Incompatibilities between 802.15.4 radio devices

Dear Control Engineering: It seems that a whole bunch of different wireless platforms use the same IEEE 802.15.4 platform. If that’s true, why don’t all these systems work together? Do vendors deliberately build in incompatibilities?

While the IEEE 802.15.4 radio is foundational to many types of low-data-rate wireless applications, that commonality is not enough to ensure interoperability between platforms. Many systems use that radio, including ZigBee, WirelessHART, ISA 100.11a, and numerous proprietary offerings, including National Instrument’s device-level radios. These are not compatible systems, and even many that exist under the same umbrella are not compatible with each other. (See an earlier posting on the topic.)

If you visit the IEEE Website and search on 802.15.4, you will turn up more than 900 articles. Or you can purchase a full copy of the standard. The official description reads: “Revision of IEEE Std 802.15.4-2003 IEEE Std 802.15.4-2003 defines the protocol and compatible interconnection for data communication devices using low data rate, low power and low complexity, short-range radio frequency (RF) transmissions in a wireless personal area network (WPAN). This revision extends the market applicability of IEEE Std 802.15.4-2003, removes ambiguities in the standard and makes improvements revealed by implementations of IEEE Std 802.15.4-2003.”

OK, maybe that isn’t very helpful. Put in less technical terms, consider that communication protocols such as Ethernet are built on multiple layers. The lowest layers actually move data and the top layers are the specific application. In this case, the radio is on the bottom. It deals with mechanical stuff like frequencies, and how the transmitters and receivers interact in the most basic terms. That alone can’t do much of anything useful. You have to build those higher layers that determine how the radios interact in greater detail, including the top layer with its application. Once you get above the bottom radio specification, these protocols all diverge in many directions because they are designed to do different things.

If you use a crude automotive analogy, consider the radio like an engine. An engine can run by itself, but has to be put into a chassis with a drive train. The chassis has to have seats and a steering wheel. In the process of adding those additional parts, you could end up with anything from a racing car to a hearse to a minivan. All those vehicles have their own structure and purpose. Communication protocols, wired and wireless alike, are much the same.

–Peter Welander, process industries editor