Abstract:
Provided is a Media Access Control (MAC) technology that may improve a data transmission performance in an asynchronous sensor network. The MAC technology may increase a waked-up time interval of a reception node to thereby continue to receive traffic from a transmission node, and change a frequency band where data is transmitted and received to thereby reduce occurrence of collision. Also, when the wake-up time interval of the reception node is similar to a wake-up time of a neighboring node, the wake-up time interval of the reception node may be moved to another time interval, thereby improving the data transmission performance.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the priority benefit of Korean Patent Application Nos. 10-2009-0042983, filed on May 18, 2009, and 10-2010-0046276, filed on May 18, 2010, in the Korean Intellectual Property Office, the disclosures of which are incorporated herein by reference. 
       BACKGROUND 
       [0002]    1. Field 
         [0003]    One or more embodiments relate to a sensor network, and more particularly, to a to method of improving a data transmission performance and reliability in a Media Access Control (MAC) protocol for an asynchronous sensor network. 
         [0004]    2. Description of the Related Art 
         [0005]    A sensor network may be a technique that provides services such as monitoring, tracking, reconnaissance, automation, and the like using sensor nodes having sensing, processing, and communication abilities. The sensor nodes included in the sensor network may be typically battery-operated. To maximize a life span of the sensor network, a duty cycle may need to be reduced to reduce an amount of battery consumption. To reduce the duty cycle, an asynchronous Media Access Control (MAC) protocol such as an X-MAC may be used. However, as for the asynchronous MAC protocol, contention may become severe along with an increase in a number of nodes desiring to transmitting data, resulting in a significant reduction in a data transmission performance and reliability. 
       SUMMARY 
       [0006]    One or more embodiments provide a method of improving a data transmission performance and reliability of an asynchronous Media Access Control (MAC) protocol used in a sensor network. 
         [0007]    According to an aspect of one or more embodiments, there may be provided a transmission node, including: a preamble transmission unit to transmit a preamble to a reception node using a first frequency band; a preamble acknowledgement character (ACK) reception unit to receive an ACK for the preamble from the reception node using the first frequency band; a data transmission unit to transmit data to the reception node, using a second frequency band different from the first frequency band; and a data ACK reception unit to receive, using the second frequency band, an ACK for the data from the reception node. 
         [0008]    According to another aspect of one or more embodiments, there may be provided a to reception node, including: a preamble reception unit to receive, using a first frequency band, a preamble from a transmission node; a preamble ACK transmission unit to transmit, using the first frequency band, an ACK for the preamble to the transmission node; a data reception unit to receive data from the transmission node, using a second frequency band different from the first frequency band; and an ACK transmission unit to transmit, using the second frequency band, an ACK for the data to the transmission node. 
         [0009]    According to still another aspect of one or more embodiments, there may be provided a reception node, including: a data reception unit to receive first data from a first transmission node; a data ACK transmission unit to transmit, to the first transmission node, an ACK for the first data; a preamble reception unit to receive, from a second transmission node, a preamble in a predetermined waiting time after transmitting the ACK for the first data; and a preamble ACK reception unit to transmit, to the second transmission node, the ACK for the preamble when receiving the preamble from the second transmission node in the predetermined waiting time. 
         [0010]    According to yet another aspect of one or more embodiments, there may be provided a reception node, including: a packet information reception unit to receive, from a transmission node, information about a data packet to be transmitted by the transmission node; and a data packet reception unit to consecutively receive a plurality of data packets from the transmission node, the plurality of data packets being based on the information about the data packet. 
         [0011]    According to further aspect of one or more embodiments, there may be provided a sensor node, including: a preamble reception unit to receive, from a transmission node, a preamble in a predetermined waiting time; a data transmission unit to transmit data to a reception node in a data transmission period in time; and a control unit to move the waiting time to enable the data to be preferentially transmitted when the data transmission period and the waiting time are overlapped. 
         [0012]    According to further aspect of one or more embodiments, there may be provided a transmission node, including: a Clear Channel Assessment (CCA) performing unit to perform a CCA procedure verifying whether a channel is available in a first period in time; a mode conversion unit to convert a reception mode to a transmission mode for a second period in time when the channel is available; a preamble transmission unit to transmit a preamble to the reception node using the channel; and a preamble ACK reception unit to receive, from the reception node, an ACK for the preamble for a third period in time, wherein a length of the first period is longer than a sum of a length of the second period and a length of the third period. 
