A MEDIUM ACCESS CONTROL (MAC) FOR A WIRELESS SENSOR NETWORK

Technical Field The disclosure concerns a medium access control (MAC) for a wireless sensor network. For example, but not limited to, the disclosure concerns a Body Area Network (BAN) comprised of low powered sensors and a gateway. Aspects include a method, a relay, a wireless sensor network and software.

Background Art

The Media Access Control (MAC) data communication protocol sub-layer is a sublayer of the data link layer (layer 2) specified in the seven-layer Open Systems Interconnection Reference (OSI model). MAC provides addressing and channel access control mechanisms that make it possible for network nodes to communicate within a multipoint network, such as a local area network (LAN).

A wireless Body Area Network (BAN) (also commonly referred to as a Body Sensor Network) is one example of a LAN. A BAN consists of a set of mobile and compact sensors, either wearable or implanted into the human body, which monitor body parameters and movements. These sensors are also referred to as physiological sensors. Due to the nature of their use the sensors used in a wireless BAN would have to be low on complexity, small in form factor, light in weight and power efficient. Usually the sensors do not store data themselves.

These devices, communicating through wireless technologies, transmit data sensed from the body or the body's immediate surroundings to a network controller, such as a gateway or base-station. The controller includes a processor and transceiver and forms a star topology with the sensors. The controller processes the received sensor data and sends it to the desired location in real time. For example the data received by the controller is sent to an external base station that using the Internet forwards the sensed information to a hospital, clinic or elsewhere. Alternatively, the processor of the controller can use the data to assist in immediate computer control medical treatment or assistance. For example, the BAN could be used in the transfer of sensed data for use by a pace maker, insulin delivery system or bionic eye information in real time. Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed in Australia before the priority date of each claim of this application.

Throughout this specification the word "comprise", or variations such as "comprises" or "comprising" will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.

Summary

A sensor node may be unable to communicate with the controller for periods of time where the data transmission to the controller from that sensor is not possible. This could be due to the movement of the sensors within the body. Of course, this is highly undesirable and in some cases also dangerous.

In a first aspect a method of medium access control (MAC) for a wireless sensor network is provided comprising a controller in communication with a plurality of sensor nodes and a relay node, the method being performed by the relay node and comprising:

(a) detecting transmission of a data packet from a sensor node to the controller;

(b) without the knowledge of the controller or sensor node, sending an acknowledgement message for the detected data packet to the sensor node on behalf of the controller, and sending the detected data packet to the controller on behalf of the sensor node; and

(c) repeating steps (a) and (b) until it is detected that the controller has sent an acknowledgement message for a data packet detected in step (a).

It is an advantage that relaying can occur where a MAC otherwise does not allow relaying. That means in most cases no modification to the MAC is required to implement this method. So should the sensor be blocked from transmitting data to the controller or receiving an acknowledgement of successful transmission from the controller, a relay can be used to cause the data to be transmitted without the sensor nodes and controller being aware that the relay exists and the transmission is only successful due to the relay's participation. At the same time, a power saving is achieved by the sensor which is now prevented from consuming power by continually resending data that has not been successfully received or acknowledged by the controller.

Detecting the acknowledgement message sent by the controller in step (c) is attempted after a repeat step (a) is performed and comprises determining whether the detected data packet of the repeated step (a) is a retransmitted data packet by the sensor node and if so, step (c) is ended with a repeat of step (a) and not step (b). .

If the data packet is a retransmitted data packet, step (c) may comprise sending a message after step (a) and before step (b) that forces a collision with the retransmitted data packet.

Step (c) may comprise repeating steps (a) and (b) until the sent acknowledgement message in step (b) is detected to have collided with an acknowledgement message sent by the controller.

For step (c), detecting that the acknowledgement message sent in step (b) collided with the acknowledgement message sent by the controller may comprise detecting that the sensor node has retransmitted the data packet associated with the acknowledgement message. In this way retransmission of a data packet by the sensor node indicates a collision of acknowledgment messages sent by the relay and the controller.

Detecting the acknowledgement message sent by the controller in step (c) is attempted after a repeat of step (a) is performed, but before a repeat step (b), and if the acknowledgement message is detected step (c) is ended by a repeat of step (a).

