KR101270684B1 - Hybrid beacon scheduling method for zigbee networks - Google Patents

Abstract

PURPOSE: A hybrid beacon scheduling method for zigbee communication is provided to efficiently use communication resources by minimizing collision between beacons. CONSTITUTION: A cluster group is selected based on a set relationship between clusters (S10). A beacon interval of the clusters included in a cluster group and a data transmission/reception section are set (S20). The relationship includes a logical distance between the different clusters, a physical distance, and a beacon section. When the duty cycle gain is generated, two select clusters are overlapped. [Reference numerals] (AA) Start; (BB) End; (S10) Select an overlappable cluster group based on a relationship between clusters; (S20) Select a beacon section and a data transmission and reception section between the clusters included in the cluster group

Description

Hybrid beacon scheduling method for zigbee communication

The present invention relates to hybrid beacon scheduling for zigbee networks. More particularly, the present invention relates to a hybrid beacon scheduling for zigbee networks. A hybrid beacon scheduling method for Zigbee communication.

For low power, low cost and low speed wireless transmission based on IEEE 802.15.4, zigbee has been rapidly used in many industrial and environmental fields, as shown in references 1 and 2. Zigbee has been used around our lives, including digital home networks, real-time location detection and factory monitoring. IEEE 802.15.4 is a standard for defining a physical layer (PHY) and a media access control layer (MAC), and is one of the standards for low rate wireless personal networks (LR-WPANs). Managed by the IEEE 802.15 working group.

The cluster tree ZigBee network disclosed in Ref. 6 consists of a number of clusters and end devices each having a cluster head (also called a router or coordinator). Among these cluster heads, one main coordinator is called a personal area network (PAN). In many cases, the PAN functions as a common sink node and is connected to a server computer, whereby routers and / or devices are connected to the PAN. Each end device may transmit data upon receiving a beacon from a router in the cluster. For example, in the cluster-tree of FIG. 1B, node N2 belonging to cluster 1 may transmit data only when receiving a beacon frame from its router N9. . FIG. 1A is a real node layout, and FIG. 1B is a diagram showing a logical cluster-tree. Here, each node means a wireless terminal, and a router is a node and plays a role of relaying transmission data of another node.

In such a cluster tree network, it should be recognized that clustering is merely a logical grouping of nodes and thus two nodes of different clusters can physically interfere with each other. In other words, if two physically adjacent clusters transmit beacons simultaneously, the beacons and data transmissions of these clusters may collide. Thus, if the beacons of different clusters can be sent at any time without being carefully scheduled, it may result in large retransmissions and eventually packet loss.

In order to avoid such intercluster collisions, beacon scheduling algorithms have been proposed.

IEEE. Looking at the beacon frame structure of 802.15.4, the IEEE 802.15.4 MAC protocol supports two modes (see Ref. 1). That is, non-beacon activation mode and beacon activation mode. In the beacon activation mode, the coordinator periodically transmits the beacon frame and the end device (data collection and scheduler device) transmits data based on the received beacon frame (see Ref. 6). As shown in Fig. 2, the beacon frame is divided into two parts. That is, the beacon interval (BI) and the superframe duration (SD). BI is the interval from one beacon frame to the next beacon frame. SD starts following the beacon frame. SD is an active part in which each node belonging to the corresponding cluster is activated and transmits data using CSMA / CA and Guaranteed Time Slots (GTS). According to Reference 1, BI and SD are officially defined as follows in Equation 1 below.

The ranges of BO (Beacon Order) and SO (SD Order) are 0 ≦ SO ≦ BO ≦ 14, and aBaseSuperframeDuration has a frequency of 2.4 GHz and 15.36 ms when the data rate is 250 Kbps.

In order to avoid inter-cluster collisions, references 4, 5, 7, 9 proposed a beacon serialization scheduling scheme. In the serialization technique, only one coordinator transmits a beacon at a time so that the beacon frames do not overlap each other. In addition, the serialization technique also organizes the beacon frames so that they do not overlap with the data frames of other clusters. Therefore, the Superframe Duration Schedule (SDS) scheme serializes both beacons and SDs in a non-overlapping fashion. For example, in FIG. 3, when beacon frames B1 and B2 of cluster 1 and cluster 2 are transmitted at the same time (t1), two beacons may collide with each other. However, if B2 is delayed by an offset, the beacon and the SD do not overlap and thus avoid inter-cluster collisions.

