WO2012011094A1

WO2012011094A1 - Dynamically synchronized distributed sensor network

Info

Publication number: WO2012011094A1
Application number: PCT/IL2011/000561
Authority: WO
Inventors: Eyal Doron; Liron Nunez Weissman; Boaz Rippin
Original assignee: Pearls Of Wisdom Advanced Technologies Ltd.
Priority date: 2010-07-22
Filing date: 2011-07-14
Publication date: 2012-01-26
Also published as: IL207147A0

Abstract

The invention provides a distributed sensor network in which each node has a processor configured to transmit a heads-up message to a recipient node in the network informing the recipient node of a data stream to be transmitted along a route including a hop between the node and the recipient node. The heads-up message further informs the recipient node of times at which data packets of the data stream are to be transmitted from the node to the recipient node. Upon receipt of a heads-up message, the processor of the recipient node cycles the receiver of the recipient node between on and off state states in a low latency mode in which the receiver of the recipient node is in the on state when the receiver is to receive a data stream packet from another node in the network.

Description

DYNAMICALLY SYNCHRONIZED DISTRIBUTED SENSOR NETWORK

FIELD OF THE INVENTION

This invention relates to wireless sensor networks.

BACKGROUND OF THE INVENTION

Wireless sensor networks (WSNs) have numerous uses in the fields of traffic, industrial and environmental monitoring, and surveillance. These networks include a large number of "nodes" that are typically dispersed in an arena to be monitored or surveilled. Each node includes one or more sensors that collect data from the vicinity of the node. The sensors may be acoustic, seismic, chemical, temperature, pressure, flow, visual, magnetic, RF, infrared, and the like. One or more of the sensors may be active sensors, such as radar, LIDAR, and ultrasonic sensors. The nodes may also include processing means, such as microprocessors, DSPs, FPGAs, or dedicated logic circuits, as well as analog conditioning and processing circuits.

In addition to sensing and processing means, wireless sensor nodes contain communication circuits, whose purpose it is to wirelessly convey any sensed information to one or more clients. Typically, the information is transmitted to one or more hubs located in the vicinity of the arena. The hubs collect the information they receive from the nodes, and forward it via wired or wireless means to the clients. The clients may be in, or adjacent to, the arena, or may be remotely located. The hubs may also contain means for processing information received from the nodes prior to transmitting it to the clients.

A common requirement of WSNs is that most nodes be relatively inexpensive, small, and to operate for extended periods of time. This severely constrains the available resources, in terms of both processing and storage resources, and, most significantly, power. In addition, the presence of a large number of nodes requires each node to minimize the amount of information it transmits to the network, otherwise the limited wireless bandwidth available becomes congested. These considerations preclude the use of the most common network topologies, such as WiFi, Bluetooth, or cellular networks. In such topologies, each node transmits directly to a hub (sometimes referred to as a "gateway" or "coordinator". However, this requires a large bandwidth, since the network may comprise thousands of nodes. More significantly, this requires each node to have a communication range which extends to the hub which often imposes unacceptable demands on both the transmission power and the processing and timing resources available to small, inexpensive, ultra-low power, nodes.

WSNs thus utilize multi-hop techniques in which the network nodes, in addition to collecting information, also function as relays for relaying information collected at other nodes towards a hub. Thus a node may only need to transmit its message to a neighboring node, instead of all the way to the hub. The receiving node, in turn, may relay a received information packet to another node, and so on until the packet arrives at the hub. Several such multi-hop standards have been devised, such as ZigBee, WirelessHART, and ISA100.

WSNs utilizing multi-hop require a routing layer, which determines the path a message must follow in order to reach its intended destination. In contrast to centrally managed networks such as WiFi, WSN are usually organized into ad-hoc networks. Such networks self-organize in accordance with local conditions, which take into account such parameters as the available nodes, their wireless connectivity at the time, and various cost functions such as path length, latency, available power, network congestion, and the like. The route connecting a certain node to the hub may also change in time, for example due to a failure of a node along the way, or conversely due to the addition of a new node which shortens the number of hops along the path.

A multi-hop WSN system has most of the communication occurring among nodes, rather than from nodes directly to hub, which forces at least some of the nodes to constantly listen for communication from other nodes. This has severe consequences for the power requirements, since, due to the fact that external forwarding requests are unpredictable, a node may be required to power up its receiving circuitry for periods which are far more frequent than is required for actual transmission of information. The dominant energy consumer in a WSN node may actually be the wireless receiver and not the sensor, sometimes by a large factor. It is therefore desirable to reduce to the extent feasible, the amount of time a node spends in passive reception, since most of the energy expended in such a state does no useful work.

