Method of controlling broadcasting in a wireless sensor network

The present invention relates to a method of controlling broadcasting in a wireless sensor network. The method includes performing broadcasting from an upper node to a lower node for a predetermined number of times, transmitting a time offset calculated according to the broadcasting from the lower node to the upper node, calculating an average of the time offsets received from at least one lower node, by the upper node and resetting the number of times of broadcasting according to the calculated average by the upper node. Accordingly, the number of times of broadcasting may be controlled according to the average of received time offsets in the wireless sensor network. In another aspect, an adjustment to the number of times of broadcasting may be received and the number of times of broadcasting may be adjusted based on an average of the received adjustments.

CLAIM OF PRIORITY

This application claims the benefit of the earlier filing date, pursuant to 35 USC 119) to that patent application entitled “METHOD OF CONTROLLING BROADCASTING IN WIRELESS SENSOR NETWORK” filed in the Korean Intellectual Property Office on Oct. 16, 2007 and assigned Serial No. 2007-0103865, the contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to wireless broadcast sensor networks, and more particularly, to a method of controlling broadcasting between nodes configured in a wireless sensor network.

2. Description of the Related Art

WSN (Wireless Sensor Network) is a network configured with a sensor node for collecting information through a sensor, and a sink node for transmitting the collected information to an external network. Unlike a conventional network, the WSN is not used for a communication purpose, but is mainly used for an automated remote information collection and is widely used for development of various applications in scientific, medical, military, and commercial fields.

Most of the wireless sensor networks are configured with hundreds of nodes, and various time synchronizing systems are used to secure a uniform time period in each node by reducing time deviations. The time synchronizing systems include TPSN (Timing-synch Protocol for Sensor Network), RBS (Reference Broadcast Synchronization), and FTSP (Flooding Time Synchronization Protocol) systems, and the FTSP system is most frequently used.

The FTSP system is a time synchronizing system in which time information is transmitted to a network at a time point of transmitting a packet by using a time stamp, and an upper node receiving the packet corrects its local time by referring to the time stamp of the received packet. The upper node then broadcasts the corrected local time to a lower node configured in a wireless sensor network.

In the FTSP system, the upper node must broadcast for a short period to improve the accuracy of time synchronization between nodes configured in a wireless sensor network. In this case, the upper node broadcasts the time information regardless of the current status of a lower node, and thereby the energy efficiency of the total wireless sensor network may be decreased. Additionally, in the case of a wireless sensor network having a large number of nodes, a conventional FTSP system may limit the number of usable nodes and may have a disadvantage that the period of a power source replacement can be shortened.

SUMMARY OF THE INVENTION

The present invention provides a method of controlling a broadcasting period.

Further, the present invention further provides a method of controlling a broadcasting period of an upper node by using a time offset.

A method of controlling broadcasting according to an exemplary embodiment of the present invention includes performing broadcasting from an upper node to a lower node for a predetermined number of times, transmitting a time offset calculated according to the broadcasting from the lower node to the upper node, calculating an average of the time offsets received from at least one lower node, by the upper node and resetting the number of times of broadcasting according to the calculated average by the upper node.

According to the present invention, the energy efficiency in a wireless sensor network may be improved by preventing unnecessary broadcasting between nodes. Additionally, the energy efficiency in the total wireless sensor network may be improved, because the “number of times of broadcasting” required for a specific area or a specific application can be controlled. Further, the structure of the wireless sensor network can be managed in various manners with an energy saving effect.

DETAILED DESCRIPTION

‘Broadcasting’ means transmitting a standard time value to a plurality of lower nodes at a corresponding data transmission time point by using a time stamp for time synchronization in an upper node.

FIG. 1is a block diagram showing a configuration of a wireless sensor network system according to the present invention.FIG. 2is a block diagram showing a configuration of a sensor node ofFIG. 1.

Referring toFIG. 1, the wireless sensor network system is configured with a sensor field100, sink200, external network300, and terminal400.

The sensor field100includes a plurality of sensor nodes151,153in a sensor array152disposed in a tree structure. That is, the sensor nodes151,153(generally referred to hereinafter as150) are disposed in a hierarchical structure, in which each level contains a predetermined number of sensor nodes150.

As shown inFIG. 2, each sensor node150includes a wireless communication unit210for supporting data communication, a control unit220for processing data, and a storage unit230for storing various data. In particular, the control unit220of the sensor node150includes a time offset calculation module223for calculating a time offset as a difference between a standard time value broadcast by an upper node150and a time value of a lower node150—i.e., node151, node153, respectively inFIG. 1, and an average time offset calculation module225for calculating an average of time offsets received from the lower nodes150. The sensor node150complies, for example, with the IEEE 802.15.4 standard and is used as a module for a small and low-power sensor network. The IEEE 802.15.4 standard is well-known in the art and a detailed description of this wireless standard protocol need not be discussed in detail herein.