         [0013]    Additional aspects of embodiments will be set forth in part in the description which follows and, in part, will be apparent from the description, or may be learned by practice of the disclosure. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    These and/or other aspects will become apparent and more readily appreciated from the following description of embodiments, taken in conjunction with the accompanying drawings of which: 
           [0015]      FIG. 1  is a diagram illustrating an example of performing a frequency-hopping according to an embodiment; 
           [0016]      FIG. 2  is a diagram illustrating an example of postponing the entry into a sleep mode during a waiting time interval according to an embodiment; 
           [0017]      FIG. 3  is a block diagram illustrating a structure of a transmission node according to an embodiment; 
           [0018]      FIG. 4  is a block diagram illustrating a structure of a reception node according to an embodiment; 
           [0019]      FIG. 5  is a diagram illustrating an example of transmitting a burst traffic according to an embodiment; 
           [0020]      FIG. 6  is a block diagram illustrating a structure of a reception node receiving a burst traffic according to another embodiment; 
           [0021]      FIG. 7  is a diagram illustrating an example of changing a starting point in time of a wake-up interval when a data packet to be transmitted is generated while receiving a preamble according to an embodiment; 
           [0022]      FIG. 8  is a diagram illustrating an example of changing a staring point in time of a wake-up interval when a preamble reception is generated while transmitting a data packet according to an embodiment; 
           [0023]      FIG. 9  is a block diagram illustrating a structure of a sensor node preferentially transmitting a data packet according to an embodiment; 
           [0024]      FIG. 10  is a diagram illustrating an example where a collision is generated because a time interval where a Clear Channel Assessment (CCA) is performed is shorter than a time interval where a turn operation is performed according to an embodiment; 
           [0025]      FIG. 11  is a flowchart illustrating a method of repeatedly performing a CCA to suppress occurrence of collision according to an embodiment; and 
           [0026]      FIG. 12  is a block diagram illustrating a structure of a transmission node that repeatedly performing a CCA to suppress occurrence of collision according to an embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0027]    Reference will now be made in detail to embodiments, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. Embodiments are described below to explain the present disclosure by referring to the figures. 
         [0028]      FIG. 1  is a diagram illustrating an example of performing a frequency-hopping according to an embodiment. 
         [0029]    A preamble packet including a preamble that is transmitted to a reception node  130  by a transmission node  160  is illustrated in a data packet (a) of  FIG. 1 , and a preamble acknowledge character (ACK) packet including a preamble ACK that is transmitted to the transmission node  160  by the reception node  130  in a data packet (b) of  FIG. 1 . In a section (c) of  FIG. 1 , the transmission node  160  and the reception node  130  transmit data using a frequency-hopping. 
         [0030]    The reception node  130  may switch a sleep mode to an active mode for each predetermined wake-up interval  140 , and receive the preamble in a waiting time interval  141 . When the reception node  130  receives the preamble from the transmission node  160 , the reception node  130  may determine that a data packet to be received from the transmission node  160  exists. 
         [0031]    When the reception node  130  fails to receive the preamble, the reception node  130  may switch the active mode to the sleep mode again, thereby reducing power consumption. 
         [0032]    Dotted lines of  FIG. 1  may signify that each of the reception node  130  and the transmission node  160  stays in the sleep mode, and time intervals illustrated as ‘On’ of  FIG. 1  may signify that each of the reception node  130  and the transmission node  160  stays in the active mode. 
         [0033]    In  FIG. 1 , the transmission node  160  may switch the sleep mode to an active mode  170 , and transmit the preamble in time intervals  171  and  172 , using a first frequency band. The transmission node  160  may transmit a first preamble in the time interval  171 , however, the reception node  130  may fail to receive the transmitted first preamble because the reception node  130  stays in the sleep mode. 
         [0034]    The transmission node  160  may transmit, in the time interval  172 , the preamble using the first frequency band. The reception node  130  may receive, in a time interval  151 , the preamble using the first frequency band. The reception node  130  may transmit, in a time interval  152 , an ACK for the preamble using the first frequency band. The transmission node  160  may receive an ACK for the preamble, in a time interval  173 , using the first frequency band. 
         [0035]    The transmission nod  160  and the reception node  130  may perform a frequency-hopping, in time intervals  174  and  153 , from the first frequency band to a second frequency band. The first frequency band and the second frequency band are different from each other. 
