A System Architecture for Tiny Networked Devices

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A System Architecture for Tiny Networked Devices

• Jason Hill
• http//www.cs.berkeley.edu/jhill
• http//tinyos.millennium.berkeley.edu
• U.C. Berkeley
• 9/22/2000

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Goals

• To develop an ultra low power networked sensor
  platform, including hardware and software, that
  enables low-cost deployment of sensor networks.
• To be a system level bridge that combines
  advances in low power RF technology with MEMS
  transducer technology.

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Key Characteristics of TNDs

• Small physical size and low power consumption
• gt Limited Physical Parallelism and Controller
  Hierarchy
• gt primitive direct-to-device interface
• Concurrency-intensive operation
• flow-thru, not wait-command-respond
• gt must handle multiple inputs and outputs
  simultaneously
• Diverse in Design and Usage
• application specific, not general purpose
• huge device variation
• gt efficient modularity
• gt migration across HW/SW boundary
• Largely Unattended Numerous
• gt robust operation
• gt narrow interfaces

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MoteThe Hardware
COTS Dust
weC Mote

• 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

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Second Generation Mote

• Two Board Sandwich
• Main CPU board with Radio Communication
• Secondary Sensor Board
• Allows for expansion and customization

• Current sensors include Acceleration, Magnetic
  Field, Temperature, Pressure, Humidity, Light,
  and RF Signal Strength.
• Can control RF transmission strength Sense
  Reception Strength

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Tiny OS The Software

• Provides a component based model abstracting
  hardware specifics from application programmer.
• Capable of maintaining high levels of
  concurrency.
• Allows multiple applications to be running.
• Services Provided Include
• RF messaging protocols.
• Periodic Timer Events.
• Asynchronous access to UART data transfers.
• Mechanism for Static, Persistent Storage.
• Can Swap Out system components to get necessary
  functionality.
• Complete applications fit in 4KB of ROM and 256B
  RAM.

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Tiny OS Internals

• Scheduler and Graph of Components
• constrained two-level scheduling model tasks
  events
• Component
• Frame (storage)
• Tasks (concurrency)
• Commands, and Handlers (events)
• Constrained Storage Model
• frame per component, shared stack, no heap
• Very lean multithreading
• Layering
• components issue commands to lower-level
  components
• event signal high-level events, or call
  lower-level commands
• Guarantees no cycles in call chain

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TOS Component
/ Messaging Component Declaration
/ //ACCEPTS char TOS_COMMAND(AM_SEND_MSG)(char
addr,char type, char data) void
TOS_COMMAND(AM_POWER)(char mode) char
TOS_COMMAND(AM_INIT)() //SIGNALS char
AM_MSG_REC(char type, char data) char
AM_MSG_SEND_DONE(char success) //HANDLES char
AM_TX_PACKET_DONE(char success) char
AM_RX_PACKET_DONE(char packet) //USES char
TOS_COMMAND(AM_SUB_TX_PACKET)(char data) void
TOS_COMMAND(AM_SUB_POWER)(char mode) char
TOS_COMMAND(AM_SUB_INIT)()
Commands
Events
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Composition to Complete Application
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
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Programming with CAD

• Can assemble overall system using structural VHDL
• Scripts pre-process VHDL at compile time
• Allows for compile time resolution of event
  handlers
• Eliminates need for registration mechanisms and
  dynamic dispatch
• Automatically allows events to be handled by
  multiple components. (TX_Done)
• Significantly more flexibility than library based
  component models

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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

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Dynamics of Events and Threads

• Message Send Transition

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Empirical Breakdown of Effort

• can take apart time, power, space,
• 50 cycle thread overhead, 10 cycle event overhead

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Migration of the Hardware Software Boundary

• TinyOS component model propagates hardware
  abstractions into software.
• Allows for a migrations of software components
  into hardware
• Example
• Bit level radio procession component could be
  implemented as specialized FIFO with complex
  pattern matching.

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Storage Breakdown (C Code)
3450 B code 226 B data
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Multi-Hop Routing Demo

• Sensors automatically assemble and determine
  routing topology
• Parallel Breadth First Search
• Shortest path to all nodes remembered
• Base station broadcasts out routing information
• Individuals listen for and propagate route update
• N messages sent
• Generational scheme to prevent cycles in routing
  table

Base
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Demo (cont.)

• Sensor information propagated up routing tree
• Statistics kept for number of readings received
  and number of packets forwarded by each node
• Sensors transmit data when significant events
  occur or when time limit is exceeded
• Must be continuously listening for packets to be
  forwarded impacts power considerations

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Future Goals

• Short Term
• Deploy sensor net for week long trials
• Target Civil Engineerings and The Center For the
  Built Environments needs for a real world
  deployment.
• Determine algorithms for aggregation and analysis
  of data inside the network.
• Support applications where data is picked up by
  roaming data collection nodes.
• Long Term
• Deploy networks of 1000s of nodes
• Year long, unattended deployments