KR20070025098A - Accurate time synchronization protocol for wireless sensor network - Google Patents

Abstract

An accurate time synchronization protocol for a wireless sensor network is provided to reduce the data exchange frequency between a sensor node of a low level and a sensor node of a high level by transmitting a response packet for a synchronization packet of the low level as a broadcast type in the high level. An accurate time synchronization protocol for a wireless sensor network includes the steps of: forming a synchronization packet including an own local time for the time synchronization when transmitting event data about a detected event to a sensor node of a high level(S100); transmitting the formed synchronization packet to the sensor node of the high level(S200); transmitting a response packet as a broadcast type when transmitting the response packet, corresponding to the synchronization packet, to a sensor node of a low level in the high level receiving the synchronization packet(S300); and correcting the own local time to the local time of the high level by using the time information included in the response packet if the compared result is identical by comparing identification information included in the response packet with own identification information included in a memory(S400).

Description

Accurate time synchronization protocol for wireless sensor network

1 is a view illustrating a time synchronization process in a time synchronization protocol (TPSN) for a sensor network among time synchronization techniques applied to a distributed system such as a conventional network.

2 is a schematic diagram illustrating a data communication process in a time synchronization protocol (TPSN).

3 is a schematic structural diagram of a system to which a time synchronization protocol according to the present invention is applied;

4 is a flowchart illustrating a time synchronization method of a time synchronization protocol according to the present invention.

5 is a schematic diagram illustrating a data communication process through a time synchronization protocol according to a preferred embodiment of the present invention.

6 is a schematic diagram illustrating a time stamping process in a time synchronization protocol according to the present invention;

7 is a view for explaining a response packet transmitted from a higher level to a lower level in the present invention.

<Explanation of symbols for the main parts of the drawings>

100: user terminal

200: external network

300: sink

400: sensor field

500: sensor node

The present invention relates to a wireless sensor network, and more particularly, to improve the time synchronization method for an existing wireless sensor network, and more precisely for a wireless sensor network to enable simple, fast, and accurate time synchronization between sensor remotes. It relates to a time synchronization method.

In general, distributed systems such as wireless sensor networks require the Time Synchronization Protocol.

For example, if an event occurs in an area with many sensor motts, if it is not synchronized, several mots will be able to detect the event at the same time, and thus it will not be possible to determine whether the event occurred at the same event or at different time points. Therefore, a time synchronization technique can be usefully used in such a case.

The time synchronization scheme applied to a distributed system such as a conventional network includes a timing-sync protocol for a sensor network (hereinafter, abbreviated as "TPSN") and a floating time synchronization protocol (Flooding Time Synchronization Protocol). (Hereinafter abbreviated as "FTSP").

First, when the TPSN is described, the TPSN takes Send time, Access time, Propagation delay, and Receive time when transmitting and receiving packets for time synchronization. The problem is that the transmission time and access time are difficult to predict. This time represents the time it takes to generate a message and send it through the protocol stack to the network, and is currently changing according to the load of the transmitter 11 or the network condition. This TPSN synchronizes time information between nodes using a method as shown in FIG.

1 is a view illustrating a time synchronization process in a TPSN among time synchronization schemes applied to a distributed system such as a conventional network, in which T1 denotes a transmission time of a node A, and T4 denotes response data of a node B in a node A. Received time, T2 represents the reception time at the NodeB, T3 represents the time when the response data transmitted from the NodeB.

For example, when T1 = 12: 50, T2 = 12: 12, T3 = 12: 15, and T4 = 12: 55, Δ = -39min and d = 1min by the formula shown in the figure.

Accordingly, when it is checked whether the time at node A becomes T1 + d = T2-Δ, it can be seen that 12: 50 + 1 = 12: 12-(− 39) = 12: 51. Therefore, it is possible to synchronize the time of node A to node B as time T = T _A + Δ of new node A. Where T _A is the local time of Node A.

However, the TPSN as described above has a problem in that an actual time spent in a packet on each node is incorrect due to the above problem.

In particular, in the case of TPSN, time packet transmission between nodes is performed by a unicast method, which increases the number of message exchanges during data communication for time packets between nodes.

That is, as shown in FIG. 2, the number of data exchanges in the three-level sensor network includes four sensor nodes at the lowest level and two sensor nodes at the upper level of this level. If there is a top level composed of a single sensor node at the top, since the communication method for the time packet between each of these sensor nodes is performed by the unicast method, the total number of data exchanges occurring in the entire network occurs 12 times. do.

As described above, as the number of message exchanges for a time packet increases due to the unicast method during data communication between the low level and the high level, that is, the number of wireless communication increases, power consumption of each sensor node increases, thereby increasing the number of sensor nodes. There is a problem that the life time is reduced.

