WO2011013879A1

WO2011013879A1 - Method and system for network configuration at wireless body area network

Info

Publication number: WO2011013879A1
Application number: PCT/KR2009/006698
Authority: WO
Inventors: Su-Ho Seo; Jong-Tae Park
Original assignee: Kyungpook National University Industry-Academic Cooperation Foundation
Priority date: 2009-07-31
Filing date: 2009-11-13
Publication date: 2011-02-03
Also published as: KR101043762B1; KR20110012885A

Abstract

An energy-efficient multi-hop network configuration method in a tree type wireless body area network (WBAN) is provided. According to the multi-hop network configuration method, a parent node is selected so that the whole power consumption becomes minimum as packet transmission delay time limit conditions are satisfied when a disconnected node of which the connection with another node has been interrupted selects its parent node. Specifically, the disconnected node broadcasts a paging message to inform candidate parent nodes of a connection request, and selects one of the candidate parent nodes which have responded to the paging message as its parent node. In this case, the parent node is a node at which the standard deviation of remaining power values of the respective nodes becomes minimum. The multi-hop network configuration method can be applied to the WBAN management for the recent ubiquitous health care services.

Description

METHOD AND SYSTEM FOR NETWORK CONFIGURATION AT WIRELESS BODY AREA NETWORK

The present invention relates to a multi-hop network configuration method and a multi-hop network system. More particularly, the present invention relates to a method and a system for network configuration at a wireless body area network, in which a node constituting a wireless body area network (WBAN) selects a node, which can make power consumption between nodes constant and can guarantee a quality of service (QoS), among reachable candidate parent nodes as a new parent node when a connection between the node and a node connected by motion of a human body is interrupted.

Recently, researches in a wireless body area network for a ubiquitous health care service have been actively made around IEEE and so on. The wireless body area network is mainly constructed in the form of a tree that is composed of parent nodes and child nodes based on multi hop communications in consideration of the power limit and biometric characteristics. In this case, unlike the existing sensor network, there is a high possibility that the connection between nodes connected to a network is interrupted due to frequent motion of a body, and the number of node connections that can be processed by respective nodes is subject to restriction due to the limit of power consumption of the respective nodes.

With the abrupt aging of society all over the world, the number of chronic invalids as well as the number of aged populations has increased to cause an abrupt increase of medical expenses. In order to reduce such medical expenses and to enjoy health management and medical services that can be used any time at any place, researches for U (Ubiquitous) - health care services have been actively made. The U-health care service has a basic function of acquiring and remotely monitoring biometric information of a body, and one scheme for acquiring biometric information is a wireless body area network (WBAN).

The WBAN is defined as wireless communications inside a human body and within three meters from the human body. That is, biometric information detected through biometric sensors is transmitted to a user’s gateway (e.g. PDA, notebook computer, PC, and the like) by using the WBAN, and thus the user’s health state can be remotely monitored. However, since the sensor node has a small-scale data accommodation capability and battery based power consumption structure, there is a need for a method capable of guaranteeing the QoS with minimum power consumption.

Although many researches have been made in power consumption in a sensor network, the WBAN environment is different from a traditional sensor network. Since a sensor at the WBAN, unlike the traditional sensor network, can be positioned within the body, the biological attribute inside the body and the height of an antenna may affect a packet loss. Also, direct single-hop communications within the body, in which the biological attribute should be considered, causes high propagation cost to be consumed to reduce the life span of the battery. Accordingly, there is an urgent need for researches in the power consumption in the multi-hop communication based WBAN.

In particular, when many child nodes request registration from a new parent node, it is impossible to process the connection request of the child nodes due to the limited power and memory of the sensor node. In addition, as the motion of the body frequently occurs, it is difficult to promptly re-connect the nodes when the connection between the nodes is interrupted. Also, in setting a new connected node when the connection with another node is interrupted, it is required there is no great difference in power consumption between the respective nodes.

Accordingly, the present invention has been made to solve the above-mentioned problems occurring in the prior art, and an object of the present invention is to provide a multi-hop network configuration method, which can prevent power consumption between nodes from differing greatly in setting a new connected node when a connection with another node is interrupted.

Another object of the present invention is to provide a multi-hop network configuration method, which can promptly set a connection of an interrupted node when a connection with another node is interrupted.

