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ISA100 and Hart Communication Foundation (HCF) chose to implement slightly different protocols to meet expressed desires from their customer bases. Users on the ISA100.11a committee requested that we include several options to optimize the protocol for high security, high reliability, high throughput or ease of deployment. In the WirelessHART specification, HCF based the transport on standard HART messaging and chose to limit the options to make deployment simpler. In either case, it is up to the vendor implementation to expose options and set defaults such that the users don't have to actually make any decisions unless they want to tune the system.
ISA100.11a users wanted to be able to extend the network to include as many as 10,000 devices and a wide variety of plant- and in-plant-related applications, so the committee implemented an IP-based network layer and a secure transport layer (using industry-standard IPV6 addressing and UDP or TCP) and an "over-the-air" provisioning mechanism (plug-and-play), since touching 10,000 devices was considered impractical. When it came to security, one of the ISA100 users manufactures diapers, so they wanted a secure mechanism to disable security for those devices deployed in their diaper plant, but not for devices deployed in their other facilities. Finally, the users on the ISA100.11a committee did not want to risk latencies above 100 ms, so the mesh networking protocol in ISA100.11a limits the depth of the mesh to the point where no user could get into trouble by setting up the network with unexpected high latencies.
WirelessHART specifies a "full mesh" network where all devices must have the capability to be a router, so that whenever possible an alternate path can be found within the network, even at the expense of higher latency or degraded battery life. The ISA100.11a standard allows for routing as well as allowing less complex devices which are not capable of routing and may have longer and more predictable battery life to participate in the network as requested by the ISA100 user participants.
Again, I don't see this as good vs evil or mine is better than yours. It is simply the case where users get to choose which protocol is most appropriate for their application. Some end users are even mixing multiple protocols for specific deployments. I think that arguing about winning and losing is missing the point. The idea that a single wireless protocol is going to "win" in the marketplace, I think, is unrealistic.
These are my views, not those of my employer, or of ISA or the ISA100 committee.