On the other hand, the FTSP is time stamped at the time of transmitting a packet from a network interface card (hereinafter, referred to as "NIC") to the network in order to reduce the error caused by the send time and the access time. Send time information using. The node receiving the packet modifies its local time with reference to the packet's time stamp and broadcasts it again to propagate the synchronization packet throughout the network.

The FTSP does not need to consider the transmission time and the access time, but ignores the propagation time, and when the network is composed of many nodes, floating occurs over several nodes, thereby causing an error in time. This phenomenon is aggravated by larger networks.

The above two protocols have a common problem. In other words, nodes of sensor network are not always active due to resource limitations, but remain in sleep or idle state, and detect an event for a specific time and perform an operation, and cannot shorten the time synchronization period.

Because of this feature, if there were nodes in sleep or idle state during time synchronization, these nodes would not be able to synchronize. In addition, when a new node is added after time synchronization is performed, the added node has a problem that the time synchronization is not synchronized until the next time synchronization is made.

The present invention has been invented to solve the above problems, and by improving the time synchronization method for the existing network, accurate time synchronization for a wireless sensor network that enables simple, fast, and accurate time synchronization between sensor remotes is possible. The purpose is to provide a protocol.

In addition, an object of the present invention is to provide an accurate time synchronization protocol for a wireless sensor network that can improve the power consumption efficiency of each sensor node constituting the sensor field to improve the life time.

In addition, an object of the present invention is to provide an accurate time synchronization protocol for a wireless sensor network that enables synchronization of non-synchronized nodes and newly added nodes during synchronization using an epidemic technique.

An accurate time synchronization protocol for a wireless sensor network according to a preferred embodiment of the present invention for achieving the above object is a protocol for synchronizing time information between sensor nodes in a wireless sensor network having a tree structure. A packet generation step of generating a synchronization packet including its local time for time synchronization when transmitting event data for an event to a sensor node of a higher level; A transmission step of transmitting the generated synchronization packet to a sensor node of a higher level; A response signal transmission step of transmitting the response packet in a broadcast manner when the response packet corresponding to the synchronization packet is transmitted from the higher level sensor node to the sensor node of the lower level receiving the synchronization packet; If the identification information included in the response packet received from the higher level sensor node is matched with the identification information stored in the memory, the local time is converted to the higher level local time using the time information included in the response packet. And a time synchronization step of modifying.

The invention will become more apparent through the preferred embodiments described below with reference to the accompanying drawings. Hereinafter will be described in detail to enable those skilled in the art to easily understand and reproduce through these embodiments.

3 is a schematic configuration diagram of a system to which a time synchronization protocol is applied according to the present invention, and includes a user terminal 100, an external network 200, a sink 300, and a sensor field 400.

In the embodiment of the present invention, the sensor field 400 has a plurality of sensor nodes 500 arranged in a tree structure. As such, the plurality of sensor nodes 500 constituting the sensor field 400 include a wireless platform for supporting data communication with the outside, a processor platform for processing data, and a memory for storing various data. In the examples, the Micaz series manufactured by US Crossbow Technology Co., Ltd. was adopted.

The Micaz follows the 2.4GHz, IEEE 802.15.4 specification and is used as a mote module for small, low power sensor networks. The Micaz features include an IEEE 802.15.4 / ZigBee-compliant RF transceiver, a 2.4-GHz to 2.4GHz compatible ISM band, and a high data rate of 250kbps. Since the configuration for Micaz having such features is a known technique, a detailed description thereof will be omitted.

The sink 300 connects the sensor field 400 to an external network including a tactical internet through an external network, for example, a private land, a mobile communication network, or a wired / wireless network. Also called a gateway or base station. In the embodiment of the present invention, the sink 300 connecting the sensor field 400 and the external network 200 is illustrated as a single unit, but may be configured as a plurality of sinks in the wireless sensor network.

Accordingly, in the wireless sensor network configured as described above, data generated in the sensor field 400 composed of the plurality of sensor nodes 500 is provided to the external network 200 through the sink 300, and the user is provided with the external network ( Various data provided by the sensor field 400 are provided through the user terminal 100 connected to the 200.

4 is a flowchart illustrating a time synchronization method of a time synchronization protocol according to the present invention.

As shown here, an accurate time synchronization protocol (hereinafter, abbreviated as "ATSP") for the wireless sensor network according to the present invention first generates and transmits a synchronization packet at a lower level sensor node ( S100 and S200, transmitting a response packet corresponding to the synchronization packet from the sensor node of the higher level (S300), and correcting its own local time using the response packet received from the sensor node of the higher level ( S400). In addition, the ATSP according to the present invention includes synchronizing local time of sensor nodes or newly added sensor nodes that are not synchronized during time synchronization.

