GB2449232A

GB2449232A - Scheduling contention-free access to a communication medium

Info

Publication number: GB2449232A
Application number: GB0708428A
Authority: GB
Inventors: Dona Gabriele; Peter Munday
Original assignee: Thales Holdings UK PLC
Current assignee: Thales Holdings UK PLC
Priority date: 2007-05-01
Filing date: 2007-05-01
Publication date: 2008-11-19
Anticipated expiration: 2027-05-01
Also published as: GB0708428D0; GB2449232B

Abstract

An ad hoc network is established between one or more deployed nodes and an access point, on the basis of each node monitoring for transmission of a beacon signal. In response to a beacon signal, the nodes transmit medium access requests to the access point, which may be on a contention basis, and then the access point schedules access to the medium on the basis of contention-free access. Medium access requests and consequent grants are repeated as necessary. Each beacon burst may include node configuration information and may be sufficiently long, with a sufficiently short beacon interval, to give a high probability that all nodes within range will intercept at least one beacon.

Description

Wireless Communications Apparatus The invention relates to Medium

Access Control (MAC) in a wireless sensor network.

It is particularly, but not exclusively, concerned with a MAC protocol for use in a network such as a Personal Area Network, or a Wireless Sensor Network such as a Distributed Environmental Data Logging (EDL) network.

It is becoming increasingly popular to deploy electronic equipment suitable for stand alone operation, and to communicate with such equipment on an ad hoc basis only as required. One example of this is the use of Environmental Data Loggers (EDL). EDLs are designed for use without continuous external control, to collect, for example, data recording temperature, humidity, atmospheric pressure, vibrations (such as due to seismological phenomena) and other externally imposed criteria. Such data can be recorded over a relatively long term, and stored in memory (preferably non-volatile memory). The data needs then to be collected, and it has proved useful to include, in an EDL, wireless communications facilities, to enable a data collector to collect such data by moving a data collection apparatus into range of the wireless facilities of the EDL.

EDLs may, in many circumstances, be installed in relatively remote, rural or otherwise inaccessible locations, predicating the need for a means of power supply other than mains power. Battery power is convenient, as this allows for a relatively compact device. However, batteries inevitably have a short lifespan, and so recharging or replacement thereof involve frequent site visits and consequent inconvenience for operators. On the other hand, such a site visit can be combined with data collection, although site location may make frequent visits of this nature highly undesirable.

As an alternative, solar power is also used, usually in conjunction with local energy storage means (such as a rechargeable battery) as required to back up during interruptions in useful sunlight. Solar powered EDLS can conveniently be combined with wireless conmiunication means for data extraction and control. However, solar power naturally places operational constraints on an EDL in terms of power consumption -it becomes highly desirable to provide any further means of keeping power consumption to a minimum. EDLs have thus been designed for their high efficiency of power consumption, particularly relative to conventional computers.

Wireless communication with an EDL has been considered, but the consequence of high power consumption has been considered an obstacle to development of this technique.

Further development of this can thus be appreciated to be desirable.

IEEE 802.15 protocol, specifically IEEE 802.15.4 as developed by Task Group 4, has been developed with a view to the establishment of Wireless Personal Area Networks (\VPAN) with low transmission rates, and thus able to accommodate very low power consumption. This would seem entirely appropriate for data loggers, as IEEE 802.15.4 recognises the need for certain equipment to operate with very long battery life, perhaps months or years. However, it was identified that even this did not meet power consumption requirements, if data loggers had to be triggered via radio.

Moreover, the approach taken to Medium Access Control in this protocol can lead to difficulties in high load applications, for instance where a network is established with an access point and many nodes, for instance in a star configuration. It may, in certain circumstances, be desirable to support hundreds or even thousands of nodes, and medium access control in such circumstances can be difficult.

According to an aspect of the invention, an ad hoc network is established between one or more deployed nodes and an access controller, on the basis of each node monitoring for transmission of a beacon signal. In response to a beacon signal, the nodes transmit, as required, medium access requests to the access controller, and then the access controller schedules access to the medium on the basis of contention free access.

