Abstract:
A plurality of interactive modules are disposed at spaced locations to form an adaptive wireless network. Each module is capable of receiving transmissions of messages or data packets from other modules, and of transmitting messages or data packets to other modules for forming selected transmission paths via one or more modules toward a base station. Upon failure of a transmission path, a module not capable of transmitting a message along a transmission path toward the base station, transmits a message to other of the plurality of modules to form a new transmission path via such other module.

Description:
RELATED APPLICATION 
     This application is a continuation-in-part of, and claims priority from, application Ser. No. 11/433,194 entitled “Adaptive Network and Method” filed on May 11, 2006 by A. Broad et al, which is a continuation-in-part of application Ser. No. 11/345,737 entitled “Interactive Surveillance Network and Method,” filed on Feb. 1, 2006 by A. Broad et al, which is a continuation-in-part of application Ser. No. 11/152,350 entitled “Adaptive Surveillance Network and Method,” filed on Jun. 13, 2005 by A. Broad, which is a continuation-in-part of application Ser. No. 11/095,640 entitled “Surveillance System and Method,” filed on Mar. 30, 2005 by A. Broad et al, which issued as U.S. Pat. No. 7,705,729 on Apr. 27, 2010 and which is a continuation-in-part of application Ser. No. 11/096,098, “Adaptive Sensing Network,” filed on Mar. 30, 2005 by A. Broad et al., which issued as U.S. Pat. No. 7,369,047 on May 6, 2008, which applications are incorporated herein in the entirety by this reference to form a part hereof. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to mesh networks and more particularly to system and method for improving probability of packet delivery over low-bandwidth, poor quality links of a wireless network using an overlay of routing protocols on single-path routing schemes for delivery of data packets to a centralized location. 
     BACKGROUND OF THE INVENTION 
     Wireless Sensor Networks (WSN) use low-bandwidth radios to create a self-healing wireless mesh network. In WSN networks, nodes route data to a centralized point in the network referred to as a base station. Periodically each node shares its local neighborhood link information with the rest of the network. Each node uses this distributed link information to find the best path from itself to the base station. Data is then sent along this optimal path. 
     Most WSN mesh algorithms use single-path routing algorithms similar to algorithms commonly found in wired networks. These conventional algorithms are commonly adapted to take advantage of the wireless nature of the network by “overhearing” neighbor traffic to form detailed link estimates. These detailed link estimates are used to form an optimal single-path route to the base station. Wireless mesh networks of this type are described in the literature (see, for example, U.S. patent application Ser. No. 11/433,194). 
     Wireless link quality is known to vary over time. Since paths are formed on the basis of link quality estimates (or hop ‘cost’), mesh algorithms must periodically use energy to recalculate the best path at the lowest hop cost to the base station. If the period between recalculations of the optimal path is too long, nodes may use bad paths. If the period used is short, the network will drain available energy (usually battery power) rapidly. Since the variation of link quality over time is difficult to determine, most algorithms choose a single network-wide period based on network lifetime. In most cases, if a path goes bad between update periods, the data sent along that path is lost. 
     Different solutions have been proposed for overcoming data loss due to poor path quality. The most widely used solution is packet retransmission. If a packet is unable to be sent over a path, then the packet is resent until it is received by the next node along a path to the base station. Most wired protocols like TCP use an end-to-end retransmission strategy. The sending node continually resends to the receiving node until an acknowledgement of receipt is delivered. However, a difficulty associated with end-to-end recovery along a single path is the unreliable characteristics of the wireless communication link. The error accumulates exponentially over multiple hops from link to link, causing a high probability of packet loss, as illustrated in the graph of  FIG. 1 . 
     This graph indicates the limitation of the end-to-end retransmission scheme. As the number of hops increases, even networks with good link quality have a lower probability of delivering data packets to a base station. 
     One known modification of the single-path routing introduces a link by link retry to the end to end retry. Forwarding nodes resend data to the parent until the parent acknowledges receipt of the packet. By retransmitting at the link level, the link quality is artificially improved at each hop. However, as the number of hops increases along the path, even a set of good quality links has a low probability of delivering data reliably, as illustrated in the graph in  FIG. 2 . 
