Patent Publication Number: US-2011069611-A1

Title: Wireless sensor network

Description:
TECHNICAL FIELD 
     The present invention relates to a wireless sensor network, and more particularly, to a wireless sensor network having a distributed TDMA based linear structure and being capable of minimizing a problem in that sensed data sensed by each sensor node are lost during transmission due to a sensor node, of which a capacity of a buffer is limited or in which an error occurs. 
     BACKGROUND ART 
     In general, a wireless sensor network (WSN) may be configured to include sensor nodes, sensor field constructed with groups of sensor nodes, sink nodes which receives information collected by the sensor field, and a gateway which routes the information transmitted from the sink nodes and transmits the information through a broadband communication network to a management control server. 
     As one of the currently used wireless sensor networks, there is a wireless sensor network having a distributed TDMA based linear structure, which can implement real-time monitored data transmission, low power consumption, and minimization of control packet overhead by using a low duty-cycle MAC algorithm and a time synchronized forwarding mechanism. However, in such a wireless sensor network having the linear structure, in the case where a capacity of a buffer memory built in a sensor node constituting the network is smaller than an amount of data transmitted from the lower level sensor nodes or an amount of sensed data sensed by the sensor node, the portion of the sensing data exceeding the capacity of the buffer memory of the sensor node cannot be transmitted up to the highest level sensor node, that is, the sink node but it may be lost during transmission thereof. 
     On the other hand, as illustrated in (a) of  FIG. 6 , conventionally in the case where an error occurs in some nodes N 7  and N 8  of the wireless sensor network, in order to prepare against a failure of communication in a portion of the sensor nodes constituting the wireless sensor network, each sensor nodes perform a recovery algorithm of recovering the failure of communication. However, as illustrated in (b) of  FIG. 6 , in the case where the failure of the communication is not recover by such a recovery algorithm, there is a problem in that data of the sensor nodes N 1  to N 6  located at the lower levels of the sensor nodes N 7  and N 8 , which are in a communication failed state, cannot be transmitted to a sink node N 15  but it is lost. 
     DISCLOSURE 
     Technical Problem 
     The present invention is to provide a wireless sensor network capable of minimizing a problem in that sensed data exceeding a capacity of a buffer of each sensor node are lost during transmission thereof. 
     The present invention is also to provide a wireless sensor network capable of minimizing a problem in that sensed data of each of the sensor nodes located at lower levels of some sensor nodes, where errors occur, among the sensor nodes constituting the wireless sensor network are lost due to the sensor nodes where the errors occur. 
     Technical Solution 
     According to a first aspect of the present invention, there is provided a wireless sensor network including: a plurality of sensor nodes connected linearly; a sink node connected to the uppermost node among the sensor nodes; a control server configured to transmit and receive data with respect to the sensor nodes and the sink node and to control operations thereof; a gateway connected between the sink node and the control server to transmit and receive data; a plurality of relay gateways connected to the control server, wherein the plurality of the sensor nodes are divided into a predetermined number of subgroups, the sensor nodes located at the highest levels in the divided subgroups are designated as relay nodes, and each of the relay nodes is connected to each of the relay gateways. 
     In the wireless sensor network according to the aforementioned first aspect, it is preferable that in the case where an amount of data stored in the buffer of the relay node exceeds a predetermined threshold value, the data stored through the connected relay gateway are transmitted to the control server. In particular, it is more preferable that the relay node determines whether or not the amount of data stored in the buffer reaches the threshold value, wherein in the case where the amount of the data is determined to reach the threshold value, the relay node determines whether or not a portion of the data stored in the buffer is the data that are to be transmitted to the sink node, and wherein in the case where the portion of the data is determined to be the data that are to be transmitted to the sink node, the relay node transmits the data toward the sink node. 
