1. Fault Tolerance in ZigBee Wireless Sensor Networks
Venkata Nagarjuna Ravulapalli

2. Fault Tolerant
Fault-tolerant design is a design that enables a system to continue operation, possibly at a reduced level, rather than failing completely, when some part of the system fails.
That is, the system as a whole is not stopped due to problems either in the hardware or the software.

3. Wireless Sensor Networks
A wireless sensor network (WSN) consists of spatially distributed autonomous sensors to monitor physical or environmental conditions, such as temperature, sound, vibration, pressure, motion or pollutants and to cooperatively pass their data through the network to a main location.

4. Architecture of a WSN

5. Example for Fault-Tolerance in a Sensor Network system

6. Fault Tolerance at different levels of the Sensor Network
Physical layer: The physical layer is responsible for establishing communication in a given medium between two nodes.
Hardware: At the hardware level, components consists of a computation engine, storage subsystem and power supply infrastructure that are all very reliable.
System Software: System software consists of the operating system (OS) and utility programs. There are several ways how one can address fault tolerance at the system software level with respect to the computational subsystem.
Middleware: Starting with the middleware level the emphasis is shifted toward data aggregation, data filtering and sensor fusion.
Application: Finally, fault tolerance can be addressed also at the application level.

7. What is ZigBee..???
ZigBee is a specification for a suite of high level communication protocols using small, low-power digital radios based on an IEEE 802 standard for personal area networks.
ZigBee and IEEE are standards-based protocols that provide the network infrastructure required for wireless sensor network applications.
defines the Physical and MAC layers, and ZigBee defines the Network and Application layers.

8. ZigBee Protocol Stack

9. Application (APL) Layer
The top layer in the ZigBee protocol stack consists of
The Application Framework
--Application Objects
ZigBee Device Object (ZDO)
Application Support (APS) Sub layer.
Security Service Provider (SSP)
ZDO Management Plane

10. Network (NWK) Layer
It Handles network address and routing by invoking actions in the MAC layer.
Its tasks include starting the network (coordinator), assigning network addresses, adding and removing network devices, routing messages, applying security, and implementing route discovery.

11. IEEE 802.15.4 Medium Access Control (MAC) Layer:
Responsible for providing reliable communications between a node and its immediate neighbors, helping to avoid collisions and improve efficiency. The MAC Layer is also responsible for assembling and decomposing data packets and frames.
Physical (PHY) Layer:
Provides the interface to the physical transmission medium (e.g. radio). The PHY layer consists of two layers that operate in two separate frequency ranges.

12. The ZigBee Network

13. Device Types Coordinator :
This device starts and controls the network. The coordinator stores information about the network, which includes acting as the Trust Center and being the repository for security keys.
Router:
These devices extend network area coverage, dynamically route around obstacles, and provide backup routes in case of network congestion or device failure. They can connect to the coordinator and other routers, and also support child devices.
End Devices :
These devices can transmit or receive a message, but cannot perform any routing operations. They must be connected to either the coordinator or a router, and do not support child devices.

14. Mesh Network Topology
Mesh topology supports “multi-hop” communications, through which data is passed by hopping from device to device using the most reliable communication links and most cost-effective path until its destination is reached.
The multi-hop ability also helps to provide fault tolerance, in that if one device fails or experiences interference, the network can reroute itself using the remaining devices.

15. ZigBee Fault Tolerance Testing
Measure mesh properties of complex ZigBee Configurations.
Determine current technology performance parameters.
Determine best way to characterize fault-tolerance behavior.
Determine optimum configurations for fault tolerance and performance.

16. Continued… Define test topologies and methods for fault injection.
Identify and ZigBee protocol packets and handshakes.
Determine timing for formation of PAN, data transfer and failover.
Measure total latency and variability for each network operation.

17. Test Measurement Parameters
End Device:
A ZigBee reduced function device that is unable to serve as a gateway or coordinator on the network.
End Device Association Time:
The time period between a ZigBee End Device sending an initial PAN Association Request to a Gateway or Coordinator and the device sending an End Device Announcement.
IEEE Address Time:
The time period between a ZigBee node sending an initial IEEE Address Request and sending an APS IEEE Address Acknowledgement.

18. Continued… Orphan Transition Time:
The time period between a ZigBee device discovering a link is broken and declaring orphan status.
PAN Reconstruction Time:
The time period between a ZigBee device discovering a link is broken and the orphaned device sending an End Device Announcement.
Date Transfer Cycle Rate:
A ZigBee Data report is sent from each end device at this rate and the coordinator replies with a ZigBee Data Acknowledgement.

19. ISM Spectrum Diagram

20. ZIGBEE RF INTERFERENCE TESTS
(1)Measure relevant parameters of the RF Physical layer
2.4 GHz ISM band Spectrum
Radiated output power and received signal strength indication (RSSI) for each ZB transmitter
Radiated output power and received power for each WLAN transceiver.
(2)Measure relevant parameters at the MAC layer nominally and in the presence of multipath interference
Packet Loss Rate
(3)Measure WSN RF compatibility at the MAC and Protocol layers with active WLANs operating within WSN channel allocation.
Data Throughput Rate

21. RF Interference Configuration Diagram

22. Proposed Fault Tolerant WSN Architecture

23. Rules for this architecture to Work
All redundant nodes must be within RF range of each
other, since the alternative paths must be supported by the Physical layer.
Certain logic must be built into the WSN mode firmware and in the WSN commissioning mechanism to setup the default nominal configuration upon startup.

24. Conclusion
It is favorable for the use of ZigBee technology for WSN applied to non-critical ancillary data collection aboard aerospace vehicles.
A router failure would require each sensor node to be re-associated with the coordinator, resulting in many ZigBee protocol layer exchanges
WSNs can be deployed within enclosed metallic volumes aboard spacecraft and aircraft.
WLANs interfere with ZB WSNs only when operated within the same area of the ISM spectrum.

25. thanQ one and all…