KR101218916B1 - Time synchronization for wireless sensor networks - Google Patents

Time synchronization for wireless sensor networks Download PDF

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KR101218916B1
KR101218916B1 KR1020060038277A KR20060038277A KR101218916B1 KR 101218916 B1 KR101218916 B1 KR 101218916B1 KR 1020060038277 A KR1020060038277 A KR 1020060038277A KR 20060038277 A KR20060038277 A KR 20060038277A KR 101218916 B1 KR101218916 B1 KR 101218916B1
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time
sensor node
frame
physical frame
reception
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KR1020060038277A
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KR20070105731A (en
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김연수
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주식회사 케이티
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Abstract

The present invention relates to a wireless sensor network time synchronization method, comprising: measuring a reception time through a timer in response to receiving a physical frame in which transmission time information of a beacon frame is recorded from a transmitting sensor node, and receiving a timestamp of the received physical frame Determining the time offset by detecting the time point information recorded in the data and comparing them to each other, and by adjusting the timer according to the calculated time offset, to solve the timing mismatch problem that causes frequent packet collisions, frequent packet retransmission between sensor nodes In addition, to provide a wireless sensor network time synchronization method for saving a battery of a sensor node via multiple hops.
To this end, the present invention provides a wireless sensor network time synchronization method, comprising: a physical frame transmitting step of a transmitting sensor node recording transmission time information of a beacon frame in a time stamp of a physical frame and transmitting the same to a receiving sensor node; A reception time measurement step of measuring a reception time through a TS timer as the reception sensor node receives the physical frame; Detecting, by the receiving sensor node, view information recorded in a timestamp of the received physical frame; A time offset calculation step of calculating, by the reception sensor node, a time offset by comparing the measured reception time point with the detected viewpoint information; And a TS timer adjusting step of adjusting the TS timer according to the calculated time offset by the receiving sensor node, wherein the physical frame transmitting step includes time information at the moment when the frame start indicator of the beacon frame is spread-modulated. Record on timestamp.
Wireless sensor network, transmitting sensor node, receiving sensor node, time synchronization, time stamp, time offset, timer control

Description

Time synchronization for wireless sensor networks

1 is a configuration diagram of an embodiment of a wireless sensor network to which the present invention is applied;

2 is a flowchart illustrating an embodiment of a wireless sensor network time synchronization method according to the present invention.

DESCRIPTION OF THE REFERENCE NUMERALS

11: WSN Coordinator 12: WSN Router

13: WSN device

The present invention relates to a wireless sensor network time synchronization method. More particularly, the present invention relates to a wireless sensor network time synchronization method, and more particularly, to receive a physical frame in which transmission time information of a beacon frame is recorded from a transmitting sensor node, and to measure a reception time through a timer. After detecting the time information recorded in the timestamp of the time stamp, the time offset is calculated by comparing with each other, and the timer is adjusted according to the calculated time offset, and thus timing mismatches causing frequent packet collisions and frequent packet retransmissions between the sensor nodes. The present invention relates to a wireless sensor network time synchronization method for solving a problem and saving battery of a sensor node via multiple hops.

Wireless sensor network technology is a wireless technology that can measure and detect various status and environmental information such as temperature, humidity, illuminance, and pressure remotely without configuring a separate wired network. These wireless sensor networks are areas or objects that are difficult to access or require constant measurement, monitoring and control, such as environmental condition monitoring, structure and building condition monitoring, volcanic activity monitoring, precise plant cultivation, and local and facility security. Can be used inexpensively and effectively.

For remote monitoring and control of an object, many wireless sensor nodes form a wireless sensor network distributed in a certain area, and the wireless sensor network is configured as PCS (Personal Communication Services), WLAN (Wireless Local Area network), By connecting with a separate central server through wired / wireless networks such as xDSL (x Digital Subscriber Line) and KORNET, users can access the central server even if they do not go to the site to monitor application and manage the area and objects. Can be used easily.

A wireless sensor network (WSN) forms a multi-hop network in which a plurality of sensor nodes having sensors and wireless communication modules are interconnected through short-range wireless links in a predetermined space. At this time, the sensor node measures the ambient state and transmits the measured state to the adjacent sensor node, or transmits data received from the adjacent sensor node to another adjacent sensor node.

