# Waterfalls Partial Aggregation in Wireless Sensor Networks

Authors: Wuyungerile Li; Bing Jia; Shunsuke Saruwatari; Takashi Watanabe
Journal: International Journal of Distributed Sensor Networks
 (2016)

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## Abstract

In wireless sensor networks (WSNs), energy saving is a critical issue. Many research works have been undertaken to save energy. Data aggregation is one of the schemes that save energy by reducing the amount of data transmission. Normally, researchers focus on saving energy by aggregating multiple data or turning to achieving short transmission delay in data aggregation; few of them are concerned with network lifetime. This work achieves an optimum network lifetime by balancing energy consumption among nodes in network. Here, we propose a waterfalls partial aggregation, controlled by a set of waterfalls pushing rate vectors. The first contribution of this paper is to propose a waterfalls partial aggregation and to model it with queuing theory. The second contribution is that the optimum network lifetime is achieved mathematically and a near optimum algorithm is proposed for a given transmission delay. The results are compared with existing energy efficient algorithms and the evaluation results show the efficiency of proposed algorithm.


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## Body

## 1. Introduction

In the recent development of wireless technology, wireless sensor networks (WSNs) [1] have attracted researchers’ attention because of the applicability in many fields for effective collection of sensing data with low cost [2]. WSN consists of a large number of sensor nodes, where sensor nodes sense events and generate event data, then transmitting the data to a sink node via intermediate sensor nodes in a multihop manner [3]. Sensor nodes are battery-powered with limited energy supply; moreover, in many of the applications, sensor nodes are deployed in harsh nature environment or vast space so that the continuous energy supplement is impossible. In this case, if one of the nodes in the network exhausted energy, the network would break down and perform reorganization, where the reorganization of the network also consumes much energy and time. For these reasons, WSNs should be energy efficient. For energy saving, many researchers are working on Medium Access Control (MAC) protocols [4], routing protocols [5], topology control [6], and data aggregation [7] in WSNs.In WSNs, data generated from neighboring sensor nodes are often redundant and highly correlated. Sensor nodes spend considerable energy for sending or relaying a large number of redundant data. Moreover, a large number of data transmissions cause data collisions and data congestion. All these lead to the turning up of data aggregation technique. Data aggregation is defined as the process of aggregating data from multiple sensor nodes to eliminate redundant transmission and provide fused information to a sink node. In data aggregation, there are mainly three kinds of aggregation ways. The first is clustering data aggregation where data are collected and aggregated at a cluster node and then transmitted to a sink node [8, 9]. The second is hop by hop aggregation, which means that data are aggregated at each intermediate node [10]. The third is partial aggregation, where data aggregation satisfies a time or energy threshold [11]. However, all these three kinds of data aggregations have their drawbacks. In clustering data aggregation, a cluster head always consumes much energy than others; WSNs always perform a cluster head selecting algorithm to decide a new cluster head, which wastes considerable time and energy [12]. In hop by hop aggregation, data suffer long transmission delay and nodes on the transmission way suffer unbalanced energy consumption [10]. Compared to these two, partial aggregation appears to be more flexible. In partial aggregation with scheduling technique always set a time period to collect data for nodes that have aggregating function. Here, time period is always decided by a given time threshold or requested data accuracy but always does not consider the energy consumption of nodes in the network [11, 13, 14]. As everyone knows, network lifetime is always decided by nodes’ lifetime; when a node’s energy is exhausted, the network has to perform reorganization. Hence, even though the partial aggregation can shorten the transmission delay, it still has the shortcoming of unbalanced nodes lifetime, which leads to short network lifetime.In this paper, to achieve optimum network lifetime, at first, we propose a waterfalls partial aggregation (WPA), which is a kind of scheduling data aggregation scheme where optimum network lifetime is achieved by controlling the data transmitting period. Secondly, by analyzing the data transmission process of proposed scheme, we determined the formulations of energy consumption and transmission delay of the network, which causes advantage for future scientific studies. Then in Section4, optimum network lifetime is analyzed mathematically and achieved by a heuristics algorithm. We present evaluation in Section 5. Finally, we show the conclusion in Section 6.


## 2. Related Work

WSNs have various applications. Some applications are required to send data as soon as possible, while in other applications energy saving is much more important. For example, in disaster monitoring or emergency rescue, immediate data transmission is more important than others; hence, data are always transmitted to neighboring nodes without aggregation; we call this nonaggregation. However, in nature monitoring, energy is more significant than transmission delay because the replacement of battery for sensor nodes is supposed to be impossible. In respect to energy saving, data aggregation technique is widely applied in WSNs that are composed of large number of sensor nodes and meanwhile it is not so easy to supply continuous energy. Hence, we call it full aggregation that processes data aggregation only for energy saving in the network. In some applications of WSNs, it is required to trade off node energy and transmission delay or some other properties. In this case, data aggregation should consider corresponding delay or some other required thresholds; we call this kind of data aggregation a partial aggregation. In this section, we introduce nonaggregation, full aggregation, and partial aggregation and then discuss their advantages and disadvantages.


### 2.1. Nonaggregation

Definition of nonaggregation is that a node transmits received data to an adjacent lower node immediately after receiving data from a neighboring node which means that all data are sent to a sink node one by one, as shown in Figure1.Figure 1
Nonaggregation.From the definition of nonaggregation, we find that data transmission rate at a node becomes larger near a sink node because of data relaying properties of a node. Hence, energy consumption of nodes near a sink node is much more than that in nodes far from the sink. As a result, nodes near the sink node exhaust energy sooner than others, which results in short network lifetime. Moreover, when event data generation rate is large, data congestions occur at nodes around a sink node and they prolong transmission delay as well as let the network lifetime become worse [4–6, 15–18]. Sensor MAC [4] (S-MAC) is proposed for energy saving in wireless sensor networks based on IEEE 802.11 MAC protocols. In S-MAC, the network is always assumed to be less in amount of data transmission; data process and data aggregation can be performed in the networks where the transmission delay is considerably tolerant. S-MAC uses a listen/sleep model and divides the time into frames; each frame has the model of listen/sleep. Besides, topology control algorithms are studied for improving network lifetime, and the minimum spanning tree is the typical one. LMST [16] is a minimum spanning tree (MST) based topology control algorithm for multihop wireless networks, called local minimum spanning tree (LMST). The topology is constructed from each node, where a node builds its local MST independently (with the use of information locally collected) and keeps only one-hop on-tree nodes as its neighbors.


### 2.2. Full Aggregation

Full aggregation is that a node processes data aggregation when new data generated at itself. In more detail, the node does not transmit any received data from neighboring node immediately; all arrival data from neighboring nodes wait for new generated data. When there are new data at a node, the node aggregates all arrival data with its own data and then transmits them to its lower node. Figure2 illustrates the full aggregation in detail. It is clear to see from the definition that full aggregation can save much energy by aggregating data at every relay node. The only condition for data transmission is that a new event data occurs at a node, which means that the data transmission rate is the same with data generation rate. If we assume data generation rates at all nodes are the same, then the number of data transmissions for each node is the same, so that full aggregation can achieve balanced energy consumption among nodes.Figure 2
Full aggregation.However, full aggregation has its inevitable defect that transmission delay is too long for data occurring at nodes far from a sink node because of the waiting time for generating data at every intermediate node. And what is more, the transmission delay is longer when data generation rate is low at nodes. PEGASIS (Power-Efficient Gathering in Sensor Information Systems) [10] is one of the energy efficiency chain based data aggregation protocols that employs a greedy algorithm. There are two assumptions in this scheme: one is that all nodes are far from a sink node; the other is that nodes except the end nodes fuse the received data and their own data and then aggregate them into one packet before transmitting them to another neighboring node. The main idea of PEGASIS is forming a chain among the sensor nodes so that each node will receive from and transmit to fused data only with a close neighbor. The fused data are sent from node by node, and all the nodes take turns to be the leader for transmission to the sink node. The disadvantage of PEGASIS is that transmission delay is too long for nodes that are at the end of a chain.


### 2.3. Partial Aggregation

In partial aggregation, data always wait a period of time to be transmitted for collecting more data at a node; all collected data are aggregated into one or several representative data and then transmitted to an upper neighboring node. The waiting time at nodes is adjustable and is always decided by corresponding applications [11, 13, 19–21]. Figure 3 illustrates a most simple class of partial aggregation in which data wait Δt time for aggregation at a node before they are transmitted (1/Δt is data transmission rate at a node). When waiting time Δt is out, a node aggregates all generated data and received data into one type of representative data and then sent it out. In partial aggregation, energy is traded off for improving transmission delay, data accuracy, network lifetime, and so forth. Since full aggregation leads to long transmission delay and nonaggregation consumes much energy, partial aggregation can balance energy with other performances in WSN. As the waiting time at nodes is always decided by applications, one can set the waiting time very short when they want data with short transmission delay and otherwise the opposite.Figure 3
Partial aggregation.In in-network cascading timeout data aggregation [13] and ATS-DA (Adaptive Timeout Scheduling for Data Aggregation) [11], a sink initially broadcasts a request to all nodes. Each node waits for a certain time period to receive data from their child nodes. The timeout period of each node is set based on the position of the node in the data aggregation tree. However, these studies are not applicable when real time data or short delay data are required. Because all data at a node have to wait at least SHDavg time, no matter whether the data are important or not. Moreover, when the data arriving rate is irregular at nodes, the node energy consumption is also irregular among nodes, which result in unbalanced network. In particular, in large scale WSN, it is very difficult to achieve fairness for all nodes from the points of transmission delay and energy consumption because of data collision and congestion as well as retransmission.