For step (c), detecting that the controller has sent an acknowledgement message for a data packet in step (a) may comprise detecting that the sensor node has transmitted a data packet that is not a retransmission. This may be achieved by comparing the detected transmitted data packet of step (a) with a data packet previously transmitted by the sensor.

In step (b) the acknowledgement message sent to the sensor node may appear to the sensor node that it was sent by the gateway. For example, it will be indistinguishable by the sensor to an acknowledge message that the controller could have sent for the same data packet. Such as, the acknowledgement message may identify the gateway as the sender. In this way the sensor node remains unaware of the relay's participation.

In step (b) the data packet sent to the controller may appear to the controller that it was sent by the gateway. For example, it will be indistinguishable by the controller to a data packet that the sensor node could have sent for the same data packet. In this way it actually identifies the sensor node as the sender. In fact, it may be identical to the detected data packet originally sent by the sensor node. In this way the controller remains unaware of the relay's participation.

The network may form a star topology with a single controller at the centre of the star topology. The MAC may not allow for multi hops and may not specifically allow for the use of relays. The method may be performed by the relay for multiple sensors in the network.

The relay node may have at least two modes of operation, a relay mode and a non-relay mode, wherein the method is only performed when the relay is in relay mode.

The sensors may be low powered devices that are unable to store data. The network may be a BAN.

ACK may be sent in a control time slot and the data packet may be sent in a data transmission time slot.

The controller may be a gateway or base-station of the network.

The method may performed independently for each of the plurality of sensors of the network. In this way a decision on whether to relay is made not only on a packet-by- packet basis but also on a sensor-by-sensor basis.

A further aspect concerns a relay of a wireless sensor network, the wireless sensor network comprising a controller in communication with a plurality of sensor nodes and the relay, wherein the relay comprises at least an input port, an output port and a processor, wherein the processor operates to: (a) detect at the input port transmission of a data packet from a sensor node to the controller; (b) without the knowledge of the controller or sensor node, send from the output port an acknowledgement message for the detected data packet to the sensor node on behalf of the controller and send the detected data packet to the controller on behalf of the sensor node; and repeating (a) and then (b) until it is detected that the controller has sent an acknowledgement message for a data packet in (a).

Yet a further aspect concerns a wireless sensor network comprising a controller in communication with a plurality of sensor nodes and the relay, wherein the relay device is as described above.

Even further, the another aspect concerns software, that is computer readable software recorded on a computer readable medium that when installed on a relay of a wireless network, the wireless sensor network comprising a controller in communication with a plurality of sensor nodes and the relay, causes the relay to operate in accordance with the method described above. The software may be firmware, application software or otherwise implemented in the hardware.

Brief Description of the Drawings Examples will now be described with reference to the following drawings, in which:

Fig. 1 is a schematic drawing of a wireless sensor network.

Fig. 2 is a flow diagram showing the method of a first and second option performed by a hidden relay.

Fig. 3 is a flow diagram showing the method of a third option performed by a hidden relay.

Examples

Examples will now be described with reference to the above listed drawings.

In these examples the wireless sensor network is a BAN that is schematically shown in Fig. 1. The sensors 10 are located in the body of a human (not shown) and are low powered. Each sensor 10 in real time wirelessly transmits sensed data in the form of data packets to a controller, which in these examples is a gateway 12 to form a star network topology with the gateway 12. A processor (not shown) of the gateway 12 then operates to concatenate the received data in real time and transmit it to a further apparatus also internal to the body (i.e. heart pace maker) or an external computer (i.e an emergency contact computer system).

In standard MAC the sensor sends a data packet (P _k ) and then tests for an acknowledgement (ACK) from the gateway. If the sensor receives ACK _k then the sensor assumes that the packet was transmitted correctly and the sensor continues to send the next data packet P _{k+ 1} at the next available time slot. If the sensor does not receive ACK _k then the sensor attempts to re-send the packet P _k until it receives ACKk or a time-out occurs. Standard MAC precludes the use of relays or multi-hop nodes in its protocol.

