energies Article
A Reconfigurable Mesh-Ring Topology for Bluetooth Sensor Networks Ben-Yi Wang 1 , Chih-Min Yu 2, * 1 2
*
ID
and Yao-Huang Kao 2
ID
Zhejiang Industry & Trade Vocational College, Wenzhou 325000, China;
[email protected] 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
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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: Section 2 proposes the motivation, mesh-ring formation algorithm, and routing algorithm to generate the desired topology. In Section 3, the problem formulation, reconfiguration mesh-ring topology, and peak-search method are presented to determine the optimal bridge for each link. In Section 4, the optimal configuration is determined and throughput performances are demonstrated via computer simulations. Finally, conclusions are drawn in Section 5. 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,
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In2018, contrast to Energies 11, 1163
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 a ring subnet generated first and each master ring subnet forms piconet individually. ring subnet is is generated first and each master in in thethe ring subnet forms its its piconet individually. A A peak-search method is then introduced in the formation phase to determine the optimal placement peak-search method is then introduced in the formation phase to determine the optimal placement of available mesh links by interconnecting interconnecting masters masters in ring subnet, subnet, thus thus achieving better network network of available mesh links by in the the ring achieving better capacity. Figure 1 illustrates an example of the RMR configuration that satisfies the design objectives capacity. Figure 1 illustrates an example of the RMR configuration that satisfies the design objectives to generate to generate networks networks into into desired desired mesh-ring mesh-ring subnets. subnets.
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Figure 1. An example. Figure 1. An illustrated illustrated Mesh-Ring Mesh-Ring topology topology 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 nn m pp== 77
(1) (1)
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
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 specifiedTherefore, by the coordinator for packet routing over the subnet. piconet. the remaining number of devices in ring a piconet 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 computes the minimum of piconet and theisnumber mesh links is 4. In Equation (1), the number maximum numberpin= a8,piconet limitedoftoavailable eight. The bridge link is The used4 to connect thedistributed other two masters in the ring subnet. To connect thelength mastersplin=the mesh linkswith can be by a maximum path criterion with path The pl / 2 .subnet, pring the number of masters is one and the maximum number of available mesh links is ml, which is value is used by the maximum path criterion in the master to connect with the farthest master,equal thus to 7 × p − n. After the scatternet topology is acquired, the minimum number of piconets is computed reducing the path length of the ring subnet. Finally, the coordinator M1 distributes the by Equation (1).piconet, After that, the coordinator computes and distributes the to mesh link, the masters piconet corresponding ring connection, and mesh connection information all the other information and the inter-piconet including M2 to M8 in Figure 2. connection information to all the other masters in the ring subnet. After receiving piconet andmaster inter-piconet information, each master starts to form its own In the second the phase, each receives connection information including piconet piconet, interconnects with the other two bridges, and connects with the other corresponding master to information, inter-piconet connection information, and the mesh link information. The piconet form the mesh-ring topology. As a result, of each master in inter-piconet the ring can be specified information contains all the slave nodesthe in identification each individual piconet. The connection by the coordinator for packet routing over the ring subnet. information includes bridges information to connect with its upward and downward masters in the a 52-node example, a coordinator M1 discovers 51 to other slave with nodes. With ncorresponding = 52, M1 then ring With backbone subnet. The mesh link information is used connect another computes the ring minimum piconet ppath = 8, criterion, and the number available links iscan 4. master in the subnet.number With theofmaximum the path of length in themesh ring subnet The 4 mesh links can be distributed by a maximum path criterion with path length pl = p/2 b c. be reduced to effectively mitigate the routing delay of packet transmission. At the same time, each The pl value is usedits bydownward the maximum path the together master toasconnect with thenetwork. farthest master starts paging master andcriterion connectsinthem a ring backbone master, thus reducing the path length of the ring subnet. Finally, the coordinator M1 distributes Then each master connects with one corresponding master in the ring to form the mesh-shaped the corresponding piconet, ring connection, and mesh connection information to all the masters subnet individually. In addition, each master pages its slaves in its piconet. As other a result, the including M2 to M8 in Figure 2. mesh-ring scatternet is generated to achieve both advantages of mesh and ring topologies.
