it can reflect and bounce off metal in a facility. There are three main ways to mitigate the risk from interference:

• Spatial diversity: Every device sends to two nodes in different locations to diversify the communication.

• Temporal diversity: A device sends data, and if the data is not received by either node, it will retry two more times, as quickly as the next millisecond.

• Frequency diversity: Every transmission is performed at a different frequency. Typical EMI interference is short, with scattered bursts, making it relatively easy to navigate around.

Matching multi-hop, wireless mesh communications with distributed control facilitates a new dimension of interactions between sensors or sensor clusters. Sensors can now communicate directly with other devices on the network. Plus, monitoring equipment can take readings from sensors without having to directly access them via wired connections. This is useful in calibration and troubleshooting.

By utilizing a single, universal, wireless mesh cloud, end users have access to one integrated platform supporting multiple field protocols and applications. With a high-speed and self-organizing mesh configuration, network users achieve flexible channel allocation and a robust architecture with latency control and redundancy for safe wireless control. They also have one scalable network that conserves power and spectrum. Best of all, plant personnel only have one system to learn, operate, and maintain.