FIELD OF INVENTION
The present invention is in the field of pervasive tracking and monitoring of enterprise objects / assets. Assets in an organization include humans, ve hicles, mobile objects, etc. Our invention is based on the newly ratified technology of IEEE 802.15.4. The technology gives complete enterprise visibility and leads to better management and better return on investments to organizations. The tracking and monitoring of objects are made through the deployment of a wireless mesh network. In this invention we present a new technique to achieve guaranteed 1-fault tolerant wireless mesh network. By 1-fault tolerant, we mean that the wireless mesh network would continue to work as before even if one device fails at any place. Further, our method guarantees the fault tolerance. As an added advantage, our method of fault tolerance doubles the life of the mesh network. The system of the invention will have various types of end uses and applications involving tracking and managing of mobile assets and people, such as tracking of the movement of persons in sensitive and high risk zones, such as in mines to improve the safety of miners and mine equipment. This improves the surveillance of the miners and the mine equipment and is of particular importance in the event of an emergency and helps in mine rescue. The tracking applications are also present in the management of a fleet of trucks in a campus thus enabling the detection of unauthorized entry into the campus or the movement of a truck in an unauthorized area of the campus. The system also has applications n the hospital sector where the movement of doctors, nurses and patients will allow better knowledge of their whereabouts. More critically, the system can track the location of hospital equipment like oxygen cylinders, defibrillator trays, first aid trays, mobile ECG machines etc. The system has applications in other sectors like retail, where the location of items in a warehouse are tracked, in logistics, where we can track the movement of inventory from one warehouse to another, to have an accurate pict are of the amount of inventory left. Thus the system of the invention has numerous industrial applications where it has the potential to provide accurate and real time information of the location of the assets of the organization thus leading to better management and improved return on investment.
BACKGROUND ART
It is well known in the art that from time to time tracking and monitoring systems have been made available. Some of the well known and presently available systems of such remote tracking and monitoring are discussed hereunder.
Standard Wi-Fi (IEEE 802.11) networks have been used to provide location tracking using customer deployed Wi-Fi hotspots. Aeroscout is a commeicial provider of such a system and offers indoor and outdoor real-time asset location, long range active RFID, choke point visibility and telemetry. A similar offering is provided by Ekahau where the core of the tracking solution is based on commercially available Wi-Fi networks. In both systems, active tags are essentially small IEEE 802.11 compliant wireless devices. These tags are attached to equipment or carried by people for real time tracking. The location of a tag is based on location of the Wi-Fi hotspots.

The above related works are based on the IEEE standard of 802.11 (Wi-Fi). Further, the solutions expect a customer deployed Wi-Fi mesh network is already in place. Using the Wi-Fi standard has severe restrictions as elaborated subsequently. In hazardous areas like the mines, safety precautions are mandatory, such as, the output power of the devices should be restricted to within 1 watt and the devices must be intrinsically safe. Restricting :he output power restricts the effective range of the Wi-Fi devices. Further, the Wi-Fi devices consume r igh power and it is therefore impractical and infeasible to run such devices on small batteries like that of size AA.
Another restriction on the solution provided 3y already deployed Wi-Fi hotspots is that the multi hop communication is not possible. Therefore, such systems; expect the hotspots to be the last mile of a traditional wired connection where data from the hotspot can then exploit the wired connection to reach the intended destination. This aspect is severely restrictive in new deployments where there have been no prior communication networks. For example, in deployments in mines, it is infeasible to deploy an additional wired network to support the Wi-Fi hotspots. Not only is this prohibitive in terms of cost, it also puts a burden on the power requirements.
A further restriction of the competing solutions is that they provide no support for the deployment of Wi-Fi Hotspots. In all these solutions it is assumed that the customer has already placed the hotspots. This is impractical, as the efficient placement on the hotspots will determine the efficacy of the tracking and the wireless coverage of the solution.
Fault tolerance has been extensively studied in computer architecture design. In wireless sensor networks, the fault tolerance has looked at five categories: Node Placement, Topology Control, Event Detection, Data Gathering/Aggregation and Sensor Surveillance. A (k-l)-fault tolerant network is one where every node is k connected and the netwo k remains connected even after k-1 node failures. Work on deploying additional relay nodes to ensure a pre determined k-connectivity between all nodes has been made. Existing sensors are assumed to be modeled as a unit-disk graph and their geographical coordinates is known. The problem of placing the minimum number of additional sensors geographically among the existing sensors is NP-hard. The problem of optimizing the deployment of additional nodes that have different transmission radii has been solved. However, in either of these works, the addressing and routing of sensors has been assumed. Further, we contend, the tracking applications in underground mines have very little probability of a single device acting as a support for multiple other devices (see Figure 1).
OBJECTS OF THE INVENTION
It is thus the basic object of the present invention to provide guaranteed 1-fault tolerant wireless mesh backbone for tracking and sensing applications usng the IEEE 802.15.4 compliant mesh network.
Another object of invention is the doubling of the network lifetime of the wireless backbone.

