Patent Description:
A fire alarm outputs an alarm when it detects a fire. By building wireless communication functions in the fire alarm and forming a multihop network by a plurality of fire alarms, it is possible, when one fire alarm detects a fire, for another fire alarm to output an alarm (see, for example, <CIT>). <CIT> discloses an information processing apparatus including a first controller that controls an IoT device to perform multihop routing via at least one relay on an upload of data from the IoT device to a base station, and a second controller that controls the base station to use, on a down-link from the base station to the IoT device, a route different from a route that uses at least one relay. <CIT> discloses a multi-hop transmission method, which, when performing multi-hop transmission between terminals in a wireless ad hoc environment, constructs a transmission path with terminals having a long lifetime. <CIT> discloses a wireless communication system, in which, when a predetermined event occurs, each of slave stations receives a radio signal transmitted by another slave station to a master station.

In the case the fire alarm is driven by a battery, low power consumption of the fire alarm is called for. Meanwhile, the fire alarm included in a relay route of a multihop network transfers signals so that the power consumption of the fire alarm increases as the frequency of transfer increases.

The present disclosure addresses this issue, and a purpose thereof is to provide a technology of suppressing an increase in power consumption in the alarm included in a multihop network.

An alarm system according to an embodiment of the present invention includes a plurality of alarms that form a multihop network extending from a relay device. The plurality of alarms include a first alarm, a second alarm, and a third alarm. The third alarm is capable of communicating with the first alarm and the second alarm, the third alarm derives a first cost for a first relay route for communicating with the relay device via the first alarm, by exchanging information on quality of link with the first alarm, and derives a second cost for a second relay route for communicating with the relay device via the second alarm, by exchanging information on quality of link with the second alarm, the third alarm selects the first relay route when the first cost is smaller than the second cost. When power consumption in the first alarm grows larger than a threshold value, the first alarm transmits a notification indicating an increase in power consumption to the third alarm, and wherein the third alarm derives the first cost based on a first indicator determined by the information on quality of link with the first alarm and a second indicator determined by power consumption in the first alarm, and wherein the third alarm derives the second cost based on a first indicator determined by the information on quality of link with the second alarm and a second indicator determined by power consumption in the second alarm, and, when the third alarm receives the notification from the first alarm, the third alarm increases the second indicator in the first cost.

Another embodiment of the present invention relates to an alarm. The alarm is an alarm of a plurality of alarms that form a multihop network extending from a relay device, including: a communication unit capable of communicating with a first alarm and a second alarm of the plurality of alarms; and a control unit that derives a first cost for a first relay route for communicating with the relay device via the first alarm, by exchanging information on quality of link with the first alarm through the communication unit, derives a second cost for a second relay route for communicating with the relay device via the second alarm (<NUM>), by exchanging information on quality of link with the second alarm through the communication unit, and then selects the first relay route when the first cost is smaller than the second cost. The communication unit receives a notification indicating an increase in power consumption from the first alarm when power consumption in the first alarm grows larger than a threshold value, and, the control unit derives the first cost based on a first indicator determined by the information on quality of link with the first alarm and a second indicator determined by power consumption in the first alarm, and wherein the control unit derives the second cost based on a first indicator determined by the information on quality of link with the second alarm and a second indicator determined by power consumption in the second alarm, when the communication unit receives the notification from the first alarm, the control unit increases the second indicator in the first cost.

Still another embodiment of the present invention relates to a relay route setting method. The method is a route setting method in an alarm of a plurality of alarms that form a multihop network extending from a relay device, the alarm being capable of communicating with a first alarm and a second alarm of the plurality of alarms, the method including: deriving a first cost for a first relay route for communicating with the relay device via the first alarm, by exchanging information on quality of link with the first alarm, and deriving a second cost for a second relay route for communicating with the relay device via the second alarm, by exchanging information on quality of link with the second alarm); selecting the first relay route when the first cost is smaller than the second cost; when power consumption in the first alarm grows larger than a threshold value, receiving a notification indicating an increase in power consumption from the first alarm; and deriving the first cost based on a first indicator determined by the information on quality of link with the first alarm and a second indicator determined by power consumption in the first alarm, and deriving the second cost based on a first indicator determined by the information on quality of link with the second alarm and a second indicator determined by power consumption in the second alarm, and when the notification from the first alarm is received, increasing the second indicator in the first cost.

Optional combinations of the aforementioned constituting elements, and implementations of the disclosure in the form of methods, apparatuses, systems, recording mediums, and computer programs may also be practiced as additional modes of the present disclosure.

According to the present disclosure, it is possible to suppress an increase in power consumption in the alarm included in a multihop network.

A brief summary will be given before describing the present disclosure in specific details. The embodiment relates to an alarm system provided in a facility such as a multi-unit apartment building, an independent housing, an office, and a hospital. In the alarm system, a relay device is connected to a management device, and a plurality of fire alarms are connected to the relay device in a wireless multihop network. In this network, the management device represents the higher level, and the fire alarm with the largest hop count from the relay device represents the lower level. Upon detecting an outbreak of a fire, the fire alarm transfers a result of detection to the relay device, and the relay device transfers the result of detection to the management device. When the result of detection is received, the management device selects one or more fire alarms that should output an alarm and transmits an instruction to output an alarm to the one or more selected fire alarms as the ultimate destinations. The relay device and the fire alarm transfer the instruction to output an alarm to the fire alarm at the ultimate destination, and the fire alarm at the ultimate destination outputs an alarm upon receiving the instruction to output an alarm.

Given that the line for a signal from the relay device to the fire alarm with the largest hop count from the relay device is referred to as "downlink line", the line for a signal from the fire alarm with the largest hop count to the relay device is referred to as "uplink line". In this embodiment, one frame is formed by arranging a plurality of time slots, and one super frame is formed by arranging a plurality of frames. Further, one fire alarm is allocated to one time slot for the downlink line (hereinafter, "downlink communication time slot") and is also allocated to one time slot for the uplink line (hereinafter, "uplink communication time slot"). The downlink communication time slot is used for transfer on the downlink line, and the uplink communication time slot is used for transfer on the uplink line.

In the downlink line, a signal for establishing synchronization on the multihop network (hereinafter, "synchronization signal") is periodically transferred in addition to the instruction to output an alarm. On the other hand, the uplink line is used mainly to transfer the result of detection. In the following description, a synchronization signal, a detection result, an instruction to output an alarm may generically be referred to as "communication signals".

Hereinafter, the embodiment will be described in the order (<NUM>) basic configuration, (<NUM>) routing, (<NUM>) construction, and (<NUM>) revision to time slot assignment.

<FIG> shows a configuration of an alarm system <NUM>. The alarm system <NUM> includes a first fire alarm 600a through a ninth fire alarm 600i, which are generically referred to as fire alarms <NUM>, a first relay device 700a through a third relay device 700c, which are generically referred to as relay devices <NUM>, and a management device <NUM>. The number of fire alarms <NUM> is not limited to "<NUM>", and the number of relay devices <NUM> is not limited to "<NUM>".

