Weighing the differences

The most important question is whether there are any differences between ISA 100.11a and WirelessHART. The answer depends on who is asked.

According to Caro, "The main differences are very technical and not easy for users to identify. Some users have an immediate need for wireless protocol, so that is one benefit for devices conforming to WirelessHART, which will be on the market some number of months in advance of products that conform to ISA 100.11a. Users needing to augment safety systems, for example, are installing whatever wireless they can put their hands on for many of these applications simply because they want the functionality and they are not willing to wait for the standard with the idea that perhaps someday they may go back through those installations and replace them with something that conforms to their plant standard. Many users, particularly those on the ISA committee, say that to them the early availability of wireless devices is not an advantage. Being first to market doesn't necessarily mean that it's the one that is going to prevail." In addition, says Caro, "the time and effort being devoted to ISA 100 has benefits in the information-security area that exceed the requirements for WirelessHART."

Ladd says the radios are the same and the frequencies are the same, but there is a difference in how they communicate and how passwords are established. "WirelessHART only allows join keys or passwords to be keyed in via hard wire, whereas [ISA] intends to transmit them via wireless. From our standpoint, this could be a security risk. Our standard requires a maintenance port that is secure and backward compatible to all handheld communicators and PC-based applications that are in the field today."

Emerson donated a lot of intellectual property to HCF and ISA. "It's the same intellectual property for security, time-synchronized mesh protocols, and 802.15.4 radios. Both of those standards have adopted those pieces. If you peel back all the noise in the system, you'll find that they look exactly the same," says Karschnia.

Double standards

ISA 100.11a and WirelessHART are built using the same chip (i.e., the physical layer of the two standards is identical). They operate at the same frequencies and use the frequencies in approximately the same way. However because of differences in the upper-layer protocols, they are not compatible. But compatibility is not the same as the ability to coexist. "The biggest questions that users ask are, 'We know they are not compatible, but can they coexist? Can I have a network of WirelessHART instruments that I've installed this year and then I go back later on in the same area of the plant and install another network of ISA 100.11a instruments?" says Caro.

Because the two standards use the same frequency, Caro says they have the potential to interfere with each other. "However, because we have recognized that the 2.4-GHz spectrum is busy, we have to provide ways for devices to share that spectrum and to recover from any collisions that may occur. And we have built in the recover mechanisms so that the user generally will not be required to intervene at all; that is, they will self recover, and the two networks will coexist peacefully within that same domain. We believe that to be true and we have additional adjustments that can be made to ISA 100, although it is not clear that any of those things can be done to WirelessHART."

Caro explains that technically, both protocols use a pattern of frequency hopping, and the patterns of frequency hopping are different. But occasionally they will hop to the same frequency at the same time. The result of that will be collision, an error message, and then a retry. The retry will occur at a different frequency and because the hopping patterns are different, then the retry in both cases will be at different frequencies. So there is an automatic recovery mechanism that will enable the two systems to coexist without the need for elegant software tools to adjust the networks.