## 2.1. Nonaggregation

Definition of nonaggregation is that a node transmits received data to an adjacent lower node immediately after receiving data from a neighboring node which means that all data are sent to a sink node one by one, as shown in Figure1.Figure 1
Nonaggregation.From the definition of nonaggregation, we find that data transmission rate at a node becomes larger near a sink node because of data relaying properties of a node. Hence, energy consumption of nodes near a sink node is much more than that in nodes far from the sink. As a result, nodes near the sink node exhaust energy sooner than others, which results in short network lifetime. Moreover, when event data generation rate is large, data congestions occur at nodes around a sink node and they prolong transmission delay as well as let the network lifetime become worse [4–6, 15–18]. Sensor MAC [4] (S-MAC) is proposed for energy saving in wireless sensor networks based on IEEE 802.11 MAC protocols. In S-MAC, the network is always assumed to be less in amount of data transmission; data process and data aggregation can be performed in the networks where the transmission delay is considerably tolerant. S-MAC uses a listen/sleep model and divides the time into frames; each frame has the model of listen/sleep. Besides, topology control algorithms are studied for improving network lifetime, and the minimum spanning tree is the typical one. LMST [16] is a minimum spanning tree (MST) based topology control algorithm for multihop wireless networks, called local minimum spanning tree (LMST). The topology is constructed from each node, where a node builds its local MST independently (with the use of information locally collected) and keeps only one-hop on-tree nodes as its neighbors.


## 2.2. Full Aggregation

Full aggregation is that a node processes data aggregation when new data generated at itself. In more detail, the node does not transmit any received data from neighboring node immediately; all arrival data from neighboring nodes wait for new generated data. When there are new data at a node, the node aggregates all arrival data with its own data and then transmits them to its lower node. Figure2 illustrates the full aggregation in detail. It is clear to see from the definition that full aggregation can save much energy by aggregating data at every relay node. The only condition for data transmission is that a new event data occurs at a node, which means that the data transmission rate is the same with data generation rate. If we assume data generation rates at all nodes are the same, then the number of data transmissions for each node is the same, so that full aggregation can achieve balanced energy consumption among nodes.Figure 2
Full aggregation.However, full aggregation has its inevitable defect that transmission delay is too long for data occurring at nodes far from a sink node because of the waiting time for generating data at every intermediate node. And what is more, the transmission delay is longer when data generation rate is low at nodes. PEGASIS (Power-Efficient Gathering in Sensor Information Systems) [10] is one of the energy efficiency chain based data aggregation protocols that employs a greedy algorithm. There are two assumptions in this scheme: one is that all nodes are far from a sink node; the other is that nodes except the end nodes fuse the received data and their own data and then aggregate them into one packet before transmitting them to another neighboring node. The main idea of PEGASIS is forming a chain among the sensor nodes so that each node will receive from and transmit to fused data only with a close neighbor. The fused data are sent from node by node, and all the nodes take turns to be the leader for transmission to the sink node. The disadvantage of PEGASIS is that transmission delay is too long for nodes that are at the end of a chain.


## 2.3. Partial Aggregation

In partial aggregation, data always wait a period of time to be transmitted for collecting more data at a node; all collected data are aggregated into one or several representative data and then transmitted to an upper neighboring node. The waiting time at nodes is adjustable and is always decided by corresponding applications [11, 13, 19–21]. Figure 3 illustrates a most simple class of partial aggregation in which data wait Δt time for aggregation at a node before they are transmitted (1/Δt is data transmission rate at a node). When waiting time Δt is out, a node aggregates all generated data and received data into one type of representative data and then sent it out. In partial aggregation, energy is traded off for improving transmission delay, data accuracy, network lifetime, and so forth. Since full aggregation leads to long transmission delay and nonaggregation consumes much energy, partial aggregation can balance energy with other performances in WSN. As the waiting time at nodes is always decided by applications, one can set the waiting time very short when they want data with short transmission delay and otherwise the opposite.Figure 3
Partial aggregation.In in-network cascading timeout data aggregation [13] and ATS-DA (Adaptive Timeout Scheduling for Data Aggregation) [11], a sink initially broadcasts a request to all nodes. Each node waits for a certain time period to receive data from their child nodes. The timeout period of each node is set based on the position of the node in the data aggregation tree. However, these studies are not applicable when real time data or short delay data are required. Because all data at a node have to wait at least SHDavg time, no matter whether the data are important or not. Moreover, when the data arriving rate is irregular at nodes, the node energy consumption is also irregular among nodes, which result in unbalanced network. In particular, in large scale WSN, it is very difficult to achieve fairness for all nodes from the points of transmission delay and energy consumption because of data collision and congestion as well as retransmission.


## 3. Waterfalls Partial Aggregation

### 3.1. Sensor Network Model

In this research, we apply queuing theory to analyze and model wireless sensor network. As in mathematical analysis, too complex network model makes it too sophisticated to get the analytical result and formulation; therefore, we define the network model to be the most basic and simplest model of tandem sensor network; however, the results can be extensible to other more complex topologies. The structure and transmission principle of tandem network are shown in Figure4.Figure 4
Tandem sensor network.In tandem networks, all the nodes deployed in a flat are allocated omnidirectional antennas with the same transmission range. Data generated at the nodes are transmitted to a sink node in multihop manner. The distance between two neighboring nodes is the same, and all the nodes are within the transmission range of their neighboring nodes:(i)
ni denotes the ith node from the sink.(ii)
n is a set of all nodes; N is natural number set.(iii)
Ei denotes the energy at node ni.(iv)
Lmax denotes maximum network lifetime.(v)
Dn denotes the total transmission delay.(vi)
μi is waterfalls pushing rate at node ni.(vii)
ti is data transmission delay for each node.


### 3.2. Definition of Waterfalls Partial Aggregation

To shorten the data transmission delay and to lengthen the network lifetime of the network as well as to suppress data congestion around a sink node, we propose a waterfalls partial aggregation (WPA). In WPA, data are transmitted to a neighboring node in two conditions: (a) if there are new local generated data at a node or (b) after waiting a holding timeΔt at a node. The inverse of the holding time Δti at node ni we call waterfalls pushing rate and it is denoted as μi where μi becomes smaller to nodes nearer a sink node; it means μi>μj, i>j, as shown in Figure 5.Figure 5
Waterfalls partial aggregation.In other words, data tends to be aggregated rarely at nodes far from a sink node to suppress delay. When there are new generated data at a node or the holding time at this node is over, all data at this node are aggregated and wait for further transmission. In more detail, the holding time for nodes nearer a sink node is longer than the ones far from the sink, so that it results in the decrease of the amount of data transmission near a sink node. Finally, it can achieve an equal amount of data transmission at each node by controlling the waterfalls pushing rate vectors. The flow chart of data aggregation and transmission is shown in Figure6.Figure 6
Flow chart of waterfalls partial aggregation.In the flow chart, after data arriving from neighboring node, nodeni starts to check if there are new generated data at itself. If yes, node ni aggregates all arrival data with the local generated data and then waits for service process; if no, node ni checks the holding time; if time is run up, it aggregates all arrival data and then goes to service process; otherwise, it checks new generated data. This process we call data arrival process to server. After aggregating data, node ni checks if the server is idle or not. If there are no other data waiting for transmitting, we say the server is idle; otherwise, server is busy; data queue up in the server and wait for further transmitting. When server is idle, node checks if the channel is idle or not. When channel is idle, data transmission process starts; otherwise, node ni checks the channel until it turns to idle. Data are sent to a neighboring node if the channel is idle.


