Abstract:
A radio communication method capable of eliminating a blocking effect caused by a collision of many pieces of data is provided. The radio communication method relaying data from a lower terminal to an upper terminal, includes: a step of determining the number of lower terminals connectable to the upper terminal; a step of connecting the lower terminal and the upper terminal based on the determined connectable number; a step of transmitting a first data from the lower terminal to the upper terminal connected to each other; a step of causing the upper terminal to receive the first data transmitted from the lower terminal; and a step of combining the first data received by the upper terminal and a second data to be transmitted from the upper terminal and creating combined data; and a step causing the upper terminal transmitting the combined data to a further upper terminal.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a radio communication method capable of eliminating a blocking effect caused by a collision of many pieces of data in a radio communication system including a plurality of radio terminals. 
       BACKGROUND ART 
       [0002]    In recent years, a small and inexpensive, IEEE 802.15.4 standard-compliant communication device capable of performing low-power digital radio communication is used in a wireless network. 
         [0003]    In an IEEE 802.15.4 standard-compliant network, a tree-type topology is adopted which is configured of a CS (collection state), and at least one node. 
         [0004]    In a tree-type topology, a lower node relays data to an upper node or the CS while combining pieces of data as necessary. 
         [0005]    In such data relaying, since a plurality of pieces of data concentrates on a node which relays data, a data collision occurs, and therefore the success rate of data collection may lower in some cases. 
         [0006]    In order to solve such a problem, collision frequency is lowered by combining a plurality of pieces of data to be relayed and thus reducing the number of times that data is transmitted (refer to Non Patent Literature 1). 
       CITATION LIST 
     Non Patent Literature 
       [0000]    
       
         Non Patent Literature 1: Kojima et al. A study on IEEE 802.15.4e Compliant Low-power Multi-Hop SUN with Frame Aggregation, Proc. ICC2013, June 2013. 
       
     
       SUMMARY OF INVENTION 
     Technical Problem 
       [0008]    Blocking at the time of transmission is eliminated by data combination as described above; however, there is a problem that in the case where many nodes relay combined data to the same node or the CS, a collision of pieces of combined data occurs and a blocking effect still occurs. 
         [0009]    The present invention is made in view of the above-described problem, and the object of the present invention is to provide a radio communication method capable of suppressing a data collision and effectively preventing occurrence of a blocking effect in an upper terminal on a reception side, as well as in a lower terminal on a transmission side. 
       Solution to Problem 
       [0010]    A radio communication method according to a first invention in a tree-type network including a collection station as a root, includes: a step of determining a maximum number of associations, which is a maximum number of lower terminals that can be associated with each of at least one upper terminal including the collection station; a step of determining a communication channel used for transmission and reception of data between each of the at least one upper terminal and each of the lower terminals; a step of causing the at least one lower terminal to be associated with each of the at least one upper terminal based on the maximum number of associations; a step of causing each of the at least one lower terminal associated with each of the at least one upper terminal to create at least one piece of data and to transmit the at least one piece of data to the each of the at least one upper terminal by using the communication channel; and a step of causing the each of the at least one upper terminal to receive the at least one piece of data transmitted from the at least one lower terminal, and in a case where a further upper terminal than the at least one upper terminal exists, further includes: a step of, in a case where the at least one upper terminal creates the data, combining the data created by the at least one upper terminal and the at least one piece of data received from the at least one lower terminal, and in a case where the at least one upper terminal does not create data, combining the at least one piece of data received from the at least one lower terminal; a step of causing the at least one upper terminal to transmit at least one piece of data including the combined data to the further upper terminal by using the communication channel; and a step of causing the further upper terminal to receive the at least one piece of data transmitted from the at least one upper terminal. 
         [0011]    According to a radio communication method according to a second invention, in the first invention, association between each of the at least one upper terminal and the at least one lower terminal is performed by the at least one lower terminal which requests association notifying the each of the at least one upper terminal of an association request, and by the each of the at least one upper terminal authorizing the at least one lower terminal one by one in order of receiving the association request from the at least one lower terminal such that a total number of the at least one lower terminal is equal to or less than the maximum number of associations. 
         [0012]    According to a radio communication method according to a third invention, in the first invention, association between each of the at least one upper terminal and the at least one lower terminal is performed by the at least one lower terminal which requests association notifying the each of the at least one upper terminal of an association request, and by the each of the at least one upper terminal authorizing the at least one lower terminal one by one in order of magnitude of transmitted power of the association request such that a total number of the at least one lower terminal is equal to or less than the maximum number of associations. 
