Patent Publication Number: US-7916017-B2

Title: Operating method of wireless sensor networks considering energy efficiency

Description:
The present application claims priority under 35 U.S.C. 119 to Korean Patent Application No. 10-2007-0057710 (filed on Jun. 13, 2007), which is hereby incorporated by reference in its entirety. 
     TECHNICAL FIELD 
     Embodiments relates to a wireless sensor network operating method in consideration of energy efficiency and, more particularly, to a wireless sensor network operating method that can increase an overall lifespan of a wireless sensor network by reducing a probability that a sensor node with a relatively low residual energy amount than other sensor nodes is selected as a cluster head that consumes much energy to thereby improve energy efficiency. 
     BACKGROUND 
     Referring to  FIG. 1 , a wireless sensor network  1  is formed of a sensor field  300  where a plurality of sensor nodes  100  are disposed, and a sink node  400  connecting the sensor field  300  to a base station  500 . The sensor nodes  100  of the sensor field  300  acquire sensing data, which are transmitted to the base station  500  through the sink node  400 . The wireless sensor network  1  is used in diverse areas such as health monitoring, military networks, home networks, environmental observation, factory management, and anti-disaster management. 
     Each of the sensor nodes  100  has their own sensor node duration set up in consideration of a power source and lifespan (i.e., device duration) of respective devices constituting the sensor node. The sensor node duration will be referred to as ‘energy,’ hereinafter. The energy of each sensor node  100  is limited and it cannot be recharged. 
     Particularly, the energy of a sensor node  100  is rapidly drained when data are transmitted/received wirelessly in the wireless sensor network  1 .
 
 E   Tx ( k,d )=( k×E   elec )+( k×ε   amp   ×d   2 )  Eq. 1
 
     where E Tx  denotes the total amount of energy consumption during data transmission; k denotes the amount of data packets; d denotes wireless transmission/reception range; ε amp  denotes a constant of the amount of energy consumed during signal amplification in an amplifier; and E elec  denotes a constant of the amount of energy consumed when data are converted into radio frequency (RF).
 
 E   Rx ( k,d )= k×E   elec   Eq. 2
 
     where E Rx  denotes the total amount of energy consumption during data reception; k denotes the amount of data packets; and Eelec denotes a constant of the amount of energy consumed when data received in the form of RF are converted. 
     As shown in  FIG. 2  and Equation 1, energy consumption increases in proportion to the amount (k) of packets and squared range during data transmission. During data reception, the energy consumption increases in proportion to the amount (k) of packets, as shown in  FIG. 2  and Equation 2. 
     Therefore, it is desirable to reduce the number of times of performing wireless communication as much as possible in order to save energy of the sensor nodes  100 . 
     Among methods for operating the wireless sensor network  1  is a Low Energy Adaptive Clustering Hierarchy (LEACH). LEACH is a method that sensor nodes  100  form a cluster  200  and a cluster head  110  randomly selected collects all informations in the cluster  200  and transmits them to the base station  500  through the sink node  400 . 
     LEACH is executed through a selection stage, a cluster setup stage, a schedule generation stage, and a data transmission stage. 
     
       
         
           
             
               
                 
