Medium Access Control

MAC layer covers three functional areas: reliable data delivery access control security

Medium Access Control

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).

DCF and PCF  DCF: fundamental access method of IEEE MAC, implemented in all STAs, known as CSMA/CA

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.

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.

DCF MAC Requirements  To avoid interference among simultaneous transmissions  But enable as many non-interfering transmission as possible  Maintain fairness among transmissions

DCF MAC Requirements  No centralized coordinators: fully distributed operations  No clock synchronization: asynchronous operations

WIRELESS SENSOR NETWORK (WSN)

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

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

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.

WSN communication architecture

Components of sensor node

Protocol Stack  Protocols should be  Power aware  Location aware  Application aware

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.

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.

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.

Application of WSN

 The applications can be divided in three categories: 1.Monitoring of objects. 2.Monitoring of an area. 3.Monitoring of both area and objects.

Monitoring area  Environmental and Habitat Monitoring  Precision Agriculture  Indoor Climate Control  Military Surveillance  Intelligent Alarms

Monitoring object  Structural Monitoring  Eco-physiology  Condition-based Maintenance  Medical Diagnostics  Urban terrain mapping

Monitoring Interaction b/n objects and space  Wildlife Habitats  Disaster Management  Emergency Response  Ubiquitous Computing  Asset Tracking  Health Care

Characteristics of WSN  Wireless Sensor Networks mainly consists of sensors. Sensors are -  low power  limited memory  energy constrained due to their small size.

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.

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.

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.

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.

Operational Challenges  Energy Efficiency  Limited storage and computation  Low bandwidth and high error rates

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