A Reconfigurable Mesh-Ring Topology for Bluetooth

energies

Article A Reconﬁgurable Mesh-Ring Topology for Bluetooth Sensor Networks

Ben-Yi Wang 1, Chih-Min Yu 2,* ID and Yao-Huang Kao 2 ID 1 Zhejiang Industry & Trade Vocational College, Wenzhou 325000, China; [email protected] 2 Department of Electronics Engineering, Chung Hua University, Hsinchu 30012, Taiwan; [email protected] * Correspondence: [email protected]; Tel.: +886-351-860-34

Received: 2 April 2018; Accepted: 2 May 2018; Published: 7 May 2018

Abstract: In this paper, a Reconfigurable Mesh-Ring (RMR) algorithm is proposed for Bluetooth sensor networks. The algorithm is designed in three stages to determine the optimal configuration of the mesh-ring network. Firstly, a designated root advertises and discovers its neighboring nodes. Secondly, a scatternet criterion is built to compute the minimum number of piconets and distributes the connection information for piconet and scatternet. Finally, a peak-search method is designed to determine the optimal mesh-ring configuration for various sizes of networks. To maximize the network capacity, the research problem is formulated by determining the best connectivity of available mesh links. During the formation and maintenance phases, three possible configurations (including piconet, scatternet, and hybrid) are examined to determine the optimal placement of mesh links. The peak-search method is a systematic approach, and is implemented by three functional blocks: the topology formation block generates the mesh-ring topology, the routing efficiency block computes the routing performance, and the optimum decision block introduces a decision-making criterion to determine the optimum number of mesh links. Simulation results demonstrate that the optimal mesh-ring configuration can be determined and that the scatternet case achieves better overall performance than the other two configurations. The RMR topology also outperforms the conventional ring-based and cluster-based mesh methods in terms of throughput performance for Bluetooth configurable networks.

Keywords: bluetooth networks; scatternet formation; topology conﬁguration; routing protocol

1. Introduction Bluetooth is currently one of the most promising available ad hoc wireless technologies [1–4], and the introduction of its advantageous features is considered to be one of the main enablers of the Internet of Things (IoT) [5–10]. 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]

Energies 2018, 11, 1163; doi:10.3390/en11051163 www.mdpi.com/journal/energies Energies 2018, 11, 1163 2 of 14 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 scatternet. 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 [20–22] 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: Section2 proposes the motivation, mesh-ring formation algorithm, and routing algorithm to generate the desired topology. In Section3, the problem formulation, reconfiguration mesh-ring topology, and peak-search method are presented to determine the optimal bridge for each link. In Section4, the optimal configuration is determined and throughput performances are demonstrated via computer simulations. Finally, conclusions are drawn in Section5.

2. Proposed Method

2.1. Motivation Until now, the Bluetooth generating architectures can generally be classiﬁed as having line, tree, ring or mesh network topologies. While the ring architecture is simple and suitable for proactive routing, it produces longer path lengths for packet transmission, especially when the size of the ring network increases. The routing protocol on a larger ring subnet needs to consider the routing path length design issue to mitigate the packet delay since most previous studies consider one-way routing in the ring subnet [25,26]. On the other hand, the mesh architecture reduces path length in terms of packet delay performance, but introduces larger scatternet formation complexity than other topologies. Another important issue in a Bluetooth scatternet is routing with a formed scatternet. Current routing solutions for mesh networks, including proactive, the reactive, and hybrid algorithms, discuss protocol design without considering the ease of routing in such a scatternet. As a result, the routing protocol with formation design [27] is challenging, due to existing performance tradeoffs between formation complexity and packet delay. In contrast to prior works on Bluetooth scatternet formation and routing algorithms, a RMR algorithm is designed to jointly address the problems of achieving the simple routing and determining the optimal placement of mesh links for a multi-hop scatternet. With local topology, Energies 2018, 11, x 3 of 14

EnergiesIn2018 contrast, 11, 1163 to prior works on Bluetooth scatternet formation and routing algorithms, a 3RMR of 14 algorithm is designed to jointly address the problems of achieving the simple routing and determining the optimal placement of mesh links for a multi-hop scatternet. With local topology, a ringa ring subnet subnet is isgenerated generated first first and and each each master master in inthe the ring ring subnet subnet forms forms its itspiconet piconet individually. individually. A Apeak peak-search-search method method is isthen then introduced introduced in in the the formation formation phase phase to to determine determine the the optimal optimal placement of available mesh links by interconnecting masters in the ring subnet, thus achieving better network capacitycapacity.. Figure1 1 illustrates illustrates anan exampleexample ofof thethe RMRRMR configurationconfiguration thatthat satisfiessatisfies thethe designdesign objectivesobjectives to generate networks into desired mesh-ringmesh-ring subnets.

S S S S S S S S S M1 M8 S S S S M2 M7 S S S S M3 M6 S S S S S M4 M5 S S S S S S S S S S

Figure 1.1. An illustrated Mesh Mesh-Ring-Ring topology example.example.

2.2. Mesh-Ring Formation Algorithm 2.2. Mesh-Ring Formation Algorithm To reduce the path length and mitigate the packet delay in the ring subnet, a Mesh-Ring To reduce the path length and mitigate the packet delay in the ring subnet, a Mesh-Ring scatternet scatternet formation algorithm with a shortest path routing algorithm is proposed in this study. The formation algorithm with a shortest path routing algorithm is proposed in this study. The algorithm algorithm designs two phases to form the Mesh-Ring scatternet. In the first phase, a coordinator designs two phases to form the Mesh-Ring scatternet. In the first phase, a coordinator advertises advertises and discovers its neighboring slave nodes, computes the Mesh-Ring topology and and discovers its neighboring slave nodes, computes the Mesh-Ring topology and distributes node distributes node connection information to the masters in the ring subnet. In the second phase, each connection information to the masters in the ring subnet. In the second phase, each master in the master in the ring subnet starts to build its corresponding piconet, connects with its neighboring ring subnet starts to build its corresponding piconet, connects with its neighboring piconets via slave piconets via slave bridges and interconnects with another corresponding piconet via one mesh link. bridges and interconnects with another corresponding piconet via one mesh link. Finally, a Mesh-Ring Finally, a Mesh-Ring topology is generated. topology is generated. In the first phase, a root designated as a coordinator discovers and queries its one-hop In the first phase, a root designated as a coordinator discovers and queries its one-hop neighboring neighboring slaves to form the multi-hop scatternet. Whenever the root discovers another device in slaves to form the multi-hop scatternet. Whenever the root discovers another device in its proximity, its proximity, it establishes a temporary piconet. Then, the root will be the master and the device that it establishes a temporary piconet. Then, the root will be the master and the device that was in was in inquiry scan state will be the slave. After a certain period of time, the root can collect all its inquiry scan state will be the slave. After a certain period of time, the root can collect all its one-hop one-hop neighboring information. neighboring information. After collecting all the neighboring information, the connection information is computed by the After collecting all the neighboring information, the connection information is computed by coordinator to form the desired multi-hop scatternet. To form a ring-based subnet, at least two the coordinator to form the desired multi-hop scatternet. To form a ring-based subnet, at least two bridge links are considered. In the ring subnet, one mesh link is introduced to reduce the packet bridge links are considered. In the ring subnet, one mesh link is introduced to reduce the packet transmission delay. In a piconet, the total number is eight and one master is used to schedule the transmission delay. In a piconet, the total number is eight and one master is used to schedule the packet packet transmission in a piconet. In order to mitigate the packet length in the ring subnet and transmission in a piconet. In order to mitigate the packet length in the ring subnet and maximize the maximize the piconet utilization, the minimum number of piconets is achieved under the formation piconet utilization, the minimum number of piconets is achieved under the formation constraint, and constraint, and the formation criterion is defined as Equation (1). the formation criterion is defined as Equation (1).