However, this beacon serialization scheduling can accommodate only a small cluster network because only one cluster can transmit a beacon for the entire network at a time. In particular, the serialization technique may determine that a given cluster tree network is inoperable (or unschedulable) and may be in a schedulable state if some overlap is allowed. For example, Reference 4 defines a necessary condition of scheduling possibility as shown in Equation 2 below. In other words, if the sum of the duty cycles is greater than 1, the target cluster tree network is not schedulable. However, if some overlap is allowed, the total utilization is lowered, thus allowing the target network to be scheduled.

IEEE 802.15.4: Wireless Medium Access Control (MAC) and Physical Lay (PHY) Specifications for Low-Rate Wireless Personal Area Networks (WPANs), IEEE, Sept 2006. 2. Zigbee-Alliance, "Zigbee specification", http://www.zigbee.org/, 2005. 3. Anis KOUBAA, Mario ALVES, Lelek ATTIA, ans Annelean VANNIEUWENHUYSE, "Collision-Free Beacon Scheduling Mechanisms for IEEE 802.15.4 / Zigbee Cluster-Tree Wireless Sensor Networks", Nov. 2006. Ashay DHARWADKER, "The Vertex Coloring Algorithm", http://www.dharwadker.org/vertex_coloring, 2006. 5.Anis Koubaa, Andre Cunha, Mario Alves, "A Time Division Beacon Scheduling Mechanism for IEEE 802.15.4 / Zigbee Cluster-Tree Wireless Sensor Networks", Real-Time Systems, 2007.ECRTS '07 / 19th Euromicro Conference. 6. Emanuele Toscano, Lucia Lo Bello, "A Multichannel Approach to Avoid Beacon Coolisions in IEEE 802.15.4 Cluster-Tree Industrial Networks", ETFA 2009. IEEE Conference. Sept. 2009. Saad A.Khan, Fahad A.Khan, "Performance analysis of a Zigbee beacon enabled cluster tree network", ICEE '09. April 2009. Anis Koubaa, Andre Cunha, Mario Alves, "Implementation Details of the Time Division Beacon Scheduling Approach for ZigBee Cluster-Tree Networks", July. 2007. 9. M.R.Garey, D.S.Johnson, "The Complexity of Near-Optimal Graph Coloring", JACM, Jan. 1976. 10.Anis KOUBAA, Mario ALVES, Eduardo TOVAR, "Modeling and Worst-Case Dimensioning of Cluster-Tree Wireless Sensor Networks", Real-Time Systems Symposium, 2006. 11. Cplex, "IBMILOG CPLEX Optimization Studio.

However, such conventional beacon scheduling methods have solved this inter-cluster collision problem by serializing all beacon frames so that no beacon intervals between cluster pairs overlap, but such beacon serialization scheduling is performed at a time for the entire network. Since only a cluster can transmit a beacon, there is a disadvantage in that only a small size cluster network can be accommodated, and resources have a disadvantage of unnecessary waste.

In particular, the beacon serialization scheduling as described above has a disadvantage in that it cannot prevent beacon collision between sibling clusters or clusters that are not logically close but are physically adjacent to each other and may cause signal interference.

Therefore, the present invention has been proposed to solve all the problems occurring in the prior art as described above,

An object of the present invention is to provide a hybrid beacon scheduling method for ZigBee communication to allow the partial overlap between the beacons to minimize the collision between the beacons and to increase the use efficiency of communication resources.

Another object of the present invention is to provide a hybrid beacon scheduling method for ZigBee communication that allows partial overlap between beacons to minimize collisions between beacons and enable efficient use of communication resources.