                   
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     where P(n) denotes a threshold value; k denotes a constant indicating a composition ratio of a cluster head; r denotes a current round (i.e., operation time unit of LEACH); C(n) denotes a group of nodes which are not cluster heads for a past 1/k round. 
     In the cluster head selection stage, a threshold P(n) acquired from the Equation 3 is compared with a random number between 0 and 1 that is randomly selected. A sensor node  100  whose threshold P(n) is greater than the random number is selected as the cluster header  110 . In a round, each sensor node  100  can be a cluster head  110  at a k probability. In the round, a sensor node  100  that becomes the cluster head  110  cannot be a cluster head  110  again for the next 1/k round. As rounds continue, an even smaller number of sensor nodes  100  are qualified to be selected as the cluster head  110 , the probability of the remaining sensor nodes  100  to be selected as the cluster head  110  increases. 
     In the cluster setup stage, selected cluster heads  110  transmit an advertisement message to the sensor nodes  100 . The sensor nodes  100  that have received the advertisement message from a plurality of cluster heads select a cluster head  110  to join based on received signal strength (RSS). A sensor node  100  transmits a join-request message (Join-REQ) to the cluster head  110  to join and set up a cluster  200 . 
     In the scheduling setup stage, the cluster head  110  performs Time Division Multiple Access (TDMA) to allocate time to the sensor node  100  that has sent out the join-request message to itself. Accordingly, each sensor node  100  transmits a sensing result to the cluster head  110  at an allocated time. 
     In the data transmission stage, the sensing results transmitted from the sensor nodes  100  at the allocated time are collected, and data fusion is performed. The fused data are transmitted to the base station  500  through the sink node  400 . 
     As described above, LEACH has an effect of extending the lifespan of the wireless sensor network  1  by randomly selecting cluster heads  110  in charge of transmitting data to the base station  500  and thereby distributing energy consumed for data transmission to all sensor nodes  100 . 
     However, since LEACH randomly selects the cluster heads  110 , even a sensor node  100  with a relatively smaller amount of residual energy than those of other sensor nodes  100  can be selected as a cluster head  110 . Since the cluster head  110  should perform data fusion onto sensing results transmitted from sensor nodes  100  in a cluster  200  and deliver the fused data to the base station  500  through the sink node  400 , it consumes more power than other sensor nodes  100 . 
     Therefore, when a sensor node  100  with a relatively smaller amount of residual energy than those of other sensor nodes  100  is selected as a cluster head  110 , it may consume all its energy for excessive data transmission and eventually complete its own lifespan. Consequently, there is a shortcoming in that when a sensor node  100  with a relatively smaller amount of residual energy than those of other sensor nodes  100  is selected as a cluster head  110 , the lifespan of the wireless sensor network  1  is shortened. 
     To overcome this shortcoming, LEACH-C is developed. LEACH-C is a method of positioning the base station  500 , the sink node  400 , and the sensor nodes  100  and selecting cluster heads  110  in consideration of residual energy amount of each sensor node  100 . In other words, since cluster heads  110  are selected in consideration of the entire residual energy amount of the wireless sensor network  1 , there is no concern in the LEACH-C that a sensor node  100  with a small amount of residual energy might be selected as a cluster head  110 . 
     Also, to figure out the positions and residual energy amounts of the base station  500 , the sink node  400 , and the sensor nodes  100  in LEACH-C, data on the positions and residual energy amounts should be transmitted/received among the sensor nodes  100 . This also causes severe energy consumption during the data transmission/reception. 
     SUMMARY 
     Embodiments have been proposed in order to provide a wireless sensor network operating method that does not transmit/receive data on positions and residual energy amounts among sensor nodes to thereby extend lifespan of the sensor nodes, reduces a probability that a sensor node with a relatively smaller amount of residual energy than those of the other sensor nodes is selected as a cluster head  110  to thereby increase the entire lifespan of a wireless sensor network. 
     In embodiments, a method for operating a wireless sensor network including clusters each formed of a plurality of sensor nodes, includes the steps of: a) selecting a cluster head based on residual energy amounts of the sensor nodes by increasing a probability that a sensor node with a greater amount of residual energy than the others is selected as a cluster head; b) transmitting an advertisement message from the selected cluster head to the sensor nodes to thereby form a cluster; and c) transmitting sensing data acquired in each sensor node to the cluster head. 
     Herein, the cluster head selection step a) may include the steps of: a-1) calculating an energy value of each sensor node based on the residual energy amount of the sensor node in comparison with an initial energy amount of the sensor node; and a-2) comparing the energy value with a random number between 0 and 1 and selecting a sensor node having an energy value greater than a random number as a cluster head. 
     The energy value comparison step a-2) may include the steps of: a-2-1) when there is at least one sensor node whose residual energy amount is less than 50% of the initial energy amount of the sensor node, comparing the energy value of the sensor node with the random number between 0 and 1 and selecting the sensor node whose energy value is greater than the random number as the cluster head; and a-2-2) when the residual energy amount of the sensor node is equal to or more than 50% of the initial energy amount of the sensor node, selecting the cluster head through Low Energy Adaptive Clustering Hierarchy (LEACH). 
     The energy value may be calculated based on an equation expressed as: 
     