     According to a second aspect of the present invention, there is provided a wireless sensor network including: a plurality of sensor nodes connected linearly; a sink node connected to the uppermost node among the sensor nodes; a control server; a gateway connected between the sink node and the control server to transmit and receive data; and a plurality of relay gateways connected to the control server, wherein the plurality of the sensor nodes are divided into a predetermined number of subgroups, the sensor nodes located at the highest levels in the divided subgroups are designated as relay nodes, and each of the relay nodes is connected to each of the relay gateways, and wherein in the case where a portion of the sensor nodes is error nodes in a communication-failed state, the relay node of lower subgroups located at lower levels of the subgroups in which the error nodes are included are converted to relay sink nodes, and the relay sink nodes transmit and receive data with respect to the control server through the connected relay gateways. 
     In the wireless sensor network according to the aforementioned second aspect, it is preferable that a communication-failed state of the sensor node occurs in the case where failure of network link occurs in the sensor node and a recovery of the failure of the network link is failed. 
     In the wireless sensor network according to the aforementioned second aspect, it is preferable that relay sink node searches for a sensor node located at a higher level, and as a result of the searching, in the case where the sensor node located at the higher level is searched, the relay sink node is reverted to the relay node to recover a communication passage with respect to the searched sensor nodes located at the higher level and to transmit the data through the recovered communication passage. 
     ADVANTAGEOUS EFFECTS 
     As described above, in a wireless sensor network according to the present invention, by using relay nodes and relay gateways, it is possible to minimize a problem in that sensed data exceeding a capacity of a buffer of each sensor node are lost during transmission thereof and to minimize a problem in that sensed data of each of the sensor nodes located at lower levels of some sensor nodes, where errors occur, among the sensor nodes constituting the wireless sensor network are lost due to the sensor nodes where the errors occur. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram illustrating a configuration of a wireless sensor network according to a first embodiment of the present invention. 
         FIG. 2  is a diagram explaining for data stored in buffers of a portion of sensor nodes according to the first embodiment of the present invention. 
         FIG. 3  is a flowchart for explaining operations of the wireless sensor network according to the first embodiment of the present invention. 
         FIG. 4  is a diagram illustrating a configuration of a wireless sensor network according to a second embodiment of the present invention. 
         FIG. 5  is a flowchart for explaining operations of the wireless sensor network according to the second embodiment of the present invention. 
         FIG. 6  is a diagram illustrating a configuration of a conventional wireless sensor network. 
     
    
    
     BEST MODE 
     Hereinafter, configurations and operations of wireless sensor networks according to embodiments of the present invention will be described with reference to the attached drawings. 
     As illustrated in  FIG. 1 , a wireless sensor network  1  according to the first embodiment of the present invention has a linear structure where data sensed by sensor nodes  110  constituting the network are transmitted toward a sink node N 15  located at the highest level. The wireless sensor network  1  is configured according to a distributed TDMA based MAC protocol. As illustrated in  FIG. 1 , the wireless sensor network  1  according to the first embodiment of the present invention is configured to include a plurality of sensor nodes  110 , a control server  120 , a gateway  130 , relay nodes N 5  and N 10 , and first and second relay gateways  140  and  150 . Herein, the relay nodes are designated as the sensor nodes located at the highest levels of subgroups of the sensor nodes, and each of the relay nodes is connected to each of the relay gateways. 
     Now, operations of the wireless sensor network  1  according to the first embodiment of the present invention are described in brief. In the case where the data sensed by the sensor nodes  110  constituting the wireless sensor network  1  are increased so that the amount of the data stored in the relay node N 5  (N 10 ) exceeds a predetermined threshold value (A in  FIG. 2 ), the relay node N 5  (N 10 ) is changed into the relay sink node N 5  (N 10 ) capable of functioning as a sink node and connected to the relay gateway  140  ( 150 ). The relay gateway  140  ( 150 ) transmits the data stored in the relay sink node N 5  (N 10 ) to the control server  120 . In addition, the relay gateway  140  ( 150 ) transmits command data, which are transmitted from the control server  120  to the relay sink node N 5  (N 10 ). 
