# Low Power Multi-Hop Networking Analysis in Intelligent Environments
## 2 Intelligent Systems at Home
### Bluetooth Multi-Hop Networking  
The Bluetooth networking capability is based on _piconets_, i.e., the smallest network that can be formed with this communication technology. Whenever two or more Bluetooth devices share a radio channel, they form such centralized networks called _piconets_. A central node (master), which controls the communications, and up to seven active nodes (slaves) are included in _piconets_. Figure 1 shows some _piconets_. For instance, the one colored in dark blue, where the \(\rm AN_{i}\) is the master, and the \(\rm AN_{i+1}\) and the \(\rm MN_{i1}\) are the slaves. Moreover, the standard includes the notion of a more complex network, i.e., the _scatternet_, but it does not define the way to form it. Such network not only helps to extend the coverage of wireless devices, but also increases the amount of active nodes in a limited area. Any Bluetooth device can play the role of slave in many _piconets_ as well as be the master in at most one _piconet_. When some nodes operate in two or more _piconets_, they are called gateways and a multi-hop network is formed, i.e., a _scatternet_. There are two types of gateways in Bluetooth, according to the roles they play in the piconets: master-slave and slave-slave. The master-slave gateway offers less delay from end to end for traffic between two adjacent piconets (inter-piconet traffic) [25]. Since the research is focused on inter-piconet traffic delivery through scatternets, the nodes in the test bench were configured as master-slave gateways, i.e., as masters of the piconet formed with the next node, and as slaves with the previous node (according to the numbers of the nodes in Figure 1). The gateways in Figure 1 are connected in linear topology and they relay the data gathered from both the sensors connected to the \(\rm AN_{i}\) in the rooms and the sensors connected to the \(\rm MN_{ij}\), which are carried by the monitored users. The first sensors can be those to measure the temperature, detect the  
Figure 1: Example of multi-hop network deployment for intelligent systems (distances expressed in meters). presence of people, or even presence of smoke; and the second ones to measure the vital signs or user's body temperature. Then, every halfway node in the wireless test bench, i.e., every gateway, participates in two piconets given that it has an open link as a master and another link as a slave. The communications in Bluetooth follow time division multiplex (TDM) scheme defined by the master node. At the baseband level, each slot lasts 625 us, so the slot rate is 1.6 kHz. Thus, there are inactivity intervals for the nodes inside the piconet. When two nodes establish a link, they negotiate the moments of activity in the piconet and the intervals in which each slave will be transmitting and receiving data. The master node of every piconet in the tests polls slave nodes every 25 ms, as it is set by default. So, nodes can take advantage of the activity breaks in the piconet, where they are slaves, to attend to the piconet where they play the master role, guaranteeing that they can attend to both of the links. Not only the slave nodes but also the master nodes can abandon the piconet while there is no activity and start playing the slave role in the other piconet. But if a node perceives the absence of the other node in the piconet, the node closes the link and the link must be re-established later if it needs to transfer data to the other node. Hence, the latency of the multi-hop communication will increase in such situations. The standard defines the Basic Data Rate that allows up to 723.2 kbps asymmetric rate with 1 bit per symbol transmission under Gaussian Frequency Shift Keying (GFSK) modulation. In addition, the standard defines the sniff and park low power modes to take advantage of the inactivity intervals. Both modes are analyzed in this paper.