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Figure 2. 2. The The formation formation topology topology of of aa Mesh-Ring Mesh-Ring backbone backbone network. network. Figure
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 Figure 2, 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,
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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 Figure 1. 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 R j 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 R j . 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. R j − 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 Figure 1, 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,
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and the final2018, one11,isx M3. On the other hand, if a slave node joins the M1 piconet, then the M3 link is first Energies 6 of 14 removed, then the M7 link, and so forth in reverse, until arriving at the second mesh link. At least Energies 2018, 11, x 6 of 14 one then the M3 link is first removed, then the M7 link, and so forth in reverse, until arriving at the mesh link in a piconet is required to maintain the Mesh-Ring topology. As a result, mesh connectivity second mesh link.is At least one mesh link in alink, piconet is required to maintain the Mesh-Ring then the M3 link first removed, then the M7 and so forth in reverse, until arriving at themesh in the ring subnet can be classified into three possible cases, including the piconet case, where the topology. As a link. result,Atmesh connectivity in theinring subnet can be classified into threethe possible cases, second mesh least one mesh link a piconet is required to maintain Mesh-Ring links are located within a piconet as shown in Figure 3. The other one is the scatternet case, where the including case, where the mesh links aresubnet located a piconet asthree shown in Figure 3. topology. the As apiconet result, mesh connectivity in the ring canwithin be classified into possible cases, meshThe links areone distributed amongcase, several piconets, aslinks shown in Figure 4.among A hybrid case combines the other is the scatternet where the mesh are distributed several piconets, as including the piconet case, where the mesh links are located within a piconet as shown in Figure 3. meshshown connectivity of Figures 3 and 4. In addition, the execution round for the first mesh link is p and in Figure hybrid case combines the mesh mesh links connectivity of Figures 3 andseveral 4. In addition, The other one is 4. theAscatternet case, where the are distributed among piconets,the as the last mesh link is 0, thus, the average execution round is p/2 to determine the optimal connectivity execution round for the first mesh link is p and the last mesh link is 0, thus, the average execution shown in Figure 4. A hybrid case combines the mesh connectivity of Figures 3 and 4. In addition, the of mesh links. round is p/2 to determine the optimal connectivity of last mesh links. execution round for the first mesh link is p and the mesh link is 0, thus, the average execution round is p/2 to determine the optimal connectivity of mesh links. M1
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(e) (d) (e) Figure 3. The possible mesh link(d) connection examples for each master 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 link connection; (b) The second mesh link connection; (c) The third mesh link connection; (d)mesh The Figure 3. The possible mesh link connection examples for each master in a piconet: (a) The first link connection; (b) The second mesh link connection; (c) The third mesh link connection; (d) The fourth fourth mesh link connection; (e) The fifth mesh link connection. link connection; (b) The second mesh link connection; (c) The third mesh link connection; (d) The mesh link connection; (e) The fifth mesh link connection.
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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 each piconet, there up toscatternet 5 links that can be asmesh meshlinks links.can In the piconet case, amongInvarious piconets for exist the other cases. In connected general, the be distributed the 5 links are concentrated within a piconet, and the mesh links can also potentially be among different piconets, and a combined configuration of piconet and scatternet cases isdistributed possible. among various piconets for the other scatternet cases. In general, the mesh links can be distributed Among the configurations above, the main research challenge issue in this study is how to among different andperformance a combinedfor configuration of piconet and scatternet cases is possible. determine the bestpiconets, throughput the Mesh-Ring topology. Among the configurations above, the main research challenge issue in this study is how to determine theReconfigurable best throughput performance for the Mesh-Ring topology. 3.2. Mesh-Ring Algorithm 3.2.The Reconfigurable Mesh-Ring Algorithm RMR algorithm introduces 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 1 ≤ I ≤ ml for the RMR formation, where ml is the maximum available mesh links. In the Figure 5 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 Formation
Network Capacity I=I+1
Optimum Bridge for I Decision no
Figure Block diagram the peak-search method. Figure 5. 5. Block diagram ofof the peak-search method.