According to another aspect of the invention, an ad hoc network is established between one or more deployed nodes and an access point, on the basis of each node monitoring for transmission of a beacon signal. In response to a beacon signal, the nodes transmit, as required, medium access requests to the access point, and then the access point schedules access to the medium on the basis of contention free access. Further aspects of the invention concern a method of achieving this, an access point and a suitably configured apparatus to be used as a node in the network.

Aspects of the invention can particularly be applied to networks in which the nodes of the network are either randomly distributed, or uniformly distributed, or both, in an area which is finite but not well specified. In particular, the invention can be applied to a network in which nodes are represented by Environmental Data Loggers (EDLs). As will be described later, application of the invention is not limited to the use of EDLs.

Another aspect of the invention provides a method of establishing a wireless communications network in a wireless medium, comprising the steps of transmitting, from an access controller, a beacon for reception by any nodes in range of said access controller, transmitting, from each node in reception of said beacon, a medium access request signal requesting access to said medium, said medium access request signals being transmitted in accordance with contention access, and, on the basis of medium access request signals received at the access controller, scheduling contention free access with each of the nodes indicated by said received medium access request signals.

It will be understood that aspects of the invention can be implemented by means of computer executable instructions, which can be introduced by way of a carrier, such as a storage medium (e.g. an optical or magnetic storage device or a mass storage device such as a flash memory) or a signal such as a software download for instance from a remote source which might be accessible to an executing device by way of the internet, or a short message compliant with, for example, the SMS service.

Further features, aspects and advantages of the invention will now be described, by way of example only, in the context of a specific embodiment thereof, with the aid of the accompanying drawings, in which: Figure 1 is a schematic diagram of an installation of nodes in a given area, each node being in accordance with a specific embodiment of the invention; Figure 2 is a schematic diagram of the installation of nodes with an access point (AP) to establish a network with certain of the nodes, the AP being in accordance with the specific embodimeilt of the invention; Figure 3 is a schematic diagram of a node of the installation illustrated in figures 1 and 2; Figure 4 is a schematic diagram of an AP as deployed in the installation as illustrated in figure 2; Figure 5 is a timing diagram illustrating operation of the A? illustrated in figures 2 and 4 in initiating communication with nodes; Figure 6 is a timing diagram illustrating operation of a node in responding to communication initiation by the AP; Figure 7 is a timing diagram illustrating coordinated access to the network established by the access point with the nodes; and Figure 8 is a timing diagram illustrating overall execution of a data query by the AP in cooperation with the nodes with which it establishes communication.

Figure 1 illustrates an arrangement of nodes 10 (e.g. environmental data loggers (EDL)) in an area defining an environmental data collection zone 12, indicated by the broken lines. The nodes are not capable of communication with each other, but are capable of communication with a cooperating access point (A?) device, when introduced into the zone 12.

It will be appreciated, by the reader skilled in the art, that the zone 12 is merely for indicative purposes, and does not represent any physical attributes such as transmission range.

As shown in Figure 2, an access point AP 20 is shown as being introduced into the zone 12. The A.P 20 has a transmission range defined by broken line 22 in Figure 2. As shown, three of the nodes 10 are within range of the AP 20. It will be appreciated that the illustrated example is but indicative, and many more nodes 10 may be within range of the AP 20 in a specific embodiment of the invention.

For the purposes of this embodiment, an EDL is a device which collects environmental data, such as temperature, humidity, pressure, vibrations and shocks, and stores them in a non-volatile memory. However, the invention can be implemented using apparatus other than EDLs as nodes of a network, as will be readily appreciated by the reader.

Each node in the exemplary installation is equipped with a radio interface in order to transmit all or part of the collected data to the AP 20. Normally, the AP is not in range of any node. As shown in Figure 2, an AP 20 can be moved into range of some or all of the nodes 10, activate them, and finally act as a sink node in order to collect all or part of the logged data. hi this embodiment, only single hop star topology is permitted.