Still another object of the present invention is to provide a multi-hop network configuration method, which can prevent the number of hops with a root node from exceeding a predetermined number of times and prevent the delay time of an urgent packet from exceeding a predetermined threshold value in setting a new connected node when a connection with another node is interrupted.

Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention.

In order to accomplish these objects, there is provided a multi-hop network configuration method, according to an embodiment of the present invention, which includes a disconnected node, of which the connection with another node is interrupted, broadcasting a paging message; and setting one of candidate parent nodes which have responded to the paging message as a parent node of the disconnected node. In this case, the parent node is a node at which the standard deviation of remaining powers of the every node in the multi-hop network becomes minimum, that is calculated on the assumption that all non-root nodes included in the network, in which the disconnected node is set as a child node of the parent node, have transmitted packets in a direction of a root node.

In another aspect of the present invention, there is provided a multi-hop network configuration method, which includes a disconnected node, of which the connection with another node is interrupted, broadcasting a paging message; and setting one of candidate parent nodes which have responded to the paging message as a parent node of the disconnected node. However, the parent node is set based on the remaining powers of the candidate parent nodes.

In still another aspect of the present invention, there is provided a multi-hop network system, which includes One or more multi-hop nodes, wherein the multi-hop nodes comprising, an interface unit performing wireless communication with another multi-hop node; a disconnection judgment unit creating and broadcasting a paging message through the interface unit if it is judged that a connection with the another multi-hop node has been interrupted; a storage unit storing remaining power data of all multi-hop nodes included in the multi-hop network system; and a parent node setting unit setting one of candidate parent nodes which have responded to the paging message as a parent node, based on the size of the remaining power stored in the storage unit.

According to the present invention as constructed above, in setting a new connected node when a connection with another node is interrupted, power consumption between nodes is prevented from differing greatly, the number of hops with a root node is prevented from exceeding a predetermined number of times, and the delay time of an urgent packet is prevented from exceeding a predetermined threshold value.

Also, in setting a new connected node when a connection with another node is interrupted, a connection of an interrupted node can be promptly set, and thus the node that stores the urgent packet can minimize the delay of the packet and transmit the packet.

The above and other objects, features and advantages of the present invention will be more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a view illustrating a multi-hop network configuration method according to an embodiment of the present invention;

FIG. 2 is a view illustrating a multi-hop network configuration method according to a second embodiment of the present invention;

FIG. 3 is a view illustrating a first state of a multi-hop network configuration method according to an embodiment of the present invention;

FIG. 4 is a view illustrating a second state of a multi-hop network configuration method according to an embodiment of the present invention;

FIG. 5 is a view illustrating a third state of a multi-hop network configuration method according to an embodiment of the present invention;

FIG. 6 is a view illustrating a fourth state of a multi-hop network configuration method according to an embodiment of the present invention; and

FIG. 7 is a view illustrating the configuration of a multi-hop network system according to an embodiment of the present invention.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. The aspects and features of the present invention and methods for achieving the aspects and features will be apparent by referring to the embodiments to be described in detail with reference to the accompanying drawings. However, the present invention is not limited to the embodiments disclosed hereinafter, but can be implemented in diverse forms. The matters defined in the description, such as the detailed construction and elements, are nothing but specific details provided to assist those of ordinary skill in the art in a comprehensive understanding of the invention, and the present invention is only defined within the scope of the appended claims. In the entire description of the present invention, the same drawing reference numerals are used for the same elements across various figures.

The present invention will be described herein with reference to the accompanying drawings illustrating block diagrams and flowcharts for explaining a multi-hop network configuration method according to embodiments of the present invention. It will be understood that each block of the flowchart illustrations, and combinations of blocks in the flowchart illustrations, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a special purpose computer or other programmable data processing apparatus, and in particular, can be executed in sensor nodes constituting a wireless body area network.

The topology of a multi-hop network configuration according to the present invention may include at least a part of a tree type topology.

Hereinafter, with reference to FIG. 1, a multi-hop network configuration method according to an embodiment of the present invention will be described. FIG. 1 is a view illustrating a multi-hop network configuration method according to an embodiment of the present invention.