     Hereinafter, the components of the wireless sensor network  1  according to the first embodiment of the present invention are described in detail. 
       FIG. 2  illustrates data stored in the buffers of the first node N 1  to the sixth node N 6 . 
     Each of the sensor nodes  110  generates data by a sensing module (not shown) of sensing given external environments, stores the data in the buffer thereof, and transmits the stored data towards higher sensor nodes. 
     The control server  120  manages the data transmitted through the sensor nodes  110  and the gateway  130 , and controls the overall network. 
     The gateway  130  connects the sink node N 15  and the control server  120 . The gateway  130  transmits the data transmitted from the sink node N 15  to the control server  120 . In addition, the gateway  130  transmits the command data transmitted from the control server  120  to the sink node N 15 . 
     In the case where a plurality of the sensor nodes  110  are divided into a predetermined number of subgroups  111 ,  112 , and  113 , the relay nodes N 5  and N 10  are designated as the sensor node N 5  and N 10  located at the highest levels of the subgroups  111  and  112 . The relay nodes N 5  and N 10  of the subgroups are connected to the relay gateways  140  and  150 , respectively. As illustrated in  FIG. 1 , the wireless sensor network  1  according to the first embodiment of the present invention is configured to include the first subgroup  111  including the first node N 1  to the fifth node N 5 , the second subgroup  112  including the sixth node N 6  to the tenth node N 10 , and the third subgroup  113  including the eleventh node N 11  to the fifteenth node N 15 . Herein, the fifth node N 5  and the tenth node N 10  correspond to the relay nodes N 5  and N 10 , and the fifteenth node N 15  corresponds to the sink node N 15 . In the case where the data stored in the buffer of the relay node N 5  (N 10 ) exceeds the predetermined threshold value A, the relay node N 5  (N 10 ) is changed into the relay sink node N 5  (N 10 ) functioning as a sink node. 
     Referring to  FIG. 2 , the reference numeral N 5  corresponds to the relay node, and a non-hatched portion B denotes the data transmitted from the lower sensor nodes N 1  to N 4 , and a hatched portion C denotes the data sensed in the relay node N 5 . Therefore, as illustrated in  FIG. 2 , in the case where the amount of the data B (C) stored in the relay node N 5  exceeds a threshold value A, the relay node N 5  transmits the stored data B (C) to a relay gateway  140 . Herein, in  FIG. 2 , although the threshold value A is about ⅓ of the capacity of the butter, it is preferable that the threshold value A is about ½ of the capacity of the buffer. 
     The relay gateway  140  ( 150 ) connects the relay node N 5  (N 10 ) and the control server  120  to transmit the data of the relay node N 5  (N 10 ) to the control server  120 . After the transmission of the data is completed, the relay sink node N 5  (N 10 ) is reverted to the relay node N 5  (N 10 ) which performs only the function as the sensor node  110 , so that the network is normalized to the original network. 
     The operations of the wireless sensor network  1  according to the first embodiment of the present invention are described with reference to  FIG. 3 . 
     First, each of the relay nodes N 5  and N 10  checks an amount of data stored in a buffer thereof, that is, an amount of data which are transmitted from lower nodes or which are detected by the relay node (S 310 ). In other words, as illustrated in  FIG. 2 , the relay node N 5  checks the amount of the data B and C stored in the buffer thereof. 
     Next, as a result of the checking of Step S 310 , it is determined whether or not the amount of the data stored in the buffer is large than a threshold value A (S 320 ). As illustrated in  FIG. 2 , it is determined whether or not the data B and C of the relay node N 5  are larger than the threshold value A. 
     If the amount of the data stored in the buffer of the relay node N 5  (N 10 ) is smaller than the threshold value A in Step S 320 , the stored data are transmitted toward the sink node N 15  (S 330 ). If the amount of the data stored in the buffer of the relay node N 5  (N 10 ) is larger than the threshold value A in Step S 320 , the relay node N 5  (N 10 ) determines whether or not there are data that are to be transmitted to the sink node N 15  among the data stored in the buffer thereof (S 340 ). 
