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Editor's Note: This is the second part of a three-part series of articles from Iboun Taimiya Sylla. The first part is called: The ISM Revolution: The Next Big Thing.
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.
Figure 1. ZigBee Network Topology
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.
Figure 2: Architecture of the ZigBee Stack
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.
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