### 3.3. Problem Definition

In WPA, the waterfalls setting of holding time is for balancing energy consumption among nodes as well as suppressing load. As we mentioned in Section2 that network lifetime is always decided by the shortest lifetime node in the network; therefore, to lengthen the shortest node lifetime is to extend network lifetime. When tolerable maximum transmission delay is given by corresponding application, we set the holding time Δti for nodes in the network according to WPA algorithm. The reason that set a maximum tolerable transmission delay is because in a finite network there would have several sets of waterfalls pushing rates vectors corresponding to different transmission delay. According to our definition, waterfalls pushing rate μi is inverse of holding time. With larger waterfalls pushing rate μi, a node transmits more data and results in much energy consumption. Thus, the problem becomes how to set the waterfalls pushing rates among nodes so that all nodes keep having the same energy consumption.In other words, how long should the holding time of arrival data for data aggregation at a node be to meet the conditions of balanced energy consumption among nodes and given transmission delay? For a node, if required transmission delay is given asDn, then what our algorithm should do is let(1)Dn=t1+t2+⋯+ti+⋯+tnn∈N.Here,ti is transmission delay for node ni and we determine the formulation of transmission delay for each node as well as total transmission delay of the network in Section 3.4. In WPA, data have to wait for Δt time at a node for gathering more data; hence, the transmission delay for a node is mainly decided by this Δt. On the other hand, to balance the energy consumption among nodes, we first obtain total energy consumption Ei for node ni which is composed of energy of data reception, data transmission, and overhearing. We show the solution of energy consumption for node ni and determine the formulations in Section 3.4. For node ni, data transmission rate is decided by itself and data receiving rate is decided by its upper neighboring node ni+1 and the overhearing rate is decided by its lower neighboring node ni-1. For node ni, data transmission rate λi′ is decided by data generation rate λi and waterfalls pushing rate μi. If we set the maximum network lifetime as Lmax, then we obtain (2) which means Lmax is decided by nodes lifetime Liorj when two arbitrary nodes i and j are balanced on energy Ei=Ej: (2)Lmax=LiorjEi=Ej1≤i,j<n.Therefore, for achieving balanced energy among nodes, we first determine the optimum set of waterfalls pushing rates μi that correspond to given Lmax in the network: (3)μi=arg⁡max⁡Lμi.From the above analysis, it is clear that we are trying to obtain a set of waterfalls pushing rates:(4)μi∣μi<μi+1,μi∈N,1<i<n.For obtaining optimum holding time for each node, it is necessary to know total delay of the network and energy consumption for a node. In the following sections, we determine the total delay and node energy consumption.


### 3.4. Analytical Model of WPA

#### 3.4.1. Queuing Theory Analysis

For determining the transmission delay, we model the data arrival process and data transmission process of WPA by queuing theory. The analytical model of nodeni is shown in Figure 7.Figure 7
Analytical model of waterfalls partial aggregation at nodeni.Before explaining the model, we introduceQueuerx, Queuetx, G, and Server. Queuerx denotes the arrival data queue in which arrival data are waiting for data aggregation at node ni. Queuetx denotes the data queue after aggregation in which aggregated data are waiting for transmission to a neighbor node. G is assumed as a virtual gate between Queuerx and Queuetx. Data transmitting process of data in Queuetx is accomplished via Server.In the analytical model,λi+1′′ is arrival data rate from upper node to Queuerx of node ni and is approximately abided by Poisson distribution. At node ni, event generation rate λi is assumed to be Poisson distribution. Waterfalls pushing rate μi is assumed to be exponential distribution. In Queuerx, all arrival data and generated data are aggregated and then join Queuetx via gate G. λi′ is data arrival rate to Queuetx in which data are waiting for further transmitting. λi′′ is the data rate upon exiting from the Server. Next, we analyze the above model start from data receiving at a node to data that are transmitted to a neighboring node.


#### 3.4.2. Event Waiting Time

In this subsection, we decide the time interval that data wait inQueuerx for aggregating at node ni; we call this time interval as event waiting time and denote it by τe(i) and it is counted from when the first data arrive to Queuerx until the data are aggregated and join Queuetx. To determine τe(i), we first determine the amount of data waiting for event data in Queuerx at node ni. The amount of data is denoted as Qrx(i) and we describe state transition rate diagram of Queuerx to determine it in Figure 8. Note that, at node ni, the arrival data rate λi+1′′ is approximately Poisson distribution, and λi is event data generation rate at node ni and is approximately Poisson distribution. In data aggregation, service time in queuing theory corresponds to data aggregating time, which is time interval from last data aggregation until next data aggregation. The basic idea of the analysis is that data waits inQueuerx for the duration according to the exponential distribution of average 1/2λi; since data aggregation rate is inverse of the mean aggregating time, hence data aggregation rate is 2λi, and when considering with waterfalls pushing rate, data aggregating rate is (5)λagg=2λi+μi.Figure 8
State transition ofQueuerx.In the diagram, the state variable is the number of data waiting for an event. Assume that a number of data are waiting for an event data at nodeni in Queuerx. If we let Pi,0 and Pi,k be the probability when the number of data waiting for an event is 0 and k in Queuerx at node ni, at state 0, according to Figure 8, we obtain (6)Pi,0λi+1′′=∑j=1∞Pi,jλagg.At state k, we obtain (7)Pi,kλi+1′′=Pi,k-1λi+1′′-Pi,kλagg.Hence, we obtain (8)Pi,k=λi+1′′λi+1′′+λaggPi,k-1Pi,k-1=λi+1′′λi+1′′+λaggPi,k-2⋮Pi,1=λi+1′′λi+1′′+λaggPi,0.Bringing these equations into (6), we determine the relationship between Pi,k and Pi,0 that (9)Pi,k=λi+1′′λi+1′′+λaggkPi,0.As we know,(10)∑k=1∞Pi,k=1.Let(11)α=λi+1′′λi+1′′+λagg.Then we determine(12)∑k=1∞αkPi,0=1.As (13)∑k=1∞αk=11-α,hence, (14)∑k=1∞αkPi,0=11-αPi,0=1.Accordingly, we obtain(15)Pi,0=1-α.Substituting α, we determine(16)Pi,0=2λi+μiλi+1′′+λagg,(17)Pi,k=λi+1′′k+λaggλi+1′′+λaggk+1.Now, we determine the amount of data that are received in Queuerx(i) as (18)Qrxi=∑k=0∞kPi,k.Taking (17) into the above equation, we obtain(19)Qrxi=∑k=0∞kλi+1′′k+λaggλi+1′′+λaggk+1.We express the above equation as (20)Qrxi=∑k=0∞kλi+1′′λi+1′′+λaggk+1λaggλi+1′′.Let β=λi+1′′/(λi+1′′+λagg), and γ=λagg/λi+1′′; then, we simplify (20) as(21)Qrxi=∑k=0∞kβk+1γ=∑k=0∞kβk-1β2γ=β2γ∑k=0∞kβk-1=β2γ∂∂β∑k=0∞βk=β2γ∂∂β11-β=β2γ1-β2.As β=λi+1′′/(λi+1′′+λagg) and λagg=2λi+μi, so we get that(22)1-β=2λi+μiλi+1′′+2λi+μi.Therefore, we determine that (23)β2γ1-β2=λi+1′′/λi+1′′+2λi+μi22λi+μi/λi+1′′2λi+μi/λi+1′′+2λi+μi2.By calculating the above equations, we determine that(24)Qrxi=λi+1′′2λi+μi.According to Little’s formula, we determine the event waiting time as follows:(25)τei=Qrxi2λi+μi=λi+1′′2λi+μi2.


#### 3.4.3. Arrival Process toQueuetx

From the analytical process ofQueuerxi, we find that arrival data rate λi′ to Queuerxi is decided by the waterfalls pushing rate μi and event data generation rate λi at node ni. The event generation rate λi and waterfalls pushing rate μi abide by discrete distribution and they are independent of one another. To determine the formulation of λi′, we calculate the property distribution of λi andμi. At first, we define that λi and μi are independent distribution X and Y as shown in Figure 9. We define the values of X as 1,2,…,EX and the values of Y as 1,2,…,EY. Here, we prove that the property Y is bigger than X.Figure 9
Property distribution ofX, Y.We determine the property as follows.PX and PY denote the properties when X=i and Y=i:(26)PY>X=PXX=1PYY=2+PYY=3+⋯+PYY=EY+PXX=2+PYY=3+⋯+PYY=EY⋮+PXX=EX+⋯+PYY=EX+1=∑i=1EXPXX=i∑j=i+1EYPYY=j.When applied to continuous distribution, we obtain(27)PY>X=∫0∞PXt∫0∞PYzdzdt.Here, if PX(t) and PY(t) are exponential distribution, then(28)PXt=λXe-λXt,PYt=λYe-λYt,PY>X=∫0∞PXt∫0∞λYe-λYzdzdt=∫0∞PXt-e-λYzt∞dt=∫0∞PXte-λYtdt=∫0∞λXe-λXte-λYtdt=∫0∞λXe-λX+λYtdt=λXλX+λY-e-λX+λYt0∞=λXλX+λY.Accordingly, we determine the data arrival rate of Queuetx:(29)λi′=λi+λiλi+μiμi.


#### 3.4.4. Service Process

Since the data generation rate is Poisson distribution and the waterfalls pushing rate abides by exponential distribution, the data arrival rate toQueuetxi approximates to Poisson distribution and each node has one server. The ACK packet transmission time is not considered. Data aggregating time is very short and negligible. Therefore, the service time is one-hop data transmission time τ. In our work, data transmission rate is Vc and generated data size is Si. Hence, the service time for generated data is (30)τ=SiVc.SinceVc and Si are constant, hence, the service time for data is fixed and constant. According to the above analysis, we can determine that the queuing system on server approximates to M/D/1 model. As data transmission time is determined and server utilization ratio isη=λi′×τtx(i), hence, the average data transmission time τtxi is(31)τtxi=τ1-λi′τ.We figure up the probability density function of the server time by means of Laplace transform in queuing theory and evaluate server waiting timeτsi that data wait for further transmitting in server:(32)τsi=λi′τtxi221-λi′τtxi.