The hybrid hybrid beacon scheduling method for ZigBee communication according to the present invention for solving the above problems,

A method of scheduling a beacon in a Zigbee communication network system comprising a cluster comprising a node for performing measurements and a coordinator for transmitting measurement data of the node, and a data collection server for performing beacon scheduling of the cluster. ,

(a) selecting an overlapping cluster set based on a predetermined correlation to allow beacon overlap between the clusters;

(b) setting a beacon period and a data transmission / reception period of the clusters included in the selected overlapping cluster set.

In the above, the correlation of step (a) is characterized in that it includes the correlation between the logical, physical distance, beacon interval between different clusters.

In the above, step (a) is characterized by identifying the condition that can be superimposed beacon frame between the cluster using the following equation.

<Formula>

Here, D _{(i, j)} corresponds to the physical distance between two Zigbee nodes i and j, and r _i , r _j are transmission ranges of nodes i and j, respectively. C _A and C _B mean clusters, and the α value represents a control factor.

In the above, step (b) is characterized in that it comprises the step of defining the clusters included in the cluster set as a single virtual cluster.

In the above, step (b) is characterized in that it comprises the step of redefining the beacon period and the data transmission and reception interval.

According to the present invention, by allowing partial overlap between two clusters, the total utilization of resources is reduced, thereby minimizing the collision between beacons, thereby increasing the use efficiency of communication resources.

1A is an actual node layout.
1B illustrates a logical cluster-tree.
2 is a structural diagram of a typical beacon frame.
3 illustrates an example of preventing collisions between two clusters through offsets.
4 is a flowchart of a hybrid beacon scheduling method for ZigBee communication according to the present invention.
5 is an exemplary diagram of a cluster tree Zigbee network.
6 illustrates SO and BO values for each cluster.
7 is a SDS schedule to grouping SDS schedule comparison.
8 is an explanatory diagram when a duty cycle gain fails when grouping two clusters.
9 is an illustration of a hybrid beacon scheduling algorithm.
10 is a beacon transmission schematic diagram of beacon serialization scheduling.
11 illustrates an example parameter for simulation of non-scheduling.
12 is an exemplary packet drop rate.
13 is an exemplary retransmission rate diagram for each device.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the following description, well-known functions or constructions are not described in detail since they would obscure the invention in unnecessary detail.

The present invention proposes a hybrid SDS algorithm that allows partial overlap. The main idea of the proposed scheme is simple and intuitive in that the two clusters can be overlapped without collision if they are geographically separated.

To this end, in the present invention, as shown in FIG. 4, the subsets of clusters define conditions that may overlap each other to develop an optimal grouping method to maximize utilization gain (S10), and to create new beacons in the grouped clusters. The collision is minimized by designating the section BI and the data transmission / reception section SD.

More specifically, it is as follows.

Although not shown in the figure, a Zigbee network system to which the present invention is applied may include a cluster including a node performing measurement and a coordinator transmitting measurement data of the node, and a data collection server performing beacon scheduling of the cluster. Include. Hereinafter, a specific algorithm of the present invention means a series of processes performed by the data collection server.

First, in step S10, it is necessary to identify the necessary condition that the beacon frames of the clusters A and B can be superimposed by Equation 3, and set a subset by grouping the superimposed clusters based on this.

In the first condition, D (i, j) corresponds to the physical distance between two Zigbee nodes i and j, and r _i , r _j are the transmission ranges of nodes i and j, respectively. In order to allow the overlap of two clusters C _A and C _B , all nodes belonging to C _A must be separated from any node belonging to C _B by the sum of the transmission range plus α. The value of α represents the control factor, and if this value is large enough, the superposition of the two clusters is safe that these clusters are sufficiently separated from each other and that the RF signals of these clusters do not interfere with each other. On the other hand, it is natural to find a pair of clusters that satisfy the condition, if the value of α is very small. The key is to control the value of α depending on how much overlap between clusters is needed. For example, if the target cluster tree ZigBee network is very crowded and becomes unschedulable by the serialization SDS algorithm, it may be necessary to overlap many clusters. In this case, the α value is set small to accommodate this necessity.

Next, in step S20, beacon sections and data transmission / reception sections of the clusters included in the selected nestable cluster set are set.

