## 3 Performance Evaluation
### Energy Consumption
In order to complement the previous study, we also measure the average current consumption of a CC2540 node configured as a slave for the one-way ATT communication. The distance between the slave and the master is 0.5 m, and the transmit power of both devices is 0 dBm. The slave is powered by a coin cell battery that has a capacity of 220 mAh and a nominal voltage of 3 V. Measurements are carried out by using an Agilent Technologies Power Analyzer (N6705A model). Figure 3 shows a picture of the experimental setup used to perform the described current consumption measurements. Figure 4 plots the measurement results obtained, for a set of _connInterval_ values within the range from 7.5 ms to 4,000 ms, and for a _connSlaveLatency_ of 0. As it can be seen, the device lifetime results shown in Figure 2 are consistent with the average current consumption results plotted in Figure 4. As _connInterval_ increases, the average current consumption decreases, since the slave remains in sleep mode for a greater fraction of the connection event. It is possible to perform multiple configurations of the _connInterval_ and _comSlaveLatency_ parameters which yield the same rate of message exchanges between master and slave under ideal conditions (_i.e._, BER = 0). However, bit errors may significantly affect performance, depending on each particular _connInterval_ and _connSlaveLatency_ tuple. When a slave does not receive a packet from the master at the beginning of a connection event (e.g., due to bit errors in the access address of the packet), the slave cannot apply slave latency and must listen for a packet from the master at the beginning of every connection event, until a packet from the master is received. This mechanism allows a fast resynchronization between master and slave, so that data can be exchanged incurring low delay, at the expense of increased energy consumption. Figure 5 illustrates the theoretically expected lifetime of a slave that transmits a notification after each poll from the master for BER values up to \(10^{-3}\) (_i.e._, the BER for which the sensitivity is defined in BLE). Note that if BER is equal to 0, the notification rate is 0.5 Hz for all the different parameter settings considered. Figure 4: Average current consumption measured in a CC2540 slave, for the ATT one-way communication and _connSlaveLatency_ = 0. Figure 5: Theoretically expected slave lifetime for various _connInterval_ and _connSlaveLatency_ settings (which yield a notification rate of 0.5 Hz for BER = 0), for different BER values. The choice of appropriate parameter values should be made by taking into consideration application requirements. While delivery delay may be tolerated for certain sensor measurement reading applications, a user that presses a button in a remote control is sensitive to delays greater than 500 ms [12]. Hence, if a slave is expected to react to commands sent by the master, setting _connInterval_ to a low value is a safe approach. In sensor measurement transmission applications, the _connSlaveLatency_ parameter can be tuned to offer the desired frequency of measurement readings and minimize energy consumption.