However, in these examples the network also includes a relay 14 that is "hidden" from the sensors 10 and gateway 12 in the sense that the relay's 14 existence and participation in communications on the network is performed without the knowledge of the sensors 10 and gateway 12. The relay 14 has processing power and operates to send data packages and messages between a sensor 10 and gateway 12 when the data packet or message otherwise would not successfully be transmitted.

The relay 14 operates to listen to the wireless channel to detect the transmission of data packets and acknowledgement messages in the same way as the sensor 10 and gateway 12 do. For example, it has an input port 16 at which it receives all transmissions (e.g. data and acknowledgement messages) made on the network. Then sends from an output port 16 ACK on behalf of the gateway 12 and data packets on behalf of the sensors 10. For each sensor 10, the relay 14 stores in internal memory 18 the sent data packet P _k and then determines using its programmed processor 19 whether to forward the data packet to the gateway 12, and whether or not to interfere with the control signals from the gateway 12. As in this example, the input and output port may be a single transceiver 16.

Three possible executions of the method of relaying data packets and ACK message by the processor 19 of the hidden relay 14 will now be described in further detail with reference to the flow charts of Figs. 2 and 3. Identical reference numerals will be used in the examples to identify the same step.

For simplicity, each example will be described in relation to one sensor 10, however the method can be repeated for each sensor 10 in the wireless network or for selected sensors 10 , such as those sending critical or priority data. The relay node 14 maintains a relay mode (described further below) for each sensor 10 in the network. The method of relaying employed by the relay node (i.e. option 1, 2, 3 or combinations of these) may also differ between nodes of the network.

As shown in the time line 20 along the length of Fig. 2 and 3 the MAC protocol is divided into two types of slots - transmission slots and control signal slots that are repeated as defined by the MAC protocol.

Option 1 - Forced collision of any data packet resent

In this option the relay 14 assesses each data packet to determine whether relay of that data packet is required.

During the first transmission slot 24, the sensor 10 transmits (i.e. sends) 26 a data packet P _k (I)- The relay 14 listening in on the wireless channel detects Pk(I) by receiving a copy and stores 28 P _k in local memory 18 of the relay 14. During the associated control slot 30 the relay 14 determines whether the gateway 12 actually received P _k (I) by listening 32 in that slot for an acknowledgement ACK _k that is sent from the gateway 12 to the sensor 10.

If the relay 14 detects ACK _k , the relay 14 continues to operate in normal mode 34 In this mode the relay 14 simply continues to store the packets as they are sent by the sensor 10 (in the same way as step 28) in transmission slots and listens for an associated ACK in the associated control slot (in the same way as step 32). The relay 14 takes no relaying steps unless the mode changes to relaying mode 46 using the method.

If the relay 14 does not detect the ACK _k, that means the sensor 10 also did not receive ACK _k in the slot 30, so the sensor 10 resends 36 the data packet P _k (2) in the next transmission slot 38. The receiver 14 monitors 40 the data sent in this slot 38 and identifies that the most recently sent packet P _k (2) is in fact a resend of Pk(I) by comparing Pk(2) with Pk(I) stored in the memory 19 of the relay 14. For example, the comparison may involve determining that the contents of the packet header and the corresponding data of the two packets are identical or identifying a resend tag in the header. Then during the same transmission slot 38 the relay 14 sends 40 a package to force a collision with Pk(2). The relay 14 may do this by transmitting noise which interferes with P _k (2). As a result of this collision the gateway 12 will not receive P _k (2) in slot 38 and in the second control slot 42 the gateway 12 will not send an ACK _k message. Instead, the relay 14 will send 44 ACK _k in the control slot 42 on the gateway's 12 behalf and without the gateway's knowledge. The ACK _k sent by the relay 14 is received by the sensor 10 and prevents the sensor 10 from resending the data packet P _k in the next transmission slot 48. In this way the relay 14 spoofs the ACKk that would usually be sent by the gateway 12 and the fact that it is the relay 14 not the gateway 14 sending the ACK _k is unknown to the sensor 10 also .