However, a more complex network using a multi step approach could be envisaged by the skilled person.

Each node 10 is implemented in hardware of known type, with software, as illustrated in figure 3. Figure 3 illustrates the node 10 as being implemented by means of a general purpose data logger with communications facilities. In this case, communications facilities are provided by means of a mixture of hardware and software, as appropriate.

More particularly, the node 10 comprises a processor 30, in communication with the wor1thg memory 32 and a bus 34. A non volatile mass storage device 36 is provided for long term storage of data and/or programs not in immediate use. A medium access controller 38 is connected to an antenna array 40, to provide the node 10 with access to the wireless communications medium.

In this case, the mass storage device 36 is a magnetic storage device, though other such storage devices would suffice.

The node 10 further comprises specific environmental instruments 46 for the collection of environmental data for storage in the mass storage device 36.

Although the illustrated arrangement has no means of audio visual output, this could be provided in certain embodiments of the invention, depending on the desired functionality of the node 10.

Between the working memory 32 and the mass storage device 36, the node 10 stores executable programs enabling the processor 30 to configure the node 10 to provide access to the wireless spectrum in accordance with the specific embodiment of the invention as will be described below.

Further, the AP 20 is of similar construction, to cooperate with the nodes 10 in communication. As illustrated in figure 4, those components of the AP 20 which correspond to components of the node are allocated the same reference numeral. The AP 20 is also implemented in hardware of known type. Figure 4 illustrates the AP 20 as being implemented by means of a general purpose computer with communications facilities.

The AP 20 also further comprises audiovisual and data output devices 44 (such as a simple display, or even merely indicator LEDs) and also will include user operable input facilities 48 such as a keyboard, mouse, or merely one or more suitable buttons or switches.

Again, between the working memory 32 and the mass storage device 36, the AP 20 stores executable programs enabling the processor 30 to configure the AP 20 to provide access to the wireless spectrum in accordance with the specific embodiment of the invention as will be described below, and particularly to establish initiation of medium access control in cooperation with nodes 10 with which the AP 20 can physically communicate.

It will be understood that the node 10, in accordance with the art, should have a highly robust operating system to enable long term use with very low risk of malfunction.

Moreover, the operating system will be configured to conserve power as far as possible.

To that end, the operating system will, as the opportunity presents itself, cause one or more of the components described above to enter a dormant "power saving" state, in which activity is minimised to avoid unnecessary power consumption. However, these features are not essential to an understanding of the invention.

Naturally, the medium access controller 38 will need to monitor the wireless communications medium, in order to identify when another device (in particular the AP 20) sends a signal as described. In order to detect the presence of an AP 20, all nodes periodically activate their Medium Access Controller 38 to listen to the wireless channel for a short period of time (wake-up time). The time interval between two consecutive activations is denoted the wake-up period, and can be selected to any suitable time period depending on network conditions and operational requirements.

The implemented MAC protocol will now be described in terms of three steps to be executed consecutively.

A first step is illustrated in figure 5. This step is initiated by an "Operator Query Event" as indicated in figure 5. Such an event may be a user input action to the AP 20 such as depression of a push button, selection by mouse click or keyboard entry of a displayed icon on screen, voice control, or any other conceivable input method.

Moreover, the event could be triggered by other automatic means, rather than user action. For instance, an embodiment of the AP 20 might include a satellite aided navigation system, such as Global Positioning System (GPS), operable to identify when the AP 20 is generally within suitable range of deployed nodes, and thus that it is appropriate to commence action to initiate the implemented MAC protocol. It will be appreciated that the invention is not limited to this, and certainly that alternatives to the GPS technology exist or are in development (such as Galileo).

During this first step the AP 20 broadcasts a short burst of uniformly distributed beacons. Each beacon contains an identification of the AP (AP ID) and can contain information identifying the type of interrogation it is pursuing, such as which nodes 10 are required to transmit data, or which type of data is required to be transmitted. The latter is useful as each node 10 may be capable of transmitting, for example, summary data, shortform data or longform data. The interrogation type field may also include selection criteria based on the node history. For instance, it may be determined that data need not be transmitted if it has already been sent recently. The question as to whether data is considered as recently transmitted or not can be predetermined or can be part of the command embedded in the beacon.