         [0013]    According to a radio communication method according to a fourth invention, in any one of the first to third inventions, association between each of the at least one upper terminal and each of the at least one lower terminal is performed by the each of the at least one upper terminal regularly notifying the each of the at least one lower terminal of a total number of the lower terminals which can be associated at present, which is a difference between the maximum number of associations and a total number of the lower terminals associated at present, and by the each of the at least one lower terminal which is notified transmitting an association request to the each of the at least one upper terminal in a case where the number of the lower terminals which can be associated at present is greater than or equal to 1. 
         [0014]    According to a radio communication method according to a fifth invention, in any one of the first to fourth inventions, the maximum number of associations is determined by setting the maximum number of associations on an upper terminal side in advance. 
         [0015]    According to a radio communication method according to a sixth invention, in any one of the first to fourth inventions, the maximum number of associations is determined by each of the at least one lower radio communication terminal notifying each of the at least one upper terminal of a capacity of a first data, and by the each of the at least one upper terminal determining the maximum number of associations such that a total of the notified capacity of the first data is less than or equal to a predetermined threshold. 
       Advantageous Effects of Invention 
       [0016]    According to the present invention configured as described above, it is possible to suppress a data collision and to effectively prevent occurrence of a blocking effect in the upper terminal on the reception side, as well as in the lower terminal on the transmission side. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0017]      FIG. 1  is a schematic view illustrating an example of a radio communication system to which the present invention is applied. 
           [0018]      FIG. 2  is a sequence chart illustrating a manner of constructing a network topology in a data transmission and reception method according to an embodiment. 
           [0019]      FIG. 3  is a view illustrating a manner of data relaying in the data transmission and reception method according to the embodiment. 
           [0020]      FIG. 4  is a schematic view illustrating a manner of combining pieces of data in the radio communication system according to the embodiment. 
           [0021]      FIGS. 5( a ) to 5( c )  are schematic views illustrating manners in which the radio communication system according to the embodiment performs the radio communication method according to the present invention.  FIG. 5( a )  is a schematic view illustrating a manner in which each meter relays data as it is.  FIG. 5( b )  is a schematic view illustrating a manner in which an upper meter combines pieces of data and relays the data.  FIG. 5( c )  is a schematic view illustrating a manner in which the number of lower meters which can be associated with an upper meter is limited and pieces of data are combined and relayed. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0022]    Hereinafter, a radio communication method as an embodiment of the present invention will be described in detail. 
         [0023]      FIG. 1  is a schematic view illustrating an example of a radio communication system  1  to which the present invention is applied. 
         [0024]    The radio communication system  1  is configured by including as radio communication terminals, a collection station CS, and four meters, a meter M 1 , a meter M 2 , a meter M 3 , and a meter M 4 . 
         [0025]    The collection station CS is a top master device, the meters M 1  and M 2  are slave devices to the collection station CS, and the meters M 3  and M 4  are slave devices to the meter M 1 . 
         [0026]    Here, an IEEE 802.15.4 standard-compliant network is configured of devices roughly classified into two types, a full function device (FFD) and a reduced function device (RFD). 
         [0027]    The FFD corresponds to the collection station CS and the meter M 1  according to the present embodiment, and is a full-function device having the function of authorizing a new device intending to join a personal area network (PAN) to which the FFD itself belongs to join the PAN, and the function of defining a superframe and a communication channel (frequency and time) used for communication with another device. 
         [0028]    From among such FFDs, the FFD further having the function of determining the ID for the entire network is referred to as a PAN coordinator. There is a single PAN coordinator for each network. In this present embodiment, the collection station CS corresponds to this. 
         [0029]    In contrast, the RFD is a device which does not have the above functions which the FFD has, that is, the function of authorizing a device to join the PAN and the function of defining a superframe. The RFD is a reduced-function device which has the same functions as those of the FFD except these functions. In this present embodiment, meters M 2 , M 3 , and M 4  correspond to this. 
         [0030]    Next, construction of a network topology in this radio communication system  1  will be described.  FIG. 2  is a sequence chart illustrating a manner of constructing a network topology in a data transmission and reception method according to the embodiment. 
