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Energy Efficient MAC Protocol for Wireless Sensor Network

Energy Efficient MAC Protocol for Wireless Sensor Network. Haozhi Xiong Department of Electrical and Computer Engineering The Ohio State University. Outline. General view on MAC protocol in WSN Requirements Problems MAC protocols for WSN Conclusion. General Purposes of MAC Protocol.

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Energy Efficient MAC Protocol for Wireless Sensor Network

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  1. Energy Efficient MAC Protocol for Wireless Sensor Network Haozhi Xiong Department of Electrical and Computer Engineering The Ohio State University

  2. Outline • General view on MAC protocol in WSN • Requirements • Problems • MAC protocols for WSN • Conclusion

  3. General Purposes of MAC Protocol • MAC protocol is to ensure that the channel can be accessed by multiple users, dealing with the situation of interference. • Has a direct bearing on how reliably and efficiently data can be transmitted

  4. Requirement of WSN • Application-level performance is the goal as opposed to per-node fairness • Long battery life • Delay • Surveillance, Low traffic, Regular update • Emergency, Quick response, Bursty heavy load

  5. Requirement of WSN cont’d • Adaptive to changing topology • Nodes die out • Mobile nodes • Applicable with limited computing capability

  6. MAC Protocol for WSN • Energy-efficient in sense of achieved throughput • Delay-optimized • Robust • As simple as possible

  7. Major problems in WSN MAC design • Idle listening • Listening when no traffic is sent • Overhearing • Receiving packets destined to other nodes • Collision • Retransmission • Overhead • Headers for signaling

  8. Overhearing • Receiving packets that are not destined to the node • Interception, waste of energy in receiving, error responding will cause potential collision

  9. Traffic Pattern • Local broadcast • Schedule exchange/update between neighbors • Omni-directional transmission is desired • Nodes to sink report • Payload and signaling • In favor of directional transmission

  10. Classification • Basic idea • Control the active time of radio • Control the times of on-off switches • Scheduling: time-slotted system, wake-up by schedule, clock shift can be disastrous • LPL (Low Power Listening): preamble sampling, wake-up tone

  11. S-MAC (Sensor-MAC)‏ • Single frequency, schedule-based • Active time interval, larger sleep interval • Duty cycle = listen interval / frame length

  12. S-MAC cont’d • Total frame length is limited by latency requirement, buffer space, and active time • Active time depends on message rate; fragmentation is used while transmitting large messages

  13. S-MAC cont’d • Synchronization period • Nodes exchange schedules by sending SYNC packets to immediate neighbors at their scheduled listen time

  14. S-MAC cont’d • Flat topology • Virtual cluster comprises nodes with common schedule • No coordination from cluster head

  15. S-MAC cont’d • Coordinating sleeping • listen for a fixed amount of time • Following the existing schedule or • Establishing new schedule if no schedule exists and announcing new schedule by SYNC

  16. S-MAC cont’d • New schedule received by node A • Discard current schedule if node A has no other neighbor • or • Adopt both schedules if node A has other neighbors

  17. S-MAC cont’d • Periodical neighbor discovery • reasons • SYNC is corrupted by collision or interference • SYNC is delayed due to busy medium • The lesser neighbors a node has, the more frequent it search for neighbors.

  18. S-MAC cont’d • Adaptive listening • To resolve sleep latency • Potential delay on every hop! • Basic idea • Let the nodes who overhear its neighbor’s transmission (ideally RTS or CTS) go to sleep and wake up for a short period at the end of transmission

  19. S-MAC cont’d • Additional collision avoidance • Record transmission time as NAV (Network Allocation Vector)‏ • Check if NAV = 0 before transmission attempt • NAV = 0 indicates ongoing transmission is over

  20. S-MAC cont’d • Message passing • Divide long messages into small fragments • Transmit fragments in a burst through reserved channel

  21. S-MAC cont’d • Features: • Loose synchronized due to large scale of intervals, no need for precise synchronization • Virtual cluster • Adaptive listen to reduce sleep delay • Message passing

  22. S-MAC cont’d • Cons: • Sleep latency • Active time must be long enough to handle expected highest load, inefficient when load is lower. • Essentially S-MAC trades energy with latency

  23. T-MAC (Timeout-MAC)‏ • Minimize idle listening • Using timeout to be adaptive to traffic during wakeup period • Transmitting all messages in burst of variable length • RTS/CTS provides both collision avoidance and reliable transmission