         [0036]    The transmission node  160  may transmit, in a time interval  175 , a data packet to the reception node  130 , using the second frequency band. The reception node  130  may receive, in a time interval  154 , the data packet from the transmission node  160  using the second frequency band. The reception node  130  may transmit, in a time interval  155 , an ACK for the data packet using the second frequency band. The transmission node  160  may receive, in a time interval  176 , the ACK for the data packet from the reception node  130  using the second frequency band. When the above described data transmission is completed, the transmission node  160  and the reception node  130  may perform, in time intervals  178  and  156 , a frequency-hopping from the second frequency band to the first frequency band. 
         [0037]    According to an embodiment, the reception node  130  may determine the second frequency band. The transmission node  160  may transmit, to the reception node  130 , information  112  about a frequency band where data is transmitted. According to an embodiment, the transmission node  160  may enable the information  112  about the frequency band to be included in a part of a preamble packet  110  including a preamble  111 , and transmit the preamble packet  110 . 
         [0038]    The reception node  130  may select, as the second frequency band, one frequency band from among frequency bands capable of transmitting data, and transmit, to the transmission node  160 , information  122  about the selected second frequency band. According to an embodiment, the reception node  130  may enable the information  122  about the second frequency band to be included in a part of a preamble ACK packet  120  including an ACK for the preamble  111 , and transmit the preamble ACK packet  120 . 
         [0039]    According to another embodiment, the transmission node  160  may select the second frequency band. The transmission node  160  may select the second frequency band, enable the information  122  about the second frequency band to be included in the part of the preamble packet  110  including the preamble  111 , and transmit the preamble packet  110 . 
         [0040]      FIG. 2  is a diagram illustrating an example of postponing the entry into a sleep mode during a waiting time according to an embodiment. 
         [0041]    A reception node  210  may switch a sleep mode to an active mode for each wake-up interval  220 , and receive a preamble. When the reception node  210  receives the preamble in a waiting time interval  221 , the reception node  210  may determine that a data packet to be received from transmission nodes  250  and  260  exists. When the reception node  210  fails to receive the preamble in the waiting time interval  221 , the reception node  210  may determine that the data to be received does not exist, and switch the active mode to the sleep mode again. 
         [0042]    At a first point in time  259 , a data packet to be transmitted to the reception node  210  by a first transmission node  250  may be generated. Also, at a second point in time  268 , a data packet to be transmitted to the reception node  210  by a second transmission node  260  may be generated. The first transmission node  250  and the second transmission node  260  may transmit preambles  252  and  262 , respectively. The preamble  252  of the first transmission node  250  and the preamble  262  of the second transmission node  260  may be competed in accordance with a carrier sense multiple access with collision detection (CSMA/CD) algorithm. 
         [0043]    It may be assumed that the reception node  210  receives the preamble  262  of the second transmission node  260 . The reception node  210  may receive, from the second transmission node  260 , a preamble  231  using a first frequency band, and receive data  234  using a second frequency band. The reception node  210  may transmit an ACK  235  for data using the second frequency band. The second transmission node  260  may receive an ACK  266  for data using the second frequency band. 
         [0044]    The reception node  210  and the second transmission node  260  may perform, in intervals  236  and  267 , a frequency-hopping from the second frequency band to the first frequency band. 
         [0045]    According to an embodiment, the reception node  210  may not switch the active mode to the sleep mode immediately after performing the frequency-hopping, and receive, from the first transmission node  250 , a preamble  241  using the first frequency band in a predetermined waiting time interval. When the reception node  210  fails to receive the preamble  241  in the predetermined waiting time interval, the reception node  210  may switch the active mode to the sleep mode. 
         [0046]    When the reception node  210  receives, from the first transmission node  250 , the preamble  241  using the first frequency band within the predetermined waiting time interval, the reception node  210  may repeatedly perform the above described data reception procedure for the first transmission node  250 . 
         [0047]    The reception node  210  may receive again, from another transmission node, a preamble using the first frequency band in a predetermined waiting time interval  247  after completing the data reception procedure for the first transmission node  250 . 
         [0048]      FIG. 3  is a block diagram illustrating a structure of a transmission node  300  according to an embodiment. 
         [0049]    The transmission node  300  includes a preamble transmission unit  310 , a preamble ACK reception unit  320 , a data transmission unit  330 , and a data ACK reception unit  340 . 
