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Wireless Sensors

Discovery in Sleeping

presented by: Ted Herman, University of Iowa

March/April 2011, Vietnam
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IntroductionTechnical ProblemsNeighbor Discovery

Plan of Presentation

Introduce topics of Wireless Sensor Networks, and show some

technical apects1 why wireless sensor networks?

2 what are the problems?

3 some ideas about neighbor discovery

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Wireless Embedded Computing

Wireless Embedded Computing Vision

Dream 1: Ubiquitous Computing - always connectedcomputing (= cloud computing + smart phone)

Dream 2: Spimes, Sensors on Web - sensing data, putting incloud (measure temperature, light, in smart phone)

Dream 3: Pervasive Computing - a world of smart objects(cars, buildings, roads, machines) - embedded computerseverywhere, sensing and adapting to changes

works even without cloud

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Wireless Embedded Computing

Wireless Embedded Computing Vision

Dream 4: Extended Sensing

what if you can know where is traffic, be aware ifgrandmother is OK, find out if heat is on/off athome, . . .

these things are like sensing from a distance, enabled by

sensors placed in our lives, with networked (wireless)communication

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Sensor Types

What kinds of sensors?

Small, low-power sensors now made for:

temperature, dark/light, CO2, vibration, pressure,ultrasound, movement

Future small sensors will include:

special chemicals, biological agents, radiation, visual

patterns, radar, . . .

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Current WSN Applications

Wireless Sensor Networks (WSN) Applications

monitor supply chain (inventory, transportation)

security (shipping containers, borders) factory equipment (overheating, vibration) infrastructure (bridges, buildings)

precision agriculture (vineyards, greenhouses)

scientific experiments (measuring precisely)

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Introduction
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Example: Jindo Bridge, South Korea

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Introduction
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Example Bridge Sensor Hardware

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Introduction
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Other Sensor Deployments

Sensors in Use

1 Aircraft: hundreds of sensors (Airbus communicates sensor

data by radio in real time)2 Vehicles: expensive vehicles may use hundreds of sensors

(lookup wireless hack)

3 RFID in supply chain: location, and even other attributes like

temperature, vibrationSome usage, but wireless sensor networks very limited!

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Introduction
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What are the Problems?

What is needed for future?

Some technical problems to solve, for more widespread use of

Wireless Sensor Networks: battery problems, radio problems,reliability, standards, . . .

Two Main Practical Problems

1 Cost. They are still too expensive (

$100)

2 Software. We need fault tolerant, flexible software for thesevery small devices

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Introduction
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Technical ProblemsNeighbor Discovery

Sensor Cost Problem

Cost Factors

1 processor, memory, radio cost $5$50 (depending onfeatures)

2 board, enclosure, batteries, antenna cost

$20$100

3 sensors (temperature, acceleration, etc) cost $1-$10

Comparison

Note: cell phone processor is 100x more powerful than

wireless sensor(But, cell phone consumes 10-100x more energy)

Currently, some cell phones cost less than a wireless sensor!

Why? Scale of manufacturing & competition

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Introduction
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Software Problems

Challenges in Wireless Sensor Networks

well-known research problems

1 Localization: how does sensor know where it is?

2 Clock Synchronization: how does sensor know what time it is?

3 Power Conservation: how can sensors optimize battery life(and perhaps gain power from environment, from vibration,heat, sun)?

4 Routing: how can sensors cooperate to forward data to theworld?

5 Neighbor Discovery: how can sensors discover other sensors?

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Introduction
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Localization

Problem: find (x, y) or even (x, y, z) coordinates of sensors.(Maybe sensors are dropped from airplane, maybe put randomly insome area.)

Some Solution Ideas

1 maybe a few sensors can have GPS they are called anchornodes

2 sensing by ultrasound, by radar, or radio strength can estimatedistances between sensors

3 from distances, each sensor can build a local map

4 by messages between sensors, local maps are exchanged

5 put together local maps to get big map of all sensor locations

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Introduction


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Localization

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IntroductionT h i l P bl
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Clock Synchronization

Problem: each sensor has a clock, but how to make all clocks innetwork the same?

Some Solution Ideas

1 one sensor node is connected to internet, or has high-quality

hardware clock that is accurate2 messages carry time from accurate clock to nearby (neighbor)

sensors

3 neighbors tell their neighbors, etc

4 technical difficulty: messages may collide, then nobody getsmessage!

