Wireless Sensor Networks

巨型机说：“我认为全球大概只需要五台计算机就够了”；PC 机说：“每个家庭的桌面上都应该有一台电脑” ;

Pocket PC 说：“太大了，应该每人口袋里放一台” ;

WSN 说：“每粒沙子都应该是一台计算机”。

Outline

• General Comments

• Wireless Sensor Network Applications

• Architecture of WSN

• Overview of Sensor Hardware

• Characteristic of WSN

• OUR WORK

Outline

• General Comments

• Wireless Sensor Network Applications

• Architecture of WSN

• Overview of Sensor Hardware

• Characteristic of WSN

• Hot Issues

• OUR WORK

Sensor network: A new research hotspot

August 2003 Business Week projects "Sensors and Sensor Networks". to be one of 4 Key Technology Waves of the Future

《国家中长期科学和技术发展规划纲要》将传感器网络列为重点研究领域

Faster, Smaller, Numerous• Moore’s Law

– “Stuff” (transistors, etc) doubling every 1-2 years

• Bell’s Law– New computing class

every 10 years

year

log

(p

eo

ple

pe

r c

om

pu

ter)

Source: The Mote Revolution:Low Power Wireless Sensor Network Devices

Streaming Data to/from the

Physical World

What’s are Wireless Sensor Networks

• Wireless networks are usually composed of small, low-cost devices that communicate wirelessly and have the capabilities of Processing, Sensing and Storing

• The purpose is to measure different physical parameters in a given environment, in order to characterize its properties, or to take decisions depending on these measurements.

Network Model

Transit Network

Basestation

Sensor Patch

Patch Network

Base-Remote Link

Data Service

Internet

Client Data Browsingand Processing

Sensor Node

Gateway

Enabling Technologies

Embedded Networked

Sensing

Control system w/Small form factorUntethered nodes

ExploitcollaborativeSensing, action

Tightly coupled to physical world

Embed numerous distributed devices to monitor and interact with physical world

Network devices to coordinate and perform higher-level tasks

Exploit spatially and temporally dense, in situ, sensing and actuation

Outline

• General Comments

• Wireless Sensor Network Applications

• Architecture of WSN

• Overview of Sensor Hardware

• Characteristic of WSN

• Hot Issues

• OUR WORK

Wireless Sensor Network Applications

• Military Applications

• Environmental Applications

• Health Applications

• Home Applications

• Industrial Applications

• Other Commercial Applications

Application <——> WSN

Military Applications• enemy tracking, battlefield surveillance• target detection and classification

An Example:Counter Sniper System

Using the arrival times of the acoustic events at different sensor locations, the shooter position can be accurately calculated using the speed of sound and the location of the sensors.

民用领域

土壤湿度、温度、成份监测，节点对土壤进行周期性采样将数据发送给远端用户

森林防火：节点实时监测周围温度情况，并在探测到温度过高时发出警报。

医疗看护：将传感器配置在身体上，可以将身体情况传输给远端监控中心。 GE 公司

科技领域

了解火山习性

研究鸟类习性

海洋环境监测

• sensors and vehicles are deployed to perform collaborative monitoring tasks over a given area under ocean.

• Large number of sensor nodes collect data from the ocean and forward to a master node.

Industrial Applications

地面上的

传感节点

立柱上的

传感节点无线传感网络原型系统拓扑图 :

矿井安全检测和防护系统

From talk of Yunhao Luo

Outline

• General Comments

• Wireless Sensor Network Applications

• Architecture of WSN

• Overview of Sensor Hardware

• Characteristic of WSN

• Hot Issues

• OUR WORK

Architecture of WSN

• Sensor nodes are scattered in a sensor field(object field)

• Sensor nodes can self organize to form a sensor network

• Data are collected by these scattered nodes and routed back to the sink in a multi-hop way

• The user communicate with the sink via Internet

Architecture of WSN(cont'd)• nodes are still

stationary.• multiple, mobile

sinks defined as users.

• sinks may collect data at any time, any place.