     As a result of the determination of Step S 340 , if there are data that are to be transmitted to the sink node N 15  among the data, Step S 330  is performed. 
     Next, as a result of the determination of Step S 340 , if there are no data that are to be transmitted to the sink node N 15  among the data, the data (B and C in  FIG. 2 ) stored in the buffer of the relay node N 5  are transmitted to the relay gateway  140  ( 150 ). 
     Finally, the relay gateway  140  ( 150 ) connects the relay node N 5  (N 10 ) with the control server  120  so as to transmit the data of the relay sink node N 5  (N 10 ) to the control server  120  (S 360 ). In  FIG. 2 , the first relay gateway  140  transmits the data B and C of the relay sink node N 5  to the control server  120 . 
     As described above, in the wireless sensor network  1  according to the first embodiment of the present invention, by using the relay nodes N 5  and N 10  and the relay gateways  140  and  150 , it is possible to minimize the problem in that the sensing data are lost during the transmission when the data of the sensor nodes are increased. 
     MODE FOR INVENTION 
     Hereinafter, configurations and operations of a wireless sensor network according to a second embodiment of the present invention are described with reference to the attached drawings. 
     As illustrated in  FIG. 4 , a wireless sensor network  2  according to the second embodiment of the present invention has the same linear structure as that of the aforementioned wireless sensor network  1  according to the first embodiment. The wireless sensor network  2  is also configured according to a distributed TDMA based MAC protocol. 
     Now, operations of the wireless sensor network  2  according to the second embodiment of the present invention are described in brief. In the case where an error occurs in a portion of the sensor nodes  410  constituting the wireless sensor network  2 , a relay node N 5  among the sensor nodes N 1  to N 6  located at a lower level of error nodes N 7  and N 8 , where the error occurs, is changed into a relay sink node N 5  to be connected to a first relay gateway  440 . The first relay gateway  440  transmits data of the relay sink node N 5  to the control server  420  and command data transmitted from the control server  420  to the relay sink node N 5 , so that a network of a first subgroup  411  can be configured by the relay sink node N 5 . 
     As illustrated in  FIG. 4 , the wireless sensor network  2  according to the second embodiment of the present invention includes a plurality of the sensor nodes  410 , the control server  420 , the gateway  430 , and relay nodes N 5  and N 10 , and the first and second relay gateways  440  and  450 . Hereinafter, the components of the wireless sensor network  2  according to the second embodiment of the present invention are described in detail. However, the description of the same components as those of the aforementioned first embodiment is omitted. 
     In the case where a plurality of the sensor nodes  410  are divided into a predetermined number of subgroups  411 ,  412 , and  413 , the relay nodes N 5  and N 10  are designated as the sensor nodes located at the highest levels of the subgroups  411  and  412 . As illustrated in  FIG. 4 , the wireless sensor network  2  according to the invention may be divided into the first subgroup  411  including the first node N 1  to the fifth node N 5 , the second subgroup  412  including the sixth node N 6  to the tenth node N 10 , and the third subgroup  413  including the eleventh node N 11  to the fifteenth node N 15 . With respect to the relay nodes N 5  and N 10 , the fifth node N 5  and the tenth node N 10  correspond to the relay nodes N 5  and N 10 , and the fifteenth node N 15  corresponds to the sink node N 15 . 
     In the case where a portion of the sensor nodes in each subgroups are the error nodes which are in a communication failed state, in order to prevent the data of the sensor nodes N 1  to N 6  located at the lower levels of the error nodes N 7  and N 8  from not being transmitted to the sink node N 15  but being lost due to the error nodes N 7  and N 8 , relay nodes of the subgroups located at the lower levels of the subgroup, in which the error nodes are included, are changed into relay sink nodes. In other words, in this case, the relay node N 5  of the first subgroup  411  located at the lower level of the second subgroup  412 , in which the error nodes N 7  and N 8  are included, is changed into the relay sink node N 5  which functions as a sink node. 
