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Published bynatan belay Modified over 7 years ago
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Medium Access Control
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MAC layer covers three functional areas: reliable data delivery access control security
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Medium Access Control
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DCF and PCF PCF and DCF operate concurrently within the same BSS. The two access methods alternate, with a contention-free period (CFP) followed by a contention period (CP).
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DCF and PCF DCF: fundamental access method of IEEE 802.11 MAC, implemented in all STAs, known as CSMA/CA
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PCF Supports time-bounded services. Lets stations to have priority access to the wireless medium. Polling stations one by one (centralized operation) Coordinated by Point Coordinator (PC), typically collocated with the AP.
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PCF PCF has higher priority than the DCF. Beacon frame is a management frame that maintains the synchronization of the timers in the stations and delivers protocol related parameters.
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DCF MAC Requirements To avoid interference among simultaneous transmissions But enable as many non-interfering transmission as possible Maintain fairness among transmissions
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DCF MAC Requirements No centralized coordinators: fully distributed operations No clock synchronization: asynchronous operations
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WIRELESS SENSOR NETWORK (WSN)
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Definition A sensor network is composed of a large number of sensor nodes that are densely deployed inside or very close to the phenomenon random deployment self-organizing capabilities
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Definition – Cont… Sensor nodes are small, low-cost, low- power devices that have following functionality: communicate on short distances sense environmental data perform limited data processing
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Definition – Cont… Network usually also contains “sink” node which connects it to the outside world. Wireless sensor networks consists of protocols and algorithms with self- organizing capabilities.
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WSN communication architecture
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Components of sensor node
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Protocol Stack Protocols should be Power aware Location aware Application aware
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Comparison of WSN & Ad hoc network Wireless sensor networks mainly use broadcast communication while ad hoc networks use point-to-point communication. Unlike ad hoc networks wireless sensor networks are limited by sensors limited power, energy and computational capability.
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Comparison of WSN & Ad hoc network – Cont… Sensor nodes may not have global ID because of the large amount of overhead and large number of sensors.
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Application of WSN They have an ever-increasing number of applications, such as infrastructure protection and security, surveillance, health-care, environment monitoring, food safety, intelligent transportation, and smart energy.
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Application of WSN
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The applications can be divided in three categories: 1.Monitoring of objects. 2.Monitoring of an area. 3.Monitoring of both area and objects.
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Monitoring area Environmental and Habitat Monitoring Precision Agriculture Indoor Climate Control Military Surveillance Intelligent Alarms
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Monitoring object Structural Monitoring Eco-physiology Condition-based Maintenance Medical Diagnostics Urban terrain mapping
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Monitoring Interaction b/n objects and space Wildlife Habitats Disaster Management Emergency Response Ubiquitous Computing Asset Tracking Health Care
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Characteristics of WSN Wireless Sensor Networks mainly consists of sensors. Sensors are - low power limited memory energy constrained due to their small size.
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Characteristics of WSN – Cont… Wireless networks can also be deployed in extreme environmental conditions and may be prone to enemy attacks. Although deployed in an ad hoc manner they need to be self organized and self healing and can face constant reconfiguration.
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Design Challenges Heterogeneity The devices deployed maybe of various types and need to collaborate with each other. Distributed Processing The algorithms need to be centralized as the processing is carried out on different nodes.
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Design Challenges – Cont… Low Bandwidth Communication The data should be transferred efficiently between sensors Large Scale Coordination The sensors need to coordinate with each other to produce required results.
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Design Challenges – Cont… Utilization of Sensors The sensors should be utilized in a ways that produce the maximum performance and use less energy. Real Time Computation The computation should be done quickly as new data is always being generated.
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Operational Challenges Energy Efficiency Limited storage and computation Low bandwidth and high error rates
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Operational Challenges – Cont… Errors are common Wireless communication Noisy measurements Node failure are expected Scalability to a large number of sensor nodes Survivability in harsh environments
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