If the beacon burst is sufficiently long and the beacon interval (i.e. the distance in time between two consecutive beacons) is sufficiently short, then there is a high probability that all nodes 10 within range of the AP 20 will intercept at least one beacon.

In order to increase the robustness to real-channel conditions, the nodes 10 are arranged to wake up twice during a beacon burst, by making the burst duration equal to or greater than twice the wake-up period. As illustrated in figure 5, node 1 enters the wake-up mode twice during the duration of the burst transmitted by the AP 20. As further noted in figure 5, the nodes are not synchronised, and the time at which each commences its wake-up period may be considerably different.

In a second step, as illustrated in figure 6, all nodes 10 that have successfully received and decoded at least one beacon during the preceding step, and that fulfil the selection criteria in the beacon transmission, are now permitted to transmit a request packet to the AP 20. It will be understood that not all nodes will necessarily transmit such a request.

This will involve activation of ("waking up") transmitting capabilities of the respective medium access controllers 38 of the nodes 10. This may already have been brought about as part of the periodic activation of the medium access controller as a whole, or may have been brought about as a result of successful reception of a burst from the AP 20. This depends on the way in which power conservation and control of hardware and software on the node is implemented.

Medium access request packets (indicated R' in figure 6) from each node 10 contain the node ID for the node 10 concerned, and other data such as the amount of data it needs to transmit. These are transmitted after a random back off interval, by following a slotted CSMA/CA algorithm, of known type. This is thus in accordance with a Contention Access communications approach, in which the occurrence of contention is reduced by use of random back off intervals.

During this "contention access period" (CAP), it is possible that some of the request packets will clash due to temporal overlapping. Temporal overlapping will result in a corrupted packet at the receiver, although the "capture effect" (i.e. when one of the two clashing packets is sufficiently stronger than the other for the receiver to decode it) may reduce the rate of lost packets.

The net result at the end of the CAP is that the AP 20 will successfully receive and decode request packets associated with a certain number of nodes 10. These request packets represent a desire, by the nodes 10 concerned, to send data to the AP 20. After this, the AP 20 can schedule the data transmission from such identified nodes 10 in the third protocol step.

As illustrated in figure 7, during this third step, the AP 20 transmits one or more grants (indicated Gi', G2' etc. in figure 7), at regular intervals. The interval between grants is known as the grant interval. Each grant is addressed to a specific one of the nodes 10.

At the same time, all nodes 10 activated during Step 1 enter a grant tracking mode.

Those nodes 10 whose medium access requests were successfully received during Step 2 are selected by the AP 20 in turn. On each occasion a node 10 receives a grant containing its ID, it transmits a data packet during the remainder of the grant interval immediately following the grant itself.

A data packet transmitted by a node contains a portion of the data requested during Step 1. It will be appreciated that, in certain cases, it may be possible to transmit the entirety of the data requested within a single data packet.

Thus, the A? establishes contention free access (CFA) using these grants. This contention free access phase ends when all nodes 10 in receipt of grants have transmitted all data requested in Step 1.

All other nodes 10 which fulfil the selection criteria in the Step 1 beacon transmission, i.e. all those that transmitted a request packet during Step 2 but for whatever reason were not granted during Step 3 (e.g. because their request was lost due to clashing), are now allowed to transmit again a medium access request packet (that is, in accordance with Step 2). Hence, Step 2 and Step 3 of the protocol are iterated until the A? 20 determines that it has received data from all nodes 10 that intend to send, and which are, for instance, activated in step 1. This will be established by the A? 20 by recognising the absence of receipt of any further medium access requests from nodes 10. This is illustrated in figure 8, which shows repetitions of the transmission of node requests and subsequent node data transmissions, until termination of the process when no further node requests are transmitted.