       
         
           
             
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     Herein, Pi(n) denotes an energy value; k denotes a constant indicating a probability to be a cluster head (which is a composition ratio of a cluster head); Er denotes a residual energy amount of a sensor node; and Ei denotes an initial energy amount of a sensor node. 
     The step b) may include the steps of: b-1) transmitting the advertisement message including information on residual energy of the cluster head from the selected cluster head to the sensor nodes; b-2) calculating a cost value in the sensor nodes based on a signal strength of the advertisement message and the residual energy of the cluster head; and b-3) transmitting a joint-request message from the sensor nodes to the cluster head having a cost value greatest among calculated cost values to thereby form a cluster. 
     A reference condition may be set up in the sensor nodes, and when the sensing data acquired in the sensor nodes satisfy the reference condition, the step c) includes a step of transmitting the sensing data to the cluster head. 
     In another embodiment, a method for operating a wireless sensor network by selecting a cluster head among a plurality of sensor nodes and transmitting an advertisement message from the selected cluster head to the sensor nodes to form a cluster, includes the steps of: transmitting an advertisement message including information on residual energy of the cluster head from the selected cluster head to the sensor nodes; calculating a cost value in the sensor nodes based on a signal strength of the advertisement message and the residual energy of the cluster head; and transmitting a joint-request message from the sensor nodes to the cluster head having a cost value greatest among calculated cost values to thereby form a cluster. 
     In another embodiment, a method for operating a wireless sensor network by selecting a cluster head among a plurality of sensor nodes includes transmitting an advertisement message from the selected cluster head to the sensor nodes to form a cluster, and transmitting sensing data acquired in the sensor nodes to the cluster head, the method which includes the steps of: setting up a reference condition in the sensor nodes, and when the sensing data satisfy the reference condition, transmitting the sensing data to the cluster head. 
    
    
     
       DRAWINGS 
         FIG. 1  illustrates a wireless sensor network. 
         FIG. 2  illustrates the consumption amount of energy generated during transmission/reception of sensing results between nodes. 
         FIG. 3  is a flowchart describing a wireless sensor network operating method in consideration of energy efficiency in accordance with an embodiment. 
         FIG. 4  is a flowchart describing a step of forming a cluster in the wireless sensor network operating method considering energy efficiency in accordance with the embodiment. 
         FIG. 5  is a flowchart describing a step of transmitting a sensing result from a sensor node to a cluster head in the wireless sensor network operating method considering energy efficiency in accordance with the embodiment. 
         FIG. 6  illustrates an operation process of  FIG. 5 . 
         FIGS. 7 and 8  are graphs showing simulation results of performing typical LEACH, typical LEACH-C, and a wireless sensor network operating method. 
     
    
    
     DESCRIPTION 
     Hereinafter, embodiments will be described in detail with reference to the accompanying drawings. 
     Referring to  FIG. 3 , a wireless sensor network operating method considering energy efficiency, which is suggested in the present invention, includes a cluster head selection stage S 10  to S 19 , a cluster forming stage (S 20 ), and a transmission stage (S 30 ). 
     In the cluster head selection stage (steps S 10  to S 19 ), a cluster head  110  is selected based on the residual energy amount of sensor nodes  100 . That is, the more residual energy a sensor node  100  has, the more highly likely the sensor node  100  is to be selected as the cluster head  110 . 
     Hereinafter, the cluster head selection stage (steps S 10  to S 19 ) will be described with reference to  FIG. 3 . 
     Referring to  FIG. 3 , sensor nodes  100  calculate their residual energy amount compared to their initial energy amount at step S 10 . At step S 14 , each sensor node  100  calculates an energy value based on the current residual energy amount with respect to its initial energy amount. 
     
