Network Embedded Systems

Sensor Networks

Review

Marcus Chang, mchang@cs.jhu.edu

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Hardware

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Microcontroller Unit (MCU)

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Multiple components in same chip CPU

Volatile memory (e.g. RAM)

Persistent memory (e.g. Flash)

Analog-to-Digital Converters

Timers (alarms, stop-watches)

Peripheral interfaces

Radio

…

System Clock

Inputs:

Low-frequency crystal (kHz range)

High-frequency crystal (MHz range)

Internal oscillator

Outputs:

Master Clock

Sub-Main Clock

Auxiliary Clock

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Power Operating Modes

5 Low-Power Modes in total

Active Mode: CPU active, all clocks active

LPM0: CPU disabled, SMCLK and ACLK active

LPM3: CPU disabled, ACLK active

LPM4: CPU, all clocks disabled

5 Image: Texas Instruments

Timer Module

16-bit counter connected to either SMCLK or ACLK

Counts number of clock cycles

Measure elapsed time

Timestamp events

Set alarms at specific points

Continuous/Up-down mode

Overflow

Capture/Compare Module

Read counter when an event occurs

Generate event when counter reaches specific value

6 Image: Texas Instruments

Watchdog Timer

Similar to Timer Module, except

Microcontroller resets when alarm is triggered

Clear counter periodically to avoid it (i.e. ‘feed the watchdog’)

Useful to reset microcontroller to a known state

The watchdog is enabled by default after reset

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General Purpose Input/Output Pins (GPIO)

Input Pin

High voltage reads ‘1’ – low voltage reads ‘0’

High – MCU supply voltage minus Δ

Low – 0V plus Δ

Output Pin

Write ‘1’ sets voltage high – write ‘0’ sets voltage low

Interrupt capable Pin

Interrupt program flow when input value changes

E.g. press button to make voltage go low

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Analog-to-Digital Converter (ADC)

12-bit ADC

Measure input voltage as a fraction of the reference voltage

Comparator

Compare voltage between two analog signals

DAC

Digital-to-Analog Converter

9 Image: Texas Instruments

External communication

GPIO pin

Turn peripherals on/off, configure peripherals

Manually transmit 1 bit at a time – ‘bit banging’

Universal asynchronous receiver/transmitter (UART)

Speed: 115 kbit/s

10 Images: Wikipedia

1-Wire Bus (Timer Module)

1 signal line, multiple devices (master/slaves) Designed for parasitic power

64-bit unique serial number/address

Speed: 16.3 kbit/s

Length: 300 m

Protocol Reset, 8-bit command, read/write

11 Images: Wikipedia

Inter-Integrated Circuit (I2C Module)

2 signal lines, multiple devices (masters/slaves)

SCL – clock line (for timing)

SDA – data line

Speed: 400 kbit/s (although 3.4 Mbit/s standard exists)

7-bit or 10-bit address space (1-bit used to signal read or write)

Protocol

Start bit, address (w/r), read/write, stop bit

12 Images: Wikipedia

Serial Peripheral Interface Bus (SPI Module)

3+1 signal lines, multiple devices (master/slaves)

Clock

MISO – master in, slave out

MOSI – master out, slave in

Slave select (enable slave when line is low)

Full duplex

Protocol

Clock polarity: base high/low

Clock phase: capture rising/falling edge

13 Images: Wikipedia

Telosb Block Diagram

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External Flash

1MiB non-volatile storage

64 KiB sector size

256 B page size

Flash operations

Erase: set all bits to ‘1’ – sector-wide operation

Write: set matching bits to ‘0’ – bit-wide operation

Write operations take longer time if sector needs to be erased first

Wear-leveling

Circular buffer

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Radio ‘CC2420’

IEEE 802.15.4 (ZigBee, 6lowPAN)

2.4 GHz (16 channels)

250 kbit/s

Auto acknowledgement and address recognition

Auto CRC and AES encryption

MPDU: 127 bytes

Frame payload: Max. 122 bytes

16 Image: Texas Instruments

Software

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Embedded Programming

“Normal” applications vs. embedded systems

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Normal

applications

Embedded

Systems

Blocking/Threading (vs. Event Driven) Function call that do not return until complete

External events can make this arbitrary long

… wget(www.google.com)

… getKey();

… read(file);

… write(file);

Non-Embedded Programming

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Blocking/Threading vs. Event driven

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read(file)

getKey()

write(file)

OS Maintenance

/ Sleep

<I/O done>

<key pressed>

Thread 1 Thread 2 Idle

Non-blocking calls

Multiple concurrent function calls

Explicit sleep

Blocking/Threading vs. Event driven

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read(file)

write(file)

<read done>

<key pressed>

Main

getKey()

sleep()

sleep()

<write done>

Polling vs. interrupts

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End

Get character

Is there a

character?