realizethe thepeak-search peak-searchmethod, method, aa novel novel algorithm ofof the ToTorealize algorithm isisimplemented implementedininthe thecoordinator coordinatornode node RMR network for both topology formation and maintenance phases.phases. At the beginning, the maximum the RMR network for both topology formation and maintenance At the beginning, the maximum of mesh link ml is introduced as of thethe counter of theAtmesh link.time, At the time, number ofnumber mesh link ml is introduced as the counter mesh link. the same thesame coordinator node collects sufficient information, eachincluding average piconet throughput in throughput the ring subnet, the coordinator node collects sufficientincluding information, each average piconet in to calculate and to compare the and average mesh-ring throughput for the optimum decision block to make the ring subnet, calculate compare the average mesh-ring throughput for the optimum a decision. decision block to make a decision. Fromthis this point designated rootroot M1 inM1 the in ring-subnet periodically executes the peak-search From point on, on,thethe designated the ring-subnet periodically executes the peak-search method tothe determine optimum connected bridge each when the grows size ofor method to determine optimumthe connected bridge for each link I for when thelink sizeI of subnets subnets grows or decreases. When the topology exceeds a threshold the located optimum is decreases. When the topology variation exceedsvariation a threshold and the optimumand I is not in Ieach not located in t,each time interval t, the designated master increases connecting sequence time interval the designated master increases the connecting sequencethe of bridges by one to locateof the bridges by one tocontinuously locate the mesh link I and continuously computes thering average throughput in theis mesh link I and computes the average throughput in the subnet. This procedure ring subnet.until Thisthe procedure repeated until thefor optimum bridge for each I is determined. repeated optimumisconnected bridge each I isconnected determined. 3.3.Peak-Search Peak-SearchMethod Method 3.3. thescatternet scatternetformation formationblock, block,the thecoordinator coordinatorcollects collectsthe thecomplete completetopology topologyand andcomputes computes InInthe theformation formationtopology topology including the ring subnet well thenumber numberofofmesh meshlinks linksfor forthe thethree three the including the ring subnet asas well asasthe configuration cases. In the network capacity block, the average Mesh-Ring throughput T is calculated configuration cases. In the network capacity block, the average Mesh-Ring throughput T is
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by Equation (3), where Ri represents the total ring throughput of each individual piconet, and p is the number of piconets: Energies 2018, 11, x 8 of 14 p
T =the∑total Ri ring throughput of each individual piconet, calculated by Equation (3), where Ri represents and p is the number of piconets:
(3)
i =1
In the optimum decision block, throughput performance Tj in Equation (3) is defined as the p average Mesh-Ring throughput for the jth connected in the network capacity block. (3) Initially, Rbridge T= i j is set as one, and each Tj is calculated. Then j = i j=1+ 1, as the topology increases for the scatternet formationInblock, while Tj+1 is computed in the optimum decision The throughput performance in Equation (3) is defined as the the optimum decision block, throughput performance Tj block. of bridge j +Mesh-Ring 1 is computed until and the decision is made average throughput forthe the maximum jth connectedTbridge in the network capacity block. Initially, j by j is determined is set(4), as one, and each j is calculated. Then j = j + bridges 1, as the in topology Equation where k is theTtotal number of available the ringincreases 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 Max Tj is determined and the decision is made by Equation Tj j=1...k in the ring subnet. (4), where k is the total number of available bridges
(4)
As a result, the bridge with the maximum throughput Max T j is regarded as the optimal connected(4)bridge j =1until ...k of mesh link I, and the algorithm is terminated all connected bridges are determined for the available As mesh links here. a result, 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 the available mesh links here.
4. Performance Results
In this Section, a discrete event simulator is written by Matlab to investigate the network routing 4. Performance Results performance, and the configuration parameters for routing performance simulation are defined as In this a discrete event is written by Matlab to investigate the network with follows. For the Section, packet transmission of simulator each node, each packet was generated in accordance routing performance, and the configuration parameters for routing performance simulation a Poisson arrival pattern. In addition, it was assumed that a single packet was sent in each are routing defined as follows. Forpacket the packet transmission of time eachslots. node,Aeach was (FIFO) generated in was session, and that each data had duration of five first packet in first out queue accordance with a Poisson arrival pattern. In addition, it was assumed that a single packet was sent provided with a length of 80 packets for each node. When the buffer is in overflow, a FIFO 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 adopted for both piconet and scatternet and packets forwardedselected, using the packets. In each routing session, thescheduling, source-destination pair were was randomly theRMR time with the designed routing protocol. The overall simulated nodes are 100, which are uniformly distributed division duplexing (TDD) scheme was adopted for both piconet and scatternet scheduling, and in a specific area. From Equations (1)the and (2),with the resulting number of protocol. piconets The is p overall = 15 and the number packets were forwarded using RMR the designed routing simulated nodes areis100, which of mesh links ml = 5. are uniformly distributed in a specific area. From Equations (1) and (2), the resulting number of piconets is p = are 15 and the number ofFigure mesh links ml ring = 5. configuration, 15 masters Three simulation configurations simulated, as in 6. Inisthe Threeare simulation configurations simulated,with as ingreen Figurecolor, 6. Inwhich the ring 15relay with red color interconnected by an are intra-bridge is aconfiguration, slave node to masters with red color are interconnected by an intra-bridge with green color, which is a slave node packets between interconnected piconets. Figure 6a is the piconet case, which is assumed 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. Figure 6b 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 among masters 3, 6, 9, 12, and 15. Figure 6c is the hybrid configuration of piconet and mesh links among masters 3, 6, 9, 12, and 15. Figure 6c is the hybrid configuration of piconet and scatternet cases. In the hybrid case, thereare aretwo two mesh links at master 3 three and three scatternet cases. In the hybrid case,ititisisassumed assumed that that there mesh links at master 3 and meshmesh linkslinks distributed at 7, 11, and 15. distributed at 7, 11, and 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).
