Wireless Sensor Networks COE 499 Design Key Challenges

Wireless Sensor Networks COE 499 Design Key Challenges Tarek Sheltami KFUPM CCSE COE http: //faculty. kfupm. edu. sa/coe/tarek/COE 499. htm 2/24/2021 1

Outline n n WSN Basic Components Key Design Challenges 2/24/2021 2

WSN Basic Components 2/24/2021 3

WSN Basic Components. . 1. Low-Power Embedded Processor § § Significantly constrained in terms of computational power Run specialized component-based embedded operating system, such as Tiny. OS 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 2/24/2021 4

WSN Basic Components. . 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 -100 kbps, <100 m), but expected to improve over time Radio communication is the most power intensive operation and hence must incorporate energy efficient sleep and wakeup modes 2/24/2021 5

WSN Basic Components. . 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 2/24/2021 6

WSN Basic Components. . 5. Geopositioning System § § Location is very important for sensor measurement The simplest way to obtain positioning is to preconfigure 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 2/24/2021 7

WSN Basic Components. . 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 2/24/2021 8

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 2/24/2021 9

Key Design Challenges 1. 2. 3. 4. 5. 6. 7. 8. Energy Efficiency Responsiveness Robustness Self-Configuration and Adaptation Scalability Heterogeneity Systematic Design Privacy and Security 2/24/2021 10

Design Key Challenges. . 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 2/24/2021 11

Design Key Challenges. . 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 2/24/2021 12

Design Key Challenges. . 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 2/24/2021 13

Design Key Challenges. . 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 2/24/2021 14

Design Key Challenges. . 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 lowcapability devices can dictate a two-tier cluster-based network architecture 2/24/2021 15

Design Key Challenges. . 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 2/24/2021 16

Sensor Network Challenges n n n 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. n n 802. 15. 4 (Zigbee) radios now available at 250 Kbps But with small packets one node can only transmit around 25 kbps 2/24/2021 I-17

Sensor Network Challenges. . n n n Limited energy budget 2 AA motes provide about 2850 m. Ah Coin-cell Li-Ion batteries provide around 800 m. Ah Solar cells can generate around 5 m. A/cm 2 in direct sunlight Must use low duty cycle operation to extend lifetime beyond a few days 2/24/2021 I-18

Sensor Network Challenges. . n n n 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 2/24/2021 19

Unique Aspects n n Number of sensor nodes can be many orders of magnitude larger than number of nodes in an ad hoc network n Tens of thousands. n But individual ID might not be needed. Sensors might be very small, cheap, and prone to failure. n Therefore, we need redundancy. Extremely limited in power, and must stay operative for long time n Energy harvesting might be considered. Sensors might be densely deployed. n Opportunity for using redundancy to improve the robustness of the system 2/24/2021 I-20

Unique Aspects. . n n n Very limited mobility n Helps with the design of the protocols Measurements might be correlated. n Example: measurements of temperature, pressure, humidity, etc. n Volume of transmitted data might be greatly reduced. For many applications, nodes are randomly deployed. n Thrown by a plane, carried by wind, etc. 2/24/2021 I-21

Location-dependent Information n Changing context n small movements may cause large changes n caching may become ineffective n dynamic transfer to nearest server for a service 2/24/2021 22

Portability n Power is key n n Risk to data due to easier privacy breach n n network integrated terminals with no local storage Small user interfaces n n long mean-time-to-recharge, small weight, volume small displays, analog inputs (speech, handwriting) instead of buttons and keyboards Small storage capacity n 2/24/2021 data compression, network storage, compressed virtual memory, compact scripts vs. compiled code 23

Low Power & Energy-awareness n n Battery technology is a hurdle… Typical laptop: 30% display, 30% CPU, 30% rest n n Low power n n circuits, architectures, protocols Power management n n wireless communication and multimedia processing incur significant power overhead Right power at the right place at the right time Battery model 2/24/2021 24

Low Power & Energy-awareness. . n n n 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. 2/24/2021 25

Source: ISI & DARPA PAC/C Program Sensor Node Energy Roadmap Average Power (m. W) 10, 000 Rehosting to Low Power COTS (10 x) Deployed (5 W) • 1, 000 PAC/C Baseline (. 5 W) • 100 (50 m. W) • 10 1 v (1 m. W) -System-On-Chip -Adv Power Management Algorithms (50 x) . 1 2002 2/24/2021 20042000 26

Battery Technology n Battery technology has historically improved at a very slow pace n n Ni. Cd improved by x 2 over 30 years! require breakthroughs in chemistry 2/24/2021 27

Computation & Communication Energy breakdown for voice Encode Decode Receive Energy breakdown for MPEG Encode Transmit Decode Transmit Receive Radio: Lucent Wave. LAN at 2 Mbps Processor: Strong. ARM SA-1100 at 150 MIPS n n Radios benefit less from technology improvements than processors The relative impact of the communication subsystem on the system energy consumption will grow 2/24/2021 28

Key Issue: Resource Awareness Inherent unpredictability Ad-hoc architecture Self-configuration Wireless communications Variability 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 n n High traffic load Limited available spectrum Focus on transmission resources 2/24/2021 Wireless Ad-Hoc Networks u u Unattended operation Limited available battery Focus on energy resources 29