WO2017018914A1

WO2017018914A1 - Power management based task distribution in a communication device cluster

Info

Publication number: WO2017018914A1
Application number: PCT/SE2015/050832
Authority: WO
Inventors: Aneta VULGARAKIS FELJAN; Azadeh BARARSANI; Keven WANG; Athanasios KARAPANTELAKIS
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 2015-07-27
Filing date: 2015-07-27
Publication date: 2017-02-02

Abstract

A communication device (12) acts as a master (D) in a cluster (10) of communication devices (12 – 28) for controlling the assigning of a work task according to a task distribution scheme, where each communication device in the cluster has short-range communication and data processing capability. The master (D) regularly obtains energy level data concerning the communication devices in the cluster, assigns the task to a communication device (W) in a group of communication devices in the cluster with capability to perform the task, sets a first energy level threshold, compares a current energy level of the currently assigned communication device with the first energy level threshold and assigns the task to another communication device in the group in case the energy level is below the first energy level threshold.

Description

POWER MANAGEMENT BASED TASK DISTRIBUTION IN A COMMUNICATION DEVICE CLUSTER

TECHNICAL FIELD

The invention relates to communication network clusters. More particularly the invention relates to a communication device acting as a master in a cluster of communication devices as well as to a method, computer program and computer program product for controlling the assigning of a work task, according to a power management based task distribution scheme in a cluster of communication devices.

BACKGROUND Mesh or clu ster networks have become popular in a variety of

applications or environments such as sensor networks for monitoring and actuation (for example, monitoring of crop growth and pesticide application, water quality, etc.), vehicular networks, geostatic cross satellite space communication .

The devices that comprise these networks are in many cases battery powered without availability to electrical power supply networks (e.g. mains electricity network) . Power management to preserve battery life is therefore an important issue.

Power management on an individual device level has been extensively investigated in for instance mobile communication systems. However, not much has been done in the field of distributed power management of a group of communication devices. Some documents that have touched on the subject are EP 1 835 668 and J P 2008078963.

EP 1 835 668 describes a method for controlling tran smission of data from a plurality of sen sor nodes to a monitor node of a network made up of the communication devices . In the method a graph is created for the sen sor nodes and monitor node based on the energy level of each node in the network. The graph is then u sed for finding an optimized routing tree.

J P 2008078963 describes a method for a wireless sen sor network that achieves low energy while allowing the wireless sen sor network to be flexibly adapted . The sen sor nodes are classified into a plurality of clu sters . A clu ster head that is to be u sed for performing designation of a gateway in side each clu ster is also selected . Each clu ster head collects available-energy information of each sen sor node in side each clu ster and intermittently and repeatedly executes inter-clu ster scheduling processing for selecting the sen sor node having the maximum available- energy as the gateway.

However, there is a need to look at power management on the system level, i.e., in a way that best suits the purpose of the network. It is therefore of interest to provide a way for collective managing of power on a more general level in order to improve systemic power con sumption .

It is in view of what has been stated above a need for providing an improved collective battery management of a clu ster of communication devices . SUMMARY

The invention is therefore directed towards providing a more efficient power supply management in a cluster of communication devices.

This object is according to a first aspect achieved by a communication device acting as a master in a cluster of communication devices. Each communication device in the cluster has short-range communication and data processing capability. Furthermore, the communication device acts as a master for controlling the assigning of a work task according to a power management based task distribution scheme. For this reason the communication device may comprise a processor acting on computer instructions whereby the communication device is configured to:

regularly obtain energy level data concerning the communication devices in the cluster,

assign the work task to a communication device in a group of

communication devices in the cluster with capability to perform the work task,

set a first energy level threshold,

compare a current energy level of the currently assigned communication device with the first energy level threshold, and

assign the work task to another communication device in the group in case the energy level is below the first energy level threshold. The object is according to a second aspect achieved through a method of controlling the assigning of a work task according to a power management based task distribution scheme in a cluster of communication devices. Each communication device in the cluster has short-range communication and data processing capability. The method is performed by one of these communication devices, which acts as a master. The method comprises: regularly obtaining energy level data concerning the communication devices in the cluster, assigning the work task to a communication device in a group of communication devices in the cluster with capability to perform the work task,

setting a first energy level threshold,

comparing a current energy level of the currently assigned communication device with the first energy level threshold, and

assigning the work task to another communication device in the group in case the energy level is below the first energy level threshold. The object is according to a third aspect achieved through a computer program for controlling the assigning of a work task according to a power management based task distribution scheme in a cluster of communication devices. Each communication device in the cluster has short-range communication and data processing capability. Furthermore, the computer program comprises computer program code intended to run in a communication device in the cluster, which communication device acts as a master. When the computer program code is running in this

communication device it causes the communication device to:

assign the work task to a communication device in a group of

communication devices in the cluster with capability to perform the work task,

set a first energy level threshold,

assign the data processing activity to another communication device in the group in case the energy level is below the first energy level threshold. The object is according to a fourth aspect achieved through a computer program product for controlling the assigning of a work task according to a power management based task distribution scheme. The computer program product comprises a data carrier with computer program code according to the third aspect.

The invention has the advantage of allowing an energy efficient

communication device usage from a system point of view.

In a first variation of the first and second aspects, at least some of the other communication devices in the group are sleeping. This measure has the advantage of prolonging the life-time of the group .

In a second variation of the first aspect, the communication device is configured to assign, for each communication device being assigned a work task, a sleeping communication device ready to take over the work task.

In a corresponding variation of the second aspect the method comprises assigning, for each communication device being assigned a work task, a sleeping communication device ready to take over the work task. The second variation has the advantage of providing redundancy.

In a third variation of the first aspect, the communication device is further configured to continue assigning the work task among the communication devices in the group until all are below the first energy level threshold.

In a corresponding variation of the second aspect the method further comprises continuing assigning the work task among the communication devices in the group until all are below the first energy level threshold. In a fourth variation of the first and second aspects, the first energy level threshold represents an energy level between a maximum level and a minimum operational level after which the communication device fails to operate.

When all communication devices in the group are below the first energy level threshold, the communication device may be further configured to set a lower energy level threshold, assign the work task among the communication devices in the group and compare the current energy level in the assigned communication devices in the group with the lower energy level threshold until all are below the lower energy level threshold and continue setting lower energy level thresholds, assigning the work task and comparing with energy level thresholds until all communication devices in the group are at the minimum level.

When all communication devices in the group are below the first energy level threshold the method then further comprises setting a lower energy level threshold, assigning the work task among the communication devices in the group and comparing the current energy level in the assigned communication devices in the group with the lower energy level threshold until all are below the lower energy level threshold and continuing setting lower energy level thresholds, assigning the work task and comparing with energy level thresholds until all communication devices in the group are at the minimum level.

The fourth variation has the advantage of prolonging the time in which all communication devices in the group are able to perform the work task, which is advantageous if redundancy is important.