         [0050]    The preamble transmission unit  310  may transmit, to the reception node  350 , a preamble using a first frequency band. The reception node  350  may receive the preamble using the first frequency band, and transmit an ACK for the preamble to the transmission node  300 . 
         [0051]    The preamble ACK reception unit  320  may receive the ACK for the preamble from the reception node  350 , using the first frequency band. 
         [0052]    According to an embodiment, the preamble transmission unit  310  may transmit, to the reception node  350 , information about at least one frequency band where the transmission node  300  transmits data to the reception node  350 , in addition to the preamble. In this case, the reception node  350  may select, as a second frequency band, an optimal frequency band where data is received from the transmission node  300 , based on information about a frequency band transmitted by the transmission node  300 . The preamble ACK reception unit  320  may additionally receive information about the second frequency band, in addition to the ACK for the preamble. The first frequency band and the second frequency band may be different from each other. 
         [0053]    According to another embodiment, the transmission node  350  may determine the second frequency band. The preamble transmission unit  310  may select, as the second frequency band, an optimal frequency band from among frequency bands where the transmission node  300  transmits data to the reception node  350 , and transmit, to the reception node  350 , the information about the second frequency band, in addition to the preamble. The reception node  350  may transmit data based on the information about the second frequency band received together with the preamble. 
         [0054]    The data transmission unit  330  may perform a frequency-hopping from the first frequency band to the second frequency band. The data transmission unit  330  may transmit data to the reception node  350  using the second frequency band. 
         [0055]    The data ACK reception unit  340  may receive, from the reception node  350 , an ACK for data using the second frequency band. The data ACK reception unit  340  may perform a frequency-hopping from the second frequency band to the first frequency band. 
         [0056]    The reception node  350  having received data may forward data to the second reception node  360 . 
         [0057]      FIG. 4  is a block diagram illustrating a structure of a reception node  400  according to an embodiment. 
         [0058]    The reception node  400  includes a preamble reception unit  410 , a preamble ACK transmission unit  420 , a data reception unit  430 , and a data ACK transmission unit  440 . 
         [0059]    The preamble reception unit  410  may receive, from a first transmission node  450 , a preamble using a first frequency band. 
         [0060]    The preamble ACK transmission unit  420  may transmit, to the first transmission node  450 , an ACK for the preamble using the first frequency band. 
         [0061]    The reception node  400  may select a second frequency band where the reception node  400  receives data from the first transmission node  450 . According to an embodiment, the preamble reception unit  410  may additionally receive information about at least one frequency band where data is received from the first transmission node  450 , in addition to the preamble. The preamble ACK transmission unit  420  may select, as the second frequency band, an optimal frequency band from among the at least one frequency band, and transmit, to the first transmission node  450 , information about the second frequency band in addition to the ACK for the preamble. 
         [0062]    According to another embodiment, the first transmission node  450  may select a second frequency band where the reception node  400  receives data from the first transmission node  450 . The first transmission node  450  may select the second frequency band from among at least one frequency band where the first transmission node  450  transmits data to the reception node  400 . The preamble reception unit  410  may receive information about the second frequency band in addition to the preamble. 
         [0063]    The data reception unit  430  may receive data from the first transmission node  450  using the second frequency band. The first frequency band and the second frequency band may be different from each other. 
         [0064]    The data ACK transmission unit  440  may transmit, to the first transmission node  450 , an ACK for data using the second frequency band. By transmitting the ACK for the data, a data reception procedure may be completed. When the data reception procedure is completed, the data ACK transmission unit  440  may perform a frequency-hopping from the second frequency band to the first frequency band. 
         [0065]    According to an embodiment, the preamble reception unit  410  may receive, from a second transmission node  460 , a second preamble using the first frequency band in a predetermined waiting time interval after transmitting the data. 
         [0066]    When the preamble reception unit  410  receives the second preamble, the preamble ACK reception unit  420  may transmit, to the second transmission node  460 , an ACK for the second preamble using the first frequency band. 
         [0067]    The data reception unit  430  may perform a frequency-hopping from the first frequency band to a third frequency band based on the second preamble, and additionally receive second data from the second transmission node  460 . According to an embodiment, the second frequency band and the third frequency band may be the same. 
