ZigBee Technology

The idea behind ZigBee is to develop a standardized specification upon which low-power wireless sensor networks can operate and be interoperable. The ZigBee specification sits on top of the physical (PHY) and medium access control (MAC) layers of the IEEE802.15.4 standard (Fig 1).

The IEEE802.15.4 standard is focused on low-rate personal area networking with key unique features of low-power, packet-based, highly-secure, large networks at low-cost that will co-exist with other wireless networks (such as Wi-Fi). These make ZigBee suitable for reliable, low-power, wireless data communications for monitoring and control devices.

The ZigBee standard provides more complex network topologies such as tree and mesh networks, standardization and compliance, profiles for various applications and marketing activities. ZigBee divides the network into three layers, layer 1 being the PHY and MAC (IEEE802.15.4), layer 2 being the mesh network and layer 3 being the profiles. Each layer has separate compliance, so for instance, Jennics silicon products with the IEEE802.15.4 MAC in ROM is layer 1 compliant and multiple ZigBee stack vendors are then level 2 compliant.

The level 1 network layer (unique to the IEEE802.15.4 PHY and MAC) was established in 2003 and provides a number of features which are keys to wireless sensor networks pervading industrial, commercial buildings and home applications. As an example, networks will not be deployed in a commercial building unless they have the features of low-power, high level of security, long range, small size and will not interfere with a Wi-Fi network.

The level 2 network layer provides tree or mesh networking and has been established since 2005. Typically a mesh network comprises controllers (or full-function devices), routers and endpoints or reduced function devices (RFD). The key feature of a mesh network is to be able to dynamically add and remove devices (whether routers or endpoints) and the network adapts around the changes. A wide variety of network topologies can be configured, from long, thin networks to wide, fat networks.

At level 3 the ZigBee Alliance creates a number of profiles for common applications. This is at an early stage and will develop over time.

Application Diversity

There are a lot of talks in wireless markets of killer applications. Wireless sensor network products and applications wont have a single killer application, but hundreds of killer applications. The key to understanding the potential diversity of applications is to start from the standpoint of how to enable a specific application, rather than how to implement the standard. In other words, dont think ZigBee, but instead, think applications.

Applications based on this standard are not just limited to one specific market. Its possible to create networks from simple point-to-point networks to ZigBee compliant mesh networks in anything from industrial and commercial buildings, to home automation, personal healthcare and more.

In a typical home automation scenario (Fig 2), intelligent sensors can provide flexible control of lighting, heating, cooling, watering, appliances and security systems C from anywhere in the home. The potential benefits include the ability to adjust the home environment to run more efficiently and to reduce utility costs. In such an environment, the interoperable nature of the ZigBee standard means that even off-the-shelf products should work together in the networked environment. The fact that ZigBee is targeted at applications requiring low power, such as light switches and sensors, means that many of the sensors and nodes can operate using standard batteries for possibly years.

Another main advantage of ZigBee-based networks in a home automation application or even any industrial or other application is that builders and contractors can easily reconfigure heating, lighting, and security systems to accommodate additional sensors and nodes.

ZigBee Evolution

When looking at the how ZigBee is doing in terms of market acceptance and the potential applications it might address, its worth looking at the key challenges that any wireless standard faces and particularly how Wi-Fi and Bluetooth evolved.

The key factors that determine the potential market of a wireless standard are:

- Market awareness: systems developers need to know how to build solutions based on the standard;
- Worldwide standards: these are necessary to overcome the interoperability and co-existence requirements of products based on the standard;
- Technical and cost challenge: the chips that enable the standard must not add significant overhead to the product in terms of size or cost;
- Ease of use: the technology must be easy to implement and use in order to develop products based on the standard.

Both Wi-Fi and Bluetooth reached a critical mass when these factors were met, and in particular they became easy to use at the right price point. It was independent specialist fabless semiconductor companies that enabled products based on these standards to come to market quickly.

The wireless sensor market has reached that same point in its critical mass, with a diverse range of applications; the standard is published, and the technical and cost challenges are now addressed with products. Pricing of under US$5 in low volumes is acceptable to early adaptors and significant price reductions (to under US$2) can be expected as high-volume mass markets are built.

One of the earlier challenges for ZigBee on the standards front was the number of proprietary systems, niche company standards, and use of inappropriate standards (Bluetooth in low-power applications). There is now clear market demand for ZigBee-based products given the availability of a worldwide standard and easy-to-use enabling silicon that meets the technical and cost challenges.

Eliminating Complexity

Despite reaching critical mass, a barrier to entry for developers is the perceived complexity of the ZigBee standard. Many other wireless standards have not met this problem as they only served a small number of high-volume applications rather than the broad range of applications and markets covered by ZigBee.

If the wireless sensor network developer is able to approach the challenge from a solution-orientated viewpoint rather that standard-based angle, then there is more of a chance that the application can be implemented using the standard.

ZigBee provides the foundation to wirelessly communicate with its mesh network stacks, interoperability and co-existence with other networks.

One of the key challenges to the adoption of the standard on a wider scale in the many different markets it is suitable for is for vendors to offer solutions for the application rather than ZigBee products. When the standard is transparent to the user, then ZigBee will surely reach mass market acceptance.

by Jim Lindop, CEO, Jennic