# Determining the Optimal Configuration of the Multi-Ring Tree for Bluetooth Multi-Hop Networks
## 1 Introduction  
Bluetooth is currently one of the most promising available ad hoc wireless connectivity technologies, and the introduction of its advantageous features is considered to be one of the main enablers of the Internet of Things (IoT) [1]. Bluetooth Low Energy (BLE) technology [2] can be embedded in existing mobile and portable devices, offering the advantages of a short-range radio, low power, and low-cost wireless connectivity. At present, globally there is wide use of smart devices such as smartphones, tablets, and laptops connected to multi-hop networks through BLE devices [3]. Many BLE data applications are emerging in daily life, such as in health networks [4], smart grid [5], wireless sensor networks [6; 7], etc. With integrating the latest innovative communications, smart grid applications become increasingly popular, as they aim to meet the energy efficiency requirements [8]. To improve the reliability and efficiency of power utilization, a smart grid is a combination of two-way data communications, distributed computing, and smart sensors. In addition, one application field is the home area network (HAN) [9], where a communication path is created among smart meters, home appliances, and plug-in electronic devices. Among wireless protocols in recent years, new technologies are emerging aiming at an energy-efficient communication. A novel research activity has focused on BLE because it is an appealing solution in becoming a key technology for smart homes in low-power, low-cost, and small devices [5; 10; 11]. However, these applications pose some important research challenges, such as how to connect BLE devices to share information among users, and how to form the desired network configurations for various types of applications. Recently, the specifications of BLE 5 have been presented to offer significant enhancements that differ in range, speed, and energy consumption compared to the earlier versions of the protocol. Another main feature of BLE is that nodes can play dual roles as relays, and such relay nodes enable inter-piconet communication. The relay node design increases the possibility of implementing a scatternet formation algorithm and multi-hop routing for BLE devices. To-date, the development of multi-hop routing networks has faced some inherent technical challenges, including research design issues related to the formation algorithms [12] and the routing protocols [13; 14]. The problem involves how to construct individual piconets and connect them together into a scatternet. In addition, the routing protocols must deal with the problem of delivering messages efficiently in such scatternets. Many scatternet formation algorithms have been developed to construct a multi-hop ad hoc network, and they can be classified into two categories. One category--the single-hop scenario [15; 16]--deals with situations in which all nodes are within radio range. The other category--the multi-hop scenario [17; 18]--handles situations in which not all nodes are necessarily within radio range. Most of the current scatternets in the multi-hop scenario are generated in a distributed manner [19; 20; 21; 22]. In addition, a number of different topology models [23] can be generated according to the desired purpose of the scatternet. Most of these scatternet models have been studied and can be generally categorized as tree-based, mesh-based, ring, or star topologies. With a non-uniform distribution application, such as a conference hall or an intensive care unit in a hospital, Bluetooth hybrid ring-tree (BlueHRT) [15] forms a ring topology in one dense area, which is connected with multiple tree subnets around the other sparsely covered areas. However, this protocol partitions networks by collecting complete topology information in a coordinator, and involves considerable formation overheads. In addition, using a ring subnet as the core network design is not reliable enough because it is vulnerable to significant failure if a node in the ring subnet fails.