         [0068]    Since the reception node  400  additionally receives data in the active mode without switching the active mode to the sleep mode after transmitting the data, a time required for switching a mode may not be consumed, and data may be effectively received. 
         [0069]    When the preamble reception unit  410  fails to receive the second preamble from the second transmission node  460  in a predetermined waiting time interval, the data reception unit  430  may switch the reception node  400  from the active mode to the sleep mode. 
         [0070]      FIG. 5  is a diagram illustrating an example of transmitting a burst traffic according to an embodiment. 
         [0071]    A preamble packet including a preamble that is transmitted to a reception node  530  by a transmission node  560  is illustrated in a data packet (a) of  FIG. 5 , and a data packet that is transmitted to the reception node  530  by the transmission node  560  is illustrated in a data packet (b) of  FIG. 5 . Also, in a section (c) of  FIG. 5 , the transmission node  560  and the reception node  530  transmit a burst traffic. 
         [0072]    The reception node  530  may switch a sleep mode to an active mode for each predetermined wake-up interval  540 , and receive a preamble in a waiting time interval  540 . 
         [0073]    To transmit data, the transmission node  560  may transmit, in a time interval  562 , a preamble to the reception node  530  using a first frequency band. When the reception node  530  receives, in the time interval  562 , the preamble from the transmission node  560 , the reception node  530  may determine that data to be received from the transmission node  560  exists, and switch the sleep mode to the active mode to receive the data. 
         [0074]    In  FIG. 5 , the reception node  530  may fail to receive the preamble transmitted, in the time interval  562 , by the transmission node  560 . The transmission node  560  may additionally transmit, in a time interval  563 , a preamble using the first frequency band. The reception node  530  may receive, in an interval  551 , a preamble, and transmit, in an interval  552 , an ACK for the preamble using the first frequency band. The transmission node  560  may receive, in a time interval  564 , an ACK for the preamble. 
         [0075]    The transmission node  560  and the reception node  530  may perform a frequency hopping, in time intervals  553  and  565 , from the first frequency band to the second frequency band. The transmission node  560  may transmit, in an interval  566 , data using the second frequency band. The reception node  530  may receive, in an interval  554 , the data, and transmit, in an interval  555 , an ACK for the data. The transmission node  560  may receive, in an interval  567 , an ACK for the data. 
         [0076]    When the data transmission performed in the time interval  566  is completed, the transmission node  560  and the reception node  530  may not perform a frequency-hopping from the second frequency band to the first frequency band. The transmission node  560  may additionally transmit, in an interval  568 , data using the second frequency band. The transmission node  560  and the reception node  530  may consecutively transmit a plurality of data without performing a frequency-hopping, and perform the frequency-hopping, in intervals  558  and  570 , from the second frequency band to the first frequency band after the data transmission is completed. 
         [0077]    According to an embodiment, the transmission node  560  may transmit, to the reception node  530 , a number  512  of data packets to be consecutively transmitted. For example, the transmission node  560  may transmit, to the reception node  530 , the number  512  of data packets that are consecutively transmitted to the reception node  530  by the transmission node  560 , in addition to a preamble  511 . The reception node  530  may receive a burst traffic based on the number of the data packets. When the transmission node  560  receives the number of the data packets, the reception node  530  may perform a frequency-hopping from the second frequency band to the first frequency band. 
         [0078]    According to another embodiment, the transmission node  560  may add, to data  521 , information  522  about whether an additional data packet is present, and transmit the data  521 . The information  522  may denote that the additional data packet to be transmitted to the reception node  530  by the transmission node  560  is absent or present. The reception node  530  may perform a frequency-hopping from the second frequency band to the first frequency band after receiving a final data packet from the transmission node  560 . 
         [0079]      FIG. 6  is a block diagram illustrating a structure of a reception node  600  receiving a burst traffic according to another embodiment. The reception node  600  includes a packet information reception unit  610  and a data packet reception unit  620 . 
         [0080]    The packet information reception unit  610  may receive, from the transmission node  630 , information about a data packet to be transmitted by the transmission node  630 . The information about the data packet to be transmitted by the transmission node  630  may be information about a number of data packets to be consecutively transmitted by the transmission node  630 . Alternatively, the information about the data packet to be transmitted by the transmission node  630  may be information about whether the additional data packet to be transmitted by the transmission node  630  is present. 
         [0081]    According to an embodiment, the packet information reception unit  610  may receive a preamble and information about the data packet using a first frequency band. 