         [0031]    First, the collection station CS is powered on, the collection station CS performs active scanning, and thus the construction of a network topology is initiated (step S 1 ) 
         [0032]    Next, the collection station CS defines a PANID and a superframe for itself and creates a PAN (step  52 ). 
         [0033]    Next, the meter M 1  is powered on, and the meter M 1  performs active scanning (step  53 ). 
         [0034]    In the active scanning performed by the meter M 1 , first, the meter M 1  broadcasts a scan request (step S 4 ). 
         [0035]    Then, the collection station CS, which is the FFD existing within a receivable range of the scan request broadcasted from the meter M 1 , unicasts a response to the received scan request (scan response) to the meter M 1  (step S 5 ). 
         [0036]    This scan response includes information of the number of meters which can be associated with the collection station CS at present, which is the difference between the association maximum number, which is the maximum number of meters that can be associated with the collection station CS and the number of meters which are associated with the collection station CS at present. The maximum number of associations is set in advance for the collection station CS. 
         [0037]    Next, the meter M 1  receives this scan response, and then the meter M 1  finds the collection station CS. In the case where the number of meters which can be associated with the collection station CS at present is not 0, the meter M 1  recognizes that the collection station CS is a device which can be a master device for itself, and active scanning of the meter M 1  is terminated. 
         [0038]    Next, the meter M 1  initiates association with the PAN of the collection station CS (step S 6 ). 
         [0039]    In the association of the meter M 1 , first, the meter M 1  unicasts an association request to the collection station CS which can be a master device (step S 7 ). 
         [0040]    Next, the collection station CS which has received the association request from the meter M 1  compares the maximum number of associations set in advance for itself and the number of meters which are connected to the collection station CS at present, and calculates the number of meters which can be associated at present. 
         [0041]    In the case where the number of meters which can be associated at present is greater than or equal to 1, the collection station CS unicasts an association response to the meter M 1  (step S 8 ). In the case where the number of meters which can be associated at present is 0, an association response is not transmitted. 
         [0042]    In the case where the association response is transmitted, the meter M 1  receives this. Then, association of the meter M 1  whose master device is the collection station CS to the PAN has been completed. 
         [0043]    When association between the collection station CS and the meter M 1  has been completed as described above, the collection station CS and the meter M 1  can transmit and receive data to and from each other (step S 9 ). 
         [0044]    Next, the meter M 2  is powered on, and the meter M 2  performs active scanning (step S 11 ). 
         [0045]    In the active scanning performed by the meter M 2 , first, the meter M 2  broadcasts a scan request (step S 12 ). 
         [0046]    Then, when the collection station CS and the meter M 1 , which are the FFDs existing within a receivable range of the scan request broadcasted from the meter M 2 , receive the scan request, the collection station CS and the meter M 1  unicast a scan response to the meter M 2  (step S 13 ). 
         [0047]    Also this scan response includes information of the number of meters which can be associated with the collection station CS or the meter M 1  at present, which is the difference between the maximum number of associations for the collection station CS or the meter M 1  and the number of meters which are associated at present. The maximum number of associations is also set for the meter M 1  in advance. 
         [0048]    In the case where the meter M 2  receives this scan request and the number of meters which can be associated with the collection station CS and the meter M 1  at present is not 0, the meter M 2  finds the collection station CS and the meter M 1  as devices which can be master devices. 
         [0049]    In the case where there is a plurality of devices which can be master devices, a device recognizes a device with a higher priority in becoming a master device according to a predetermined standard. In the present embodiment, the meter M 2  recognizes according to the predetermined standard that the collection station CS is a device with a higher priority from among the two meters which can be master devices. Thus, active scanning performed by the meter M 2  is terminated. 
         [0050]    This priority is determined according to a predetermined standard such as the distance to the collection station CS, the distance from a device which transmits a scan request, transmission order of scan requests, or the like. In the present embodiment, priority is determined according to the distance to the collection station CS, and the collection station CS itself is the device with the highest priority. As described, since the priority is determined according to a predetermined standard, it is possible to quickly construct a PAN. 
         [0051]    Next, the meter M 2  initiates association to the PAN of the collection station CS. 
         [0052]    In association of the meter M 2 , first, the meter M 2  unicasts an association request to the collection station CS, which is the device with the above higher priority (step S 15 ). 
         [0053]    Next, the collection station CS which has received the association request from the meter M 2  compares the maximum number of associations set for the collection station CS itself in advance and the number of meters connected to the collection station CS at present. 