ln m pp==   (1(1)) 7  where p is the minimum number of formation piconets, n is the total number of discovered nodes, where p is the minimum number of formation piconets, n is the total number of discovered nodes, including the coordinator and all discovered slaves. In general, the piconet number should be equal to including the coordinator and all discovered slaves. In general, the piconet number should be equal n/8 in Equation (1), where 8 is the limit of the number of connected devices. However, each bridge node needs to connect with 2 piconets in the ring subnet, and this bridge can be counted twice in each Energies 2018, 11, x 4 of 14

to n/8 in Equation (1), where 8 is the limit of the number of connected devices. However, each bridge node needs to connect with 2 piconets in the ring subnet, and this bridge can be counted twice in each piconet. Therefore, the remaining number of devices in a piconet is 6; thus, Equation (1) is divided by 7. In Equation (1), the maximum number in a piconet is limited to eight. The bridge link is used to connect with the other two masters in the ring subnet. To connect the masters in the ring subnet, the number of masters is one and the maximum number of available mesh links is ml, which is equal to 7× p − n . After the scatternet topology is acquired, the minimum number of piconets is computed by Equation (1). After that, the coordinator computes and distributes the mesh link, the piconet information and the inter-piconet connection information to all the other masters in the ring subnet. After receiving the piconet and inter-piconet information, each master starts to form its own Energies 2018, 11, 1163 4 of 14 piconet, interconnects with the other two bridges, and connects with the other corresponding master to form the mesh-ring topology. As a result, the identification of each master in the ring can be piconet.specified Therefore, by the coordinator the remaining for packet number routing of devices over the in ring a piconet subnet. is 6; thus, Equation (1) is divided by 7.With a 52-node example, a coordinator M1 discovers 51 other slave nodes. With n = 52, M1 then computesIn Equation the minimum (1), the number maximum of piconet number p in = a8, piconetand the isnumber limited of to availabl eight. Thee mesh bridge links link is 4 is. The used 4 tomesh connect links withcan be the distributed other two mastersby a maximum in the ring path subnet. criterion To connectwith path the length masters pl in= thep ring/ 2. subnet, The pl thevalue number is used of by masters the maximum is one and path the criterion maximum in numberthe master of availableto connect mesh with links the farthest is ml, which master is equal, thus toreduc7 ×ingp − nthe. After path the length scatternet of topologythe ring is subnet. acquired, Finally, the minimum the coordinator number of piconetsM1 distributes is computed the bycorresponding Equation (1). piconet, After that,ring connection, the coordinator and mesh computes connection and distributes information the to mesh all the link, other the masters piconet informationincluding M2 and to M8 the inter-piconetin Figure 2. connection information to all the other masters in the ring subnet. AfterIn the receiving second thephase, piconet each and master inter-piconet receives information, connection each information master starts including to form itspiconet own piconet,information, interconnects inter-piconet with theconnection other two information, bridges, and connectsand the withmesh the link other information. corresponding The master piconet to forminformation the mesh-ring contains topology. all the slave As a result,nodes the in identiﬁcationeach individual of eachpiconet. master The in inter the ring-piconet can beconnection speciﬁed byinformation the coordinator includes for bridge packets routinginformation over to the connect ring subnet. with its upward and downward masters in the ring Withbackbone a 52-node subnet. example, The mesh a coordinator link information M1 discovers is used 51 to other connect slave with nodes. another With n corresponding= 52, M1 then computesmaster in the the ring minimum subnet. numberWith the of maximum piconet p path= 8, criterion and the, numberthe path oflength available in the meshring subne linkst iscan 4. Thebe reduced 4 mesh to links effectively can be distributedmitigate the by routing a maximum delay of path packet criterion transmission. with path At lengththe samepl time,= b peach/2c. Themasterpl valuestarts paging is used its by downward the maximum master path and criterion connects in them the together master to as connecta ring backbone with the network. farthest master,Then each thus master reducing connects the path with length one corresponding of the ring subnet. maste Finally,r in the thering coordinator to form the M1 mesh distributes-shaped thesubnet corresponding individually. piconet, In addition, ring connection, each master and mesh pages connection its slaves information in its piconet. to all theAs othera result, masters the includingmesh-ring M2 scatternet to M8 in is Figuregenerated2. to achieve both advantages of mesh and ring topologies.

M1 M8

M2 M7

M3 M6

M4 M5

Figure 2.2. The formation topologytopology of a Mesh-RingMesh-Ring backbone network.

In Figure 2, each master in the ring subnet from M2 to M8 receives piconet, inter-piconet In the second phase, each master receives connection information including piconet information, connection, and mesh link information from M1. Then M2 starts to connect with its downward inter-piconet connection information, and the mesh link information. The piconet information contains master M3, M3 connects with M4, M4 connects with M5. In this way, the ring backbone subnet is all the slave nodes in each individual piconet. The inter-piconet connection information includes generated until M8 connecting with M1. With the maximum path length pl = 4, M1 connects with bridges information to connect with its upward and downward masters in the ring backbone subnet. The mesh link information is used to connect with another corresponding master in the ring subnet. With the maximum path criterion, the path length in the ring subnet can be reduced to effectively mitigate the routing delay of packet transmission. At the same time, each master starts paging its downward master and connects them together as a ring backbone network. Then each master connects with one corresponding master in the ring to form the mesh-shaped subnet individually. In addition, each master pages its slaves in its piconet. As a result, the mesh-ring scatternet is generated to achieve both advantages of mesh and ring topologies. In Figure2, each master in the ring subnet from M2 to M8 receives piconet, inter-piconet connection, and mesh link information from M1. Then M2 starts to connect with its downward master M3, M3 connects with M4, M4 connects with M5. In this way, the ring backbone subnet is generated until M8 connecting with M1. With the maximum path length pl = 4, M1 connects with M5, Energies 2018, 11, 1163 5 of 14

M2 connects with M6 until the M4 connects with M8 to form the mesh-ring backbone network. Finally each master from M2 to M8 connects with its corresponding slaves and the mesh-ring topology is generated as shown in Figure1.

