Zigbee wireless mesh technology uses a simple, cost-effective and battery-efficient approach to provide sensor and control applications with robust, reliable, self-configuring and self-healing networks. The Zigbee Alliance ratified a specification last year that adds mesh networking with security to the IEEE 802.15.4 short-range wireless protocol.
Zigbee networking sits atop the physical and media-access layers to provide the required functionality to create and manage mesh networks. When a node is activated for the first time, the network commands the media access control to search all channels for an available network. Once a net is found, the MAC can provide that information to the network layer and let the end application determine whether to join, or the node can join the net automatically. The network's Zigbee coordinator-or one of the routers-then assigns a 16-bit address to the node according to rules based on the parameters contained in the Zigbee stack profile.
Zigbee networks use the concept of generations of a family. Only fully functional 802.15.4 devices may "procreate," and at Layer Lmax no device may procreate. Zigbee devices that may procreate are considered Zigbee routers (ZRs). Those that may not or cannot procreate are Zigbee end devices (ZEDs). Layer 0 contains only the Zigbee coordinator (ZC).
The coordinator determines the network addressing according to the three network parameters-layers, children and routers. Layers specify the maximum radius for any Zigbee network. For an Lmax = 3 network, no node may be more than three physical RF hops from the coordinator.
Children define the number of nodes in a given layer that may be directly connected to a parent router in Layer n-1. Routers (R) can be limited in number. For any procreating Zigbee parent, the first R address blocks are reserved for router children, while the remaining Cmax-Rmax addresses are reserved for end-device children.
Most products that are mains-powered, fully functional IEEE 802.15.4 devices can be parents. But, as defined above, any device in the outermost layer may not procreate by network rule, whether a fully functional device or not.
defines the maximum number of nodes in a given Zigbee network. In an example network where Lmax = 3, Cmax = 20 and Rmax = 6, a designer can have a total of 861 nodes. There is no limit defined to Lmax, Cmax and Rmax except that Ntotal cannot exceed approximately 216.
In a network with Lmax = 6, Cmax = 6 and Rmax = 6, all children may be routers. The maximum number of nodes in the network exceeds 55,000. The physical radius of the net-even assuming only 10 meters per hop-could be 120 meters, or about 400 feet, potentially covering many acres of office or factory floor.
For a network described by Lmax = 10, Cmax = 60 and Rmax = 2, where there are a lot of sensors and a well-placed routing infrastructure, the maximum number of nodes is well over 61,000. Networks that are physically larger than this are generally broken into multiple subnetworks, just as Internet addresses are divided into subdomains, partially for ease of use and also for added robustness and network capacity.
While an initial network address allocation creates a default tree network, very soon afterward the network routing devices begin to learn additional routes among themselves. In this manner Zigbee nets create a mesh network that may actually do the majority of the routing, depending on routing performance.
Because of the topology of a mesh network, latency varies depending on the number of hops required. But latency may not be an issue for most systems that interact with humans.
Single-hop times, including acknowledgement, are in the several-millisecond range. So even with network processing overhead, typical hop times are on the order of tens of milliseconds, allowing quick reaction times even in multihop networks.
In short, Zigbee technology provides a strong yet simple networking technology. It runs on simple microcontrollers, can last for months to years on standard batteries, and has the built-in ability to self-configure and self-heal robust mesh networks.
Jon Adams (firstname.lastname@example.org), director of radio technology for the Wireless and Mobile Systems Group of Freescale Semiconductor Inc. (Tempe, Ariz.)