Start Rx

Wait

Yes

No

Start Rx Interrupt

End Rx Interrupt

Get character

Sleep

Interrupt

SP – Stack Pointer

TOS – Top of the stack

PC – Program Counter

SR – Status Register

Interrupt Vector

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read(file)

getKey()

write(file)

<I/O done>

<key pressed>

Main

getKey()

Source: Texas Instruments

Priority Address Module Register

Race conditions

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Problem:

Accessing the same variables in both main program and asynchronous interrupt context

Images: Frank Duignan

Race conditions

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Disable interrupts during atomic operations

Might miss interrupts!

Images: Frank Duignan

Memory Map

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Memory Mapped I/O

Peripheral modules are accessed as regular variables (pointers)

Flash read, extension of RAM

Flash write, ‘peripheral module’

Pros and cons

Global variables, no overhead

Buffer swapping

No memory protection

Images: Doina Bucur

Data types

Common data types:

char, short, int, long, float, etc.

But, the MSP430 is a 16-bit CPU

How big is a short? int? long? float? double?

Determined by the compiler

Specific data types (stdint.h): int8_t, uint8_t, int16_t, uint16_t, int32_t, uint32_t, int64_t, uint64_t

E.g. 12-bit ADC reading uint16_t

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#include <msp430x14x.h> static char string1[] = { "Hello World\n\r" }; char i; void main(void) { WDTCTL = WDTPW + WDTHOLD; // Stop watchdog P3SEL = 0x30; // P3.3,4 = USART0 TXD/RXD // Control registers ME1 |= UTXE0 + URXE0; // Enable USART0 module UCTL0 |= CHAR; // 8-bit character UTCTL0 |= SSEL0; // UCLK = ACLK UCTL0 &= ~SWRST; // Initialize USART state machine // Speed registers UBR00 = 0x03; // 9600 baud UBR10 = 0x00; // UMCTL0 = 0x4A; // IE1 |= URXIE0 + UTXIE0; //Enable RX/TX interrupt IFG1 &= ~UTXIFG0; // Clear initial flag on POR while(1) _BIS_SR(LPM3_bits + GIE); // Enter LPM3 w/ interrupt }

Hello World\n\r // UART0 TX ISR #pragma vector=USART0TX_VECTOR __interrupt void usart0_tx (void) { if (i < sizeof string1-1) TXBUF0 = string1[i++]; } // UART0 RX ISR #pragma vector=USART0RX_VECTOR __interrupt void usart0_rx (void) { if (RXBUF0 == 'u') // 'u' received? { i = 0; TXBUF0 = string1[i++]; } }

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Code: Texas Instruments

Meet sensor network characteristics AND run on mote-class device

TinyOS

Event driven

Two-level scheduling: Tasks and events

Scalable hardware/software boundary

No blocking functions or polling

(No dynamic memory)

Embedded OS

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Interfaces

Provides (up)

Uses (down)

Mutators

Call command (down)

Signal event (up)

Tasks

Low priority

Functions, command calling, event signaling

Asynchronous events

High priority

Hardware interrupt handler

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Components

Component Graph

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Concurrency

Tasks run atomically with regard to other tasks

Race conditions can be detected at compile time

Split-phase operation

Commands are non-blocking

E.g. call command ‘start’, signal event ‘startDone’

Modular

Code reuse

Flexible hardware/software boundary

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TinyOS

Radio

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Physical Layer

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PHY Layer Motivation

Nodes communicate over wireless channel

Characteristics of wireless channel and transceiver have impact on all upper layer protocols

Transmission range

Loss rate

Energy consumption

Why?

Understand performance of existing systems

Guide design of future systems

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Bit Error Rate

Noise can corrupt received signal to the point that it is incorrectly decoded

Bit errors

For a given modulation scheme, bit error rate (BER) is a function of Signal-to-Noise ratio (SNR)

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Signal to Noise Ratio

Noise of power N0 arrives at the sensor

SNR= Pr / N0

Usually measured in decibels: SNRdB= 10logSNR

When SNR drops below threshold signal cannot be detected

How can we increase detection range?