A disconnected node, of which the connection with another node is interrupted, senses such disconnection S100. The disconnection can be sensed in the case where synchronization data, which is received in a specified period, is not received even after the lapse of the specified period. The reason why the connection is interrupted may be an excess of a communication radius of the nodes for pre-defined time due to the movement of the disconnected node. For example, if a user, who forms a wireless body area network (WBAN) in which a node installed on the tip of the user’s hand is connected with a node installed on the side of the user, moves his/her hand to cause the node installed on the tip of his/her hand to escape from the communication radius with the node installed on the side of the user, the node installed on the tip of the user’s hand senses that the connection with the node installed on the side of the user has been interrupted S100. The disconnected node may be a leaf node.

The disconnected node broadcasts a paging message S102. The paging message may be a response request message of a predefined format. The paging message is received in another node existing within the communication radius of the disconnected node. The node, having received the paging message, transmits a response message.

The disconnected node receives the response message S104. The disconnected node judges that nodes having received the response message are candidate parent nodes.

The disconnected node sets one among the candidate parent nodes that have responded to the paging message as a parent node of the disconnected node. However, the disconnected node should be a node at which the standard deviation of the remaining powers of the respective nodes becomes minimum, that is calculated on the assumption that all non-root nodes included in a network, in which the disconnected node is set as a child node of the parent node, have transmitted packets in a direction of a root node S106. Hereinafter, it should be noted that setting as a parent node includes forming of a communication channel between the parent node and the child node.

It is preferable that the disconnected node stores data of powers of respective nodes to obtain the standard deviation in the case where the disconnected node is assigned as the child node of the respective candidate parent nodes. It is preferable that power data includes data of the currently remaining powers of the respective nodes and data that is consumed when a packet is transmitted/received. Also, the power data may be obtained by a root node, and may be distributed to all nodes in the network.

The standard deviation can be calculated by an equation noticed in Table 1 below.

Table 1

Symbols indicated in Table 1 will now be described. A set of nodes, of which the number is s, among nodes included in the whole network is {N₁, N₂, … , N_s}. Since other nodes, which are not currently connected, may exist in the nodes included in the whole network, it would be understood that the standard deviation is calculated in consideration of the currently connected nodes only.

“P(N_i)” denotes a value of the remaining power calculated after a node i transmits a packet in a direction of a root node. “m” denotes an average of the remaining powers after the whole nodes included in the network transmit packets in a direction of a root node. Accordingly, by obtaining the value, the standard deviation of the remaining power values of the respective nodes, which are calculated on the assumption that all non-root nodes included in the network transmit packets in a direction of a root node, can be calculated.

The disconnected node calculates the standard deviation on the assumption that it becomes a child node of the candidate parent nodes, and sets the candidate parent node having the smallest standard deviation value as the parent node. For example, if three candidate parent nodes exist, i.e., if three nodes have responded to the paging message, the disconnected node calculates the standard deviation value three times, and sets the candidate parent node having the smallest standard deviation value as the parent node.

In the case of setting the parent node, the disconnected node may set the parent node in further consideration of whether the number of hops between the parent node and the root node is smaller than the hop threshold value. That is, since the disconnected node sets the parent node so that the number of hops up to the root node does not exceed the predetermined hop threshold value, the QoS can be guaranteed.

The hop threshold value may be a value set based on an ordered pair of the root node and the parent node. That is, the hop threshold value is not designated in advance as one value, but different values may be designated according to the ordered pair of the root node and the parent node.

The parent node may be set in further consideration of whether the communication delay time, which is calculated on the assumption that the disconnected node has transmitted an urgent packet to the parent node in the network configuration where the disconnected node is set as the child node of the parent node, is below a preset delay time threshold value. The delay time threshold value may be 100 ms proposed by the IEEE 802.15 TG (Task Group) 6. The urgent packet means a packet of which the priority is over a predetermined level. The priority may be included in a packet that the respective node in the network transmits/receives.

Hereinafter, with reference to FIG. 2, a multi-hop network configuration method according to another embodiment of the present invention will be described. FIG. 2 is a view illustrating a multi-hop network configuration method according to a second embodiment of the present invention.

A disconnected node, of which the connection with another node is interrupted, senses such disconnection S200. The disconnection can be sensed in the case where synchronization data, which is received in a specified period, is not received even after the lapse of the specified period. The reason why the connection is interrupted may be an excess of a communication radius of the nodes for pre-defined time due to the movement of the disconnected node. For example, if a user, who forms a wireless body area network (WBAN) in which a node installed on the tip of the user’s hand is connected with a node installed on the side of the user, moves his/her hand to cause the node installed on the tip of his/her hand to escape from the communication radius with the node installed on the side of the user, the node installed on the tip of the user’s hand senses that the connection with the node installed on the side of the user has been interrupted S200. The disconnected node may be a leaf node.