         [0054]    Then, in the case where the number of meters which can be associated with the collection station CS at present is greater than or equal to 1, the collection station CS unicasts an association response to the meter M 2  (step S 16 ). In the case where the number of meters which can be associated at present is 0, an association response is not transmitted. 
         [0055]    In the case where the association response is transmitted, the meter M 2  receives this. Then, association of the meter M 2  whose master device is the collection station CS to the PAN has been completed. 
         [0056]    When association between the collection station CS and the meter M 2  has been completed as described above, the collection station CS and the meter M 2  can transmit and receive data to and from each other (step S 17 ). 
         [0057]    Next, the meter M 3  is powered on, and the meter M 3  performs active scanning (step S 21 ). 
         [0058]    In the active scanning performed by the meter M 3 , first, the meter M 3  broadcasts a scan request (step S 22 ). 
         [0059]    Then, when the meter M 1  existing within a receivable range of the scan request broadcasted from the meter M 3  receives the scan request, the meter M 1  unicasts a scan response to the meter M 3  (step S 23 ). 
         [0060]    Also this scan response includes information of the number of meters which can be associated with the meter M 1  at present, which is the difference between the maximum number of associations, which is the maximum number of meters that can be associated with the meter M 1  and the number of meters which are associated with the meter M 1  at present. 
         [0061]    In the case where the meter M 3  receives this scan request and the number of meters that can be associated with the meter M 1  at present is not 0, the meter M 3  finds the meter M 1  which is a device that can be a master device, and active scanning performed by the meter M 3  is terminated. 
         [0062]    Next, the meter M 3  initiates association to the PAN of the collection station CS. 
         [0063]    In association of the meter M 3 , first, the meter M 3  unicasts an association request to the meter M 1  which can be a master device (step S 25 ). 
         [0064]    Next, the meter M 1  which has received the association request from the meter M 3  compares the maximum number of associations set for the meter M 1  itself in advance and the number of meters connected to the meter M 1  at present. 
         [0065]    Then, in the case where the number of meters which can be associated with the meter M 1  at present is greater than or equal to 1, the meter M 1  unicasts an association response to the meter M 3  (step S 26 ). In the case where the number of meters which can be associated with the meter M 1  at present is 0, an association response is not transmitted. 
         [0066]    In the case where the association response is transmitted, the meter M 3  receives the association response. Then, association of the meter M 3  whose master device is the meter M 1  to the PAN has been completed. 
         [0067]    When association between the meter M 1  and the meter M 3  has been completed as described above, the meter M 1  and the meter M 3  can transmit and receive data to and from each other. Association between the meter M 1  and the meter M 4  is performed in the same manner as in the association between the meter M 1  and the meter M 3 . 
         [0068]    In the above-described present embodiment, a coordinator, which is an upper terminal, establishes connection with meters, which are lower terminals, one by one in order of arrival, that is, in chronological order of transmission of association requests from the meters until the maximum number of associations is reached. 
         [0069]    Next, an example of data relaying in the data transmission and reception method according to the present embodiment will be described.  FIG. 3  is a view illustrating a manner of data relaying in the data transmission and reception method according to the present embodiment. 
         [0070]    As illustrated in  FIG. 3 , in the data transmission and reception method according to the present embodiment, transmission and reception of data between the collection station CS as a master device and the meter M 1  or M 2  as a slave device to the collection station CS is performed in accordance with a superframe defined by the collection station CS and a communication channel (frequency, time) defined by the collection station CS. 
         [0071]    In addition, transmission and reception of data between the meter M 1  as a master device and the meter M 3  or M 4  as a slave device to the meter M 1  is performed in accordance with a superframe defined by the meter M 1  and a communication channel (frequency, time) defined by the meter M 1 . 
         [0072]    In the above-described transmission and reception of data between the collection station CS and the meter M 1  or M 2  and the above-described transmission and reception of data between the meter M 1  and the meter M 3  or M 4  as a slave device, an active period is defined as superframe duration (SD), and is configured of a contention access period (CAP) and a contention free access period (CFP). 
         [0073]    The CAP is a period during which the master device and all the slave devices that can perform communication are allowed to transmit and receive information to and from each other. 
         [0074]    In contrast, the CFP is a period during which only one slave device assigned by the master device is allowed to transmit and receive information. During the CFP, each slave device can transmit and receive information to and from the master device only within a guaranteed time slot (GTS), which is a slot assigned to the slave device itself. 