Quadrupling the power doubles the range

Double the carrier wavelength

Improve detection algorithm

Link Layer

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MAC and its Classification

Medium Access Control (MAC)

When and how nodes access the shared channel

Classification of multiple access MAC protocols

Scheduled protocols

Schedule nodes onto different sub-divisions

Examples: Time (TDMA), Frequency (FDMA), Code (CDMA)

Contention-based protocols

Nodes compete in probabilistic coordination

Examples: ALOHA (pure & slotted), Carrier Sense (CSMA)

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MAC Attributes

Collision avoidance

Basic task of a MAC protocol

Energy efficiency

Scalability and adaptivity

Network size, node density and topology change

Channel utilization

Latency

Throughput

Fairness

Primary

Secondary

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Energy Efficiency in MAC Design

What causes energy waste?

Packet collisions

Control packet overhead

Overhearing unnecessary traffic

Long idle time

Bursty traffic in sensornet applications

Idle listening consumes 50—100% of the power for receiving

Terminology

Wakeup period (time between wakeups)

Duty Cycle = listen period/Wakeup period

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Scheduled Protocols

TDMA

Advantages

No collisions

Energy efficient — easily support low duty cycles

Disadvantages

Bad scalability and adaptivity

Difficult to accommodate node changes

Difficult to handle inter-cluster communication

Requires time synchronization

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CSMA — Carrier Sense Multiple Access

Listening before transmitting

Collisions can still occur

Examples

IEEE 802.11 – CSMA/CA

Collision Avoidance – random back-off time

Problem

Solution Explicit Request-to-Send and Clear-to-Send (RTS/CTS) packets

Contention-Based Protocols

a b c

Hidden terminal: a is hidden from c’s carrier sense

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Case Study: S-MAC

S-MAC — by Ye, Heidemann and Estrin

Tradeoffs

Increase latency and decrease fairness to improve energy efficiency

Major components in S-MAC

Periodic listen and sleep

Collision avoidance

Overhearing avoidance

Message passing

From “Medium Access Control With Coordinated Adaptive

Sleeping for Wireless Sensor Networks” by Ye et al.

Principle

Node periodically wakes up, turns radio on and checks channel

Wakeup time fixed, “Check time” variable

If energy is detected, node powers up in order to receive the packet

Noise floor estimation used to detect channel activity during LPL

Low Power Listening (B-MAC)

45 From “Versatile Low Power Media Access for Wireless Sensor Networks” by Polastre et al.

X-MAC

Preamble contains destination ID Other receivers can

return to sleep

Strobed preamble Receiver sends ACK after

receiving short preamble

Receiver stays awake after packet reception Transmissions from

pending senders can proceed without additional preambles

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From “X-MAC: A Short Preamble MAC Protocol for

Duty-Cycled Wireless Sensor Networks” by

Buettner et al.

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Scheduled Channel Polling (SCP-MAC)

SCP synchronizes neighbor’s channel polling time A short wake up tone wakes up receiver

It is efficient for both unicast and broadcast packets

From “Ultra-Low Duty Cycle MAC with Scheduled Channel Polling”

by Ye et al.

Receiver Initiated MAC

Goal:

Reduce idle overhearing in dense networks

RI-MAC

Sender does the idle listening

Receiver transmits beacons

48 From “RI-MAC: A Receiver-Initiated Asynchronous Duty Cycle MAC Protocol for Dynamic

Traffic Loads in Wireless Sensor Networks” by Sun et al.

Low Power Probing

Receiver Initiated Wakeup

Nodes send periodic beacons while sleeping

After each beacon, nodes listen for acknowledgement

Node remains awake when acknowledgement is received

Nodes that are awake acknowledges other beacons

One node initializes network wide wakeup by acknowledging the other nodes’ beacons

Result: wakeup spreads like flood wave from origin

Benefits:

Nodes can collect neighborhood information from beacons

Use neighborhood information to detect missing nodes

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Routing

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Network Layer

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Radio

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Range

Unit Disc (simplification)

MAC

Broadcast

Unicast

Link

Asymmetric

A

B

C

D

Multi-hop Routing

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B

A?