The relay 14 now switches to relay mode 46 where it sends 45 an exact copy of the most recently stored packet P _k (either P _k (I) or P _k (2)) in the next transmission slot 48 to the gateway 12 without the knowledge of the sensor 10. In this way the relay 14 spoofs P _k that would usually be sent by the sensor 10 and the fact that it is the relay 14 not the sensor 10 sending P _k is unknown to the gateway 10.

In this example, in relay mode 46 the method essentially repeats itself with the relay 14 making a decision after a packet resend is detected to force a collision of the resent packet and instead relay it to the gateway. That is, in the same transmission slot 48 the sensor 10 having received ACK _k from what it thinks is from the gateway 12 will send P _k+ i(l) 26a which the relay 14 stores 28a. In the associated control slot 50 the relay 14 will wait to see if the gateway 12 sends ACK _k+ ] 32a _.

If the relay 14 does not see ACK _k+ i it essentially remains in relay mode 46 and will monitor 40a the next transmission slot 52 for P _k+ i(2) 40a sent 36a by the sensor 10. If P _k+ i(2) is detected, relay 12 will send a packet P _k+! to force a collision 40a, send ACK _k+1 message (similarly to 44) in the next control slot and relay P _k+ i (similarly to step 45) in the next transmission slot (not shown).

The relay 14 returns to normal mode 34 when it sees an ACK sent from the gateway 12 to the sensor 10 to any data packet sent by sensor 10 that is not a resent packet.

Option 2 - Possible collision of ACK messages after data resent In this option the relay 14 again assesses each packet separately to determine whether relay is required but unlike option 1 does not force a collision on any resent packet. Steps 26, 28, 32, 34, 36 and 44 remain the same as option 1 but step 40 is not performed. As a result P _k (2) sent 36 from the sensor 10 may have been received by the gateway 12, and the gateway 12 may have also sent an ACK _k message in slot 42 and the relay 14 checks 52 for this.

If yes, then the ACK _k sent 44 from the relay 14 and the ACK _k sent from the gateway 12 in slot 42 will have collided and the sensor 10 will not have received either ACKk. Accordingly, the sensor 12 will resend 54 the P _k (3) in the next transmission slot 48.

If no, then the sensor 10 would have received the ACK _k sent from the relay 14 and accordingly will send 56 the next packet Pk+i(l) in the next transmission slot 48. The relay 14 will listen 58 in that transmission slot 48 to detect whether the sensor 10 sent either P _k (3) 54 or P _k+ i(l) 56. The relay 14 does this by comparing the data sent in slot 48 with the most recent data packet it has stored 28 locally.

If P _k (3) was sent that means that ACK _k messages collided in slot 42 and confirms that the gateway 12 is in fact receiving the transmissions from the sensor 10. So in the associated control slot 50 the gateway 12 will again send 60 an ACK _k that will be received by the sensor 10 (as there will be no collision this time) and the relay 14 does not relay any data packets in the next transmission slot 52. The sensor 10 will then send 26a P _k+ i(l) in the next transmission slot 52 and the relay 14 operates in normal mode as described above, such as storing 28 P _k+ i(l) and checking for ACKk+i from the gateway 12 (similarly to step 32).

If P _k+ i(l) was sent 56 that means that ACKk messages in slot 42 did not collide as the gateway 14 did not send an acknowledgement message in that slot 42. In relay mode 46 the relay 14 will now send 62 ACK _k+! in the control slot 50 and then relay 64 Pk+ _Ϊ in the next transmission slot 52. The sensor 60 will also send a packet in that transmission slot 52 which the relay 14 will store and monitor in accordance with this method.

Option 3 - Possible collision of ACK messages before data resent In this option the relay 14 actively sends ACK and then checks to see what effect that has had from the beginning. In this way the relay 14 defaults to relay mode 34 until it is changed to normal mode 34.

Again steps 26 and 28 are performed. Then in slot 30 the relay 14 automatically sends an acknowledgement ACK _k 44. In the same slot 30 the relay 14 determines whether the gateway 12 actually received P _k (I) by listening 32 for an acknowledgement ACK _k that is sent from the gateway 12 to the sensor 10.

If the relay 14 does not see 32 the ACK _k from the gateway 12, that means the sensor 10 did receive ACK _k in the slot 30 as there was no collision with the ACKk sent from the relay 14. So the sensor 10 sends 56 the data packet P _k+ i(l) in the next transmission slot 38.