Another object of invention is the unique way of sharing the load among pairs of wireless routers where the sleep state is toggled among them.
DETAILS OF THE INVENTION
To achieve fault tolerance, we introduce duplicate routers for every router present. Each of these routers has the same address as its counterpart. One router is logically designated as the active and the other as a standby. The active-standby pair decides among themselves to toggle the active and stand-by roles. The active node is completely functional and the standby is in sleep state (thus conserving power). By having the same addresses for the active-standby pair, the parent and children nodes need not be aware of which of the active-standby nodes is currently active. The physical layout of the standby node is made close to the active. For example, we would place the standby routers on the other wall of the mine facing the active.
Upon boot, a node sends an "I am Active" packet with its destination address set to its own address. The nodes running IEEE 802.15.4 (CC 2430) cannot receive and send packets at the same time and hence the transmitting node does not receive the packet it is sending. If the corresponding node (of the active-standby pair) is up at this time, upon receiving of the "Active" packet goes to sleep (thus conserving power). The former device starts a timer and expects to receive the "Active" packet from its counterpart. Once this is received, it in-turn gous into sleep and the standby now takes the role of the active node. In case, the standby does not wal e up from its sleep, the active node sends a "Standby Failure" packet to the gateway and continues to carry the active role.
The active-standby scheme has two important properties that make it ideal for deployment in resource constrained networks. The distributed sleep mechanism is localized to only the active-standby pair and no communication needs to be made with their parent or their children. This significantly reduces the control packet overhead and correspondingly increases the network lifetime (figure 2). Second, the scheme is completely scalable. If we wish to achieve 2 fault tolerance, a third device with the same address is included. This also triples the network lifetime. There are however two issues with this scheme- a) there will be a small but finite amount of time when both the devices in an active-standby pair are active. This will lead to duplicate messages being propagated. This is a relatively harmless issue; b) a more important point is of a node failure as soon as it sends the "I am Active" message. In such a scenario, the standby device (which is now in sleep state) will continue to sleep until its timer expires. During this time, network connectivity is lost. This problem can be abated by having a small sleep-awaken cycle. However, by having a smaller cycle power consumption is increased in two ways-increase in control packets and the power needed to save the data registers while entering sleep and retrieve the data while coming out of sleep. An optimal wake up cycle is to be chosen depending on the application criticality.
The designed 1 Fault tolerant network was implemented on Tl supplied SOCs, CC2430, running TIMAC (802.15.4). The network layer was implemented as a simple queue where messages with higher priority from the MAC layer (MLME and MCPS) were put to the front of the queue and messages of lower

priority (UART) were put at the back of the queue. These messages were processed by raising an interrupt and the writing and reading from the queue were interrupt-shielded. We avoided the use of an operating system like Tiny-OS as the performance of the queue based system was impressive and scaled well.
A total of 10 devices were used, including one gateway, 6 Routers (Active-Standby) and 3 tags. The topology was made as shown in figure 3. The devices were programmed with different loads of messages per second and for each setting, were left to run overnight where the interruption due to people moving in the area is non existent. The devices, for each run, were placed in the exact location as before. Thus the randomness of the wireless medium was reduced as much as possible. We measured the packet drop and variance in the packet a rival rate at the gateway. The measure of variance as a function of the frequency of packets sent is shown in figure 5. The variance increases dramatically as the packet rate crosses one every three seconds. However, in the entire experiments carried out the packet loss rate was 0.003 % for 3 hops with the interval of 1.5s. In all other cases, there were no packet drops. We thus conclude, in such scenarios where wireless connectivity can be ensured and there is relatively low frequency of packet generation, the MAC support provided by 802.15.4 is robust enough to prevent developing intricate control mechanisms in higher layers.

BRIEF DESCRIPTION OF ACCOMPANYING FIGURES
Figure 1: is shows the topology that is created by using existing optimization schemes for network deployment.
Figure 2: shows our new scheme for achieving fault tolerance where the child nodes need not be aware of the load sharing capability of its parent.
Figure 3: shows how the topology of our sample implementation for the verification of our method. Figure 4: shows the success of our scheme in terms of variance in the packet arrival rate.

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Node 6 acts as a backup for nodes 2 and 5. In underground mines and other constrained areas, there is very little probability of such a scheme.
Figure 1


[The Active-Standby Pair (Eg. 6 - 6) behave as a logical entity. Therefore, its parent (8) and children ( 4 & 5) need not know which of {6 - 6} is currently active.
Figure 2

Topology of the pilot implementation. The nodes are placed on facing walls of the pathway

WE CLAIM:
1. A new method for achieving guaranteed 1-fault tolerant wireless mesh backbone for IEEE
802.15.4 compliant real-time pervasive tracking, monitoring and management system.
2. A unique way for enhancing the lifetime of the wireless mesh network by making pairs of
wireless routers toggle between active and standby states.
3. A new way to achieve load balancing with very little communication overhead where the
balancing of load is achieved in a local fashion without having to inform of other devices.

Our invention presents a new way to achieve a guaranteed fault tolerant wireless mesh network for IEEE 802.15.4 compliant devices. Our invention does not need global information as in other optimization schemes. We achieve fault tolerance by plating redundant wireless routers for every router that is present. However, the redundant router that is placed has the same address as that it is paired with. This pair of routers toggles between active and standby states. When a node comes up, it sends a "Active" packet with the destination address set to its own address. The limitation of IEEE 802.15.4 compliant devices prevents the node from receiving its own sent message. The corresponding node of the pair, on receiving the "Active" message goes into a low power standby mode thus conserving power. The standby node comes into active state after a finite amount of time and repeats the "Active" packet so that the other node can no go into standby state. In this way, a local mechanism for load sharing is achieved where the children of any node does not need to know which of the specific pair is currently active. Thus the inter node communication to achieve load sharing is minimized. Through this invention, the management of mobile assets is made efficient and tolerant to faults. In particular, our invention allows for applications such as informing mine workers of hazards in spite of certain device failures. This simple example shows how our system can be critical in rescue operations in cases of accidents leading to saving lives and equipment.