The alarm system <NUM> is a system applied to facilities such as houses, offices, and commercial facilities to detect a fire and alert that a fire has broken out. The plurality of fire alarm <NUM> are, for example, home fire alarms and are provided with fire detection sensors. The plurality of fire alarm <NUM> are provided on, for example, the ceilings of facilities but may be provided on the walls, etc..

The first fire alarm 600a through the sixth fire alarm 600f form a wireless multihop network extending from the first relay device 700a. For example, a relay route that links the first relay device 700a, the first fire alarm 600a, and the second fire alarm 600b and a relay route that links the first relay device 700a, the fourth fire alarm 600d, the fifth fire alarm 600e, and the third fire alarm 600c are formed. Further, a relay route that links the first relay device 700a, the fourth fire alarm 600d, the fifth fire alarm 600e, and the sixth fire alarm 600f and a relay route that links the first relay device 700a and the seventh fire alarm <NUM> are also formed. These relay routes are determined by the respective fire alarm <NUM> and are shared by the first relay device 700a and the management device <NUM>.

In these relay routes, the first fire alarm 600a, the fourth fire alarm 600d, and the seventh fire alarm <NUM> can communicate with the first relay device 700a in "<NUM>" hop. The second fire alarm 600b and the fifth fire alarm 600e can communicate with the first relay device 700a in "<NUM>" hops. The third fire alarm 600c and the sixth fire alarm 600f can communicate with the first relay device 700a in "<NUM>" hops.

The second relay device 700b, the third relay device 700c, the eighth fire alarm <NUM>, and the ninth fire alarm 600i are configured in a manner similar to that of the first relay device 700a, the first fire alarm 600a, etc. For example, a multihop network starting from the first relay device 700a is provided on the first floor of a facility, a multihop network starting from the second relay device 700b is provided on the second floor of the facility, and a multihop network starting from the third relay device 700c is provided on the third floor of the facility. Different frequencies are used in the multihop network starting from the first relay device 700a, the multihop network starting from the second relay device 700b, and the multihop network starting from the third relay device 700c. Further, the first relay device 700a, the second relay device 700b, and the third relay device 700c communicate with each other wirelessly or by wire.

Thus, the relay device <NUM> communicates with the plurality of fire alarm <NUM> that form the multinetwork wirelessly and communicates with the other relay devices <NUM> wirelessly or by wire. It can be said that the relay device <NUM> relays communication between the plurality of fire alarm <NUM> included in the multihop network. Further, the first relay device 700a is connected to the management device <NUM> by a cable and communicates with the management device <NUM> by wire.

The management device <NUM> is, for example, a controller of a home energy management system (HEMS) provided in the facility. The management device <NUM> can communicate with a plurality of appliances provided in the facility. The plurality of appliances include, for example, air conditioners, illumination appliances, hot water dispensers, etc. having a communication function. Further, the management device <NUM> can communicate with the first relay device 700a provided in the facility. The management device <NUM> can also communicate with the second relay device 700b, the third relay device 700c, and the fire alarms <NUM> via the first relay device 700a.

<FIG> shows a configuration of the fire alarm <NUM>. The fire alarm <NUM> includes a communication unit <NUM>, a processing unit <NUM>, a control unit <NUM>, a fire detection sensor <NUM>, and a buzzer <NUM>. A publicly known technology may be used in the fire detection sensor <NUM>. For example, the fire detection sensor <NUM> may be an optical smoke detection sensor and may detect a fire by detecting the smoke in a fire by utilizing diffuse reflection of light. For example, the fire detection sensor <NUM> may be a heat detection sensor and may detect a fire by detecting the heat from a fire. For example, the fire detection sensor <NUM> may be a carbon monoxide detection sensor and may detect a fire by detecting the density of carbon monoxide generated by combustion in a fire. For example, the fire detection sensor <NUM> may be an infrared detection sensor and may detect a fire by detecting infrared rays radiated by combustion in a fire.

The communication unit <NUM> communicates with the other fire alarm <NUM> or the relay device <NUM> wirelessly. The processing unit <NUM> processes a signal received by the communication unit <NUM> or generates a signal that should be transmitted from the communication unit <NUM>. The control unit <NUM> controls the operation of the communication unit <NUM> and the processing unit <NUM>. The detail of the control unit <NUM> will be described later. The buzzer <NUM> can output a buzzer sound. The fire alarm <NUM> may be configured not to include the buzzer <NUM> and include the fire detection sensor <NUM>. In other words, the fire alarm <NUM> may be configured to have the detection function and the communication function. The fire alarm <NUM> configured as described above can be said to be a sensor capable of alerting that a fire is detected.

<FIG> show a configuration of a super frame used in the alarm system <NUM>. As shown in <FIG>, a predefined period of time is defined as the super frame <NUM>. The super frame <NUM> is arranged repeatedly. The super frame <NUM> is divided into a plurality of frames <NUM>. As shown in <FIG>, one frame <NUM> is divided into a plurality of time slots <NUM>. <FIG> shows one time slot <NUM>. The communication signal is transmitted in the time slot <NUM>. The duration of the communication signal is shorter than the duration of one time slot <NUM>.

<FIG> shows the usage of the plurality of time slots <NUM> included in the frame <NUM> shown in <FIG>. Of the plurality of time slots <NUM>, one or more time slots <NUM> in the leading portion are used as "downlink communication time slots". One time slot <NUM> following the downlink communication time slots is used as an "uplink communication time slot". One or more time slots <NUM> following the uplink communication time slot are used as "backup slots". The number of downlink communication time slots and the number of uplink communication time slots are identical and are equal to or larger than the number of fire alarm <NUM> included in the multihop network. Backup slots may not be provided.

<FIG> shows a configuration of the relay device <NUM>. The relay device <NUM> can be said to be a controller for the plurality of fire alarms <NUM> that form the multihop network. The relay device <NUM> includes a communication unit <NUM>, a control unit <NUM>. The communication unit <NUM> includes an output unit <NUM>. The control unit720 includes an assignment unit <NUM>. The communication unit <NUM> has the communication function for communicating with the plurality of relay devices <NUM> and also has the communication function for communicating with the management device <NUM>. The control unit <NUM> controls the operation of the relay device <NUM>.

By communicating with the plurality of fire alarms <NUM> that form the multihop network, the communication unit <NUM> receives the result of routing performed in the respective fire alarms <NUM>. Routing performed in the respective fire alarms <NUM> will be described later. The result of routing indicates the relay routes as shown in <FIG>.

The assignment unit <NUM> assigns a combination of one downlink communication time slot and one uplink communication time slot shown in <FIG> to one fire alarm <NUM> based on the result of routing. Assignment by the assignment unit <NUM> will also be described later. The combination of the downlink communication time slot and the uplink communication time slot is changed depending on the fire alarm <NUM>. The output unit <NUM> outputs the result of assignment by the assignment unit <NUM> to the plurality of fire alarms <NUM>. The result of assignment shows the correspondence between the combination of the downlink communication slot/the uplink communication slot and the fire alarm <NUM>.

