Microwave Engineering Europe - January/February 2008 - (Page 20)

20 WIRELESS SENSOR NETWORKS — ZigBee Table 1: Comparison of ZigBee and ZigBee PRO feature sets. The different ZigBee Feature Sets offer developers new choices for implementing robust wireless control networks, which are summarized in Table 1. Some ZigBee platform suppliers are providing only the ZigBee Feature Set, or may provide both in the future. Ember Corporation, a ZigBee supplier, has chosen to focus on the ZigBee PRO Feature Set only, available now in Ember’s EmberZNet PRO software offering, since the beneﬁts of the additional features are so strong. It is important that developers understand the new ZigBee PRO capabilities as they evaluate their ZigBee choices. Improved scalability: Stochastic addressing and route aggregation One of ZigBee’s greatest virtues is scalability, allowing networks of thousands of nodes. While this capability is obviously important for large building automation and neighborhood utility metering networks, it has also proven important in home-oriented networks that may grow to 100 nodes or more. ZigBee has emerged as the choice for advanced Home Automation networks in large part because proprietary wireless technologies aimed at the home have failed to deliver such scalability. The ZigBee PRO Feature Set signiﬁcantly improves ZigBee’s ability to scale through a number of new capabilities, including Stochastic Addressing and Route Aggregation. Stochastic Addressing is a new method for assigning the addresses used by individual nodes for routing in the network. The original ZigBee scheme used a “cluster-tree” routing algorithm where a single ZigBee Coordinator node acts as the root of a network and address tree, and each node’s address is assigned based on its position within the tree. While having the node’s location implicit in the address can allow relatively simple routing algorithms, this also limits the practical address space, leading to the possibility of address exhaustion down long branches of the tree structure. Also, changes to the tree topology may cause potentially disruptive re-addressing of signiﬁcant portions of the network. By contrast, Stochastic Addressing allows new nodes to randomly pick an address when they join the network. In the rare case of a collision (two nodes picking the same address), the network stack provides an address conﬂict resolution mechanism utilizing the unique IEEE MAC address assigned to each node. Thus the entire 16-bit address space is available to all nodes, anywhere in the network, and the assignment is persistent even if RF conditions change (such as placement of new obstacles in a building or if a node moves). Another new ZigBee PRO scalability feature is the ability to aggregate routes in the network through the use of many-to-one routing and source routing techniques. In most wireless sensor networks, many or most nodes in the network need to communicate with one or more centralized nodes, such as a centralized home automation controller, a sensor data collection gateway, or a security trust center. For illustration purposes, assume many devices are communicating to a centralized gateway. Each device issues a route discovery broadcast to ﬁnd the gateway, and all the intervening nodes form a routing table entry for each device (see Figure 1). The nodes near the gateway will see many, many such requests, likely overﬂowing the relatively constrained routing tables available within the small RAMs typical of low-cost ZigBee silicon. Such overﬂows cause the devices to issue new route request broadcasts, causing continued routing table churn and additional network broadcast trafﬁc. ZigBee PRO mitigates this behavior through a “many-to-one” route discovery mechanism that allows each device seeking a route to the gateway (continuing the example above) to share the same route and associated routing table entry. For trafﬁc returning from the gateway to the devices, a source routing technique is used, where the gateway remembers the path used from the device to the gateway, and embeds that path in the returning packets enabling the intervening nodes to forward the packets without requiring a routing table entry (see Figure 2). These mechanisms dramatically improve the efﬁciency and stability of the routing tables and lower the amount of broadcast trafﬁc in the network. Improved resiliency: Asymmetric link handling, frequency agility, and PAN ID conﬂict resolution ZigBee-based networks are inherently selfforming and self-healing in their operation. ZigBee PRO provides a number of new features that enhance network resiliency in certain situations. In ZigBee mesh networks, paths are chosen based on the quality of the individual RF links. Unfortunately, the link quality between any two nodes is often not symmetric (i.e. the link quality in one direction is the same as the other direction). This can cause suboptimal routes to be chosen through the mesh, resulting in lower network stability. ZigBee PRO provides an Asymmetric Link Handling capability that uses regular information exchanges between neighbors to enable the selection of the best symmetric links. One of the advantages of 2.4GHz-based ZigBee networks is the choice of 16 different Figure 1: In larger networks, many routes to a centralized mode may cause routing table overﬂows near the central node. Microwave Engineering Europe ● January/February 2008 ● www.mwee.com 019_020_021_022_MWEE.indd 20 24/01/08 15:21:19 http://www.mwee.com

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Microwave Engineering Europe - January/February 2008 Contents News Comment Radio: Raising the Bar for the Radio: Making 802.11n Work Cover Feature: The RF-System-In-Package Trend - Efficient Design with Advanced Design System 2008 Wireless Sensor Networks: The Zigbee PRO Feature Set: More of a Good Thing Very Fast Measurements of Wireless Devices with Small Antennas in Reverberation Chambers WiMAX Update 2008 Bridging the Gap from the CMOS DSP to the Antenna in OFDM Systems Products Calendar

Microwave Engineering Europe - January/February 2008

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