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Wireless SensorNetworks(WSNs)

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Topics

• Wireless Sensor Networks (WSNs)

• Research topics

• Networking sensors in WSNs

• Coverage of sensor networks

• Location service

• Sensor databases

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Wireless Sensor Networks (WSNs)

• What is a sensor?– Light, noise,

acoustic, temperature ,pressure, humidity sensors

– Magnetometers, accelerometers

– biosensorsBerkeley Motes(WeC)

light, temperature,10 kbps @ 20m

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What is a Sensor Node?

• Sensor is a tiny electronic devices with four major components– Sensing (外接

sensor)– Communication– Processing– Power

Motes (UC Berkeley) : 8-bit CPU at 4MHz,

8KB flash, 512B RAM

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• BASIC MOTE-KITS – 4 MICA2 Processor/Radio Boards – 4 MICA2DOT Quarter-Sized Processor Radio Boards – 3 MTS310 Sensor Boards (Acceleration, Magnetic, Light,

Temperature, Acoustic, and Sounder) – 2 MTS510 Sensor Boards (Acceleration, Light, Microphone) – 2 MDA500, MICA2DOT Prototype and Data Acquisition Boards – 1 MIB510 Programming and Serial Interface Board .

• MICAz– 802.15.4/ZigBee compliant Motes, sensor and data acquisition

boards, and two different gateway/interface boards.

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What is a WSN?

• WSN is a multi-hop wireless network consisting of a large number of small, low-cost, low-power sensor nodes to perform intended monitoring functions in the target area.

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Wireless Sensor Network

Sensor Field Sensor Nodes

Base station(Sink, Gateway)

Internet

User

CooperationExtended coverage

Fault toleranceExtended lifetime

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(1324,1245)

Data

WakeupLine of Bearing (LOB)Fusion center

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Limitations of a Sensor Node• Modest processing power(4MHs)

• Little storage(512byte)

• Short communication range (p d4)

• Limited power source (< 1.2v, < 0.5Ah)

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Sensor Networks v.s. MANETs• The number of sensor nodes in WSNs 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.

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Sensor Networks v.s. MANETs• Sensor nodes mainly use broadcast

communication paradigm whereas most ad hoc networks are based on point-to-point communications

• Sensor modes are limited in power, computational capacities, and memory

• Sensor nodes may not have global identification (ID)

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Applications

• Military application• Monitoring friendly forces, equipment and ammunition• Battlefield surveillance• Targeting• Battle damage assessment• Nuclear, biological and chemical attack detection and

reconnaissance(偵察 )• C4ISRT systems: military command, control,

communications, computing, intelligence, surveillance(監視 ), reconnaissance(偵察 ) and targeting

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Applications (cont.)• Environment application

Forest fire detection‧Biocomplexity mapping of the environment‧Flood detection‧Precision agriculture‧

• Health applicationTelemonitoring of human physiological(‧ 生理上的 ) dataTracking and monitoring doctors and patients‧

inside a hospitalDrug administration in hospital‧

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Applications (cont.)

• Context-aware computingintelligent home, smart environment‧

• Other commercial application– tracking chemical plumes(羽狀煙） : Ad Hoc, Just-in-

time deployment for mitigating disasters (Berkeley team)

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• A chemical gas leak has been detected• Need to get a real-time assessment of extent and

movement of the gas and inform the evacuation• 3 UAVs (unmanned aerial vehicle) are immediately

launched, each with 1000 chemical sensing nodes• Upon flying over the attack site, sensor nodes are

released• Nodes self-organize into an ad hoc network, and

relay tracking result back to emergency response command center.

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Design Issues of WSNs• Fault tolerance• Scalability• Production costs• Hardware constraints• Topology• Environment• Transmission media• Power consumption

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Fault Tolerance & Scalability

• The failure of sensor nodes should not affect the overall task of the sensor network.

• The system should be scalable enough to work with large number of nodes.

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Production Cost• Cost of individual node plays an important role.• Sensor nodes are densely deployed. Nodes

must be cheap!• Use Bluetooth RF, US$10, should be less than

US $1.

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Hardware Constraints

Location Finding System Mobilizer

Sensor ADCProcessor

StorageTransceiver

Power Unit Power Generator

Processing Unit

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The Mote Family

Ref: from Levis & Culler, ASPLOS 2002

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Topology• Topology should be carefully maintained.• Three phases:

Pre-deployment and deployment phase:‧ Sensor nodes are either thrown in mass or placed one by one in the sensor field.Post-deployment phase:‧

After deployment, topology changes due to the change in sensor nodes’ position or reach ability or failure.Re-deployment of additional nodes phase:‧

Additional sensor nodes can be re-deployed at any time to replace the malfunctioning nodes or due to changes in task dynamics.

