# On the Benefits of Listen before Talk Scheme for NB-Fi Networks
## 7 Numerical Results
### Results for a Circle with Radius of 0.4 km
#### 7.1.4 Energy Consumption per Successfully Transmitted Frame  
Figure 6(a) shows the dependency of \(E\) on the load for a 25.6 kbps rate. We see that for all modes, \(E\) is almost the same at low loads because the sensors mostly transmit their frames at different times. At high loads, we see that Aloha and persistent CSMA become inefficient because of frequent collisions. In addition, we see that at high loads, the difference between different kinds of CSMA is not very high, but p-persistent CSMA with \(p=0.1\) shows the best performance. This result is explained by the fact that at 25.6 kbps, all sensors use the same subchannel, which is mostly occupied, and the winning strategy is to wait for the channel to become idle and transmit after a random delay. At the same time, if the sensors wait too long, they lose their frames because of buffer overflows but waste energy on channel assessment, which is why non-persistent CSMA loses to p-persistent CSMA.  
Figure 6(b) shows the dependency of \(E\) on the load for a 3.2 kbps rate. The energy consumption is also almost the same for all modes at low loads, but at high loads, collisions  
Figure 6: Throughput, R = 0.4 km, rates: (a) 25,600 bps, (b) 3200 bps, (c) 400 bps, (d) 50 bps.  
cause the sensors in the Aloha mode to retransmit their packets and thus waste much energy. In addition, the difference in energy efficiency between CSMA variants becomes greater due to different algorithms of listening to the channel and collision probability. Persistent CSMA is the least energy efficient because of the high collision probability (see PLR results in Figures 4a,b). After persistent CSMA, at high loads, we see that the energy consumption suddenly increases for non-persistent CSMA with frequency hopping, which is caused by frequent channel assessment and mostly occupied channels. The \(p\)-persistent CSMA with \(p=0.1\) exhibits the highest energy consumption, and the slightly lower energy consumption is seen when \(p=0.01\). Similarly to persistent CSMA, the sensor in the \(p\)-persistent CSMA mode wastes energy by sensing the channel and waiting until it becomes idle. When several sensors generate frames during someone else's transmission, when the transmission ends, these sensors may attempt transmission, which will lead to a collision. The collision probability grows with \(p\). Non-persistent CSMA is the most energy efficient because, in this mode, the sensor consumes the least amount of energy to listen to the channel while avoiding collisions.  
Figure 7c,d show the dependency of \(E\) on the load for 50 and 400 bps rates, respectively. Again, the energy consumption for different modes is similar at low loads because (i) the sensors mostly transmit their frames at different times, and (ii) they rarely select overlapping subchannels for narrowband transmission. As a result, collisions are rare with Aloha, while with CSMA, the channel is usually idle and the sensor consumes little energy listening to the channel. At high loads, Aloha and persistent CSMA consume much more energy due to collisions. We also see that non-persistent CSMA consumes much  
Figure 7: Energy consumption, R = 0.4 km, rates: (**a**) 25,600 bps, (**b**) 3200 bps, (**c**) 400 bps, (**d**) 50 bps.  
energy at high loads because in this mode, transmissions are long and the channel is mostly occupied by transmissions of other sensors, so when a sensor switches from one subchannel to another one, it likely finds the channel busy and spends much energy on frequent channel assessment.  
From the obtained results, we can conclude that the lowest energy consumption is obtained with a 25,600 bps data rate and non-persistent CSMA at loads up to 100 packets/s and p-persistent CSMA with properly configured \(p\) for higher loads.