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Wireless Sensor Networks
Aν. Καθηγητής Συμεών Παπαβασιλείου
Εθνικό Μετσόβιο Πολυτεχνείο
Τμήμα Ηλεκτρολόγων Μηχανικών και Μηχανικών
Υπολογιστών
[email protected]
Τηλ: 210 772-2550
Sensors
What is a sensor:
Device that measures a physical
quantity and converts it into a signal
which can be read by an observer or
by an instrument.
What is a sensor node:
Node in a wireless sensor network that is capable of performing
some processing, gathering sensory information and
communicating with other connected nodes in the network.
• small size
• energy constrained
• limited capabilities
Consists of:
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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
etc. and to cooperatively pass their data
through the network to a main location
Operation:
•Every sensor node collects data
(acoustic, seismic etc.) from its
environment
• Data is sent to the collection center
(aka sink) for further processing
• Data gathering is realized through
multi-hop routing
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Wireless Sensor Network Architecture
Base
Station
B
Internet
and/or
Satellite
E
D
A
C
Control &
Management
Station
Sensor
Network
Sensor
Nodes
End User
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Sensor Network Applications
o Environment detection and
monitoring –Great Duck Island, Maine,
SEA-LABS
o Disaster Prevention
o Medical Care – Mercury project, Harvard
o Home Intelligence
o Scientific exploration
o Surveillance
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WSNs vs Ad hoc networks I
Wireless Sensor Networks differ from traditional ad hoc networks:

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The number of sensor nodes in a sensor network can be several
orders of magnitude higher than the nodes in an ad hoc network.
Sensor nodes are densely deployed.
Sensor nodes are prone to failures.
The topology of a sensor network changes very frequently.
Sensor nodes mainly use a broadcast communication paradigm,
whereas most ad hoc networks are based on point-to-point
communications.
Sensor nodes are limited in power, computational capacities, and
memory.
Sensor nodes may not have global identification (ID) because of the
large amount of overhead and large number of sensor
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WSNs vs Ad hoc networks II
Why not port ad hoc protocols?



Ad Hoc networks require significant amount of routing data
storage and computation
 Sensor nodes are limited in memory and CPU
Topology changes due to node mobility are infrequent as in
most applications sensor nodes are stationary
 Topology changes when nodes die in the network due to
energy dissipation
Scalability with several hundred to a few thousand nodes
not well established
Therefore: Need for development of new protocols specific for WSN
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WSN general requirements
Wireless ad hoc sensor network requirements include the following
1. Large number of (mostly stationary) sensors: Aside from the deployment
of sensors on the ocean surface or the use of mobile, unmanned, robotic sensors
in military operations, most nodes in a smart sensor network are stationary.
2. Low energy consumption: Since in many applications the sensor nodes will be
placed in a remote area, service of a node may not be possible. In this case, the
lifetime of a node may be determined by the battery life, thereby requiring the
minimization of energy expenditure.
3. Ease of installation and maintenance. In case of a malfunction, it is difficult
to visit in-situ and check the problem.
4. Network dynamic self-organization: Given the large number of nodes and
their potential placement in hostile locations, it is essential that the network be
able to self-organize; manual configuration is not feasible.
5. Querying ability: A user may want to query an individual node or a group of
nodes for information collected in the region.
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WSN requirements summary
Technical Challenges and/or
Requirements
Design Objectives & Directions
Massive and Random deployment
Cheap and small sensor node:
scalable, flexible architecture
Data redundancy
Localized processing & data fusion
Limited resources
Resource efficiency design
Unattended operation
Self-configuration & coordination
Dynamic surrounding
Adaptability
Error-prone medium
Reliability & fault tolerance
Diverse applications
Application specific design
Safety and privacy
Security
QoS concerns
QoS design with resource
constraint;localization;attributebased naming and data centric
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routing
Sensor Network Architectures
Layered Architecture
Clustered Architecture
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Sensor Localization
Sensor Localization
It is essential, in some applications, for each node
to know its location


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Sensed data coupled with loc. data and sent
We need a cheap, low-power, low-weight, low
form-factor, and reasonably accurate mechanism
Global Positioning Sys (GPS) is not always
feasible
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GPS cannot work indoors, in dense foliage, etc.
GPS power consumption is very high
Size of GPS receiver and antenna will increase node
form factor
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Indoor Localization


