Mesh networks on the factory floor

5.  Once a faulty communication connection is remedied, the network automatically resumes communications without the action of external personnel.

Try that with a conventional network, Saunders seems to be saying.

Throw in another advantage: wired mesh networks conform to common practices and standards such as IEEE 802.1W Commercial Rapid Spanning Tree Protocol (RSTP) technology, 802.3 Ethernet, 802.1D MAC Bridges, and so on, because they are based on COTS devices. Some conventional networks conform to these standards but others are still proprietary. Wireless mesh networks are still being standardized.

Wireless: Stick to Monitoring
Wireless systems promise amazing benefits. As costs come tumbling down and more products become available, it becomes more and more attractive to consider wireless monitoring and control.

But wireless mesh networks still have a long way to go before they meet your criterion of “Prove to me that it works, and I’ll buy it.”

Craig Skarpiak, product manager of wireless products at B&B Electronics says, “There’s little doubt that wireless mesh sensor networks currently are in the early adaptor stage of the market.” He is seeing products coming from major equipment OEMs including building automation, lighting, HVAC, electric motors, security, and home appliances.

Skarpiak is concerned about the lack of finalized and approved standards. “There are several flavors of mesh networks, including standards-based ZigBee Alliance and ‘enhanced-featured’ networks from Dust Networks, Ember, Helicomm, Millennial Net, and Sensicast, to name a few.”

To industrial users, a lack of standards can be the kiss of death for a new technology. “Standards organizations such as ZigBee, the 802.15.4 committee, and the Wireless Industrial Networking Alliance (WINA) all are working to make wireless standards and products more appropriate for industrial applications,” says Hesh Kagan, Consulting Engineer, at Invensys Process Systems.

“Realizing that adopting a standard such as ZigBee will enable the marketplace to move forward more rapidly,” Skarpiak adds, “several of the current 802.15.4 enhanced-featured networks providers created ‘agnostic’ platforms that will incorporate the benefits of their system, while at the same time leverage the customer economics of a ZigBee standards-based system.”

Another major problem is speed. Wireless mesh sensor networks are superb for non-critical monitoring applications, because those systems don’t care if a temperature or a level value takes one microsecond or 5 seconds to arrive. In fact, many wireless monitoring systems sample at glacial speeds, if only to preserve the batteries in wireless systems.

Such a long response time is not suitable for real-time control applications in a machine or a fast process loop. Richard McGrail, COO Avek/B&B Electronics puts it bluntly: “Wireless is useful for turning on and off a building light, but I wouldn’t trust it to turn off a machine just before it takes my hand off.”

And there is not much you can do to speed up a wireless system. The problem, according to Francis de Costa, founder and CTO of MeshDynamics is that radio is a shared medium, and it forces all stations to remain silent while one node transmits.

A “Request-to-Send” by one node silences all nodes within range. “That does not sound too bad in a wireless sensor network, but it is disastrous if you wish to provide the level of latency and throughput people are accustomed to with wired networks,” says de Costa. He explains that a system with 10 nodes could be reduced to where one node is operating at as little as 1/16,000 of the total bandwidth.

Why has this problem not been noticed before? “Because, first, there are not a lot of mesh networks around and, second, they have not been tested under high-use conditions,” he explains.

Wireless mesh sensor networks seem ideal for non-critical applications such as monitoring wastewater plants, tank farms, pipelines, HVAC systems, energy management, vibration and similar tasks. But, until IEEE, Zigbee and WINA standardize wireless, and figure out how to make wireless fast and deterministic, you may want to put off developing wireless machine controls. The scenario portrayed in Figure 1 is here to stay, at least for a while.
From what we’ve seen, it appears that hard-wired mesh networks are the future of process control and automation. Wired mesh networks seem to be the next step in the never-ending evolution of Ethernet.

Wireless mesh networks save wiring and installation costs, but have yet to prove themselves for control. The ideal solution may well be a mixture of the two—once vendors prove to you that their systems work, of course.

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Inside mesh networks

esh networks based on hardwired Ethernet switches and routers have been used for several years, but they were called “switched networks.” The term “mesh network” really didn’t become popular until wireless mesh sensor networks attracted our attention in the past year or so.

In a mesh network, any single device can link to many other devices, and the connection path from one device to another can take many different paths. This is also called “peer to any peer” connectivity, and often is diagrammed like a big spider web, or “mesh,” to illustrate the many paths possible.

If one device fails, “self-healing” software finds another path through the mesh. This is accomplished with special routing software algorithms that find or “discover” an alternate path when the primary path fails.

In cases where time is not an issue, the mesh network reconfigures itself constantly, always discovering the strongest link between nodes. This is known as a proactive-discovery network, because it is always looking for links.

This approach might not be suitable for industrial control, where speed is a major issue. For speed, the mesh network can use single-path routing; that is, the link between nodes A and B is always the same, and the system never seeks an alternate route except when a failure occurs. In some cases, the entire route from A to B to Z is defined and used at all times. If the link fails for some reason, then the self-healing software will find another route.

In a traditional star network, devices communicate through a central hub. In a ring network, devices are linked by a cable that runs from device to device. For redundancy, these networks often use a second cable, so if a break occurs, the device automatically switches to the alternate wire. Because a wired mesh network simply finds another path, it does not need redundant wires.

Two types of mesh networks exist: wired and wireless. The primary difference between the two is that a wireless network node can directly communicate with any other node that’s close enough to receive its RF signal; wired mesh networks must go through an Ethernet switch to reach other devices. Some mesh networks can combine wired and wireless.

Wireless nodes contain all the software needed for self-healing and discovering alternative communications paths. In wired systems, all such intelligence is built into the switch.