Microwave Engineering Europe - March 2008 - (Page 32)

32 WIRELESS INFRASTRUCTURE point-to-point, star and tree (as shown in the examples in ﬁgures 2(a), (b) and (c)). For example, ﬁgure 3 illustrates how the network in ﬁgure 1 could be reconﬁgured to work just as well as a tree network. The key difference is that data is routed via an intermediary before reaching its intended destination. Such practical pre-determined networks will always be more efﬁcient than a complex mesh and hence less costly to set up and maintain, and less power hungry. ZigBee WSNs are constructed using nodes of varying functionality. These different types of nodes were introduced to reduce costs by allowing the use of the less capable but cheaper types when possible. ZigBee’s node family starts with a ZigBee Coordinator (ZC). This is the most functional device and is used to start the network and provide the bridge to other networks. Then there’s a ZigBee Router (ZR). This device is able to run an application function and act as an intermediate router, passing data from other devices. Finally, the ZigBee hierarchy includes a ZigBee End Device (ZED). This has just enough functionality to talk to its parent node (either the ZC or ZR) but cannot relay data from other devices. ZEDs can be in a low power sleep mode for much of the time and are the device often used to support ZigBee’s claims of long battery life. However, ZR’s and ZC’s are needed too for a functioning network and these are more complicated and power hungry than ZEDs so their battery life ﬁgures are a little less impressive. When setting up a ZigBee network it is ﬁrst necessary to form a subset cluster around a ZC. This also handles requests from neighbouring coordinator nodes wishing to attach their clusters to the mesh (see ﬁgure 4). Such networks aren’t easy to extend on an ad hoc basis, because it is difﬁcult for nodes to casually join and leave the network unless they are the right type. There are technologies where all nodes are identical and thus equally capable of acting as “slaves” or “masters” within a practical network and swapping roles at any time. In such a network, nodes can act as transmitters, receivers or transceivers to route trafﬁc to other nodes and can leave or join the network in an ad hoc fashion. In addition, every node is capable of determining the best time to transmit based on the activity of its neighbours, so no special “coordinator” (such as ZigBee’s ZC) or “router” (ZR) node is required. ANT is an example of this type of network solution. It has an inherent ability to support ad hoc interconnection of tens or hundreds of nodes. This means nodes can easily join and leave the network and required system resource overhead is kept low. Low power consumption is essential for a practical wireless network because the batteries powering nodes need to last for months (or even years) to minimise maintenance. The ZigBee Alliance describes ZigBee as a “low power” alternative. This is certainly the case when compared with Bluetooth. However, classic Bluetooth wireless technology is designed for rapid transfer of large amounts of data from devices with relatively large batteries (i.e. AA or Li ion), rather than the small coin cells typical of WSNs. (Note: a lower power version of Bluetooth will be available with the release of Ultra Low Power Bluetooth in the next few years. However, this technology is not suitable for WSNs being able to support only one master and seven slaves like its bigger brother). But how does ZigBee’s power consumption stack up against a proprietary ultra-low power alternative such as ANT? Power consumption is inﬂuenced by many factors; one important one is the bandwidth of the radio. The bandwidth of the radio broadly correlates to how much time it will need to spend transmitting in a relatively high power ‘on’ mode for a given amount of data. Theoretically, the wider the bandwidth, the faster the transmission and the less time the radio will need to spend out of sleep mode. In the real world, bandwidth costs power and the optimal trade-off point is generally considered Figures 2(a) (top), (b) (middle) and (c) (bottom): Examples of practical point-to-point, star and tree WSNs. Table 1: A comparison of ANT and ZigBee. Microwave Engineering Europe ● March 2008 ● www.mwee.com 031_032-033_034_MWEE.indd 32 22/02/08 12:07:22 http://www.mwee.com

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Microwave Engineering Europe - March 2008 News Contents Comment Wireless Infrastructure: A Direct Conversion I/Q Demodulatordrives Favorable Basestation Cost-performance Metrics Wireless Infrastructure: Mobile World Set to Reshape the Internet RF Amplifiers: Latest Advances in RF Amplifiers Include a CMOS PA Operating at 77 GHz and Significant Advances in PAs for WiMAX and Broadband Applications Many Applications Still Require Unique Performance Benefits of BeO ACE Automated Circuit Extraction Returns to Real Design by Exploring Design Alternatives and Changes in Seconds Exceeding the Standard for Wireless Sensor Networks Products Calendar

Microwave Engineering Europe - March 2008

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