5 hardware radio inserts random delay, to reduce collisions6 but random delay makes clocks unsynchronized!

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IntroductionT h i l P bl s


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Synchronization

Best Case: random delay is known (access part is random amount)

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IntroductionTechnical Problems
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Synchronization

Other Case: random delay is discovered later

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Power Conservation and Harvesting

What Consumes or Supplies Power?

1 sensors, local computing, radio consume power

2

batteries supply limited power ( 1.2 Amp Hours typical)3 ultra low-power sleep modes available (no radio, no sensing)

sensor node could last for years, if mostly sleeping

4 duty-cycle alternate sleep, wake periods (coordinated innetwork)

5 active research: harvest solar power, energy from vibration,heat, . . .

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Routing

1 sensors must forward data to base station

2 for efficiency, sensor data can be aggregated in network

3 some research: sensor network is queried like SQL data base!

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Neighbor Discovery

Purpose: we look more in depth at one problem what if mostsensors use duty cycle to sleep most of time, to save power

How can a sensor know if a new sensor was added tonetwork? What happens if sensor has batteries replace?What if a sensor reboots and forgets who areneighbors?

Such events motivate neighbor discovery (hard, because radio isturned off most of time)

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Neighbor Discovery

BREAK OF PRESENTATION

BREAK OF PRESENTATION: A Puzzle!Puzzle of Election

1 n sensors, in line network (only communicate with limitedrange)

2 each sensor has ID from [0, 264

]3 all have synchronized clocks, starting from 0

4 puzzle: choose one sensor as leader using only n messages

5 solution done when every sensor knows ID of leader

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Neighbor Discovery

Methods of Neighbor Discovery

Survey of Techniques

Hardware Ideas

Wake by Radio (likeRFID) - messagesreceived wake up sensor

Tiny Samples - very briefwakeup to sample radiowaves

Software Ideas (ClassicalComputer Science) - use

patterns of wake/sleep todiscover awake neighbor

patterns must intersecteach other

assume radio on forknown interval (longenough for discovery)

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Neighbor Discovery

Basic Notions in Discovery Problem

Slotted Time (relax this later)

although node clocks may be unsynchronized, assume time isdivided into slots and the slots are aligned

Awake vs Asleep Slots

for each node, some slots are asleep (radio off), others are awake

Duty Cycle

over time, ratio of awake time to sleep time should be /1duty cycles of 5%, 1%, 0.02%, . . . desired for good lifetime(goals depend on application requirement)

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IntroductionTechnical ProblemsN i hb Di


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Neighbor Discovery

B-MAC: Radio Sampling

Radio Activity Sample (CCA)

(CCA designed for sensing to prevent collisions) CCA quick, low-power wakeup to sample radio Idea: if sample looks like probable activity, then wake fully &

read

not a feature of all radios (only a few)

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IntroductionTechnical ProblemsN i hb Di


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Neighbor Discovery

B-MAC Low-level Behavior

Leftmost peak is the sample, laterpeak is start of reading. If the sam-ple sees no radio activity, then crystaloscillator does not start.

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Neighbor Discovery

B-MAC Tradeoff

Problem of Sampling

The time taken after a sample, to turn on the radio ( 2ms)makes it too late to receive the message!

Solution: longer messages

1 change message format: increase the frame preamble (bytesfor synchronizing the start of a frame) enough to enablelisteners to receive the message

2 alternative: back-to-back duplicate frames sent3 implicit tradeoffs:

transmittors use more power, listeners use less power some bandwidth loss due to more bytes transmitted

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Neighbor Discovery

B-MAC Evaluation

B-MAC implemented in TinyOS as LPL (Low-PowerListening)

duty cycles reported in 0.52.5% range packet delivery success reported 98% (carefully tuned

sampling) better than previous MAC protocols using CTS/RTS (5-10%

duty cycles)

success depends on hardware factors (CCA, bandwidth,

transmission power costs) favors some application types over others (somecompute-sense tasks would interfere with real-time radiosampling; other applications need longer sleep periods andlower duty cycles)

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Neighbor Discovery

IEEE 802.11 Research

Slot types: AW (active window), BW (beacon window), MTIM(RTS, CTS, ACK, data)

