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Wireless Sensor Networks The MAC sublayer

Wireless Sensor Networks The MAC sublayer. Supervised by professor: Ivan Stojmenovic Done by: Yaser Fouad. Outline. Introduction The Medium Access Control sublayer Problem definition different Medium Access protocols overview Conclusion References. Characteristics of sensor networks.

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Wireless Sensor Networks The MAC sublayer

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  1. Wireless Sensor NetworksThe MAC sublayer Supervised by professor: Ivan Stojmenovic Done by: YaserFouad

  2. Outline • Introduction • The Medium Access Control sublayer • Problem definition • different Medium Access protocols overview • Conclusion • References

  3. Characteristics of sensor networks • heavily deployment • Limited power • Changing network topology • Multihop routing and low range communications • Non uniform data distribution

  4. The MACsublayer • Medium Access Control • Subdivision of the data link layer • Different from Multiple access • Measures of Performance for MAC protocols • Throughput efficiency • Stability • Scalability • Access delay • Power conservation

  5. The MAC sublayer for WSN • Changes due to WSNs characteristics: • Throughput efficiency • Latency • Power consumption

  6. Problem definition • How to preserve the energy to increase the lifetime of the WSN ? • Use Protocols that aim mainly to increase the sleep periods as much as possible

  7. Other problems • Hidden/Exposed terminal problem • Deadlock/Livelock problem

  8. MAC layer protocols for WSNs • Active research area for three decades • Different categorization criteria • One criteria is based on type of assignment • Some protocols use a combination

  9. Sparce Topology and Energy Managemnt(STEM) • Reduction of power consumption is the main target • Designed mainly for networks with bursts of data • Uses two transceivers (one low power and one high power(

  10. STEM continued • STEM-T • Single tone(simple) • High latency • Results in overhearing • STEM-B • More complex • Lower Latency • High energy consumption

  11. Carrier Sense Multiple Access (CSMA) • Based on channel sensing • No controller needed • Two major categories: • Persistent and non persistent • Two variation of persistent came along after • P persistent • 1 persistent

  12. CSMA continued • Major problem is the hidden/exposed terminal • A busy tone was suggested as a solution • Results in higher complexity • Another technique was suggested(CSMA/CA) • Based on RTS ,CTS and random back off • Might still have some collisions in RTS

  13. Low Energy Adaptive Cluster Hierarchy (LEACH) • Based on some assumptions • Heavy deployment • Ability to contact the sink in one hop • Reduce Cluster head and following nodes • More load on cluster heads

  14. LEACH continued • We have two phases for this protocol

  15. Self-organizing Medium Access Control for Sensor networks(SMACS) • Distributed infrastructure algorithm • Based on some assumptions • Available spectrum is much larger • Most ofthe nodes are stationary • Uses unidirectional links • Uses four messages format for link building

  16. SMACS continued • How links are built:

  17. SMACS continued • SMACS protocol matlab simulation:

  18. SMACS improvement • Problems with algorithm: • should accommodate largest depth(Latency and limitation on number of nodes) • Nodes wake up in their assigned slots even with no data (wasted power) • Suggestion( adaptive frame usage) A B C D E F Code 1 Code 2 Y X A B C D E F

  19. 802.15.4 • Expected to have commercial impact • Features • Based on low data rate communications • Scalable and self maintained • Enable low power and cost operation • Combines scheduled and contention based schemes • Divides nodes into two main categories • Reduced Function Device • Full Function Device

  20. 802.15.4 continued • Characteristics • RFDs can only communicate with the FFDs • Routing is done through FFDs communications • Slots are unidirectional • FFDs are awake much more than RFDs • Has two main modes of operation • Beaconed mode • Nonbeaconed mode

  21. 802.15.4 (Beaconed mode) • Frame structure

  22. 802.15.4 (Nonbeaconed mode) • No beacons and therefore no GTS • Depends mainly on CSMA/CA • Checks the channel once for being idle • Coordinators are always awake • Small active period after transmission for devices

  23. Conclusion • Different protocols available • Medium Access is different from Multiple Access • All MAC protocols for WSN aim at reducing energy usage • Sleeping is the only state that consumes almost no energy • The 802.15.4 protocol has a potential to hit the commercial market

  24. References • Ian F.Akyildiz, Weilian Su, YogeshSankarasubramaniam and ErdalCayirci, Georgia institute of technology,”A survey on sensor networks " IEEE communications magazine , August 2002 • Holger Karl, Andreas Willig, ”Protocols and Architectures for wireless sensor networks”, John Wiley &sons , Ltd , 2005 • G. Lu, B. Krishnamachari, and C. S. Raghavendra. Performance Evaluation of the IEEE 802.15.4 MAC • for Low-Rate Low-Power Wireless Networks. pages 701–706, Phoenix, AZ, April 2004. • K. Sohrabi, J. Gao, V. Ailawadhi, and G. J. Pottie. Protocols for Self-Organization of a Wireless SensorNetwork. IEEE Personal Communications, 2000. • W. B. Heinzelman, A. P. Chandrakasan, and H. Balakrishnan. An Application-Specific Protocol Architecturefor Wireless Microsensor Networks. IEEE Transactions on Wireless Networking, 1(4): 660–670,2002.

  25. Thank you for your attention

  26. My questions • Why did the 802.15.4 protocol adopt Carrier Sensing Multiple Access to work with? • Because CSMA doesn’t have significant overhead , suitable for scalability and doesn’t waste energy if no data is to be sent

  27. My questions continued • If A FFD consumes 1 watt per day to remain active and receive data from nodes when working in the beaconed mode with an active period of 10 slots and the available power in its battery is 15 watts and it has on average 10 followers with an activity probability of 40%. what would you suggest to enable the FFD to function for at least 20 days period?

  28. Solution: 1 day 1 watts 20 days 20watts >15 we should reduce the activity period to preserve power 10 slots 1 watts ?? slots 15/20 (total available power/number of days of operation) Number of slots needed=10*(15/20)=7.5 which should be floored to 7 because we have at least20 days number of active followers at a time=40%* number of followers =0.4*10=4 nodes then we should assign 4 slots to the GTS and 3 slots for the GAP period

  29. My questions continued • for 802.14.5 with beacon mode. assuming the slot size to be able to accommodate a 50 kbyte frame define the procedure the RFD will follow to send: • A) 25Kbyte frame to the FFD • B) 100Kbyte frame to the FFD

  30. Solution • Case A • since frame < available slot size • RFD sends a transmit request in the CAP period • Wait for the assigned slot number in the beacon • Wake up in the assigned slot and start transmitting • Case B • Since frame size >available slot size • A node uses slotted CSMA to transmit the frame in the CAP period • wait for immediate acknowledgment as it is not a preserved slot for it

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