The ZigBee specification just turned three in December 2007. ZigBee has come a long way since 2004, becoming more mature, better defined, and more focused. The milestone provides an interesting opportunity to reflect not just on the state of the ZigBee standard, but on the state of wireless embedded control (WiEC) technologies in general.
Promoters of the competing WiEC technologies have promised many things to design engineers over the past few years, and the question should be asked: where do we stand?
First, a definition of what qualifies as a WiEC technology should be given. At the transceiver level, they are low power radios that typically have a range between 10m and 50m, data rates under 4Mbps, and operate in any of several Industrial, Scientific, and Medical (ISM) frequency bands.
Network protocols used to control communication between the wireless nodes range from simple point-to-point topologies for machine-to-machine (M2M) communication, through star topologies for basic wireless sensor networks (WSN), to advanced self-healing mesh networks, where all nodes are able to communicate with each other.
Any design engineer who has spent sufficient time in the embedded space is likely familiar with the promises that the promoters of WiEC technologies have made.
The promises varied in their boldness and scope. But some key terms were used by all WiEC technology promoters, almost without exception, to describe their offerings: low power, low cost, high reliability, high security, ease of design-in, and ease of use.
Along with the technical promises came bold forecasts of rapid returns on investment to all involvedthe vendors would sell hundreds of millions of units within a few short years, customers would realize increased efficiencies and lower costs, and the world would be covered with small, low power transceivers that connect everything to everything else.
After the hype
Unfortunately, a few years into the hype, no single wireless technology has delivered on all those promises simultaneously. As so often happens in engineering, compromises have to be made.
How can one technology be equally suited to both controlling the lights in a home and controlling a safety valve in a factory? The promoters of some WiEC technologies did make that very promise to a hopefully optimistic market.
However, after the promises made in PowerPoint presentations were vetted in real hardware, the market became increasingly skeptical. Perhaps the promoters over-promised. Perhaps the silicon and stack vendors under-delivered. Perhaps engineers should have not believed that any single technology could solve all problems.
There are several reasons for the failure to deliver on all those promises (though some were indeed fulfilled). The first is that some target performance goals are fundamentally opposed to one another, and provide formidable engineering challenges. Consider low cost versus high reliability.
Engineering a low-cost solution requires a holistic approach to reducing expenses. First, silicon size must be reduced, requiring compromises in transceiver architecture (open-loop versus closed-loop modulation, as an example, with the latter providing higher reliability at increased die size).
Next, network stack size has to be trimmed down, to minimize the amount of code space needed in the processor running the RF transceiver. Slimming down the network stack would likely mean that intelligent features like complete node-to-node routing and network self-healing have to go. Need those features? You must pay the expense in silicon area.