ZigBee BASICS

Zowie! What’s ZigBee? A hot new personal area network radio communications standard with a catchy name, lots of hype, and no real product associated with it until years after the marketing push hits the mainstream? Not likely. I’m talking about ZigBee here, not Bluetooth. Actually, that’s pretty unfair. People are always slighting Bluetooth, but the complaints are often specifically related to an application Bluetooth wasn’t designed for. Remember, Bluetooth was originally designed to replace cables between cell phones, laptops, and other devices within a range of 10 m.

Some people say ZigBee got its name from the way bees zig and zag while tracking between flowers and relaying information to other bees about where to find resources (router bees!). It is designed for mesh networking (see Figure 1). The applications are targeted toward groups of unattended wireless systems in homes, offices, and factories. ZigBee is optimized for low-cost, low-power systems. The compromise is fairly modest bit rates—a maximum of 250 kbps versus the 1 Mbps of Bluetooth version 1.2.

Mesh networking makes up for the limited power of each individual node by leveraging the ability to relay data through nearby cooperating nodes. This happens transparently and provides redundancy and reliability, assuming the density of nodes is high enough. It’s a case of the value of the network growing at a greater rate than the rate at which you add nodes to it. The overhead of occasional network reconfiguration takes only a few tens of milliseconds.

Nodes can be full-function devices (FFDs), which embody all the 802.15.4 functionality and features. This allows them to act as a network coordinator or router. An FFD used as a coordinator needs sufficient memory to hold the network configuration, data, and processing power to self-configure the network in addition to its application task. At least one coordinator is required for a network to form. A router stores and forwards messages to and from devices that can’t directly swap messages. A coordinator or router would use a lot more power than a simple node at the edge of the network and may require line power or be powered from a device with a substantial power supply. For example, a cell phone would be a good choice for a coordinator for a network carried entirely by a person.

Reduced-function devices (RFDs) are limited to a star topology and can only talk to a full-function device. They have a low level of complexity and are found at the edge of the network.

ZigBee uses direct sequence spread spectrum (DSSS) modulation in mixed-mesh, star, and peer-to-peer topologies (including cluster-free) to deliver a reliable data service with optional acknowledgments. The range per node is a nominal 10 m, but popular implementations have a single-hop range of up to 100 m per node line of sight (and farther if relaying through other nodes). ZigBee employs 64-bit IEEE addresses and shorter 16-bit ones for local addressing, which allows thousands of nodes per network.

Association, disassociation, and CSMA- CA channel access with an optional guaranteed time slot for high-priority, low-latency transmissions are transparently handled from the application’s point of view, as is AES 128-bit security. Association is the process used to establish a device’s membership in the network. With 16 channels at 2.4 GHz offering 250 kbps, 10 channels at 915 MHz offering 40 kbps, or one channel at 868 MHz offering 20 kbps, ZigBee provides modest bandwidth that enables multi-year battery life from a coin cell in designs with a low duty-cycle (less than 0.1%).