# Performance Evaluation of Bluetooth Low Energy: A Systematic Review
## 3 BLE Performance
### Latency  
In the context of communication, the latency is the interval of time a data packet needs to be transmitted from a node to another one, and vice versa. In some studies, the latency has been modeled and evaluated during the discovery phase, where a node scans other nodes to connect with and also during a connection [79; 88; 89; 90]. The discovery latency refers to the interval for the _advertiser_ to enter into advertising mode up to the point when the advertising information is received by an _initiator_. The discovery latency as function of the _AdvInterval_ and the _SccanInterval_, described in Section 2.2.1, has been modeled and evaluated in [79]. The authors defined two new parameters:  
\[\alpha=\frac{AdvInterval}{T} \tag{4}\]  
\[\beta=\frac{ScanInterval}{T} \tag{5}\]  
They obtained the average latency, both theoretical and simulated, for fixed values of \(\alpha\) and variations of \(\beta\) and vice versa, as shown in Figures 20 and 21. Figure 19: Current consumption as a function of throughput for different BLE ICs. This value is computed using the maximum and the minimum number of packets per _connEvent_ (PPCE) each device may support. Adapted from [85]. A similar study about the discovery latency has been done in [78]. They also proposed a general model for analyzing BLE in the discovery process. This model is also validated using a simulation compared with the experimental results obtained by [79]. They found that the results of the two works are comparable, noticing that the average discovery latency, varying _advInterval_, _scanInterval_ and _Scan Window_, goes from 1 to 30 s. They also highlighted that [79] fails to model the discovering process when _advInterval_ is higher than _Scan Interval_. These results are shown in Figures 16, 17, 22 and 23. Figure 21: Average latency comparison with varied scanning duty ratio (\(\beta\)) and a fixed advertising duty ratio (\(\alpha\) = 0.3). Adapted from [79]. Figure 20: Average latency comparison with varied advertising duty ratio (\(\alpha\)) and a fixed scanning duty ratio (\(\beta\) = 0.75). Adapted from [79]. In order to decrease collisions and unsuccessful discoveries, [PERSON] et al. [80] proposed a new discovery mechanism, already mentioned in Section 3.3, which improves BLE performance in terms of latency. Furthermore, [PERSON] et al., in [88; 89], studied in theory and in simulation how the discovery latency varies in relation to some network parameters (shown in Figure 24), in addition to the parameters observed above. For example, they evaluate the latency in comparison with the number of _scanners_ and  
Figure 23: Average discovery latency of the _advertiser_ according to different values of _Scan Interval_. Blue and black curves indicate the theoretical model and the simulation proposed in [78], while the red lines represent the latency model of [79]. \(M\) is the number of pairs of _scanners_ and _advertisers_, which can communicate with each other, while \(p_{0}\) is the failure probability due to the interference with other devices. Adapted from [78]. Figure 22: Average discovery latency and energy consumption of the _advertiser_ according to \(T_{a}\) (\(T_{s}\) = 1.28 s and \(T\) = 10.24 s) [78]. The green line shows the model proposed by [76], while the black and blue lines positioned in the lower part show the models analyzed in [78]. The crosses represent the simulation based on the theoretical model. As can be noticed, when \(T_{a}>T_{s}\), the two results do not agree. On the other side, the red line [79] and the two blue and black lines positioned in the upper part of the figure represent the data relative to the latency, analyzed in Section 3.4. Adapted from [78]. advertisers_. Their results are shown in Figure 24. Similar analyses have also been done by [PERSON] et al. in [91], in which they evaluate the discovery latency changing the _advertising_ and _scanning_ parameters.