     This graph illustrates the number of retransmissions required to make a poor link into a high quality link. However, even after 8 retransmissions, such link-level retransmission scheme cannot make a poor-quality link into a high-quality link, and there is still possibility for data to be lost over the path. 
     Another known modification of the single-path routing may be implemented in systems where delay in delivery of data packets is acceptable. Packets which were to be dropped due to poor path quality, can be stored locally until a better path is found. Thus, data is stored either at the originating node or the forwarding node, until a good path is found, at which time the packet is forwarded. 
     A Delay Tolerant Network (DTN) is an appropriate solution for data which can be stored and retrieved at a later time. However, in networks where data is time-sensitive and must be delivered by a hard deadline, storage for a later date is inappropriate. 
     For time-sensitive data to be delivered over a single-path mesh network, DTN is an inappropriate solution and an alternative is to then fall back to using retransmissions in order to force the data to the base station hop by hop, expending large amounts of energy. 
     However, even packet retransmissions have limitations. Long-hop networks and/or networks where link quality can become excessively poor can still lose data along poor quality paths while expending large amounts of energy. 
     In addition, retransmissions have a detrimental consequence in high density networks. Due to the low-bandwidth radios used at each node, retransmissions can saturate the bandwidth, causing congestion. Congestion decreases the link quality on all surrounding paths with the consequence that all paths within radio distance of the retransmitting node will also begin dropping packets, and this can lead to a full network collapse. 
     SUMMARY OF THE INVENTION 
     In accordance with one embodiment of the present invention, a wireless network and method of operation form a transmission route for data packets or messages from child node or module to parent node or module progressively along a single path toward a central or base station. In addition, if a data packet or message may be dropped during operation due to poor link quality, an embodiment of the present invention facilities “jumping” to alternative paths within the network toward the base station. Also, data that may have been stored because of poor quality or disconnected link for later forwarding, may be aggregated with other data packets for forwarding along a new path established by such a “jump”. In this manner, time-sensitive data transmitted over long-hop networks can be more reliably delivered to the base station along an established route, or along a “jump” to an alternative route. Similarly, in high-density networks, the formation of an alternative “jump” route in accordance with the present invention obviates formation of congested parallel paths with concomitant savings of available energy and avoidance of repetitious attempts to retransmit data packets over a poor-quality link. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a graph illustrating an operating characteristic of end-to-end retransmission schemes; 
         FIG. 2  is a graph illustrating the detrimental effect of multiple retransmissions of data packets over poor quality links; 
         FIG. 3  in a flow chart illustrating an overview of network operation in accordance with the present invention; 
         FIG. 4  is a graphic illustration of Flood operation of distributed nodes in a wireless network; 
         FIG. 5  is a graphic illustration of a data path formed among selected nodes; 
         FIG. 6  is a graphic illustration of Flood operation at and about a bad link; 
         FIG. 7  is a flow chart illustrating operation of a network according to an embodiment of the present invention; 
         FIG. 8  is a graphic illustration of operation of a disconnected node with memory; 
         FIG. 9  is a graphic illustration of operation of a wireless network with re-connected nodes; 
         FIG. 10  is a graphic illustration of operation among isolated nodes; 
         FIG. 11  is a graphic illustration of data packets moving laterally among nodes isolated from the network; 
         FIG. 12  is a graphic illustration of transmissions among re-connected nodes; 
         FIG. 13  is a flow chart illustrating network operation according to an embodiment of the present invention; 
         FIG. 14  is a graph illustrating hop count v. success rate for different error factors; 
         FIGS. 15 and 16  are graphic illustrations of long-hop networks fragmenting into sub-networks; 
         FIGS. 17-19  are graphic illustrations of operations of nodes in a segment of a high-density network; 