         [0082]    The data packet reception unit  620  may consecutively receive the data packet from the transmission node  630 , using the information about the data packet. According to an embodiment, the data packet reception unit  620  may perform a frequency-hopping from the first frequency band to a second frequency band, and receive the data packet using the second frequency band. According to an embodiment, after a reception of a single data packet is completed, the data packet reception unit  620  may consecutively receive subsequent data packets using the second frequency band without performing the frequency-hopping from the second frequency band to the first frequency band. 
         [0083]    When the information about the data packet is the information about the number of data packets to be transmitted by the transmission node  630 , the data reception unit  620  may perform a frequency-hopping from the second frequency band to the first frequency band after receiving all data packets to be transmitted by the transmission node  630 . 
         [0084]    When the information about the data packet is the information about whether the additional data packet to be transmitted by the transmission node  630  is present, the data reception unit  620  may verify whether the additional data packet is present while receiving each of the data packets. When receiving a final data packet, the data reception unit  620  may perform the frequency-hopping from the second frequency band to the first frequency band. 
         [0085]    According to an embodiment, the consecutively performed data reception may be impossible due to an error. When the data reception unit  620  fails to receive the data packet in a sufficient period in time, it may be determined that an error in the data reception occurs. In this case, the reception node  600  may perform the frequency-hopping from the second frequency band to the first frequency band, and re-receive the preamble from the transmission node  630  using the first frequency band. 
         [0086]    Also, when the data reception unit  620  fails to receive the data packet in a predetermined period in time, the packet information reception unit  610  may control the reception node  600  to perform the frequency-hopping from the second frequency band to the first frequency band. 
         [0087]      FIG. 7  is a diagram illustrating an example of changing a wake-up point in time when a data packet to be transmitted is generated while receiving a preamble according to an embodiment. 
         [0088]    A sensor node  720  may ascertain a point in time when a mode of a reception node  710  is switched from a sleep mode to an active mode, and transmit a preamble starting from immediately before the mode of the reception node  710  is switched to the active mode. Thus, the sensor node  720  may not transmit unnecessarily large number of preambles, thereby reducing power consumption. 
         [0089]    In  FIG. 7 , the sensor node  720  may switch a mode of the reception node  710  from the sleep mode to the active mode at the end of a wake-up interval  721 . The sensor node  720  may receive the preamble from a neighboring transmission node adjacent to the sensor node  720  in a waiting time interval  722 . When receiving the preamble from the neighboring transmission node, the sensor node  720  may receive data from the neighboring transmission node. 
         [0090]    When data to be transmitted to the reception node  710  by the sensor node  720  exists, the sensor node  720  may transmit, to the reception node  710 , the preamble starting from immediately before the waiting time interval  711  of the reception node  710  starts. When the preamble is successfully transmitted, the sensor node  720  may transmit the data to the reception node  710 . 
         [0091]    In this case, a time interval during which the sensor node  720  transmits the preamble to the reception node  710  and a time interval during which the sensor node  720  receives the preamble from the neighboring transmission node may be overlapped. 
         [0092]    In this case, the sensor node  720  may change a starting point in time of the wake-up interval to another starting point in time, so that the time interval during which the sensor node  720  receives the preamble from the neighboring transmission node may be moved. 
         [0093]    Specifically, when the time interval during which the sensor node  720  transmits the preamble to the reception node  710  and a time interval during which the sensor node  720  receives the preamble from the neighboring transmission node are overlapped, the sensor node  720  may move the time interval where the preamble is received, and thus preferentially transmitting the data. 
         [0094]      FIG. 8  is a diagram illustrating an example of changing a starting point in time of a wake-up interval when a preamble reception is generated while transmitting a data packet according to an embodiment. 
         [0095]    In general, when a wake-up interval  831  is expired, a sensor node  830  may receive a preamble from a neighboring transmission node adjacent to the sensor node  830 . When receiving the preamble from the neighboring transmission node, the sensor node  830  may perform a procedure for receiving data from the neighboring transmission node. 
         [0096]    When tracking a starting point of a wake-up interval of a reception node  810 , the sensor node  830  may transmit, in a time interval  841 , a preamble to the reception node  810  before the starting point in time of the wake-up interval of the reception node  810 . 
