RF Designline
The explosion of wireless technologies in recent years has allowed the emergence of several standards, especially in the Industrial Scientific & Medical (ISM) band. Among these emerging standards ZigBee is considered one of the most promising. Analysts are forecasting several hundreds of millions of ZigBee-enabled devices in coming years. Without a clear understanding of whether or not the ZigBee standard is the right fit for their application, many engineers have been developing new products based on the ZigBee platform. This article aims to help engineers answer a fundamental question when faced with selecting ZigBee technology: Is ZigBee the right technology platform for the next product to be developed?
ZigBee Platform Overview
The ZigBee standard is supported by a consortium of over 200 companies grouped under the name of ZigBee Alliance. The goals driving the ZigBee Alliance are the creation of a reliable, low-cost, low-power, open global standard for low data rate wireless solutions, while allowing multi-hop routing of data. The ZigBee standard through mesh network capability and AES 128-bit encryption provides support for self-healing and high security. Figure 1 describes a ZigBee network topology which typically includes three types of devices or nodes:
Coordinator : One coordinator exists in each network. It starts the network and handles management functions as well as data routing functions. These functions require that the coordinator always be powered. Therefore, this type of node is recommended to be main-powered.
Routers: In most cases, routers are also main-powered. They help carry data across multi-hop ZigBee networks including a variable number of routers and, in some cases, are without routers, thus, transforming the network into a point-to-multipoint.
End Devices: These are devices that are battery-powered due to their low-power consumption. They sleep most of the time and wake up regularly to collect and transmit data. Devices such as sensors are configured as end devices. They are connected to the network through the routers.
The type of node is assigned during the commissioning process. The main-powered requirement for coordinators can be a limiting factor for ZigBee, especially if minimizing power consumption is actively targeted for each and every device.
As the number of nodes in a ZigBee network increases, potential communication bottlenecks can occur in some parts of the network. Two main techniques can be used to limit the congestion issue within ZigBee networks:
- Through node placement and adequate ZigBee router coverage within an installation area. This provides multiple paths for messages to reach a concentrator and alleviates potential bottleneck points in the network.
- Use several data concentrators instead of a single concentrator. This can reduce the number of hops required for nodes for their messages to reach a concentrator, and also reduces the chance of having a single point of failure.
In addition to the three methods mentioned earlier, techniques such as network partitioning through channel or PANID (or network ID) are available to deal with the ZigBee network congestion. These techniques, however, can be resource intensive.
Taking a close look at the ZigBee stack can help better understand challenges that implementing ZigBee can pose, especially when choosing a hardware platform. The ZigBee technology stack architecture has utilized the IEEE 802.15.4 standard, adding a set of layers to it to achieve the targeted features. Figure 2 describes the ZigBee stack architecture topology.
Two lower layers, the physical layers (PHY) and the media access layer (MAC) are defined by the IEEE 802.15.4 specification. The PHY deals with the implementation of the direct sequence spread spectrum (DSSS) radio hardware in both 2.4GHz and sub-1GHz band, while the MAC handles access to the PHY layer. The above layers are defined by the ZigBee Alliance, except the application layer which is defined by the end user.
The layers defined by the ZigBee Alliance are respectively: the network layer, the application framework layer, and the application profile layer. The routing and the mesh capability are defined within the network layer. The security features implemented within the ZigBee stack are very flexible as it can be implemented in any of the layers. Security also can be defined for the application framework by the profile. The application profile plays a big role in the standard interoperability by helping implement a common data exchange protocol as well as a set of processing actions.
The application profile layer specifies the application domains and allows devices from different manufacturers to communicate with each other. Profiles for a specific application regroup different related ZigBee clusters library that specifies functional domains within that application. As of today, the ZigBee Alliance has defined three application profiles: Smart Energy, Home Automation and Personal Home and Hospital Care (PHHC). Several other profiles are expected to be completed in the near future. It is always possible for the users to implement their own profile, therefore, making the application proprietary.
The application layer in which the application code is implemented, is fully owned by the end user to control the specific application.
By analyzing the ZigBee stack architecture, you can appreciate its role when the end user chooses a processor. It is imperative that you use a high code efficiency, and large memory size processor when dealing with ZigBee in certain applications such as Smart Energy. The performance required from the processor can vary from one processor vendor to another for the same ZigBee stack. This factor should be carefully weighted during the ZigBee platform selection process.
To ZigBee or Not to ZigBee?
While ZigBee technology presents attractive features, several factors need to be analyzed before selecting the technology. A poor analysis of these factors could result in a very cost-inefficient product. Consider the following factors when selecting the technology process:
- Interoperability: The need for interoperability is a major factor in choosing ZigBee , especially when the product developed is expected to communicate with other manufacturer's devices. If interoperability is not required or desired, then a proprietary solution could be considered for the sake of cost efficiency.
- Power consumption: As the coordinator and routers in the network always need to be "ON," using the main power is highly recommended.
- Software overhead: ZigBee has the characteristic of high software overhead due to the stack size. ZigBee is not recommended for applications targeting very low CPU resource use such as CODE/RAM.
- Full mesh network: The benefit of the mesh configuration is to enable data exchange between points through multi-hoping link. A network of sensors in a large building could be a good example. In such a case ZigBee might be a perfect platform. However, a smart use of a star network provided by IEEE 802.15.4 standard or some other proprietary protocols can help route data at a cheaper cost.
Many engineers are snared by the attractiveness of the ZigBee technology, and possibly overlook solutions that could have helped design more cost-efficient products. In this article we presented an overview of the ZigBee technology as well as possible limitations. We also explained the main factors to considered, avoiding any pitfalls during the technology selection process.
References:
- To learn more about the ZigBee Alliance, visit: http://www.ZigBee.org/
- For more information about the IEEE 802.15.4 standard, visit: http://standards.ieee.org/getieee802/802.15.html.
- For more ZigBee solutions from Texas Instruments, visit: www.ti.com/ZigBee .
About the Author
Iboun Sylla is currently managing business development in the Americas for Texas Instruments Low Power RF products. Prior to this position, Iboun was a senior RF design engineer. Iboun received his Bachelor in Telecommunications Engineering from ESPT (Tunis-Tunisia), and his Master's and PhD in Electrical Engineering from Ecole Polytechnique de Montreal, Canada. Iboun also holds a Master's in Business Administration from the University of Texas at Dallas with focus on Corporate Finances and Strategic Leadership. Iboun can be reached at: ti_ibounsylla@list.ti.com.
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