ENVIRONMENTAL MONITORING: FROM SENSORS TO DATABASE Jerry Yang

Overview  Design Requirement  System Framework  Wireless Sensor Networks Communications Protocols Data Interpolating Energy Harvesting Over-the-Air Programming  Telecommunication system Single Board Computer GPRS modem Networking sensors, data loggers and data servers  Database and Client Interfaces Database Data Visualization  Conclusion and Future Work

Project Object  Design and implement a fully functional environmental monitoring system  Collect and report temporal and spatial soil moisture data with required accuracy  Provide near-real-time data about monitored variables to the public  Monolithic weather stations  Wired Sensors (Data loggers) limited Spatial Coverage  Field Study Data is acquired every month

Go Wireless  Wireless Sensors could fulfill this mission  Unprecedented temporal and spatial granularities  Near-real-time data is accessible via the Internet  Besides…  Robust and accurate through dense deployment  Minimize disturbance to the monitored site  Cover larger area (Multihop)  Low installation cost  Ease of deployment and relocation

System Architecture Database Server Internet Client Data Browsing and Processing Data loggers In the Field: Download Data In the Lab: Upload Data

System Architecture Database Server Internet Client Data Browsing and Processing Base Station Node Wireless Sensor Nodes GPRS Modem Gateway (Single Board Computer) GPRS Link Data loggers

An introduction to WSNs  A wireless sensor mote is a battery-operated embedded system including various hardware and software components. For MicaZ motes:  Processing Unit 7.37MHz micro-controller 4KB RAM 512KB Flash  Sensors 16-bit ADC with MDA 300 Data Acquisition Board EC-5 Soil Moisture Sensors  Transceiver 2.4 GHz, IEEE compliant, 250 kbps  Powered by 2 AA batteries

Constraints of Sensor Motes  Limited processing, storage and communication capabilities  100 = 25kbps (data sampled every second)  WILL be solved in the near future year Streaming Data to/from the Physical World

Fundamental Problem  Sensor network is un-tethered, and will be operating for a long time.  Replacing batteries is difficult and expensive if not impossible For MicaZ, typical current drawing is 30mA. Powered by 2.4V 3000mAh Batteries, a MicaZ mote could run for 100 hours continuously. Communicating 1 bit data over the wireless medium consumes far more energy than processing it. Operating Current (mA)MicaZ ATMega128L, full operation12 (7.37 MHz) ATMega128L, sleep0.010 Radio, receive19.7 Radio, transmit (0dBm)17 Radio, sleep0.001

Software Support  TinyOS and NesC  An open-source operating system designed for wireless embedded sensor networks  Component-based architecture which enables rapid innovation and implementation while minimizing code size  Event-driven execution model

Communication Protocols  Design requirement  Energy Efficient Radio communication is the most expensive operation in terms of energy usage  Robust, scalable and adaptive Dynamic topology changes due to unstable links, node failures and network disconnections  Unique characteristics of our project Long-term operation with very low data rate A single sink node At most of the time, data flow is uni-directional  Layered Architecture  Physical/Link Layer  Medium Access Control  Routing

Physical/Link Layer  Radio Propagation  Path Loss - signal strength attenuates as distance to a constant exponent  However, radio connectivity is not a simple disk  Shadowing (due to obstructions) and Multipath Fading  Wireless Channel Characteristics  Great spatial variability  Non-isotropic propagation  Asymmetric links are common due to hardware calibration

Link Quality Over Space Packet reception over distance has a heavy tail. There is a non-zero probability of receiving packets at distances much greater than the average cell range 169 motes, 13x13 grid, 2 ft spacing, open area, RFM radio, simple CSMA

Medium Access Control  MAC protocol decides when and how nodes access the shared wireless channel  Collision avoidance  Duty-cycle control MAC layer protocols directly controls radio activities, significantly affect the overall node lifetime  MAC in Wireless Networks  Contention-based protocols CSMA/CA – node compete for a single channel On-demand allocation provides more flexibility and adaptivity  Scheduled protocols C/T/FDMA – divide wireless channel into different sub-channels Collision-free and energy-efficient

MAC for Sensors  Sources of energy waste in radio communication  Idle listening Costs as much power as transmitting or receiving dominant factor of energy consumption especially in low data rate systems  Collision – retransmit when packets collide  Build on CSMA but also adopt TDMA-like sleep/wakeup duty cycle  S-MAC, T-MAC, B-MAC, Z-MAC  Reduce idle listen and minimize collision  Improve power efficiency while retaining flexibility  Sacrifice throughput, increase latency

