Wireless Sensor Networks

Aν. Καθηγητής Συμεών Παπαβασιλείου

Εθνικό Μετσόβιο ΠολυτεχνείοΤμήμα Ηλεκτρολόγων Μηχανικών και

Μηχανικών Υπολογιστών

papavass@mail.ntua.gr Τηλ: 210 772-2550

Sensors

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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 NetworksA 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

Wireless Sensor Network Architecture

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Internet and/or

Satellite

Control & Management

Station

Base Station

Sensor Network Sensor

Nodes

AB

CD

E

End User

Sensor Network Applications

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o Environment detection and monitoring –Great Duck Island, Maine, SEA-LABSo Disaster Preventiono Medical Care – Mercury project, Harvardo Home Intelligenceo Scientific explorationo Surveillance

WSNs vs Ad hoc networks IWireless Sensor Networks differ from traditional ad hoc networks:

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 IIWhy 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

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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.  

WSN requirements summaryTechnical Challenges and/or Requirements

Design Objectives & Directions

Massive and Random deployment Cheap and small sensor node: scalable, flexible architecture

Data redundancy Localized processing & data fusionLimited resources Resource efficiency designUnattended operation Self-configuration & coordinationDynamic surrounding AdaptabilityError-prone medium Reliability & fault toleranceDiverse applications Application specific designSafety and privacy SecurityQoS concerns QoS design with resource

constraint;localization;attribute-based naming and data centric routing 9

Sensor Network Architectures

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Layered Architecture Clustered Architecture

Sensor Localization

Sensor LocalizationIt is essential, in some applications, for each node

to know its location 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 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

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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. SystemsRADAR (MSR), Cricket (MIT), BAT (AT&T), SPA

Sensor Network Localization I No fixed infrastructure available Prior measurements are not always

possible Basic idea:

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 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 Based on the time diff. of arrival Atomic Multi-lateration:

If a node receives 3 beacons, it can determine its location (similar to GPS)

Iterative ML: 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

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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, ordering the users accordingly

Multiple Access Control (MAC) Protocols

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– 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

Highly constrained resources and functionality Small packet size Deep multi-hop dynamic topologies The network tends to operate as a collective structure, rather

than supporting many independent point-to-point flows Traffic tends to be variable and highly correlated Little or no activity/traffic for longer periods and intense traffic

over shorter periods

Media Access in Sensor Networks

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Energy Consumption in Sensor Networks

• Transmission and reception of data require the highest energy consumption

FACT: 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 Networks4 main reasons for expenditure of energy

1. Collisions: Packets form neighboring nodes conflict and require retransmission

2. Overhearing: Sensor nodes listen and receive packets not destined to them

3. Control Packet Overhead: Many protocols require the exchange of control packets

4. 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

Energy efficient Scalable Adaptive Mean delay & throughput can be of

secondary importance

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Contention based MAC protocols I

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

Node Α Node B

RTS / DATA

CTS / ACK

IEEE 802.11 (DCF) was the 1ο standard protocols for the communication of wireless devices

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Contention based MAC protocols IIPros Simple Scalable – insertion / deletion of nodes easy Robust No synchronization required Knowledge of the topology not needed

Cons 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

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 ListeningIts main use is for decreasing the energy consumption caused by idle listening

sleeplisten listen sleep

Nodes “sleep” periodically and turn off their radio

Less energy consumption but increased delay in data gathering

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SMAC IBasic features:

Periodic listen and sleep Collision and overhearing avoidance Message passing

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SMACIIPeriodic listen and sleep sleep

listen

sleep

listen

sleepSYNC CS for RTS/CTSDATA/ACK

listenSYNC CS for RTS/CTS

DATA/ACK

listen

Listen + Sleep = Frame• Nodes synchronize with each other by sending their schedule• Neighboring nodes follow the same schedule• Border nodes follow 2 or more schedules

Schedule 2Schedule 1

At the beginning of each listen period, nodes synchronize by sending SYNC

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SMAC IIICollision & Overhearing Avoidance

Use of RTS/CTS Use of physical & virtual carrier sense Use of NAV (Neighbor Allocation Vector)

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 IVTransmission 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 VMain drawback

Increased delay because of the periodic sleep of nodes

Partial solution by adaptive listening method

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SMAC variations ITMAC: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 IIDMAC:DMAC: 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 ΖΖMAC:MAC: It is adaptive to the level of contention and the

load of the network 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 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 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

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 two-

hop 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