The disconnected node broadcasts a paging message S202. The paging message may be a response request message of a predefined format. The paging message is received in another node existing within the communication radius of the disconnected node. The node, having received the paging message, transmits a response message.

The disconnected node receives the response message S204. The disconnected node judges that nodes having received the response message are candidate parent nodes.

The disconnected node sets one among the candidate parent nodes that have responded to the paging message as a parent node of the disconnected node. However, the parent node is set based on the remaining powers of the candidate parent nodes S206. For example, the disconnected node may set the node having the largest remaining power among the candidate parent nodes as the parent node, or may set the node having the smallest remaining power among the candidate parent nodes as the parent node.

However, the disconnected node is newly assigned as the child node of the parent node, and if the parent node communicates with too many child nodes, the parent node consumes much more power than other nodes, and this causes the battery replacement period to be shortened. According to the multi-hop network configuration method according to an embodiment of the present invention, it is judged whether the parent node exceeds a limited degree S208 after the parent node setting S206. The degree is an integer that means the number of child nodes owned by one node. It is preferable that the limited degree is a predefined value. For example, if the limited degree is designated as 5, the respective node can own 5 child nodes at maximum.

If the parent node exceeds the limited degree as the result of judgment S208, a connection of one child node is canceled. It is preferable to cancel the connection of the child node having the lowest packet priority S210. That is, since the transmission of the current packet can be guaranteed by maintaining the connection of the child node having a high priority, the QoS can be guaranteed.

The node of which the connection is canceled broadcasts a paging message for its reconnection. The connection-canceled node sets one of the candidate parent nodes that have responded to the paging message as the parent node of the connection-canceled node. It is preferable that the parent node is a node at which the standard deviation of remaining powers of the every node in the multi-hop network becomes minimum, that is calculated on the assumption that all non-root nodes included in the network, in which the disconnected node is set as a child node of the parent node, have transmitted packets in a direction of a root node S212.

Since the standard deviation may be calculated by the equation noted in Table 1, or may be calculated in the same method as that described in the previous embodiment of the present invention, the detailed description thereof will be omitted.

In the case of setting the parent node, the connection-canceled node may set the parent node in further consideration of whether the number of hops between the parent node and the root node is smaller than the hop threshold value. That is, since the disconnected node sets the parent node so that the number of hops up to the root node does not exceed the predetermined hop threshold value, the QoS can be guaranteed.

The parent node may be set in further consideration of whether the communication delay time, which is calculated on the assumption that the connection-canceled node has transmitted an urgent packet to the parent node in the network where the connection-canceled node is set as the child node of the parent node, is below a preset delay time threshold value. The delay time threshold value may be 100 ms proposed by the IEEE 802.15 TG (Task Group) 6. The urgent packet means a packet of which the priority is over a predetermined level. The priority may be included in a packet that the respective node in the network transmits/receives.

Hereinafter, with reference to FIGS. 3 to 6, a multi-hop network configuration method according to still another embodiment of the present invention will be described.

Table 2 below indicates the remaining powers and the priorities of the owned packets to be applied to the embodiment of the present invention.

Table 2

In Table 2, “N” denotes a non-urgent packet, and “U” denotes an urgent packet.

First, FIG. 3 illustrates an initial node configuration of a multi-hop network according to an embodiment of the present invention. Hereinafter, it is assumed that the limited degree is 2, and the communication radius of the respective node is 40 cm.

As illustrated in FIG. 2, it is assumed that the user, who forms the wireless body area network (WBAN), lifts up his/her hand.

Node J broadcasts a paging message, and nodes K, H, and L, which are within the communication radium of node J, transmit response messages. In this case, nodes K, H, and L become candidate parent nodes.

Node J sets node H having the smallest remaining amount of current among nodes K, H, and L as its parent node. The selection of the node having the smallest amount of current is merely exemplary, and a node having the largest amount of current may be selected instead.

As node J sets node H as its parent node, it can be known that node H exceeds the limited degree.

