Venkata Nagarjuna Ravulapalli
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.
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.
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
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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 Objects
 ZigBee 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 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.
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.
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.
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 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.
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 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.
 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.
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.
thanQ
one and all…