In a fifth variation of the first and second aspects, the first energy level threshold represents a minimum operational level after which

communication devices fail to operate. This has the advantage of allowing the time in which the work task is possible to be carried out in the group to be prolonged as long as possible. In a sixth variation of the first and second aspects, the work tasks comprise a data handling task, a sensing task and a long-range communication task and there is one group of communication devices for each type of work task

In this case the communication device is configured to assign work tasks according to the task distribution scheme in all groups. Tassigning of work tasks according to the task distribution scheme made in the method is also performed in all groups.

In a seventh variation of the first and second aspects a subset of the communication devices in the cluster have long-range communication capability and at least some of the communication devices with long-range communication capability are excluded from the group being assigned data handling tasks.

In an eighth variation of the first and second aspects a subset of the communication devices of the cluster have sensor capability and at least some of the communication devices having sensor capability are excluded from the group being assigned data handling tasks.

In a ninth variation of the first aspect, the communication device is further configured to hand over the role of master to another communication device.

In a corresponding variation of the second aspect the method further comprises handing over the role of master to another communication device.

The ninth variation has the advantage in providing redundancy and flexibility also in relation to the role of master. In a tenth variation of the first aspect, the communication device is configured to hand over the role of master in case it itself is to be assigned the work task.

In a corresponding variation of the second aspect the handing over of the role of master to another communication device is done in case the current master itself is to be assigned the work task. In an eleventh variation of the first aspect, the communication device is further configured to perform initial communication with neighbouring communication devices and form the cluster with at least some of the neighbouring communication devices based on the initial communication . In a corresponding variation of the second aspect the method further comprises performing initial communication with neighbouring

communication devices and forming the cluster with at least some of the neighbouring communication devices based on the initial communication . In a twelfth variation of the first aspect, the communication device is further configured toassume the role of master after negotiating with the other communication devices of the cluster

In a corresponding variation of the second aspect the method comprises assuming the roll of master after negotiations with the other

communication devices of the cluster.

It should be emphasized that the term "comprises/ comprising" when used in this specification is taken to specify the presence of stated features, integers, steps or components, but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof. BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail in relation to the enclosed drawings, in which : fig. 1 schematically shows a clu ster of communication devices, where the communication devices have different roles,

fig. 2 schematically shows the same clu ster of communication devices, where some of the communication devices have changed roles, fig. 3 schematically shows one way of realizing a communication device that is able to take on a roll of master communication device,

fig. 4 shows a block schematic of another way of realizing a

communication device that is able to take on the roll of master

communication device,

fig. 5 shows a flow chart of a number of method steps in a first

embodiment of a method of controlling assigning of a work task according to a power management based task distribution scheme,

fig. 6 shows a flow chart of a number of method step s in a second embodiment of the method of controlling assigning of a work task according to a power management based task distribution scheme, fig. 7 shows a signalling chart of signals between three communication devices in the method according to the second embodiment,

fig. 8 shows a flow chart of a number of method steps in a third embodiment of the method of controlling assigning of a work task according to a power management based task distribution scheme, fig. 9 shows a signalling chart of signals between three communication devices in the method according to the third embodiment,

fig. 10 shows a computer program product comprising a data carrier with computer program code for implementing the method, and

fig. 11 shows one way of realizing a task distributing unit of a

communication device acting as master. DETAILED DESCRIPTION

In the following description, for purposes of explanation and not limitation, specific details are set forth such as particular architectures, interfaces, techniques, etc. in order to provide a thorough understanding of the invention . However, it will be apparent to those skilled in the art that the invention may be practiced in other embodiments that depart from these specific details . In other instances, detailed descriptions of well-known devices, circuits, and methods are omitted so as not to obscure the description of the invention with unnecessary detail.

The present invention concerns communication devices that together form clu sters or mesh networks. Such a network may in turn form a system that may be u sed in a variety of applications or environments such as a remote sen sing and actuation sensor system, vehicular system, geostatic cross satellite space communication etc.

Fig. 1 schematically shows one clu ster CL 10 of communication devices forming a local network or local system . The network is local in that all communication devices are able to communicate with each other within the clu ster u sing a clu ster limited communication technique. The range of the short range communication is thu s limited to within the clu ster. The clu ster limited communication technique may for in stance be a short-range communication technique such as Bluetooth, IEEE

6LowPAN (IPv6 over Low power Wireless Personal Area Networks), WirelessHart, ZigBee etc. One or more of the communication devices may also have long range communication capability, i.e. the ability to communicate outside of the clu ster. The u se of the long-range

communication is thu s beyond the clu ster limit.

As can be seen the communication devices in the clu ster CL 10 therefore make up nodes in a local network, such as a local mesh network. There is in this exemplifying network a first communication device 12 A, a second communication device 14 B, a third communication device 16 C, a fourth communication device 18 , a fifth communication device 20 , a sixth communication device 22, a seventh communication device 24 , an eighth 5 communication device 26 and a ninth communication device 28 .

The first and second communication devices 12 and 14 are here shown as exchanging information with each other, where the first communication device 12 is also shown as exchanging information with the fifth

1 0 communication device 20. The second communication device 14 is also shown as exchanging information with the third communication device 16 as well as with the fourth communication device 18 . The third and fourth communication devices 16 and 18 are also exchanging information with each other. Also the fourth communication device 18 is shown as

15 exchanging information with the fifth communication device 20 , which in turn exchanges information with the sixth communication device 22. The sixth communication device 22 exchanges information with the seventh communication device 24, which in turn exchanges information with the eighth communication device 26. The eighth communication device 26

2 0 finally exchanges information with the ninth communication device 28 .

In the clu ster 10 the communication devices have different roles . The first communication device 12 is here acting as a master communication device and is therefore a decision node D, while the second, third and

25 eighth communication devices 14, 16 and 26 perform some work task and are therefore working nodes W. One of the work tasks, the task

performed by the second communication device 14 , is long-range communication , which is thu s communication outside of the clu ster (e.g. towards the Internet) . For this reason the second communication device

3 0 14 is shown as exchanging information with an external network 30. The other communication devices 18 , 20 , 22, 24 and 28 are not involved in performing any work task. They are thu s non-active N, which are sleep nodes .

Fig. 2 shows the same clu ster and the same communication path s within the clu ster as in fig. 1. However, here it can be seen that some of the nodes have changed roles . The first communication device 12 is still acting as a decision node D or master . Also the fifth , sixth and ninth communication devices 20 , 22 and 28 are still non-active N or sleeping nodes . However the rest of the communication devices have changed . The second communication device 14 is now a non-active node N, as is the third and eighth communication device 16 and 26. On the other hand, the fourth, and seventh communication devices 18 and 24 are now working nodes W, where the seventh communication device 24

furthermore has taken over the work task of long range communication from the second communication device 14 and is therefore connected to the external communication network 30. Another observation that can be made is that the number of working nodes has been reduced . This may be due to the fact that the computational demands of the devices (i.e.

number of tasks needed to be performed) may be dynamic and vary. In the case illu strated in Fig. 2, a task may have ju st been completed and communication device 14 may no longer need to have the worker role.