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In Figure 6a of master 3, there exist various connection opportunities for the 5 mesh links via a In Figure 6a of 3, Each therebridge exist various forsimulated the 5 mesh via bridge node from IDmaster 16 to 30. node is connection sequentiallyopportunities connected and by links master 3 a bridge node from ID 16 to 30. Each bridge node is sequentially connected and simulated by master to determine the highest throughput point. With a peak-search method, the 5 determined bridge3 to determine the highest throughput point. Withfor a peak-search 5 determined nodes with highest throughput performance the piconet method, case arethe shown as Figurebridge 7. Thenodes first with highest throughput performance for the piconet case are shown as Figure 7. The first peak point peak point is generated at bridge 25, which is shown by the blue line. The other determined peak is generated bridge 25, which shown by the bluedemonstrate line. The other determined peak points are works 24, 16, points are 24,at16, 19, and 20. Theissimulation results that the peak-search method 19, The simulation results demonstrate that the peak-search methodwith works determine welland to 20. determine the optimal placement for each mesh link connection thewell besttothroughput the optimal placement for each mesh link connection with the best throughput performance. performance. 380
Throughput (packets/second)
360
340
320
300 Peak-search method performance 1st peak point 2nd peak point 3rd peak point 4th peak point 5th peak point
280
260 16
20
24 ID of intra-bridge node
28
32
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.
With the determined determinedmesh meshlinks, links,three threeconfigurations configurations simulated, their performances With the areare simulated, andand their performances are are shown in Figure piconet case,three threemesh meshlinks links achieve achieve the the maximum maximum throughput shown in Figure 8. 8. ForFor thethe piconet case, throughput performance. For the the scatternet scatternet and and hybrid hybrid cases, cases, the the throughput throughput performance performance increases increases as mesh performance. For as the the mesh link increases. In addition, more mesh links achieve better throughput performances. As a result, link increases. In addition, more mesh links achieve better throughput performances. As a result, the the scatternet achieves better throughput performance than theother othertwo twocases casesas as the the packet scatternet casecase achieves better throughput performance than the packet generation rate increases. The packet generation rate is defined as the number of packets generated in generation rate increases. The packet generation rate is defined as the number of packets generated each node at aatgiven time slot. in each node a given time slot. In [15], the cluster-based In [15], the cluster-based mesh mesh topology topology demonstrates demonstrates better better routing routing performance performance than than the the conventional full mesh topology with the on-demand routing protocol. Our proposed Mesh-Ring conventional full mesh topology with the on-demand routing protocol. Our proposed Mesh-Ring topology is is aatype typeofofcluster-based cluster-based mesh architecture, routing protocol can operate topology mesh architecture, andand thethe ringring routing protocol can operate for a for a larger network. After the RMR determines the optimal Mesh-Ring configuration, Figure larger network. After the RMR determines the optimal Mesh-Ring configuration, Figure 99 demonstrates the the packet packet successful successful probability probability (PSP) (PSP) for for RMR RMR with with three three cases, cases, Ring Ring and and Mesh demonstrates Mesh topologies. The RMR achieves almost 100% packet delivery ratio when the packet generation rate topologies. The RMR achieves almost 100% packet delivery ratio when the packet generation rate in in each node node is than 4. The PSP PSP in in each each node node is is defined defined as as the the total total successful successful reception reception packets packets each is smaller smaller than 4. The over the generated packets. TheThe piconet case achieves better better PSP than thethan Ringthe network over thetotal totalnetwork network generated packets. piconet case achieves PSP Ring when the packet generation rate is greater than 5. The scatternt case achieves the highest PSP than the network when the packet generation rate is greater than 5. The scatternt case achieves the highest other two cases. As a result, the RMR achieves superior performance on the PSP than the conventional PSP than the other two cases. As a result, the RMR achieves superior performance on the PSP than Ringconventional and the cluster-based Mesh topologies. Mesh topologies. the Ring and the cluster-based
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10 of 14 10 of 14 440 440
Mesh performance of three configurations Mesh performance of three configurations Scatternet case Scatternet case Piconet case Piconet case Hybrid case Hybrid case
Network Network Throughput Throughput (packets/second) (packets/second)
400 400
360 360
320 320
280 280
240 240
200 200
0 0
1 1
2 3 2 3 Number of mesh links Number of mesh links
4 4
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Figure 8. The three Figure 8. The determined determined mesh mesh link link with with highest highest throughput throughput of of peak peak search search method method for for three Figure 8. Thecases. determined mesh link with highest throughput of peak search method for three configuration configuration cases. configuration cases.