MAC Protocol Design Parent…SyncSleepRcv 1Rcv2SleepTransmitSleep Child 1…SyncSleepTransmitSleep Child 2…SyncSleepTransmitSleep  We implement a tree-structure data report hierarchy, rooted at the sink node  A global clock is also maintained by time synchronization  All nodes begin with a Sync slot  Synchronize time, manage neighbor list, select parent  Parent nodes then allocate time slots for their children  All nodes are awake, but only broadcasting very short control packets  A node will report its latest readings to its parent in transmit slot, while the parent node will become active and listen to the channel  Nodes sleep for the rest of time

Network Layer - Routing  Establishing and maintain the multi-hop routing hierarchy  Link Quality Estimation  Neighbor Management  Discover, update, remove neighboring nodes  Parent selection  Shortest Path, Minimal Transmission, Geo-Routing, Energy-Aware routing

Link Quality Versus Distance

Time Synchronization  Why do we need network-wise clock?  Time stamp data samples  Set up radio schedule  TOA, TDOA in Localization  Pair-wise Synchronization  Estimate communication delays Send time, access time, propagation time, receive time, etc.  Estimate clock skew Perform linear regression on past local/global time pairs  Multihop Synchronization  Minimize control overhead

Application Layer  Energy Efficient Map Interpolation for Sensor Fields using Kriging (E2K)  an energy efficient and error bounded framework for interpolating maps from sensor fields  Environmental dynamics, such as temperature and soil moisture, are continuous  Should be represented as a continuous surface over the sensor fields through interpolating  Spatial and temporal autocorrelation could be utilized to reduce sample points

Data Interpolating

Localization  Knowing the exact location where information was collected is critical  A reading is represented by vectors (x,y,t,v)  Self-localization vs Tracking  Ranging Methods  Radio, acoustic/ultrasound, laser, etc.  RSS, TOA, TDOA  Lateration and Triangulation

Solar Harvesting Sub-System  Energy Storage Module  Ultra Capacitors and Rechargeable Batteries  Choosing Batteries NiMH, NiCd, Li-ion  Solar Harvesting Module  Solar Cells  Regulators and Switches  Circuit Design  Smart Battery Monitoring  Energy-Aware Protocol and Considerations

Over-the-Air Programming  Loading a new application or upgrading an existing application on a sensor node  via a serial port or some physical connections to the node  Reprogram nodes one by one  However, physical access to nodes is in many cases extremely limited following deployment  Even when access were possible, manually updating hundreds or thousands of nodes would be a tedious task indeed  Network reprogramming protocols have recently emerged as a way to distribute application updates without requiring physical access to sensor nodes.

Multi-hop Over-the-Air Programming  MOAP divides a program image into packets, and these packets are distributed through the network. Once received, packets are placed in stable storage until the entire update has been completed.  In MOAP, sources advertise updated code images to their neighbors. A node having received a full image become publishers and propagate the image to other nodes out of range of the original source.  This process is applied iteratively until the update has propagated across the network.  Packet loss and retransmission  Receiver uses a sliding window to keep track of lost packets.  When a missing packet is detected, the receiver sends a uni-cast retransmission request.  If the source does not respond within a certain amount of time, the receiver broadcasts a retransmission request to which all nodes within range reply. This allows the receiver to choose a new source in case the original source fails. Duplicate requests arriving at a source within a given time period are suppressed.

Cross Layer Protocol Design  No standard protocol for sensor nets  Sensor protocol design is task-specific  Resource constraints even demand cross-layer integration  While some protocols can achieve very high performance in terms of the metrics related to each of the individual layer, they are not jointly optimized in order to maximize the overall network performance and minimize energy expenditure  When designing communication schemes, we can not simply pick the best protocol in each layer and pile them up.

Tele-Communication System  The needs for telemetry  Provides near-real-time data feeding  Enables remote control of sensor nets and data loggers Change monitoring parameters Update sensor motes/data logger programs after deployment  Single Board Computer (SBC)  200Mhz ARM processor, 64MB RAM, 1GB SD Card  Linux support  Bridge between sensors and Internet  Local Database Server  GPRS modem  PPP and PPP Daemon a data link protocol commonly used to establish a direct connection between two nodes over serial cable, phone line, cellular phone, or dial-up network to get access to the Internet

Conclusion  Data flows from sensors to remote database  System Architecture  Research areas  Energy-Aware Design  Cross Layer Protocol Design  Over the air programming  Localization  Questions?