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A System Architecture for Networked Sensors

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Title: A System Architecture for Networked Sensors


1
A System Architecture for Networked Sensors
  • Jason Hill, Robert Szewczyk, Alec Woo, Seth
    Hollar, David Culler, Kris Pister
  • http//tinyos.millennium.berkeley.edu
  • U.C. Berkeley
  • 11/13/2000

2
Computing in a cubic millimeter
  • Advances in low power wireless communication
    technology and micro-electromechanical sensors
    (MEMS) transducers make this possible
  • How do you combine sensing, communication and
    computation into a complete architecture
  • What are the requirements of the software?
  • How do you evaluate a given design?

3
Ad hoc sensing
  • Autonomous nodes self assembling into a network
    of sensors
  • Sensor information propagated to central
    collection point
  • Intermediate nodes assist distant nodes to reach
    the base station
  • Connectivity and error rates used to infer
    distance

Base Station
4
Organization
  • The Vision
  • Hardware of today
  • Software Requirements
  • TinyOS system architecture
  • System evaluation

5
Todays Hardware
  • Assembled from off-the-shelf components
  • 4Mhz, 8bit MCU (ATMEL)
  • 512 bytes RAM, 8K ROM
  • 900Mhz Radio (RF Monolithics)
  • 10-100 ft. range
  • Temperature Sensor Light Sensor
  • LED outputs
  • Serial Port

1.5 x 1.5
6
No dedicated I/O controllers
  • Bit by bit interaction with Radio
  • Software must process bit every 100µs
  • No buffering
  • missed deadline ? lost data
  • Must time share on granularity of 2x sampling rate

7
Key Software Requirements
  • Capable of fine grained concurrency
  • Small physical size
  • Efficient Resource Utilization
  • Highly Modular

8
TinyOS system architecture
9
State Machine Programming Model
  • System composed of state machines
  • Command and event handlers transition a module
    from one state to another
  • Quick, low overhead, non-blocking state
    transitions
  • Many independent modules allowed to efficiently
    share a single execution context

10
Simple State Machine Logic
Bit_Arrival_Event_Handler
State bit_cnt
Start
Send Byte Eventbit_cnt 0
Yes
bit_cnt8
bit_cnt
Done
No
  • Dont you have to do computational work
    eventually?
  • Tasks used to perform computational work

11
TinyOS component model
  • Component has
  • Frame (storage)
  • Tasks (computation)
  • Command and Event Interface
  • Constrained Storage Model allows compile time
    memory allocation
  • Provides efficient modularity
  • Explicit Interfaces help with robustness

Messaging Component
Internal State
Internal Tasks
Commands
Events
12
TinyOS The Software
  • Scheduler and graph of components
  • constrained two-level scheduling model tasks
    events
  • Provides a component based model abstracting
    hardware specifics from application programmer
  • Capable of maintaining fine grained concurrency
  • Can interchange system components to get
    application specific functionality

13
Composition
Route map
router
sensor appln
application
Active Messages
packet
Serial Packet
Temp
Radio Packet
SW
byte
HW
UART
Radio byte
i2c
photo
bit
clocks
RFM
14
Event Based Programming Model
  • System composed of state machines
  • Each State Machine is a TinyOS component
  • Command and event handlers transition a component
    from one state to another
  • Quick, low overhead, non-blocking state
    transmissions
  • Allows many independent components to share a
    single execution context
  • Emerging as design paradigm for large scale
    systems
  • Tasks are used to perform computational work
  • Run to completion, Atomic with respect to each
    other

15
Dynamics of Events and Threads
  • Message Send Transition

16
Analysis
  • Lets take apart Space, Power and Time

17
Space Breakdown
Code size for ad hoc networking application
Scheduler 144 Bytes code Totals 3430 Bytes
code 226 Bytes data
18
Power Breakdown
  • But what does this mean?
  • Lithium Battery runs for 35 hours at peak load
    and years at minimum load!
  • Thats three orders of magnitude difference!
  • A one byte transmission uses the same energy as
    approx 11000 cycles of computation.

19
Sample tradeoffs
20
Time Breakdown
  • 50 cycle thread overhead (6 byte copies)
  • 10 cycle event overhead (1.25 byte copes)

21
Hardware Kits
  • Two Board Sandwich
  • Main CPU boardwith Radio Communication
  • Secondary Sensor Board
  • Allows for expansion andcustomization
  • Current sensors includeAcceleration, Magnetic
    Field,Temperature, Pressure,Humidity, Light,
    and RF Signal Strength
  • Can control RF transmission strength Sense
    Reception Strength

22
Real time operating systems
  • QNX context switch 2400 cycles on x86
  • pOSEK context switch gt 40 µs
  • Creem -gt no preemption

23
Ad hoc networking
  • Each node needs to determine its parent and its
    depth in the tree
  • Each node broadcasts out ltidentity, depth, datagt
    when parent is known
  • At start, Base Station knows it is at depth 0
  • It send out ltBase ID, 0, gt
  • Individuals listen for minimum depth parent

0
Base
24
Planned Data Collection Experiment
  • March, 2001
  • UAV mote deployment
  • Vehicle detection and tracking
  • Pick-up data from network at a later date

25
Test Scenario
  • delivery of motes
  • network discovery
  • position estimation
  • time base synchronization
  • vehicle tracking
  • reporting to UAV
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