         FIG. 20  is a graph illustrating the cost of transmission along a path v. the expected cost per data packet; 
         FIG. 21  is a graph illustrating the underlying network dropped or lost data packets; 
         FIG. 22  is a chart illustrating the data packets captured and retransmitted according to the present invention; and 
         FIG. 23  is a graph illustrating the delay times of transmissions of all data packets in accordance with the present invention during operation over a 20-hop network. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to  FIG. 3  there is shown a graphic illustration of a segment of a wireless network in which a plurality of nodes or modules  9  are spatially oriented substantially the radio transmission distance away from each toward a base station (not shown). Each such node or module  9  of the Wireless Sensor Network (WSN) uses low-bandwidth radios to create a self-healing wireless mesh network in which each node or module is capable of receiving and transmitting data packets or messages, and of sending a signal indicative of having received a data packet or message from another node. In WSN networks, nodes route data toward a centralized point in the network referred to as a base station. Nodes are battery operated and attempt to increase their lifetime by minimizing radio communication, a highly energy-expensive action. Thus, as illustrated in  FIG. 3 , routing through the network must attempt to find the most reliable path to deliver data via one or more nodes to a base station while minimizing communication. Data reliability and energy conservation are inverse objectives of routing data packets for delivery through the WSN. Usually improving reliability is at the cost of energy, or vice versa. A node  9  that receives a data packet retransmits  10  (and possibly aggregates its own data) toward a base station. If not successful in retransmitting to the base station (or to another node in a path toward the base station), then additional processing  12 , as described later herein, is required prior to the node  9  returning  14  to a wait state for receiving subsequent data packets. 
     For highly reliable routing, a Flood scheme, as illustrated in  FIG. 4 , attempts to send data packets on all possible paths to a base station. Thus, if there exists a single path from a node to the base station, Flood will find it since it attempts all permutations. Of course, if all permutations are tried then the entire network expends energy in forwarding the data from the node to the base station. 
     As shown in the figure, though a single good path exists among the nodes  9 , all nodes  9 ,  11  are expending energy to attempt to find a path to a base station. In this case the nodes  11  are wasting energy in attempting to find a path to the base station. 
     For energy efficiency, path routing is preferred with minimal energy expended to find and form a single good path to the base station. Then, that path can be reused for every data packet sent. Since data travels only over a single path, only the nodes  9  along the path expend energy to send the data toward a base station, as illustrated in  FIG. 5 . The data travels along a single path resulting in the least amount of energy expended in sending the data. However, reliability issues arise at any breaks along the path that will cause the data to be lost. 
     Wireless link quality is known to vary over time, and links that were good when the path was established can become poor. Best-path routing can be affirmed by periodically using energy to recalculate the best path to the base station. If the period used to recalculate the optimal path is too long, nodes may attempt to use bad paths. If the period used is short, the network will drain its energy rapidly. Since the variation of link quality over time is difficult to determine, most routing schemes use a single network-wide period based on optimizing network lifetime, but if a path goes bad between update periods, the data sent along that path is lost. 
     In accordance with an embodiment of the present invention, a path that exists as formed to transmit data packets through the WSN will be used. However, if a link along the path degrades, an embodiment of the present invention uses the Flood scheme in such a way as to provide the reliability of Flooding but with minimum expenditure of energy by localizing the Flood to the area where the link has degraded. 
     Thus, as graphically illustrated in  FIG. 6  and in the flow chart of  FIG. 7 , the data follows along the path  13 - 15  until it reaches a point where it cannot be forwarded due to link degradation, at which point the unsuccessful node  15  begins a localized Flood by broadcasting  16  to all neighboring nodes in the network. Each node  17 ,  19  that receives the data attempts to send it over its own path. In the case that a receiving node cannot forward the data over its own path, the unsuccessful node  15  again attempts to Flood the data out over the WSN. Eventually the data is forwarded  18  to a node  21  that has a good path, and the data is sent over that good path  21 ,  23 ,  25  toward the base station. 
