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WSN Time Synchronization Chap. 3 other material

Chapter 3 of [text]. Title: FireFly: A Time-Synchronized Real-Time Sensor Networking PlatformA complete wireless sensor networking platform developed at Carnegie Mellon University (CMU) in Pittsburg, PennsylvaniaFireFly sensor nodes (hardware platform)Real-time multitasking operating system: nano

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WSN Time Synchronization Chap. 3 other material

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    1. WSN Time Synchronization (Chap. 3 + other material) Prof. Sunggu Lee EE Dept., POSTECH

    2. Chapter 3 of [text] Title: FireFly: A Time-Synchronized Real-Time Sensor Networking Platform A complete wireless sensor networking platform developed at Carnegie Mellon University (CMU) in Pittsburg, Pennsylvania FireFly sensor nodes (hardware platform) Real-time multitasking operating system: nano-RK Time-division multiple-access (TDMA) communication method Known primarily for its demo implementation of a WSN system for a coal mine safety application 2

    3. Fig. 3.21 of [text]: Rescue sensor network in coal mine 3

    4. Fig. 3.22 of [text]: Coal mine map 4

    5. FireFly Design Objectives Low cost Energy efficiency Scalability Extensibility Ease of programmability Self-configuring Support for time-sensitive applications Hierarchical architecture 5

    6. FireFly Sensor Node Similar to mote in appearance Fig. 3.1 Atmel Atmega32L 8-bit microcontroller 8MHz clock, 32KB Rom, 2KB RAM Chipcon CC2420 IEEE 802.15.4 (ZigBee) wireless transceiver Attached sensors Light, temperature, audio, dual-axis acceleration, passive infrared motion sensors Table 3.1 of [text] shows typical energy consumption of various components on the FireFly board 6

    7. Table 3.1 of [text] 7

    8. Time Synchronization Benefits of time synchronization Energy-efficient communication Can periodically put sets of nodes to sleep Bounded message latency Allows messages to be scheduled so that they are collision-free Assuming no external radio interference High throughput Deterministic lifetime (energy lifetime) Total event ordering 8

    9. Out-of-Band Time Synchronization in FireFly Nodes Synchronization mechanism must be low-power, inexpensive, and require a simple receiver Two out-of-band time synchronization sources used for time synchronization WWVB atomic clock broadcast Pulse-width modulated (PWM) signal with a bit starting every second Atomic clock receivers cannot be used inside buildings Carrier-current AM transmitter Carrier-current uses a buildings power infrastructure as an antenna to radiate the time synchronization pulse AM transmitter locally rebroadcasts the atomic clock time signal Synch pulse for AM transmitter is line-balanced 50 ms square wave Implementation Results Jitter bounded to 200 ms in worst-case (expect to improve to 50 ms) 9

    10. Fig. 3.2 of [text]: Time synch modules used in FireFly 10

    11. General Time Synchronization Problem Time synchronization has been studied in computer networking systems since the start of the Internet era Standard Internet time synch protocol: Network Time Protocol (NTP) Clock synchronization in parallel computer systems Time synchronization in automatic control systems Time synch problem is much more difficult in wireless networks 11

    12. 2012-07-21 Computer Architecture Laboratory, POSTECH 12 Medium Access in IEEE 802.11 CSMA/CA (Carrier Sense Multiple Access / Collision Avoidance) is used. Before accessing the medium, station must sense the carrier. If medium is idle, station can transmit its packet after backoff timer becomes zero. Backoff timer used to reduce probability of collision with other packet transmissions If medium is not idle, station freezes decrement of backoff timer to reduce the probability of conflict. Deferral period Also increment retry counter Random backoff scheme is used for fairness. Binary exponential backoff algorithm Pick random number from a contention window, which doubles after every deferral

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