The sink200connects the sensor field100to the external network300, and is also referred to as a gateway or a base station.

The external network300may be a public network, such as the Internet, a mobile communication network or a wired/wireless network, and is used to transmit data generated by the sensor field100to the terminal400.

The terminal400is a data processing device and may be any instrument identifying data generated by the sensor field100, such as a mobile terminal and a computer for transmitting and receiving data.

FIG. 3is a flow chart showing a method of controlling broadcasting between nodes151and153shown inFIG. 1according to an exemplary embodiment of the present invention.

Referring toFIG. 3, the upper node151sets a standard time value based on a data transmission time point when transmitting data, and broadcasts to the lower node153for a predetermined number of times (S310). In the lower node153, a time offset is calculated as a difference between the standard time value received through broadcasting and the time value of the lower node153(S320). Subsequently, the lower node153transmits the calculated time offset to the upper node151(S330).

When the time offset is received, the upper node151calculates an average value of the time offsets received from at least one lower node153(S340). Here, the upper node151forms a group by connecting to a plurality of lower nodes153in the wireless sensor network. In the case that the broadcast is performed at the upper node151, the time point of receiving broadcasting at each lower node153may be different. Accordingly, the time offset for each lower node is calculated as a difference between the standard time value and the current time of the corresponding lower node153. Time offset is then transmitted to the upper node151. The upper node151then calculates the average of the time offsets received from the plurality of lower nodes153.

The upper node151compares the calculated average with a predetermined average value (S350). The predetermined average may be set at different value according to environmental factors of the wireless sensor network or according to a user setting.

The upper node151performs broadcasting according to the result of comparison at step S350(S360). If the calculated average is less than the predetermined average, the upper node151performs broadcasting for a number of times less than a predetermined number of times. If the calculated average is greater than the predetermined average, the upper node151performs broadcasting for a number of times greater than the predetermined number of times.

FIG. 4is a flow chart showing a procedure of controlling broadcasting in the upper node151ofFIG. 3.

Referring toFIG. 4, the upper node151performs broadcasting for a predetermined number of times (S410). The upper node151determines how many lower nodes153are connected thereto, then determines whether time offsets are received from a select number of numbers (e.g., more than half) of the lower nodes153(S420). For example, if the upper node151communicates with 5 lower nodes153, the number of time offsets received from the lower nodes153after broadcasting must be at least 3. For each lower node153, the time offset is the difference between the time value of the lower node153and the standard time value received from the upper node151.

If time offsets are not received from more than half of lower nodes153, the upper node151performs broadcasting for a number of times greater than the predetermined number of times (S457). Additionally, if time offsets are not received from more than half of lower nodes153, the upper node151may transmit the standard time value to the lower node153by unicasting, namely, by a 1:1 transmission (not shown). In the case that a large number of lower nodes153are included in the upper node151, unicasting may decrease the accuracy of time synchronization or the energy efficiency. Accordingly, in the case that the number of lower nodes153returning a time offset value is greater than a predetermined value, broadcasting may be performed by using an average calculated from the time offsets in a group having the greatest number of nodes. Alternatively, the upper node151may perform broadcasting by using an average calculated from time offsets having a higher value than those received from the lower nodes153.

If time offsets are received from more than the predetermined number of lower nodes of lower nodes153at step S420, the upper node151calculates an average of the time offsets received from the lower nodes153(S430). Here, the predetermined number is half the number of lower nodes153and the average may be calculated by calculating a sum of all received time offsets and dividing the sum by the number of lower nodes153. For example, it is assumed that the upper node151includes 5 lower nodes153from which the time offsets are received. The upper node151identifies whether time offsets are received from more than half of lower nodes153after performing broadcasting. If time offsets are received from more than half of lower nodes153and the time offsets received from the lower node153are respectively 7 μs, 9 μs, 8 μs, 6 μs, and 7 μs, the upper node151calculates an average from the received time offset. In this case, the average is 7.4 μs, because the sum of the received time offsets is 37 μs and the number of lower nodes153is 5. In another aspect, it is assumed that the upper node151includes 5 lower nodes153and the time offsets are received from 4 of lower nodes153e.g. respectively 7 μs, 9 μs, 8 μs and 6 μs. The upper node151identifies whether time offsets are received from more than half of lower nodes153after performing broadcasting. The upper node151calculates an average from the received time offset because the time offsets are received from more than half the number of lower nodes153. In this case, the average is 7.5 μs, because the sum of the received time offsets is 30 μs and the number of lower nodes153is 4.