       
         
           
             
               
                 
                   
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                   4 
                 
               
             
           
         
       
     
     where Pi(n) denotes an energy value; k denotes a constant indicating a probability to be a cluster head (which is a composition ratio of a cluster head); Er denotes a residual energy amount of a sensor node; and Ei denotes an initial energy amount of a sensor node. 
     The Equation 4 calculates an energy value in each sensor node  100 . As shown in Equation 4, the energy value of the sensor node  100  is decided based on the residual energy amount (Er) of the sensor node in the present embodiment, which is different from a threshold value (see Equation 3) used to select the cluster head  110  in the conventional LEACH technology. 
     At step S 15 , a random number between 0 to 1 is extracted. At step S 16 , the calculated energy value is compared with the random number between 0 to 1. When the energy value of the sensor node  100  is greater than the random number at step S 17 , the sensor node  100  is selected as the cluster head  110  at step S 18   a.    
     When the energy value of the sensor node  100  is smaller than the random number at the step S 17 , the sensor node  100  is excluded from a candidate group for the cluster head  110  at step S 18   b.    
     Meanwhile, each sensor node  100  analyzes its current residual energy amount in comparison with its initial energy amount and see if the current residual energy amount exceeds 50% of the initial energy amount at step S 11 . When the current residual energy amount is less than 50% of the initial energy amount at step S 11 , the sensor node  100  calculates it energy value based on the Equation 4, and it may be selected as the cluster head  110  through the process of the steps S 15  to S 17 . 
     Also, when the current residual energy amount is more than 50% of the initial energy amount at the step S 11 , the sensor node  100  may be selected as the cluster head  110  through a typical LEACH method at steps S 12  and S 13 . 
     Also, a cluster  200  is formed at step S 20  only when the number of cluster heads  110  selected through the steps S 10  to S 18  is equal to or more than 5% of the total number of sensor nodes  100 . When the number of cluster heads  110  selected through the steps S 10  to S 18  is less than 5% of the total number of sensor nodes  100 , the steps S 11  to S 18  are repeated. This is because when the number of the cluster heads  110  is less than 5% of the total number of sensor nodes  100 , the energy efficiency of the wireless sensor network  1  is low. 
     Thus, in the cluster head selection stage (steps S 10  to S 19 ) of the present embodiment, when the residual energy amount of each sensor node  100  is as low as less than 50%, the lifespan of the wireless sensor network  1  can be extended by decreasing a probability that a sensor node  100  with a low residual energy amount is selected as a cluster head  110  based on a cluster head selection equation of the present embodiment, which is the Equation 4. 
     Also, although the present embodiment is described by taking an example where either the cluster head selection equation of the present embodiment (which is Equation 4) or a conventional LEACH is used according to the residual energy amount of each sensor node  100 , it is possible to select a cluster head  110  only by using the cluster head selection equation of the present embodiment (which is Equation 4). 
     When the cluster heads  110  are selected, a cluster  200  is formed in the cluster formation stage (step S 20 ). 
     In the cluster formation stage (step S 20 ), the cluster heads  110  selected through the cluster head selection stage (S 10  to S 19 ) transmit advertisement messages to the sensor nodes  100  to thereby form a cluster  200 . 
     The cluster formation stage is described in detail in  FIG. 4  more specifically than in  FIG. 3 . 
     Referring to  FIG. 4 , when cluster heads  110  are selected, they transmit advertisement messages to the sensor nodes  100  at step S 21 . The advertisement messages transmitted from the cluster heads  110  include information on residual energy amount of the cluster heads  110 .
 