无线传感网络无线传感网络无线传感网络无线传感网络 接入网络接入网络接入网络接入网络 核心网络核心网络核心网络核心网络

Architecture of WSN(cont'd)

From Talk of YunHao Luo

A general work process of WSN

Deploy Organize into network

Sensing and monitoring Data collection and dissemination

Outline

• General Comments

• Wireless Sensor Network Applications

• Architecture of WSN

• Overview of Sensor Hardware

• Characteristic of WSN

• Hot Issues

• OUR WORK

Sensor Hardware

• Fundamental Components– Various Sensing,Processing,Storing, Transceiver,Power

• Application dependent components – Locating, Mobilizer, Power generator

Sensor Node SamplesLWIM III

UCLA, 1996

Geophone, RFM

radio, PIC, star

network

AWAIRS I

UCLA/RSC 1998

Geophone, DS/SS

Radio, strongARM,

Multi-hop networks

Sensor Mote

UCB, 2000

RFM radio,

Atmel

Medusa, MK-2

UCLA NESL

2002

Mote Evolution

Source: The Mote Revolution:Low Power Wireless Sensor Network Devices

Outline

• General Comments

• Wireless Sensor Network Applications

• Architecture of WSN

• Overview of Sensor Hardware

• Characteristic of WSN

• Hot Issues

• OUR WORK

Characteristic of WSN• Resource Constraints

– battery equipped,recharging the batteries is impossible or unfeasible

– Radio and embedded CPU

• Self configuring– Randomly deployed, unattended.

• Dynamic Topology• Data centric

– Different from traditional network

• Unique traffic model• Application specific

Power Consumption• Power is of most important and directly influencing the lifetime

of WSN• Consumption in three domains:

– Sensing, communication and data processing– Energy consumed by Comm. is dominating

• The energy cost of transmitting 1Kb a distance of 100 m is approximately the same as that for executing 3 million instructions by a 100 million instructions per second (MIPS)/W processor.

S Dd

kTx/Rc electronics

Tx amplifier

MANET vs WSN: Differences

• Low density vs. high density• Address centric (IP) vs. content

centric (no IP)• Resource (constraint vs critical)• Mobile vs stationary• First criterion of performance (QoS

vs. Power)

Outline

• General Comments

• Wireless Sensor Network Applications

• Architecture of WSN

• Overview of Sensor Hardware

• Characteristic of WSN

• Hot Issues

• OUR WORK

Communication Protocols

• MAC Protocol– Fairness vs. Energy

• Routing Protocol– Energy-aware routing– Geo-routing

• Transport Control Protocol– Congestion Control– Reliability– End-to-end vs. Hop-by-hop

Event

Event

Sensor sources

Sensor sink

Directed Diffusion

A sensor field

Coverage Control• Problem

– Given a set of sensors deployed in a target area, we want to determine if the area is sufficiently k-covered, in the sense that every point in the target area is covered by at least k sensors, where k is a predefined constant.

• Two Motivations– One of the measurements of the QoS– Energy efficient

• Two conflicting objectives:– minimizing the number of active sensors to

minimize the energy consumption.– maintaining the coverage.

• Two metrics– Connectivity and Coverage

Sensing radius

Communicating radius

Data and Query Dissemination

• Problem– The sensor network is a distributed

database.– How to collect or query the interested data

detected by some nodes in a energy-efficient way?

• Application-specific– Area-based– Attribute-based

• Pull vs. Push

Time Synchronization

• Link to the physical world– When does an event take place?

• Key basic service of sensor networks– Fundamental to data fusion

• Crucial to the efficient working of other basic services– Localization, Calibration, In-network processing, …

• Several protocols require time synchronization– Cryptography, Topology management.