Node H should cancel the connection with one of nodes J, L, and K. Since only node K has a non-urgent packet among nodes J, L, and K, the connection with node K is canceled.

As illustrated in FIG. 3, K node broadcasts a paging message. Nodes G, H, and J, which exist within the communication radius of node K, transmit response messages to the paging message, and become candidate parent nodes of node K.

Node K simulates nodes G, H, and J as parent nodes, and if node K becomes the child node of a certain node, it performs simulation to judge whether the standard deviation of the remaining power values of the respective nodes becomes minimum, that is calculated on the assumption that all the non-root nodes included in the network have transmitted packets in the direction of the root node. In the embodiment of the present invention, it is assumed that the standard deviation becomes minimum when node K becomes the child node of node G.

As illustrated in FIG. 4, node K sets node G as its parent node.

Hereinafter, the configuration and operation of a multi-hop network system according to an embodiment of the present invention will be described. FIG. 7 is a view illustrating the configuration of a multi-hop network system according to an embodiment of the present invention.

As illustrated in FIG. 7, the multi-hop network system according to an embodiment of the present invention includes one or more multi-hop nodes 700. The multi-hop node 700 communicates with other multi-hop nodes 700 to constitute the multi-hop network system according to an embodiment of the present invention. It is preferable that the multi-hop node 700 performs wireless communication with another multi-hop node 700. The multi-hop node 700 may be, for example, a node constituting a wireless body area network (WBAN).

As illustrated in FIG. 7, the multi-hop node 700 includes a disconnection judgment unit 702, a parent node setting unit 704, a child node management unit 706, a storage unit 708, and an interface unit 710.

The interface unit 710 performs wireless communication with another multi-hop node 700.

The disconnection judgment unit 702 creates and broadcasts a paging message through the interface unit 710 if it is judged that the connection with another multi-hop node 700 has been interrupted. It is preferable that the disconnection judgment unit 702 judges that the connection with another multi-hop node 700 is interrupted if a synchronization message, which is periodically transmitted/received between the disconnection judgment unit 702 and another multi-hop node 700, is not received.

The parent node setting unit 704 sets one of the candidate parent nodes which have responded to the paging message as the parent node. In this case, the parent node setting unit sets the parent node based on the size of the remaining power stored in the storage unit. For example, the parent node setting unit sets the node having the largest size of the remaining power as the parent node, or sets the node having the smallest size of the remaining power as the parent node.

The storage unit 708 stores the remaining power data of all multi-hop nodes included in the multi-hop network system. The storage unit 708 may further store consumed power data for transmitting/receiving packets, and priority data of packets stored in for every multi-hop nodes included in the multi-hop network system.

The child node management unit 706, if the node, of which the connection with another multi-hop node 700 is judged to be interrupted, becomes the child node of the parent node, and the degree of the parent node exceeds the limited degree, cancels one connection of the child node with the parent node. The degree means the number of child nodes. The limited degree means the maximum number of child nodes that one node can own. The child node management unit 706 may cancel the connection of the child node having the lowest priority of the packet stored in the storage unit 708. In the case of the node having the lowest priority of the stored packet, a temporary cancellation of the connection would not cause a big problem in the QoS of the network.

The multi-hop node 700, of which the connection has been cancelled, should be reconnected to the network, setting a new multi-hop node 700 as its parent node. Hereinafter, the reconnection operation will be described.

If it is judged that the connection with the parent node has been canceled, the disconnection judgment unit 702 creates and broadcasts a paging message through the interface unit 710.

The parent node setting unit 704 sets one of the candidate parent nodes which have responded to the paging message as a parent node of the disconnected node. In this case, the parent node becomes the multi-hop node at which the standard deviation of the remaining powers of the every node in the multi-hop network becomes minimum, that is calculated on the assumption that all the non-root nodes included in the network, in which the disconnected node is set as a child node of the parent node, have transmitted packets in a direction of the root node.

That is, the multi-hop node 700, of which the connection is initially interrupted, broadcasts a paging message, and promptly connects with the network as setting the multi-hop node 700 having the largest or smallest remaining power among other multi-hop nodes 700, which have responded to the paging message, as its parent node. If the parent node is connected to the child node that exceeds the limited degree, the child node management unit may cancel the connection of the child node having the lowest priority of the owned packet. The connection-canceled node broadcasts the paging message again, and sets a new parent node among other multi-hop nodes 700 which have responded to the paging message. In this case, it would be understood that the new parent node is the multi-hop node at which the standard deviation of the remaining power values of the nodes becomes minimum.