2.3. Routing Algorithm In general, the packet routing of the Mesh-Ring network can be classified into two cases, namely the intra-piconet routing and the inter-piconet routing cases. In the intra-piconet routing case, a source node and a destination node are located at the same piconet under a master. First, the packet is forwarded to the master, and then the master forwards to the destination in the corresponding slots of its piconet. In the inter-piconet routing case, the packets will be forwarded to the master of the source node in the piconet. Then the source master Ri checks and delivers the packets to the immediate downstream master along the interconnecting mesh or the ring subnet, as well as passing the packets to the master of the destination. Finally, the destination master Rj forwards packets to reach the final destination. To reduce the hop length of a larger ring subnet and efficiently deliver packets, a criterion is defined in Equation (2) to minimize the hop distance between master i as Ri and master j as Rj. As long as a master in the ring subnet receives a new packet either from the piconet or the ring subnet, the master checks the mesh link first and the hop distance from the source ID to the destination ID will be computed by Equation (2). If Equation (2) is satisfied, the packet will be forwarded in the counterclockwise direction in the ring subnet. Otherwise, the packet will be forwarded in the clockwise direction.

Rj − Ri ≤ c/2; j > i (2) To create a full RMR network, the computation complexity is O (n2) and the formation message complexity is O (n). During scatternet formation, the connection messages do not affect packet transmission. On the other hand, the computational complexity of the optimal mesh connectivity is equal to O (p × ml) during the maintenance phase, where p is the number of piconets and ml is the number of available mesh links. In addition, the message complexity of the mesh connection is equal to O (ml), since only ml links need to be established. As a result, the impact on the connection message over the sending packets is quite small, since the connection message is triggered only as the topology changes.

3. Conﬁgurable Algorithm During the topology maintenance phase, the size of a network dynamically changes as nodes join or leave the Mesh-Ring network. When the network size increases or decreases dramatically, the mesh size can be adjusted to achieve the desired network performance. Therefore, this study aims to determine the optimum number of mesh links for achieving the desired routing performance for various sizes of networks. After scatternet formation, the coordinator node acquires the local topology to reconﬁgure the Mesh-Ring topology as long as the topology variation exceeds a threshold. In order to determine the optimum placement of mesh link I in the ring subnet by the designated root, a heuristic algorithm called the peak-search method is proposed to achieve the best capacity performance of the RMR topology.

3.1. Problem Statement During the topology maintenance phase, each master locally in the ring subnet can be connected with up to ﬁve more mesh links due to node leaves. As shown in Figure1, the master initially connects with the other master that has the maximum hop length, such as M1 to M5. If a slave node leaves the M1 piconet, then the master is able to connect with one additional master with a mesh link via the maximum path criterion. The second connected master is M6, the third one is M4, the fourth one is M7, Energies 2018, 11, 1163 6 of 14

and theEnergies final 2018 one, 11 is, x M3. On the other hand, if a slave node joins the M1 piconet, then the M3 link6 of 14 is first removed,Energies then 2018, the 11, x M7 link, and so forth in reverse, until arriving at the second mesh link. At 6 leastof 14 one meshthen link the in aM3 piconet link is is first required removed, to maintain then the theM7 Mesh-Ringlink, and so topology.forth in reverse, As a result, until arriving mesh connectivity at the in thesecondthen ring the subnet mesh M3 link can link. beis Atfirst classified least removed, one into mesh then three linkthe possible M7 in alink, piconet cases, and includingisso requiredforth in the reverse, to piconet maintain until case, thearriving where Mesh-Ring at the the mesh topology. As a result, mesh connectivity in the ring subnet can be classified into three possible cases, linkssecond are located mesh within link. At a piconet least one as meshshown link in Figure in a piconet3. The isother required one is to the maintain scatternet the case, Mesh-Ring where the includingtopology. theAs apiconet result, meshcase, whereconnectivity the mesh in the links ring are subnet located can within be classified a piconet into as three shown possible in Figure cases, 3. mesh links are distributed among several piconets, as shown in Figure4. A hybrid case combines the Theincluding other onethe piconetis the scatternet case, where case, the where mesh the links mesh are links located are withindistributed a piconet among as severalshown piconets,in Figure as3. mesh connectivity of Figures3 and4. In addition, the execution round for the first mesh link is p and shownThe other in Figure one is 4.the A scatternet hybrid case case, combines where thethe meshmesh linksconnectivity are distributed of Figures among 3 and several 4. In addition, piconets, the as the lastexecutionshown mesh in link Figureround is 0,4.for A thus, the hybrid first the case averagemesh combines link execution is p theand mesh the round lastconnectivity mesh is p/2 link to of determineis Figures 0, thus, 3 theand the average 4. optimal In addition, execution connectivity the of meshroundexecution links. is p /2round to determine for the first the optimalmesh link connectivity is p and the of lastmesh mesh links. link is 0, thus, the average execution round is p/2 to determine the optimal connectivity of mesh links. M1 M8 M1 M8 M1 M8

M1 M8 M1 M8 M1 M8 M2 M7 M2 M7 M2 M7

M2 M7 M2 M7 M2 M7 M3 M6 M3 M6 M3 M6

M3 M6 M3 M6 M3 M6 M4 M5 M4 M5 M4 M5

M4(a) M5 M4 (b) M5 M4 (c) M5

(a)M1 M8 (b) M1 M8 (c) M1 M8 M1 M8 M2 M7 M2 M7

M2 M7 M2 M7 M3 M6 M3 M6

M3 M6 M3 M6 M4 M5 M4 M5

(e) M4((dd) M5 M4 (e) M5

(d) (e) Figure 3. The possible mesh link( dconnection) examples for each master (e) in a piconet: (a) The first mesh Figure 3. The possible mesh link connection examples for each master in a piconet: (a) The ﬁrst mesh linkFigure connection; 3. The possible (b) The mesh second link meshconnection link connection;examples for (c )each The master third meshin a piconet: link connection; (a) The first (d )mesh The link connection; (b) The second mesh link connection; (c) The third mesh link connection; (d) The fourth fourthlink connection; mesh link (connection;b) The second (e) The mesh fifth link mesh connection; link connection. (c) The third mesh link connection; (d) The mesh link connection; (e) The ﬁfth mesh link connection. fourth mesh link connection; (e) The fifth mesh link connection. M1 M8 M1 M8 M1 M8 M1 M8 M2 M7 M2 M7 M2 M7 M2 M7 M3 M6 M3 M6 M3 M6 M3 M6 M4 M5 M4 M5