Cost( i )= CH ( i )remainenergy+ CH ( i )signal energy  Eq. 5
 
     where Cost(i) denotes a cost value; CH(i) denotes the residual energy amount of a cluster head; and CH(i) signal energy denotes a signal strength of an advertisement message of a cluster head measured in a sensor node. 
     As shown in the Equation 5, each sensor node  100  that has received an advertisement message calculates a cost value at step S 23  by summing the residual energy amount of a cluster head  110  with the signal strength of the advertisement message. 
     Herein, the sensor node  100  can estimate the strength of signals transmitted/received between cluster heads  110  based on the signal strength of the advertisement message transmitted from the cluster head  110 . This signifies that as the range is short, the signal strength increases. As the signal strength of a selected cluster head  110  is higher, the energy loss amount is reduced when signals are transmitted/received between the sensor node  100  and the cluster heads  110 . 
     Also, the residual energy amount may differ among cluster heads  110 . As described above, when the cluster head  110  is selected and sensing data are transmitted/received between the sensor node  100  and the sink node  400 , the amount of energy consumed in the transmission of the sensing data is affected by the amount of the sensing data, that is, the amount of data packets. In short, the smaller the number of the sensor nodes  100  is in a cluster  200 , the smaller the amount of data packets becomes. Thus, a small number of the sensor nodes  100  in the cluster  200  signifies a reduced amount of energy consumed in the cluster head  110 . 
     Therefore, when a cluster head  110  having much residual energy amount than the other cluster heads  110  has more sensor nodes  100  in its cluster  200  than the other cluster heads  110 , it is possible to reduce a difference between residual energy amounts of the cluster heads  110 . This brings about an increased lifespan of the wireless sensor network. 
     Therefore, the sensor nodes  100  calculate the residual energy amounts of the respective cluster heads  110  and signal strengths of the advertisement messages of the cluster heads  110  that are measured in the sensor nodes  100  at step S 22 , and calculate cost values at step S 23 . At step S 24 , the acquired cost values are compared with each other, and the sensor nodes  100  transmits join-request messages to a cluster head  110  having a greatest cost value at step S 25 . 
     In response, the cluster head  110  collects the sensor nodes  100  that have transmitted the join-request messages to thereby form a cluster  200 . 
     Herein, the formation of the cluster  200  according to the present embodiment can reduce the total amount of energy consumed in the wireless sensor network  1  by decreasing the amount of energy consumed according to the range between the sensor nodes  100  and the cluster head  110 . 
     Also, the formation of the cluster  200  according to the present embodiment can reduce the total amount of energy consumed in the wireless sensor network  1  by allocating more sensor nodes  100  to a cluster head  110  having a greater residual energy amount than to the other cluster heads  110  having a smaller residual energy amount in their cluster  200 . 
     Herein, it is apparent to those skilled in the art to which the present embodiment pertains that the total amount of energy consumed in the wireless sensor network  1  can be reduced only by executing the cluster formation stage (steps S 21  to S 26 ) without the above-described cluster head selection stage (S 10  to S 19 ) and thereby the energy efficiency of the wireless sensor network  1  can be improved. 