Once you decide which pairs to group, you need to assign new BI and SI values to the grouped clusters for further SDS scheduling. In particular, grouped clusters are treated as a single virtual cluster for scheduling. Therefore, the present invention carefully designates new BI and SI for each group based on the following rules. That is, the rules are constrained below.

1. The total duty cycle (utility value in Equation 2) should be reduced.

2. The initial BI and SI of the clusters must be maintained, ie the newly assigned BI and SI of the group can accommodate the initial BI and SI of the clusters inside.

For example, in the cluster tree Zigbee network as shown in FIG. 5, assume a situation in which BO and SO values are designated as shown in FIG. 6 and clusters 3 and 5 are selected to be grouped. In this situation, specify BO and SO values for the group as 4 and 1, respectively. As a result, a beacon and an SD schedule as shown in FIG. 7 are obtained. The upper portion of FIG. 7 illustrates an initial serialization schedule without overlap, while the lower portion illustrates a modified schedule after grouping. By allowing overlap between clusters 3 and 5, total utilization is reduced from 21/32 to 19/32 while BI and SD in clusters 3 and 5 are the same as before.

In general, the following four cases are presented respectively.

1. If SO _i = = SO _j : The BO _group is assigned to the smaller of BO _i and BO _j . Otherwise, clusters with smaller BOs will not be able to be scheduled properly and will collide with other clusters. Of course, SO _group = SO _i = SO _{j .}

2. BO _i = = BO _j if: SO _group is SO _i and SO _j Is assigned to a large value. Otherwise, clusters with larger SO values will not be able to be scheduled properly and will collide with other clusters. Of course, BO _group = BO _i = BO _{j .}

3. BO _i > BO _j and SO _i <SO _j In this case, the beacon and the SD of the cluster i may be included in the schedule of the cluster j, as in the example of FIG. 7. Thus, SO _group = SO _j and BO _group = BO _{j .}

4. BO _i > BO _j and SO _i > SO _j If: In this case, the beacons and SDs of one cluster cannot be included in each other and no common multiple can be found to accommodate both cluster i and cluster j. Therefore, clusters belonging to this category cannot be grouped. For example, suppose that clusters 1 and 2 of FIG. 6 are grouped. In this case, to accommodate the initial BI and SD of the cluster 1 and cluster 2 of the group, the BI and SD for the group are designated as 4 and 2, respectively. In this way, the initial BI and SD of clusters 1 and 2 can remain the same. However, the total duty cycle increases from 21/32 to 23/32 as shown in FIG. In this case, grouping these two clusters does not gain any benefit and worsens the situation.

To reflect the fourth case described above, other overlapping conditions are presented. In other words, the overlapping of clusters is allowed only when the overlap gives a duty cycle gain.

By excluding the fourth case because of the above overlap condition, the generalized rule for specifying BO _group and SO _group when grouping cluster _i with BO _i and SO _i and cluster _j with BO _j and SO _j as follows: (Equation 5 and these conditions are for nesting two clusters. The rules and conditions can be easily extended for nesting more than two clusters.)

Once determining whether each pair of clusters can overlap based on the two conditions described above, it is now necessary to "carefully" select the subset (s) of the non-interfering pairs. To do this, you can think of applying a graph coloring algorithm simply. However, reducing the number of colors to be used to cover the entire graph, ie maximizing overlap between clusters, is not the purpose of the beacon overlap algorithm of the present invention. Instead, it is more important to minimize the total duty cycle. To reflect this fact, we develop the following ILP (Integer Linear Programming) based Equation 6.

This is subject to the following conditions.

The purpose of the optimization equation described above is to minimize the total duty cycle defined in Equation 2. There are four constraints on the equation. The first and second constraints correspond to the fact that each cluster can belong to only one group. The third constraint requires that two clusters u and v cannot belong to the same group c if neither of the two overlapping conditions mentioned in the previous section is met. In particular, the term edge in this constraint indicates that the two clusters are in violation of at least one of the overlapping conditions. The fourth constraint reflects that the duty cycle of each group is defined as the maximum value of SD / BI of clusters belonging to the group. To implement the "maximum" concept in the ILP, the constraints in Equation 9 are used. In addition, the duty cycle is multiplied by MaxBI, the maximum of the BIs of all clusters, so that the duty cycle is an integer value. This value is known in advance and is therefore treated as an integer in the ILP.

