Technology
Trade-offs selecting a wireless instrumentation protocol The larger wireless landscape has changed since the introduction of WirelessHART and ISA100.11a. These two standards have changed the process manufacturing landscape, but some companies are still trying to decide which protocol to adopt. What considerations should guide a facility choosing between the two today? SOURCE: EMERSON
IT’S BEEN OVER A DECADE since the introduction of the two most comprehensive and widely adopted wireless instrumentation protocols: ISA100.11a (IEC 62734) and WirelessHART (IEC 62591). Given the tens of thousands of wireless instrumentation networks in use globally, there should be no question that these protocols work as advertised when deployed correctly with a high degree of cyber security. The question as to which protocol to choose is less clear. Those wanting to argue the selection question on technical minutia are welcome to do so, but for purposes of this article, we will focus mainly on the issues of network topology and network management, which are the key differences in the context of how the larger industrial wireless landscape is changing.
Similarities vs. differences
To begin, it is important to keep in mind that ISA100.11a and WirelessHART are alike in many ways, although still incompatible: • Both are open standards maintained by independent industry organizations and recognized by the IEC. • Both specify end devices that have similar battery life estimates. • Both use the IEEE 802.15.4 radio (as do Zigbee, MiWi and other wireless protocols). • Both are designed for communication of low-data rate devices over short distances with low power consumption. • Both use AES-128 encryption and a secure network joining mechanism. • Both have extensive catalogs of compatible process instruments, actuators and accessories provided by multiple vendors. • Both provide multi-vendor interoperability with certified devices. The overall operational scope of both is very similar and, in that respect, they compete head-to-head in process manufacturing environments. Where differences emerge is in user implementation.
Differing network topology
Companies implementing ISA100.11a typically use a star topology where groups of individual end devices cluster around a router which collects their data and sends it to a central
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ISA100.11a uses a star topology where groups of individual end devices communicate with a router, which sends data to and from the gateway.
gateway. Multiple routers can communicate with a single gateway. Routers are normally externally powered and can therefore have more powerful transmitters than end devices. They are helpful for gateway communications with a over longer distances and with greater bandwidth but have the potential to limit throughput into and out of a single subnet. While this approach minimizes the need for meshing between individual end devices, there can be side effects. Due to typical ISA100.11a network topologies, communication is lineof-sight and devices need to be visible to a router. This means routers need to be mounted in relatively high locations where they can communicate with individual end devices while maintaining a clear path to the gateway. WirelessHART takes a different approach and embraces the meshing concept, using it to create a dynamic self-organizing and self-optimizing network where end devices can communicate directly with the gateway when possible, or via hops from device to device where they are out of range. While meshing can introduce some latency, it is typically a minor consideration and does not affect the overall performance of the network.
Network management software constantly monitors network traffic and adjusts meshing patterns automatically as necessary to avoid bottlenecks and transmission delays. This is one of the biggest differences between the two networks, and while its effect is nuanced, it can still have a major effect on implementation and day-to-day use.
Network management
Hand-in-hand with the difference in networking topology is the difference in network management. ISA100.11a requires manual network management to the extent that when a new end device is added, it needs to be assigned to a given router and configured to follow specific communication paths as primary and backup. Network management personnel have the tools to control these interactions, and diagnostic software can indicate performance as desired. WirelessHART, on the other hand, is self-organizing and self-optimizing. When a new end device is added to a network, the management software automatically determines how it will communicate, without the need for any manual management or
in d u s t r ial et h er ne t b o o k
10.2019