     When the cluster  200  is formed through the cluster formation step S 20 , the sensor nodes  100  enters a transmission stage, which is a step S 30 ) to transmit the sensing data to the cluster head  110 . 
       FIG. 5  illustrates the transmission stage (S 30 ) of FIG.  3  in detail. Referring to  FIG. 50 , when a cluster  200  is formed, the cluster head  110  allocates an available transmission time to each sensor node  100  in the cluster  200  at step S 31 . As shown in  FIG. 6 , transmission of the sensing data from the sensor nodes  100  to the cluster head  110  is carried out through Time Division Multiple Access (TDMA). This is to reduce the energy consumption amount of each sensor node  100  by turning it in an active mode at the possible transmission time allocated to each sensor node  100  and having it transmit the sensing data to the cluster head  110  only at the allocated transmission time. 
     Therefore, as illustrated in  FIG. 6 , a steady-state phase where the sensing data are divided into a plurality of frames and transmitted from each sensor node  100  to the cluster head  110  comes in after a set-up phase where the cluster head  110  and the cluster  200  are set up in one round. Herein, each sensor node  100  of one frame can transmit the sensing data to the cluster head  110  only at the allocated transmission time. 
     The transmission stage (S 30 ) of the present embodiment includes a reference condition setup step S 32  where reference conditions are set up to decide whether to transmit sensing data of the sensor node  100 . When the reference conditions are set up at the reference condition setup step S 32 , the sensor node  100  operates to output the sensing data at step S 33 . At step S 34 , the sensor node  100  decides whether the sending data satisfy the reference conditions. 
     The reference conditions may be changed according to a field where the wireless sensor network  1  is used. For example, when it is assumed that sensor nodes  100  are disposed in rooms to measure the temperature of each room and the reference conditions are set up to “transmit sensing data only when the room temperature is higher than 30□,” the sensor nodes  100  transmit sensing data to the cluster head  110  only when the sensing data are equal to or higher than 30□. 
     Therefore, the sensor nodes  100  with reference conditions set up therein do not transmit sensing data to the cluster head  110  if the reference data are not fulfilled, although the allocated transmission time comes. Accordingly, the amount of energy consumption of each sensor node  100  can be reduced by decreasing the frequency of transmitting the sensing data to the cluster head  110 . 
     Therefore, at step S 35 , the sensor node  100  transmits the sensing data to the cluster head  110  only when the sensing data satisfy the reference condition and the allocated transmission time comes. At step S 36 , the cluster head  110  receives the sensing data from the sensor node  100  and transmits them to the sink node  400 . 
     Herein, it is apparent to those skilled in the art to which the present embodiment pertains that the total amount of energy consumed in the wireless sensor network  1  can be reduced only by executing the transmission stage (steps S 31  to S 36 ) without the cluster head selection stage (S 10  to S 19 ) and the cluster formation stage (steps S 21  to S 26 ) which are described above and thereby the energy efficiency of the wireless sensor network  1  can be improved. 
     Hereinafter, simulation results of the wireless sensor network operating method proposed in the present embodiment will be compared with the typical LEACH and LEACH-C by referring to  FIGS. 7 and 8 , hereinafter. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
             