It can thu s be seen that the work tasks performed by the communication devices have been shifted . Assignment of work tasks to communication devices is controlled by the master communication device 12 based on a power management based task distribution scheme. According to aspects of the invention , this task distribution scheme distributes work tasks within a group of communication devices in the clu ster based on the remaining energy levels of the power sources of these communication devices . How this may be done will shorty be described . The master node D thu s decides which of the nodes in the network will perform work tasks (e.g. some of them may relay data of other nodes via a long range radio to the network) , whereas others simply do

computation on data of other nodes . The rest of the nodes, simply communicate work tasks to the working nodes .

Before the task distribution is described, a short description of a communication device that is able to take on any type of working node role and a decision node role will be given with reference to fig. 3 and 4. The communication device will thu s be able to perform a number of work tasks as well as act as a master.

Fig. 3 shows a block schematic of one way in which the communication device, here exemplified by the first communication device 12, may be implemented. The communication device 12 may comprise a processor PR 32, such as central processing unit (CPU) as well as a program memory MEM 34, which memory may comprise computer program code with computer instruction s that cau se the processor 32 to implement a task distributing unit carrying out a task distribution scheme when the communication device 12 is operating as a master communication device. The program code may also make the processor 32 implement a remaining power metering and reporting unit, a clu ster forming unit and a master negotiating unit, which are active also when the communication device is not acting as a master. In the communication device 12 there is also a power source PS 35, for in stance in the form of a battery, a long range communication unit LRC 36 for long range communication outside of the clu ster, a short range communication unit SRC 38 for

communication within the clu ster and a sen sing unit SENSE 39 implementing one or more sensors . A short range communication unit 38 may as an example be a Bluetooth Communication unit, a Wireless Hart communication unit or a ZigBee communication unit, while a long range communication unit 36 may be realized as a mobile radio communication unit, such as a unit communicating with a Long Term Evolution (LTE) network. It should here be realized that it is possible that the sen sor unit 39 is only present in some of the communication devices. It is likewise possible that the long-range communication unit 36 is only present in some communication devices .

Fig. 4 shows a block schematic of another way in which the

communication device 12 may be implemented. Also in this case there is a power source 35, a long range communication unit 36 , a short range communication unit 38 and a sensing unit 39. In addition there is also a clu ster forming unit CF 40 , a task distributing control unit TDC 41, a remaining power level storage PLS 42, a remaining power metering and reporting unit PMR 43 , a task handling unit TH 44 and a master negotiating unit MN 45. As can be seen in fig. 4, the remaining power measurement and reporting unit 42 and remaining power level storage are shown as two logically separate units . However, it should be realized that they may be combined in the same physical component. Furthermore, in some cases the remaining power measurement and reporting unit may only be a remaining power metering unit..

The task distribution control unit 41 of a master communication device has the purpose to distribute tasks within a group of communication devices of the clu ster based on the energy levels of the power sources of the communication devices . How this may be done will now be described also with reference being made to fig. 5, which shows a flow chart of a first embodiment of a method of controlling the assigning of work tasks according to a task distribution scheme being performed by a master communication device. The communication devices in fig. 1 and 2 together form a clu ster u sed to provide a local network. A clu ster may be pre-determined in that a planning may have been made in advance, which communication devices that are to form the clu ster. Alternatively communication devices may form clu sters themselves, for instance at initial power-on . When forming clu sters, the communication devices in an area may exchange information about their capabilities, such as the capabilities to perform variou s tasks as well as their short-range communication capabilities . This information can then be u sed for forming a clu ster, such as the clu ster in fig. 1 and 2. The capability information may also be u sed to form groups within clu sters . A clu ster will then typically comprise communication devices that have good short-range communication capabilities with each other, for instance through communicating over high quality communication links, such as links with low signal to noise ratio. It is for in stance possible that the signal to noise ratio of the short-range communication between all

communication devices is calculated and that the signal to noise ratio of the communication links between the communication devices of a clu ster should all be above a clu ster forming threshold.

The clu ster forming unit 40 of the first communication device 12 may thu s perform initial communication with neighbouring communication devices 14 - 28 u sing the short-range communication unit 38 and form the clu ster with at least some of these neighbouring communication devices based on the initial communication . The forming of a clu ster may then be based on which neighbouring communication devices that are within range and which of these have functionality required in the clu ster. The range may be determined u sing a link qu ality being above the clu ster forming threshold. A communication device that is within range of another may be directly in range or possible to connect to via one or more hop s.

The communication devices of such a clu ster are arranged for performing work tasks . One work task may be the sen sing of a physical property such as temperature, humidity, rainfall, luminosity, gas concentration (e.g. carbon dioxide, carbon monoxide, oxygen) etc. u sing a sen sing unit 39 , another task may be the processing of data, such as the processing of one or more sen sed physical properties. Processing may in this case be made by the processor of one communication device in respect of functionality in another communication device. As an example of this one node may perform the task of sen sing a physical property. Another node may perform the task of processing the physical property in order to obtain a measure of the state of the environment in which the physical property is sen sed. Yet another node may perform the task of storing physical property measurements and/ or the results of such data processing.

Another node may finally perform the task of communicating the sensed properties and/ or processing results outside of the local network u sing long-range communication . The handling of the task, may thu s be performed by the task handling unit 44, which may u se a sen sing unit 39 comprising a sen sor if the task involves sen sing a physical property or the long-range communication unit in case the task is long-range

communication .

Each communication device that is to be able to act as master may have short-range communication and data processing capability. This is the minimum functionality needed. A communication device that is to handle the task of processing a physical property may not need any further functionality than this either.

However, a communication device that is to provide a sensing function would need to also comprise a sen sing unit 39. A communication device that is to store data would of course need a memory, while a

communication device that is to provide long-range communication would need a long-range communication unit 36.

The work tasks may thu s comprise data handling tasks, sensing tasks and long-range communication tasks, where a data-handling task may comprise data processing and/ or data storing. The carrying out of these tasks requires power. As the devices are battery powered, this is a scarce resource. In order to control the resource in a way that is most beneficial for the system or local network formed by the clu ster, one of the communication devices in the clu ster is acting as a master. In fig. 1 it can be seen that the first communication device 12 is acting as a master D. There are a number of ways in which a

communication device may take on the role of master. It is possible that one communication device is always acting as a master. It may thu s be set to be master from the start. A master may also be selected among the communication devices. There are several ways in which a master may be selected. It is possible that a master is selected as a node having the lowest task processing capability. It is for in stance possible to select a node that lacks sensor and/ or long-range communication or exten sive processing capability. It is also possible to select a master that has the best

communication quality with the other nodes. It is for instance possible that the signal to noise ratio of the short-range communication between all communication devices is calculated and an average is determined for each communication device. The device having the lowest average signal to noise ratio may then be selected to be a master. In other cases it is possible that the communication devices negotiate among each other about which one is to be a master. A master is thu s elected among the communication devices . The elections may u se the same criteria as that u sed for a selection . Therefore the master negotiating unit 45 of the first

communication device 12 may negotiate with the other communication devices 14 - 28 of the clu ster 10 via the short-range communication unit

38 in order to determine or elect master. It may more particularly assume the role of master after such a negotiation , i.e. if it wins the election .