#### 3.4.5. Channel Waiting Time

In general queuing system, customers can be served if there are no other customers waiting in front of them. However, when we apply queuing theory to model wireless communication, it is necessary to consider the impact of wireless channel caused by its properties. In wireless sensor networks, because of the overhearing of omnidirectional antenna, node has to wait a period of time if its upper or lower neighbors are transmitting data. Here, we call this time as channel waiting timeτci and it is determined as follows:(33)τci=2×η×τtxi=2λi′×τtx2i.


#### 3.4.6. Total Delay

Total delayDn in WPA is the time interval from when an event data is generated at a node until the data are received by a sink node in N hops network. The formulation is as follows:(34)Dn=∑i=1Nτei+τsi+τci+τtxi.


#### 3.4.7. Energy Consumption

The energy consumptionEi for node ni is the sum of transmission energy consumption, reception energy consumption, and overhearing energy consumption. Now, we determine the three kinds of energy consumption in the following paragraphs. Note that Ptx and Prx are energy consumption of transmitting or receiving for each data packet and Etxi, Erxi, and Eohi are energy consumption of data transmitting, data receiving, and overhearing of node ni in Dn time. As we are trying to obtain the balanced energy consumption among nodes, hence we calculate energy consumption per Dn time for each node. Therefore, we get the data transmission energy consumption according to Little’s formula as follows:(35)Etxi=λi′DnPtx.The reception energy consumption is(36)Erxi=λi+1′′DnPrx.Here, λi+1′′ is data arrival rate from upper neighboring node ni+1; as the service time is one data packet transmission time, hence the data rate getting out from server is same with data transmission rate, which means (37)λi+1′′=λi+1′.The overhearing energy consumption is (38)Eohi=λi-1′DnPrx.Here, λi-1′ is data transmission rate of node ni-1. Hence, we determine total energy consumption Ei for a node in timeDn as (39)Ei=Etxi+Erxi+Eohi.


## 3.1. Sensor Network Model

In this research, we apply queuing theory to analyze and model wireless sensor network. As in mathematical analysis, too complex network model makes it too sophisticated to get the analytical result and formulation; therefore, we define the network model to be the most basic and simplest model of tandem sensor network; however, the results can be extensible to other more complex topologies. The structure and transmission principle of tandem network are shown in Figure4.Figure 4
Tandem sensor network.In tandem networks, all the nodes deployed in a flat are allocated omnidirectional antennas with the same transmission range. Data generated at the nodes are transmitted to a sink node in multihop manner. The distance between two neighboring nodes is the same, and all the nodes are within the transmission range of their neighboring nodes:(i)
ni denotes the ith node from the sink.(ii)
n is a set of all nodes; N is natural number set.(iii)
Ei denotes the energy at node ni.(iv)
Lmax denotes maximum network lifetime.(v)
Dn denotes the total transmission delay.(vi)
μi is waterfalls pushing rate at node ni.(vii)
ti is data transmission delay for each node.


## 3.2. Definition of Waterfalls Partial Aggregation

To shorten the data transmission delay and to lengthen the network lifetime of the network as well as to suppress data congestion around a sink node, we propose a waterfalls partial aggregation (WPA). In WPA, data are transmitted to a neighboring node in two conditions: (a) if there are new local generated data at a node or (b) after waiting a holding timeΔt at a node. The inverse of the holding time Δti at node ni we call waterfalls pushing rate and it is denoted as μi where μi becomes smaller to nodes nearer a sink node; it means μi>μj, i>j, as shown in Figure 5.Figure 5
Waterfalls partial aggregation.In other words, data tends to be aggregated rarely at nodes far from a sink node to suppress delay. When there are new generated data at a node or the holding time at this node is over, all data at this node are aggregated and wait for further transmission. In more detail, the holding time for nodes nearer a sink node is longer than the ones far from the sink, so that it results in the decrease of the amount of data transmission near a sink node. Finally, it can achieve an equal amount of data transmission at each node by controlling the waterfalls pushing rate vectors. The flow chart of data aggregation and transmission is shown in Figure6.Figure 6
Flow chart of waterfalls partial aggregation.In the flow chart, after data arriving from neighboring node, nodeni starts to check if there are new generated data at itself. If yes, node ni aggregates all arrival data with the local generated data and then waits for service process; if no, node ni checks the holding time; if time is run up, it aggregates all arrival data and then goes to service process; otherwise, it checks new generated data. This process we call data arrival process to server. After aggregating data, node ni checks if the server is idle or not. If there are no other data waiting for transmitting, we say the server is idle; otherwise, server is busy; data queue up in the server and wait for further transmitting. When server is idle, node checks if the channel is idle or not. When channel is idle, data transmission process starts; otherwise, node ni checks the channel until it turns to idle. Data are sent to a neighboring node if the channel is idle.


## 3.3. Problem Definition

In WPA, the waterfalls setting of holding time is for balancing energy consumption among nodes as well as suppressing load. As we mentioned in Section2 that network lifetime is always decided by the shortest lifetime node in the network; therefore, to lengthen the shortest node lifetime is to extend network lifetime. When tolerable maximum transmission delay is given by corresponding application, we set the holding time Δti for nodes in the network according to WPA algorithm. The reason that set a maximum tolerable transmission delay is because in a finite network there would have several sets of waterfalls pushing rates vectors corresponding to different transmission delay. According to our definition, waterfalls pushing rate μi is inverse of holding time. With larger waterfalls pushing rate μi, a node transmits more data and results in much energy consumption. Thus, the problem becomes how to set the waterfalls pushing rates among nodes so that all nodes keep having the same energy consumption.In other words, how long should the holding time of arrival data for data aggregation at a node be to meet the conditions of balanced energy consumption among nodes and given transmission delay? For a node, if required transmission delay is given asDn, then what our algorithm should do is let(1)Dn=t1+t2+⋯+ti+⋯+tnn∈N.Here,ti is transmission delay for node ni and we determine the formulation of transmission delay for each node as well as total transmission delay of the network in Section 3.4. In WPA, data have to wait for Δt time at a node for gathering more data; hence, the transmission delay for a node is mainly decided by this Δt. On the other hand, to balance the energy consumption among nodes, we first obtain total energy consumption Ei for node ni which is composed of energy of data reception, data transmission, and overhearing. We show the solution of energy consumption for node ni and determine the formulations in Section 3.4. For node ni, data transmission rate is decided by itself and data receiving rate is decided by its upper neighboring node ni+1 and the overhearing rate is decided by its lower neighboring node ni-1. For node ni, data transmission rate λi′ is decided by data generation rate λi and waterfalls pushing rate μi. If we set the maximum network lifetime as Lmax, then we obtain (2) which means Lmax is decided by nodes lifetime Liorj when two arbitrary nodes i and j are balanced on energy Ei=Ej: (2)Lmax=LiorjEi=Ej1≤i,j<n.Therefore, for achieving balanced energy among nodes, we first determine the optimum set of waterfalls pushing rates μi that correspond to given Lmax in the network: (3)μi=arg⁡max⁡Lμi.From the above analysis, it is clear that we are trying to obtain a set of waterfalls pushing rates:(4)μi∣μi<μi+1,μi∈N,1<i<n.For obtaining optimum holding time for each node, it is necessary to know total delay of the network and energy consumption for a node. In the following sections, we determine the total delay and node energy consumption.


## 3.4. Analytical Model of WPA

### 3.4.1. Queuing Theory Analysis

For determining the transmission delay, we model the data arrival process and data transmission process of WPA by queuing theory. The analytical model of nodeni is shown in Figure 7.Figure 7
Analytical model of waterfalls partial aggregation at nodeni.Before explaining the model, we introduceQueuerx, Queuetx, G, and Server. Queuerx denotes the arrival data queue in which arrival data are waiting for data aggregation at node ni. Queuetx denotes the data queue after aggregation in which aggregated data are waiting for transmission to a neighbor node. G is assumed as a virtual gate between Queuerx and Queuetx. Data transmitting process of data in Queuetx is accomplished via Server.In the analytical model,λi+1′′ is arrival data rate from upper node to Queuerx of node ni and is approximately abided by Poisson distribution. At node ni, event generation rate λi is assumed to be Poisson distribution. Waterfalls pushing rate μi is assumed to be exponential distribution. In Queuerx, all arrival data and generated data are aggregated and then join Queuetx via gate G. λi′ is data arrival rate to Queuetx in which data are waiting for further transmitting. λi′′ is the data rate upon exiting from the Server. Next, we analyze the above model start from data receiving at a node to data that are transmitted to a neighboring node.


