Wireless Sensor Networks COE 499 Design Key Challenges

04/21/23

Wireless Sensor Networks COE 499

Design Key ChallengesTarek Sheltami

KFUPMCCSECOE

http://faculty.kfupm.edu.sa/coe/tarek/COE499.htm

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Outline

WSN Basic Components Key Design Challenges

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WSN Basic Components

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1. Low-Power Embedded Processor Significantly constrained in terms of

computational power Run specialized component-based embedded

operating system, such as TinyOS May include nodes with greater computational

power due to heterogeneity Nodes incorporate advanced low-power design

techniques, such as efficient sleep modes and dynamic voltage scaling to provide significant energy savings

WSN Basic Components..

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2. Memory/Storage Storage in the form of random access and read only

memory includes both program memory and data memory

The memory and storage on board are often limited but most likely to improve over time

3. Radio Transceiver Low-rate, short range wireless radio (10-100kbps,

<100m), but expected to improve over time Radio communication is the most power intensive

operation and hence must incorporate energy efficient sleep and wakeup modes


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4. Sensors BW is very limited, so only low data rate

applications are supported Due to multi-model sensing, some devices my

have several sensors on board Sensors used are highly dependant on the

application


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5. Geopositioning System Location is very important for sensor measurement The simplest way to obtain positioning is to pre-

configure sensor location at deployment, but this is not the case in many applications

WSN is mostly deployed in ad hoc fashion for outdoor operations, where fraction of the sensor nodes may be equipped with GPS

When some nodes equipped with GPS, other nodes must obtain their locations indirectly through network localization algorithms


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6. Power Sources WSN devices are battery powered for flexibility Some fixed nodes may be wired to a continuous

power source in some applications Energy harvesting techniques may provide a

degree of energy renewal in some cases The finite battery energy, which is almost

always the case in WSN, is the most critical resource bottleneck in most WSN applications


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In a basic data-gathering applications, there is a node referred to as the sink to which all data from source sensor nodes are directed

The simplest logical topology for communication of gathered data is a single hop star topology, where all nodes send their data directly to the sink

In large area, a multi-hop tree structure may be used for data-gathering, in this case some nodes must act as routers


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1. Energy Efficiency2. Responsiveness3. Robustness4. Self-Configuration and

Adaptation5. Scalability6. Heterogeneity7. Systematic Design8. Privacy and Security

Key Design Challenges

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1. Extended Lifetime WSN devices are severely energy constrained due

to limitation of batteries A typical alkaline battery provides about 50 watt-

hours of energy, which lasts to less than a month of continuous operation for each node in full active mode

Replacing batteries for a large scale network is very expensive and infeasible

In many applications, it is necessary to provide guarantee that a network of unattended wireless sensors can remain operational for several years

Design Key Challenges..

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1. Extended Lifetime.. Hardware improvements in battery design and energy

harvesting will offer only partial solutions As a result, most protocols are design explicitly with

energy efficient as a primary goal

2. Responsiveness One simple solution to extending network lifetime is to

coordinate the efforts by switching sleep and wakeup modes periodically

Synchronizing such sleep schedules is challenging in itself Long sleep periods can reduce the responsiveness and

effectiveness of the sensor


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3. Robustness WSN is supposed to provide large-scale and fine

grained coverage using large numbers of inexpensive devices

However, inexpensive devices can often be unreliable and prone to failures, especially if deployed in harsh or hostile environment

Therefore, protocols designers must have a built-in mechanisms to provide robustness

Performance of the network shouldn’t be sensitive to individual devices failures


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4. Synergy Moore’s law-type advances in technology have ensured

that devices capabilities in terms of processing power, memory, storage, radio transceiver performance and even accuracy of sensing improve rapidly (given a fixed cost)

The challenge is to design synergistic protocols with ensure that the system as a whole is more capable than sum of the capabilities of its individual components

The protocol must provide as efficient collaborative use of storage, computation and communication resources


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5. Scalability Protocols have to be inherently distributed, involving

localized communication, and sensor network must utilize hierarchical architectures in order to provide such scalability

6. Heterogeneity Can have a number of important design consequences The presence of a small number of devices of higher

computational capability along with a large number of low-capability devices can dictate a two-tier cluster-based network architecture


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7. Systematic Design There is a challenging tradeoff between ad hoc and

more flexible, easy-to-organize design methodologies that sacrifice some performance

Given severe resources constraints in WSN, systematic design methodologies are necessitated by practical considerations

8. Privacy and Security The large scale, prevalence and sensitivity of

information collected by WSN give rise to both privacy and security


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Sensor Network Challenges Low computational power Current mote processors run at < 10 MIPS (Million

instructions per second) Not enough horsepower to do real signal processing Memory not enough to store significant data Poor communication bandwidth, current radios achieve

about 10 Kbps per mote Note that raw channel capacity is much greater

Overhead due to CSMA backoff, noise floor detection, start symbol, etc.