<FIG> shows an exemplary assignment of time slots in the alarm system <NUM> as similarly shown in <FIG>. The figure shows the assignment of a plurality of time slots <NUM> to the first relay device 700a, the first fire alarm 600a through the seventh fire alarm <NUM> of <FIG>. "M" in <FIG> denotes the first relay device 700a, and "S1" through "S7" denote the first fire alarm 600a through the seventh fire alarm <NUM>, respectively. The first relay device 700a, the first fire alarm 600a, the fourth fire alarm 600d, the seventh fire alarm <NUM>, the second fire alarm 600b, the fifth fire alarm 600e, the third fire alarm 600c, and the sixth fire alarm 600f are sequentially allocated to downlink communication time slots, with the first relay device 700a preceding the rest. As described above, the hop count from the first fire alarm 600a, the fourth fire alarm 600d, and the seventh fire alarm <NUM> to the first relay device 700a is "<NUM>". The hop count from the second fire alarm 600b and the fifth fire alarm 600e to the first relay device 700a is "<NUM>", and the hop count from the third fire alarm 600c and the sixth fire alarm 600f to the first relay device 700a is "<NUM>". In other words, the smaller the hop count to the first relay device 700a, the earlier the downlink communication slot assigned to the fire alarm <NUM>.

The the sixth fire alarm 600f, the third fire alarm 600c, the fifth fire alarm 600e, the second fire alarm 600b, the seventh fire alarm <NUM>, the fourth fire alarm 600d, the first fire alarm 600a, and the first relay device 700a are sequentially allocated to uplink communication time slots, with the sixth fire alarm 600f preceding the rest. In other words, the larger the hop count to the first relay device 700a, the earlier the uplink communication time slot assigned to the fire alarm <NUM>.

To highlight the fifth fire alarm 600e with the hop count "<NUM>", the downlink communication slot earlier than the time slot assigned to the sixth fire alarm 600f with the hop count "<NUM>" is assigned to the fifth fire alarm 600e. The downlink communication time slot is used when a signal (communication signal) is transferred in a direction away from the first relay device 700a in the multihop network. Further, the uplink communication slot later than the time slot assigned to the sixth fire alarm 600f is assigned to the fifth fire alarm 600e. The uplink communication time slot is used when a signal (communication signal) is transferred in a direction toward the first relay device 700a in the multihop network. In other words, of the plurality of time slots <NUM>, the relay device <NUM> determines the time slot <NUM> assigned to each fire alarm <NUM> in accordance with the hop count between the fire alarm <NUM> and the relay device <NUM>.

The downlink communication time slot "S5" and the uplink communication time slot "S5" are assigned to the fifth fire alarm 600e, and the fifth fire alarm 600e transmits a signal (communication signal) in the downlink communication time slot "S5" or the uplink communication time slot "S5". The downlink communication time slot "S6" and the uplink communication time slot "S6" are assigned to the sixth fire alarm 600f, and the sixth fire alarm 600f transmits a signal (communication signal) in the downlink communication time slot "S6" or the uplink communication time slot "S6".

The assignments of time slots <NUM> is determined by the assignment unit <NUM> of the first relay device 700a but may be determined by the management device <NUM>. For example, the first relay device 700a or the management device <NUM> determines the assignment of the time slots <NUM> based on the information on the relay routes. The first relay device 700a or the management device <NUM> notifies the fire alarms <NUM> of the assignment of the time slots <NUM> thus determined. Therefore, the fire alarms <NUM> also have the knowledge of the assignment of the time slots <NUM>. As a result, each fire alarm <NUM> has the knowledge of the time slot <NUM> in which the communication signal should be transmitted and which is assigned to the fire alarm <NUM>. The fire alarm <NUM> also has the knowledge of the time slot <NUM> in which the communication signal from the adjacent fire alarm <NUM> or the relay device <NUM> on the relay route can be received.

In this setup, the communication unit <NUM> of the fire alarm <NUM> may perform an intermittent receiving operation to reduce power consumption. In the intermittent receiving operation in the communication unit <NUM>, a receiving operation is performed during a predefined period of time in the leading portion of the time slot <NUM>, and the receiving operation is suspended in the remainder of the time slot <NUM> if a signal (communication signal) is not received during the predefined period of time. When a signal is received during the predefined period of time in the leading portion of the time slot <NUM>, the receiving operation is continued in the remainder of the time slot <NUM>.

<FIG> shows an outline of downlink communication in the alarm system <NUM>. The figure shows downlink communication time slots of <FIG>. The first relay device 700a periodically transmits a synchronization signal to the plurality of fire alarm <NUM> that form the multihop network. The synchronization signal is, for example, a beacon signal. The synchronization signal is transmitted in, for example, the leading frame <NUM> of the super frame <NUM> shown in <FIG> and is not transmitted in the remaining frames <NUM>. The first relay device 700a transmits the synchronization signal in the time slot <NUM> "M" in the leading frame <NUM> of the super frame <NUM>. When the fourth fire alarm 600d receives the synchronization signal in the time slot <NUM> "M", the fourth fire alarm 600d transfers the synchronization signal in the time slot <NUM> "S4". Further, the fourth fire alarm 600d transmits a response signal to the first relay device 700a in the time slot <NUM> "S4". The response signal is, for example, Ack (ACKnowledgment). The response signal may be included in a portion of the synchronization signal.

The first relay device 700a receives the response signal in the time slot <NUM> "S4". When the fifth fire alarm 600e receives the synchronization signal in the time slot <NUM> "S4", the fifth fire alarm 600e transfers the synchronization signal and transmits the response signal to the fourth fire alarm 600d in the time slot <NUM> "S5". The fourth fire alarm 600d receives the response signal in the time slot <NUM> "S5". The fourth fire alarm 600d transfers the response signal from the fifth fire alarm 600e to the first relay device 700a in the time slot <NUM> "S4" of the next frame (not shown in <FIG>).

When the third fire alarm 600c receives the synchronization signal in the time slot <NUM> "S5", the third fire alarm 600c transfers the synchronization signal and transmits the response signal to the fifth fire alarm 600e in the time slot <NUM> "S3". When the sixth fire alarm 600f receives the synchronization signal in the time slot <NUM> "S5", the sixth fire alarm 600f transfers the synchronization signal and transmits the response signal to the fifth fire alarm 600e in the time slot <NUM> "S6".

The fifth fire alarm 600e receives the response signal in the time slots <NUM> "S3" and "S6". The fifth fire alarm 600e transfers the response signal from the third fire alarm 600c and the response signal from the sixth fire alarm 600f to the fourth fire alarm 600d in the time slot <NUM> "S5" of the next frame (not shown in <FIG>). The fourth fire alarm 600d transfers the response signal from the fifth fire alarm 600e to the first relay device 700a in the time slot <NUM> "S4" in the frame after next.