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Transmission Media

• In a multi-hop sensor network, communicating nodes are linked by a wireless medium.Radio Frequency (FR)‧

Do not need Line of Sight.WINS , PAMS

‧Infrared (IR) License-free and robust to interference form electrical device.Optical media‧

Require line of sight (smart dust mote)

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Power Consumption

• The wireless sensor node, being a micro-electronic device, can only be equipped with a limited power source (<0.5 Ah, 1.2V).

• Two major power consumptionCommunication‧

A sensor node expends maximum energy in data communication. ( transmit > receive)Data processing‧

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Identifying the Energy Consumers

• Need to shutdown the radio

From Tsiatis et al. 2002

SENSORS

Power consumption of node subsystems

0

5

10

15

20

Po

wer

(m

W)

CPU TX RX IDLE SLEEP

RADIOSLEEPIDLERXTX EEEE

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Research Issues

• Localization and tracking

• Time synchronization

• Networking sensors (MAC, Network)

• Topology control (network coverage)

• WSN security

• Sensor network databases

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1) .Data-centric paradigm:The operating paradigm of WSNs is centered around information

retrieval from the underlying network, usually referred to as a data-centric paradigm.

– Compared to the address-centric paradigm exhibited by traditional networks, the data-centric paradigm is unique in several ways.

– New communication patterns resemble a reversed multicast tree.– In-network processing extracts information from raw data and

removes redundancy among multiple source data. – Cooperative strategies among sensor nodes are used to replace

the non-cooperative strategies for most Internet applications.– The development of appropriate routing strategies that take the

above factors into consideration is challenging.

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2). Collaborative information processing and routing:

The data-centric paradigm involves two fundamental operations in WSNs: information processing and information routing.

– Many research efforts are motivated by the fact that information

processing and routing are mutually beneficial. While information processing helps reduce the data volume to be routed, information routing facilitates joint information compression (or data aggregation) by bringing together data from multiple sources.

– It is often non-trivial to model and analyze the inter-relationship

between information processing and routing. In many situations, the problem of finding a routing scheme in conjunction with joint compression for energy minimization turns out to be NP-hard.

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3). Energy-efficient design: Once deployed, it is often infeasible or un-desirable to recharge sensor

nodes or replace their batteries.

– Energy conservation becomes crucial for sustaining a sufficiently long network lifetime. Among the various techniques proposed for improving energy-efficiency, cross-layer optimization has been realized as an effective approach.

– Due to the nature of wireless communication, one performance metric of the network can be affected by various factors across layers.

– Hence, a holistic approach that simultaneously considers the optimization at multiple layers enables a larger design space within which cross-layer tradeoffs can be effectively explored.

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4). Network discovery and organization– Localization – Time synchronization – Deployment of sensor– Coverage

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5). Security: – Since WSNs may operate in a hostile environment, security

is crucial to ensure the integrity and confidentiality of sensitive information. To do so, the network needs to be well protected from intrusion and spoofing.

– The constrained computation and communication capability of sensor nodes make it unsuitable to use conventional encryption techniques. Lightweight and application-specific architectures are preferred instead.

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Networking WSNs• Power limitation of a sensor node plays an

important role.

• The sensor MAC protocol---S-MAC

IEEE/ACM transactions on networking, Vol. 12, No. 3, June 2004

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• Major sources of energy waste

– Idle listening• Long idle time when no sensing event happens• Collisions• Control overhead• Overhearing

• Try to reduce energy consumption from all above sources

• TDMA requires slot allocation and time synchronization• Combine benefits of TDMA + contention protocols

Energy Efficiency in MAC

Common to all wireless

networks

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Sensor-MAC (S-MAC) Design(Wei et al. 2002)

• Tradeoffs

• Major components of S-MAC• Periodic listen and sleep• Collision avoidance• Overhearing avoidance• Message passing

Latency

FairnessEnergy

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Periodic Listen and Sleep

• Problem: Idle listening consumes significant energy– Nodes do not sleep in IEEE 802.11 ad hoc mode

• Solution: Periodic listen and sleep– Turn off radio when sleeping– Reduce duty cycle to ~10% (200 ms on/2s off)– Increased latency for reduced energy

sleeplisten listen sleep

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Periodic Listen and Sleep• Schedules can differ

