# A Reconfigurable Mesh-Ring Topology for Bluetooth Sensor Networks
## 1 Introduction  
Bluetooth is currently one of the most promising available ad hoc wireless technologies [1; 2; 3; 4], and the introduction of its advantageous features is considered to be one of the main enablers of the Internet of Things (IoT) [5; 6; 7; 8; 9; 10]. Until now, Bluetooth Low Energy (BLE) technology [11] has been embedded in existing mobile and portable devices, offering the advantages of short-range radio, low power, and low cost wireless connectivity. At present, there is wide use of smart devices, such as smart phones, tablets and laptops connected to multi-hop networks through BLE devices. BLE data applications are increasingly emerging in daily life, such as in health networks [12], industry automation [13], wireless sensor networks [14], etc. However, these applications pose some important research challenges, such as how to connect BLE devices in order to share information between users, and how to form the desired network configurations for various types of applications. The main feature of BLE version 4.1 is that nodes can play dual roles as relays, and such relay nodes can enable inter-piconet communications. The relay node design increases the possibility of implementing the scatternet formation algorithm and multi-hop routing for BLE devices. To date, the development of multi-hop routing networks has faced some inherent challenges. The main technical challenges include research design issues with respect to the formation algorithms [15]and routing protocols [16; 17]. The scatternet formation problem involves how to construct individual piconets and connect them together into a scatternet. On the other hand, the routing protocols deal with the problem of delivering messages efficiently in such a generated scattemet. To construct a multi-hop ad hoc network, many scatternet formation algorithms have been developed; they can be classified into two categories. One category, the single-hop scenario [18; 19], deals with the situations in which all nodes are within radio range. The other category, the multi-hop scenario [20; 21; 22] handles situations in which not all nodes are necessarily within radio range. In addition, a number of different topology models can be generated, according to the desired purpose of the scatternet. Currently, most scatternet formation methods partition networks by collecting complete topology information, introducing considerable computation and communication overheads [23]. In general, the generating architectures can be classified as tree, ring and mesh topologies [24]. While the ring architecture is simple for proactive routing, it produces longer path lengths for packet transmission, especially when the size of the ring network increases. On the other hand, the mesh architecture reduces path length in terms of packet delay, but introduces larger formation complexity than other topologies. In order to utilize the advantages of mesh-subnet and ring-subnet, this paper proposes a ring-based subnet with mesh interconnection, called the Reconfigurable Mesh-Ring (RMR), to construct a scatternet. RMR simultaneously considers scatternet formation and routing design issues to generate a configurable mesh-ring topology. RMR constructs a ring-shaped topology as a backbone subnet, which is then associated with a mesh-shaped topology. However, the placement of the available mesh connection is adjustable, so as to determine the optimal configuration with better throughput for various network sizes. To maximize network capacity, three configurations (including piconet, scatternet, and hybrid) are investigated to determine the optimal placement of the mesh links. In seeking to determine the optimum configuration, a heuristic peak-search method is presented in order to achieve the optimal placement of the available mesh links. The remainder of this paper is arranged as follows: Section 2 proposes the motivation, mesh-ring formation algorithm, and routing algorithm to generate the desired topology.