# Improving Route Selections in ZigBee Wireless Sensor Networks  
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1 Underwriters Laboratories (UL) Inc., 333 Pringsten Rd., Northbrook, IL 60062, USA; [EMAIL_ADDRESS]  
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1 Underwriters Laboratories (UL) Inc., 333 Pringsten Rd., Northbrook, IL 60062, USA; [EMAIL_ADDRESS]  
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1 Underwriters Laboratories (UL) Inc., 333 Pringsten Rd., Northbrook, IL 60062, USA; [EMAIL_ADDRESS]  
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2 Department of Electrical Engineering, Northern Illinois University, 590 Garden Rd. In this paper, we focus on one particular aspect that the ZigBee specifications do not fully specify: the estimation of the probability of successful transmission on a link. Such an estimation is important because it is used by nodes to select multihop routes. Often, a node has multiple candidate routes to reach another node; for example, node 3 in Figure 1 can reach node 0 through the nodes 1 or 2; and nodes are supposed to choose the route with the lowest cost. As discussed in Section 2.3, there are several ways to define the cost of a route; and ZigBee specifies the cost of a route as the sum of the cost of each link that compose a route; and the cost of each link is an inverse function of the probability of successfully transmitting on the link [13]. Although the ZigBee specifications do not specify how nodes should estimate the probability of successful transmission on a link, two possibilities are suggested (see page 338 of [13]): estimating the probability of successful transmission at the network layer by counting link status (LS) and data frames; or estimating the probability of successful transmission indirectly through the use of a physical level indicator that reflects the quality (or SINR) of a received packet. In ZigBee, such an indicator is the link quality indicator (LQI) that the IEEE 802.15.4 medium access layer (MAC) includes in every packet received and forwarded to the network layer; and the ZigBee's network layer averages the received LQIs and maps the result into a probability of successful transmission to determine the cost of a link. As discussed in Section 3.1, previous authors have shown that estimating link costs from beacons, such as ZigBee's LS packets, is less accurate than estimating them from unicast data packets and have proposed alternative procedures to improve route selection [14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28]. Although these procedures are valuable and would improve link cost estimation and route selection, their implementation in ZigBee devices would require significant changes in the ZigBee specifications, protocol, and frame formats. The question that motivated this paper was: can we improve link cost estimation and select better routes without changing the ZigBee protocol or frame formats? With this question in mind, our paper has two contributions:  
1. We studied and compared the performances of the LS-based and LQI-based link cost estimation procedures suggested by the ZigBee specifications. We confirmed the findings of previous studies--that selecting routes based on solely the exchange of link status messages or LQI measurements can lead to poor route selections in ZigBee. 2. We proposed and evaluated the performance of a link cost estimation procedure that can be implemented without changes to the ZigBee protocol. As described in Section 6, the method that we propose is founded on estimating the probability of successful transmission by using information from the medium access control (MAC) layer regarding unicast retransmissions. Although the use of MAC layer information to estimate link costs has been considered before in other wireless networks [16; 17; 18; 21; 23; 24; 25; 26; 27; 28], our method has procedures tailored to ZigBee. Furthermore, our procedure defines how to select among routes with the same cumulative costs, which is common in ZigBee links because ZigBee frame formats require the quantization of link costs into three bits.