Reducing the fraction of time that a node engages in passive reception involves cycling the receiver between on and off states. However, this must be carefully managed so that requests to the node can still be received. There are various schemes to coordinate between a node intending to transmit a message and an intended recipient node. For example, the ZigBee standard makes use of beacons, in which a node periodically transmits a beacon pulse just prior to switching on its receiver. Any node wishing to send a packet to an intended recipient node first listens for the beacon from the node, and upon receipt of the beacon sends its information to the node. This method has the advantage that very little time is spent in idle reception. Also, this scheme removes the need to synchronize between nodes, and so simplifies the deployment and maintenance of the network. However, the method requires relatively frequent beacon transmissions. If the beacons are spaced far apart in time, the network latency becomes large, since the mean time it takes to transmit a packet over a single hop is half the beacon spacing. If, on the other hand, the beacon transmissions are more frequent, the nodes expend a large amount of energy in mostly sterile transmission, thus negating the advantage of the low reception duty cycle. In addition, frequent beacon transmissions increase the likelihood of congestion over the limited available bandwidth.

Alternative schemes, such as TSMP which is used by WirelessHART, operate by cycling the receiver in a manner which is known by its potential client. Thus a node may operate its receiver for, say, 1% of the time. The node's potential clients will then be required to have at least partial knowledge of the operating schedule of the node. If a client node wishes to transmit information to the node, it will wait until the node's receiver is known to be operational, and then transmit the packet at that time. The advantage of this scheme is the absence of non-essential transmissions in the shared RF medium, thus easing congestion and increasing the effective bandwidth. The disadvantages of this method include the requirement of synchronization between nodes, so that the reception schedule of one is known by the others. This requires a certain amount of maintenance communication, since the internal clocks of the nodes tend to drift with respect to each other. Such communication takes up valuable power and bandwidth. As with the use of beacons, also with this method there is a tradeoff between power usage and latency (as the frequency of reception cycles increases, power usage goes up and the chance for congestion of the band with increases, while the network latency decreases).

An additional factor in the performance of WSN is reliability. WSN by their nature cannot ensure very high reliability of every wireless link, since the nodes may be deployed in a non-optimal configuration. In addition, environmental conditions may change with time, thus varying the strength of point-to-point links or modifying interference between multiple RF paths. Finally, individual nodes may fail unpredictably, thus modifying the topology of the network.

There are various methods to enhance the end-to-end reliability of WSN transmissions. One such method is the inclusion of acknowledgements at various steps along the way. For example, when one node forwards a packet to its neighbor, it may require an "ACK" message in return. If this message is not received, the node may retransmit its message, possibly along a different route. In addition, the node may require an ACK message from the ultimate destination, which in this case may be the hub. The disadvantage of these and similar methods is that they increase the system load, and thus reduce the throughput and increase power consumption.

From the above it is clear that a low power, ad-hoc, multi-hop network must balance between many conflicting requirements, for example between power utilization, bandwidth, latency and reliability. This can cause difficulties in systems which require more than one simultaneous mode of operation. For example, an alarm system based on WSN may contain a mixture of low bandwidth and high bandwidth components. Low bandwidth elements could include passive IR sensors, door opening magnets or active Doppler sensors. Such sensors transmit a short message (e.g. "door opened at 10:31 AM") at relatively rare occasions. These kinds of messages contain very little information, and are not sensitive to network latencies of less than" a few seconds. However, when the alarm system contains a large number of such sensors, and a very low duty cycle, ultra-low power network protocol would nonetheless be appropriate. The alarm system may also contain a smaller number of high-bandwidth devices, such as intercoms or cameras. These devices may be larger, and perhaps mains powered. They are much less tolerant of delays and latencies, as well as dropped packets. A network designed for such devices would have a different design than the one built for a larger network of low-bandwidth devices. Another alternative involves two or more different networks in an arena, each operating independently and using a different modality, for the different needs of the sensors. However, this incurs additional cost and complexity, as well as a dual system for message forwarding, dual hubs and processing. Moreover, any data that has to be transmitted from a node of one system to a node of another system has to flow to the first system's hub, then to the second systems hub and then to the recipient node, thus increasing network load and energy consumption in both networks.

SUMMARY OF THE INVENTION

The present invention provides a distributed sensor network. The network of the invention comprises a plurality of nodes, a processor, a communication device and a power source. Two or more of the nodes further comprises a sensor. The receiver of each node is cycled between on and off states in order to reduce passive reception. This may be done by any method known in the art, for example, using beacons, in which a node transmits a beacon pulse just prior to switching on its receiver, or by cycling the receiver in a predetermined manner which is known to other nodes.