M4 (a) M5 M4 (b) M5

M1 (a) M8 M1 (b) M8 M1 M8 M1 M8 M2 M7 M2 M7 M2 M7 M2 M7 M3 M6 M3 M6 M3 M6 M3 M6 M4 M5 M4 M5

M4 (c) M5 M4 ((dd)) M5

(d) Figure 4. The possible mesh link connection(c) example for each (d master) in a scatternet: (a) The first meshFigure link 4. Theconnection; possible ( bmesh) The link second connection mesh link example connection; for each (c) Themaster third in mesha scatternet: link connection; (a) The first (d) Figure 4. The possible mesh link connection example for each master in a scatternet: (a) The ﬁrst mesh Themesh fourth link connection;mesh link connection. (b) The second mesh link connection; (c) The third mesh link connection; (d) link connection;The fourth mesh (b) The link second connection. mesh link connection; (c) The third mesh link connection; (d) The fourth mesh link connection.

Energies 2018, 11, x 7 of 14 Energies 2018, 11, 1163 7 of 14 In each piconet, there exist up to 5 links that can be connected as mesh links. In the piconet case, the 5 links are concentrated within a piconet, and the mesh links can also potentially be distributed amongIn various each piconet, piconets there for existthe other up to scat 5 linksternet that case cans. beIn connectedgeneral, the as mesh mesh links links. can In thebe distributed piconet case, amongthe 5 linksdifferent are concentratedpiconets, and within a combined a piconet, configuration and the mesh of piconet links can and also scatternet potentially cases be is distributedpossible. Amongamong the various configurations piconets for above the other, the scatternet main research cases. Inchallenge general, theissue mesh in this links study can be is distributed how to determamongine different the best piconets,throughput and performance a combined for configuration the Mesh-R ofing piconet topology. and scatternet cases is possible. Among the configurations above, the main research challenge issue in this study is how to determine 3.2the. Reconfigurable best throughput Mesh performance-Ring Algorithm for the Mesh-Ring topology. 3.2.The Reconfigurable RMR algorithm Mesh-Ring introduces Algorithm a peak-search method to locate the optimum placement of the available mesh links. The peak-search method contains three functional blocks, namely the topology The RMR algorithm introduces a peak-search method to locate the optimum placement of the formation block, the network capacity block, and the optimum decision block. To determine the available mesh links. The peak-search method contains three functional blocks, namely the topology optimal Mesh-Ring configuration for the three configuration cases of various sizes of networks, a formation block, the network capacity block, and the optimum decision block. To determine the optimal peak-search method is presented. This scheme is a systematic approach, and is implemented by Mesh-Ring configuration for the three configuration cases of various sizes of networks, a peak-search three functional blocks: the topology formation block generates the mesh-ring topology, the network method is presented. This scheme is a systematic approach, and is implemented by three functional capacity block computes the throughput performance, and the optimum decision block introduces a blocks: the topology formation block generates the mesh-ring topology, the network capacity block decision-making criterion to determine the optimum connected bridge node of each mesh link. computes the throughput performance, and the optimum decision block introduces a decision-making Figure 5 shows a block diagram of the peak-search method. Initially, parameter I is set as one criterion to determine the optimum connected bridge node of each mesh link. and ≤ ≤ for the RMR formation, where ml is the maximum available mesh links. In the 1FigureI 5ml shows a block diagram of the peak-search method. Initially, parameter I is set as one and network capacity block, the average throughput performance is defined to reflect the scatternet 1 ≤ I ≤ ml for the RMR formation, where ml is the maximum available mesh links. In the network capacity of total Mesh-Ring topology by summing up the piconet throughput on the ring. Finally, capacity block, the average throughput performance is defined to reflect the scatternet capacity of the total throughput is calculated and compared with its previous connected bridge in the optimum total Mesh-Ring topology by summing up the piconet throughput on the ring. Finally, the total decision block. This algorithm is iterated until the optimum connected bridge of each mesh link I is throughput is calculated and compared with its previous connected bridge in the optimum decision determined. block. This algorithm is iterated until the optimum connected bridge of each mesh link I is determined.

I=1 Topology Network Optimum Bridge for I Formation Capacity Decision no I=I+1

FigureFigure 5. 5.BlockBlock diagram diagram of ofthe the peak peak-search-search method. method.

ToTo realize realize the the peak peak-search-search method, method, a novel algorithm isis implementedimplemented in in the the coordinator coordinator node node of of the theRMR RMR network network for bothfor both topology topology formation formation and maintenance and maintenance phases. phase At thes. beginning,At the beginning, the maximum the maximumnumber of number mesh link of meshml is introduced link ml is introduced as the counter as ofthe the counter mesh link.of th Ate mesh the same link. time, At the the same coordinator time, thenode coordinator collectssufﬁcient node collects information, sufficient including information, each including average piconeteach average throughput piconet in throughput the ring subnet, in theto calculatering subnet and, to compare calculate the and average compare mesh-ring the average throughput mesh for-ring the optimumthroughput decision for the block optimum to make decisiona decision. block to make a decision. FromFrom this this point on,on, the the designated designated root root M1 inM1 the in ring-subnet the ring- periodicallysubnet periodically executes theexecutes peak-search the peakmethod-search to determinemethod to the determine optimum the connected optimum bridge connected for each bridge link Iforwhen each the link size I ofwhen subnets the growssize of or subnetsdecreases. grows When or decreases. the topology When variation the topology exceeds variation a threshold excee andds the a threshold optimum andI is notthe locatedoptimum in I each is nottime located interval in t,each the designatedtime interval master t, the increases designated the connecting master increases sequence the of bridgesconnecting by one sequence to locate of the bridgesmesh linkby oneI and to continuouslylocate the mesh computes link I and the averagecontinuously throughput compute ins the the ring average subnet. throughput This procedure in the is ringrepeated subnet until. This the procedure optimum is connectedrepeated until bridge the for optimum each I is connected determined. bridge for each I is determined.

