EE-MAC PROTOCOL FOR WSN
By,
Aswathi R Nair
S7 Applied Electronics and Instrumentation
College Of Engineering, Trivandrum
2007-11 batch


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overview
WSN

KEY PERFORMANCE METRICS

IEEE 802.11 DCF MAC PROTOCOL

CC-MAC PROTOCOL

ADVANTAGES OF THE ENHANCED VERSION

WHAT IS A WSN?
WHERE IS IT USED?
WITHIN A SENSOR NODE
Sensor nodes - low power devices equipped with one or more sensors, a processor, memory, a power supply, a radio, and an actuator.

HOW IT WORKS
KEY PERFORMANCE METRICS
Energy efficiency: Due to the limited capacity of the sensor nodes’ batteries and to improve the network lifetime.


Reliability : The accuracy of the reported information.


Latency: The maximum tolerable time to report an event.

THE CURRENT SCENARIO
Energy-efficient MAC protocols such as IEEE 802.11 distributed coordination function (DCF) with its optional request-to-send/clear-to-send (RTS/CTS) scheme.

Takes care of energy wastage due to collisions and idle listening.

Enhanced version takes care of redundant packet transmission using representative nodes.

IEEE 802.11 DCF MAC PROTOCOL
HOW THE PROTOCOL WORKS
Based on the CSMA/CA technique.

The N active reporting nodes compete to access the common data channel to report the event to the sink.

After DIFS and back off it sends RTS, followed by CTS, DATA and ACK each frame after an SIFS.
WHY A NEW PROTOCOL?
Supplementary energy conservation


Additional flexibility to attain desired reliability.


Also ensures maximum tolerated latency

THE KEY IDEA
Number of active reporting nodes N < Nmin decreases energy wastage due to collision and idle listening.

Associated number of reports R(N) > Nmin requires more energy to report the event.

The protocol tries to optimize N and R(N) so as to obtain required event detection reliability.


AN EXAMPLE OF A SENSOR NETWORK
INFORMATION RELIABILITY Vs. NO. OF REPORTING NODES
INFERENCES FROM GRAPH
Distortion decreases with the increase in r, but distortion becomes relatively constant when the transmitted data to the sink becomes highly redundant.

The lower the value of N, the greater the number of reports R(N) required to attain the same reliability.

For each distortion value, there exist a minimum number of reporting nodes to achieve the desired information reliability.

THE ENHANCED IEEE PROTOCOL
To support the Reporting Nodes Selection Scheme

The CC-MAC protocol

The optimal number of reporting nodes is to be broadcast to all the sensor nodes.

The selection process is based on correlation radius Rcorr, which indicates the average distance allowable between selected representative nodes.

HOW IT WORKS
All the sensor nodes contend for the medium access.

Once a sensor node accesses the medium, all the other nodes within the Rcorr radius stop their transmission attempt and undergo the sleep mode.

The selection process is executed until all the representative nodes are selected.

N*pi*Rcorr(N)2= pi* Rc2

ADVANTAGES OF THE ENHANCED VERSION
Simplicity since it needs no modifications in the existing DCF MAC protocol.

It only introduces an additional mechanism to limit the medium access to a small subset of N nodes.

Ntot sensor nodes choose their representatives based on the information about N sent by the sink, without requiring any explicit internode communication.
AVERAGE AMOUNT OF ENERGY CONSUMED BY THE WSN PER UNIT TIME AND AVERAGE TIME REQUIRED TO REPORT RELIABLY AN EVENT
SENSOR NETWORK LIFETIME Vs. NO. OF REPORTING NODES
CONCLUSION
The optimal way of consuming energy is to activate only Nopt energy reporting nodes. Activating more or less than Noptenergy sensor nodes increases energy expenditure.

The adjustment of the N parameter and the choice of the appropriate regime in order to achieve energy-reliability-latency trade-offs depends on the specific QoS requirement of the WSN application, i.e., whether more priority is given to latency or energy constraints.

FUTURE ENHANCEMENTS
For instance this paper has not addressed the scenario where multiple events occur concurrently in the network.

Also this paper has not considered the mobility of the same event over time. e.g., mobile target detection applications.

REFERENCES
Fatma Bouabdallah, Nizar Bouabdallah, And Raouf Boutaba, Senior Member, IEEE, ‘Toward Reliable And Efficient Reporting In Wireless Sensor Networks’ IEEE Transactions On Mobile Computing, Vol. 7, No. 8, August 2008.
A Survey On Sensor Networks By Ian F. Akyildiz, Weilian Su, Yogesh Sankarasubramaniam, And Erdal Cayirci Georgia Institute Of Technology, IEEE Communications Magazine August 2008
A MAC Protocol To Reduce Sensor Network Energy Consumption Using A Wakeup Radio Matthew J. Miller, Student Member, IEEE, And Nitin H. Vaidya, Senior Member, IEEE, IEEE Transactions On Mobile Computing, VOL. 4, No. 3, May/June 2008
Medium Access Control With Coordinated Adaptive Sleeping for Wireless Sensor Networks Wei Ye, Member , IEEE, John Heidemann, Member , IEEE, and Deborah Estrin, Fellow, IEEE, IEEE/ACM Transactions On Networking, VOL. 12, NO. 3, JUNE 2009



DISTORTION
D(N,r)= σS 2 – (σ S 4 / r*(σS 2 + σN2 )){2*∑ρ(s,n(k))-1} + (σ S 6 / r2*(σS 2 + σN2 )2)∑∑ρ((n(k),n(m))


applications
AVERAGE TIME TO REPORT AN EVENT Vs. REPORTING FREQUENCY
PERFORMANCE EVALUATION
AVERAGE TIME TO REPORT AN EVENT Vs. NO. OF REPORTING NODES
AVERAGE AMOUNT OF ENERGY CONSUMED Vs. NO. OF REPORTING NODES