# Opportunistic Sensor Data Collection with Bluetooth Low Energy
## 5 Evaluation
### Sensor Node Current Consumption
#### 5.1.2 Connection-Based Approach
As an example, we consider _advInterval_ = 0.02 s and \(T_{contact}\) = 150 s. A tendency similar to the one observed in Figure 11 is visible, which affects the _connInterval_ periods within \(T_{comm}\). Finally, Figure 14 shows the average current consumption of the sensor node, assuming one contact between the sensor node and the mobile entity per day. Average current consumption of the sensor node decreases with _advInterval_, since time between advertising events, during which the sensor node sleeps, increases. For low _advInterval_ values, the number of advertising packets sent per advertising event (\(N\)) determines current consumption, as advertising packet transmission, and subsequent receive intervals for possible incoming Connection Request packets, occur frequently. However, as _advInterval_ increases, the relative contribution of the advertising event to the average current consumption decreases. For high _advInterval_ values, \(T_{contact}\) is the most relevant parameter, as data exchange during \(T_{contact}\) dominates current consumption over infrequent advertising and sleep intervals between advertising events. Current consumption increases with \(T_{contact}\), as expected. Figure 12: Average current consumption of the sensor node within a \(T_{comm}\) period in the connection-based approach, as a function of _connInterval_, and for \(N\) = 3 and BER = 0. For BER = 0, the _comInterval_ setting has a negligible effect on the average current consumption of the sensor for one contact per day. However, for non-zero BER values, _comInterval_ becomes relevant. When _comInterval_ is low, non-zero BER leads to negligible difference in terms of sensor node average current consumption, compared to a BER = 0 scenario (not shown in Figure 14 for clarity). However, such difference increases with _comInterval_, _advInterval_ and BER, and decreases with \(N\). For example, Figure 15 illustrates sensor node average current consumption for BER = \(10^{-4}\), BER = \(10^{-5}\), and for the highest _comInterval_ value (i.e., 4 s). The sensor node average current consumption difference becomes  
Figure 14: Average current consumption of the sensor node in the connection-based approach, for a time between contacts of one day, as a function of _advInterval_, and for different \(N\) and \(T_{contact}\), and for BER = 0. A theoretical value of \(T_{contact}\) = 0 has been evaluated, however depicted results in the logarithmic representation used in the figure are very close to those of \(T_{contact}\) = 45 s. Thus they have been excluded from the figure for the sake of clarity. Figure 13: Average current consumption of the sensor node within a \(T_{comm}\) period in the connection-based approach, as a function of _comInterval_, for several BER values, and for \(N\) = 3, _advInterval_ = 0.02 s, and \(T_{contact}\) = 150 s.  
significant for high _connInterval_, since sleep intervals after connection event termination are then large, and for high _advInterval_ and low \(N\). The latter happens because the performance difference during \(T_{conn}\) (i.e., the active portion of the time between contacts), as a function of BER becomes more significant when low energy is consumed in the rest of time (in which only advertising activities consume more energy than sleep periods).