### 3.4.2. Event Waiting Time

In this subsection, we decide the time interval that data wait inQueuerx for aggregating at node ni; we call this time interval as event waiting time and denote it by τe(i) and it is counted from when the first data arrive to Queuerx until the data are aggregated and join Queuetx. To determine τe(i), we first determine the amount of data waiting for event data in Queuerx at node ni. The amount of data is denoted as Qrx(i) and we describe state transition rate diagram of Queuerx to determine it in Figure 8. Note that, at node ni, the arrival data rate λi+1′′ is approximately Poisson distribution, and λi is event data generation rate at node ni and is approximately Poisson distribution. In data aggregation, service time in queuing theory corresponds to data aggregating time, which is time interval from last data aggregation until next data aggregation. The basic idea of the analysis is that data waits inQueuerx for the duration according to the exponential distribution of average 1/2λi; since data aggregation rate is inverse of the mean aggregating time, hence data aggregation rate is 2λi, and when considering with waterfalls pushing rate, data aggregating rate is (5)λagg=2λi+μi.Figure 8
State transition ofQueuerx.In the diagram, the state variable is the number of data waiting for an event. Assume that a number of data are waiting for an event data at nodeni in Queuerx. If we let Pi,0 and Pi,k be the probability when the number of data waiting for an event is 0 and k in Queuerx at node ni, at state 0, according to Figure 8, we obtain (6)Pi,0λi+1′′=∑j=1∞Pi,jλagg.At state k, we obtain (7)Pi,kλi+1′′=Pi,k-1λi+1′′-Pi,kλagg.Hence, we obtain (8)Pi,k=λi+1′′λi+1′′+λaggPi,k-1Pi,k-1=λi+1′′λi+1′′+λaggPi,k-2⋮Pi,1=λi+1′′λi+1′′+λaggPi,0.Bringing these equations into (6), we determine the relationship between Pi,k and Pi,0 that (9)Pi,k=λi+1′′λi+1′′+λaggkPi,0.As we know,(10)∑k=1∞Pi,k=1.Let(11)α=λi+1′′λi+1′′+λagg.Then we determine(12)∑k=1∞αkPi,0=1.As (13)∑k=1∞αk=11-α,hence, (14)∑k=1∞αkPi,0=11-αPi,0=1.Accordingly, we obtain(15)Pi,0=1-α.Substituting α, we determine(16)Pi,0=2λi+μiλi+1′′+λagg,(17)Pi,k=λi+1′′k+λaggλi+1′′+λaggk+1.Now, we determine the amount of data that are received in Queuerx(i) as (18)Qrxi=∑k=0∞kPi,k.Taking (17) into the above equation, we obtain(19)Qrxi=∑k=0∞kλi+1′′k+λaggλi+1′′+λaggk+1.We express the above equation as (20)Qrxi=∑k=0∞kλi+1′′λi+1′′+λaggk+1λaggλi+1′′.Let β=λi+1′′/(λi+1′′+λagg), and γ=λagg/λi+1′′; then, we simplify (20) as(21)Qrxi=∑k=0∞kβk+1γ=∑k=0∞kβk-1β2γ=β2γ∑k=0∞kβk-1=β2γ∂∂β∑k=0∞βk=β2γ∂∂β11-β=β2γ1-β2.As β=λi+1′′/(λi+1′′+λagg) and λagg=2λi+μi, so we get that(22)1-β=2λi+μiλi+1′′+2λi+μi.Therefore, we determine that (23)β2γ1-β2=λi+1′′/λi+1′′+2λi+μi22λi+μi/λi+1′′2λi+μi/λi+1′′+2λi+μi2.By calculating the above equations, we determine that(24)Qrxi=λi+1′′2λi+μi.According to Little’s formula, we determine the event waiting time as follows:(25)τei=Qrxi2λi+μi=λi+1′′2λi+μi2.


### 3.4.3. Arrival Process toQueuetx

From the analytical process ofQueuerxi, we find that arrival data rate λi′ to Queuerxi is decided by the waterfalls pushing rate μi and event data generation rate λi at node ni. The event generation rate λi and waterfalls pushing rate μi abide by discrete distribution and they are independent of one another. To determine the formulation of λi′, we calculate the property distribution of λi andμi. At first, we define that λi and μi are independent distribution X and Y as shown in Figure 9. We define the values of X as 1,2,…,EX and the values of Y as 1,2,…,EY. Here, we prove that the property Y is bigger than X.Figure 9
Property distribution ofX, Y.We determine the property as follows.PX and PY denote the properties when X=i and Y=i:(26)PY>X=PXX=1PYY=2+PYY=3+⋯+PYY=EY+PXX=2+PYY=3+⋯+PYY=EY⋮+PXX=EX+⋯+PYY=EX+1=∑i=1EXPXX=i∑j=i+1EYPYY=j.When applied to continuous distribution, we obtain(27)PY>X=∫0∞PXt∫0∞PYzdzdt.Here, if PX(t) and PY(t) are exponential distribution, then(28)PXt=λXe-λXt,PYt=λYe-λYt,PY>X=∫0∞PXt∫0∞λYe-λYzdzdt=∫0∞PXt-e-λYzt∞dt=∫0∞PXte-λYtdt=∫0∞λXe-λXte-λYtdt=∫0∞λXe-λX+λYtdt=λXλX+λY-e-λX+λYt0∞=λXλX+λY.Accordingly, we determine the data arrival rate of Queuetx:(29)λi′=λi+λiλi+μiμi.


### 3.4.4. Service Process

Since the data generation rate is Poisson distribution and the waterfalls pushing rate abides by exponential distribution, the data arrival rate toQueuetxi approximates to Poisson distribution and each node has one server. The ACK packet transmission time is not considered. Data aggregating time is very short and negligible. Therefore, the service time is one-hop data transmission time τ. In our work, data transmission rate is Vc and generated data size is Si. Hence, the service time for generated data is (30)τ=SiVc.SinceVc and Si are constant, hence, the service time for data is fixed and constant. According to the above analysis, we can determine that the queuing system on server approximates to M/D/1 model. As data transmission time is determined and server utilization ratio isη=λi′×τtx(i), hence, the average data transmission time τtxi is(31)τtxi=τ1-λi′τ.We figure up the probability density function of the server time by means of Laplace transform in queuing theory and evaluate server waiting timeτsi that data wait for further transmitting in server:(32)τsi=λi′τtxi221-λi′τtxi.


### 3.4.5. Channel Waiting Time

In general queuing system, customers can be served if there are no other customers waiting in front of them. However, when we apply queuing theory to model wireless communication, it is necessary to consider the impact of wireless channel caused by its properties. In wireless sensor networks, because of the overhearing of omnidirectional antenna, node has to wait a period of time if its upper or lower neighbors are transmitting data. Here, we call this time as channel waiting timeτci and it is determined as follows:(33)τci=2×η×τtxi=2λi′×τtx2i.


### 3.4.6. Total Delay

Total delayDn in WPA is the time interval from when an event data is generated at a node until the data are received by a sink node in N hops network. The formulation is as follows:(34)Dn=∑i=1Nτei+τsi+τci+τtxi.


### 3.4.7. Energy Consumption

The energy consumptionEi for node ni is the sum of transmission energy consumption, reception energy consumption, and overhearing energy consumption. Now, we determine the three kinds of energy consumption in the following paragraphs. Note that Ptx and Prx are energy consumption of transmitting or receiving for each data packet and Etxi, Erxi, and Eohi are energy consumption of data transmitting, data receiving, and overhearing of node ni in Dn time. As we are trying to obtain the balanced energy consumption among nodes, hence we calculate energy consumption per Dn time for each node. Therefore, we get the data transmission energy consumption according to Little’s formula as follows:(35)Etxi=λi′DnPtx.The reception energy consumption is(36)Erxi=λi+1′′DnPrx.Here, λi+1′′ is data arrival rate from upper neighboring node ni+1; as the service time is one data packet transmission time, hence the data rate getting out from server is same with data transmission rate, which means (37)λi+1′′=λi+1′.The overhearing energy consumption is (38)Eohi=λi-1′DnPrx.Here, λi-1′ is data transmission rate of node ni-1. Hence, we determine total energy consumption Ei for a node in timeDn as (39)Ei=Etxi+Erxi+Eohi.


## 3.4.1. Queuing Theory Analysis

For determining the transmission delay, we model the data arrival process and data transmission process of WPA by queuing theory. The analytical model of nodeni is shown in Figure 7.Figure 7
Analytical model of waterfalls partial aggregation at nodeni.Before explaining the model, we introduceQueuerx, Queuetx, G, and Server. Queuerx denotes the arrival data queue in which arrival data are waiting for data aggregation at node ni. Queuetx denotes the data queue after aggregation in which aggregated data are waiting for transmission to a neighbor node. G is assumed as a virtual gate between Queuerx and Queuetx. Data transmitting process of data in Queuetx is accomplished via Server.In the analytical model,λi+1′′ is arrival data rate from upper node to Queuerx of node ni and is approximately abided by Poisson distribution. At node ni, event generation rate λi is assumed to be Poisson distribution. Waterfalls pushing rate μi is assumed to be exponential distribution. In Queuerx, all arrival data and generated data are aggregated and then join Queuetx via gate G. λi′ is data arrival rate to Queuetx in which data are waiting for further transmitting. λi′′ is the data rate upon exiting from the Server. Next, we analyze the above model start from data receiving at a node to data that are transmitted to a neighboring node.