     In this embodiment of the invention the energy is expended on exploring multiple paths only within an area where the paths are poor. Once the data reaches a node  21  which has a good single path, the Flooding exploration stops and the data is forwarded over the good path. 
     This embodiment of the invention attempts to overcome limitations in path routing by attempting to find alternate paths when the optimal path has degraded, and this assumes that a path exists. In some cases, portions of the network including one or more nodes may be periodically disconnected from the WSN. 
     In a delay-tolerant network using path routing, when no path exists to the base station for a portion of the network, each node stores  20 ,  26  its own data locally, as illustrated in  FIGS. 7 and 8 , until a path is discovered. In a time-sensitive network, the data must arrive at the base station by a deadline. Storing the data locally until a path from that node can be discovered may take too long, particularly if portions of the network are continually connecting and disconnecting. 
     In a delay-tolerant network, stored data is localized  26  at a single node as illustrated in  FIG. 8 . Depending upon the network conditions, end-to-end path formation may be very improbable. This is particularly possible in long-hop networks where error rate can accumulate to be very high. However, portions of the network may regain connectivity while others may remain detached, as illustrated in  FIG. 9 . The improbable nature of complete path formation in some networks make storing time-sensitive data impractical since time deadlines will likely pass before a path is formed. 
     In accordance with another embodiment of the present invention, storage is a practical solution for moving time-sensitive data quickly through the WSN. If data is unable to progress toward the base station along a path, as illustrated in  FIG. 10 , the data moves laterally looking for alternate paths toward the base station, as illustrated in  FIG. 11 , and in addition to forwarding the data laterally, each node  27 - 31  that receives the data will also store  26  it locally. Then, if a path becomes available from any node that has stored the data, it will be forwarded toward the base station. 
     Thus, as illustrated in  FIGS. 10 ,  11 , data that is initially created by a node in a disconnected portion of the network is not able to route toward the base station, but attempts to move laterally through the WSN being stored  26  at each receiving node. Later, as the network connectivity changes, as illustrated in  FIG. 12 , a portion of the nodes which contain the data are now connected to the network and can route toward the base station. The original node may still be disconnected but its data has found a path to the base station through the spatial distribution and storage  26  of the original data. 
     Storage efficiency and energy efficiency of the WSN may be improved according to the present invention by the nodes periodically removing old data which has passed its deadline, making room for more recent data. As shown in  FIG. 11 , the disconnected group of nodes which stored the original data may eventually remove the stale data that passes its lifetime, as indicated in the flow chart of  FIG. 13 . Thus, a periodic timer  22  in a node  15  processes stored data  26  to determine  24  whether it is ‘stale’ or expired and, if so, deletes  28  the old data. However, contemporary data may be transmitted  30  at least to a neighboring node within radio range along a connected route toward a base station. If such transmission is not successful, then the data remains stored  32  until a subsequent periodic timer cycle during which the data will be determined to be expired or still contemporary. This conserves also the energy which would be spent on sending expired packets. 
     To improve radio efficiency, data is aggregated into a single packet. Once a path has formed, and stored data can be aggregated into a single packet along with data which is currently originating from the node. Stored data is thereby essentially “piggybacked” on packets of data which the node would otherwise send. These embodiments of the present invention thus enhance the probability of successfully sending a data packet from a node toward a base station by overcoming packet drops at any link along a route. 
     In conventional mesh networks such success probability may be analyzed along [n] hops, as follows: 
     p(l)=probability of success over link  1   
     P(n)=Probability of successfully transmitting a packet over n hops, with each link having a probability p(1 . . . n). 
     R(n)=the number of retransmissions to achieve 100% success over a route
 