Operation of the preferred embodiment of the present invention as described above will be described in detail with reference to FIGS. 4 to 8.

First, the ATSP according to the preferred embodiment of the present invention is a network system in which a plurality of sensor nodes 500 (see FIG. 3) constituting the sensor field 400 (see FIG. 3) are arranged in a tree structure as shown in FIG. 5. In this case, the local time of the sensor nodes 520, 521, 530, 531, 532, and 533 disposed at a lower level based on the highest level is synchronized with the local time of the sensor node 510 of the highest level.

To this end, when an event is detected at the lowest level sensor nodes 530, 531, 532 and 533, the lowest level sensor nodes 530, 531, 532 and 533 perform time synchronization for data communication with the sensor node 510 located at the highest level.

That is, when event data for the detected event is sequentially transmitted to the sensor nodes 520 and 521 of the intermediate level LEVEL1 and the sensor node 510 of the highest level LEVEL0, as shown in FIG. 6, the lowest level LEVEL2. Each sensor node 530, 531, 532, 533 and the sensor node 520, 521 of the intermediate level LEVEL1 generate a synchronization packet including their local time for time synchronization in the NIC immediately before transmission of the synchronization packet (S100). In this way, the local time included in the synchronization packet is stored in the internal memory.

The sensor node transmits the synchronization packet generated through the NIC to a sensor node of a higher level of the sensor node (S200).

Accordingly, synchronization packets generated at the sensor nodes constituting the lowest level are sequentially transmitted to the higher levels, such as LEVEL2 → LEVEL1 → LEVEL0. In this case, the synchronization packet data is transmitted from the lowest level sensor nodes 530, 531, 532, 533 and the middle level sensor nodes 520, 521 to the highest level sensor node 520, 521 and the highest level sensor node 510. Is done by way.

Meanwhile, the highest level sensor node that receives the synchronization packet from the lower level sensor nodes through the above transmission process synchronizes the response packet to the synchronization packet immediately after receiving the synchronization packet from the lower level sensor nodes. The packet data is transmitted to the sensor node of the lower level (S300).

That is, as shown in the figure, the sensor node 510 of the highest level receives the response packet for the synchronization packet after receiving the synchronization packet from the sensor nodes 520 and 521 which are the lower level. Transmit to (520,521).

In this case, unlike the unicast method of transmitting data from the intermediate level to the highest level, data transmission from the highest level sensor node 510 to the intermediate level sensor nodes 520 and 521 is arranged at the intermediate level as shown in FIG. 5. The broadcast is performed by transmitting a response packet to all the sensor nodes 520 and 521.

As described above, the response packet transmitted from the highest level to the lower level, that is, from the upper level to the lower level is at least the identification information of the corresponding sensor node receiving the response packet and the lower level transmitting the synchronization packet as shown in FIG. Time information including the transmission time information of the level sensor node, the reception time information of the higher level sensor node that has received the synchronization packet, and the response packet transmission time information of the higher level sensor node.

Therefore, by broadcasting the response packet from the higher level sensor node to the lower level sensor node as described above, the number of data communication for the synchronization packet can be reduced. That is, as shown in FIG. 2, when transmitting and receiving data in the unicast method in the conventional TPSN, the total number of message exchanges between each sensor node located at LEVEL 0 to LEVEL 2 is 12, whereas the ATSP according to the present invention is used. The total number of message exchanges by the broadcast method in is reduced to 9 times.

Therefore, the number of message exchanges between the sensor nodes can be reduced, thereby reducing the power consumption of wireless communication of the nodes, thereby improving the power consumption efficiency of each sensor node, thereby improving the life time of the sensor nodes.

Meanwhile, the intermediate level sensor nodes 520 and 521 receiving the response packet from the highest level sensor node 510 synchronize their local time to the highest level time using the time information included in the response packet (S400). .

That is, each sensor node 520 or 521 of the intermediate level compares its own identification information stored in the memory with the identification information included in the received response packet, and the response packet received from the sensor node 510 of the highest level is itself. It is determined whether it is a response signal to the transmitted synchronization packet. As a result of determination, if the two identification information does not match, the received response packet is discarded. If the two identification information is matched, the response time of the sensor node 510 disposed at the upper level, that is, the highest level, is used by using the time information included in the response packet. Synchronize with time.

As such, the intermediate level sensor nodes 520 and 521 receiving the response signal transmitted from the highest level sensor node 510 are shown in FIG. 1 by using the time information included in the response packet and the reception time when the response packet is received. It is modified by the formula, which is already described and will be omitted below.

