Fault Tolerance in ZigBee Wireless Sensor Networks

Venkata Nagarjuna Ravulapalli

Fault TolerantFault-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.

Wireless Sensor NetworksA 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.

Architecture of a WSN

Example for Fault-Tolerance in a Sensor Network system

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.

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 802.15.4 are standards-based protocols that provide the network infrastructure required for wireless sensor network applications.

802.15.4 defines the Physical and MAC layers, and ZigBee defines the Network and Application layers.

ZigBee Protocol Stack

Application (APL) Layer The top layer in the ZigBee protocol stack

consists of The Application Framework --Application ObjectsZigBee Device Object (ZDO) Application Support (APS) Sub layer.

Security Service Provider (SSP) ZDO Management Plane

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.

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.

The ZigBee Network

Device TypesCoordinator : 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.

Mesh Network TopologyMesh 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.

ZigBee Fault Tolerance TestingMeasure 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.

Continued…Define test topologies and methods for fault

injection.Identify 802.15.4 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.

Test Measurement ParametersEnd 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.

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.

ISM Spectrum Diagram

ZIGBEE RF INTERFERENCE TESTS(1)Measure relevant parameters of the RF Physical layer 2.4 GHz ISM band SpectrumRadiated output power and received signal strength indication

(RSSI) for each ZB transmitterRadiated 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.

Packet Loss Rate Data Throughput Rate

RF Interference Configuration Diagram

Proposed Fault Tolerant WSN Architecture

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.

ConclusionIt 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.

thanQ one and all…