Successful Successful packet packet delivery delivery ratio ratio
1 1
0.8 0.8
0.6 0.6
Packet successful probability Packet successful probability Ring Ring case Piconet Piconet case Scatternet case Scatternet Hybrid casecase Hybrid Mesh case Mesh
0.4 0.4
0.2 0.2
0 0
2 4 6 8 2Packet generation 4 rate (packets/node/second) 6 8 Packet generation rate (packets/node/second)
10 10
Figure 9. The packet successful probability (PSP) performance of the Reconfigurable Mesh-Ring Figure 9. The the Reconfigurable Reconfigurable Mesh-Ring Mesh-Ring (RMR) Figure algorithm. 9. The packet packet successful successful probability probability (PSP) (PSP) performance performance of of the (RMR) algorithm. (RMR) 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 the examine the overall network capacity. average packet of throughput in each device is defined ratio of total To reflect the The scatternet capacity RMR, a packet throughput is used as to examine thethe overall network capacity. The average packet throughput in each device is defined as the ratio of the total number of successfully routed packets over the total simulation time in seconds. The simulation network capacity. The average packet throughput in each device is defined as the ratio of the total number of successfully routed packets over the total simulation time in seconds. The simulation results the packet throughput for the three cases oftime RMR, conventional Ring and numberofof successfully routed packets over configuration the total simulation inthe seconds. The simulation 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 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 case performance, compared to the conventional cluster-based Mesh scheme. mesh In RMR, theamong scatternet case achieves the highest throughput performance since the even distributed links masters achieves the highest throughput performance since the even distributed mesh links among masters
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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 masters effectively the connected configuration than the On hand, the other effectively improvesimproves the connected configuration than the other twoother cases.two On cases. the other the hand, the packet throughput of conventional Ring is promptly saturated, as the packet generation packet throughput of conventional Ring is promptly saturated, as the packet generation rate rate increases above 4 since the traditional topology increases the hop length reduces increases above 4 since the traditional ring ring topology increases the hop length and and thusthus reduces the the throughput performance inring the ring subnet. According the simulation results, the proposed throughput performance in the subnet. According to thetosimulation results, the proposed RMR RMR scatternet with scatternet improves by almost throughput performance,asascompared compared to to the the with case case improves by almost 70%70% throughput performance, conventional ring scheme. conventional ring scheme. 600
Network throughput (packets/seconds)
Mesh-Ring Throughput Ring Piconet case Scatternet case Hybrid case Mesh 400
200
0 0
2 4 6 8 Packet generation rate (packets/node/second)
10
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 to estimate the multi-hop routing performance, an average packet metric was used In to order measure the end-to-end delay of a scatternet. The packet delay of each delay routing packet is used to measure the end-to-end delay of a scatternet. The packet delay of each routing packet is defined as the average packet transmission time from the first transmitted bit at the source node to defined as the average packet transmission time from the first bit atdelay the source node to the last received bit at the destination node. Figure 11 shows thetransmitted average packet performances the last received bit at the destination node. Figure 11 shows the average packet delay performances of the three cases of RMR, the conventional Ring and the cluster-based Mesh schemes. The packet of the increases three cases RMR, thegeneration conventional and the cluster-based Mesh schemes. The packet delay asof the packet rateRing increases since more packets have to be processed at delay increases as the packet generation rate increases since more packets have to be processed at network. The RMR scatternet case generates the smallest average delay, as it generates the even network. The RMR scatternet case generates the smallest average delay, as it generates the even mesh mesh connectivity than all the simulated