Use a fixed infrastructure
 Beacon nodes are
strategically placed
Nodes receive beacon signals
and measure:
 Signal Strength
 Signal Pattern
 Time of arrival; Time
difference of arrival
 Angle of arrival
 Nodes use measurements from multiple beacons and use
different multi-lateration techniques to estimate locations
 Accuracy of estimate depends on correlation between
measured entity and distance
Examples of Indoor Loc. Systems
RADAR (MSR), Cricket (MIT), BAT (AT&T), SPA
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Sensor Network Localization I
No fixed infrastructure available
 Prior measurements are not always
possible
 Basic idea:

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Have a few sensor nodes who have known
location information
These nodes sent periodic beacon signals
Other nodes use beacon measurements and
triangulation, multi-lateration, etc. to estimate
distance
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Sensor Network Localization II

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Receiver Signal Strength Indicator (RSSI) was used to
determine correlation to distance
 Suitable for RF signals only
 Very sensitive to obstacles, multi-path fading,
environment factors (rain, etc.)
 Was not found to have good experimental correlation
 RF signal had good range, few 10metres
RF and Ultrasound signals
 The beacon node transmits an RF and an ultrasound
signal to receiver
 The time difference of arrival between 2 signals is used
to measure distance
 Range of up to 3 m, with 2cm accuracy
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Localization Algorithms
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Based on the time diff. of arrival
Atomic Multi-lateration:
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Iterative ML:
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If a node receives 3 beacons, it can determine its
location (similar to GPS)
Some nodes not in direct range of beacons
Once an unknown node estimates its location, will send
out a beacon
Multi-hop approach; Errors propagated
Collaborative ML:

When 2+ nodes cannot receive 3 beacons (but can
receive say 2), they collaborate
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Sensor MAC Protocols
Multiple Access Control (MAC) Protocols
 MAC allows multiple users to share a common channel.
 Conflict-free protocols ensure successful transmission. Channel can be
allocated to users statically or dynamically.
 Only static conflict-free protocols are used in cellular mobile
communications
- Frequency Division Multiple Access (FDMA): provides a fraction of the
frequency range to each user for all the time
- Time Division Multiple Access (TDMA) : The entire frequency band is
allocated to a single user for a fraction of time
- Code Division Multiple Access (CDMA) : provides every user a portion of
bandwidth for a fraction of time
 Contention based protocols must prescribe ways to resolve conflicts
- Static Conflict Resolution: Carrier Sense Multiple Access (CSMA)
- Dynamic Conflict Resolution: keeps track of various system parameters,
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ordering the users accordingly
Media Access in Sensor Networks
–
Why STUDY MAC protocols in sensor networks?

Application behavior in sensor networks leads to very
different traffic characteristics from that found in conventional
computer networks
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Highly constrained resources and functionality
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Small packet size
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Deep multi-hop dynamic topologies
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The network tends to operate as a collective structure, rather
than supporting many independent point-to-point flows
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Traffic tends to be variable and highly correlated
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Little or no activity/traffic for longer periods and intense
traffic over shorter periods
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Energy Consumption in Sensor Networks
• Transmission and reception of data require the
highest energy consumption
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FACT:
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Energy required for the transmission of 1 bit in100 m =
Energy required for the performance of 300 operation
(Pottie & Kaiser, 2000)
Increase the network lifetime
New techniques need to be found for decreasing the
energy consumption within the sensor network
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Energy Consumption in Sensor Networks
4 main reasons for expenditure of energy
1.
2.
3.
4.
Collisions: Packets form neighboring nodes
conflict and require retransmission
Overhearing: Sensor nodes listen and receive
packets not destined to them
Control Packet Overhead: Many protocols
require the exchange of control packets
Idle listening: Sensor nodes wait and listen for
packets that may not arrive eventually
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MAC Protocols – Properties
Wireless Sensor Networks due to their unique
nature and characteristics call for development of
new MAC protocols
Desired Properties
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Energy efficient
Scalable
Adaptive
Mean delay & throughput can be of
secondary importance
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Contention based MAC protocols I
IEEE 802.11 (DCF) was the 1ο standard protocols
for the communication of wireless devices
Based on CSMA/CA
(Carrier Sense Multiple
Access / Collision
Avoidance)
 Use of RTS/CTS packets
for avoiding the hidden
terminal problem
 Use of DATA/ACK packets