## 3.4.2. Event Waiting Time

In this subsection, we decide the time interval that data wait inQueuerx for aggregating at node ni; we call this time interval as event waiting time and denote it by τe(i) and it is counted from when the first data arrive to Queuerx until the data are aggregated and join Queuetx. To determine τe(i), we first determine the amount of data waiting for event data in Queuerx at node ni. The amount of data is denoted as Qrx(i) and we describe state transition rate diagram of Queuerx to determine it in Figure 8. Note that, at node ni, the arrival data rate λi+1′′ is approximately Poisson distribution, and λi is event data generation rate at node ni and is approximately Poisson distribution. In data aggregation, service time in queuing theory corresponds to data aggregating time, which is time interval from last data aggregation until next data aggregation. The basic idea of the analysis is that data waits inQueuerx for the duration according to the exponential distribution of average 1/2λi; since data aggregation rate is inverse of the mean aggregating time, hence data aggregation rate is 2λi, and when considering with waterfalls pushing rate, data aggregating rate is (5)λagg=2λi+μi.Figure 8
State transition ofQueuerx.In the diagram, the state variable is the number of data waiting for an event. Assume that a number of data are waiting for an event data at nodeni in Queuerx. If we let Pi,0 and Pi,k be the probability when the number of data waiting for an event is 0 and k in Queuerx at node ni, at state 0, according to Figure 8, we obtain (6)Pi,0λi+1′′=∑j=1∞Pi,jλagg.At state k, we obtain (7)Pi,kλi+1′′=Pi,k-1λi+1′′-Pi,kλagg.Hence, we obtain (8)Pi,k=λi+1′′λi+1′′+λaggPi,k-1Pi,k-1=λi+1′′λi+1′′+λaggPi,k-2⋮Pi,1=λi+1′′λi+1′′+λaggPi,0.Bringing these equations into (6), we determine the relationship between Pi,k and Pi,0 that (9)Pi,k=λi+1′′λi+1′′+λaggkPi,0.As we know,(10)∑k=1∞Pi,k=1.Let(11)α=λi+1′′λi+1′′+λagg.Then we determine(12)∑k=1∞αkPi,0=1.As (13)∑k=1∞αk=11-α,hence, (14)∑k=1∞αkPi,0=11-αPi,0=1.Accordingly, we obtain(15)Pi,0=1-α.Substituting α, we determine(16)Pi,0=2λi+μiλi+1′′+λagg,(17)Pi,k=λi+1′′k+λaggλi+1′′+λaggk+1.Now, we determine the amount of data that are received in Queuerx(i) as (18)Qrxi=∑k=0∞kPi,k.Taking (17) into the above equation, we obtain(19)Qrxi=∑k=0∞kλi+1′′k+λaggλi+1′′+λaggk+1.We express the above equation as (20)Qrxi=∑k=0∞kλi+1′′λi+1′′+λaggk+1λaggλi+1′′.Let β=λi+1′′/(λi+1′′+λagg), and γ=λagg/λi+1′′; then, we simplify (20) as(21)Qrxi=∑k=0∞kβk+1γ=∑k=0∞kβk-1β2γ=β2γ∑k=0∞kβk-1=β2γ∂∂β∑k=0∞βk=β2γ∂∂β11-β=β2γ1-β2.As β=λi+1′′/(λi+1′′+λagg) and λagg=2λi+μi, so we get that(22)1-β=2λi+μiλi+1′′+2λi+μi.Therefore, we determine that (23)β2γ1-β2=λi+1′′/λi+1′′+2λi+μi22λi+μi/λi+1′′2λi+μi/λi+1′′+2λi+μi2.By calculating the above equations, we determine that(24)Qrxi=λi+1′′2λi+μi.According to Little’s formula, we determine the event waiting time as follows:(25)τei=Qrxi2λi+μi=λi+1′′2λi+μi2.


## 3.4.3. Arrival Process toQueuetx

From the analytical process ofQueuerxi, we find that arrival data rate λi′ to Queuerxi is decided by the waterfalls pushing rate μi and event data generation rate λi at node ni. The event generation rate λi and waterfalls pushing rate μi abide by discrete distribution and they are independent of one another. To determine the formulation of λi′, we calculate the property distribution of λi andμi. At first, we define that λi and μi are independent distribution X and Y as shown in Figure 9. We define the values of X as 1,2,…,EX and the values of Y as 1,2,…,EY. Here, we prove that the property Y is bigger than X.Figure 9
Property distribution ofX, Y.We determine the property as follows.PX and PY denote the properties when X=i and Y=i:(26)PY>X=PXX=1PYY=2+PYY=3+⋯+PYY=EY+PXX=2+PYY=3+⋯+PYY=EY⋮+PXX=EX+⋯+PYY=EX+1=∑i=1EXPXX=i∑j=i+1EYPYY=j.When applied to continuous distribution, we obtain(27)PY>X=∫0∞PXt∫0∞PYzdzdt.Here, if PX(t) and PY(t) are exponential distribution, then(28)PXt=λXe-λXt,PYt=λYe-λYt,PY>X=∫0∞PXt∫0∞λYe-λYzdzdt=∫0∞PXt-e-λYzt∞dt=∫0∞PXte-λYtdt=∫0∞λXe-λXte-λYtdt=∫0∞λXe-λX+λYtdt=λXλX+λY-e-λX+λYt0∞=λXλX+λY.Accordingly, we determine the data arrival rate of Queuetx:(29)λi′=λi+λiλi+μiμi.


## 3.4.4. Service Process

Since the data generation rate is Poisson distribution and the waterfalls pushing rate abides by exponential distribution, the data arrival rate toQueuetxi approximates to Poisson distribution and each node has one server. The ACK packet transmission time is not considered. Data aggregating time is very short and negligible. Therefore, the service time is one-hop data transmission time τ. In our work, data transmission rate is Vc and generated data size is Si. Hence, the service time for generated data is (30)τ=SiVc.SinceVc and Si are constant, hence, the service time for data is fixed and constant. According to the above analysis, we can determine that the queuing system on server approximates to M/D/1 model. As data transmission time is determined and server utilization ratio isη=λi′×τtx(i), hence, the average data transmission time τtxi is(31)τtxi=τ1-λi′τ.We figure up the probability density function of the server time by means of Laplace transform in queuing theory and evaluate server waiting timeτsi that data wait for further transmitting in server:(32)τsi=λi′τtxi221-λi′τtxi.


## 3.4.5. Channel Waiting Time

In general queuing system, customers can be served if there are no other customers waiting in front of them. However, when we apply queuing theory to model wireless communication, it is necessary to consider the impact of wireless channel caused by its properties. In wireless sensor networks, because of the overhearing of omnidirectional antenna, node has to wait a period of time if its upper or lower neighbors are transmitting data. Here, we call this time as channel waiting timeτci and it is determined as follows:(33)τci=2×η×τtxi=2λi′×τtx2i.


## 3.4.6. Total Delay

Total delayDn in WPA is the time interval from when an event data is generated at a node until the data are received by a sink node in N hops network. The formulation is as follows:(34)Dn=∑i=1Nτei+τsi+τci+τtxi.


## 3.4.7. Energy Consumption

The energy consumptionEi for node ni is the sum of transmission energy consumption, reception energy consumption, and overhearing energy consumption. Now, we determine the three kinds of energy consumption in the following paragraphs. Note that Ptx and Prx are energy consumption of transmitting or receiving for each data packet and Etxi, Erxi, and Eohi are energy consumption of data transmitting, data receiving, and overhearing of node ni in Dn time. As we are trying to obtain the balanced energy consumption among nodes, hence we calculate energy consumption per Dn time for each node. Therefore, we get the data transmission energy consumption according to Little’s formula as follows:(35)Etxi=λi′DnPtx.The reception energy consumption is(36)Erxi=λi+1′′DnPrx.Here, λi+1′′ is data arrival rate from upper neighboring node ni+1; as the service time is one data packet transmission time, hence the data rate getting out from server is same with data transmission rate, which means (37)λi+1′′=λi+1′.The overhearing energy consumption is (38)Eohi=λi-1′DnPrx.Here, λi-1′ is data transmission rate of node ni-1. Hence, we determine total energy consumption Ei for a node in timeDn as (39)Ei=Etxi+Erxi+Eohi.