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Neighbor Discovery

Intersecting Patterns

Power-Saving Modeintroduce sleeping as part of beacon interval (periodic) for dutycycle

unsynchronized nodes may never see each other!Simple Solution

Require AW > BI/2 + BW

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Quorum Solution

1 not all periods contain sleep intervals

2 for some parameter n, let 1/n of the periods be totally awake3 assumption: periods overlap enough for unsynchronized nodes

to see each other

4 n > space of node IDs

5 nodes use fixed quorum to choose all-awake intervals, basedon ID

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The Birthday Protocol

Fact: given enough persons, some two will probably have samebirthday

randomized sleeping

node states: T, L, S (transmit, listen, sleep)

modes: BLT, BL (birthday-listen-transmit or birthday-listen) each node randomly choses (depending on mode) which of T,

L, S to do

upon entry to BL: pt = 0, p = , ps = 1

p

upon entry to BLT: pt = t, p = , etcLike birthdays, high probability some are awake at same time(what to do after that . . . ?)

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Birthday Example

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Theory of Patterns

Model Schedules as Block Designs

each node has on/off schedule of T slots goal: discover neighbors within one scheduling period (one

duty cycle)

goal: deterministic, static cycle definition

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Theory of Patterns

definitions

schedule for node v is given by fv =T1

i=0 aixi where

ai {0, 1} for on/off, xi is slot number f

kv (x) = x

k

fv(x) mod (xT

1) cyclic shift by k slots C(u, v) = minj,k | fju fkv | the (worst case) overlap Problem: design f so that C(u, v) m for all u, v let kv = fv(1) (duty-cycle) for symmetric designs, ku = kv

for all u, v necessary condition: C(u, v) m km kv m T corollary: k m T (for symmetric designs)

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Theory of Patterns

Schedules follow a symmetric (v, k, ) design (node

row)

(Block Design theory found in most Discrete MathematicsTextbooks 36/46



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Theory of Patterns

considerations for block design protocol

fast, deterministic discovery

can tune amount of overlap desired in design but, design requires knowing number of nodes each node needs individual pattern patterns irregular (difficult for some hardware/applications)

higher duty cycle than other protocols: (T) (note, T hasto be large enough to accommodate number of nodes)

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Discovery using Primes

The DISCO Protocol

1 wake schedule based on Chinese Remainder Theorem (note:schedules based on primes are in literature of wakeup problem)

2 choose two primes p, q such that 1/p+ 1/q duty cycle3 cycle length at least max(p, q)

4 number slots of cycle 1,2,3,. . . , then wake in slot i wheni mod p = 0 or i mod q = 0 (in a cycle, a node may wake

more than once)5 eventually, neighbors must wake in common slot

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discovery rate may not be uniform ?!

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Temporal Partition

no-cost vs probing

no-cost protocols: each cycle has awake and sleep intervals

(each period is contiguous, unlike several other schemes) no-cost schedules based on primes selection of primes randomized (periodically) to assure

intersection

probing protocols add additional awake slots to speed updiscovery

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Temporal Partition

no-cost protocol

parameter S is upper bound on how long a node may sleep duty cycle is 1/z for z S each synchronized group of nodes act identically duty cycle determined by max(p, q), for primes p, q each synchronized group pseudorandomly chooses from {p, q} choice is held for 2z cycles (after that, choose again)

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Temporal Partition

no-cost protocol results

discovery of neighbor occurs in O(z2) slot times discovery for graph occurs in O(diam z2) slots times

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Temporal Partition

extra-cost protocols

each cycle can have two awake intervals, normal andprobing

probing interval consists of c consecutive slots trivial: c> z/2 assures fast discovery but with smaller c, need to increment start position in each

cycle

duty cycle is (1 + c)/z (normal is 1 slot) discovery upper bound is O(z2/c) slot times

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Temporal Partition

randomized probing protocols

use one extra slot for probing selection of probing slot is random (per node) in each cycle result for clique topology: expected discovery of all nodes is

O((log z + log log n) z) slot times

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END OF PRESENTATION

END OF PRESENTATION: A Puzzle

1 There are 5 doors, numbered 1, 2, 3, 4, 5

2 Behind each door is a room doors are closed

3 Each door is labeled by its number, on both sides

4 Two persons are in different rooms; other rooms are empty

5 Operation: person can send a message to any numbered room

6 Time is synchronous, starting with 1, then 2, 3, . . . with bothpersons starting at time 1

Challenge: devise a protocol guaranteeing that one person willsend a message to the other with at most 3 total messages (not 3per person, but 3 in total)

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