In addition, the intermediate level sensor nodes 520 and 521 receiving the response packet from the highest level sensor node 510 use the time information included in the response packet and the reception time information receiving the response packet to propagate the packet data. Calculate the time Therefore, when performing data communication with a higher level sensor node using the propagation delay time information obtained, communication can be performed more efficiently.

When the local time of each sensor node 520, 521 of the intermediate level is synchronized with the time of the sensor node 510 of the highest level through the above process, each sensor node 520, 521 of the intermediate level is the sensor node 530, 531, 532, 533 of the lowest level. In response to the synchronization packet received from the ()) is transmitted to the lower sensor node (530,531,532,533).

Data transmission between the sensor node 520,521 of the middle level and the sensor nodes 530,531,532,533 of the lowest level, and the local time of the sensor node 530,531,532,533 of the lowest level, the time of the sensor node 520,521 of the middle level. Since the synchronization process proceeds in the same manner as the above-described highest level and intermediate level, a description thereof will be omitted.

Therefore, the sensor nodes arranged at the lowest level and the highest level constituting the sensor network through the above process are synchronized with the time of the sensor nodes arranged at the highest level to perform the network.

Accordingly, according to the configuration as described above, the ATSP according to the present invention sends its local time to the packet immediately before transmitting the time synchronization packet to the sensor node of the higher level. Stamp error can be eliminated, enabling accurate time synchronization.

In particular, by transmitting a response packet to a synchronization packet of a lower level at a higher level, the number of data exchanges between a lower level sensor node and a higher level sensor node can be reduced as compared with the conventional unicast method.

Therefore, the power consumption of wireless communication of the sensor nodes for time synchronization can be reduced, thereby improving the power consumption efficiency of the sensor nodes, thereby improving the life time of the sensor nodes.

Meanwhile, according to an additional embodiment of the present invention, the ATSP according to the present invention includes synchronizing local time of sensor nodes or newly added sensor nodes which are not synchronized during time synchronization.

That is, when there is a request for a synchronization packet from a sensor node that is not time-synchronized or a newly added sensor node, it transmits its local time information, which is stored at a high level of local time in memory, to a corresponding sensor node that has requested a synchronization packet.

Accordingly, sensor nodes that are not time synchronized or newly added sensor nodes modify their local time based on time information included in the response packet received from the sensor node that requested the synchronization packet.

As described in detail above, the accurate time synchronization protocol for the wireless sensor network according to the present invention transmits its local time in the packet immediately before transmitting the time synchronization packet to the higher level sensor node. Time stamp errors due to transmission time and access time can be eliminated, which provides the benefit of simple, fast and accurate time synchronization between distributed sensor motors.

In particular, by transmitting a response packet to a lower level synchronization packet at a higher level, the number of data exchanges between a lower level sensor node and a higher level sensor node can be reduced compared to the conventional unicast method. Since the power consumption of wireless communication for the sensor nodes can be reduced, the power consumption efficiency of the sensor nodes can be improved, thereby improving the life time of the sensor nodes.

Although the present invention has been described with reference to the accompanying drawings, it will be apparent to those skilled in the art that many different and obvious modifications are possible without departing from the scope of the invention from this description. Therefore, the scope of the invention should be construed by the claims described to include many such variations.

Claims (4)

In a protocol for synchronizing time information between sensor nodes in a wireless sensor network having a tree structure, A packet generation step of generating a synchronization packet including its own local time for time synchronization when transmitting event data for a detected event to a sensor node of a higher level; A transmission step of transmitting the generated synchronization packet to a sensor node of a higher level; A response signal transmission step of transmitting the response packet in a broadcast manner when the response packet corresponding to the synchronization packet is transmitted from the higher level sensor node to the sensor node of the lower level receiving the synchronization packet; If the identification information included in the response packet received from the higher level sensor node is matched with the identification information stored in the memory, the local time is converted to the higher level local time using the time information included in the response packet. Modifying time synchronization steps; Accurate time synchronization protocol for a wireless sensor network comprising a. The method of claim 1, The reply packet includes at least identification information of a corresponding sensor node that receives the response packet, transmission time information of a lower level sensor node that has transmitted the synchronization packet, and a reception time of a higher level sensor node that has received the synchronization packet. Time information including information and response packet transmission time information of the higher level sensor node. The method of claim 2, The time synchronizing step further includes calculating a propagation delay time using time information of the received response packet. 4. The method according to any one of claims 1 to 3, wherein the time synchronization protocol is: If there is a synchronization packet request from a non-time-synchronized sensor node or a newly added sensor node, transmitting the local time information of its synchronization time stored in the memory at a high level local time to the corresponding sensor node requesting the synchronization packet. An accurate time synchronization protocol for a wireless sensor network further comprising.

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