## 4. Optimum Network Lifetime

### 4.1. Mathematical Solution

According to the analysis in Section3.2, the longest network lifetime is decided by the total transmission delay and waterfalls pushing rate vectors. Here, we illustrate the relationship between transmission delay, energy consumption, and network lifetime, so that at last we determine the formulations of a set of waterfalls pushing rates. Accordingly, energy consumption can be written as follows:(40)Ei=λi′Ptx+λi+1′′Prx+λi-1′Prx=λi′Ptx+λi+1′′+λi-1′Prx.To balance energy among nodes, we have (41)E1=E2=⋯=Ei=⋯=En.As the service time is one-hop data transmitting time, hence(42)λi′′=λi′.Here, Ei is the energy consumption for node in time Dn. Hence, we assume the energy consumption for each unit of time is E. We assume that λi=λ and the network in a finite network. Hence, there are no arrival data to the last node in the network so that it does not need a waterfalls pushing rate, and moreover, there is no overhearing for the first node that counted from the sink node. Therefore, for node nn, we have(43)μn=0,λn′=λ,Erxn=0,Eoh1=0.Accordingly, we obtain equations as follows:(44)λ+λμ1λ+μ1Ptx+λ+λμ2λ+μ2Prx=Eλ+λμ2λ+μ2Ptx+2λ+λμ1λ+μ1+λμ3λ+μ3Prx=E⋮λ+λμiλ+μiPtx+2λ+λμi-1λ+μi-1+λμi+1λ+μi+1Prx=E⋮λPtx+λ+λμn-1λ+μn-1Prx=E.By calculating the above equations, we have(45)μn-1=λPrxλ+λμn-2/λ+μn-2λμn-2/λ+μn-2Prx-λPtx(46)μn-2=λPrxλμn-3/λ+μn-3+λPrx-Ptxλμn-1/λ+μn-1Prx-Ptxλ-λμn-1/λ+μn-1-Prxλμn-3/λ+μn-3⋮(47)μi=λ2μi+2Prx-Ptx/λ+μi+2-λ2μi+1Prx-Ptx/λ+μi+1-λ2μi+3Prx/λ+μi+3λμi+3Prx/λ+μi+3-λμi+2Prx-Ptx/λ+μi+2+λμi+1Prx-Ptx/λ+μi+1-λPrx⋮μ1=λ2μ2Prx-Ptx/λ+μ2-λ2μ3Prx/λ+μ3-λ2Prxλμ2Ptx-Prx/λ+μ2-λμ3Prx/λ+μ3+2λPrx-λPtx.From the above equations, we obtained the relationship among waterfalls pushing rate. Equation (45) shows that if μn-1 is given, then μn-2 can be determined. In (46), if μn-1 and μn-2 are given, then μn-3 can be determined. In this way, all μi can be determined if μn-1 is known. To determine the values of μi, we designed a heuristics algorithm as follows. In light of real applications of WSN where the threshold of transmission delay is always given, in the heuristics algorithm, we assume that the transmission delay is predefined by application. Hence, we set the value of μn-1 and then determine the others. However, the value of μn-1 being too large or too little makes the algorithm too complex to process. Therefore, it is critical to determine the maximum value of μn-1.


### 4.2. Heuristics Algorithm

In a given application, we assume that data generation rates at all nodes are known and areλ. Thus, at node nn-1, if the arrival data from node nn just does not keep pace with the generated data, then the node needs waterfalls pushing rate to transmit the arrival data. However, the processing period of waterfalls pushing rate is decided by node nn-1; hence, there is again the case that the arrival data just does not catch up the pace of the waterfalls pushing rate μn-1. Therefore, we determine simply that, for the arrival data, the average catching up rate of waterfalls pushing rate at node nn-1 is λ/2. Accordingly, for transmitting all arrival data, the maximum value of μn-1 at node nn-1 is (48)μmax=2λ.Hence, a heuristics algorithm is described as shown in Algorithm 1.Algorithm 1:Heuristics algorithm.
Input:λ, μn-1, η, Dapp
Output: [μ1:μn-1]
μ
m
a
x
=
2
λ
μ
n
-
1
=
μ
m
a
x
μ
1
:
μ
n
-
1
=
f
μ
n
-
i
D
h
e
u
=
g
μ
1
:
μ
n
-
1
While|Dapp-Dheu|>η do
μn-2=μn-1-Δ
[μ1:μn-1]=f(μn-i)
Dheu=g(μ1:μn-1)
End
Return[μ1:μn-1]


## 4.1. Mathematical Solution

According to the analysis in Section3.2, the longest network lifetime is decided by the total transmission delay and waterfalls pushing rate vectors. Here, we illustrate the relationship between transmission delay, energy consumption, and network lifetime, so that at last we determine the formulations of a set of waterfalls pushing rates. Accordingly, energy consumption can be written as follows:(40)Ei=λi′Ptx+λi+1′′Prx+λi-1′Prx=λi′Ptx+λi+1′′+λi-1′Prx.To balance energy among nodes, we have (41)E1=E2=⋯=Ei=⋯=En.As the service time is one-hop data transmitting time, hence(42)λi′′=λi′.Here, Ei is the energy consumption for node in time Dn. Hence, we assume the energy consumption for each unit of time is E. We assume that λi=λ and the network in a finite network. Hence, there are no arrival data to the last node in the network so that it does not need a waterfalls pushing rate, and moreover, there is no overhearing for the first node that counted from the sink node. Therefore, for node nn, we have(43)μn=0,λn′=λ,Erxn=0,Eoh1=0.Accordingly, we obtain equations as follows:(44)λ+λμ1λ+μ1Ptx+λ+λμ2λ+μ2Prx=Eλ+λμ2λ+μ2Ptx+2λ+λμ1λ+μ1+λμ3λ+μ3Prx=E⋮λ+λμiλ+μiPtx+2λ+λμi-1λ+μi-1+λμi+1λ+μi+1Prx=E⋮λPtx+λ+λμn-1λ+μn-1Prx=E.By calculating the above equations, we have(45)μn-1=λPrxλ+λμn-2/λ+μn-2λμn-2/λ+μn-2Prx-λPtx(46)μn-2=λPrxλμn-3/λ+μn-3+λPrx-Ptxλμn-1/λ+μn-1Prx-Ptxλ-λμn-1/λ+μn-1-Prxλμn-3/λ+μn-3⋮(47)μi=λ2μi+2Prx-Ptx/λ+μi+2-λ2μi+1Prx-Ptx/λ+μi+1-λ2μi+3Prx/λ+μi+3λμi+3Prx/λ+μi+3-λμi+2Prx-Ptx/λ+μi+2+λμi+1Prx-Ptx/λ+μi+1-λPrx⋮μ1=λ2μ2Prx-Ptx/λ+μ2-λ2μ3Prx/λ+μ3-λ2Prxλμ2Ptx-Prx/λ+μ2-λμ3Prx/λ+μ3+2λPrx-λPtx.From the above equations, we obtained the relationship among waterfalls pushing rate. Equation (45) shows that if μn-1 is given, then μn-2 can be determined. In (46), if μn-1 and μn-2 are given, then μn-3 can be determined. In this way, all μi can be determined if μn-1 is known. To determine the values of μi, we designed a heuristics algorithm as follows. In light of real applications of WSN where the threshold of transmission delay is always given, in the heuristics algorithm, we assume that the transmission delay is predefined by application. Hence, we set the value of μn-1 and then determine the others. However, the value of μn-1 being too large or too little makes the algorithm too complex to process. Therefore, it is critical to determine the maximum value of μn-1.


## 4.2. Heuristics Algorithm

In a given application, we assume that data generation rates at all nodes are known and areλ. Thus, at node nn-1, if the arrival data from node nn just does not keep pace with the generated data, then the node needs waterfalls pushing rate to transmit the arrival data. However, the processing period of waterfalls pushing rate is decided by node nn-1; hence, there is again the case that the arrival data just does not catch up the pace of the waterfalls pushing rate μn-1. Therefore, we determine simply that, for the arrival data, the average catching up rate of waterfalls pushing rate at node nn-1 is λ/2. Accordingly, for transmitting all arrival data, the maximum value of μn-1 at node nn-1 is (48)μmax=2λ.Hence, a heuristics algorithm is described as shown in Algorithm 1.Algorithm 1:Heuristics algorithm.
Input:λ, μn-1, η, Dapp
Output: [μ1:μn-1]
μ
m
a
x
=
2
λ
μ
n
-
1
=
μ
m
a
x
μ
1
:
μ
n
-
1
=
f
μ
n
-
i
D
h
e
u
=
g
μ
1
:
μ
n
-
1
While|Dapp-Dheu|>η do
μn-2=μn-1-Δ
[μ1:μn-1]=f(μn-i)
Dheu=g(μ1:μn-1)
End
Return[μ1:μn-1]


## 5. Evaluation

### 5.1. Validation of the Queuing Theory Model

We implement WPA in the original network simulator written in C++ language. Analytic results in the previous section are shown as well as simulated result. The parameters are shown in Table1.Table 1
Simulation parameters of WPA.
Node distance
10 (m)
Transmission range
11 (m)
Transmission rate
250 (kbps)
Data size
4096 (bit)
Current consumption for transmission
17.4 (mA)
Current consumption for reception
19.7 (mA)
MAC
CSMA/CA
Routing
DSRIn the simulation, data occurs at each node randomly and independently. Buffer size of each node is infinite. Although analytic model assumes that transmission error is negligible, transmission errors and retransmission may occur in the simulation. We set data generation rateλ=5 and waterfalls pushing rate as μi={1,2,3,4,5} for 5-hop tandem network.From Figures10 and 11, we find that our analytical model meets with simulation results, so that we basically confirm the correctness of queuing theory model of WPA.Figure 10
Total delay of WPA.Figure 11
Node energy consumption of WPA.