         [0097]    When the wake-up interval is expired while performing a preamble transmission in the time interval  841  and performing a data transmission in a time interval  845 , a time interval during which the sensor node  830  transmits the data to the reception node  810  and a time interval during which the sensor node  830  receives the preamble from the neighboring transmission node may be overlapped. In this case, the sensor node  830  may move the time interval where the preamble is received, and thus preferentially transmitting the data. 
         [0098]      FIG. 9  is a block diagram illustrating a structure of a sensor node  900  preferentially transmitting a data packet according to an embodiment. The sensor node  900  includes a preamble reception unit  910 , a control unit  920 , and a data transmission unit  930 . 
         [0099]    The preamble reception unit  910  may receive a preamble from a transmission node  940  in a waiting time interval. 
         [0100]    The data transmission unit  930  may transmit data to a reception node  950  in a time interval where the data is transmitted. 
         [0101]    When the time interval where the data is transmitted and the waiting time interval are overlapped, the control unit  920  may move the waiting time interval so that the time interval where the data is transmitted and the waiting time interval are not overlapped. In this case, the data transmission unit  930  may preferentially transmit the data to the reception node  950 . 
         [0102]    According to an embodiment, the control unit  920  may move the waiting time interval to another time interval subsequent to the time interval where the data is transmitted. 
         [0103]      FIG. 10  is a diagram illustrating an example where a collision is generated because a time interval where a Clear Channel Assessment (CCA) is performed is shorter than a time interval where a Turn operation is performed according to an embodiment. 
         [0104]    In  FIG. 10 , a first sensor node  1010  and a second sensor node  1020  may communicate with each other, and the second sensor node  1020  and a third sensor node  1030  may communicate with each other. However, the first sensor node  1010  and the third sensor node  1030  may not communicate with each other. 
         [0105]    The second sensor node  1020  may perform a CCA in a time interval  1021 . The CCA may be a procedure for determining whether other sensor nodes use a channel, and the second sensor node  1020  may ascertain whether a signal is transmitted to the channel in a predetermined period in time to thereby determine whether the other sensor nodes use the channel. 
         [0106]    When it is determined that the other sensor nodes do not use the channel in the predetermined period in time, the second sensor node  1020  may perform a Turn operation in a time interval  1022 . 
         [0107]    The time interval  1022  where the turn operation is performed may be a time required for switching a reception mode to a transmission mode by the second sensor node  1020 . Based on an Institute of Electrical and Electronics Engineers (IEEE) 802.15.4 standard, the second sensor node  1020  may perform the turn operation in the time interval  1022 , that is, a time interval of 1946. 
         [0108]    The second sensor node  1020  having been switched to the transmission mode may transmit, to the first sensor node  1010 , a preamble in a time interval  1023 . The first sensor node  1010  may receive the preamble in a time interval  1011 , and perform the turn operation in a time interval  1012  to thereby switch the reception mode to the transmission mode. Also, the second sensor node  1020  may perform the turn operation in a time interval  1024  to thereby switch the transmission mode to the reception mode again. 
         [0109]    The first sensor node  1010  staying in the transmission mode may transmit, in a time interval  1013 , an ACK for the preamble to the second reception node  1020 . Also, the second sensor node  1020  staying in the reception mode may receive the ACK for the preamble in a time interval  1025 . 
         [0110]    In this case, the preamble may be transmitted so that the third sensor node  1030  transmits data to the second sensor node  1020 . Specifically, the third sensor node  1030  may perform a CCA in a time interval  1031  while the first sensor node  1010  and the second sensor node  1020  perform the turn operation. Accordingly, it may be ascertained that the first sensor node  1010  and the second sensor node  1020  use the channel, however, the third sensor to node  1030  does not use the channel. 
         [0111]    The third sensor node  1030  may switch the reception mode to the transmission mode by performing the turn operation in a time interval  1032 , and transmit the preamble in a time interval  1033 . In this case, a time interval during which the second sensor node  1020  receives, from the first sensor node  1010 , the ACK for the preamble in a time interval  1025 , and a time interval during which the third sensor node  1030  transmits the preamble in a time interval  1033  may be overlapped. Specifically, the ACK for the preamble transmitted, in the time interval  1013 , by the first sensor node  1010  and the preamble transmitted, in the time interval  1033 , by the third sensor node  1030  may be collided with each other. 