The network allows for the transmission of data along a route between an origin node or the hub to a destination node or the hub. A data item may be generated by a node, for example, when the node detects some suspicious activity. This data item may comprise, for example, a short message, e.g. "intrusion detected at 12:30". Alternatively, it might comprise a stream, such as an audio or video stream. In comparison to a data packet, a data stream requires low latency and real-time transmission, since the network typically does not include large-scale buffering capacity. Consequently, in accordance with the invention, prior to transmission of the data stream along a predetermined route, the nodes of the route coordinate on-off cycling schedules so as to reduce latency in the transmission of the data stream between the nodes in the route. The latency along the predetermined route may further be reduced by reducing the time that the nodes along the route are in their off state. . In this way, data flowing along the predetermined route is transmitted in a high-bandwidth, low-latency mode, , while data flowing along other routes are in a high latency mode.

The sensors may be, for example, any one or more of an acoustic, seismic, electromagnetic, IR, chemical, pressure, temperature, contact breaker, or similar passive sensor, or one or more active sensor such as active optical, radar or ultrasound sensor, a video or still camera, a scanned imaging sensor such as scanning radar or ultrasonic device, or a microphone picking up an audio stream. A single node may comprise two or more sensors.

The communication devices may be wireless communication devices, such as an RF device, an active or passive optical or IR communication device, or an acoustic communication device.

The nodes may be configured for enhancing the reliability of transmission to the hub, at the expense of power, bit rate or processing complexity. For example, the high throughput mode may include additional communication between nodes, to ensure safe delivery of a packet and to allow retransmission as required. This communication could also include an acknowledgement of reception from the hub, similar to the procedure used in TCP/IP systems. Alternatively, the physical protocol used in the high throughput mode could differ from the one used in the low throughput mode, for example the bit rate could be reduced and the symbol length increased, to improve the signal-to-noise ratio (SNR).

The invention thus provides a distributed sensor network comprising:

(a) two or more nodes, each node comprising a sensor, a transmitter, a receiver, and a processor configured to:

(i) cycle the receiver between an on state and an off state in a high latency mode;

(ii) transmit a heads-up message to a recipient node in the network, the heads-up message informing the recipient node of a data stream to be transmitted along a route including a hop between the node and the recipient node, and further informing the recipient node of times at which data packets of the data stream are to be transmitted from the node to the recipient node;

(iii) upon receipt of a heads-up message, cycle the receiver of the node between the on state and the off state in a low latency mode in which the receiver is in the on state when the receiver is to receive a data stream packet from another node in the network.

In the distributed sensor network of the invention, one of the communication devices may be a wireless communication device. The network may further comprise one or more nodes not having a sensor and may further comprise a hub. The processor of one or more nodes may be configured to decrease the duration of the off state of the node.

In the network of the invention, one or more of the nodes may comprise a processor configured to analyze a signal generated by a sensor of the node for the occurrence of one or more predetermined patterns in the signal indicative of suspicious activity in a vicinity of the node. A node may transmit a data stream upon detection of suspicious activity in the vicinity of the node.

The network of the invention network may further comprise a routing layer. In this case, the heads-up message can include the route of the data stream packets from an origin node or the hub to a destination node or the hub.

The times at which data packets of the data stream are to be transmitted from the node to the recipient node can be specified by a time of an initial data packet and a frequency of subsequent data packet transmission.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to understand the invention and to see how it may be carried out in practice, embodiments will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:

Fig. 1 shows a distributed sensor network in accordance with one embodiment of the invention; and

Fig. 2 shows a node for use in the distributed sensor network of Fig. 1.

DETAILED DESCRIPTION OF EMBODIMENTS

Fig. 1 shows schematically a distributed sensor network 2 in accordance with one embodiment of the invention. The network 2 has been deployed in an arena 4. The sensor system 2 comprises one or more nodes 6. The nodes 6 can communicate with each other as well as with a hub 10 that may be located inside the arena 4 or outside of the arena 4. The network 2 also comprises a routing layer (not shown) that determines a communication route involving one or more hops between nodes from each of one or more origin nodes or from the hub to one or more destination nodes or to the hub, when a message is to be sent from the origin to the destination. Fig. 2 shows schematically the structure of a node 6. The node 6 comprises a sensor 12, a processor 14, a communication device 16 and a power source 18. The communication device 16 comprises a receiver 17 and a transmitter 19 that allows a node 6 to communicate with at least one other node 6. The node 6 also includes a clock 21.

The receiver 17 of each node 6 is cycled between on and off states in order to reduce passive reception. This may be done by any method known in the art, for example, using beacons, in which a node transmits a beacon pulse just prior to switching on its receiver, or by cycling the receiver in a predetermined manner which is known to other nodes.