 P ( n )=Π p ( l ) [from  l= 1 −n] 
 
 R ( n )=1 /P ( n )
 
     In accordance with the present invention two improvements are achieved in Route Level Retransmissions and Spatial Distribution. Route Level Retransmissions improve the R(n) for any given path. The expected hop to drop a packet (thereby causing a retransmission) is calculated as follows: 
     N=number of hops 
     p(l)=probability of success over link  1   
     E(D)=the expected number of hops before a packet is dropped
 
 E ( D )=Σ( n*p ( l )^( n− 1)*(1 −p ( l )) [from  n= 1− N  and  l=n] 
 
     According to the present invention, Route Level Retransmission resends the packet from the node it was dropped at instead of retrying from the originating node. This changes the R(n) to the following:
 
 R ( n )=(1 /Πp ( l ) [from  l=n−E ( Dn )])+ R ( E ( Dn )) where  E ( Dn )&gt;1
 
 R ( n )=1 /P ( n ) where  E ( Dn )=&lt;1
 
     Thus, in a conventional WSN, a node continues to retransmit from the originating node, not taking advantage of the fact that the data packet may have gotten quite far along a path toward a base station. In contrast, the present invention takes advantage of the fact that a data packet has gone Dn number of hops to a node that then tries to resend from there. Since Dn is closer to a base station than the original node, its probability of success should be higher. This makes a retransmission from Dn much more likely to arrive at the base station than a retransmission from the originating node n. 
     The present invention also improves the P(n) of a single packet by spatially distributing the packet over a subset of the neighboring nodes in radio range of the dropping node. These nodes can then resend the packet along their own paths, improving the probability of packet delivery to the base station. The number of nodes receiving a distributed packet is as follows: 
     S(x)=number of nodes in radio range of node x 
     p(ls)=the link quality between node x and a node in S(x) 
     M(x)=the number of nodes which receive a spatially distributed packet sent by node x.
 
 M ( x )=Σ p ( ls ) [from  ls= 1 −S ( x )]
 
     Since the M(x) nodes also forward the packet along their paths, the new P(n) becomes:
 
 P ( n )=(Π p ( l ) [from  l=n−E ( Dn )])*(1−(Π(1− P ( x ))) [from  x= 1 −M ( E ( Dn ))]
 
     The present invention thus improves the probability of successful transmission of a data packet over n hops by attempting to send data to M neighbors from the point of failure E(Dn). The probability of success is increased because there are M more routes the data is simultaneously taking. 
     The present invention is particularly beneficial and advantageous in Long Hop Networks and in High Density Networks. One implication of P(n) is that as n gets larger the P(n) gets smaller. This means that the greater number of hops in a network lowers the probability of successfully sending the packet over the entire route. 
     The graph of  FIG. 14  illustrates Hop Count (n) vs Success Rate for different Line Errors Rate (1−p(x)). 
     As shown, even good link qualities (90%) tend to have poor success rates (60%) as the hop count increases (10 hops). 
     For this reason, long hop networks tend to become sets of disconnected sub-networks, and these sub-networks can merge with other sub-networks or can fracture into more sub-networks, as illustrated in  FIGS. 15 and 16 . 
     In these situations the present invention improves the WSN by reliably forwarding the data:
         1) by routing data outside the given path: As illustrated in  FIGS. 15 and 16 , an embodiment of the present invention attempts to jump the gap by sending a broadcast message out to any node within radio range that can “hear” it. This can potentially be any of the nodes  33 - 36 , as shown.   2) by also spatially distributing the data over all the nodes in the sub-network: As illustrated in  FIGS. 15 and 16 , the nodes  37 - 40  spatially distribute the data amongst each other so that when one of these nodes is once again a part of the network, it will forward the data. This can actually be used to forward data in a staged manner from sub-network to sub-network until the data reaches the base station.       

     Another implication of P(n) is that a very poor p(x) can reduce P(n). This means a single bad link can cause a path of good links to fail. In most cases this requires retransmissions to overcome the bad link, but in high-density networks retransmissions can have substantial impact. 
     In high-density networks, retransmissions of data along a path can lead to congestion. High-density networks, with frequent communication needs, have a tendency to mistake data loss due to congestion as data loss due to environment. In doing so, the nodes retransmit lost packets multiple times. Packet retransmissions in turn cause more congestion, leading to a collapse of the network. 
       FIGS. 17-19  illustrate a segment of a high-density WSN. 
     As the node  41  retransmits to overcome its poor link, it begins to cause collisions with the packets transmitted by nodes  42 ,  43 . The nodes  42 ,  43  then retransmit received packets and saturate the bandwidth of the radios, causing a congestion collapse. In essence, poor link quality of one node causes the entire high-density WSN to stop transmitting. 
     In accordance with the present invention, this situation is avoided by:
         1) Reducing the number retransmissions: Once a node  41  has done a minimum number of retransmissions to attempt to overcome a poor link, an embodiment of the present invention attempts to “jump” to another path, e.g., the data will jump to one of the nodes  42 ,  43 .   2) Aggregate the “jumped” packets and “piggyback” them on existing packets: Thus, the nodes  42 ,  43  will aggregate the “jumped” data from the node  41  and forward it within its own packets.       