• Preferable if neighboring nodes have same schedule

— easy broadcast & low control overhead

Border nodes: two schedules broadcast twice

Node 1

Node 2

sleeplisten listen sleep

sleeplisten listen sleep

Schedule 2

Schedule 1

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Periodic Listen and Sleep

• Schedule maintenance– Remember neighbors’ schedules

— to know when to send to them

– Each node broadcasts its schedule every few periods (Sync packet, saying when it will enter sleep, relative to the Sync)

– Refresh on neighbor’s schedule when receiving an update

– Schedule packets also serve as beacons for new nodes to join a neighborhood

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Collision Avoidance

• Problem: Multiple senders want to talk• Options: Contention vs. TDMA• Solution: Similar to IEEE 802.11 ad hoc

mode (DCF)實際上只用 RTS, CTS– Physical and virtual carrier sense– Randomized backoff time– RTS/CTS for hidden terminal problem– RTS/CTS/DATA/ACK sequence

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Overhearing Avoidance

• Problem: Receive packets destined to others

• Solutions: see the paper

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Message Passing• Problem: In-network processing requires

entire message• Solution: Don’t interleave different messages

– Long message is fragmented & sent in burst– RTS/CTS reserve medium for entire message– If a fragment lost, re-transmit it.(802.11 will abort

the whole message)

• Other nodes sleep for whole message time

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Routing

• Given a topology, how to route data?– MANET: Reactive[DSR], proactive[AODV],

TORA, GPSR[KarpKung00]

• Address Centric– Distinct paths from each source to sink.– Usually has address concept

• Data Centric Routing

Advantages

• Communication overhead for binding is minimized

• In-network processing is enabled because the content moving through the network is identifiable by intermediate nodes. This allows further energy saving through data aggregation and compression.

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Data Centric Routing• Basic idea

– name data (not nodes) with externally relevant attributes

• Data type, time, location of node, SNR, etc

– diffuse requests and responses across network using application driven routing

• Data sources publish data, Data clients subscribe to data

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Routing of WSNs

• Most of previous work focuses data centric routing

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Data gathering/Routing schemesFlooding Broadcasts data to all neighbor nodes regardless if

they receive it before or not

Gossiping Sends data to one randomly selected neighbor

SPIN Sends data to sensor nodes only if they are interested; has three types of messages (I.e., ADV, REQ, and DATA)

LEACH Forms clusters to minimize energy dissipation

Direct Diffusion

Sets up gradients for data to flow from source to sink during interest dissemination

Data aggregation

Nodes aggregate data before sending, using aggregate functions, min, max, etc

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Flooding• Each node receiving a data or

management packet repeats it by broadcasting, unless a maximum number of hops for the packet is reached or the destination of the packet is the node itself.

• It does not require costly topology maintenance and complex route discovery algorithms.

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Disadvantages• Implosion: duplicated messages are sent

to the same node

• Overlap: two nodes share the same observation region, both send the sensed results, neighbor nodes receives duplicated messages

• Resource blindness: does not take into account the available energy resource.

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Gossiping• Derived from flooding by randomly selects

another sensor node to send the data.

• Disadvantages: it takes a long time to propagate the message to all sensor nodes.

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SPIN• Stands for Sensor Protocols for Information via

Negotiation• Before sending a DATA message, the sensor

node broadcasts an ADV message containing a descriptor (meta-data) of the DATA

• If a neighbor is interested in the data, it sends a REQ message for the DATA and DATA is sent to this neighbor sensor node.

• The neighbor sensor node then repeats this process.

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SPIN

ADV REQ DATA

ADV REQ DATA

Step1 Step2 Step3

Step4 Step5 Step6

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Directed Diffusion• Nodes push named data (using tuple space) into

the network• The result is return through a most efficient path

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Interests Propagation

Sink

Event

Source

Interests

User

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Initial Gradients Setup

Sink

Event

Source

Gradients

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Reinforcement

Sink

Event

Source

Reinforce

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Interest Propagation

• a vehicle-tracking task might be described as

type = wheeled vehicle // detect vehicle location

interval = 10 ms // send events every 10 ms

duration = 10 min // for the next 10 min

rect = [-100; 100; 200; 400] // from sensors within rectangle.

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Interest Propagation

• For each active task, the sink periodically broadcasts an interest message to each of its neighbors.This initial interest contains the specified rect and duration attributes, but contains a much larger interval attribute.

type = wheeled vehicleinterval = 1 srect = [-100; 200; 200; 400]timestamp = 01 : 20 : 40 // hh:mm:ssexpiresAt = 01 : 30 : 40.