3.33.3.. Peak Peak-Search-Search Method Method InIn the the scatternet scatternet formation formation block, block, the the coordinator coordinator collects collects the the complete complete topology topology and and computes computes thethe formation formation topology topology including including the the ring ring subnet subnet as as well well as as the the number number of of mesh mesh links links for for the the three three configurationconﬁguration cases cases.. In thethe network network capacity capacity block, block, the averagethe averag Mesh-Ringe Mesh throughput-Ring throughputT is calculated T is

Energies 2018, 11, 1163 8 of 14

by Equation (3), where Ri represents the total ring throughput of each individual piconet, and p is the numberEnergies of piconets:2018, 11, x 8 of 14 p T = R (3) calculated by Equation (3), where Ri represents the∑ totali ring throughput of each individual piconet, i=1 and p is the number of piconets: In the optimum decision block, throughput performance T in Equation (3) is defined as the p j average Mesh-Ring throughput for the jth connected bridge in the network capacity block. Initially, T = ∑ Ri (3) j is set as one, and each Tj is calculated. Then j =ij=1+ 1, as the topology increases for the scatternet formationIn block, the optimum while T j+decision1 is computed block, throughput in the optimum performance decision Tj block.in Equation The throughput (3) is defined performance as the of bridgeaveragej + Mesh 1 is-Ring computed throughput until for the the maximumjth connectedT bridgej is determined in the network and capacity the decision block. Initially, is made j by Equationis set(4), as one where, andk iseach the T totalj is calculated. number ofThen available j = j + bridges1, as the intopology the ring increases subnet. for the scatternet formation block, while Tj+1 is computed in the optimum decision block. The throughput performance of bridge j + 1 is computed until the maximum Tj is determined and the decision is made by Equation Max Tj (4) (4), where k is the total number of available bridgesj=1...k in the ring subnet. As a result, the bridge with the maximum throughput is regarded as the optimal connected bridge MaxTj (4) of mesh link I, and the algorithm is terminatedj=1 until...k all connected bridges are determined for the availableAs mesh a result, links here.the bridge with the maximum throughput is regarded as the optimal connected bridge of mesh link I, and the algorithm is terminated until all connected bridges are determined for 4. Performance Results the available mesh links here. In this Section, a discrete event simulator is written by Matlab to investigate the network routing performance,4. Performance and the Results configuration parameters for routing performance simulation are defined as follows. ForIn this the Section, packet transmissiona discrete event of simulator each node, is written each packet by Matlab was generatedto investigate in accordancethe network with a Poissonrouting arrival performance pattern., and Inaddition, the configuratio it wasn assumed parameters that for a singlerouting packet performance was sent simulation in each are routing session,defined and thatas follows. each data For packet the packet had durationtransmission of five of timeeach slots.node, A each first packet in first outwas (FIFO)generated queue in was providedaccordance with awith length a Poisson of 80 packetsarrival pattern. for each In node.addition, When it was the assumed buffer that is in a overflow,single packet a FIFO was sent buffer is in each routing session, and that each data packet had duration of five time slots. A first in first out used in each node, and a tail drop mechanism is designed to drop the packets. In each routing session, (FIFO) queue was provided with a length of 80 packets for each node. When the buffer is in the source-destination pair was randomly selected, the time division duplexing (TDD) scheme was overflow, a FIFO buffer is used in each node, and a tail drop mechanism is designed to drop the adoptedpackets. for both In each piconet routing and scatternetsession, the scheduling, source-destination and packets pair werewas randomly forwarded selected, using thetheRMR time with the designeddivision routingduplexing protocol. (TDD) scheme The overall was simulatedadopted for nodes both arepiconet 100, whichand scatternet are uniformly scheduling, distributed and in a specificpackets area. were From forwarded Equations using (1) the and RMR (2), with the resultingthe designed number routing of protocol. piconets The is p overall= 15 and simulated the number of meshnodes links are is100ml, =which 5. are uniformly distributed in a specific area. From Equations (1) and (2), the Threeresulting simulation number of configurations piconets is p = are15 and simulated, the number as in of Figuremesh link6. Ins is the ml ring = 5. configuration, 15 masters with red colorThree aresimulation interconnected configuration by ans a intra-bridgere simulated, with as in green Figure color, 6. In whichthe ring is aconfiguration, slave node to15 relay packetsmasters between with red interconnected color are interconnected piconets. by Figure an intra6a is-bridge the piconet with green case, color which, which is is assumed a slave node to have to relay packets between interconnected piconets. Figure 6a is the piconet case, which is assumed to 5 available mesh links in master 3. Figure6b is the scatternet case with a uniformly distribution for have 5 available mesh links in master 3. Figure 6b is the scatternet case with a uniformly distribution meshfor links mesh among links among masters master 3, 6,s 3, 9, 6, 12, 9, and12, and 15. 15. Figure Figure6 c6c is is the the hybridhybrid configuration configuration of piconet of piconet and and scatternetscatternet cases. cases. In theIn the hybrid hybrid case, case, it it is is assumed assumed that there there are are two two mesh mesh links links at master at master 3 and 3 three andthree meshmesh links links distributed distributed at 7,at 11,7, 11, and and 15. 15.

(a) (b) (c)

Figure 6. The piconet case of Mesh-Ring topology (a); the scatternet case of Mesh-Ring topology (b); Figure 6. The piconet case of Mesh-Ring topology (a); the scatternet case of Mesh-Ring topology (b); the hybrid case of Mesh-Ring topology (c). the hybrid case of Mesh-Ring topology (c).

Energies 2018,, 11,, 1163x 9 of 14

In Figure 6a of master 3, there exist various connection opportunities for the 5 mesh links via a bridgeIn node Figure from6a of ID master 16 to 30. 3, thereEach bridge exist various node is connection sequentially opportunities connected and for simulated the 5 mesh by links master via 3 toa bridge determine node the from highest ID 16 tothroughput 30. Each bridge point. node With is sequentiallya peak-search connected method, andthe simulated5 determined by master bridge 3 nodeto determines with highest the highest throughput throughput performance point. With for a peak-searchthe piconet method,case are theshown 5 determined as Figure bridge 7. The nodes first withpeak highestpoint is throughput generated at performance bridge 25, forwhich the is piconet shown case by arethe shownblue line. as Figure The other7. The determined ﬁrst peak point peak pointsis generated are 24, at 16, bridge 19, and 25, which20. The is simulation shown by theresults blue demonstrate line. The other that determined the peak-search peak points method are works 24, 16, 19,well and to 20.determine The simulation the optimal results placement demonstrate for each that themesh peak-search link connection method with works the well best to throughput determine performance.the optimal placement for each mesh link connection with the best throughput performance.

380

360 )

d 340 n o c e s / s t e k c

a 320 p (

t u p h g u o r h

T 300

Peak-search method performance 1st peak point 280 2nd peak point 3rd peak point 4th peak point 5th peak point 260

16 20 24 28 32 ID of intra-bridge node Figure 7. The determined bridge node with highest throughput of peak search method for piconet Figure 7. The determined bridge node with highest throughput of peak search method for piconet case. case.

