# Low Power Multi-Hop Networking Analysis in Intelligent Environments
## 1 Introduction
Second, the transmission medium is usually unreliable. Finally, there is a significant variability in the communication delay coming from factors as the random delay added whenever the data must be retransmitted and the variable wait since the data are delivered to the Host Controller Interface (HCI) until they are transmitted by radio. As a result, latency is a key metric when defining the QoS of a communication system [17]. There are many wireless communication standards that can be used to implement networks for intelligent environments [18; 19]. Bluetooth is a worldwide communication standard without radio emission license costs. It allows for multi-hop networking with low energy consumption. Such features made this wireless technology the chosen one as the communication standard to implement multi-hop networks for intelligent systems. Even though there are differences among the releases of the Bluetooth standard, the chosen release, i.e., the 2.0 one, is still on the cutting edge of the technology in terms of multi-hop transmission performance [20]. Then, the goal was set on obtaining the data that lead to the empirical model for the latency in the wireless communications. Focusing on that purpose, a test bench based on commercially available off-the-shelf (COTS) products was developed. Many prototype nodes were developed with some Bluetooth modules. Such nodes can be integrated into electronic systems for intelligent environments so that they can interconnect via wireless links. Furthermore, it was built the middleware to manage the performance of the node at the HCI level of the radio module. Thanks to the middleware, the nodes were configured using high-level commands. So, the performance of Bluetooth gateways inside multi-hop networks was analyzed in the tests by measuring the end-to-end latency QoS metric. Not only the tests were repeated a lot of times, but also the data analyzed statistically to limit the errors made in examining a finite set of samples. This paper analyzes the end-to-end latency of the network in order to evaluate the time performance of the multi-hop communication network for intelligent systems. Bluetooth provides several low power modes for the radio transceptor of the nodes. They have been considered two representative Bluetooth low power modes: _sniff_, which keeps the biggest activity and connectivity while nodes are in low power mode; and _park_, which loses the most connectivity when nodes are parked. To the best of the authors' knowledge, there is no empirical analysis of the latency in Bluetooth multi-hop communications with low powered modes. This paper introduces and discusses the empirical models for the upper limit of the transmission latency from end-to-end in a multi-hop wireless network when working with Asynchronous Connectionless Links (ACL) in both, park and sniff low power modes. The models will benefit the designers of intelligent systems and wireless networks providing them with the estimation of the latency that every hop in the communication adds. The paper is organized as follows: Section 2 introduces a case study of intelligent systems networking, as well as the Bluetooth standard multi-hop networking capability and the way it is applied to such systems. Next, Section 3 gives the features of the test bench developed. In addition, Section 4 explains the details of the methodology of the tests. The results of the tests are discussed in the Section 5, as well as some models are proposed for the latency in the case study of multi-hop wireless communications networking. After that, Section 6 discusses the suitability of the low power modes under test for real world applications. Finally, Section 7 summarizes the conclusions and points out the potential future work.