Sensor Network Overview

Taekyoung Kwon

tk@mmlab.snu.ac.kr

For starters

• The problems of engineering education– Problem solving– English– Communication skills

For starters

• What you can achieve by taking this course– Problem solving

• Problem definition– Topics in the wireless/sensor network

• Idea• Verify/evaluate

– sensor network • Ubiquitous computing• standardization

Evolution (size and number)

Confluence of technologies

Ubiquitous computing

• 21st century computers– Embedded in our world (ubiquitous, pervasive)

• They weave themselves into the fabric of everyday life until they are indistinguishable from it

[Mark Weiser, 1991]• The anti-thesis of “virtual reality”• Like motor technology, embedding computers

everywhere and having them “disappear in the background” is easy

Wired vs. wireless

• Bandwidth

• Reliability

• CSMA/CD vs CSMA/CA

Wireless networks

• Wireless network ad hoc network

• Ad hoc network sensor network?

• Wireless WAN: Cellular

• Wireless MAN: IEEE 802.16

• Wireless LAN: IEEE 802.11 series

• Wireless PAN: IEEE 802.15 family

What is sensor?

• Sensor: a transducer that converts a physical, chemical, or biological parameter into an electrical signal

• Actuator: a transducer that accepts an electrical signal and converts it into a physical, chemical, or biological action

• Transducer: a device converting energy from one domain into another. The device may either be a sensor or an actuator

Sensor network

• Tens of thousand nodes– Densely deployed

Internet, Internet, Satellite, Satellite, etcetc

Sink

Sink

TaskManager

Sensor node hardware

Power UnitPower Unit

Sensor ADCProcessorProcessor

MemoryMemoryTransceiverTransceiver

Location Finding SystemLocation Finding System MobilizerMobilizer• Small

• Low power

• Low bit rate

• High density

• Low cost (dispensable)

• Autonomous

• Adaptive

Sensor network• Power constraint

– Battery powered mains powered– Energy harvest

• Light(solar), vibration, temperature

• Tradeoff between energy and QoS– Prolong network lifetime by sacrificing application

requirements• Delay, throughput, reliability, data fidelity,…

– Still QoS is attractive• Deterministic or probabilistic bound

Sensor network• Traffic type: streaming, periodic, event• Low cost, Low bit rate, low duty cycle• IEEE 802.15.4: 250Kbps

Application LayerApplication Layer

Transport LayerTransport Layer

Network LayerNetwork Layer

Data Link LayerData Link Layer

Physical LayerPhysical Layer

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Ad hoc vs. sensor• Number of sensor nodes can be several orders of

magnitude higher• Sensor nodes are densely deployed and are prone to

failures• The topology of a sensor network changes very

frequently due to node mobility and node failure• May leverage broadcasting than point-to-point

communications• May operate in aggregate fashion• In-network processing• Sensor nodes are limited in power, computational

capacities, and memory• May not have global ID like IP address• Need tight integration with sensing tasks

Design issues• Fault tolerance

– Battlefield application

• Scalability– Node density: (NR^2)/A (transmission)

• Production costs• Hardware constraints• Topology

– Deployment phase– Post-deployment phase

• Environment• Transmission media: ISM, IR• Power consumption: sensing, processing,

communication

PHY layer

• Sync• Self-organization

– Beacon scheduling (periodic)

• Directional/smart antenna• Ultra-wideband (UWB)• Transmit-only device

– pros: cost, energy– Cons: uncontrollable, communications/networking

overhead

MAC layer• TDMA vs. CSMA

– TDMA: inter-cluster, scalability– CSMA: idle listening, overhearing

• Sleep cycle• Coordination

– Spatial correlation– Clustering (MAC vs NWK)

• Additional control channel– FDMA or TDMA

• Location awareness– Exposed terminal problem

network layer

• Attribute-based addressing– Information-centric delivery

• Routing– Route discovery

• Data aggregation/coordination• Location awareness

– Directional antenna (AOA)– UWB (distance measure via signal flight time)– GPS

routing

• Route discovery (AODV, DSR,…)– Route selection metric: hop count– Metric can be generalized to cost

• Hierarchical tree routing

• Gradient routing: data broadcasting

Transport layer

• Goodput decreases drastically as the offered traffic exceeds the network capacity

• Flow control vs. Congestion control– open loop vs closed loop– Proactive vs. reactive

Transport layer

• Reliability concept should be relaxed– Event-to-sink reliability

• Not all event-sensing nodes need to report• N reception among M transmission might be

OK (M > N)

• Hop-by-hop approaches

Middleware/Language/Appl.

• query/advertisement – Publish/subscribe

• nesC, Mate, SQTL – Declarative rather than procedural– TEDS (IEEE 1451)

Some of the commercial applications

– Industrial automation (process control)

– Defense (unattended sensors, real-time monitoring)

– Utilities (automated meter reading),

– Weather prediction

– Security (environment, building etc.)

– Building automation (HVAC controllers).

– Disaster relief operations

– Medical and health monitoring and instrumentation

What to consider: application requirements

• Energy-saving• QoS

– Throughput/Goodput– Reliability– timeliness

• Traffic/application scenario– Amdahl’s law– Every possible case

• Self-organization

What to consider: enabling technologies

• Directional (smart, MIMO) antenna– Multi-hop reachability– AoA– Hidden node problem

• Heterogeneous node type– E.g., Transmit-only device

• GPS: too costly• UWB (distance measurement)

– Location aware

• Energy harvesting device• Additional (separate) control channel

Possible approaches• Conservative vs. aggressive• Pessimistic vs. opportunistic vs. optimistic• Proactive (a priori) vs reactive (on demand)• Information amount vs. performance (better

control/decision)– History– Neighbors within some hops

• Deterministic (e.g. threshold) vs. probabilistic– N * p = 1?

• Reservation vs. random access• Heterogeneous functionalities

– E.g, cluster head, member

Possible enhancements:

• Flexibility vs. efficient – adaptivity

• Stability vs. throughput (utilization)– Goodput

• Reliable vs. fault-tolerant vs. error-resilient vs. robust

• fairness• Legacy-system support, standard-compliant,

backward compatibility

Final goal

• Tradeoff

• Quantitative trend

• Qualitative feature

• How to verify?– Analysis– Simulation – Implementation

analysis

• assumptions

• Whole system vs key element

• Steady state probability

• Upper/lower bound

• Worst/average case

• Complexity: O()– Temporal vs. spatial

Simulation

• Arbitrary level of detail

• Still too many ambiguities– Follow the norm, other reference

• How to emphasize the strength?

• Also show the weakness

Implementation

• Most time and energy consuming

• Good luck!

Leverage other techniques

• Algorithm • Combination theory• AI

– e.g., self-learning

• Operations Research– optimization

• Network Flow, scheduling theory• Probability

– Queuing theory

Let’s make team!