cases. Another observation from the results is that the connectivity allconventional the simulatedRing cases.quickly Another observation from results is that theaspacket delay packet delaythan of the increases versus thethe other three cases, the packet of the conventional Ring quickly versus the other three cases, as the packet generation rate generation rate increases above 4,increases because the traditional ring topology greatly increases the routing increases above 4, because the traditional ring topology greatly increases the routing path length. path length. A packet packet drop drop probability probability(PDP) (PDP)metric metricisisused usedtotoevaluate evaluatethe theimpact impactofoftraffic trafficcongestion congestion A inin a a scatternet. When the FIFO buffer overflows in each node, the damaged routing packets, including scatternet. When the FIFO buffer overflows in each node, the damaged routing packets, including both the the currently dropped. The PDP is defined as both currently received receivedand andnewly newlygenerated generatedrouting routingpackets, packets,were were dropped. The PDP is defined thethe ratio of the number of dropped packets over the totalthe generated packets for all nodes. Figure 12 as ratio of total the total number of dropped packets over total generated packets for all nodes. shows the PDP of the three cases of RMR, the conventional Ring and the cluster-based mesh schemes. Figure 12 shows the PDP of the three cases of RMR, the conventional Ring and the cluster-based Here, schemes. the RMR Here, scatternet case achieves lowest PDPthe than all other since thecases, scatternet mesh the RMR scatternetthe case achieves lowest PDPcases, than all other sincecase 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
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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 2018, 11,generation x of 14 evenEnergies the packet rate, which increases when the packets start to be dropped in12Figure 12. Energies 2018, 11, x 12 of 14 As a result, the tail drop mechanism can effectively manage the FIFO buffer and maintain throughput dropped in Figure 12. As a result, the tail drop mechanism can effectively manage the FIFO buffer performance for network congestion conditions. dropped in Figure 12. As a result, the tail drop mechanism canconditions. effectively manage the FIFO buffer and maintain throughput performance for network congestion and maintain throughput performance for network congestion conditions. Average packet delay Average packet Ring delay Piconet case Ring Scatternet case Piconet case Hybrid case Scatternet case Mesh Hybrid case Mesh
End-to-end packet (slots) End-to-end packet delaydelay (slots)
4000 4000
2000 2000
0 0
0 0
2 4 6 8 Packet generation 2 4 rate (packets/node/second) 6 8 Packet generation rate (packets/node/second)
10 10
Figure 11. The average packet delay performance of the 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 Packet dropRing probability Piconet Ring Scatternet case Piconet Hybrid case Scatternet case Mesh Hybrid case Mesh
0.6
Packet probability Packet drop drop probability
0.6
0.4 0.4
0.2 0.2
0 0
0 0
2 4 6 8 Packet generation rate (packets/node/second) 2 4 6 8 Packet generation rate (packets/node/second)
10 10
Figure 12. The packet drop probability (PDP) performance of the RMR algorithm. Figure 12. The packet drop probability (PDP) performance of the RMR algorithm.
Figure 12. The packet drop probability (PDP) performance of the RMR algorithm. 5. Conclusions 5. Conclusions In this study, a RMR topology is proposed for Bluetooth multi-hop networks. The RMR constructs ring topology as a backbone subnet,for which is then extendednetworks. by a mesh-shaped In thisa study, a RMR topology is proposed Bluetooth multi-hop The RMR topology. network three configurations (including piconet, and constructsTo a maximize ring topology as acapacity, backbone subnet, which is then extended by ascatternet, mesh-shaped topology. maximizeand network capacity, to three configurations (including piconet, scatternet, hybrid) areToexamined demonstrated determine the optimal placement of mesh links.and In addition, theexamined number ofand mesh link is adjustable to achieve optimal networkof configuration. hybrid) are demonstrated to determine thethe optimal placement mesh links. In addition, the number of mesh link is adjustable to achieve the optimal network configuration. In
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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.
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