WithWith the determined determined mesh mesh links, links, three three configurations configurations are are simulated simulated,, and and the theirir performances performances are shownare shown in Figure in Figure 8. 8F.or For the the piconet piconet case, case, three three mesh mesh links links achieveachieve thethe maximummaximum throughputthroughput performance. F Foror the scatternet and hybrid cases, the throughput performance increases as the mesh link increases. increases. In In addition, addition, more more mesh mesh links links achieve achieve better better throughput throughput performances. performances. As a As result, a result, the scatternetthe scatternet case case achieves achieves better better thr throughputoughput performance performance than than the the other other two two cases cases as as the the packet generation raterate increases.increases. TheThe packetpacket generation generation rate rate is is defined defined as as the the number number of of packets packets generated generated in ineach each node node at aat given a given time time slot. slot. In [[15],15], the cluster-basedcluster-based mesh topology demonstrates better routingrouting performance than the conventional full mesh topology with the on-demandon-demand routing protocol. Our proposed Mesh-RingMesh-Ring topology is is a a type type of of cluster cluster-based-based mesh mesh architecture architecture,, and and the thering ring routing routing protocol protocol can operate can operate for a laforrger a larger network network.. After Afterthe theRMR RMR det determinesermines the the optimal optimal M Mesh-Ringesh-Ring configuration, configuration, Figure Figure 99 demonstrates the packet successful probability (PSP) (PSP) for for RMR with three cases cases,, Ring and Mesh topologies. The RMR achieves almost 100% packet delivery ratio when the packet generation rate in each node is smaller than 4. The PSP in each node is defined defined as the total successful reception packets over thethe totaltotal network network generated generated packets. packets. The The piconet piconet case achievescase achieves better better PSP than PSP the than Ring the network Ring whennetwork the when packet the generation packet generation rate is greater rate than is greater 5. The than scatternt 5. The case scatternt achieves case the achieves highest PSP the thanhighest the PSPother than two the cases. other As atwo result, cases. the A RMRs a result, achieves the superiorRMR achieves performance superior on performance the PSP than theon the conventional PSP than theRing conventional and the cluster-based Ring and the Mesh cluster topologies.-based Mesh topologies.

Energies 2018, 11, 1163 10 of 14 Energies 2018, 11, x 10 of 14 Energies 2018, 11, x 10 of 14 440 440 Mesh performance of three configurations Mesh performance of three configurations Scatternet case Scatternet case 400 Piconet case 400 HybridPiconet case case Hybrid case ) d n ) o d c n

e 360 o s / c s e 360 t s e / k s t c e a k p c (

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k 280 w r t o e 280 w N t e N

240 240

200 200 0 1 2 3 4 5 0 1 Num2 ber of mesh lin3ks 4 5 Number of mesh links Figure 8. TheThe determined determined mesh mesh link link with with highest highest throughput throughput of of peak search method for three Figure 8. The determined mesh link with highest throughput of peak search method for three configurationconﬁguration cases.cases. configuration cases. 1 1

0.8 0.8 o i t a o r i

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S Packet successful probability 0.4 Packet successful probability 0.4 Ring PiconetRing case ScatternetPiconet case case HybridScatternet case case MeshHybrid case Mesh 0.2 0.2 0 2 4 6 8 10 0 2Packet generatio4n rate (packets/6node/second) 8 10 Packet generation rate (packets/node/second) Figure 9. The packet successful probability (PSP) performance of the Reconfigurable Mesh-Ring (FigureRMR) algorithm 9.9. ThThee packet packet. successful probability (PSP) performance of the ReconfigurableReconfigurable Mesh-RingMesh-Ring ((RMR)RMR) algorithm.algorithm. To reflect the scatternet capacity of RMR, a packet throughput is used to examine the overall To reflect the scatternet capacity of RMR, a packet throughput is used to examine the overall networkTo reflect capacity the. The scatternet average capacity packet ofthroughput RMR, a packet in each throughput device is defined is used as to the examine ratio of the the overall total network capacity. The average packet throughput in each device is defined as the ratio of the total numbernetwork of capacity. successfully The average routed packetpackets throughput over the tot inal each simulation device istime defined in seconds. as the ratioThe ofsimulation the total number of successfully routed packets over the total simulation time in seconds. The simulation resultsnumber of of the successfully packet throughput routed packetsfor the three over configuration the total simulation cases of time RMR in, the seconds. conventional The simulation Ring and resultsMesh schemesof the packet are asthroughput shown in for Fig theure three 10. configurationThe piconet casescase achievesof RMR, theapproximate conventional throughput Ring and performanceMesh schemes, compare are as dshown to the conventionalin Figure 10 .cluster The piconet-based Meshcase scheme.achieves Inapproximate RMR, the scatternet throughput case performanceachieves the ,highest compare throughputd to the conventional performance cluster since- basedthe even Mesh distributed scheme. meshIn RMR, links the among scatternet masters case achieves the highest throughput performance since the even distributed mesh links among masters

Energies 2018, 11, 1163 11 of 14 results of the packet throughput for the three conﬁguration cases of RMR, the conventional Ring and Mesh schemes are as shown in Figure 10. The piconet case achieves approximate throughput performance, compared to the conventional cluster-based Mesh scheme. In RMR, the scatternet Energies 2018, 11, x 11 of 14 case achieves the highest throughput performance since the even distributed mesh links among effectivelymasters effectively improves improves the connected the connected configuration conﬁguration than the other than thetwo other cases. two On cases.the other On hand, the other the packethand, thethroughput packet throughput of conventional of conventional Ring is Ringpromptly is promptly saturated saturated,, as the aspacket the packet generation generation rate increasesrate increases above above 4 since 4 since the traditional the traditional ring ringtopology topology increases increases the hop the hoplength length and andthus thus reduces reduces the throughputthe throughput performance performance in the in ring the ringsubnet. subnet. According According to the to simulation the simulation results, results, the proposed the proposed RMR withRMR scatternet with scatternet case caseimproves improves by almost by almost 70% 70% throughput throughput performance performance,, as as compared compared to to the conventional ring scheme.

600 Mesh-Ring Throughput

Ring Piconet case Scatternet case Hybrid case

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t u p h g u o r h t

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0 2 4 6 8 10 Packet generation rate (packets/node/second) Figure 10. The network throughput performance of the RMR algorithm. Figure 10. The network throughput performance of the RMR algorithm.

