Courtesy of Emerson’s VP of wireless technology, Bob Karschnia, I received a draft copy of a whitepaper circulating about redundancy in WirelessHART device networks. It’s not yet finished so I don’t have a link, but here are some of the key thoughts I gleaned from it.
Redundancy, in this context, is defined as a duplication of critical system components to reduce the probability of a failure caused by a single component. This redundancy is available at three levels including the network of wireless field devices, the access points and the gateways to the control and/or asset management systems.
Starting with the wireless field devices, the WirelessHART standard supports communications redundancy through multiple paths (spatial diversity), multiple transmission frequencies (frequency diversity) and multiple timing possibilities (time diversity).
Consider a wireless temperature transmitter mounted in your process communicating with other wireless devices–say a pressure and level transmitter. This device creates a self-organized communications path through one of the other devices back to an access point or directly to a wireless gateway. If the path through this device is obstructed, the temperature transmitter will retry at a slightly different time, frequency and path to the other device. If it fails, it will retry–again adjusting time, frequency, and path.
As we discussed in an earlier post, Planning Your Wireless Instrument Installation, it’s important that each wireless device have at least two other devices to communicate with to provide alternative paths when needed.
An access point is a specialized WirelessHART device with a high-bandwidth communication interface to the gateway. Multiple access points can be connected to the gateway to provide path diversity and increased bandwidth across the network. There is no limit to the number of access points in the field network.
At the highest level of the field network is the gateway, network manager software and security manager software. The network manager performs scheduling and routing services. The security manager performs security key generation and storage as well as field device authentication services. All three components can reside within a physical gateway or can be distributed in separate gateways.
Using a mechanism similar to redundant controller pairs available in most automation systems, primary/backup redundancy management is being developed and stress-tested for these gateways.
I’ll keep a sharp eye out for the finished whitepaper and update this post with a link.