The processor 14 may be a microprocessor, DSP, discrete logic circuits, or FPGA. The processor 14 is connected to the sensor 12 via an interface 20. The interface 20 may be a digital interface or an analog-to-digital converter. The processor 14 is connected to the communication device 16. The communication device 16 may be a wireless communication device, such as an RF device, an active or passive optical or IR communication device, or an acoustic communication device. A wireless communication device would comprises a receiver 17 for receiving messages from other nodes and a transmitter 19 for transmitting messages to other nodes. The power source may be a rechargeable or non rechargeable battery. The power source 18 may be energized by environmental energy such as light, wind, vibration or electromagnetic energy. The power sources 18 may be electrical mains.

The processor 16 may configured to analyze signals generated by the sensor 12 to detect instances of one or more predefined "suspicious activities". The suspicious activity may be, for example, a level of noise above a noise level in a recent time window of the analyzed signal, or a level of noise above a predetermined threshold. As another example, the suspicious activity may be motion of objects in the vicinity of the node.

The network 2 allows for the transmission of a data stream along a route between an origin node or the hub to a destination node or the hub. A data stream may be transmitted, for example, when a node detects some suspicious activity.

When an origin node or the hub wishes to transmit a data stream to a destination node or to the hub, the origin node waits for the first node in the route to the destination determined by the routing layer to be turned on. When the first node is known to be on, the origin sends a "heads-up" message to the first node. The heads-up message informs the first node of the origin's intention to transmit the data stream, and further provides the first node with information relating to the timing of the transmission of the data packets of the data stream from the origin to the first node. The heads-up message may also include the destination node or hub of the data stream as well as the route of the data stream from the origin to the destination. The timing information may include the clock time of the first data packet in the stream and the frequency of the transmission of subsequent data packets. The first node then resets its cycling schedule so as to be in the on state at the times that it is to receive a data packet from the origin. The processor of the first node then determines the times at which it is to transmit the data stream packets to the second node in the route. The first node waits for the second node in the route to be turned on and then sends a heads-up message to the second node. The heads-up message informs the second node of the first node's intention to transmit the data stream to the second node, and further provides the second node with information relating to the timing of the transmission of the data packets of the data stream from the first to the second node. The second node then resets its cycling schedule so as to be in the on state at the times that it is to receive a data packet from the first node. The processor of the second node then determines the times at which it is to transmit the data stream packets to the third node in the route. The process is repeated until the heads-up message reaches the destination.

After the cycling schedules of all of the nodes in the route have been coordinated as explained above, the origin begins transmitting the data stream. By coordinating the cycling schedules, the latency of data stream transmission can be reduced. The origin may also wait for an acknowledgement message from the destination, to avoid transmission of a long data stream in the absence of a reliable connection. In addition, during low-latency transmission along the route, the nodes may be configured to ignore transmissions from nodes not along the route. Alternatively, external packets may be allowed to enter the data stream and either integrate with data stream or be buffered until a free time segment becomes available.

In another embodiment, the "heads-up" tracing packet may cause the nodes along the path to increase their wake-up frequency for the duration of the stream. For example, the wake-up frequency may be increased by an integral factor to further reduce the packet latencies. Use of an integral factor ensures that the schedules of the nodes which are not along the route are not altered will enable passage of additional packets from elsewhere. In an extreme case, the receivers of the nodes along the route may be powered continuously for the duration of the data stream.

The low-latency route may be in existence for a predetermined period of time, which may be preprogrammed or may be conveyed by the "heads-up" packet, or may be terminated by a further packet from the origin or the destination. Upon reaching the termination of the low-latency period of the route, the nodes along the route may reset their timing and protocol parameters to their original settings and the network can then resume operating as before.

Claims

CLAIMS:

1. A distributed sensor network comprising:

2. The network according to Claim 1 further comprising one or more nodes not having a sensor.

3. The network according to Claim 1 wherein at least one of the communication devices is a wireless communication device.

4. The network according to any one of the previous claims further comprising a hub.

5. The network according to any one of the previous claims wherein one or more of the nodes comprises a processor configured to analyze a signal generated by a sensor of the node for the occurrence of one or more predetermined patterns in the signal indicative of suspicious activity in a vicinity of the node.

6. The network according to Claim 5 wherein a node transmits a data stream upon detection of suspicious activity in the vicinity of the node.

7. The network according to any one of the previous claims further comprising a routing layer.

8. The network according to Claim 3 wherein the heads-up message includes the route of the data stream packets from an origin node or the hub to a destination node or the hub.

9. The network according to any one of the previous claims wherein times at which data packets of the data stream are to be transmitted from the node to the recipient node are specified by a time of an initial data packet and a frequency of subsequent data packet transmission.

10. The network according to any one of the previous claims wherein the processor of one or more nodes is configured to decrease the duration of the off state of the node.