WO2011141911A1

WO2011141911A1 - Distributed sensor network having subnetworks

Info

Publication number: WO2011141911A1
Application number: PCT/IL2011/000372
Authority: WO
Inventors: Liron Nunez Weissman; Raanan Ben Shachar; Eyal Doron
Original assignee: Pearls Of Wisdom Advanced Technologies Ltd.
Priority date: 2010-05-13
Filing date: 2011-05-09
Publication date: 2011-11-17
Also published as: IL205727A0

Abstract

The invention provides distributed sensor network comprising two subnetworks having different throughputs. Nodes of the low throughput subnetwork are configured to activate sensors of the high throughput subnetwork. The network may include a hub that is configured to send a message through the low throughput subnetwork to a specified first node instructing the specified first node to activate one or more sensors of the high throughput subnetwork.

Description

DISTRIBUTED SENSOR NETWORK HAVING SUBNETWORKS.

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 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 maintenance communication increases, power usage goes up and the chance for congestion of the bandwidth 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, 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 comprising two or more subnetworks. A first subnetwork is a low throughput subnetwork and comprises one or more first nodes where each node comprises a sensor and a communication device. A second subnetwork is high throughput subnetwork and comprises one or more second nodes where each node comprises a sensor and a communication device. As used herein, the term "throughput" of a subnetwork refers to the maximum rate that data can be transmitted over the subnetwork.

The low throughput subnetwork comprising the is configured for low-power, low throughput, high latency operation, while the high throughput subnetwork is configured for high throughput, low latency, operation. For example, the low throughput subnetwork may be based on beacons transmitted between 100msec and a few seconds apart, thus drawing little power but having a latency of up to a few seconds per hop. Alternatively, the low-power subnetwork may be based on low duty cycle (e.g. <5%) periodic activation of the receiver, and partial synchronization of neighboring nodes.

In accordance with the invention, at least one of the first nodes can communicate with at least one of the second nodes.

A low-throughput sensor 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 an active optical, radar or ultrasound sensor, or a combination thereof. A high throughput sensor may be, for example, any one or more of a video or still camera, a scanned imaging sensor such as a scanning radar or ultrasonic device, or a microphone picking up an audio stream. The communication devices are preferably wireless communication devices, such as an RF device, an active or passive optical or IR communication device, or an acoustic communication device.

The subnetworks may have similar structures and physical layer (PHY) characteristics, while having different parameters such as duty cycle, packet length, radio channel, timing, bit rate, spreading sequence or the like. Alternatively, the subnetworks may have substantially different architectures, for example a beacon based structure and a synchronized time-domain modulation (TDM) scheme such as TSMP.

A node of the low throughput power subnetwork may carry commands for switching on and off a node of the high-throughput subnetwork. For example, the hub of the network may send a message through the low throughput subnetwork to a specified node instructing the specified node to activate one or more specified nodes of the high throughput subnetwork. Data collected by the activated node can be transmitted over the high throughput subnetwork to the hub. Alternatively, a trigger for operation and configuration of a high-throughput node may originate from one or more of the low throughput nodes, rather than from a central hub. For example, a low-throughput sensor, such as a passive IR sensor, may detect some suspicious activity. The node then sends a message along the low throughput subnetwork to a node which supports both networks, such as a node which includes a camera. The node then activates the high-throughput network and uses it to send a video stream to its hub.

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;

Fig. 2a shows a low-throughput node and Fig. 2b shows a high-throughput node, for use in the distributed sensor network of Fig. 1 ;

Fig. 3 shows a distributed sensor network in accordance with a second embodiment of the invention; and

Fig. 4 shows a node for use in the distributed sensor network of Fig. 3. 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 two or more subnetworks. A first subnetwork, comprises one or more first nodes indicated by squares 6. A second subnetwork, comprises one or more second nodes indicated by circles 8. Two subnetworks are shown in Fig. 1. This is by way of example only, and the network 2 may comprises any number of subnetworks that is at least two. All subnetworks communicate with a common hub 10 that may be located inside the arena 4 or outside of the arena 4.

Fig. 2a shows schematically the structure of a node 6, and Fig. 2b shows schematically the structure of a node 8. The node 6 comprises a low throughput sensor 12a, a processor 14a, a communication device 16a and a power source 18a. The node 8 comprises a high throughput sensor 12b, a processor 14b, a communication device 16b and a power source 18b. The communication device 16a comprises a receiver 17a and a transmitter 19a that allows a node 6 to communicate with at least one other node 6. The communication device 16b comprises a receiver 17b and a transmitter 19b that allows a node 8 to communicate with at least one other node 8. In accordance with the invention, at least one of the nodes 6 can communicate with at least one of the nodes 8.

Fig. 3 shows schematically a distributed sensor network 52 in accordance with another embodiment of the invention. The network 52 has been deployed in an arena 54. The sensor network 52 comprises two or more subnetworks. A first subnetwork comprises one or more first nodes 56. A second subnetwork comprises one or more second nodes 58. In this embodiment, at least one node 56 is integral with a node 58 in a common node 53. Two subnetworks are shown in Fig. 3. This is by way of example only, and the network 2 may comprises any number of subnetworks that is at least two. All subnetworks communicate with a common hub 60 that may be located inside the arena 54 or outside of the arena 54.