The master then assign s work tasks according to a task distribution scheme. In order to do this it keep s track of all the work tasks being carried out in the clu ster. It may thu s be aware of which tasks are being

performed. If the clu ster is to perform a known set of function s, the variou s work tasks that are to be performed may be known beforehand and therefore the master may easily keep track of them . In other in stances some work tasks, especially some processing tasks and data storing tasks, may be dynamic. They may occur in the system during the carrying out of the set of functions . In those cases a demand for a task will occur in one of the communication devices, for in stance the communication device in charge of long range communication or a communication device involved in a computational task. In this case a request for carrying out the task may be sent from the communication device where the need for the task occurred to the master, where the master is then respon sible for handing out the new task to a communication device. A task may furthermore be passed from communication device to communication device. In this passing, it is then possible that the master comes to the conclu sion that it itself is to perform the task. In this case it is also possible that the role of master is tran sferred to another communication device.

A task distribution scheme may furthermore be provided for a group of communication devices . A task distribution scheme may thu s be provided for a group of devices that are to share the performing of the work task between each other, where a group may be all the communication devices of the clu ster or a subset of the communication devices of the clu ster. A group of communication devices may be provided for each type of task. Furthermore the task distribution scheme is power management based. The task distribution scheme is more particularly based on the remaining power or energy levels of the power sources 35 in the individu al

communication devices . In case the power sources are batteries, the energy or power level is thu s the battery level of the battery. In order to base the task distribution on energy levels, the communication devices all report their battery levels to the master. Therefore, the remaining power metering and reporting unit 43 in a communication device may regularly meter the energy level of the power source 35. This may be done periodically such as once a minute, once every 15 minutes, once every thirty minutes or once an hour.

The remaining power metering and reporting unit 43 may thereafter report the measurement to the task distribution control unit 41 of the master communication device via the short-range communication unit 38 . This may also be done regularly, such as periodically, for in stance u sing the same time intervals . The regular reporting may as an alternative be linked to a sen sed change from one energy level to another. It is for in stance possible that reporting is made when the power source 35 goes from one energy level to another. As an example a change of remaining battery from 90 to 89% or from 90 to 85% may trigger a reporting.

The reporting may furthermore be done in different ways . The master may poll the other communication devices . Alternatively the

communication devices may pu sh energy level data to the master.

It is thu s possible to u se a pu sh based mechanism , where the

communication devices report their reserve battery level to the master by them selves, either periodically or asynchronou sly when their remaining power measurement and reporting units 43 sen se a change in the reserve power or energy level of some granularity. Alternatively a pull based mechanism may be u sed , where the communication devices are polled for their battery statu s from the task distribution control unit 41 of the master communication device, either periodically, or asynchronou sly, whenever the master node re-computes a work schedule.

Therefore, when acting as a master, the task distribution control unit 41 of the master communication device D regularly obtains energy level data concerning the communication devices in the clu ster via the short-range communication unit 38 , step 46. In this way the master is aware of the battery life of each node and u ses this as a con straint in the decision process .

For a work task needing to be assigned, the task distribution control unit 41 then assign s the task to a communication device in a group of communication devices in the clu ster able to perform the task, step 47. As stated above a task may be a special type of task, such as sen sing, heavy computing, exten sive data storing or long range communication . This mean s that the whole clu ster is not necessarily able to perform the task, but only a sub set. In this case a communication device in a group that forms this subset is assigned the task. The selected communication device starts to perform the task u sing the task handling unit 44. It is thereby working. In other in stances all communication devices in the clu ster may be able to perform the task. In this case it is possible that the communication device is selected from a group that is the same as the whole clu ster. In both these cases it is furthermore possible that the master is excluded from the group . Alternatively it may be a part of the group . It is in any event possible that all or at least some of the other communication devices of the group are sleeping. It is also possible that for each communication device being assigned a work task, that a sleeping communication device is ready to take over the task.

It is furthermore possible that a subset of communication devices in the clu ster that have long-range communication units 36 and thereby have long-range communication capability are excluded from a group assigned a data-handling task. It is also possible that a subset of the

communication devices of the clu ster having a sen sor unit and thereby having a sen sing capability are excluded from a group assigned data handling tasks .

The task distribution control unit 41 also sets a first energy level threshold, step 48 . It is possible that this threshold is set before the task is being assigned . In this case it is also possible that the group from which a communication device is selected only comprises communication devices capable of performing the task having power source energy levels that exceed this first threshold . The initial selection may as an example be a random selection of a device with a battery level above the threshold . It may also be a device that is unoccupied, i.e. a device that is not involved in any other tasks . The first energy level threshold may be set somewhere between a maxim power source energy level and minimum power source energy level, where a maximum level may be the level of a full battery and a minimum energy level may be a minimum operational level at which the communication device is able to tran sfer a task to another communication device ju st before failing to operate, i.e. ju st before the battery is depleted . Thereby the communication device will fail after this minimum operational level. The first threshold may be set above the midpoint between these levels, such as 90 , 80 , 70 or 60 percent of the full battery. It may also be set at 50 %. In one variation the first threshold is set to the minimum energy level.

When the level has been set and the task has been distributed, the task distribution control unit 41 then compares the current energy level of the currently assigned communication device with the first energy level threshold . This mean s that the current energy level of the working communication device is compared with the first energy level threshold, step 49. The current energy level may here be the energy level of the latest energy level data received from the working communication device.

As an alternative it is possible that the current energy level is estimated . The task distribution control unit 41 may estimate the current energy level of the working communication device at a current point in time based on the latest received energy level and an estimation of the power con sumption required for performing the task between the time of this most recently received energy level and the current time. If the task distribution control unit 41 then finds that the energy level of the working communication device W is below the threshold, it then assign s the task to another communication device in the group , step 50. This may involve the task distribution control unit 41 informing the investigated working communication device to stop performing the task and to forward any intermediate processing results followed by in structing the newly selected communication device to continue performing the task and forwarding the possible intermediate processing results . Alternatively the task distribution control unit 41 may in struct the investigated communication device to ask the selected new

communication device to continue performing the task and to also forward the possible intermediate processing results . Here the energy level of the power source of the newly selected communication device is above the first energy level threshold . Also this level of the new

communication device may be a directly received or estimated level.