         [0075]    Devices transmit and receive a data frame to and from each other only within the above-described CAP or CFP. 
         [0076]    In the data transmission and reception method according to the present embodiment, transmission and reception of a data frame is initiated within the CAP. In contrast, the transmission and reception has not necessarily been completed within the CAP. In the case where transmission and reception of a data frame has not been completed within the CAP, transmission and reception of data continues even after completion of the CAP until the transmission and reception has been completed. 
         [0077]    Therefore, since the CAP can be made shorter than that in a conventional data transmission and reception method, the active period can be reduced and a non-active period, that is, the period during which a device can be in a sleep mode can be increased by the reduced amount of time. Thus, power consumption of the device can be effectively suppressed. 
         [0078]    In addition, a beacon signal is transmitted from the master device only when synchronization between the master device and the slave device is established according to a beacon interval. After the synchronization is established, transmission of a beacon signal is stopped. 
         [0079]    Therefore, power consumption caused by unnecessary transmission and reception of a beacon signal after establishment of synchronization can be effectively suppressed. 
         [0080]    Note that transmission of a beacon signal may be resumed as necessary in the case where synchronization is no longer maintained, or the like. 
         [0081]    Next, data combination performed in the above-described radio communication system  1  will be described.  FIG. 4  is a schematic view illustrating a manner of combining pieces of data in a radio communication system  1  according to the present embodiment. 
         [0082]    The radio communication system  1  illustrated in  FIG. 4  is different from the radio communication system  1  illustrated in  FIG. 1  in that it is configured of a collection station CS and 10 meters, meters M 1  to M 10 . A piece of data created and transmitted by each of the meters M 1  to M 10  is denoted by the same number as that of the meter which creates and transmits the piece of data. That is, pieces of data are referred to as data  1  to  10 . 
         [0083]    In the data transmission and reception method according to the present embodiment, every time data is sequentially relayed and collected from the lowest meter to the collection station CS in the data transmission and reception method according to the above-described first embodiment, a sync part and a header part are newly created and payload from each meter is combined with the data. 
         [0084]    Therefore, the size of redundant parts such as the sync part and the header part can be reduced, and the time required for data transmission and reception can be reduced. 
         [0085]    In addition, by reducing the time required for data transmission and reception, the device which transmits and receives data after completion of the CAP can be put into a sleep mode in a shorter time. Therefore, power consumption can be suppressed. 
         [0086]    Next, the manner in which the above-described radio communication system  1  performs the radio communication method according to the present invention will be described. 
         [0087]      FIGS. 5( a ) to 5( c )  are schematic views illustrating manners in which the radio communication system  1  according to the present embodiment performs the radio communication method according to the present invention.  FIG. 5( a )  is a schematic view illustrating a manner in which each meter relays data as it is.  FIG. 5( b )  is a schematic view illustrating a manner in which an upper meter combines pieces of data and relays the data.  FIG. 5( c )  is a schematic view illustrating a manner in which the number of lower meters which can be associated with an upper meter is limited and pieces of data are combined and relayed. 
         [0088]    In the state illustrated in  FIG. 5( a ) , many pieces of data  1 ,  2 ,  3 ,  4 ,  5 ,  6 , and  7  are transmitted at the same time to the collection station CS. These pieces of data collide with each other, and a blocking effect occurs. 
         [0089]    In addition, in the state illustrated in  FIG. 5( b ) , by combining pieces of data transmitted from lower meters at an upper meter, the number of times that data is transmitted is reduced; however, a blocking effect still occurs when the collection station CS receives data. 
         [0090]    Therefore, in the present invention, as illustrated in  FIG. 5( c ) , control is performed so that the maximum number of associations of lower meters which can be associated with the collection station CS or an upper mater which serves as a coordinator is limited to a predetermined number or less. In the example of  FIG. 5( c ) , the maximum number of associations is set to 2. 
         [0091]    Therefore, only two meters can be connected to the collection station CS. In  FIG. 5( c ) , two meters, the meters M 1  and M 2 , are connected. 
         [0092]    In addition, from among the meters which serve as coordinators and to which a plurality of meters is connected, only one meter, the meter M 4 , is connected to the meter M 1 , only two meters, the meters M 3  and M 5 , are connected to the meter M 2 , and only two meters, the meters M 6  and M 7 , are connected to the meter M 4 . 