If the relay does see 32 the ACK _k from the gateway 12 _; that means the sensor 10 did not receive ACK _k in the slot 30 as there was a collision with the ACK _k sent from the relay 14. So the sensor 10 resends 36 the data packet P _k (2) in the next transmission slot 38. The relay 14 monitors 70 the data sent in this slot 38 and identifies whether the most recently sent packet 36 or 56 is different to the packet stored 28 in local memory.

If yes, then the relay 14 knows that there was a collision in control slot 30 as the gateway 12 received the packet and sent ACK _k also. Accordingly the relay 14 returns to normal mode 72 while the gateway 12 sends 74 ACK _k in slot 42.

If no, then the relay 14 knows that P _k+ i(l) was sent 56 and that there was no collision at control slot 30 as the gateway 12 did not received the packet Pk(I) and did not send ACK _k at 30. The relay 14 then sends 76 ACK _{k+ 1} in control slot 42 and operates in relay mode 78.

Since each sensor is low powered, the aim is to save the power consumption by the sensor. It is an advantage that by relaying data when it would otherwise not be transmitted (i.e. sent and acknowledged) successfully, the sensor is prevented from continuously resending the same data in an attempt to get it to transmit successfully. In this way a power saving is achieved which is important to low powered sensors.

For each of the options above, when a relay sends a data packet to the gateway on the sensor's behalf, the address of the sensor from which the data packet originated is spoofed in the relayed data packet. In that way the gateway has no knowledge of the relay's involvement. At the same time, the acknowledgement messages that are sent to the sensor on behalf of gateway are also spoofed so that the sensor is unable to distinguish that it came from the relay rather than the gateway. The relay would send an exact replica of the standard acknowledgement message, including an identifier for the gateway.

It is a further advantage of this example that the MAC does not need to be altered to allow for the use of relays. In particular there is no guaranteed slot for all relays to simultaneously transmit.

It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the scope of the invention as broadly described.

In operation, the relay may in fact operate according to a mix of one or more of the processes described in the examples above. For example, once the relay is in relay mode from Option 1 or 2, the relay can then use the method of Option 3 which is more assertive in assuming that on a balance relaying will be necessary. Alternatively, a single option can be applied consistently. Even further, the relay can make a decision of which relaying method to apply as each packet arrives. This decision could be random or based on pattern detection that determines which is the best option to apply to the next packet sent by the sensor.

Multiple relays may be used and the selection of which relay would also need to be performed. The relays may be accommodated having each relay add a random offset in their activity The particular relay chosen is selected by algorithms in the open literature, such as Ghasem Naddafzadeh Shirazi, Peng-Yong Kong, and Chen-Khong Tham. A cooperative retransmission scheme for IR-UWB networks. In ICUWB, pages 207-210, 2008 or A. Bletsas, A. Khisti, D. P. Reed, and A. Lippman. A simple cooperative diversity method based on network path selection. 24(3):659-672, March 2006. Alternatively, a higher powered relay could be implemented.

The transmissions may be encrypted.

Applications include medical vital signs monitoring, elite sports use, elite sport spectating, consumer sports and electronics. It should be understood that the techniques of the present disclosure might be implemented using a variety of technologies. For example, the methods described herein may be implemented by a series of computer executable instructions residing on a suitable computer readable medium. Suitable computer readable media may include volatile (e.g. RAM) and/or non-volatile (e.g. ROM, disk) memory, carrier waves and transmission media. Exemplary carrier waves may take the form of electrical, electromagnetic or optical signals conveying digital data streams along a local network or a publicly accessible network such as the internet.

It should also be understood that, unless specifically stated otherwise as apparent from the following discussion, it is appreciated that throughout the description, discussions utilizing terms such as "listening" or "detecting" or "sending" or "processing" or "reading" or "generating" or "recognising" or "determining" or the like, refer to the action and processes of a computer system, or similar electronic computing device of a network, that processes and transforms data represented as physical (electronic) quantities within electronic registers and memories into other data similarly represented as physical quantities within registers or other such information storage, transmission or display devices.

The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.