Time Synchronization

• Characteristics of SN– Cheap and Small

• No Accurate Oscillator

– Limited Energy• Need to Sleep

– Work Together• Data Fusion

• Conclusion– must initiate T-Sync in WSN

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Difficulties in Sensor Networks

• No periodic message exchange is guaranteed– There may be no links between two nodes at all

• Transmission delay between two nodes is hard to estimate– The link distance changes all the time

• Energy is very limited – Nodes sleep most of the time to conserve power

• Node need to be small and cheap– No expensive clock circuitry

Basic Mechanism

• Pair-wise synchronization

• T2=T1+delay+offset

• T4=T3+delay-offset

• offset=((T2-T1)-(T4-T3))/2• delay=((T2-T1)+(T4-T3))/2

A B

h ih j

distance

T1

T4

T2T3

Localization• Critical service

– A sensor reading consists of <time, location, measurement>

– E.g., target tracking, disaster recovery, fire detection, patient location in a smart hospital, …

– Needed for geographic routing

• Too expensive for an individual sensor to have a GPS (Global Positioning System)– Reference nodes (called anchor or

beacon nodes) + sensor nodes

Range-based localization schemes

• TOA (Time of Arrival)– Get range info via signal propagation delay– E.g., GPS– Expensive, power consuming, inaccurate

• TDOA (Time Difference of Arrival)– Transmit both radio and ultrasonic signals at the

same time to observe the arrival time difference– Extra hardware, i.e., ultrasonic channel, is

required– Not only radio but also sound signals have

multipath effects affected by humidity, temperature, …

• Received signal strength (RSS) – Distance estimation based on RSS– Hard due to radio signal vagaries

• AoA (Angle of Arrival)– A node estimates the relative angles

between neighbors– Requires directional antennae

Range-based localization schemes

Range-free localization

• Centroid algorithm– Anchors beacon their positions to

neighbors (single hop broadcast)– A sensor node computes the centroid using

all received beacon messages

• DV-HOP– Anchor locations are flooded through the

network– Keep the running hop count– Estimate average one hop distance

• Amorphous Positioning– Similar to DV-HOP– Use offline one hop distance estimation

Outline

• General Comments

• Wireless Sensor Network Applications

• Architecture of WSN

• Overview of Sensor Hardware

• Characteristic of WSN

• Hot Issues

• OUR WORK

自组织的无源无线 Zigbee － WiFi输电线路在线监测

• 监测功能– 图像采集：线下图像、塔架塔基图像、导线及绝缘

子图像– 弧垂和导线温度的采集

• 两级数据传输– 监测数据和报警信息从采集终端上传到塔架上监测

子站采用无线 Zigbee 技术– 数据和信息从塔架监测子站接力上传到监控中心

（运行值班、变电站）采用无线 WiFi 技术– 监控中心的控制命令可以下传

• 监控功能– 监控中心采集监测数据、存储并进行综合分析处理

监测平台运行示意图

两层网络架构

骨 干 网 络

子 网 子 网

采集终端

线路监测子站

变电站

监控中心

图例：

Zigbee

802.11b/g

能源紧缺、环境污染

电荒、煤荒、油价上涨

新能源

• 电能是与我们严密不可分开的能源，还存在着很大的节能空间。 积少成多

• 同时，产生电能需要释放大量二氧化碳，造成环境污染。如果一个煤燃料的发电能力为 44M 瓦的发电厂，使用后会释放 15 吨二氧化碳。

• 有报告称， 72% 的电能都消耗在家庭和商业建筑方面，其中 30% 的电能被浪费了。

• 研究表明，实时细粒度电器耗能反馈将能改变用户行为，使用电量降低 10% 到 20% 。

• 然而，当前的电表计量方式 为人们节能提供信息很少。

细粒度计量：节能的第一步

• 目的– 以可视化的方法实现每个电器任意时段的能耗– 为实现节能减排提供基础

• 目前系统已稳定工作，并投入到与能耗相关的科研工作中。

细粒度能耗计量无线传感器网络

细粒度能耗计量无线传感器网络

细粒度能耗计量无线传感器网络

Sensor-enabled Mobile Phone

accelerometer

digital compass

microphone

WiFi/bluetooth

GPS

…

Capture environment information

Capture environment information

What if sensors

automatically infer various

aspects of our lives ?

What if sensors

automatically infer various

aspects of our lives ?

Having conversationHanging out with friends…