The ZigBee standard now stands along side Wi-Fi and Bluetooth as an established and critical wireless technology for an increasingly interconnected world. ZigBee's ability to simply and reliably form self-configuring, self-healing, mesh networks of low-cost, very low-power nodes is unique among these wireless standards sporting fanciful names.
As a result, ZigBee has found widespread applications in home and building automation and new energy management systems that not only offer new conveniences but promise to contribute to solving some of society's most pressing problems.
The ZigBee Alliance has recently introduced an important update to the ZigBee specification leveraging the experience gained from the millions of ZigBee nodes deployed by multiple vendors since the specification was first published in 2004.
The specification now includes two distinct Feature Sets "ZigBee" and "ZigBee PRO"that define key aspects of how a mesh network operates. The new ZigBee PRO Feature Set offers significant improvements in network scalability, resiliency, security, and ease-of-use, especially for larger, more complex networks.
The ZigBee Feature Set is essentially the same as the 2006 version ZigBee specification, with some new optional features adopted from the ZigBee PRO capabilities. Networks are generally built using one Feature Set or the other, though end devices implementing either Feature Set may operate in any ZigBee network.
It is worth noting that with this update of the specification, the focus of the ZigBee Alliance is shifting from further networking stack development toward expanding the higher-level Application Profiles, so users can be assured of continuing stability in the underlying standard.
The different ZigBee Feature Sets offer developers new choices for implementing robust wireless control networks, which are summarized in Table 1.
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Table 1: Comparison of ZigBee and ZigBee PRO feature sets.
Some ZigBee platform suppliers are providing only the ZigBee Feature Set, or may provide both in the future. Ember Corporation, a ZigBee supplier, has chosen to focus on the ZigBee PRO Feature Set only, available now in Ember's EmberZNet PRO software offering, since the benefits of the additional features are so strong. It is important that developers understand the new ZigBee PRO capabilities as they evaluate their ZigBee choices.
Improved scalability: Stochastic addressing and route aggregation
One of ZigBee's greatest virtues is scalability, allowing networks of thousands of nodes. While this capability is obviously important for large building automation and neighborhood utility metering networks, it has also proven important in home-oriented networks that may grow to 100 nodes or more.
ZigBee has emerged as the choice for advanced Home Automation networks in large part because proprietary wireless technologies aimed at the home have failed to deliver such scalability. The ZigBee PRO Feature Set significantly improves ZigBee's ability to scale through a number of new capabilities, including Stochastic Addressing and Route Aggregation.
Stochastic Addressing is a new method for assigning the addresses used by individual nodes for routing in the network. The original ZigBee scheme used a "cluster-tree" routing algorithm where a single ZigBee Coordinator node acts as the root of a network and address tree, and each node's address is assigned based on its position within the tree.
While having the node's location implicit in the address can allow relatively simple routing algorithms, this also limits the practical address space, leading to the possibility of address exhaustion down long branches of the tree structure. Also, changes to the tree topology may cause potentially disruptive re-addressing of significant portions of the network.
By contrast, Stochastic Addressing allows new nodes to randomly pick an address when they join the network. In the rare case of a collision (two nodes picking the same address), the network stack provides a address conflict resolution mechanism utilizing the unique IEEE MAC address assigned to each node.
Thus the entire 16-bit address space is available to all nodes, anywhere in the network, and the assignment is persistent even if RF conditions change (such as placement of new obstacles in a building or if a node moves).
Another new ZigBee PRO scalability feature is the ability to aggregate routes in the network through the use of many-to-one routing and source routing techniques. In most wireless sensor networks, many or most nodes in the network need to communicate with one or more centralized nodes, such as a centralized home automation controller, a sensor data collection gateway, or a security trust center.
For illustration purposes, assume many devices are communicating to a centralized gateway. Each device issues a route discovery broadcast to find the gateway, and all the intervening nodes form a routing table entry for each device (see Figure 1).
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Figure 1: In larger networks, many routes to a centralized mode may cause routing table overflows near the central node.
The nodes near the gateway will see many, many such requests, likely overflowing the relatively constrained routing tables available within the small RAMs typical of low-cost ZigBee silicon. Such overflows cause the devices to issue new route request broadcasts, causing continued routing table churn and additional network broadcast traffic.
ZigBee PRO mitigates this behavior through a "many-to-one" route discovery mechanism that allows each device seeking a route to the gateway (continuing the example above) to share the same route and associated routing table entry.
For traffic returning from the gateway to the devices, a source routing technique is used, where the gateway remembers the path used from the device to the gateway, and embeds that path in the returning packets enabling the intervening nodes to forward the packets without requiring a routing table entry (See Figure 2).
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Figure 2: Through shared many-to-one routes and source routing, less routing table space is used.
These mechanisms dramatically improve the efficiency and stability of the routing tables and lower the amount of broadcast traffic in the network.