Wireless Sensor Networks COE 499 Design Key Challenges

1. Wireless Sensor Networks COE 499Design Key Challenges Tarek Sheltami KFUPM CCSE COE http://faculty.kfupm.edu.sa/coe/tarek/COE499.htm

2. Outline • WSN Basic Components • Key Design Challenges

3. WSN Basic Components

4. WSN Basic Components.. • 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

5. WSN Basic Components.. • 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 • 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

6. WSN Basic Components.. • 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

7. WSN Basic Components.. • 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

8. WSN Basic Components.. • 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

9. WSN Basic Components.. • 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

10. Key Design Challenges • Energy Efficiency • Responsiveness • Robustness • Self-Configuration and Adaptation • Scalability • Heterogeneity • Systematic Design • Privacy and Security

11. Design Key Challenges.. • 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

12. Design Key Challenges.. • 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 • 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

13. Design Key Challenges.. • 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

14. Design Key Challenges.. • 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

15. Design Key Challenges.. • Scalability • Protocols have to be inherently distributed, involving localized communication, and sensor network must utilize hierarchical architectures in order to provide such scalability • 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

16. Design Key Challenges.. • 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 • Privacy and Security • The large scale, prevalence and sensitivity of information collected by WSN give rise to both privacy and security

17. 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

18. 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

19. Sensor Network Challenges.. • 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

20. 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

21. 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.

22. Location-dependent Information • Changing context • small movements may cause large changes • caching may become ineffective • dynamic transfer to nearest server for a service

23. 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

24. 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

25. 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.

26. Source: ISI & DARPA PAC/C Program Sensor Node Energy Roadmap 10,000 1,000 100 10 1 .1 Rehosting to Low Power COTS (10x) • Deployed (5W) • PAC/C Baseline (.5W) Average Power (mW) • (50 mW) -System-On-Chip -Adv Power Management Algorithms (50x) (1mW) 2002 2004 2000

27. Battery Technology • Battery technology has historically improved at a very slow pace • NiCd improved by x2 over 30 years! • require breakthroughs in chemistry

28. Computation & Communication Energy breakdown for MPEG Energy breakdown for voice Decode Transmit • Radios benefit less from technology improvements than processors • The relative impact of the communication subsystem on the system energy consumption will grow Decode Encode Encode Receive Receive Transmit Radio: Lucent WaveLAN at 2 Mbps Processor: StrongARM SA-1100 at 150 MIPS

29. Key Issue: Resource Awareness Inherent unpredictability Wireless Backbone Networks • High traffic load • Limited available spectrum Focus on transmission resources Solution: adaptation Resource awareness “right resource at the right time and the right place” • Wireless Ad-Hoc Networks • Unattended operation • Limited available battery • Focus on energy resources