It is possible that the comparison is stopped after this change of working node. When the first energy level threshold is set at the minimum level, it may for in stance be possible that only two devices are to operate on a task until both fail. Alternatively it is possible that more communication devices are to operate on the task when their power sources have energy levels above the first energy level threshold . In this case also the energy level of the power source of the new working communication device is compared with the first energy level threshold and the task passed on to yet another communication device when the energy level of this power source falls below the first energy level threshold . It is thu s possible that the task distribution control unit 4 1 continues and assign s the task among the communication devices in the group until the power sources all have energy levels that are below the first energy level threshold . The task may thereby be passed around among the communication devices in the group until either the task is finished or all are below the first energy level threshold . The communication between the master D and the different

communication devices in the group described above is furthermore performed u sing the short-range communication unit 38 .

As indicated above it is possible that the first energy level threshold is the only energy level threshold that is u sed . Alternatively it is possible to u se more energy level thresholds, where the first energy level threshold may be followed by other lower energy level thresholds .

Thereby it is clear that the task distribution control unit 4 1 may, when all communication devices in the group are below the first energy level threshold, set a lower energy level threshold, assign the task among the communication devices in the group and compare the current energy level in the assigned communication devices in the group with the lower energy level threshold until all are below the lower energy level threshold and continue setting lower thresholds assigning the task and comparing with energy level thresholds until all communication devices in the group are at the minimum level.

Furthermore in the distribution of tasks it is possible that the master u ses a first or second distribution strategy, where the first strategy may be a strategy to smoothen out the power con sumption across communication devices and the second is a strategy to prolong the lifetime of the group as long as possible. It should here be realized that it is possible to u se one strategy, for in stance the first strategy, for a first group of communication devices in the clu ster and to u se another strategy, such as the second strategy, for a second group of the clu ster. While both described strategies help out with extending battery lifetime for nodes of the local network, each might be suitable for different scenarios . The strategy may be chosen in an initial phase, when deploying the system, depending on the domain of operation of the communication devices. The smoothening strategy may be set as default in scenarios that require additional reliability for critical services. For example, in the case of sensors located in a forest for reporting a possible fire, even though all sensors are performing the same task, it may still be important to have sensors alive as long as possible in different locations of the forest in case there is a fire. Therefore the smoothening strategy makes sure that all the devices are alive for a long time in a diversity of locations. Also, the smoothening strategy might reduce the latency of data in applications where communication devices transmit large amounts of data and/ or have large computation requirements.

In the maximize network lifetime strategy, the master always activates one or more idling (e.g. "sleeping") devices when other one or more device is running out of battery. At this moment, all data from the device with the depleting battery needs to be transferred to the newly activated device or devices. On the other hand, the smoothening strategy lets every device to work for a while, and subsequently do the data transfer and role reassignment. In this case, when transferring tasks/ data, the whole data set is not transferred, but only what has been changed. Since only the delta of the changed data is transferred between communication devices, the time required to transfer the data, in other words the latency of the system, will be lower as opposed to the maximize network lifetime strategy. This may make the smoothening strategy more suitable for time-critical (e.g. real-time) applications.

In the first embodiment it can thus be seen that the master is aware of the energy level of the power source and work task performed in every communication device in the clu ster and that this energy level is periodically or asynchronously reported. It can also be seen that based on the energy level, work task information and system requirement, the communication devices can decide an appropriate strategy, which may be the first or the second strategy, where the first strategy evens out the battery consumption across the communication devices of a group by delegating tasks that affect energy consumption (e.g. transmission, computation, storage of data) of a communication device with lower energy levels to another device with higher energy level. The second strategy on the other hand prolongs device network lifetime by allowing only one communication device from a group to perform a task such as to transmit data and subsequently allowing another one of the

communication devices in the group to transmit data when the former working device is about to fail, for instance because its battery is depleted.

Now a second embodiment where the smoothing strategy is used will be described with reference being made to fig. 6 and 7, where fig. 6 shows a flow chart of a number of method steps in the method of distributing tasks and being preformed by one or more communication devices acting as master and fig. 7 shows a signaling chart with signals passed between three communication devices. The sequence diagram illu strates the flow on how to smoothen the task evenly out over the communication devices in a small cluster with only three communication devices; the first communication device A, the second communication device B and the third communication device C. However, the same principle also applies to bigger clu sters. Furthermore, the method is described in relation to a single task that can be performed by the whole cluster. The principles used may easily be extended to more devices and more tasks.

Furthermore in the example, the device acting as a master is a part of the group .

The communication devices may first form a cluster. The communication devices may exchange information about their capabilities, such as the capabilities to perform certain tasks as well as their short-range communication capabilities. This information is then u sed for forming a clu ster of communication devices having a required functionality and required short-range communication capability. A clu ster may be formed in any of the ways described in the first

embodiment. The communication devices in the clu ster may also form one or more group s for handling variou s types of tasks . In this example there is only group for handling one task. Therefore the group will be the whole clu ster.

The formed clu ster may at this point also decide on a suitable strategy for assigning work tasks . Alternatively the strategy may be pre-determined .

Furthermore, the strategy is a strategy for preserving battery on a system level. A schedule that implements the strategy therefore con siders the system requirements, which may be the requirments of an application running on the device clu ster . For example, if the clu ster is u sed by a critical service that requires high availability, the master should make sure at any point of time that the working devices should be redundant.

Thereafter an election process may be started to find a master. The role of a master is to collect the battery life of each individual communication device in the group and coordinate them to execute the strategy implemented through the task distribution s scheme. In this respect, the master can continuou sly plan the best schedule and inform the other devices when there is a change in schedule.

Thu s, after the most suitable strategy has been determined for the group or for the clu ster, an election process is started to find a master (to avoid single point of failure, there could also be two masters , one active and another on standby in case the aforementioned active master fails) .

Typically all communication devices of the clu ster could be involved in this election . This mean s that there is an election 72 between the first and second communication device A and B, an election 74 between the second and third communication devices B and C and an election 76 between the first and third communication devices A and C. The device that win s the election s then assumes the role of master. In the example of fig. 5, the first communication device A assumed the role of or was elected 78 master D.

Thereafter the master D regularly obtain s the energy levels of the communication devices in the group , step 52. This may be done through the communication devices B and C being set to regularly report the energy levels . Alternatively it may involve the master sending an in struction to all other devices in the clu ster on how frequently they should report battery information . One way in which this may be done, which is shown in fig. 7, involves the master D informing the

communication devices B and C through indicating a time interval to be u sed to the master D. The time interval may be the time between battery level reports. Thereby the master D inform s the communication devices in the group about the rate with which battery level reports are to be given . The master D may thu s indicate 80 a reporting time interval to the second communication device B in which the second communication device B is to send to send battery statu s as well as indicate 84 a reporting time interval to the third communication device C. The second communication device B may then accept 82 this reporting and also the third communication device C may accept 86. These devices B and C then continue and periodically report battery statu s 88 to the master according to the times of the reporting time intervals . The battery statu s reporting may then be stored by the task distribution control unit 41 in the remaining power level storage 42. It is thu s possible that only the most recent levels are stored . Thereafter the master distributes the task according to the task

distribution scheme. This may be performed through iterating through different energy level thresholds corresponding to different battery levels . For each energy level threshold the master would be keeping some communication devices working and some devices sleeping to save battery. When the working devices pass the threshold , they tran sfer the task to sleeping devices to continue the service. This is then repeated for increasingly lower thresholds . In this second embodiment this is exemplified by one working device and one sleeping device.