     Operation of the present invention was analyzed using EmStar, an open-source simulator specifically configured for analyzing Wireless Sensor Networks. In the analyses, EmStar was configured to create a Long Hop Chain Network of 20 nodes, and was configured to simulate a path-loss radio model that emulates the following real radio characteristics:
         1) Exponential Decay over distance: The radio strength degrades exponentially as the nodes get farther apart; and   2) Normalized Probability of Packet Loss, i.e., links quality (indicated by packet loss) varies over time. In other words, a good link will stay good, a bad a link will stay bad, and links will vary between to the two states gradually.       

     Nodes were positioned for operation and analyses of the present invention in such a way that they had on average a good link to their 1-hop neighbors, a poor link to their 2-hop neighbors, and no link to their 3-hop neighbors. This configuration emulates real-life Long Hop Network conditions experienced in real-life deployment of nodes. 
     The test involved having the node farthest from the base station (node  20 ) to send a data packet every 30 seconds. A conventional scheme (e.g., XMesh algorithm commercially available from Crossbow Technology, San Jose, Calif.) was used to form mesh and route data along a specified path among nodes. In an overlay of XMesh, an embodiment of the present invention was installed to augment XMesh. During the test, the following information was recorded per packet:
         1) The path cost of the route at the time of packet transmission;   2) Whether XMesh was scheduled to drop the packet, and whether the present invention was able to recover the packet; and   3) The delay between when the packet was sent vs. when it was received by the base station.       

     The results, as illustrated in the graph of  FIG. 20 , indicate the path cost in terms of the expected transmission cost (ETX). ETX measures the quality of a path by indicating the expected number of times a packet will need to be transmitted along a path. For a 20-hop network, a perfect path will have an ETX of 20 cost units. In this analysis of more realistic operation of the present invention, the ETX to node  20  remained between 20-40 cost units for most of the experiment. Some time after packet  599 , the route completely dissolved. This means that at least node  20  was disconnected from the base station. 
     Referring now to  FIG. 21 , this graph illustrates that the underlying operation of XMesh dropped particular packets. As expected, a large number of packets were dropped during the time when the Path Cost indicates XMesh was disconnected, as illustrated in  FIG. 20 . It is noted that XMesh also drops a small number of packets during the good route. This is believed to be due to the variability of the link quality over time. That is, by the probabilistic nature of radio communication, a good route does not guarantee that a packet will be delivered. 
     Operation of the present invention overlaying XMesh was able to deliver 100% of the packets, and was able to overcome the probabilistic nature of the radio communication and deliver all of the data, as illustrated in the chart of  FIG. 22 . 
     Referring now to  FIG. 23 , this graph illustrates the delay between sending the packet and finally receiving the packet at a base station after 20 hops. Packets which were not scheduled to be dropped by XMesh had a sub-second delay attributable to the expected delays in transmitting a packet over 20 hops. Higher order delays were the result of recovering a packet, in accordance with the present invention, that the underlying XMesh dropped. These packets have a varying degree of delay based on the state of the network 
     Thus, considering the network conditions by which the route was established, and including packet loss due to probabilistic transmission conditions, the present invention was able to deliver a packet in under 10 minutes, with an average delivery time of about 3 minutes. And, despite network conditions, even in which the route was disconnected, the present invention was able to deliver all of the data packets in under 45 minutes. 
     Therefore, operation of the present invention with respect to a wireless sensor network (WSN) recovered 100% of data packets intended to be transmitted over a 20-hop segment of a WSN, with associated increased reliability of packet delivery in the network. For time-sensitive data the present invention balances urgency of delivery with efficiency based on the state of the network.