After spending almost four years to find a technical path to converge the WirelessHART specification (IEC 62591) with that of ISA 100 Wireless (ISA/ANSI 100.11a, IEC 62734), the ISA100.12 WirelessHART Convergence Subcommittee has abandoned its work without finding a single convergence solution. The subcommittee had prepared a request for proposal (RFP) asking any organization or group of organizations to propose a technical solution for creating a single specification for a wireless process control network. The hoped-for network specification would replace both WirelessHART and ISA 100 Wireless, but would define ways to address backwards compatibility. Three proposals were received:
- Team E—Based on WirelessHART (IEC 62591); many features of ISA 100 Wireless (ISA100.11a or IEC 62734) added to meet user requirements.
- Team H—Based on ISA 100 Wireless with a HART-based application layer added.
- Team G—Specified a common network management entity that would allow the union of networks based on both WirelessHART and ISA 100 Wireless.
An evaluation team was formed to determine which, if any, of the proposals was acceptable. The selection criteria were based on a set of common user requirements (CURT) developed with the User Working Group. The evaluation team prepared a matrix showing how each user requirement could be met by each of the three proposals. The following is a summary of the evaluation team's findings:
- Team E's proposal could meet all user requirements, but only after certain agreed-upon extensions to the current WirelessHART standard and the Team E proposal were added. No backward compatibility with ISA 100 Wireless recommended.
- Team H's proposal could meet all user requirements after the addition of an application function layer patterned after the one used in the HART specification. No backward compatibility with WirelessHART recommended.
- Team E's proposal could meet all user requirements because it would allow both the existing and extended WirelessHART and ISA 100 Wireless networks to interoperate through a common network manager. No backward compatibility with either WirelessHART or ISA 100 Wireless.
Since the Team G proposal could use any version of WirelessHART or ISA 100 Wireless, including the newly proposed versions, it was also found to meet all of the CURT requirements.
In the end, no proposal addressed the core problem: definition of a single network specification that could replace both WirelessHART and ISA 100 Wireless and offer backward compatibility with both prior existing networks. The basic incompatibilities between the two networks remained, and neither proposal team could accept the requirements to modify their own base network to adopt the other proposal method.
The following summarizes the basic technical incompatibilities preventing interoperability between WirelessHART and ISA 100 Wireless:
- Time synchronization: ISA 100 Wireless uses the IEEE 1588 method for distributed network time that is also used by IEEE 802.15.4e, the latest version of the personal area network standard. WirelessHART uses a fixed slot time interval of 10 ms and derives time from that base.
- Slot time: While the ISA 100 Wireless protocol separates slot times from time synchronization, allowing variable slot times, WirelessHART depends on a single slot time to support time synchronization.
- Meshing methods: WirelessHART uses a dynamic proprietary method to form mesh associations. ISA 100 Wireless also forms mesh associations dynamically in a proprietary way, including the formation of duocast links. WirelessHART supports redundant paths for the mesh, but does not transmit to the redundant node during the same slot time. The duocast method used by ISA 100 Wireless sends redundant messages to the primary node and the duocast node in the same slot time.
- Network addressing: WirelessHART uses network addresses based on a unique number series originating with the HART Communications Foundation. The low-order 16 bits are in the device, while the high order 48 bits are in the gateway. ISA 100 Wireless is similar except that the number series originates with the IEEE Standards Registration Authority. In addition, ISA 100 Wireless fully supports Internet Protocol version 6 (IPv6) at the link layer.
- Transport Layer: WirelessHART created a unique transport layer to assure end-to-end delivery of messages. ISA 100 Wireless uses the Internet-standard UDP method to assure end-to-end delivery of messages.
Unless all five of these differences are resolved, interoperability between ISA 100 Wireless and WirelessHART is not possible without messages being stored in a gateway common to both networks, and forwarded to a receiving node on the other network. Direct communications between nodes on different networks is not possible.