ZigBee has come a long way, especially since many first heard about the new wireless standard poised to revolutionize the home automation market. For those new to ZigBee or who have not yet had a chance to digest the latest specification, this article serves to explore the status quo of ZigBee, provides an overview of the ZigBee and ZigBee PRO feature sets, and discusses ZigBee in the context of various markets and applications, including a discussion of ZigBee's applicability to the medical market.

How ZigBee and ZigBee PRO Feature Sets Differ
ZigBee has recently ratified the ZigBee-2007 specification which defines two feature sets called ZigBee and ZigBee PRO. Building on the ZigBee-2006 specification, the new 2007 specification provides enhanced functionality and is backward compatible under certain network scenarios. The ZigBee feature set provides tree addressing, AODV mesh routing, unicast, broadcast and group communication, security, and much more. ZigBee PRO replaces tree addressing with stochastic addressing. It includes the same AODV routing used in ZigBee (2006 and 2007), but provides many-to-one source routing as an alternative. ZigBee PRO also adds limited broadcast addressing and adds support for "high" level security. Both ZigBee and ZigBee PRO feature sets provide additional support for optional frequency agility and fragmentation.

ZigBee's tree addressing assigns addresses hierarchically, while ZigBee PRO utilizes a stochastic addressing scheme which randomly assigns addresses to devices and sorts out conflicts by continuously monitoring and reacting to "management" traffic. ZigBee benefits from guaranteed unique addressing and is free from the overhead of constantly monitoring communication and dealing with address conflicts. However, PRO benefits from scaling such as when communication limitations result in a network comprised of many (five +) hops, or when a network is comprised of potentially many mobile end devices. This benefit comes with the price of increased startup delay as ZigBee PRO must allow time for address conflict resolution, which is not necessary for tree addressing.

Both ZigBee and ZigBee PRO routing utilize ad-hoc on-demand distance vector (AODV) routing, but only PRO also supports the many-to-one source routing option. At the expense of a larger protocol stack, many-to-one source routing allows fast route establishment where many devices (e.g., sensors) are all reporting to a single sink (e.g., network gateway device). For unsolicited bi-directional and peer-to-peer communication (e.g., light switch and light), the many-to-one feature becomes less effective and in some cases inappropriate.

Both ZigBee and ZigBee PRO support group addressing, but PRO adds support for limited broadcast group addressing that prevents unnecessary flooding of the entire network when all group members are in relative close proximity. This feature can be useful in reducing network wide communication overhead for large networks, but comes at the expense of using additional precious code space.

Although several additional minor differences exist, the last feature difference between ZigBee and ZigBee PRO is support for high security. High security provides a mechanism for establishing link keys between peer-to-peer connections, and adds additional security when devices on a network may not be trusted at the application layer. Like many PRO features high security can be helpful for certain applications. But it comes at a significant expense with respect to utilizing precious code space that could be used by the application.

While ZigBee and ZigBee PRO share a majority of their features, it is important to know that ZigBee and ZigBee PRO devices play together on the same network only under limited circumstances. If the established network (established by the coordinator) is a ZigBee network, then ZigBee PRO devices will only be able to join and participate in that network under the end device role. This means that the device will communicate into that network through a parent (router or coordinator), and will not take part in the routing or allowing additional devices to join the network. Similarly, if the network is initially established as a ZigBee PRO network, then ZigBee devices will also only be capable of participating under the limited end device role.

Many of the products using ZigBee will, in essence, be proprietary or implement a private profile or application specification. This means that any given network will likely only contain devices sold by a specific company or conglomerate of companies. This is especially the case for medical where no public profile currently has been defined and ratified. In this case, ZigBee will be used for features such as its mesh routing protocol, communication redundancy, and low power operation. However, for markets where devices from various vendors must function as a single cooperating network to perform the designated service or function, public profiles have been specified to ensure that all devices on the network speak a common language and can achieve interoperability. Interoperability in this context means that a product from some company A (light switch) will be able to communicate with a product from a second company B (light ballast) without those companies working together a-priori to ensure the devices work together (Figure 1).

Figure 1: Typical ZigBee products for home automation

During the initial years of ZigBee, the technology's main focus remained on the home and commercial building automation market. As more companies and professionals are exposed to ZigBee and better understand its capabilities and nuances, ZigBee has burgeoned and is being designed into quite an extensive range of additional markets. Most prominently ZigBee has made its way into advanced metering infrastructure (AMI), as well as being designed into products that focus on elderly care and assisted living, medical, telecom, asset tracking, entertainment, and more.