## 5 Evaluation  
### Simulations  
In this section, CENet is evaluated in simulations. A prototype was implemented in TinyOS [18]. CENet system is compared to standard TinyOS networking stack (MintRoute [5] on top of B-MAC [6]), standard stack for short, in terms of reliability and energy efficiency. TOSSIM [19] is used in all simulations, which is a discrete event-based simulator for TinyOS. TOSSIM requires a user specify the signal attenuation levels for every network link. We used the Log Distance Path Loss model to calculate these attenuations [20]. One hundred and one nodes are deployed in an area of (50 m, 50 m). There were 70 slave nodes, 30 master nodes, and one BS placed at (0, 0).  
The number of beacons for CENet and standard stack are shown in Figure 9. The beaconing rate of CENet is high during network startup because every node is busy with exchanging beacons to discover the routes and construct the network. The beaconing rate decreases after a period of time, because in  
Figure 9: The number of beacons for CENet decreases after a period of time. It is significantly less than that of standard stack.  
CENet beacons are only sent when the route needs be maintained. In MintRoute, beacons are sent at a fixed interval of 1 minute, which is a standard stack routing protocol. CENet has much less number of control packet transmissions than MintRoute.  
Figure 10 shows the simulation results when CENet and standard stack are run for over 8 hours, respectively. The maximum number of retransmissions for CENet and standard stack are both 4. The delivery ratio for CENet and standard stack are 93% and 76%, respectively. Therefore CENet has higher reliability than the standard stack.  
The ability of route failure recovery is studied by running CENet and the standard stack for 25 minutes with a packet transmission interval of 10 s. After 10 minutes, we remove five master nodes that are forwarding most of the packets in the network. The simulation results are shown in Figure 11. It shows that CENet has only a small drop in delivery ratio at 12 min. This is because when a node detects a route failure, it transmits beacons periodically with an interval of several hundreds milliseconds. Thus the failure route can be repaired quickly. However, the delivery ratio for standard stack is 55%. This is because its routing protocol is MintRoute that sends beacons every 1 minute. Therefore it needs several minutes to repair the failure.  
Figure 11: Robustness of CENet and standard stack when five master nodes are removed in the 10 th minute.  
Figure 10: CENet has a consistently higher delivery ratio than standard stack.  
The energy consumption for CENet and standard stack is studied by the following simulations. Slave nodes generate sensing data every 30 seconds and MintRoute beaconing interval is 1 minute. Figure 12 shows that the average node energy consumption is 9.1 mAh for the standard stack and 5.65 mAh for CENet over 3 hours. Therefore CENet is about 38% more energy efficient than the standard stack. Figure 13 compares the energy consumption of delivering each data packet from source node to BS for CENet and standard stack. It shows that control packets for CENet are much less than those for the standard stack (3% _vs._ 7.2%). The decrease in the number of control packets is due to the data-aided routing protocol, where beacon exchange happens only when route needs to repair.  
serve 51 days for a node running CENet and 29 days for running the standard stack. When the sample interval is 5 minutes, the average energy consumption of each node is 6.9 mAh/day for CENet and 12.8 mAh/day for standard stack. So the network lifetime can be longer than one year for a CENet network.