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Interest Propagation

• Every node maintains an interest cache. Each item in the cache corresponds to a distinct interest. Two interests are distinct.

• Interest entries in the cache do not contain information about the sink, but just about the immediately previous hop.

• An entry in the interest cache has several fields.

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Interest Propagation

1. A timestamp field indicates the timestamp of the last received matching interest.

2. The interest entry also contains several gradient fields,up to one per neighbor.

3. Each gradient contains a datarate field requested by the specified neighbor, derived from the interval attribute of the interest.

4. It also contains a duration field, derived from the timestamp and expiresAt attributes of the interest and indicating the approximate lifetime of the interest.

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What is a gradient

• Gradient contains two things--- the next hop address (can be 802.11 MAC address, or Bluetooth cluster address) and a data rate

• Gradient keep the return path for data

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Interest Propagation

• When a node receives an interest, it checks to see if the interest exists in the cache.

1. If no matching entry exists, the node creates an interest entry. The parameters of the interest entry are instantiated from the received interest.

2. If there exists an interest entry, but no gradient for the sender of the interest, the node adds a gradient with the specified value.

3. if there exists both an entry and a gradient, the node simply updates the timestamp and duration fields.

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Data Propagation

• a sensor that detects a wheeled vehicle might generate the following data message

type = wheeled vehicle // type of vehicle seen

interval = truck // instance of this type ?

location = [125; 220] // node location

intensity = 0.6 // signal amplitude measure

confidence = 0.85 // confidence in the match

timestamp = 01 : 20 : 40 // event generation time.

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Data Propagation• A node that receives a data message from its neighbors

attempts to find a matching interest entry in its cache.1. If no match exists, the data message is silently dropped.2. If a match exists, the node checks the data cache

associated with the matching interest entry. This cache keeps track of recently seen data items. It has several potential uses, one of which is loop prevention.1) If a received data message has a matching data cache

entry, the data message is silently dropped.2) Otherwise, the received message is added to the data

cache and the data message is resent to the node’s neighbors (i.e., gradients).

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Data Propagation

MatchingInterest

entry

Receivesa data

message

MatchingData entry

Silentlydropped

added to the datacache and resent tothe node’s neighbors

yes

yes

no

no

Down conversion: the data rate is down convert to lower rate according to the rate of a gradient

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Reinforcement• Sink receives a previously unseen event. To reinforce

this neighbor, the sink resends the original interest message but with a smaller interval (higher data rate).

• Every intermediate nodes chose a neighbor to reinforce

type = wheeled vehiclesinterval = 10 msrect = [-100; 200; 200; 400]timestamp = 01 : 22 : 35expiresAt = 01 : 30 : 40.

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Reinforcement

Sink

Event

Source

1

11

1 1

1

11

11

100

100

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LEACH (Low-Energy Adaptive Clustering Hierarchy)

• Clustering-based protocol

- Cluster-head election

- Organizing cluster

- Data transmission

• A round consisting of the above three steps

Base Station

Cluster-head

Cluster

• The cluster-heads periodically collect and aggregate/compress the data from nodes within the cluster using TDMA.

• The cluster-heads many send to the sink through a direct transmission or through multiple hops.

• Cluster-heads are rotated periodically for load balancing

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Expanding ring search

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A

1 hop

2 hop3 hop

Less than10 percent

energy savingas compared with

flooding

Acquire

• Active query forwarding• Treat the query as an intelligent entity that moves through the

network searching for the desired response.• Three steps:

– Examine cache: when then query arrives at a node, the node checks its cache to see if its cache can be used to answer the query, if yes, return the response to the sink, else next steps;

– Request updates: if the cache does not contain the information desired, the active node issues a request for updates from nodes within a d-hop neighborhood. The responses from the controlled flood are then gathered back and used to see if the query can be resolved.

– Forward: if it has not already been resolved, the query is then forwarded to another active node by a sufficient number of hops so that the controlled flood phases do not overlap significantly.

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Acquire

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Request for updates within d-hop if cache missed

Active query forwarded if unresolved

Response To the sink

d is adjusted according to the update/query frequency ratio

Rumor routing

• Sinks desiring information send queries through the network• Source generating important events send notifications through

the network.• Queries and event notifications are mobile agent• Event notifications leave trail of state information through the

network• When query agent meets a trail, it knows the event source, and

route to the event source• rendezvous

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Source event notification

source

sink

Pointer to source located

Sink interest

75 112/04/20

Comb-Needle

Query comb

Event needle