Thus, the synchronization signal is transferred in the frame <NUM> in which the first relay device 700a transmitted the synchronization signal. Further, the fire alarm <NUM> receiving the synchronization signal from the first relay device 700a establishes timing synchronization with the first relay device 700a based on the synchronization signal. A publicly known technology may be used for timing synchronization so that a description thereof is omitted.

<FIG> shows an outline of uplink communication in the alarm system <NUM>. The figure shows uplink communication time slots of <FIG>. It is assumed here that the fire detection sensor <NUM> of the sixth fire alarm 600f detects an outbreak of a fire. The processing unit <NUM> of the sixth fire alarm 600f causes the communication unit <NUM> to transmit a detection result. The detection result includes identification information on the sixth fire alarm 600f that has detected the fire. The communication unit <NUM> of the sixth fire alarm 600f transmits the detection result in the time slot <NUM> "S6".

The fifth fire alarm 600e receives the detection result in the time slot <NUM> "S6". Subsequently, the fifth fire alarm 600e transfers the detection result in the time slot <NUM> "S5". Further, the fifth fire alarm 600e transmits a response signal to the sixth fire alarm 600f in the time slot <NUM> "S5". The response signal may be included in a portion of the detection result.

The sixth fire alarm 600f receives the response signal in the time slot <NUM> "S5". The fourth fire alarm 600d receives the detection result in the time slot <NUM> "S5". The fourth fire alarm 600d transfers the detection result and transmits a response signal to the fifth fire alarm 600e in the time slot <NUM> "S4".

The fifth fire alarm 600e receives the response signal in the time slot <NUM> "S4". The fifth fire alarm 600e transfers the response signal from the fourth fire alarm 600d to the sixth fire alarm 600f in the time slot <NUM> "S5" of the next frame <NUM> (not shown in <FIG>).

The first relay device 700a receives the detection result in the time slot <NUM> "S4". In a manner as already described, the first relay device 700a transmits a response signal in the time slot <NUM> "M". The response signal is transferred by the fourth fire alarm 600d and the fifth fire alarm 600e and is received by the sixth fire alarm 600f.

When the first relay device 700a receives a detection result from the fourth fire alarm 600d, the first relay device 700a transfers the detection result to the management device <NUM>. When the management device <NUM> receives the detection result, the management device <NUM> identifies the fire alarm <NUM> that should output an alarm based on the identification information included in the detection result. The correspondence between the identification information and the information on the fire alarm <NUM> that should output an alarm is stored in the management device <NUM> in advance. The management device <NUM> transmits an instruction to output an alarm to the first relay device <NUM>, designating the identified fire alarm <NUM> as the ultimate destination.

When the fire alarms <NUM> identified by the management device <NUM> are the third fire alarm 600c and the sixth fire alarm 600f, an instruction to output an alarm is received by the third fire alarm 600c and the sixth fire alarm 600f by transferring the signal in the same manner as illustrated in <FIG>. In this case, the instruction to output an alarm is transmitted instead of the synchronization signal of <FIG>. When the instruction to output an alarm is received from the management device <NUM> by way of the first relay device 700a, the second relay device 700b and the third relay device 700c transfer the instruction to output an alarm to the fire alarm <NUM>. When the communication units <NUM> of the third fire alarm 600c and the sixth fire alarm 600f receive the instruction to output an alarm, the control unit <NUM> causes the buzzer <NUM> to sound an alarm. The control unit <NUM> may cause a light-emitting device to flash.

In the above description, it is assumed that the relay routes as shown in <FIG> are formed. A description will be given here of the formation and modification of relay routes with reference also to <FIG> shows an outline of routing in the alarm system <NUM>. <FIG> shows an n+1th fire alarm 600n+<NUM>, an n+2th fire alarm 600n+<NUM>, an n+3th fire alarm 600n+<NUM>, and the relay device <NUM> of the alarm system <NUM>. The n+1th fire alarm 600n+<NUM>, the n+2th fire alarm 600n+<NUM>, and the n+3th fire alarm 600n+<NUM> each corresponds to one of the fire alarms <NUM> of <FIG>. Fire alarms <NUM> other than the n+1th fire alarm 600n+<NUM> and the n+2th fire alarm 600n+<NUM> may be located around the n+3th fire alarm 600n+<NUM>. For example, an n+4th fire alarm 600n+<NUM> (not shown) may be located.

Hereinafter, (<NUM>-<NUM>) formation of relay route and (<NUM>-<NUM>) modification of relay route will be described in the stated order.

<FIG> is a sequence chart showing steps of routing in the alarm system <NUM>. A routing process will be described by highlighting the n+3th fire alarm 600n+<NUM>. Each fire alarm <NUM> broadcasts a HELLO message an predetermined intervals. The HELLO message includes information on quality of route to the relay device <NUM>. The communication unit <NUM> of the n+3th fire alarm 600n+<NUM> receives the HELLO message from the n+1th fire alarm 600n+<NUM>, the n+2th fire alarm 600n+<NUM>, and the n+4th fire alarm 600n+<NUM> (S10, S12, S14).

The communication unit <NUM> of the n+3th fire alarm 600n+<NUM> measures the reception power of each HELLO message received, and the processing unit <NUM> derives the link quality for each fire alarm <NUM> based on the measured reception power. The link quality varies in accordance with the reception power. The larger the reception power, the smaller the value of link quality. The route quality mentioned above is defined in a manner similar to the link quality. The control unit <NUM> derives a tentative route cost as follows by adding the link quality of the n+1th fire alarm 600n+<NUM> to the route quality included in the HELLO message from the n+1th fire alarm 600n+<NUM>. <MAT> where Ka and Kb are coefficients, and C is a predetermined constant. Given that Kb is "<NUM>" when the relay route is formed, Kb×C is neglected.

The control unit <NUM> also derives a tentative route cost for the other fire alarms <NUM>. The control unit <NUM> compares a plurality of tentative route costs and selects several fire alarms <NUM> as preferential link destinations in the ascending order of tentative route cost. In this case, the n+1the fire alarm 600n+<NUM> and the n+2th fire alarm 600n+<NUM> are selected as preferential link destinations, for example.

The communication unit <NUM> of the n+3th fire alarm 600n+<NUM> includes the addresses of the selected fire alarm <NUM> and the link quality identified in the reception in the LINK_REQ submessage of the HELLO message and transmits the resultant message (S16, S18). The n+1th fire alarm 600n+<NUM> and the n+2th fire alarm 600n+<NUM> include the link quality in the reverse direction in the LNK_REP submessage and transmits the resultant message (S20, S22).

The communication unit <NUM> of the n+3th fire alarm 600n+<NUM> receives the LINK_REP submessage. The control unit <NUM> of the n+3th fire alarm 600n+<NUM> compares the link quality included in the LINK_REP submessage from the n+1th fire alarm 600n+<NUM> with the link quality derived based on the reception power already measured and selects the link quality with a larger value. The control unit <NUM> also derives a definitive route cost as follows by adding the selected link quality and the route quality for the n+1th fire alarm 600n+<NUM>. <MAT> where Max indicates selecting the maximum link quality.

