## 3 Routing  
Most existing routing protocols such as MintRoute [5], MultihopLQI [7], Directed Routing [8], Backbone Routing [9], etc. use periodic beaconing to estimate the link quality and maintain routing topology. Each node periodically measures the link qualities between itself and its neighbors by calculating the expected transmission (ETX [10]). The periodic beaconing has the following drawbacks:  
(1) Periodic beaconing has a slow response to network variation. The link quality is a statistical result, so it must to be estimated after many beacon exchanges for more accuracy. In some situations, these beacon exchanges should be finished in a short time. For example, when a node parent has failed, the node needs to find a new parent right away. However, taking into account the energy efficiency, the protocols using periodic beaconing should not use short intervals. As shown in Figure 3, when the link from C to A is failed, C needs to select a new parent. Assume at least eight rounds of beaconing are needed to accurately estimate link quality. The default beaconing interval in MintRoute is 8 s and at least 64 s are needed to recover the failure route. CENet solves the problem by considering a network change as an event. When an event is detected, route maintenance is triggered. Many beacons are exchanged in a short time and failed routes can be quickly repaired.  
(2) Periodic beaconing is not energy efficient. A node periodically broadcasts beacons to its neighbors without considering link quality and data transmission. We denote the link from a node to its  
\begin{table}
\begin{tabular}{|l|l|l|} \hline \multicolumn{1}{|c|}{**Mote Type**} & \multicolumn{1}{c|}{**Dot**} & \multicolumn{1}{c|}{**Mica2**} \\ \hline Microcontroller & MSP430F1611 & ATmega128 \\ Radio & CC1101 & CC1000 \\ Data rate (kbps) & 38.4 & 38.4 \\ Power dissipation for reception (mA) & 15.0 & 15.0 \\ Power dissipation for transmission (0 dBm) (mA) & 16.0 & 16.0 \\ Battery & A button battery & A pair of rechargeable batteries \\ Integrated sensors & Yes & No \\ \hline \end{tabular}
\end{table}
Table 1: Node parameters.  
Figure 3: (a) The link from C to A is failed. (b) C selects G as its parent.  
parent as L2P. A node selects the neighbor node with the best link as its parent. When L2P quality is good enough to support data transmission reliability, it is not necessary to change parent. As shown in Figure 4 (a), the parent of node C is A and only the bidirectional link between C and A effects their communication reliability. Let's focus on the nodes inside the circle, as shown in Figure 4 (b). Node C broadcasts beacons to its neighbors and estimates the link qualities periodically. But for the link between C and A, the beacon exchanges between node G, D, E and F are redundant and waste energy. CENet solves the problem as follows: when a node is trying to find a parent, the link qualities of all the routes connected to the BS are estimated using beacons. When the node has found its parent, it stops issuing beacons. The link quality variation of L2P is monitored by transmitted data.  
Our data-aided routing protocol can overcome the shortcomings of the current periodic beaconing routing protocols. It is more energy efficient and reliable for data packet transmission.