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In October 2007, the ZigBee Alliance announced an expanded set of features for the ZigBee protocol.
This new stack profile is universally referred to as ZigBee Pro and for the most part defines specific stack settings and makes mandatory many of the features that are optional in the ZigBee stack that was ratified in 2006.
ZigBee Pro also adds some new application profiles such as automatic meter reading, commercial building automation, and home automation.
In general, ZigBee Pro features implement support for larger networks, which includes anticipating—and addressing—all of the challenges that larger networks encounter.
The Alliance likes to position ZigBee Pro as a seamless extension of 2006 ZigBee but making some formerly optional features mandatory means it is not completely backward compatible. The incompatibilities are all involved with router functions. A ZigBee 2006 node can join a 2007 network, and vice-versa but designers cannot mix 2006 routers with 2007 routers.
Larger networks
Some of the specific problems addressed by ZigBee Pro include doing away with the highly complex routing tables that are typical of larger 2006 ZigBee networks. Multiple methods are now available to simplify routing structures. In addition, routing tables are size-configurable and limited in size only by available RAM.
2006 ZigBee uses a tree structure to assign addresses and this can lead to a massive reassignment of addresses when a new device is introduced to a large network. This technique has been replaced by stochastic addressing, which uses probability analysis to simplify network formation and operation. In addition, devices do not need a new address when joining a new parent.
Another benefit of stochastic addressing is that it does no lead to address exhaustion by churning network tables as tree-structure addressing could do. ZigBee Pro also adopts asymmetric link handling techniques to improve network reliability and boost throughput.
Asymmetric link handling involves first detecting asymmetric links and then using these observations to optimize path selection. In most cases, RF links between two devices are asymmetric (one direction is more efficient than the other). ZigBee Pro keeps track of which links are asymmetric and uses them in favor of symmetric links when it is advantageous. This provides more reliable routing and less route rediscovery in the network.
Figure 1 shows an example of asymmetric link handling. The 10% link in the A-to-B-to-D path might at first glance appear to have an advantage because the outbound path from A to B to D has 99% reliability. But the 10% figure on the return path is an example of asymmetry that substantially reduces the bidirectional efficiency.
Figure 1. Keeping track of inefficient links and avoiding them makes routing more efficient.
ZigBee Pro also adopts the techniques of many-to-one routing and source routing, both of which help minimize traffic particularly when centralized nodes are involved.
Many-to-one routing has the effect of providing a single route of discovery for all the devices in the network to an aggregation point. This minimizes route table size for all the network devices and minimizes route discovery broadcasts, which provides more bandwidth for data.
Source routing also makes the network more efficient because the aggregator generates the complete route from the aggregator to the destination (based on the topology map data in the initiators memory) and places the route information into the frame which needs to be transmitted.
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