The control unit <NUM> also derives a definitive route cost for the n+2th fire alarm 600n+<NUM>. The control unit <NUM> compares the definitive route cost for the n+1th fire alarm 600n+<NUM> with the definitive route cost for the n+2th fire alarm 600n+<NUM> and selects the route with a smaller cost as the relay route. The relay route not selected may be used as a substitute route.

In other words, the n+3th fire alarm 600n+<NUM> derives the definitive route cost (hereinafter, "first cost") for the relay route for communication with the relay device <NUM> by way of the n+1th fire alarm 600n+<NUM> (hereinafter, "first relay route") by exchanging link quality information with the n+1th fire alarm 600n+<NUM>. Further, the n+3th fire alarm 600n+<NUM> derives the definitive route cost (hereinafter, "second cost") for the relay route for communication with the relay device <NUM> by way of the n+2th fire alarm 600n+<NUM> (hereinafter, "second relay route") by exchanging link quality information with the n+2th fire alarm 600n+<NUM>. Further, the n+3th fire alarm 600n+<NUM> compares the first cost and the second cost and selects the relay route with a smaller cost preferentially. By performing a process like this in the respective fire alarms <NUM>, the relay routes are formed. Information relating to the relay route (substitute route) formed in the respective fire alarms <NUM> is transmitted to the management device <NUM> by way of the relay device <NUM>. The management device <NUM> determines the assignment of the time slots <NUM> in accordance with the hop count, based on the information relating to the relay route (substitute route).

When the relay route is formed as described above, the fire alarms <NUM> included in the relay route transfer the signal. Transfer of the signal increases the power consumption in the fire alarm <NUM>. In the case the fire alarm <NUM> is driven by a battery, it is preferred that the power consumption is small. In order to suppress an increase in the power consumption in the fire alarm <NUM>, the relay route is modified.

The control unit <NUM> of the n+1th fire alarm 600n+<NUM> included in the first relay route connecting the n+3th fire alarm 600n+<NUM> and the relay device <NUM> of <FIG> measures the communication frequency based on the number of times of communication of the communication unit <NUM> in a predetermined period of time. The number of times of communication includes at least one of the number of times of transmission or the number of times of reception. The control unit <NUM> maintains the correspondence between the communication frequency and the power consumption and derives the power consumption based on the communication frequency. In the correspondence, the larger the communication frequency, the larger the power consumption.

Further, the control unit <NUM> of the n+1th fire alarm 600n+<NUM> may count the number of other fire alarms <NUM> that the n+1th fire alarm 600n+<NUM> directly communicates with. The control unit <NUM> maintains the correspondence between the number of other fire alarms <NUM> and the power consumption and derives the power consumption based on the number of other fire alarms <NUM>. In the correspondence, the larger the number of other fire alarms <NUM>, the larger the power consumption. Further, the control unit <NUM> of the n+1th fire alarm 600n+<NUM> may measure the battery level of the n+1th fire alarm 600n+<NUM>. The control unit <NUM> maintains the correspondence between the battery level and the power consumption and derives the power consumption based on the battery level. In the correspondence, the lower the battery level, the larger the power consumption.

The control unit <NUM> maintains a threshold value for the power consumption. <FIG> show a data structure of a table maintained in the n+1th fire alarm 600n+<NUM>. <FIG> shows conditions for power consumption and threshold value and shows operations determined by the conditions. When the power consumption grows larger than the threshold value, the control unit <NUM> determines to transmit a notification indicating an increase in the power consumption. When the power consumption is equal to or smaller than the threshold value, on the other hand, the control unit <NUM> determines not to transmit a notification. <FIG> will be described later, and reference is made back to <FIG>. When it is determined to transmit a notification, the communication unit <NUM> of the n+1th fire alarm 600n+<NUM> transmits a notification to the n+3th fire alarm 600n+<NUM>.

As described above, the control unit <NUM> of the n+3th fire alarm 600n+<NUM> determines the relay route based on the definitive route cost of expression (<NUM>). Depending on whether a notification is received from the n+1th fire alarm 600n+<NUM>, the control unit <NUM> controls the values of Ka, Kb of expression (<NUM>). <FIG> show a data structure of a table maintained in the n+3th fire alarm 600n+<NUM>. <FIG> show values of the coefficients Ka, Kb responsive to the case of reception of a notification and the case of non-reception of a notification. The coefficients Ka and Kb are related such that Ka+Kb=<NUM>.

When a notification is not received, the coefficient Ka will be "A1" and the coefficient Kb will be "B1". The case of non-reception of a notification includes the case of (<NUM>-<NUM>) formation of relay route. For example, "A1" is "<NUM>" and "B1" is "<NUM>". When a notification is not received, therefore, the third term on the right side of expression (<NUM>) is neglected.

When a notification is received, the coefficient Ka will be "A2" and the coefficient Kb will be "B2". "B2" is a value larger than "<NUM>" so that "A2" is a value smaller than "<NUM>". A2 and B2 may be such that A2>B2, A2=B2, or A2<B2. When a notification is received, therefore, the impact from the third term on the right side of expression (<NUM>) will be large, and the definitive route cost will be larger as compared with the case of non-reception of a notification. As a result, it will be less likely that the first relay route including the n+1th fire alarm 600n+<NUM> is selected. In other words, the n+3th fire alarm 600n+<NUM> makes it less likely that the first relay route including the n+1th fire alarm 600n+<NUM> is selected when a notification from the n+1th fire alarm 600n+<NUM> is received.

The second term on the right side of the definitive route cost given by expression (<NUM>) is "Ka×Max (link quality)" and so can be said to be an indicator (hereinafter, "first indicator") determined by the information on quality of link with the n+1th fire alarm 600n+<NUM>. The third term on the right side of the definitive route cost given by expression (<NUM>) is "Kb×C" and so can be said to be an indicator (hereinafter, "second indicator") determined by the power consumption in the n+1th fire alarm 600n+<NUM>. When a notification is received, the control unit <NUM> makes it less likely that the first relay route is selected, by increasing the impact from the second indicator in the definitive route cost. Such a process is also performed in relation to the other fire alarms <NUM>.

The values of the coefficients Ka, Kb are hitherto adjusted in two steps. However, the values of the coefficients Ka, Kb may be adjusted in three or more steps. <FIG> shows conditions for power consumption and threshold value and shows operations determined by the conditions. The control unit <NUM> of the n+1th fire alarm 600n+<NUM> defines the first threshold value and the second threshold value larger than the firs threshold value for power consumption. When the power consumption is larger than the first threshold value and equal to or smaller than the second threshold value, the control unit <NUM> determines to transmit the first notification indicating an increase in the power consumption. When the power consumption grows larger than the second threshold value, the control unit <NUM> determines to transmit the second notification indicating a further increase in the power consumption. When the power consumption is equal to or smaller than the first threshold value, on the other hand, the control unit <NUM> determines not to transmit the first notification or the second notification. Reference is made back to <FIG>. When it is determined to transmit the first notification, the communication unit <NUM> of the n+1th fire alarm 600n+<NUM> transmits the first notification to the n+3th fire alarm 600n+<NUM>. When it is determined to transmit the second notification, the communication unit <NUM> transmits the second notification to the n+3th fire alarm 600n+<NUM>.