In order to distribute the task according ot the scheme, the master D sets the first energy level threshold, step 54, and in this second embodiment, the first energy level threshold is set between a minimum and a maximum battery level, such as for in stance at 50 % of the maximum level. The level may with advantage be set ju st after the first battery reports are received . As an alternative the threshold may be set at the initiation stage when master is selected or even before that when strategy is determined . The threshold level may also be a default level. The master D then decides which of the two communication devices B and C is to perform the task. In this case it may select a device that is not occupied with other tasks, i.e. one that is sleeping. As an example this selected device is the second communication device B. The master then assign s the task to the selected communication device in the group , step 56. In this example it sends a direction 90 to work to the second

communication device B, which is accepted 92. At the same time it may send 94 a direction to sleep to the third communication device C, which may also be accepted 96.

Thereby the first communication device A is master D, the second communication device is working W and the third communication device C is sleeping 98 . The master D then compares the energy level of the battery of the working communication device B with the first energy level threshold, step 58 . As long as the battery level is above the threshold, step 60 , the second communication device B continues working on the task and the master continues comparing the energy level of the battery with the threshold , step 58 . As the second communication device B is processing the task it con sumes power. The battery level will then eventually go below the threshold of 50 % 100. The master will thu s detect the battery level being below the first energy level threshold, step 60 , which indicates that a change needs to be made. The master then investigates if the device B was the last in the group operating at this level. If it was not, step 62, then another communication device in the group is assigned the task, step 64. In this example, there were two more communication devices, the first A and the third C. Therefore the master assign s the task to another device in the group , step 64 , which in fig. 7 is exemplified by the third communication device C. The master D thu s in structs 102 the second communication device B to hand over the task to the third communication device C. The second communication device B responds to this request by handing over the task 104 and accepting 106 that it is to stop working. The handing over may also involve handing over any intermediate processing results of the handling of the task, such as intermediate data needed for carrying out the task. The master D then orders 108 the third communication device C to start to work, which the third communication device accepts 110. The master D then in structs 112 the second communication device B to sleep , which it accepts 114. The first communication device A is now still master D, the second communication device B is sleeping or non-working N and the third communication device C is working W on the task 116. The master now compares the battery levels of the third communication device C with the first energy level threshold , step 58 and as long it is above, step 60 , the third communication device C continues working and the first communication device A keep s on performing the comparison . However, also here the battery of the third communication device C will after a while go below the threshold of 50 % 118 . This mean s that the master will find that the battery level goes below the first energy level threshold, step 60 .

A master may then again investigate if the working device is the last and if it was not, step 62, assign the task to another communication device, step 64. In the example there is one communication device left in the group , namely the first communication device A itself. Therefore, when the current master is to be assigned the task itself, it in structs another device to take over the role of master. The current master thu s hands over the role of master to another communication device. It may with advantage in struct the device that came second in the election process to be master. The new master may also need to be a sleeping device, i.e. one that is not working or acting as a master. In this example the current master, i.e. the first communication device A, selects to tran sfer the role of master to the second communication device B as it is not working. Therefore the first communication device A hands over the master role to the second communication device B 120 and the second communication device B accepts 122. Now the second communication device B is master D. In the handing over it is also possible that the new master is informed about the current reported battery levels as well as the current battery level of the first communication device and the current energy level threshold . Thereafter the communication devices in the group report battery levels to the new master D. The second communication device B is thu s now the device that receives battery level reports according to the reporting time intervals from the first and the third communication devices A and C. The first communication device A may thereby u se the same reporting time interval as that previou sly u sed by the second communication device B. In the example in fig. 7, the new master will only have one communication device to hand over the task to, namely the first communication device A. The new master B thu s in structs 126 the third communication device C to hand over the task to the first communication device A. The third communication device C responds to this by handing over the task 128 to the first communication device A, which in turn accepts 130 to the third communication device 132. Also here the handing over may involve handing over any intermediate data needed for performing the task. The third communication device C also accepts 132 the order from the new master D. Thereafter the master sends an order to start to work to the first communication device A 134, which accepts 136. The new master D also sends an in struction to sleep 137 to the third communication device C, which accepts 138 .

This mean s that now the first communication device A is working, the second communication device is master D and the third communication device is sleeping 139.

The battery level of the first communication device is now continually compared with the first energy level threshold and when it falls below it, step 60 , the master investigates if it is the last device at the threshold level. In this example it is, step 62, and therefore the master then investigates if the threshold is at the minimum working level.

If it is not, step 66 , then a next lower threshold is set, for in stance at 25% of a full battery, and the energy level of the working device now compared with this new energy level, step 58 .

It is here possible that the role of master is only changed once per level. It is also possible that the role of master is changed last after all other communication devices of the group have been working at the task at the investigated energy level. The second communication device B may therefore assign the task to all other communication devices in the group until it is the only one in the group that has not been working. A master starting to control task assignment at an investigated energy level may thu s be the last to be working on the task at this energy level. The task may in this way be rotated between the communication devices in the group until the threshold has been set to be the minimum working threshold . The communication devices of the group will then work one by one until they fail. At this last level, when the last communication device reaches the last energy level threshold, step 66 , the operation is ended, step 70. It is in relation to this last step possible that when the master has no more communication device to turn to becau se all others are no longer able to work, then there is no need for any master. The last master may then itself continue to work on the task without tran sferring the role of master and without checking battery level until it itself fails .

Alternatively it is possible that the master does not start to work on the task.

It can be seen that in the second embodiment the number of sleeping communication devices in the group may be kept as high as possible as long as possible. This mean s that redundancy is retained until all communication devices reach the minimum threshold . This maximizes the system lifetime with regard to task assignment flexibility.

Now a third embodiment where the maximize network lifetime strategy is u sed will be described with reference being made to fig. 8 and 9 , where fig. 8 shows a flow chart of a number of method step s in the method of distributing tasks and being performed by a master and fig. 9 shows a signaling chart with signals passed between three communication devices . It is in this embodiment possible that the master is excluded from the group of communication devices that are to be working on the task.

The sequence diagram of fig. 9 illu strates the flow on how to maximize the network lifetime in a small clu ster with the same three devices as in fig. 7. However, also here the principles u sed may easily be extended to more devices and more tasks .

Furthermore, also here there may or may not be cluster forming process as well as a master electing process. If there are such processes, these may be the same or different than the ones u sed in the first and/ or second embodiments.