Here, the sensor node is a device that uses a battery and is discarded in the event of power consumption or failure in consideration of application in a difficult maintenance environment, and has a low cost and a small form, and various communication protocol technologies for efficient use of battery power. This applies.

These sensor nodes have the functions of a WSN coordinator, a WSN router, and a WSN device. First, only one WSN coordinator exists in an independent wireless sensor network and broadcasts various information on the wireless sensor network through a beacon. The WSN coordinator also connects sensor networks with existing wired and wireless networks.

Next, the WSN router broadcasts the beacon in the same manner as the WSN coordinator, and also transmits data received from an adjacent sensor node to another sensor node. Next, the WSN device only transmits the sensing information to the adjacent sensor node and does not broadcast the beacon or relay the data.

In a large multi-hop wireless sensor network, there is one WSN coordinator, and many WSN routers and WSN devices, so information measured by any WSN device is usually delivered to its destination through hopping by several WSN routers. .

Wireless sensor networks use wake-up / sleep modes based on time slots to increase the efficiency of limited energy. That is, by repeating the active state and the inactive state, communication is required only for a predetermined time to suppress energy use of the sensor node.

Time synchronization is important for time slot timing, wakeup scheduling, channel access timing, and data transmission / reception timing of sensor nodes in this mode of operation. In particular, in applications such as adjusting for future operation, measuring position of sensor node, correlation between measurement information of several sensor nodes, recognizing duplicate detection of same event by different sensor nodes, removing redundant data, and measuring moving speed of sensor node It requires precise time synchronization that is maintained throughout the wireless sensor network.

Conventional time synchronization is based on the time point at which the broadcast beacon is received. Sensor nodes that broadcast beacons periodically generate beacons on their own clocks and perform all times necessary for time slot timing and data communication.

On the other hand, the sensor node receiving the beacon executes all necessary timings on its own based on the reception point. In this way, the multi-hop wireless sensor network uses the WSN coordinator as the reference time for all timings to receive the surrounding WSN routers and WSN device beacons.

That is, the WSN coordinator broadcasts the beacon, and the WSN router executes its operation timing based on the reception point of the beacon. The lower WSN router is based on the beacon reception point of the upper WSN router, and the WSN device uses the beacon reception point of the upper sensor node, that is, the WSN coordinator or the WSN router as the reference of the operation timing.

However, since the timing reference of each sensor node is the beacon reception point, a constant time offset occurs due to the time difference between each sensor node by the delay caused by the generation, processing, and propagation of the beacon, and the intermediate path in the outermost sensor node of the wireless sensor network. The time offset is further magnified by the delay accumulation of the sensor nodes on the image.

Therefore, the conventional time synchronization method has a problem that all the sensor nodes of the network do not have the same timing even for the same slot time. As a result, the timing mismatch caused by the time offset increases the energy consumption such as frequent packet collision and frequent packet retransmission among the sensor nodes, and causes a problem of shortening the lifespan of the wireless sensor network.

The present invention has been proposed to solve the above-mentioned problem, and when receiving a physical frame in which transmission time information of a beacon frame is recorded from a transmitting sensor node, a reception time is measured through a timer, and the time stamp of the received physical frame is measured. After detecting recorded time information, compare time and calculate time offset, and adjust timer according to the calculated time offset to solve the timing mismatch problem that causes frequent packet collisions and frequent packet retransmission among sensor nodes. It is an object of the present invention to provide a wireless sensor network time synchronization method for saving battery of a sensor node via multiple hops.

Other objects and advantages of the present invention can be understood by the following description, and will be more clearly understood by the embodiments of the present invention. It will also be readily apparent that the objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.