In order to implement the above, a modification of the firmware already provided to implement IEEE8O2. 15.4-compliant ICs can be carried out. This will be understood by the skilled reader.

It will be appreciated by the reader that the above protocol, and others like it, will establish a temporary network with devices (and potentially a very high number of devices) via a wireless interface, while maintaining a very low power consumption.

This is possible because the protocol allows for a very low duty cycle listening operation.

The protocol also maximises data throughput, and hence minimises latency, in high load applications. That is, if there are a large number of nodes 10 that are required to transmit a data packet, the A? 20 is configured to grant access to the medium to each node in turn. If a node fails to gain access to the medium in one attempt, it can continue to send requests at the appropriate time until access is achieved.

Once all data packets have been transmitted to the access point (the AP 20), the medium access protocol can be terminated very quickly.

While the invention has been exemplified by description of a node in general terms (and with reference to environmental data loggers), it will be appreciated by the skilled person that the invention can be implemented in a number of ways, without divergence from the scope of the invention. For instance, while a node could encompass an unattended weather station recording device (for recording one or more measurements of environmental data such as wind speed / direction, temperature, relative humidity, solar radiation), other devices could also benefit from aspects of the present invention.

For instance, hydrographic or oceanographic data could also be collected by data loggers, suitable for measuring water level/depth, flow, pH, or concentrations of contaminants. Soil moisture could also be monitored, as could flora or fauna related data, such as tree sap flow, or animal footfall past a particular point.

Industrial use (i.e. non-environmental use) is also contemplated, such as unattended recording of gas or oil pressure in natural resources extraction and transmission facilities. Also commercial or economic applications, such as road traffic counting or railway signalling status recording can be implemented using embodiments of the present invention.

The invention is also envisaged for use in collecting security data from sensors installed in or with storage/transportation containers or in goods vehicles.

Further aspects and embodiments of the invention will be understood to be envisaged by the skilled reader on the basis of the foregoing disclosure of specific embodiments thereof. The scope of the invention is not intended to be limited by any specific embodiment given in this disclosure, but should be read on the basis of the appended claims.