Fig. 4 shows schematically the structure of a node 53. The node 53 comprises a node 56 and a node 58. The nodes 56 and 58 have elements in common with the nodes 6 and 8, respectively, which are indicated by the same reference numerals without further comment. Thus, the node 56 comprises the low throughput sensor 12a, the processor 14a, the communication device 16a and the power source 18a. The node 58 comprises the high throughput sensor 12b, the processor 14b, the communication device 16b and the power source 18b. The communication device 16a comprises the receiver 17a and the transmitter 19a that allows a node 56 to communicate with at least one other node 56. The communication device 16b comprises the receiver 17b and the transmitter 19b that allows a node 58 to communicate with at least one other node 58. In this embodiment, a node 56 can communicate with the node 58 of the same network node 53.

The processors 14a and 14b may be a microprocessor, DSP, discrete logic circuits, or FPGA. The processors 14a and 14b are connected to the sensors 12a and 12b, respectively, via an interface 20a and 20b, respectively. The interfaces 20a and 20b may be digital interfaces or analog-to-digital converters. The processors 14a and 14b are connected to the communication device 16a and 16b, respectively. The communication devices 16a and 16b are preferably wireless communication devices, such as an RF device, an active or passive optical or IR communication device, or an acoustic communication device. The power sources 18a and 18b may be rechargeable or non rechargeable batteries. The power sources 18a and 18b may be energized by environmental energy such as light, wind, vibration or electromagnetic energy. The power sources 18a and 18b may be electrical mains.

The subnetwork comprising the low throughput nodes 6 is configured for low- power, low throughput, high latency operation, while the subnetwork comprising the high throughput nodes 8 is configured for high throughput, low latency, operation. For example, the low throughput subnetwork may be based on beacons transmitted between 100msec and a few seconds apart, thus drawing little power but having a latency of up to a few seconds per hop. Alternatively, the low-power subnetwork may be based on low duty cycle (e.g. <5%) periodic activation of the receiver, and partial synchronization of neighboring nodes.

The receiver 17b of the high throughput node 8 may be activated continuously or at a high duty cycle, so that the inter-hop delay is reduced substantially. Alternatively or in parallel, the high throughput subnetwork may utilize a higher transmission power or lower frequency of operation, to reduce the number of hops and with it the resulting delays. The high-throughput subnetwork may use sophisticated coding and processing algorithms to increase the network bit rate at the expense of power.

At least one of the nodes 6 can communicate with at least one of the nodes 8. A node 6 of the low-power subnetwork may carry commands for switching on and off a node 8 of the high-throughput subnetwork. For example, in the network 2 or 52, the hub 10 or 60, respectively, may send a message through the low throughput subnetwork to a specified node 6 instructing the specified node 6 to activate one or more specified nodes 8. Data collected by the activated node 8 would be transmitted over the high throughput subnetwork to the hub. After receipt of the data at the hub, the hub may send a second message over the low throughput subnetwork to same specified node 6 instructing the node 6 to deactivate the activated node 8 to conserve power.

In another embodiment, the trigger for operation and configuration of a high- throughput node 8 or 58 may come from one or more of the low throughput nodes 6 or 56, rather than from a central hub. For example, a low-throughput sensor, such as a passive IR sensor, of a node 6 or 56, may detect some suspicious activity. The node then sends a message along the low throughput subnetwork to its neighbors. The message can make its way along the low throughput subnetwork to the hub, thus signaling an event. Concurrently, the message may also reach a node which supports both networks, such as a node which includes a camera. The node then activates the high-throughput network and uses it to send a video stream to its hub. Alternatively, the message may only activate and prime the high-throughput network, and wait for a command from the hub to send the actual data. Another option would have the dual-network node monitor the low-power network for additional triggers, for example reception of an alarm from more than one low-power node, before either activating the high-throughput network, or sending the data, or both. In yet another alternative, when a node of the low-power network detects suspicious activity detected, the node may activate the entire high bandwidth network, or a subregion of the high bandwidth network.

The network 52 shown in Fig. 3 may include several types of nodes, where each type of node supports a particular combination of subnetworks. Thus, some of the nodes may contain one or more low-throughput sensors, or one or more high-throughput sensors, or both, or neither. In addition, each node type may be connected to a different combination of networks. Nodes with no sensors serve as communication relays in the networks they are members of, while nodes which do contain sensors can also serve as signal sources. The interaction between the networks can occur wherever a node supports more than one network.

One of the subnetworks may include provision for enhancing the reliability of transmission to the hub, at the expense of power, bit rate or processing complexity. For example, such a network may include additional communication between nodes, to ensure safe delivery of a packet and 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 by the network could differ from the one used by other networks, for example the bit rate could be reduced and the symbol length increased, to improve the signal-to- noise ratio (SNR).

Claims

CLAIMS:

1. A distributed sensor network comprising:

(a) at least a first subnetwork and a second subnetwork, the first subnetwork comprising at least one first node and the second subnetwork comprising at least one second node;

wherein at least one first node comprises a first sensor and a first communication device, and at least one second node comprises at least one second sensor and a second communication device;

wherein the first subnetwork has a lower throughput than the second subnetwork; and

wherein at least one first node of the first subnetwork is configured to activate at least one second sensor of a second node.

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

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

4. The system according to any one of the previous claims wherein one or more of the nodes comprises a processor.

5. The system according to any one of the previous claims wherein at least one first node is integral with a second node.

6. The system according to anyone of the previous claims wherein a first node is configured to activate a second sensor of a second node.

7. The system according to Claim 3 wherein the hub is configured to send a message through the first subnetwork to a specified first node instructing the specified first node to activate one or more second sensors of one or more specified second nodes.

8. The system according to Claim 7 wherein an activated second sensor is configured to send data through the second subnetwork to the hub.

9. The system according to any one of the previous claims wherein a first node is configured to detect one or more predetermined events and to activate one or more predetermined second nodes upon detection of one or more of the predetermined events.