Depending on whether the first notification or the second notification from the n+1th fire alarm 600n+<NUM> is received, the control unit <NUM> of the n+3th fire alarm 600n+<NUM> controls the values of Ka, Kb of expression (<NUM>). <FIG> show values of Ka, Kb responsive to the case of non-reception of the first notification or the second notification and the case of reception of the first notification or the second notification. In this case, too, the coefficients Ka and Kb are related such that Ka+Kb=<NUM>.

When the first notification or the second notification is not received, the coefficient Ka will be "A1" and the coefficient Kb will be "B1". For example, "A1" is "<NUM>" and "B1" is "<NUM>". When the first notification is received, the coefficient Ka will be "A2" and the coefficient Kb will be "B2". When the second notification is received, the coefficient Ka will be "A3" and the coefficient Kb will be "B3". "B3" is a value larger than "B2" and "A3" is a value larger than "A2". In other words, the control unit <NUM> increases the impact from the second indicator in the definitive route cost when the second notification is received from the n+1th fire alarm 600n+<NUM> by an amount larger than than when the first notification is received.

A description will now be given of a technology for facilitating the construction of a multihop network of the alarm system <NUM>. In particular, a technology of providing information for determining at which position the fire alarm <NUM> should be constructed. <FIG> show an outline of the construction of the alarm system <NUM>. <FIG> shows the first example. The alarm system <NUM> includes an external appliance <NUM> in addition to the features of <FIG>. The external appliance <NUM> is, for example, a computer and can communicate with the management device <NUM>.

Routing in a multihop network is performed after several fire alarms <NUM> are provided near the relay device <NUM> instead of being performed after all of the fire alarms <NUM> are provided. After routing for the several fire alarms <NUM> is completed, several additional fire alarms <NUM> are provided, and routing is updated. Thus, routing is updated to adapt to the fire alarms <NUM> additionally provided in steps.

It is assumed here that the n+1th fire alarm 600n+<NUM> and the n+2th fire alarm 600n+<NUM> are provided before the n+3th fire alarm 600n+<NUM> is provided. Each of the n+1th fire alarm 600n+<NUM> and the n+2th fire alarm 600n+<NUM> derives the definitive route cost through the aforementioned process before selecting a relay route based on the definitive route cost. The communication unit <NUM> of each of the n+1th fire alarm 600n+<NUM> and the n+2th fire alarm 600n+<NUM> transmits the information relating to the relay route. The information relating to the relay route includes the definitive route cost. The information relating to the relay route may also include the definitive route cost for a relay route other than the selected relay route (e.g. the substitute route).

The information relating to the relay route transmitted from the n+1th fire alarm 600n+<NUM> and the n+2th fire alarm 600n+<NUM> is transferred along the relay route and received by the relay device <NUM>. The relay device <NUM> transmits the information relating to the relay route to the management device <NUM>. The management device <NUM> receives the information relating to the relay route.

<FIG> shows a configuration of the external appliance <NUM>. The external appliance <NUM> is, for example, a personal computer or a table terminal device. The external appliance <NUM> includes a communication unit <NUM>, a control unit <NUM>, and a display unit <NUM>. The communication unit <NUM> performs a communication process for communicating with the management device <NUM>. The constructor manipulates the external appliance <NUM> to access the management device <NUM>, and the communication unit <NUM> receives the information relating to the relay route from the management device <NUM>. The control unit <NUM> generates a screen based on the information relating to the relay route and displays the generated screen on the display unit <NUM>.

<FIG> shows a screen displayed on the display unit <NUM>. The identification information on each fire alarm <NUM>, the cost, etc. are displayed as the information relating to the relay route. The constructor checks the status of the relay route by checking the information relating to the relay route displayed on the display unit <NUM>.

The constructor additionally provides the n+3th fire alarm 600n+<NUM> as a new fire alarm <NUM> in the multihop network. The plurality of fire alarms <NUM> including the n+1th fire alarm 600n+<NUM>, the n+2th fire alarm 600n+<NUM>, and the n+3th fire alarm 600n+<NUM> update the definitive route cost when the new fire alarm <NUM> is added and updates the relay route based on the updated definitive route cost. The communication unit <NUM> of each of the plurality of fire alarms <NUM> transmits the information relating to the relay route. In a manner as already described, the management device <NUM> receives the information relating to the relay route, and the external appliance <NUM> displays the updated information relating to the relay route.

<FIG> shows the second example. The alarm system <NUM> includes an external appliance <NUM> and an information processing device <NUM> in addition to the features of <FIG>. The external appliance <NUM> is a communication device capable of receiving a signal transmitted from the fire alarm <NUM> and the relay device <NUM>. The information processing device <NUM> is, for example, a computer and is connected to the external appliance <NUM>. The same process as described above is performed in the multihop network. The external appliance <NUM> receives the information relating to the relay route, and the information processing device <NUM> displays the updated information relating to the relay route. The information processing device <NUM> displays the updated information relating to the relay route.

Derivation of the definitive route cost may be started or terminated by an instruction of the constructor. For example, the external appliance <NUM> or the external appliance <NUM> may transmit an instruction to search for a relay route to each fire alarm <NUM> in response to a user operation of the constructor in the user operation unit (not shown) provided in the external appliance <NUM> or the information processing device <NUM>. In that process, each of the plurality of fire alarms <NUM> starts deriving the cost upon receiving the instruction to search for a relay route from the external appliance <NUM> or the external appliance <NUM>.

When a plurality of fire alarms <NUM> are searching for a relay route, the external appliance <NUM> or the external appliance <NUM> may transmit the instruction to search for a relay route to each fire alarm <NUM> in response to a user operation of the constructor in the user operation unit (not shown) provided in the external appliance <NUM> or the information processing device <NUM>. In that process, each of the plurality of fire alarms <NUM> terminates the derivation of the cost upon receiving the instruction to terminate the search for a relay route from the external appliance <NUM> or the external appliance <NUM>.

The user operation unit (not shown) that receives an instruction from the constructor may be provided in each fire alarm <NUM>. Each of the plurality of fire alarms <NUM> starts deriving the cost upon receiving the instruction to search for a relay route. Further, each of the plurality of fire alarms <NUM> terminates the derivation of the cost upon receiving the instruction to terminate the search for a relay route.

As described above, the time slots <NUM> should be assigned to the respective fire alarms <NUM> in accordance with the hop count between the relay device <NUM> and the fire alarm <NUM>. However, assignment may not be in accordance with the hop count in the construction of the alarm system <NUM>. In such an assignment of the time slots <NUM>, a large transfer delay in the communication signal could result. A description will now be given of the process of revising the assignment when an assignment in accordance with the hop count is not made after the alarm system <NUM> is constructed and while the alarm system <NUM> is in operation.