Although preferred embodiments of the present invention have been described for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.

According to the present invention as constructed above, in setting a new parent WBAN node when a connection with another WBAN node is interrupted, a WBAN node which enables minimizing power consumption difference for each node in the whole WBAN network, and preventing from exceeding a predetermined number of times, and from the delay time of an urgent packet exceeding a predetermined threshold value can be chosen as a new parent WBAN node.

Claims

A multi-hop network configuration method, comprising:

a disconnected node, of which the connection with another node is interrupted, broadcasting a paging message; and

setting one of candidate parent nodes which have responded to the paging message, as a parent node of the disconnected node,

wherein the parent node is a node at which the standard deviation of remaining powers of the every node in the multi-hop network becomes minimum, that is calculated on the assumption that all non-root nodes included in the network, in which the disconnected node is set as a child node of the parent node, have transmitted packets in a direction of a root node.
The method of claim 1, wherein the broadcasting comprises the disconnected node, of which the connection has been interrupted due to an excess of a communication radius of the another node for pre-defined time, broadcasting the paging message.
The method of claim 1, wherein the parent node is set in further consideration of whether the number of hops to the root node is smaller than or equal to a hop threshold value.
The method of claim 3, wherein the hop threshold value is a value set based on an ordered pair of the root node and the parent node.
The method of claim 1, wherein the parent node is set in further consideration of whether a communication delay time, which is calculated on the assumption that the disconnected node has transmitted an urgent packet to the parent node in the network configuration where the disconnected node is set as the child node of the parent node, is below a preset delay time threshold value.
A multi-hop network configuration method, comprising:

a disconnected node, of which the connection with another node is interrupted, broadcasting a paging message; and

setting one of candidate parent nodes which have responded to the paging message, as a parent node of the disconnected node,

wherein the parent node is set based on the remaining powers of the candidate parent nodes.
The method of claim 6, further comprising canceling one connection of the child node of the parent node if the degree of the parent node exceeds a limited degree due to the setting.
The method of claim 7, wherein the canceling comprises canceling a connection of the child node having the lowest priority of stored packets among the child nodes.
The method of claim 7, further comprising:

the connection-canceled node broadcasting the paging message; and

setting one of candidate parent nodes which have responded to the paging message, as the parent node of the connection-canceled node,

wherein the parent node is a node at which the standard deviation of remaining powers of the every node in the multi-hop network becomes minimum, that is calculated on the assumption that all non-root nodes included in the network, in which the disconnected node is set as a child node of the parent node, have transmitted packets in a direction of a root node.
A multi-hop network system comprising:

One or more multi-hop nodes,

wherein the multi-hop nodes comprising,

an interface unit performing wireless communication with another multi-hop node;

a disconnection judgment unit creating and broadcasting a paging message through the interface unit if it is judged that a connection with the another multi-hop node has been interrupted;

a storage unit storing remaining power data of all multi-hop nodes included in the multi-hop network system; and

a parent node setting unit setting one of candidate parent nodes which have responded to the paging message as a parent node, based on the size of the remaining power stored in the storage unit.
The system of claim 10, wherein the multi-hop node further comprises a child node management unit canceling one connection of a child node,if the degree of the multi-hop node exceeds a limited degree due to the setting parent node of the parent node setting unit.
The system of claim 11, wherein the storage unit further stores consumed power data for transmitting/receiving packets, and priority data of packets stored in for every multi-hop nodes included in the multi-hop network system; and the child node management unit cancels the connection of the child node having the lowest priority of the packets stored in the storage unit.
The system of claim 12, wherein the disconnection judgment unit creates and broadcasts the paging message through the interface unit if it is judged that the connection with the parent node has been interrupted;

the parent node setting unit sets one of the candidate parent nodes which have responded to the paging message, as the parent node of the connection-canceled node; and

the parent node is the multi-hop node at which the standard deviation of the remaining powers of the every node in the multi-hop network becomes minimum, that is calculated on the assumption that all non-root nodes included in the network, in which the connection-released multi-hop node is set as a child node of the parent node, have transmitted packets in a direction of a root node.