     Referring to  FIG. 4 , the seventh node N 7  and the eighth node N 8  correspond to the error nodes N 7  and N 8 , and the fifth node N 5  corresponds to the relay sink node N 5 . In the case where the relay node N 5  is changed into the relay sink node, the relay gateway  440  connects the relay sink node N 5  and the control server  420  to transmit the data of the relay sink node N 5  to the control server  420  and to transmit the command data transmitted from the control server  420  to the relay sink node N 5 . 
     The operations of the wireless sensor network  2  according to the second embodiment of the present invention are described with reference to  FIG. 5 . 
     First, in the case where error nodes N 7  and N 8  occur among the sensor nodes  410  constituting the wireless sensor network  2 , sensor nodes where failure of network link occurs due to the error nodes N 7  and N 8  performs a predetermined network link recovery algorithm (S 510 ). Referring to  FIG. 4 , in the case where the seventh node N 7  and the eighth node N 8  are the error nodes N 7  and N 8 , the sixth node N 6  and the ninth node N 9  performs the network link recovery algorithm. 
     Next, it is determine whether or not the network link recovery of Step S 510  is completed (S 520 ). As a determination result of Step S 520 , if the network link recovery is completed, the procedure proceeds to Step S 590 . This means that the original wireless sensor network except for the error nodes N 7  and N 8  is recovered. 
     As a determination result of Step S 520 , if the network link recovery is not completed, the relay node N 5  of the first subgroup  411  located at the lower level of the second subgroup  412 , in which the error nodes N 7  and N 8  are included, is changed into the relay sink node N 5  which functions as a sink node (S 530 ). 
     Next, the first relay gateway  440  connects the changed relay sink node N 5  and the control server  420  to transmit the data of the relay sink node N 5  to the control server  420  and to transmit the command data transmitted from the control server  420  to the relay sink node N 5 , so that the network of the first subgroup  411  is configured (S 540 ). 
     Next, in order to recover the network from the communication failed state, the relay sink node N 5  determines whether or not to perform a process of searching for the sensor nodes located at the higher levels (S 550 ). 
     As a determination result of Step S 550 , if the process of searching for the sensor nodes located at the higher levels is not performed, the procedure proceeds to Step S 550 . This means that the network of the first subgroup  411  configured in Step S 540  is maintained. 
     As a determination result of Step S 550 , if the process of searching for the sensor nodes located at the higher levels is performed, and if a higher level sensor node N 9  is searched within a predetermined distance, the relay sink node N 5  changed in Step S 530  is reverted to the relay node N 5  (S 560 ). 
     Next, the relay node N 5  reverted in Step S 560  performs the same network link recovery algorithm as that of Step S 510  (S 570 ). 
     Next, it is determined whether or not the network link recovery performed in Step S 570  is completed (S 580 ). 
     As a result of the determination of Step S 580 , if the network link recovery is completed, the wireless sensor network  2  where the relay node N 5  and the upper node N 9  searched in Step S 550  are connected to each other is recovered. Therefore, the network of the first subgroup  411  configured in Step S 540  is decomposed. 
     As a result of the determination of Step S 580 , if the network link recovery is not completed, the procedure returns to Step S 550 . This denotes that the network of the first subgroup  411  configured in Step S 40  is maintained. 
     In this manner, in the wireless sensor network  2  according to the second embodiment of the present invention, the network of the first subgroup  411  is configured by using the relay node N 5  and the first relay gateway  440 , it is possible to minimize the problem that the error node N 7  (N 8 ) causes the loss of the sensing data of the sensor nodes N 1  to N 6  located in the lower level of thereof. 
     INDUSTRIAL APPLICABILITY 
     A wireless sensor network according to the present invention which can transmit sensed data efficiently and safely may be widely used in the field of the wireless sensor network.