<FIG> show a partial configuration of the alarm system <NUM>. <FIG> shows a configuration of the first stage of the alarm system <NUM>. The m+1th fire alarm <NUM>+<NUM> is connected to the relay device <NUM>, and the m+2th fire alarm <NUM>+<NUM> is connected to the m+1th fire alarm <NUM>+<NUM> to form a multihop network. The hop count between the relay device <NUM> and the m+1th fire alarm <NUM>+<NUM> is "<NUM>", and the hop count between the relay device <NUM> and the m+2th fire alarm <NUM>+<NUM> is "<NUM>".

<FIG> shows an outline of downlink communication in the alarm system <NUM>. "M" denotes the time slot <NUM> assigned to the relay device <NUM>, "S1" denotes the time slot <NUM> assigned to the m+1th fire alarm <NUM>+<NUM>, and "S2" denotes the time slot <NUM> assigned to the m+2th fire alarm <NUM>+<NUM>. As in the foregoing examples, the smaller the hop count, the earlier the time slot <NUM> assigned to the fire alarm <NUM>.

The relay device <NUM> transmits the communication signal in the time slot <NUM> "M", and the m+1th fire alarm <NUM>+<NUM> receives the communication signal in the time slot <NUM> "M". The m+1th fire alarm <NUM>+<NUM> transmits the communication signal in the time slot <NUM> "S1", and the m+2th fire alarm <NUM>+<NUM> receives the communication signal in the time slot <NUM> "S1". The m+2th fire alarm <NUM>+<NUM> transmits the communication signal in the time slot <NUM> "S2". A description of transmission and reception of a response signal is omitted in the above description.

<FIG> shows a configuration of the second stage of the alarm system <NUM>. The configuration is derived from adding the m+3th fire alarm <NUM>+<NUM> to <FIG>. The m+3th fire alarm <NUM>+<NUM> is connected to the relay device <NUM>, the m+1th fire alarm <NUM>+<NUM> is connected to the m+3th fire alarm <NUM>+<NUM>, and the m+2th fire alarm <NUM>+<NUM> is connected to the m+1th fire alarm <NUM>+<NUM> to form a multihop network.

<FIG> show an outline of downlink communication in the alarm system <NUM>. The time slot <NUM> "S3" in <FIG> is provided later than the time slot <NUM> "S2". The newly added m+3th fire alarm <NUM>+<NUM> is allocated to the time slot <NUM> "S3".

The relay device <NUM> transmits the communication signal in the time slot <NUM> "M", and the m+3th fire alarm <NUM>+<NUM> receives the communication signal in the time slot <NUM> "M". The m+3th fire alarm <NUM>+<NUM> transmits the communication signal in the time slot <NUM> "S3", and the m+1th fire alarm <NUM>+<NUM> receives the communication signal in the time slot <NUM> "S3". The m+1th fire alarm <NUM>+<NUM> transmits the communication signal in the time slot <NUM> "S1" of the next frame <NUM>, and the m+2th fire alarm <NUM>+<NUM> receives the communication signal in the time slot <NUM> "S1". The m+2th fire alarm <NUM>+<NUM> transmits the communication signal in the time slot <NUM> "S2".

In other words, the time slot <NUM> "S3", which is later than the time slot <NUM> "S1" assigned to the m+1th fire alarm <NUM>+<NUM> with the hop count "<NUM>", is assigned to the m+3th fire alarm <NUM>+<NUM> with the hop count "<NUM>" to the relay device <NUM> so that a transfer delay is produced. A description of transmission and reception of a response signal is omitted in the above description.

To suppress such a transfer delay, the assignment is modified by the relay device <NUM> or the management device <NUM> after the m+3th fire alarm <NUM>+<NUM> is added. When the time slot <NUM> "S1" and the time slot <NUM> "S2" are provided earlier than the time slot <NUM> "S3" in the downlink communication, for example, the assignment unit <NUM> of the relay device <NUM> modifies the assignment such that the time slot <NUM> "S1" and the time slot <NUM> "S2" are provided later than the time slot <NUM> "S3". The downlink communication is communication on the downlink line, and the signal is transferred in a direction away from the relay device <NUM> in the multihop network.

Further, when the time slot <NUM> "S1" and the time slot <NUM> "S2" are provided later than the time slot <NUM> "S3" in the uplink communication, the assignment unit <NUM> modifies the assignment such that the time slot <NUM> "S1" and the time slot <NUM> "S2" are provided earlier than the time slot <NUM> "S3". The uplink communication is communication on the uplink line, and the signal is transferred in a direction toward the relay device <NUM> in the multihop network.

An identification number for identifying the fire alarm <NUM> is assigned to each fire alarm <NUM>. The identification number is assigned in the order of construction. Referring to <FIG>, therefore, the m+3th fire alarm <NUM>+<NUM> with the identification number "<NUM>", the m+1th fire alarm <NUM>+<NUM> with the identification number "<NUM>", and the m+2th fire alarm <NUM>+<NUM> with the identification number "<NUM>" are arranged in the stated order. To manage a plurality of fire alarms <NUM>, it is preferred that the identification numbers are arranged in the order that the fire alarms <NUM> are arranged along the relay route. The relay device <NUM> or the management device <NUM> determines the identification number of each fire alarm <NUM> in accordance with the hop count. Referring to <FIG>, the identification number "S1" is assigned to the m+3th fire alarm <NUM>+<NUM>, the identification number "<NUM>" is assigned to the m+1th fire alarm <NUM>+<NUM>, and the identification number "<NUM>" is assigned to the m+2th fire alarm <NUM>+<NUM>.

The device, the system, or the entity that executes the method according to the disclosure is provided with a computer. By causing the computer to run a program, the function of the device, the system, or the entity that executes the method according to the disclosure is realized. The computer is comprised of a processor that operates in accordance with the program as a main hardware feature. The disclosure is non-limiting as to the type of the processor so long as the function is realized by running the program. The processor is comprised of one or a plurality of electronic circuits including a semiconductor integrated circuit (IC) or a large-scale integration (LSI). The plurality of electronic circuits may be integrated in one chip or provided in a plurality of chips. The plurality of chips may be aggregated in one device or provided in a plurality of apparatuses. The program is recorded in a non-transitory recording medium such as a computer-readable ROM, optical disk, and hard disk drive. The program may be stored in a recording medium in advance or supplied to a recording medium via wide area communication network including the Internet.