         [0112]    This collision between the ACK for the preamble and the preamble may be generated because the time interval  1031  where the third sensor node  1030  performs the CCA is shorter than the time intervals  1012  and  1013  where the first sensor node  1010  performs the turn operation, and transmits the ACK for the preamble. 
         [0113]    Accordingly, the sensor node according to an embodiment may perform the CCA in a time interval longer than a time interval during which another sensor node performs the turn operation and transmits the ACK for the preamble. When the channel is observed in the time interval longer than the time interval during which the other sensor node performs the turn operation and transmits the ACK for the preamble, a collision interval  1034  illustrated in  FIG. 10  may be prevented from being generated. 
         [0114]      FIG. 11  is a flowchart illustrating a method of repeatedly performing a CCA to suppress occurrence of collision according to an embodiment. Some of sensor nodes may not change a time interval where the CCA is performed, to be longer. In this case, by repeatedly performing the CCA, the sensor nodes may observe a channel in a time interval longer than a time interval during which another sensor node performs a turn operation, and transmits an ACK for a preamble. 
         [0115]    In operation  1110 , the sensor node may observe the channel in a predetermined period in time. 
         [0116]    In operation  1120 , the sensor node may determine whether the channel is available. When the sensor node fails to receive a signal transmitted by another sensor node in operation  1110 , the sensor node may determine that the channel is available. 
         [0117]    When the sensor node determines that the channel is unavailable, the sensor node may terminate the CCA procedure in operation  1140 , and wait for a termination of the data transmission performed by the other sensor node. 
         [0118]    When the sensor node determines that the channel is available, the sensor node may compare a time interval during which the channel is observed and a time interval during which the other sensor node performs the turn operation and transmits the ACK for the preamble. 
         [0119]    When the time interval during which the channel is observed is longer than the time interval during which the other sensor node performs the turn operation and transmits the ACK for the preamble, the sensor node may complete the CCA procedure in operation  1150 , because the sensor node observes the channel in a sufficient time interval. 
         [0120]    When the time interval during which the channel is observed is shorter than the time interval during which the other sensor node performs the turn operation and transmits the ACK for the preamble, the sensor node may not observe the channel in the sufficient time interval. Accordingly, the sensor node may additionally perform the CCA procedure in operation  1110 . 
         [0121]      FIG. 12  is a block diagram illustrating a structure of a transmission node  1200  that repeatedly performing a CCA to suppress occurrence of collision according to an embodiment. The transmission node  1200  includes a CCA performing unit  1210 , a mode switch unit  1220 , a preamble transmission unit  1230 , and a preamble ACK transmission unit  1240 . 
         [0122]    The CCA performing unit  1210  may perform a CCA procedure for verifying whether a channel is available in a first time interval. For example, the CCA performing unit  1210  may observe the channel in the first time interval, and determine whether a signal transmitted by another sensor node exists. When the sensor node fails to receive the signal transmitted by the other sensor node, the CCA performing unit  1210  may determine that the other sensor node does not the channel. In this case, the channel may be available. 
         [0123]    When the channel is available, the mode switch unit  1220  may switch a reception mode to a transmission mode. To perform the CCA procedure, the sensor node may need to receive the signal transmitted from the other sensor node. Accordingly, the sensor node may maintain the reception mode while performing the CCA procedure. When the channel is available, the sensor node may switch the reception mode to the transmission mode to transmit a preamble and data. 
         [0124]    The preamble transmission unit  1230  may transmit the preamble to a reception node  1260  in a third time interval, using the available channel. 
         [0125]    The preamble ACK reception unit  1240  may receive the preamble from the reception node  1260 . 
         [0126]    In this case, the first time interval where the CCA performing unit  1210  performs the CCA procedure may be longer than a sum of a second time interval where the mode switch unit  1220  switches a mode and the third time interval where the preamble transmission unit  1230  transmits the preamble. 
         [0127]    Some of the sensor nodes may not adjust the time interval where the CCA procedure is performed. In this case, the time interval where the CCA performing unit  1210  performs the CCA procedure may be shorter than the sum of the second time interval and the third time interval. 
         [0128]    In this case, the CCA performing unit  1210  may repeatedly perform the CCA procedure, so that the CCA performing unit  1210  may adjust the first time interval, where whether the channel is available is observed, to be longer than the sum of the second time interval and the third time interval. 
         [0129]    Although a few embodiments have been shown and described, it would be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the disclosure, the scope of which is defined by the claims and their equivalents.