In the method of the present invention for achieving the above object, in the wireless sensor network time synchronization method, a physical frame transmitting step of transmitting a transmission sensor node records the transmission time information of the beacon frame in the time stamp of the physical frame and then transmits it to the receiving sensor node. ; A reception time measurement step of measuring a reception time through a TS timer as the reception sensor node receives the physical frame; Detecting, by the receiving sensor node, view information recorded in a timestamp of the received physical frame; A time offset calculation step of calculating, by the reception sensor node, a time offset by comparing the measured reception time point with the detected viewpoint information; And a TS timer adjusting step of adjusting the TS timer according to the calculated time offset by the receiving sensor node, wherein the physical frame transmitting step includes time information at the moment when the frame start indicator of the beacon frame is spread-modulated. Record on timestamp.

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In addition, the present invention achieves precise time synchronization between two sensor nodes through time stamps, and even between two sensor nodes via multiple hops, and maintains / provides a consistent timing throughout the wireless sensor network, thereby causing a sensor to be caused by timing mismatch. It eliminates problems such as frequent packet collisions between nodes and frequent packet retransmissions, and maximizes battery life of sensor nodes.

The foregoing and other objects, features and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings, in which: There will be. In the following description, well-known functions or constructions are not described in detail since they would obscure the invention in unnecessary detail. Hereinafter, a preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings.

1 is a configuration diagram of an embodiment of a wireless sensor network to which the present invention is applied.

Multi-hop wireless sensor networks provide periodic channel access intervals so that wireless sensor nodes distributed in a certain area can transmit their own data frames. Here, the channel access section includes a beacon frame broadcast and a channel active section capable of transmitting and receiving packets. In this case, a shared channel access technique such as carrier sensing multiple access collision avoidance (CSMA / CA) is applied to the channel active interval to provide the wireless channel occupancy and use opportunities to all the sensor nodes in the wireless coverage.

As shown in FIG. 1, the sensor nodes (WSN coordinator 11 and WSN router 12) except for the WSN device 13 positioned at the edge of the wireless sensor network are adjacent to the sensor node and their own information and network information. A beacon frame is broadcast periodically to provide time interval information for data transmission and reception. Here, the beacon frame is composed of fields such as a preamble, a frame start indicator, a frame length, and a payload.

Looking at this in more detail, the preamble is used by the receiving sensor node to check the occupancy state of the radio channel, determine the existence of the data frame, and recognize the start of the receiving frame to provide frame synchronization. At this time, a bit sequence of constant length is defined, and usually a "0" sequence is used.

The frame start indicator is information indicating that a complete data frame is beginning to be received, and is defined as a bit sequence of a specific pattern.

The frame length field indicates the length of the data sequence corresponding to the payload.

Therefore, after receiving the data frame, the receiving sensor node transfers a bit sequence corresponding to the length to the MAC entity.

On the other hand, all sensor nodes (WSN coordinator 11, WSN router 12, WSN device 13) maintains its own clock. Here, the clock is generated by its own oscillator and has a resolution of 1 us as a reference clock that can provide timing of all operations and processes of the sensor node.

In addition, all sensor nodes have TS timers of the same resolution to provide time synchronization timing. At this time, the TS timer is an N timer and repeats a count from 0 to N-1. In other words, the TS timer is a swept timer that counts up to N-1 and then starts again from 0. It can be slowed down or adjusted quickly in 1us increments and works with its own clock.

2 is a flowchart illustrating an embodiment of a wireless sensor network time synchronization method according to the present invention.

First, the transmitting sensor node (WSN coordinator) records the transmission time information of the beacon frame in the time stamp of the physical frame and transmits it to the receiving sensor node (201). In this case, it is preferable that the transmitting sensor node records the time information of the instant when the frame start indicator of the beacon frame is spread-modulated (the moment when the header of the beacon frame is modulated) in the time stamp.

Thereafter, the reception sensor node measures the reception time through the timer as the reception of the physical frame from the transmission sensor node (202). In this case, the reception sensor node preferably measures the reception time of the timer at the moment of receiving the frame start indicator of the physical frame.

Thereafter, the receiving sensor node detects the viewpoint information recorded in the time stamp of the received physical frame (203).

Thereafter, the reception sensor node compares the measured reception time and the detected time information to calculate a time offset (time difference) (204).