According to the embodiment, the relay route with a smaller cost is preferentially selected. When the power consumption in the fire alarm <NUM> included in the relay route grows large, selection of that relay route is made less likely. Therefore, an increase in the power consumption in the fire alarm <NUM> included in a multihop networks is suppressed. Further, the cost is derived based on the first indicator determined by the link quality information and the second indicator determined by the power consumption in the other fire alarm <NUM>. When a notification is received from the other fire alarm <NUM>, the impact from the second indicator on the cost is increased so that selection of the relay route including the other fire alarm <NUM> can be made less likely. Further, the cost is changed merely by increasing the impact from the second indicator so that the process is simplified. Further, the first threshold value and the second threshold value are defined for power consumption, and the impact from the second indicator on the cost is changed in accordance with the magnitude of power consumption relative to the first threshold value and the second threshold value. Therefore, detailed setting of the relay route is enabled. Further, the power consumption is derived based on the communication frequency so that the power consumption can be estimated easily. Further, the power consumption is derived based on the number of other fire alarms <NUM> directly in communication so that the power consumption can be estimated easily. Further, the power consumption is derived based on the battery level of the fire alarm <NUM> so that the power consumption can be estimated easily.

Further, each of the plurality of fire alarms <NUM> transmits the information relating to the relay route to the external appliance <NUM> or the external appliance <NUM> so that it is possible to provide information for determining at which position the fire alarm <NUM> should be constructed to build a multihop network. Further, the information for determining at which position the fire alarm <NUM> should be constructed is provided so that the multihop network can be built easily. Further, when a new fire alarm <NUM> is added, each of the plurality of fire alarms <NUM> updates the information relating to the relay route and transmits the updated information to the external appliance <NUM> or the external appliance <NUM> so that it is possible to provide information for determining at which position the new fire alarm <NUM> should be constructed. Further, the information relating to the relay route includes the cost so that it is easy to understand the situation of routing. Further, the information relating to the relay route is displayed on the external appliance <NUM> so that it is easy to check the information relating to the relay route.

Further, upon receiving an instruction to search for a relay route from the external appliance <NUM> or the external appliance <NUM>, each of the plurality of fire alarms <NUM> starts deriving the cost. Therefore, a trigger to start deriving the cost can be provided. Further, upon receiving an instruction to terminate the search for a relay route from the external appliance <NUM> or the external appliance <NUM>, each of the plurality of fire alarms <NUM> terminates deriving the cost. Therefore, a trigger to terminate deriving the cost can be provided. Further, upon receiving an instruction to search for a relay route in the user operation unit, each of the plurality of fire alarms <NUM> starts deriving the cost. Therefore, a trigger to start deriving the cost can be provided. Further, upon receiving an instruction to terminate the search for a relay route in the user operation unit, each of the plurality of fire alarms <NUM> terminates deriving the cost. Therefore, a trigger to terminate deriving the cost can be provided.

Further, the order of the time slots <NUM> assigned to the respective fire alarms <NUM> is determined in accordance with the hop count between the fire alarm <NUM> and the relay device <NUM> so that the delay time in transfer in a multihop network can be reduced. Further, the larger the hop count of the fire alarm <NUM>, the later the time slot <NUM> assigned to the fire alarm <NUM> in downlink communication so that the delay time in transfer in a multihop network can be reduced. Further, the larger the hop count of the fire alarm <NUM>, the earlier the time slot <NUM> assigned to the fire alarm <NUM> in uplink communication so that the delay time in transfer in a multihop network can be reduced.

Further, when the fire alarm <NUM> with a large hop count is assigned to an earlier time slot <NUM> in downlink communication, the assignment is modified such that a later time slot <NUM> is assigned to that fire alarm <NUM>. Therefore, the delay time in transfer in a multihop network can be reduced. Further, when the fire alarm <NUM> with a large hop count is assigned to a later time slot <NUM> in uplink communication, the assignment is modified such that an earlier time slot <NUM> is assigned to that fire alarm <NUM>. Therefore, the delay time in transfer in a multihop network can be reduced.

Further, the assignment is modified after the construction of the alarm system <NUM> so that the frequency of assignment modification can be reduced. Further, the identification number of the fire alarm <NUM> is determined in accordance with the hop count so that it is easy to manage the fire alarm <NUM>. The relay device <NUM> performs the assignment so that the relay device <NUM> can manage the assignment. Further, the management device <NUM> performs the assignment so that the management device <NUM> can manage the assignment.

Described above is an explanation based on an exemplary embodiment. The embodiment is intended to be illustrative only and it will be understood by those skilled in the art that various modifications to constituting elements and processes could be developed and that such modifications are also within the scope of the present invention.

The n+3th fire alarm 600n+<NUM> according to the embodiment receiving a notification from the n+1th fire alarm 600n+<NUM> makes it less likely that the fist relay route including the n+1th fire alarm 600n+<NUM> is selected. Alternatively, for example, the n+3th fire alarm 600n+<NUM> receiving a user operation makes it less likely that the first relay route is selected. The user operation is received by, for example, the management device <NUM>, the external appliance <NUM>, the external appliance <NUM>, or the fire alarm <NUM>. According to this variation, the relay route can be modified in accordance with the user's intent.

The fire alarms <NUM> of the embodiment exchange link quality to determine the relay route. Alternatively, however, the fire alarms <NUM> may exchange the value of power consumption. The value of power consumption is reflected in C, i.e., the third term on the right side of expression (<NUM>) and expression (<NUM>). For example, the larger the value of power consumption, the larger the value of "C". According to this variation, the impact of power consumption can be reflected in the tentative route cost or the definitive route cost.

Claim 1:
An alarm system (<NUM>) comprising a plurality of alarms (<NUM>) that form a multihop network extending from a relay device (<NUM>), wherein
the plurality of alarms (<NUM>) include a first alarm (600n+<NUM>), a second alarm (600n+<NUM>), and a third alarm (600n+<NUM>),
the third alarm (600n+<NUM>) is capable of communicating with the first alarm (600n+<NUM>) and the second alarm (600n+<NUM>),
the third alarm (600n+<NUM>) derives a first cost for a first relay route for communicating with the relay device (<NUM>) via the first alarm (600n+<NUM>), by exchanging information on quality of link with the first alarm (600n+<NUM>), and derives a second cost for a second relay route for communicating with the relay device (<NUM>) via the second alarm (600n+<NUM>), by exchanging information on quality of link with the second alarm (600n+<NUM>),
the third alarm (600n+<NUM>) selects the first relay route when the first cost is smaller than the second cost,
when power consumption in the first alarm (600n+<NUM>) grows larger than a threshold value, the first alarm (600n+<NUM>) transmits a notification indicating an increase in power consumption to the third alarm (600n+<NUM>), and
wherein the third alarm (600n+<NUM>) derives the first cost based on a first indicator (Ka×Max) determined by the information on quality of link with the first alarm (600n+<NUM>) and a second indicator (Kb×C) determined by power consumption in the first alarm (<NUM>), and wherein the third alarm (600n+<NUM>) derives the second cost based on a first indicator (Ka×Max) determined by the information on quality of link with the second alarm (600n+<NUM>) and a second indicator (Kb×C) determined by power consumption in the second alarm (600n+<NUM>),
when the third alarm (600n+<NUM>) receives the notification from the first alarm (600n+<NUM>), the third alarm (600n+<NUM>) increases the second indicator in the first cost.