In case theses process are in place, then the communication devices form a clu ster , which also here corresponds to the forming of a group, and may also decide the most suitable strategy . The strategy is in this case to allow a work task to be performed as long as possible in the group .

A scheme implementing the strategy could in this case generally be to keep some devices working and other sleeping to save battery. When working devices are running out of battery, then the tasks are transferred to sleeping devices to continue the service.

After forming of clusters and deciding strategy, an election process is started to find a master in the same way as the second embodiment Thu s, in the example in fig. 9 the same type of election is used as in the second embodiment. There is thus an election 156 between the first and second communication device A and B, an election 158 between the second and third communication devices B and C and an election 160 between the first and third communication devices A and C. The device that wins the election then assumes the role of master. In the example of fig. 9 , the first communication device A was elected 162 and assumed the role of master D.

Thereafter the master D regularly obtains the energy levels of the power sources of the communication devices in the group . This may also be done in the same way as in the second embodiment. The communication device A acting as a master D may thu s indicate 164 a reporting time interval to the second communication device B in which it is to send to send battery statu s as well as indicate 168 a reporting time interval to the third communication device C. The second communication device B may accept 166 this reporting and also the third communication device C may accept 170 . These devices B and C then continue and periodically report battery statu s 172 according to the times of the reporting time intervals . Thereby the master regular obtain s the energy levels of the power sources in the group , step 140. The battery statu s reporting may or may not be stored by the task distribution control unit 4 1 in the remaining power level storage 42.

The master D then computes a schedule to maximize network lifetime 174. This involves only u sing one threshold level, the minimum threshold level. The master thu s sets the first threshold to be the minimum battery level, i.e. the level at which the corresponding communication device will be able to handover a task before failing, step 142. The master D then decides which of the two other communication devices B and C that is to perform the task. It may select in the same way that was described in the second embodiment. In the example in fig. 9 , the selected device is the second communication device B. The master then assign s the task to the selected communication device, step 144. In this example it sends a direction 176 to work to the second communication device B, which is accepted 178 . At the same time it may send 180 a direction to sleep to the third communication device C, which may also be accepted, 182.

The master then compares the energy level of the battery of the working communication device with the first threshold, step 146 , which working device in this case is the second communication device B. As long as the battery level is above the threshold, step 148 , the second communication device B continues working on the task and the master continues comparing the energy level of the battery with the threshold , step 146. As the second communication device B is processing the task it con sumes power. The battery level will then eventu ally go below the threshold , step 148 . The second communication device B will thu s run out of battery 184 , which mean s that a change needs to be made. The master then

investigates if the device B was the last in the group operating at this level. If it was not, step 150 , then another communication device in the group is assigned the task, step 152. In this example, there was one more communication device, the third communication device C.

Therefore the master assign s the task to another device in the group , step 152, which in fig. 9 is exemplified by the third communication device C. The master thu s in structs the second communication device B to hand over 186 the task to the third communication device C. The second communication device B responds to this request by handing over the task 188 and accepting 190 that it is to stop working. The handover may also here comprise a tran sfer of intermediate processing results . The master D then orders 192 the third communication device C to start to work, which the third communication device accepts 194. The master D then in structs the second communication device B to terminate 196 operation , which it accepts 198 . The second communication device B has now terminated operation and the third communication device C has started working on the task 200. Thereafter the master compares the battery levels of the third

communication device C with the first energy level threshold and as long it is above, step 148 , the third communication device C continues working and the first communication device A keep s on performing the

comparison , step 146. However, also here the master will eventu ally find that the battery level goes below the threshold, step 148 . It may then again investigate if the working device is the last and if it was not, step 150 , assign the task to another one, step 152. However, if it was , then the operation is ended, step 154. In this case it may in struct the third communication device to terminate operation . Now all other communication devices in the group have terminated operation . It is then possible that also the master terminates operation . As can be seen the life time of the performing of the task may be prolonged in the third embodiment. This mean s that the time that the system is able to perform a work task may extended .

The computer program code of the task distribution control unit and optionally also that of the task handling unit, the remaining power metering and reporting unit, the master negotiating unit and the clu ster forming unit may be in the form of computer program product for instance in the form of a data carrier, such as a CD ROM disc or a memory stick. In this case the data carrier carries a computer program with the computer program code, which will implement the functionality of the task distribution control unit and optionally also of the task handling unit and the power metring and reporting unit. One such data carrier 202 with computer program code 204 is schematically shown in fig. 10. As is schematically shown in fig. 11, the task distributing unit 41 of a communication device acting as master may be realized as :

means 206 for regularly obtaining energy level data concerning the communication devices in the clu ster.

means 208 for assigning the task to a communication device in a group of communication devices in the clu ster with capability to perform the task, means 210 for setting a first energy level threshold, means 212 for comparing a current energy level of the currently assigned communication device with the first energy level threshold, and

means 214 for assigning the task to another communication device in the group in case the energy level is below the first energy level threshold.

The means are in one embodiment corresponding to software instructions. In another embodiment the means are implemented as hardware units in one or more hardware circuits, like ASICs or FPGAs.

The means for assigning the task to another communication device in the group may here comprise means for assigning, for each communication device being assigned a work task, a sleeping communication device ready to take over the task.

The means 208 and 214 for assigning may comprise means for continuing assigning the task among the communication devices in the group until all are below the first energy level threshold.

When the first energy level threshold represents an energy level between a maximum level and a minimum operational level after which the communication device fails to operate, the means 210 for setting a first energy level threshold may comprise means for setting, when all communication devices in the group are below the first energy level threshold, a lower energy level threshold, the means 208 and 214 for assign the task to a communication device may comprise means for assigning the task among the communication devices in the group and the means 212 for comparing a current energy level of the currently assigned communication device with the first energy level threshold may comprise means for comparing the current energy level in the assigned

communication devices in the group with the lower energy level threshold until all are below the lower energy level threshold. The task distribution control unit may furthermore comprise means for continuing setting lower thresholds, assigning the task and comparing with energy level thresholds until all communication devices in the group are at the minimum level.

When the tasks comprise data handling task, a sensing task and a long- range communication task and there is one group of communication devices for each type of task, the means 208 and 214 for assigning tasks may furthermore comprise means for assigning tasks according to the task distribution scheme in all group s. The task distribution control unit may furthermore comprise mean s for handing over the role of master to another communication device, which may furthermore be a means for handing over the role of master in case the communication device in which the task distribution control unit is provided itself is to be assigned the task.

The short-range communication unit may comprise means for performing initial communication with neighbouring communication devices and the clu ster forming unit may be realized as means for forming the clu ster with at least some of the neighbouring communication devices based on the initial communication .

The master negotiating unit may furthermore be realized as mean s for assuming the role of master after negotiating with the other

communication devices of the clu ster.

Also these latter mean s may be software instruction s or hardware units in one or more hardware circuits, like ASICs or FPGAs .

While the invention has been described in connection with what is presently con sidered to be most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiments, but on the contrary, is intended to cover variou s modifications and equivalent arrangements. Therefore the invention is only to be limited by the following claims.