### 5.2. Evaluation of Optimum Network Lifetime

In this section, we evaluate the effectiveness of the proposed waterfalls partial aggregation. Here, we consider network environment as nature monitoring network, so that we obtain the maximum network lifetime, which is the same with network lifetime of full aggregation. We setE in (44) which is equal to Eful-largest, where Eful-largest denotes the energy consumption of the node that consumes most energy in the network. The optimal results can be achieved by numerical packets; however, the mathematical calculation results are too complex. For more understanding of our algorithm, we obtain the pushing rate vectors applying Excel when the event generation rate is λ=1 for 5-hop tandem network. Nodes are counted from the sink node, which means that the nearest node with the sink is node n1 and its waterfalls pushing rate is μ1. The optimum waterfalls pushing rates are shown in Table 2 when WPA has the same network lifetime with full aggregation. The analytical model of nonaggregation and full aggregation is realized by queuing theory; however, for the brevity of the paper, we omitted the derivation process.Table 2
Waterfalls pushing rate for optimum network lifetime.
Waterfalls pushing rate
Values
μ
1
0.02
μ
2
0.04
μ
3
0.08
μ
4
0.1
μ
5
0.5Figure12 shows the lifetime of all nodes in the network where we set the waterfalls pushing rate of nodes as the same with Table 2. From the figure, we find that all nodes almost have the same lifetime in WPA. In this case, we can say that the node energy is utilized optimally in WPA. In full aggregation, the second and the third node consumed most energy and the other nodes have less energy consumption which results in the suboptimum utilization of energy. The nonaggregation has very large energy consumption at the second node, which results in the short lifetime of the entire network. We find from the figure that nonaggregation and full aggregation have the largest energy consumption at their second node. The reason is that the first node has no adjacent upper node, which means that it does not consume overhearing energy compared with others.Figure 12
Lifetime of nodes.Figure13 shows the corresponding total delay of the network where we set the same waterfalls pushing rates with Table 1. From the figure, we find that when it has the same network lifetime with full aggregation, WPA can shorten the transmission delay by considerable amount. When data generation rate becomes larger (larger than 6 types of data/sec), transmission delay of nonaggregation gets longer rapidly due to data congestion at nodes. However, in WPA, the rapid increase of transmission delay arises later (at data generation rate of 30 types of data/sec) than nonaggregation and full aggregation on data generation line, which indicates that WPA can relieve data congestion at nodes. This is because, with waterfalls pushing rates, proposed WPA can adjust the number of data transmissions. Moreover, we find from the figure that with the given waterfalls pushing rates there is minimum transmission delay and we can determine it in WPA.Figure 13
Transmission delay of network.


### 5.3. Summary of Evaluation

Firstly, from the presentation of Section5.1, we conclude that our analytical model of WPA meets with simulation results; the diversity of energy consumption is that we did not consider retransmission in analytical model and considered it in simulation. Secondly, in Section 5.2, compared to nonaggregation and full aggregation, proposed WPA has the superiorities as follows: nonaggregation consumes much energy and data congestion occurs easily when data generation rate is larger. In WPA, after data are sent to a neighboring node, the data have to be aggregated together or aggregated with local data for further transmitting; hence, WPA is more energy efficient than nonaggregation. When data generation rate is larger at a node, data congestion occurs at nonaggregation, which leads to long transmission delay; however, proposed WPA can relieve data congestion via reducing the number of data transmissions by aggregating received data at nodes. On the other hand, full aggregation can save much energy consumption and suffer long transmission delay. Compared to full aggregation, proposed WPA has similar energy consumption and is superior in balancing energy consumption among nodes. Moreover, with waterfalls pushing rates, WPA can control data waiting time more reasonably and efficiently than full aggregation at nodes, so that transmission delay of WPA is superior to full aggregation.


## 5.1. Validation of the Queuing Theory Model

We implement WPA in the original network simulator written in C++ language. Analytic results in the previous section are shown as well as simulated result. The parameters are shown in Table1.Table 1
Simulation parameters of WPA.
Node distance
10 (m)
Transmission range
11 (m)
Transmission rate
250 (kbps)
Data size
4096 (bit)
Current consumption for transmission
17.4 (mA)
Current consumption for reception
19.7 (mA)
MAC
CSMA/CA
Routing
DSRIn the simulation, data occurs at each node randomly and independently. Buffer size of each node is infinite. Although analytic model assumes that transmission error is negligible, transmission errors and retransmission may occur in the simulation. We set data generation rateλ=5 and waterfalls pushing rate as μi={1,2,3,4,5} for 5-hop tandem network.From Figures10 and 11, we find that our analytical model meets with simulation results, so that we basically confirm the correctness of queuing theory model of WPA.Figure 10
Total delay of WPA.Figure 11
Node energy consumption of WPA.


## 5.2. Evaluation of Optimum Network Lifetime

In this section, we evaluate the effectiveness of the proposed waterfalls partial aggregation. Here, we consider network environment as nature monitoring network, so that we obtain the maximum network lifetime, which is the same with network lifetime of full aggregation. We setE in (44) which is equal to Eful-largest, where Eful-largest denotes the energy consumption of the node that consumes most energy in the network. The optimal results can be achieved by numerical packets; however, the mathematical calculation results are too complex. For more understanding of our algorithm, we obtain the pushing rate vectors applying Excel when the event generation rate is λ=1 for 5-hop tandem network. Nodes are counted from the sink node, which means that the nearest node with the sink is node n1 and its waterfalls pushing rate is μ1. The optimum waterfalls pushing rates are shown in Table 2 when WPA has the same network lifetime with full aggregation. The analytical model of nonaggregation and full aggregation is realized by queuing theory; however, for the brevity of the paper, we omitted the derivation process.Table 2
Waterfalls pushing rate for optimum network lifetime.
Waterfalls pushing rate
Values
μ
1
0.02
μ
2
0.04
μ
3
0.08
μ
4
0.1
μ
5
0.5Figure12 shows the lifetime of all nodes in the network where we set the waterfalls pushing rate of nodes as the same with Table 2. From the figure, we find that all nodes almost have the same lifetime in WPA. In this case, we can say that the node energy is utilized optimally in WPA. In full aggregation, the second and the third node consumed most energy and the other nodes have less energy consumption which results in the suboptimum utilization of energy. The nonaggregation has very large energy consumption at the second node, which results in the short lifetime of the entire network. We find from the figure that nonaggregation and full aggregation have the largest energy consumption at their second node. The reason is that the first node has no adjacent upper node, which means that it does not consume overhearing energy compared with others.Figure 12
Lifetime of nodes.Figure13 shows the corresponding total delay of the network where we set the same waterfalls pushing rates with Table 1. From the figure, we find that when it has the same network lifetime with full aggregation, WPA can shorten the transmission delay by considerable amount. When data generation rate becomes larger (larger than 6 types of data/sec), transmission delay of nonaggregation gets longer rapidly due to data congestion at nodes. However, in WPA, the rapid increase of transmission delay arises later (at data generation rate of 30 types of data/sec) than nonaggregation and full aggregation on data generation line, which indicates that WPA can relieve data congestion at nodes. This is because, with waterfalls pushing rates, proposed WPA can adjust the number of data transmissions. Moreover, we find from the figure that with the given waterfalls pushing rates there is minimum transmission delay and we can determine it in WPA.Figure 13
Transmission delay of network.


## 5.3. Summary of Evaluation

Firstly, from the presentation of Section5.1, we conclude that our analytical model of WPA meets with simulation results; the diversity of energy consumption is that we did not consider retransmission in analytical model and considered it in simulation. Secondly, in Section 5.2, compared to nonaggregation and full aggregation, proposed WPA has the superiorities as follows: nonaggregation consumes much energy and data congestion occurs easily when data generation rate is larger. In WPA, after data are sent to a neighboring node, the data have to be aggregated together or aggregated with local data for further transmitting; hence, WPA is more energy efficient than nonaggregation. When data generation rate is larger at a node, data congestion occurs at nonaggregation, which leads to long transmission delay; however, proposed WPA can relieve data congestion via reducing the number of data transmissions by aggregating received data at nodes. On the other hand, full aggregation can save much energy consumption and suffer long transmission delay. Compared to full aggregation, proposed WPA has similar energy consumption and is superior in balancing energy consumption among nodes. Moreover, with waterfalls pushing rates, WPA can control data waiting time more reasonably and efficiently than full aggregation at nodes, so that transmission delay of WPA is superior to full aggregation.


## 6. Conclusion

In this paper, we proposed a waterfalls partial aggregation that can achieve optimum network lifetime in WSN via applying data aggregation. At first, we modeled WPA in queuing theory and then determined transmission delay of the network and energy consumption of a node. Then, we continued to model balanced energy consumption among nodes. The evaluation parts show the correctness of our analytical model and demonstrate the superiorities of proposed WPA.



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*Source: 2392149-2016-05-23.xml*