• IEEE 802.15.4 LR-WPAN(Low data Rate -
Wireless Personal Area Networks) standard is
widely used because of its position
independent sensing capabilities even in toxic
and inaccessible regions to humans.
• Sensors are low cost and can be randomly
placed as long as they are in some sensors
• Area Monitoring
– WSN is deployed over a region where some
phenomenon is to be monitored like detecting
enemy intrusion etc.,
• Forest Fire Detection
– A network of Sensor Nodes can be installed in a
forest to detect when a fire has started.
• Battlefield surveillance
• Reference: Wikipedia
Devices in WSNDevices in WSN
• WSN has mainly two types for devices;FFD,RFD
• FFD or Full Functional Device
– Senses the agent
– Routes packets and
– Initialize the Network(act as PAN Coordinator)
– Can communicate with another RFD or FFD
• RFD or Reduced Functional Device
– Can only sense the agent
– Can communicate with FFD only
Sink nodeSink node
• Sink node is the principal controller in WPAN
and there is only one PAN Coordinator in a
• If Sink node uses Beacon enabled mode then
nodes use Slotted CSMA/CA algorithm for
transmitting packets else nodes use Unslotted
• Sensors sense the Agent and generate a packet
containing its position if the agent is in its
• In NetSim, Agent mobility is modeled by
RandomWalk and RandomWayPoint which are
followed internationally to test WSN.
• For each of the models we can set its velocity and
pause time(only random way point) and also we
can capture the agents path for the whole
• WSN consists of spatially distributed sensors
to monitor physical or environmental
conditions, such as temperature, sound,
pressure, etc. and to cooperatively pass their
data through the network to a main
Beacon FrameBeacon Frame
• Beacons occupy the first slot in SuperFrame.
• Beacons are used to synchronize the attached
devices, to identify the PAN, and to describe the
structure of the super frames.
• Beacon frames are transmitted periodically to
announce the presence of a network.
SuperFrame StructureSuperFrame Structure
The superframe is contained in a Beacon Interval bounded
by two beacon frames, and has an active period and an
Active PeriodActive Period
• Active period consists of two periods
– Contention Access Period (CAP)
– Contention Free Period (CFP)
• Length of active period is determined by the
• Active Period Length =
aBaseSuperframeDuration * symbols
Contention Access Period (CAP)Contention Access Period (CAP)
• In CAP all the nodes having a packet to transmit
compete for the channel and follows Slotted CSMA/CA
• A sufficient portion of the CAP remains for contention-
based access of other networked devices or new
devices wishing to join the network.
Contention Free Period (CFP)Contention Free Period (CFP)
• CFP is used to allocate slots for the nodes which require low latency
i.e., PAN coordinator allocates guaranteed time slots (GTS) for such
• A node issues GTS allocation request to the PAN coordinator, which
can allocate available GTS to nodes.
• In the CFP, the node requested for GTS can transmit during its GTS
if it is allocated, without any contention with other devices.
• CFP can be disabled by disabling GTS.
Inactive periodInactive period
• In the inactive period, the coordinator may enter
a low-power mode and doesn’t interact with its
PAN which helps in reduced energy consumption
and so extends the network lifetime
• Inactive period can be remove by setting
SuperFrame Order same as Beacon Order
• Packet is generated by the sensor whenever agent comes
with in its sensor range and for every sensing interval.
• Each packet is characterized by 3 variables NB,CW,BE
– NB, Number of backoffs the node has underwent while
attempting the current transmission, initialized to 0 before
every new transmission
– CW, Contention Window Length, which defines the number of
backoff periods that need to be clear of channel activity before
starting transmission. CW is only used with the slotted CSMA/CA
version. This value is initialized to 2 before each transmission
attempt and reset to 2 each time the channel is assessed to be
– BE, Backoff exponent is related to backoff periods (0 to 2BE
device has to wait before attempting to assess the channel.
Clear Channel Assessment (CCA)Clear Channel Assessment (CCA)
• CCA is responsible for sensing channel as busy or idle.
• The CCA is performed in three operational modes:
– Energy Detection mode: If the Channel energy is less than ED
threshold then the channel is reported as Idle else channel is
reported as busy.
– Carrier Sense mode: CCA reports a busy medium only if it
detects a signal with the modulation and the spreading
characteristics of IEEE 802.15.4 and which may be higher or
lower than the ED threshold.
– Carrier Sense with Energy Detection mode: This is a
combination of the fore mentioned techniques. CCA reports the
medium is busy only if it detects a signal with the modulation
and the spreading characteristics of IEEE 802.15.4 and with
energy above the ED threshold.
Unslotted CSMA/CA Slotted CSMA/CA
Step1 NB(=0),CW(=1),BE are initialized. NB(=0),CW(=2),BE are initialized.
Locate the backoff boundary and
attempt for transmission or channel
assess at the start of the slot.
Step2 MAC layer shall delay for a random number of
backoff periods in the range (0 to )
Step3 MAC will request PHY to perform CCA.
If the frame transmission and acknowledgment
can be completed before the end of the current
CAP then MAC sub layer shall proceed else it shall
wait until the start of the CAP in the next
SuperFrame and repeat the evaluation.
MAC sub layer shall increment both NB and BE by
one, ensuring that BE shall be no more than
aMaxBE and if NB is greater than
maxMaxCSMABackoffs then the packet is
discarded else return to step 2.
Same and CW is reset to 2.
MAC sub layer starts transmission. If CW is zero then start transmission
else return to step 3
Data Link Layer ParametersData Link Layer Parameters
Parameter Standard Name Standard Value
Beacon Order BO [0 to 15]
SuperFrame Order SO [0 to 15]
Base SuperFrame Duration aBaseSuperframeDuration 15.36 milliseconds
Min Backoff Exponent macMinBE 3
Max Backoff Exponent aMaxBE 5
Max Frame Retries aMaxframeRetries 3
Max CSMA Backoffs macMaxCSMABackoffs 4
PHY ParametersPHY Parameters
Parameter Standard Value
Frequency Band 2.4 GHz
Chip Rate 2000 McPS
Symbol Rate 62.5 kSymbolsPS
Modulation Technique O-QPSK
Unit Backoff Time 20 Symbols
Turn Around Time 12 Symbols
Transmitter Power 100 mW
Sensor Range 100 m
Receiver Sensitivity -85 dbm
ED Threshold -95 dbm
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