Thereafter, the receiving sensor node adjusts its timer according to the calculated time offset (205). At this time, the channel access adjusts the time offset at the first time slot boundary after the beacon frame transmission in consideration of the CSMA / CA feature that is attempted at the time slot boundary. In this case, when the receiving sensor node is a WSN router, the received physical frame is transferred to a neighboring WSN router and a WSN device.

Here, the receiving sensor node may repeat the above process to calculate a plurality of time offsets and then calculate the average to adjust the timer.

In more detail, the time synchronization of the wireless sensor network begins with the transmission of beacons. That is, the WSN coordinator 11 first generates a beacon frame before transmitting the beacon frame. At this time, the format and detail fields of the beacon frame are separately defined and parameters defined by the user among the related parameters are determined by the application field, application environment, network size, shape, beacon period, etc. of the sensor network.

The beacon frame generated by the MAC entity of the WSN coordinator 11 is made of a physical frame that is easily transmitted through a wireless channel. Here, the physical frame has a structure in which a time stamp is added after the beacon frame. In this case, the time stamp is an information element indicating a time point at which the beacon frame is propagated through radio frequency. The time stamp has no value at the moment when the physical frame is made, but has only a constant length, and displays the frame start of the beacon frame. The time information at that time is stored by the WSN coordinator 11 at the time of self spreading modulation.

That is, the WSN coordinator 11 measures the TS timer value at the moment when the last bit of the frame start indicator is spread. This is converted to a binary value of Tn bit length and stored in a separate buffer. In this case, the transmitted physical frame is subjected to a modulation process in order of a frame length field, followed by a bit sequence corresponding to a payload, followed by a frame start indicator. The timestamp buffer is triggered the moment the last bit of the payload is spread modulation, the stored timestamp value is filled in the timestamp field, and the last bit of the payload is propagated through spread modulation and carrier modulation.

Thereafter, the reception sensor node measures a reception timestamp that is the reception point of the beacon frame, and then compares the transmission timestamp with the reception timestamp to calculate a time offset that is a time difference between the two sensor nodes. The offset is used to determine the adjustment amount of the receiving sensor node TS timer.

The measurement of the reception time stamp is performed through a process opposite to that of the frame transmission. When the preamble is received, it is determined whether the following field is a frame start indicator. The frame start indicator is a unique pattern known to all sensor nodes as mentioned above. Therefore, the existence of the frame start indicator can be known through correlation with the reception sequence, and the time can be measured as the reception time of the reception frame.

The measurement of the received timestamp is once triggered to the output of the correlator. The sensor node stores its TS timer value in the TS timer buffer at the correlator output time above a certain threshold. Then, the transmission timestamp value following the received payload is converted into a time in us in the bit sequence.

The receiving sensor node uses the receiving timestamp value and the transmitting timestamp to calculate the time offset between the two sensor nodes. At this time, the radio channel propagation delay time of the beacon frame is negligible because it is less than 1us small enough not to be measured in all application environments. Therefore, the two timestamp values are considered to be about the same point in time. Here, the time offset can be determined by the difference between the two values.

The calculated offset is then used to slow or advance the receiving TS timer by the offset. In this case, when the receiving sensor node is a WSN router, the time offset adjustment point takes into account the CSMA / CA feature that channel access attempts at the time slot boundary, and then the first time after transmitting the beacon frame received from the WSN coordinator to the adjacent sensor node. At the slot boundary.

Thereafter, the receiving side sensor node applies the adjusted TS timer to perform beacon transmission and reception and channel access.

The time offset may include drift caused in a somewhat unstable radio channel according to the sensor network environment. Considering this aspect, we consider moving averaging using several time stamps measured at different times or using several time offsets calculated at each moment. In other words, the time offset to be adjusted currently is obtained by averaging from two time stamps or n time offsets measured previously n times. At this time, the n value is considered to be a period of the beacon, etc., but is usually set to 2-3 times.

The adjustment of the TS timer by the calculated time offset is slowed or advanced through the time-lag or time-lead function of the TS timer. The time lag function delays the TS timer by the time offset. This stops the TS timer by the clock time for a time offset. Since the TS timer is a counter, its value is delayed as long as it is interrupted and time synchronization with the beacon transmitting side can be obtained. In contrast, the timelead function precedes the TS timer by the time offset. That is, the TS timer is added by the time corresponding to the time offset. By adding a time offset to the value of the current timer, the time difference is adjusted to achieve time synchronization between the two sensor nodes.