         [0093]    Furthermore, as illustrated in  FIG. 5( c ) , pieces of data transmitted from lower meters are combined at an upper coordinator. Thus, the number of pieces of data that an upper meter simultaneously receives can be reduced. 
         [0094]    As described above, since an upper coordinator combines pieces of data transmitted from lower meters and relays the combined data and the number of lower meters which can be connected to the coordinator is limited, a data collision can be suppressed and occurrence of a blocking effect can be effectively prevented in an upper terminal on a reception side as well as in a lower terminal on a transmission side. 
         [0095]    Note that in the above-described embodiment, a coordinator and lower meters are connected in chronological order of transmission of association requests from the meters. 
         [0096]    However, the present invention is not limited to this, and connection is performed in order of magnitude of transmitted power of the association requests until an association possible number is reached. 
         [0097]    In addition, in the above-described embodiment, the maximum number of associations is set in advance on the side of a coordinator which is an upper terminal. 
         [0098]    However, the present invention is not limited to this. The maximum number of associations may be determined as follows. A lower meter notifies an upper coordinator of the capacity of data to be transmitted from itself, and the coordinator determines the maximum number of associations so that the total capacity of data notified by the respective lower meters becomes less than or equal to a predetermined threshold. 
         [0099]    In addition, an aspect may be adopted where in the above-described radio communication system  1 , in the case where each mater which relays data does not create data by itself, each meter combines at least one piece of data received from lower meters and transmits the data to an upper meter. In addition, the number of pieces of data that each meter creates and transmits at a time is not limited to 1 but may be 2 or greater. 
       REFERENCE SIGNS LIST 
       [0000]    
       
           1 : radio communication system 
         CS: collection station 
         M 1 , M 2 , M 3 , M 4 : meter 
         M 5 , M 6 , M 7 , M 8 : meter 
         
           FIG. 2 
         
         COLLECTION STATION CS 
         METER M 1   
         METER M 2   
         METER M 3   
         (PAN COORDINATOR) 
         M 1 , M 2 , M 3 : OTHER METERS 
         (COORDINATOR OR TERMINAL) 
         SCAN 
         POWER-ON 
         S 1  ACTIVE SCANNING 
         S 2  PAN IS STARTED 
         S 3  ACTIVE SCANNING 
         S 4  SCAN REQUEST 
         S 5  SCAN RESPONSE 
         S 6  ASSOCIATION 
         S 7  ASSOCIATION REQUEST 
         S 8  ASSOCIATION RESPONSE 
         S 9  DATA TRANSMISSION AND RECEPTION 
         S 11  ACTIVE SCANNING 
         S 12  SCAN REQUEST 
         S 13  SCAN RESPONSE 
         S 14  ASSOCIATION 
         S 15  ASSOCIATION REQUEST 
         S 16  ASSOCIATION RESPONSE 
         S 17  DATA TRANSMISSION AND RECEPTION 
         S 21  ACTIVE SCANNING 
         S 22  SCAN REQUEST 
         S 23  SCAN RESPONSE 
         S 24  ASSOCIATION 
         S 25  ASSOCIATION REQUEST 
         S 26  ASSOCIATION RESPONSE 
         S 27  DATA TRANSMISSION AND RECEPTION 
         
           FIG. 3 
         
         BEACON INTERVAL 
         SUPERFRAME DURATION 
         BEACON (INACTIVE) 
         ACTIVE PERIOD 
         DATA FRAME 
         NON-ACTIVE PERIOD (SLEEP PERIOD) 
         RECEPTION BY CS 
         BI SET BY CS 
         DATA TRANSMITTED FROM M 1  TO CS 
         SD SET BY CS 
         TRANSMISSION FROM M 1 /M 2   
         RECEPTION BY M 1   
         BI SET BY M 1   
         DATA TRANSMITTED FROM M 3  TO M 1   
         SD SET BY M 1   
         TRANSMISSION FROM M 3 /M 4   
         STATE OF M 1   
         RECEPTION/STANDBY PERIOD 
         TRANSMISSION PERIOD 
         SLEEP PERIOD 
         
           FIG. 5 
         
         COLLECTION STATION 
         NODE 
         COORDINATOR 
         LINKS SHARING SUPERFRAME 
         NORMAL FRAME 
         RELAYED FRAME 
         COMBINED FRAME