I attended the first of a five-part lecture series on ZigBee today, offered through the DigiKey Continuing Education Center at Design News. The speaker, Charles J. Lord, offered some interesting tips on how to leverage ZigBee technology without paying the high cost of certification.
First, it is important to realize that ZigBee is a specific superset of the IEEE 802.15.4 networking standard, which itself builds on the RF transceiver basics as IEEE 802.15.1 (Bluetooth). What distinguishes ZigBee from 802.15.4 is that the IEEE spec defines a simple packet data protocol for lightweight wireless networks, while ZigBee defines a series of interoperability specs for all defined messages, handshaking, and operations within specific application profiles.
The differences are subtle but important. IEEE 802.15.4, for instance, defines the protocol only to the link layer, and allows different network topologies including star, cluster tree, and mesh. ZigBee uses mesh only, and its interoperability requirements reach higher in the OSI model to include definitions of message content and format. To ensure interoperability, the ZigBee Alliance has an independent certification process that is open only to alliance members. Thus to create a certified ZigBee device you need to pay for both membership and a certification test.
But not all applications that can use ZigBee need to ensure interoperability. You might want, for instance, to leverage 802.15.4 radio hardware as well as all the messaging and related work inherent in ZigBee profiles, but don't care about interoperability because you are creating a unique system that only needs to interact with its own kind.
Lord pointed out in his lecture that you can do just that, and not bother with Alliance membership or certification. The IEEE 802.15.4 implementation has no royalty or other fees. You can even get a free ZigBee stack to play with and learn on (but not use in the final product). But unless you need the ZigBee mark and stamp of interoperability, you do not need to conform to any specific ZigBee profile. You can create your own.
One useful attribute of following the ZigBee standards, even though you have a proprietary profile, is that you can have your system mesh with a certified ZigBee system for message-passing purposes. The ZigBee devices won't be able to talk to your proprietary devices, but they will carry the mail. This will require registration with the ZigBee Alliance, but that is a much less expensive proposition. Lord indicates that more than 450 such devices have already been registered.
Following the guidelines of an industry standard while deviating in the details is an approach worth considering. It allows you to leverage the hardware and software designed to support that standard, while still giving you the flexibility to make customizations that your application needs. As long as your system does not need to be interoperable with standard systems, you can realize significant savings.
Have you ever deviated from a standard to meet specific application requirements while using standards-based hardware or software?