In order to estimate the multi-hop routing performance, an average packet delay metric was used Into ordermeasure to estimate the end- theto-end multi-hop delay of routing a scatternet. performance, The packet an average delay of packet each delayrouting metric packet was is definedused to as measure the average the end-to-end packet transmission delay of a time scatternet. from the The first packet transmitted delay ofbit each at the routing source packet node to is thedefined last received as the average bit at the packet destination transmission node. Fig timeure from 11 shows the first the transmitted average packet bit at delay the source performance node tos ofthe the last three received cases bit of atRMR the, destination the conventional node. FigureRing and 11 showsthe cluster the average-based Mesh packet schemes delay performances. The packet delayof the increases three cases as ofthe RMR, packet the generation conventional rate Ring increases and the since cluster-based more packets Mesh have schemes. to be processed The packet at networkdelay increases. The RMR as the scatt packeternet generationcase generates rate increasesthe smallest since average more packetsdelay, as have it generates to be processed the even at meshnetwork. connectivity The RMR than scatternet all the case simulated generates cases. the smallest Another average observation delay, asfrom it generates the results the is even that mesh the packetconnectivity delay thanof the all conventional the simulated Ring cases. quickly Another increases observation versus from the the other results three is thatcases the, as packet the packet delay generationof the conventional rate increases Ring quicklyabove 4, increases because the versus traditional the other ring three topology cases, asgreatly the packet increases generation the routing rate pathincreases length. above 4, because the traditional ring topology greatly increases the routing path length. A packet drop drop probability probability (PDP) (PDP) metric metric is is used used to to evaluate evaluate the the impact impact of of traffic traffic congestion congestion inin a scatternet.a scatternet. When When the the FIFO FIFO buffer buffer overflows overflows in in each each node, node, the the damaged damaged routing routing packets packets,, including both the currently receivedreceived andand newlynewly generated generated routing routing packets, packets were, were dropped. dropped. The The PDP PDP is definedis defined as asthe the ratio ratio of the of totalthe total number number of dropped of dropped packets packets over the over total th generatede total generated packets forpackets all nodes. for all Figure nodes. 12 Figshowsure the12 shows PDP of the the PDP three of cases the ofthree RMR, cases the of conventional RMR, the conventional Ring and the cluster-basedRing and the meshcluster schemes.-based mHere,esh schemes the RMR. Here, scatternet the RMR case achievesscatternet the case lowest achieves PDP the than lowest all other PDP cases, than all since other the cases scatternet, since case the scatternet case achieves better mesh connectivity than the other ones. From the simulation results, it is clear that all masters begin to drop packets when the FIFO buffers overflow. Moreover, the packet generation rates are greater than 3 for the piconet case and the cluster-based mesh, 4 for the conventional ring and the scatternet case, and 5 for the hybrid case. From Figure 10, the throughput performance saturates even the packet generation rate, which increases when the packets start to be

Energies 2018, 11, 1163 12 of 14 achieves better mesh connectivity than the other ones. From the simulation results, it is clear that all masters begin to drop packets when the FIFO buffers overﬂow. Moreover, the packet generation rates are greater than 3 for the piconet case and the cluster-based mesh, 4 for the conventional ring and the scatternet case, and 5 for the hybrid case. From Figure 10, the throughput performance saturates evenEnergies the packet 2018, 11 generation, x rate, which increases when the packets start to be dropped in12 Figure of 14 12. As a result,Energies 2018 the, 11 tail, x drop mechanism can effectively manage the FIFO buffer and maintain throughput12 of 14 dropped in Figure 12. As a result, the tail drop mechanism can effectively manage the FIFO buffer performanceanddropped maintain forin Figure networkthroughput 12. A congestions performancea result, the conditions. tailfor networkdrop mechanism congestion can condition effectivelys. manage the FIFO buffer and maintain throughput performance for network congestion conditions. Average packet delay

Average paRingcket delay Piconet case Ring Scatternet case Piconet case Hybrid case Scatternet case Mesh 4000 Hybrid case

) Mesh s t 4000 o l s ) ( s

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t a l e e k d c

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0 0 2 4 6 8 10 0 P2acket generatio4n rate (packets/n6ode/second) 8 10 Figure 11. The average Ppacketacket ge delayneration performancerate (packets/nod eof/se thecond RMR) algorithm. Figure 11. The average packet delay performance of the RMR algorithm. Figure 11. The average packet delay performance of the RMR algorithm.

Packet drop probability

0.6 Packet dropRing probability Piconet Ring 0.6 Scatternet case Piconet Hybrid case Scatternet case Mesh Hybrid case

y Mesh t i l i b a y t b i 0.4 l i o r b p a

p 0.4 o o r r p d

t p e o r k c d

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0 0 2 4 6 8 10 Packet generation rate (packets/node/second) 0 2 4 6 8 10 Figure 12. The packet drop probabilityPacket generatio (PDP)n rate (p aperformanceckets/node/secon dof) the RMR algorithm. Figure 12. The packet drop probability (PDP) performance of the RMR algorithm. 5. ConclusionFigures 12. The packet drop probability (PDP) performance of the RMR algorithm. 5. Conclusions In this study, a RMR topology is proposed for Bluetooth multi-hop networks. The RMR constructsIn this a study,ring topology a RMR topologyas a backbone is proposed subnet, for which Bluetooth is then multi extended-hop networks. by a mesh The-shaped RMR topologyconstructs. To a maximizering topology network as a capacity, backbone three subnet, configurations which is then(including extended piconet, by ascatternet, mesh-shaped and hybrid)topology are. To examined maximize and network demonstrated capacity, tothree determine configurations the optimal (including placement piconet, of meshscatternet, links. and In additionhybrid) ,are the examined number of and mesh demonstrated link is adjustable to determine to achieve the theoptimal optimal placement network ofconfiguration mesh links.. InIn addition, the number of mesh link is adjustable to achieve the optimal network configuration. In

Energies 2018, 11, 1163 13 of 14

5. Conclusions In this study, a RMR topology is proposed for Bluetooth multi-hop networks. The RMR constructs a ring topology as a backbone subnet, which is then extended by a mesh-shaped topology. To maximize network capacity, three configurations (including piconet, scatternet, and hybrid) are examined and demonstrated to determine the optimal placement of mesh links. In addition, the number of mesh link is adjustable to achieve the optimal network configuration. In order to determine the optimum bridge of each mesh link, a heuristic peak-search method is presented to achieve the desired throughput performance of the RMR network. The peak-search method is implemented by three functional blocks: the scatternet formation, the network capacity and the optimum decision blocks. Simulation results demonstrate that the optimal Mesh-Ring configuration can be determined by the peak-search scheme and the scatternet case achieves better overall network performances than the other two cases. Moreover, that RMR demonstrates better network routing performance than the conventional Ring and the cluster-based Mesh schemes for Bluetooth sensor networks.