C?

Routing: A B Problems:

Optimal route?

Metric?

Route discovery?

Network changes?

Asym. links?

Hop Count is a bad metric

Reception rate deteriorates quickly as transmission range grows

Hop count tends to pick long links lossy links

Instead, one should select high quality links

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Estimating Link Quality

What link characteristics can we measure?

Hardware (radio dependent):

RSSI: Received Signal Strength Indication

Energy (dbm)

SNR: Signal-to-Noise Ratio

LQI: Link Quality Indicator

Packet correctness confidence

Software

PRR: Packet Reception Ratio

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Metric Idea #2: Avoiding static thresholds

Using a static PRR threshold for selecting links can lead to disconnections, suboptimal performance

Ideally we want to dynamically select the links with the highest PRRs

ETX metric:

Expected number of Transmissions required to successfully send a packet over link/path

Proposed in “A High-Throughput Path Metric for Multi-Hop Wireless Routing” by DeCouto et al. (MobiCom 2003)

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Metric idea #4: : Use information from multiple

layers

4-bit Link Estimator Physical layer

If white bit is set, medium quality is high during reception Used to quickly decide whether a

link should be even considered

Link layer Ack bit is set when an

acknowledgement is received Increases frequency of ETX

estimation

Network layer Set pin bit for entries that are

important

Compare bit indicates whether new neighbor is better than one of the current entries

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RPL: IPv6 Routing Protocol for Low power

and Lossy Networks

Nodes form a Destination Oriented Directed Acyclic Graph (DODAG) using RPL control ICMPv6 messages

Very similar to CTP

Trickle-timer reduces maintenance overhead

Exponentially growing resend timer

Timer resets when “disagreement” occur

“Objective functions” determine the rank of a node in the DODAG

ETX, hop-count, etc.

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Energy

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Power Management

Where does all my power go?

Simulation

Collect power measurements during deployment

Collect run-time traces from deployment

What is the most efficient way to use the available power?

Fixed lifetime requirement?

Local power usage policy

Global energy optimization

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Mote Power Supply

Switching Regulator

Takes variable input voltage Vin and changes it into stable Vout

Charge-discharge cycle

Analogy: filling and emptying a bucket of water

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Energy Profiling

Combine application profiling with energy measurements

Log power state changes

Log power consumption in each stage

Log application profile

Estimate each components power consumption by solving linear equations: XΠ = Y

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Virtual Battery

Energy Reserve Manager = Bank

Each task requests an account with conditions

W – fraction of real battery allocated

N – number of energy installments

L – expected lifetime of the task

C – credit line, fraction of remaining energy

B – maximum energy burn rate

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Lance Architecture

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Solar Power

Photovoltaic (PV) panels

200 mA, $50

Diurnal generation (predictable)

Cloud dependent (unpredictable)

Plan after expected sun hours

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Wind Power

Wind turbine

65 mA, $75

Moving parts maintenance

Minimum and maximum wind speed

Plan after expected wind conditions

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Air 30, Southwest Windpower

Hymini, Miniwiz

Other Sources

Kinetic Energy Recovery System

Hybrid cars!

Self-charging watch

Radio waves

RFID

Wireless power

Steam

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nPower Peg, www.npowerpeg.com

Qi Wireless Power

Energy Storage Problem

Energy harvesting output is variable Solar and wind follow diurnal cycles

Store surplus energy in battery Limited lifetime: ~1,000 cycles

Variable efficiency: 70-99%

Slow charging

(Super)capacitors Long lifetime: ~1,000,000 cycles

Constant efficiency: ~90%

Fast charging

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Wireless Identification and Sensing Platform

RFID meets MCU

Convert RF signal to DC voltage

Receive: Amplitude Modulated signal

Send: Change antenna characteristic reflected energy

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“I don’t need luck, I got ammo” - Wrex

Good Luck!

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Schedule

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Week 1: Introduction and Hardware

Week 2: Embedded Programming

Week 3: Medium Access Control

Week 4: Link Estimation and Tree Routing

Week 5: IP Networking

Week 6: JHU Special feat. Doug Carlson

Week 7: (seminar, no lecture)

Week 8: Energy Management and Harvesting

Week 9: Review and Midterm

Week 10: Time Synchronization

Week 11: Localization

Week 12: TBD

Week 13: (seminar, no lecture)

Week 14: TBD