RTS / DATA
Node Α
CTS / ACK
Node B
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Contention based MAC protocols II
Pros

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Simple
Scalable – insertion / deletion of nodes easy
Robust
No synchronization required
Knowledge of the topology not needed
Cons

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Multiple conflicts – το carrier sense does not work for more
than one hop
Great amount of control packets (RTS/CTS) – 40%-75% of
channel utilization
Long idle listening (~75% of total time)
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WiseMAC

Use of np-CSMA with preamble sampling
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Τhe preamble proceeds the data packet in
order to notify the receiver node (no RTS/CTS)
Method to dynamically determine the length of
the preamble packet
Every node has a sleep-wake program
Cons: Collisions because of the hidden
terminal problem
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Periodic Listening
Its main use is for decreasing the energy
consumption caused by idle listening
Nodes “sleep” periodically and turn off their radio
listen
sleep
listen
sleep
Less energy consumption but increased delay in data
gathering
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SMAC I
Basic features:
Periodic listen and sleep
 Collision and overhearing avoidance
 Message passing

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SMAC II
Periodic listen and sleep
listen
listen
SYNC
CS for RTS/CTS
DATA/ACK
listen
sleep
SYNC
CS for RTS/CTS
DATA/ACK
sleep
Listen + Sleep = Frame
listen
• Nodes synchronize with each
other by sending their
schedule
• Neighboring nodes follow the same schedule
• Border nodes follow 2 or more schedules
Schedule 1
Schedule 2
At the beginning of each listen period, nodes synchronize by sending
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SYNC
SMAC III
Collision & Overhearing Avoidance
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Use of RTS/CTS
Use of physical & virtual carrier sense
Use of NAV (Neighbor Allocation Vector)
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When a nodes listens the transmission of its neighbor, it can
determine how long it will last and become “silent”
This value is saved into NAV and then decreases
For a node to transmit, it has to succeed in CS but also
holds NAV=0
When a node listens to RTS/CTS, then by knowing how long
the transmission will last, it can be put to sleep
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SMAC IV
Transmission of long packets
Break the packet into smaller junks
 Transmission of only one pair of RTS/CTS
 Neighboring nodes sleep for the whole
duration of the transmission

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SMAC V
Main drawback
Increased delay because of the periodic
sleep of nodes
 Partial solution by adaptive listening
method

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SMAC variations I
TMAC:
 Decrease in idle listening by transmitting
the packets in burst and then sleep
 The Listen period is adapted based on the
network load
 Use of RTS/CTS/ACK & FRTS (Future
Request to Send) control packets for
dealing with the delay caused of the sleep
period
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SMAC variations II
DMAC:
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Adapt the listen period when a node has many
packets to send
Inform the receiver nodes in order to adjust their
schedule too
No use of RTS/CTS
Use of Data prediction method – a node expects
data form its children
Use of MTS (More to Send) control packet - sent
by the children of a node to it in order to adjust
its schedule
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SMAC variations III
ΖMAC:

It is adaptive to the level of contention and the
load of the network
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Under low contention it behaves as CSMA
Under heavy load behaves as TDMA
Every node picks its slots and decides the length
of its frame
Use of control packets (no RTS/CTS) but ECN
(Explicit Contention Notification)

ECN is used to notify for two-hop contention
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TDMA based MAC protocols

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These protocols use the notion of timeslots
Each nodes transmits during its own slot
Solve the hidden terminal problem without the
use of control packets
Drawbacks

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Require tight synchronization
It is hard to find a conflict-free program (NP hard
when channel reuse is wanted)
Difficult to scale
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Spatial TDMA and CSMA

Use of 2 separate channels
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Use of TDMA for transmission of data packets
Use of CSMA (low power- preamble) for
signaling (transmission of control packets)
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TRAMA I
Time is divided into “Scheduled Access” and “Random Access”
• Random Access: for signaling
• Scheduled Access: for regular traffic
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TRAMA II
Nodes have knowledge for all their twohop neighbors
 This information is exchanged during the
signaling which is contention based
 Each node announces the slots in which it
will transmit as well as its receivers
 When a node is not transmitting or
receiving, it is put to sleep

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