Author Contributions: C.-M.Y. designed the RMR formation algorithm, routing protocol, formulated the mesh connection models, performed the simulation, and wrote the paper; B.-Y.W. and Y.-H.K. investigated the topology, validated the algorithms, analyzed the results and cooperated in designing the simulation. Acknowledgments: This work was supported by the Ministry of Science and Technology of Taiwan, under Grants MOST 106-2221-E-216-002 and MOST 106-2221-E-009-050. Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

1. Todorović,B.M.; Samardžija, D. Road lighting energy-saving system based on wireless sensor network. Energy Effic. 2017, 10, 239–247. [CrossRef] 2. Hu, J. Application of ZigBee wireless sensor network in gas monitoring system. Acta Tech. CSAV 2017, 62, 255–264. 3. Leccese, F.; Cagnetti, M.; Trinca, D. A smart city application: A fully controlled street lighting isle based on Raspberry-Pi card, a ZigBee sensor network and WiMAX. Sensors 2014, 14, 24408–24424. [CrossRef] [PubMed] 4. Leccese, F. Remote-control system of high efficiency and intelligent street lighting using a zig bee network of devices and sensors. IEEE Trans. Power Deliv. 2013, 28, 21–28. [CrossRef] 5. With an Installed Base of 10 Billion Devices Expected in 2018, Bluetooth Will Be an Essential Tool for Building the INTERNET of Everything. Available online: https://www.abiresearch.com/press/with-an-installed- base-of-10-billion-devices-expec/ (accessed on 4 May 2018). 6. Fiandrino, C.; Capponi, A.; Cacciatore, G.; Kliazovich, D.; Sorger, U.; Bouvry, P.; Kantarci, B.; Granelli, F.; Giordano, S. CrowdSenSim: A simulation platform for mobile crowd sensing in realistic urban environments. IEEE Access 2017, 5, 3490–3503. [CrossRef] 7. Satrya, G.B.; Reda, H.T.; Kim, J.W.; Daely, P.T.; Shin, S.Y.; Chae, S. IoT andw public weather data based monitoring & control software development for variable color temperature LED street lights. Int. J. Adv. Sci. Eng. Inf. Technol. 2017, 7, 366–372. 8. Masek, P.; Masek, J.; Frantik, P.; Fujdiak, R.; Ometov, A.; Hosek, J.; Andreev, S.; Mlynek, P.; Misurec, J. A harmonized perspective on transportation management in smart cities: The novel IoT-driven environment for road traffic modeling. Sensors 2016, 16, 1872. [CrossRef][PubMed] 9. Calvo, I.; Gil-García, J.M.; Recio, I.; López, A.; Quesada, J. Building IoT applications with raspberry Pi and low power IQRF communication modules. Electronics 2016, 5, 54. [CrossRef] 10. Pau, G.; Collotta, M.; Maniscalco, V. Bluetooth 5 energy management through a fuzzy-pso solution for mobile devices of internet of things. Energies 2017, 10, 992. 11. Baker, C.; Almodovar-Faria, J.; Juste, P.S.; McNair, J. Low Energy socially cognizant routing for delay tolerant mobile networks. In Proceedings of the IEEE Military Communications Conference MILCOM 2013, San Diego, CA, USA, 18–20 November 2013; pp. 299–304. Energies 2018, 11, 1163 14 of 14

12. Etxaniz, J.; Aranguren, G. Modeling of the Data Transportation Network of a Multi-hop Data-content-sharing Home Network. IEEE Trans. Consum. Electron. 2015, 61, 31–38. [CrossRef] 13. Hung, C.H.; Bai, Y.W.; Tsai, R.Y. Design of blood pressure measurement with a health management system for the aged. IEEE Trans. Consum. Electron. 2012, 58, 619–625. [CrossRef] 14. Goh, H.L.; Tan, K.K.; Huang, S.; de Silva, C.W. Development of Bluewave: A Wireless Protocol for Industrial Automation. IEEE Trans. Ind. Inform. 2006, 2, 221–230. [CrossRef] 15. Zhang, X.; Riley, G.R. Energy-aware on-demand scatternet formation and routing for Bluetooth-based wireless sensor networks. IEEE Commun. Mag. 2005, 43, 126–133. [CrossRef] 16. Kamkuemah, M.N.; Le, H. A Study of Different Routing Protocols for Mobile Phone Ad Hoc Networks Connected Via Bluetooth. In Proceedings of the International Conference on Computer Modeling and Simulation (UKSim), Cambridge, UK, 10–12 April 2013; pp. 681–686. 17. Jung, C.; Kim, K.; Seo, J. Topology Conﬁguration and Multihop Routing Protocol for Bluetooth Low Energy Networks. IEEE Access 2017, 5, 9587–9598. [CrossRef] 18. Sharafeddine, S.; Al-Kassem, I.; Dawy, Z. A Scatternet Formation Algorithm for Bluetooth Networks with a Non-uniform Distribution of Devices. J. Netw. Comput. Appl. 2012, 35, 644–656. [CrossRef] 19. Chen, H.; Sivakumar, T.V.L.N.; Huang, L.; Kashima, T. Controlling network topology in forming bluetooth scatternet. IEICE Trans. 2005, 88, 943–949. [CrossRef] 20. Yu, C.; Yu, Y. Reconﬁgurable Algorithm for Bluetooth Sensor Networks. IEEE Sens. J. 2014, 14, 3506–3507. [CrossRef] 21. Zaruba, G.; Basagni, S.; Chlamtac, I. Bluetrees-scatternet formation to enable Bluetooth-based ad hoc networks. In Proceedings of the 2001 IEEE International Conference on Communications (ICC 2001), Helsinki, Finland, 11–14 June 2001; Volume 1, pp. 273–277. 22. Cuomo, F.; Melodia, T.; Akyildiz, I. Distributed Self-Healing and Variable Topology Optimization Algorithms for QoS Provisioning in Scatternets. IEEE J. Sel. Areas Commun. 2004, 22, 1220–1236. [CrossRef] 23. Salonidis, T.; Bhagwat, P.; Tassiulas, L.; LaMaire, R. Distributed topology construction of Bluetooth personal area networks. IEEE J. Sel. Areas Commun. 2005, 23, 633–643. [CrossRef] 24. Persson, K.; Manivannan, D.; Singhal, M. Bluetooth scatternet formation: Criteria, models, and classification. In Proceedings of the First IEEE Consumer Communications and Networking Conference, Las Vegas, NV, USA, 5–8 January 2004. 25. Liu, C.-H.; Dai, S.-W. The study for the extension of Bluetooth Ring network. In Proceedings of the International Conference on Multimedia and Information Technology, Kaifeng, China, 24–25 April 2010; pp. 127–130. 26. Lin, T.; Tseng, Y.; Chang, K.; Tu, C. Formation, Routing, and Maintenance Protocols for the BlueRing Scattemet of Bluetooths. In Proceedings of the International Conference on System Sciences, Big Island, HI, USA, 6–9 January 2003; pp. 313–322. 27. Guo, Z.; Harris, I.; Tsaur, L.F.; Chen, X. An on-demand scatternet formation and multi-hop routing protocol for BLE-based wireless sensor networks. In Proceedings of the 2015 IEEE Wireless Communications and Networking Conference (WCNC), New Orleans, LA, USA, 9–12 March 2015; pp. 1590–1595.

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