The method of the present invention as described above may be embodied as a program and stored in a computer-readable recording medium (such as a CD-ROM, a RAM, a ROM, a floppy disk, a hard disk, or a magneto-optical disk). Since this process can be easily implemented by those skilled in the art will not be described in more detail.

It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or scope of the invention. The present invention is not limited to the drawings.

According to the present invention as described above, the reception time is measured by a timer according to the reception of the physical frame in which the transmission time information of the beacon frame is recorded from the transmitting sensor node, and the viewpoint information recorded in the time stamp of the received physical frame is measured. By detecting and comparing each other, the time offset is calculated, and the timer is adjusted according to the calculated time offset to solve the timing mismatch problem that causes frequent packet collisions and frequent packet retransmissions between sensor nodes. There is an effect to save the battery of the sensor node.

Claims (11)

  1. delete
  2. In the wireless sensor network time synchronization method,
    Transmitting, by the transmitting sensor node, the transmission time information of the beacon frame to the receiving sensor node after recording the transmission time information of the beacon frame in the time stamp of the physical frame;
    A reception time measurement step of measuring a reception time through a TS timer as the reception sensor node receives the physical frame;
    Detecting, by the receiving sensor node, view information recorded in a timestamp of the received physical frame;
    A time offset calculation step of calculating, by the reception sensor node, a time offset by comparing the measured reception time point with the detected viewpoint information; And
    And a TS timer adjusting step of adjusting the TS timer according to the calculated time offset by the receiving sensor node.
    The physical frame transmission step,
    And recording time information at the moment when the frame start indicator of the beacon frame is spread-modulated in a timestamp.
  3. The method of claim 2,
    The physical frame transmission step,
    And transmitting transmission time information of the beacon frame through a TS timer value and storing the transmission time information of the beacon frame in a timestamp buffer of the physical frame.
  4. The method of claim 3, wherein
    The physical frame transmission step,
    And a timestamp triggered when the payload of the physical frame is modulated, subsequent to the payload.
  5. 5. The method according to any one of claims 2 to 4,
    The receiving point measurement step,
    And a time point at which the TS timer is received at the moment of receiving the frame start indicator of the physical frame.
  6. 6. The method of claim 5,
    The TS timer adjusting step,
    When the receiving sensor node is a wireless sensor network router, the first time slot after transmitting a beacon frame to an adjacent sensor node in consideration of the carrier sensing multiple access collision avoidance (CSMA / CA) feature that channel access attempts at a time slot boundary. A wireless sensor network time synchronization method for adjusting a time offset at a boundary.
  7. 6. The method of claim 5,
    The time offset calculation step and the TS timer adjustment step,
    The receiving sensor node repeats the physical frame transmission step to the time offset calculation step to calculate a plurality of time offsets, and then calculates an average of the calculated plurality of time offsets to control the TS timer. Network time synchronization method.
  8. 6. The method of claim 5,
    The TS timer adjusting step,
    And stopping the TS timer by a time offset in the case of time lag adjustment.
  9. 6. The method of claim 5,
    The TS timer adjusting step,
    And adding the TS timer by a time offset when the time lead is adjusted.
  10. 6. The method of claim 5,
    The TS timer,
    A wireless sensor network time synchronization method of repetition counting with N (natural number).
  11. delete
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CN101471766B (en) 2007-12-28 2011-10-05 中国科学院软件研究所 Time synchronization method of wireless sensor network
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KR100952281B1 (en) * 2008-02-28 2010-04-12 주식회사 케이티 Method of deciding a delay for network synchronization in network system
KR100920211B1 (en) * 2008-04-21 2009-10-05 경북대학교 산학협력단 Method for synchronizing timers of sensor nodes in the wireless sensor network
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KR101662232B1 (en) * 2011-09-16 2016-10-04 한국전자통신연구원 Method of synchronization and link access for low energy critical infrastructure monitoring network
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