If time offsets are not received from more than the predetermined number of lower nodes153at step S420, the upper node151performs broadcasting by adjusting the number of times of broadcasting to an increased number of times (S457).

After calculating the average of the time offsets, the upper node151compares the calculated average with a predetermined value (S440). The predetermined value may be set at different values according to environmental factors of the wireless sensor network. Alternatively, the predetermined value may be set by a user.

The upper node151performs broadcasting by resetting (adjusting) the number of broadcasts according to the result of comparison at step S440/S450. If the calculated average is less than the predetermined value, the upper node151resets (adjusts) the number of times of broadcasting to a decreased number of times (S453). For example, if the predetermined average value is 10 μs and the average calculated in a wireless sensor network broadcasting 8 times per period is 5 μs, the upper node151performs broadcasting by resetting the number of times of broadcasting to a decreased value (for example, 4 times per period).

If the calculated average is similar to or the same as the predetermined average value, the upper node151performs broadcasting for a predetermined number of times (S455). If the calculated average is greater than the predetermined average, the upper node151performs broadcasting by resetting (adjusting) the number of times of broadcasting to an increased number of times (S457). For example, if the predetermined average is 10 μs and the average calculated in a wireless sensor network performing broadcasting 8 times per period is 12 us, the upper node151performs broadcasting by resetting the predetermined number of times of broadcasting (8 times per period) to an increased number of times (for example, 10 times per period).

In the process of comparing the calculated average with the predetermined average value and increasing or decreasing the number of times of broadcasting according to the result of comparison, the increment or decrement of the number of times may be gradually adjusted according to the comparison result between the calculated average and the predetermined average.

While performing broadcasting, the upper node151may control the number of times of broadcasting according to the time offset. Because the time offset is a difference between a standard time value of the upper node151and a time value of a lower node153, a lower average of time offsets offers better time synchronization when the upper node151broadcasts to the lower nodes153. Accordingly, unnecessary broadcasting may be prevented by decreasing the number of times of broadcasting at the upper node151.

FIG. 5is a flow chart showing a procedure of calculating a time offset in the lower node153ofFIG. 3.

Referring toFIG. 5, the lower node153receives broadcasting from the upper node151(S510). The lower node153identifies a standard time value included in the broadcast, and calculates a time offset using the difference between the identified standard time value and the time value of the lower node153(S520). The lower node153then transmits the calculated time offset to the upper node151(S530).

In performing broadcasting according to the result of comparison between the average of time offsets and the predetermined average, the average of time offsets may be calculated according to the number of times of broadcasting. Additionally, time synchronization between sensor nodes150in the wireless sensor network starts at a starting point of sensing, and the time synchronization may be performed periodically by identifying the time offset between the sensor nodes150.

Hereinabove, a method of controlling broadcasting by using an average of time offsets transmitted from lower nodes153is described. That is, a method of controlling broadcasting by calculating a time offset of broadcasting at a lower node153and transmitting the calculated time offset to an upper node151has been described. However the present invention is not limited thereto, and other methods of controlling broadcasting may be applied. For example, the lower node153may transmit an increment or decrement of the number of times of broadcasting to the upper node151by using the time offset. That is, the lower node153may calculate a time offset, determine an increment or decrement of the number of times of broadcasting according to the calculated time offset, and transmit the increment or decrement to the upper node151. The upper node151then may calculate an average of the increments and decrements of the number of times of broadcasting that have been received from the lower nodes153, and may thereby reset the number of times of broadcasting for each lower node153.

The above-described methods according to the present invention can be realized in hardware or as software or computer code that can be stored in a recording medium such as a CD ROM, an RAM, a floppy disk, a hard disk, or a magneto-optical disk or downloaded over a network, so that the methods described herein can be rendered in such software using a general purpose computer, or a special processor or in programmable or dedicated hardware, such as an ASIC or FPGA. As would be understood in the art, the computer, the processor or the programmable hardware include memory components, e.g., RAM, ROM, Flash, etc. that may store or receive software or computer code that when accessed and executed by the computer, processor or hardware implement the processing methods described herein.

Although exemplary embodiments of the present invention have been described in detail hereinabove, it should be understood that many variations and modifications of the basic inventive concept herein described, which may appear to those skilled in the art, will still fall within the spirit and scope of the present invention as defined in the appended claims. For example, although the invention is describe with regard to receiving information from at least one-half the number of lower nodes, it would be within the knowledge of those skilled in the art to adjust this value to a select number of lower nodes providing the required information without having to perform undue experimentation.