            
               
                   
                 Size of Network 
                 100 m × 100 m 
               
               
                   
                 Size of Data Packets 
                 500 bytes 
               
               
                   
                 E elec  (Radio electronic energy) 
                  50 nJ/bit 
               
               
                   
                 E amp  (Radio amplification energy) 
                 100 pJ/bit/m 2   
               
               
                   
                 E init  (Initial energy of each node) 
                  2 J 
               
               
                   
                 Number of Nodes 
                 100 
               
               
                   
                   
               
            
           
         
       
     
     The above Table 1 describes an environment for simulating the wireless sensor network operating method according to an embodiment. The size of the network for simulating the wireless sensor network operating method of the present embodiment is a square of 100 m×100 m and the size of data packets is 500 bytes. Also, the initial energy of each sensor node  100  is 2 J, and the amplification energy during data transmission is 100 pJ/bit/m 2 . The energy basically used for data transmission/reception is 50 nJ/bit. 
     The lifespan of a wireless network was measured in the simulation to assess the wireless sensor network operating method of the present embodiment, and the result was compared with results obtained through the typical LEACH and LEACH-C. 
       FIG. 7  are graphs showing simulation results of performing the wireless sensor network operating method of the present embodiment, typical LEACH, and typical LEACH-C. It presents graphs showing the lifespan of 100 sensor nodes  100  after performing the simulation. 
     Herein, X axis denotes time marked as rounds, whereas Y axis denotes the number of surviving sensor nodes  100  among the  100  sensor nodes  100 . 
     The result showed that the lifespan of the sensor nodes  100  operated according to the wireless sensor network operating method (see a graph indicated as ⋄) of the present embodiment was longer than the lifespan of the sensor nodes  100  operated according to the LEACH (see a graph indicated as □) and the LEACH-C (see a graph indicated as □). 
     It took a time of about 800 seconds in the network of LEACH until the sensor nodes  100  of 10 J die, and it took a time of 70 rounds in the network of LEACH-C until the first sensor node  100  dies. On the other hand, it took a time of 420 rounds in the network operated in the wireless sensor network operating method of the present embodiment until the first sensor node  100  dies. 
     Also, it took a time of 420 rounds in the network of LEACH until all the sensor nodes  100  die, and it took a time of 430 rounds in the network of LEACH-C until all the sensor nodes  100  die. On the other hand, it took a time of 640 rounds in the network operated in the wireless sensor network operating method of the present embodiment until all the sensor nodes  100  die. 
       FIG. 8  are graphs showing simulation results of a wireless sensor network operating method of the present embodiment, typical LEACH and typical LEACH-C while varying the initial energy amount of the sensor nodes  100 . 
     Herein, X axis denotes variation of initial energy amount of the sensor nodes  100 , whereas Y axis denotes time (sec) until the sensor nodes  100  die. 
     The result showed that the lifespan of a wireless sensor network operated according to the wireless sensor network operating method (see a graph indicated as ⋄) of the present embodiment was longer than the lifespan of networks operated according to the LEACH (see a graph indicated as □) and the LEACH-C (see a graph indicated as □). 
     It took about 800 seconds in the network of LEACH until 10 J sensor nodes  100  die, and it took about 1200 seconds in the network of LEACH-C until 10 J sensor nodes  100  die. On the other hand, it took about 3500 seconds in the network operated in the wireless sensor network operating method of the present embodiment until 10 J sensor nodes  100  die. 
     Also, it took about 2500 seconds in the network of LEACH until 25 J sensor nodes  100  die, and it took about 3000 seconds in the network of LEACH-C until 25 J sensor nodes  100  die. On the other hand, it took about 8500 seconds in the network operated in the wireless sensor network operating method of the present embodiment until 25 J sensor nodes  100  die. 
     As shown in the simulation results, the wireless sensor network operating method of the present embodiment can reduce the amount of energy consumed in each sensor node  100  more than the conventional LEACH and LEACH-C technologies. 
     In consequences, the wireless sensor network operating method of the present embodiment can increase the lifespan of a wireless sensor network by more than 37%, compared to the conventional LEACH technology, and increase the lifespan of a wireless sensor network by more than 30%, compared to the conventional LEACH-C technology. Therefore, the wireless sensor network operating method of the present embodiment is excellent in the respect of energy efficiency. 
     The wireless sensor network operating method considering energy efficiency, which is proposed in the present embodiment, reduces a probability that a sensor node  100  having a relatively small amount of residual energy than the other sensor nodes  100  to thereby increase the entire lifespan of a wireless sensor network. 
     Also, the wireless sensor network operating method of the present embodiment can reduce difference in energy amounts among cluster heads by increasing the number of sensor nodes in a cluster of a cluster head having much residual energy to thereby increase the entire lifespan of a wireless sensor network. 
     In addition, the wireless sensor network operating method of the present embodiment can reduce the amount of energy consumed for wireless data transmission/reception by transmitting sensing data appropriate for a reference condition to a cluster head to thereby increase the entire lifespan of a wireless sensor network. 
     It will be apparent to those skilled in the art that various modifications and variations can be made to embodiments without departing from the spirit or scope of the disclosed embodiments. Thus, it is intended that the present invention covers modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.