Claims

1. A communication device ( 12) acting as a master (D) in a clu ster ( 10 ) of communication devices ( 12 - 28 ), where each communication device in the clu ster has short-range communication (38 ) and data processing capability, said communication device acting as a master (D) for controlling the assigning of a work task according to a task distribution scheme and comprising a processor (32) acting on computer in structions whereby said communication device ( 12) is operative to:

regularly obtain energy level data concerning the communication devices in the clu ster,

assign the work task to a communication device in a group of

communication devices in the clu ster with capability to perform the work task,

set a first energy level threshold,

assign the work task to another communication device in the group in case the energy level is below the first energy level threshold .

2. The communication device ( 12) according to claim 1, wherein at least some of the other communication devices in the group are sleeping.

3. The communication device ( 12) according to 2, being further configu to assign , for each communication device being assigned a work task, a sleeping communication device ready to take over the work task.

4. The communication device ( 12) according to any previou s claim, bein further operative to continue assigning the work task among the communication devices in the group until all are below the first energy level threshold .

5. The communication device ( 12) according to claim 4, wherein the first energy level threshold represents an energy level between a maximum level and a minimum operational level after which the communication device fails to operate and being further operative to, when all

communication devices in the group are below the first energy level threshold, set a lower energy level threshold, assign the work task among the communication devices in the group and compare the current energy level in the assigned communication devices in the group with the lower energy level threshold until all are below the lower energy level threshold and continue setting lower energy level thresholds assigning the work task and comparing with energy level thresholds until all communication devices in the group are at the minimum level .

6. The communication device ( 12) according to any of claim s 1 - 4, wherein the first energy level threshold represents a minimum operational level after which communication devices fail to operate.

7. The communication device ( 12) according to any previou s claim, wherein the work tasks comprise a data handling task, a sen sing task and a long-range communication task, there being one group of communication devices for each type of work task and the communication device acting as master being operative to assign work tasks according to the task distribution scheme in all group s.

8. The communication device ( 12) according to claim 7, wherein a subset of the communication devices in the clu ster have long-range

communication capability (36) and at least some of the communication devices with long-range communication capability are excluded from the group being assigned data handling tasks .

9. The communication device ( 12) according to claim 7 or 8 , wherein a subset of the communication devices of the clu ster ( 10 ) have sen sor capability (39) and at least some of the communication devices having sensor capability are excluded from the group being assigned data handling tasks .

10. The communication device ( 12) according to any previou s claim, being further operative to hand over the role of master to another

communication device.

11. The communication device ( 12) according to claim 10 , which when being operative to hand over the role of master is operative to hand over the role of master in case it itself is to be assigned the work task.

12. The communication device according to any previou s claim, being further operative to perform initial communication with neighbouring communication devices and form the clu ster with at least some of the neighbouring communication devices based on said initial

communication .

13. The communication device according to any previou s claim, being further operative to assume the role of master after negotiating with the other communication devices of the clu ster.

14. A method of controlling the assigning of a work task according to a power management based task distribution scheme in a clu ster ( 10) of communication devices ( 12 - 28 ), where each communication device in the clu ster has short-range communication (38 ) and data processing capability, the method being performed by a communication device in the clu ster acting as a master (D) and comprising:

regularly obtaining (42; 52, 88 ; 140 , 172) energy level data concerning the communication devices in the clu ster, assigning (44 ; 56 , 90 ; 144, 176) the work task to a communication device in a group of communication devices in the clu ster with capability to perform the work task,

setting a first energy level threshold (48 ; 54 ; 142),

comparing(49 ; 58 ; 146) a current energy level of the currently assigned communication device with the first energy level threshold, and assigning (50 ; 64, 102; 152, 186) the work task to another communication device in the group in case the energy level is below the first energy level threshold.

15. The method according to claim 14 , wherein at least some of the other communication devices in the group are sleeping.

16. The method according to claim 15, further comprising assigning, for each communication device being assigned a work task, a sleeping communication device ready to take over the work task.

17. The method according to any of claims 14 - 16 , further comprising continuing assigning (64 ; 152) the work task among the communication devices in the group until all are below (60 ; 148 ) the first energy level threshold.

18 . The method according to claim 17, wherein the first energy level threshold represents an energy level between a maximum level and a minimum operational level after which the communication device fails to operate and when all communication devices in the group are below (60) the first energy level threshold further comprising setting (68 ) a lower energy level threshold, assigning (64) the work task among the communication devices in the group and comparing (58 ) the current energy level in the assigned communication devices in the group with the lower energy level threshold until all are below the lower energy level threshold and continuing setting lower energy level thresholds, assigning the work task and comparing with energy level thresholds until all communication devices in the group are at the minimum level (66) .

19. The method according to any of claims 14 - 17, wherein the first energy 5 level threshold represents a minimum operational level after which the communication device fails to operate.

20. The method according to any of claims 14 - 19, wherein there is a data handling task, a sensing task and a long-range communication task, there

1 0 being one group of communication devices for each type of work task and the assigning of work tasks according to the task distribution scheme is performed in all groups .

21. The method according to claim 20 , wherein a subset of the

15 communication devices in the clu ster have long-range communication capability and at least some of the communication devices with long-range communication capability are excluded from the group being assigned data handling tasks.

2 0 22. The method according to claim 20 or 21, wherein a subset of the

communication devices of the clu ster have sensor capability and at least some of the communication devices having sen sor capability are excluded from the group being assigned data handling tasks .

25 23. The method according to any of claims 14 - 22, further comprising handing over ( 120) the role of master to another communication device.

24. The method according to claim 23 , wherein the handing over the role of master to another communication device is done in case the current

3 0 master itself is to be assigned the work task.

25. The method according to any of claims 14 - 24, further comprising performing initial communication with neighbouring communication devices and forming the clu ster with at least some of the neighbouring communication devices based on said initial communication .

5

26. The method according to any of claims 14 - 25, further comprising assuming the roll of master after negotiation s with the other

communication devices of the clu ster.

1 0 27. A computer program for controlling the assigning of a work task

according to a power management based task distribution scheme in a clu ster ( 10 ) of communication devices ( 12 - 28 ) , where each

communication device in the clu ster has short-range communication (38 ) and data processing capability, the computer program comprising

15 computer program code (204) which when run in a communication device ( 12) in the clu ster acting as a master (D) , cau ses the communication device ( 12) to:

2 0 assign the work task to a communication device in a group of

communication devices in the clu ster with capability to perform the work task,

set a first energy level threshold,

compare a current energy level of the currently assigned communication

25 device with the first energy level threshold, and

assign the data processing activity to another communication device in the group in case the energy level is below the first energy level threshold.

28 . A computer program product for controlling the assigning of a work

3 0 task according to a power management based task distribution scheme in a clu ster of communication devices, the computer program product comprising a data carrier (202) with computer program code (204) according to claim 23.