  24. T-MAC cont’d • An active period ends when no activation event has occurred for a time TA

  25. T-MAC cont’d • Definition of activation event: • The firing of a periodic frame timer (the beginning) • The reception of any data on the radio • The sensing of traffic (during collision)‏ • The end-of-transmission of a node’s own data packet or acknowledgement • End of neighbor’s transmission (knowledge from prior overhearing)‏

  26. T-MAC cont’d • Node successfully transmitting 3 packets

  27. T-MAC cont’d • Determining TA • RTS starts transmission, the TA should be at least long enough to hear the CTS • TA > RTS + CTS + Turn_around_Time

  28. T-MAC cont’d • Early sleeping problem • Reason: asymmetric communication • Node to sink • Border of highly active part of network

  29. T-MAC cont’d • Solution1 • Future request-to-send

  30. T-MAC cont’d • Use of FRTS • Send FRTS when overhearing CTS destined to another nodes • Do not send FRTS if communication is sensed right after CTS • Do not send FRTS if the node has already been prohibited by prior RTS/CTS • Receiver of FRTS wakes up as dictated by FRTS (FRTS contains transmission time from CTS)

  31. T-MAC cont’d • DS (data-send) packet is used to occupy the channel while giving time for FRTS to be sent

  32. T-MAC cont’d • Solution 2 • Full buffer priority

  33. T-MAC cont’d • Nodes with buffer almost full may prefer sending to receiving • After a node receiving RTS, it sends its own RTS without responding CTS. • A node may take advantage of full buffer priority only after several failed contentions (for example 2)‏

  34. T-MAC cont’d • Pros: • Adaptive active time • Cons: • Early sleeping problem

  35. P-MAC (Pattern-MAC)‏ • A node’s sleep-wakeup schedule is determined by its own traffic and the traffic of its neighbors.

  36. P-MAC cont’d • Pattern • Pattern is a string of bits indicating the tentative sleep-wake plan and is subject to change. • ‘1’: wake at a slot • ‘0’: sleep at a slot • Pattern is NOT equal to schedule. • Schedule is derived from a node’s own pattern and its neighbors’ pattern.

  37. P-MAC cont’d • Pattern generation • Let Pj be the pattern of node j • A sequence of N slots is called a period • Pj repeats over N slots if the length of Pj is lesser than N

  38. P-MAC cont’d • Example of a pattern • N = 5, Pj =01, then the tentative plan will be 01010 • 01010 is denoted as 031 • 0m1, m = 0,1,...,N-1

  39. P-MAC cont’d • Pattern growing • If the node has no data to send, the number of ‘0’ doubles in current pattern until threshold th. • Beyond threshold, the number of ‘0’ increases by 1. • 1, 01, 021, 041, …, 0th1, 0th01, 0th021, 0th031, …, 0N-11 • If the node has data to send, the pattern is restored to default ‘1’

  40. P-MAC cont’d • Pattern exchange • STF (Super Time Frame) • PRTF (Pattern Repeat Time Frame)

  41. P-MAC cont’d • PETF (Pattern Exchange Time Frame) • The last generated pattern during PRTF becomes the pattern for next PRTF, and will be advertised to neighbors during PETF.

  42. P-MAC cont’d • w : a slot in PRTF for all nodes to stay awake, downstream nodes can update pattern to ‘1’ and wake up quickly

  43. P-MAC cont’d • TR is chosen long enough to handle a complete data transmission (CW+RTS+CTS+DATA+ACK) • TE is chosen long enough to broadcast a pattern.

  44. P-MAC cont’d • Schedule generation

  45. X-MAC • Asynchronous protocols like B-MAC and Wise-MAC, rely on LPL (low power listening) also called preamble sampling.

  46. X-MAC (cont’d) • LPL drawbacks: • Extended waiting time even if the receiver has already wake up • Cannot tell who’s the intended receiver until the end of preamble

  47. X-MAC (cont’d) • Strobed preamble • Allowing interruption and wake up faster • Short preamble • embedded with address information of the target

  48. X-MAC (cont’d) • Energy-efficient • Low latency (reduced preamble length) • No need for synchronization • Low overheads • Less complex

  49. Conclusion • The special requirement for MAC design in WSN • Energy-efficient • Delay-optimized • Robust • Simple • Synchronous MAC based on schedule • SMAC, TMAC, PMAC • Asynchronous MAC based on preamble • XMAC (BMAC, WiseMAC) Big Mac ?

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