System Architecture Directions for Networked Sensors

1
System Architecture Directions for Networked
Sensors

• Jason Hill, Robert Szewczyk, Alec Woo, Seth
  Hollar, David Culler, Kris Pister
• Presenter James

2
Computing in millimeters

• Advances in low power wireless communication
  technology and micro-electromechanical sensors
  (MEMS) make this possible
• How to combine sensing, communication and
  computation into a complete architecture
• What are the requirements of the software?

3
Organization

• Hardware of today
• Software Requirements
• TinyOS system architecture
• System evaluation

4
Todays Hardware

• Assembled from off-the-shelf components
• 4Mhz, 8bit MCU (ATMEL)
• 512 bytes RAM, 8K ROM
• 916.5Mhz Radio (RF Monolithics)
• 10-100 ft. range
• Temperature Sensor Light Sensor
• LED outputs
• Serial Port

1.5 x 1.5
5
Todays Hardware (cont.)

• Bit by bit interaction with Radio
• No buffering
• missed deadline ? lost data
• No dedicated I/O controllers

6
Key Software Requirements

• Capable of fine grained concurrency
• Small physical size
• Efficient Resource Utilization
• Highly Modular

7
Real time operating systems

• VxWorks in hundreads of kilobytes
• QNX context switch 2400 cycles on x86
• Creem no preemption

8
TinyOS system architecture
9
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
10
TinyOS Component

• //AM.comp//
• TOS_MODULE AM
• ACCEPTS
• char AM_SEND_MSG(char addr, char type,
  char data)
• void AM_POWER(char mode)
• char 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 AM_SUB_TX_PACKET(char data)
• void AM_SUB_POWER(char mode)
• char AM_SUB_INIT()

AM_SEND_MSG
AM_INIT
AM_POWER
AM_MSG_SEND_DONE
AM_MSG_REC
Messaging Component
Internal State
Internal Tasks
AM_SUB_POWER
AM_TX_PACKET_DONE
AM_SUB_TX_PACKET
AM_RX_PACKET_DONE
AM_SUB_INIT
Commands
Events
11
State Machine 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 are allowed to
  efficiently share a single execution context

12
A Complete Application
sensing application
application
Routing Layer
routing
Messaging Layer
messaging
Radio Packet
packet
Radio byte
Temp
byte
photo
SW
HW
RFM
i2c
ADC
bit
clocks
13
Analysis

• Lets take apart Space, Power and Time

14
Space Breakdown
Code size for ad hoc networking application
Scheduler 144 Bytes code Totals 3430 Bytes
code 226 Bytes data
15
Energy

• It turns out energy is your most valuable
  resource
• Traditional notions of resources memory, CPU,
  I/O become expenses, not resources
• All components must support low power modes

16
Power Breakdown
Energizer CR2450 575 mAh

• What does this mean?
• Lithium Battery runs for 28 hours at peak load
  and years at minimum load!
• All its energy can only support 144KB data

17
Time Breakdown

• 50 cycle thread overhead (6 byte copies)
• 10 cycle event overhead (1.25 byte copes)

18
An Example Timing Diagram

• Message Send Transition

19
Conclusions

• TinyOS is a highly modular software environment
  tailored to the requirements of Network Sensors,
  increasing efficiency, modularity and concurrency

20
Discussions

• Transmitting encoding
• Zeros and Ones
• Acks and retransmissions
• Data compression, aggregation, etc.
• Single MCU vs. multiple MCUs
• Concurrency-intensive operation?
• Power allocation
• Other ways of power conservation
• Duty cycle
• Lower accuracy