Claims

CLAIMS: 1. A method of establishing a wireless communications network

in a wireless medium, comprising the steps of transmitting, from an access controller, a beacon for reception by any nodes in range of said access controller, transmitting, from each node in reception of said beacon, a medium access request signal requesting access to said medium, said medium access request signals being transmitted on the basis of contention in the wireless medium, and, on the basis of medium access request signals received at the access controller, scheduling contention free access with each of the nodes indicated by said received medium access request signals.
2. A method in accordance with claim 1 wherein the step of transmitting a beacon comprises transmitting a beacon comprising node configuration information, said node configuration information being receivable by a node to configure operation of said node.
3. A method in accordance with claim 1 or claim 2 and including the step of repeatedly performing a step of monitoring at one or more nodes for reception at said node of a beacon transmitted by said access controller.
4. A method in accordance with claim 3 including, between repeatedly performing said step of monitoring, causing said node or nodes to enter a power saving state in which said medium is not monitored.
5. A method in accordance with claim 3 or claim 4 wherein the step of monitoring is performed periodically.
6. A method in accordance with claim 5 wherein the step of monitoring is performed with a constant period between repetitions thereof.
7. A method in accordance with claim 6 wherein said constant period is known to said access controller and said step of transmitting a beacon comprises transmitting a burst of beacons, said burst having a duration greater than said constant period.
8. A method in accordance with claim 7 wherein said burst is of a duration equal to or greater than twice said constant period.
9. A method in accordance with claim 7 or claim 8 wherein consecutive beacons, transmitted in said step of transmitting, commence periodically with a beacon repetition period, said beacon repetition period being shorter than a period for which said step of monitoring is performed.
10. A method in accordance with any one of the preceding claims wherein the step of transmitting a medium access request signal from one or more of the nodes comprises, at each node performing said step, backing off from transmission for a back off period, and then transmitting a medium access request signal to said access controller.
11. A method in accordance with claim 10 and including the step, at each node performing said step, of randomly or pseudo randomly generating a back off period.
12. A method in accordance with claim 10 or claim 11, wherein said medium access request signal comprises information identifying the node transmitting said medium access request signal, and information identifying requirements for access to said medium.
13. A method in accordance with claim 12 wherein said medium access requirements identifying information comprises an indication of an amount of data to be transmitted by said node.
14. A method in accordance with any preceding claim wherein said step of scheduling contention free access comprises sending a grant message, said grant message identifying a node permitted to transmit, and then permitting said node to transmit in a subsequent time penod, and repeating said steps of sending a grant message and permitting a node for all nodes indicated by means of said medium access request signals received at said access controller.
15. A method in accordance with any preceding claim and including, on completion of said scheduling step, repetition of said medium access request signal transmitting step from any further node or nodes remaining to transmit data in response to reception of said beacon and subsequent repetition of said scheduling step in response to reception of further medium access request signals.
16. A method in accordance with claim 15 wherein said medium access request signal transmitting step and said subsequent scheduling step are iterated until no further medium access request signals are received at said access controller.
17. Wireless communications apparatus operable to establish a wireless network with a plurality of nodes monitoring a wireless medium for a beacon, the apparatus comprising beacon transmitting means operable to transmit a beacon for reception by any nodes in range of said apparatus, medium access request signal receiving means for receiving a medium access request signal indicating a request for access by one or more nodes to said medium, and scheduling means operable, on the basis of medium access request signals received at the apparatus, to schedule contention free access with each of the nodes indicated by said received medium access request signals.
18. Apparatus in accordance with claim 17 wherein said beacon transmitting means is operable to transmit a beacon comprises node configuration information, said node configuration information being receivable by a node to configure operation of said node.
19. Apparatus in accordance with claim 17 or claim 18 wherein said beacon transmitting means is operable to transmit a beacon a plurality of times, at regular intervals defined by a beacon interval period.
20. Apparatus in accordance with any one of claims 17 to 19 wherein said scheduling means is operable to send a grant message, said grant message identifying a node permitted to transmit, and is then operable to configure said access controller to allow said permitted node to transmit in a subsequent time period, the scheduling means being operable to repeating said grant message sending and node transmitting permitting operations for all nodes indicated by means of said medium access request signals received at said access controller.
21. A wireless communications device, operable to establish wireless communication on receipt of a beacon from a further device, said device comprising beacon detecting means for detecting a beacon transmitted in a wireless medium, medium access requesting means for transmitting, on detection of said beacon, a medium access request signal requesting access to said medium, and transmitting means operable to transmit on the basis of medium access control by said further device.
22. A wireless communications device in accordance with claim 21 wherein the beacon detecting means is operable to detect node configuration information carried in said beacon and to configure said device on the basis of said node configuration information.
23. A wireless communications device in accordance with claim 21 or claim 22 wherein the beacon detecting means is operable to repeatedly monitor for reception at said node of a beacon transmitted by said further device.
24. A wireless communications device in accordance with claim 23, wherein said beacon detecting means is operable to enter a power saving state in which said medium is not monitored between repeatedly performing said operation of monitoring said medium.
25. A wireless communications device in accordance with claim 23 or claim 24 wherein said beacon detecting means is operable to perform said monitoring operation periodically.
26. A computer program product comprising computer executable instructions operable, when executed by a general purpose communications apparatus, to cause said apparatus to operate as a wireless communications device in accordance with any one of claims 17 to 20.
27. A computer program product comprising computer executable instructions operable, when executed by a general purpose communications apparatus, to cause said apparatus to operate as a wireless communications device in accordance with any one of claims 21 to 25.
28. A carrier medium carrying a computer program product in accordance with claim 26 or claim 27.
29. A carrier medium in accordance with claim 28 wherein said carrier medium comprises a computer accessible storage medium.
30. A carrier medium in accordance with claim 28 wherein said carrier medium comprises a computer receivable signal.