802.15.4 (Zigbee) radios now available at 250 Kbps But with small packets one node can only transmit

around 25 kbps

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Sensor Network Challenges.. Limited energy budget 2 AA motes provide about 2850 mAh Coin-cell Li-Ion batteries provide around 800

mAh Solar cells can generate around 5 mA/cm2 in

direct sunlight Must use low duty cycle operation to extend

lifetime beyond a few days

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Portable, energy-efficient devices End-to-end quality of service Seamless operation under context

changes Context-aware operation Secure operation Sophisticated services for simple

clients

Sensor Network Challenges..

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Unique Aspects

Number of sensor nodes can be many orders of magnitude larger than number of nodes in an ad hoc network

Tens of thousands. But individual ID might not be needed.

Sensors might be very small, cheap, and prone to failure. Therefore, we need redundancy.

Extremely limited in power, and must stay operative for long time

Energy harvesting might be considered. Sensors might be densely deployed.

Opportunity for using redundancy to improve the robustness of the system

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Unique Aspects .. Very limited mobility

Helps with the design of the protocols Measurements might be correlated.

Example: measurements of temperature, pressure, humidity, etc.

Volume of transmitted data might be greatly reduced.

For many applications, nodes are randomly deployed. Thrown by a plane, carried by wind, etc.

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Location-dependent Information

Changing context small movements may cause large changes caching may become ineffective dynamic transfer to nearest server for a

service

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Portability Power is key

long mean-time-to-recharge, small weight, volume Risk to data due to easier privacy breach

network integrated terminals with no local storage Small user interfaces

small displays, analog inputs (speech, handwriting) instead of buttons and keyboards

Small storage capacity data compression, network storage, compressed

virtual memory, compact scripts vs. compiled code

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Low Power & Energy-awareness

Battery technology is a hurdle… Typical laptop: 30% display, 30% CPU, 30% rest

wireless communication and multimedia processing incur significant power overhead

Low power circuits, architectures, protocols

Power management Right power at the right place at the right time

Battery model

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Low Power & Energy-awareness..

There are many means for powering nodes, although the reality is that various electrical sources are by far the most convenient.

Technology trends indicate that within the lifetime of CENS, nodes will likely be available that could live off ambient light.

However, this cannot be accomplished without aggressive energy management at many levels; continuous communications alone would exceed the typical energy budgets.

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Sensor Node Energy Roadmap

20022002 2004200420002000

10,00010,000

1,0001,000

100100

1010

11

..11

Ave

rag

e P

ow

er

(mW

)

• Deployed (5W)

• PAC/C Baseline (.5W)

•( 50 mW)

(1mW)

Rehosting to Rehosting to Low Power Low Power COTSCOTS (10x)(10x)

-System-On-Chip-System-On-Chip-Adv Power -Adv Power ManagementManagementAlgorithms (50x)Algorithms (50x)

Source: ISI & DARPA PAC/C Program

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Battery Technology

Battery technology has historically improved at a very slow pace

NiCd improved by x2 over 30 years! require breakthroughs in chemistry

Battery Rechargeable? Gravimetric Density(Wh/lb)

Volumetric Density(Wh/l)

Alkaline-MnO2(typical AA)

NO 65.8 347

Silver oxide NO 60 500Li/MnO2 NO 105 550Zinc Air NO 140 1150NiCd YES 23 125Li-Polymer YES 65-90 300-415

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Computation & Communication

Radios benefit less from technology improvements than processors

The relative impact of the communication subsystem on the system energy consumption will grow

TransmitReceive

Encode DecodeTransmit

Receive

EncodeDecode

Energy breakdown for voiceEnergy breakdown for MPEG

Processor: StrongARM SA-1100 at 150 MIPSRadio: Lucent WaveLAN at 2 Mbps

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Key Issue: Resource Awareness

Ad-hoc architectureSelf-configuration

Wireless communications Variability

Inherent unpredictability

Solution: adaptation

Select required performance level Operate always at peak performance

Settings based on external conditions

Fixed settings set by worst case conditions

Resource awareness“right resource at the right time and the right place”

Wireless Backbone Networks High traffic load Limited available spectrum

Focus on transmission resources

Wireless Ad-Hoc Networks Unattended operation Limited available battery

Focus on energy resources04/21/23 29

Documents

Wireless Sensor Networks COE 499 Design Key Challenges