A hybrid beacon scheduling algorithm in accordance with the present invention is disclosed in FIG.

1. Calculate the utilization of the target cluster network.

2. If utilization> 1 (ie non-schedulable),

3. For all possible pairs of clusters, determine whether two clusters interfere with each other (see first overlap condition).

4. Determine whether the overlap of the two clusters provides a duty cycle gain (see second overlap condition).

5. Perform ILP using the cplex [11] solver to get a set of clusters to be grouped.

6. Specify BO and SO for each group.

7. Verify that the modified utilization is still less than one. If still less than 1, decrease the value of α and go to step 3 above.

8. Run the SDS algorithm.

Looking at the simulation results of the proposed method as follows.

Specifically, ns-2 simulations were performed on two test sets as shown in FIG. 10. For all of these test sets, the same BO and SO values were used as shown in FIG. 6. Detailed simulation parameters are described in FIG. 11.

Using three metrics: 1) packet drop rate, 2) utilization, and 3) retransmission rate, the scheme of the present invention was compared with non-scheduling and beacon serialization scheduling.

In the case of a non-scheduling algorithm, all coordinators transmit beacons simultaneously. On the other hand, in the case of beacon serialization scheduling, each coordinator transmits a beacon according to the schedule shown in the upper part of FIG. In the case of using the present invention, as shown in the lower part of FIG. 7, clusters 3 and 5 are grouped to transmit beacons simultaneously.

12 shows the packet drop rate of each algorithm for two test sets. The packet drop rate of the proposed scheme is very similar to that of serialized SDS, but the proposed hybrid scheme provides lower utilization than serialized SDS. This means that the proposed algorithm can reduce the utilization without sacrificing non-collision characteristics. It is natural that the non-scheduling case exhibits the highest packet drop rate because beacons and data transmissions between adjacent clusters interfere with each other without scheduling. We also observed that the packet drop rate of non-beacon scheduling was not significant in Test 1 but was severe in Test 2. This can be explained by the fact that Test 1 consists of a small number of devices and therefore each device is less congested with a high chance of attempting to send packets successfully.

This fact was further analyzed by examining the retransmission rate for each device as shown in FIG. As a result, it was confirmed that for the test 1, the packet drop rate of the non-scheduling case is not serious, and the retransmission rate is almost four times higher than that of other scheduling cases at the congestion node (ie, D4). This additional retransmission can result in additional power consumption and end-to-end delays.

In conclusion, we can see that the proposed hybrid algorithm provides 1) better utilization than serialized SDS and 2) better packet drop rate (and retransmission) than non-scheduling.

It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined in the appended claims and their equivalents. Of course, such modifications are within the scope of the claims.

N1 ~ N11: Node

Claims (7)

A method of scheduling a beacon in a Zigbee communication network system comprising a cluster comprising a node for performing measurements and a coordinator for transmitting measurement data of the node, and a data collection server for performing beacon scheduling of the cluster. ,
(a) selecting an overlapping cluster set based on a predetermined correlation to allow beacon overlap between the clusters;
and (b) setting a beacon period and a data transmission / reception period of the clusters included in the selected nestable cluster set.
The method of claim 1, wherein the correlation of the step (a) comprises a correlation between the logical, physical distance, beacon interval between different clusters.
delete The method of claim 1, wherein step (a) allows overlapping of clusters only when the overlapping of two clusters results in a duty cycle gain.
The method of claim 1, wherein the step (a) calculates the utilization of the target cluster network when selecting a nestable cluster set, and determines whether to schedule by comparing a preset value to determine the utilization and scheduling. Hybrid Beacon Scheduling Method for Zigbee Communication.
The method of claim 1, wherein step (b) comprises defining clusters included in the cluster set as a single virtual cluster.
The method according to claim 1, wherein step (b) comprises the step of scheduling and redefining the beacon period and the data transmission and reception intervals for the hybrid beacon scheduling method for Zigbee communication.

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