# Low Power Multi-Hop Networking Analysis in Intelligent Environments
## 6 Selection of the Most Suitable Low Power Mode  
This paper introduces research on the performance of the park and sniff low power modes defined in the Bluetooth standard specification [24]. More precisely, it was the time performance the one under test in the experiments carried out during the research. Both low power modes, sniff and park, provide a useful way to reduce the power bill of a pair of connected Bluetooth devices, a master and a slave node. They are eligible when no active participation in the piconet is needed, but the slave device wants to remain synchronized. The results from the experiments summarized and discussed in Section 5 demonstrate that there is no only one low power mode that provides the best performance when operating in Bluetooth multi-hop networks. The choice depends on the following factors discussed in this section. To begin with, the key points of sniff and park modes are summarized. On the one hand, while in sniff mode, the master and slave devices exchange ACL packets only on specific timeslots. Therefore, the slave needs to be ready only for these timeslots. In fact, the absences scheduled throughout extended periods of time allow the slave to save power by turning off its radio. As a result, the battery operating time is reduced and the power bill decreased. Alternatively, sniff mode may also be used by a device that participates in multiple piconets, also known as scatternet, when it needs to perform some action in another piconet. \begin{table}
\begin{tabular}{c c c} \hline
**Coefficient** & **Master-to-Slave** & **Slave-to-Master** \\ \hline \(t_{\text{1s\it{inf}}}\) (ms) & 230 & 320 \\ \(\delta_{\text{a}}\) (ms) & 14 & 15 \\ \(\alpha\) (\%) & 82 & 78 \\ R\({}^{2}\) & 0.9773 & 0.9801 \\ MSE & 0.1161 & 0.1088 \\ \hline \end{tabular}
\end{table} 
Table 2: Values of the coefficients in Equation (2) for master-to-slave and slave-to-master multi-hop communication with nodes in sniff mode. On the other hand, while in park mode, even though a slave does not actively participate in a piconet, it remains synchronized to the master. Hence, the slave must wake up at regular intervals to resynchronize and listen for packets. When slaves are in park mode all traffic is stopped, ACL and Synchronous Connection Oriented (SCO). Actually, only small data transmission is enabled for maintaining the synchronization. The master supports a beacon channel to allow the broadcast traffic, the piconet synchronization, and the unpark negotiation. Then, the data traffic is only enabled right after finishing the unpark procedure. Park mode offers some advantages to the management of nodes in a piconet. On the one hand, park mode allows more than seven slaves to participate in a single piconet, and on the other hand, it allows, as sniff mode also does, to reduce the power consumption of the nodes as they are only active at regular intervals. However, park mode shows inherent problems in terms of QoS [51]. For instance, the traffic scheduling to more than the seven active slave nodes requires simultaneous indirect scheduling for park/unpark procedures, making it unpredictable and leading to data packet losses. Moreover, the unpark procedure is not deterministic and it can last as much time as the supervision timeout, which is set to 20 s per default. Therefore, since no latency guarantees on the unpark procedure are provided, instead of parking a node, it could be better to disconnect it and reconnect it just when the node needs to be addressed once more. But another solution can be found in the specification [24]. It consists of operating in sniff mode with the master node transmitting _null_ packets instead of _poll_ packets, so that the slave does not have to respond to them and, therefore, it can save power. When the network operates in such way, the slave node has to respond only when the master requires an update of the supervision timeout.