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Clock Synchronization Issue in the IEEE 802.11 TGs

Clock Synchronization Issue in the IEEE 802.11 TGs. Berlin, Germany 12-19, September Yeonkwon Jeong, Joongsoo Ma Mobile Multimedia Research Center Information and Communications University, KOREA {ykwjeong, jsma@icu.ac.kr}. Presentation Outline. An Example Configuration for WLAN Mesh

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Clock Synchronization Issue in the IEEE 802.11 TGs

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  1. Clock Synchronization Issue in the IEEE 802.11 TGs Berlin, Germany 12-19, September Yeonkwon Jeong, Joongsoo Ma Mobile Multimedia Research Center Information and Communications University, KOREA {ykwjeong, jsma@icu.ac.kr}

  2. Presentation Outline • An Example Configuration for WLAN Mesh • Why the Clock Synchronization is needed? • The TSF of IEEE 802.11 • What is the IEEE802.11’s state? • How to Fix it? • Backup Slides

  3. An Example Configurationfor WLAN Mesh Mesh Point Mesh Portal Mesh AP WLAN Mesh Mesh Client BSS(*Infrastructure Mode)

  4. Why the Clock Synchronization is needed? • Frequency Hopping Spread Spectrum • QoS • New Applications : Multi-Channel MAC Protocols, Location based Service, etc. • Power Save Mode

  5. Beacon Interval Windows=[0,W] 12:01 12:0012:01 faster adopts 12:01 12:02 slower not adopts The TSF of IEEE 802.11 • Each station calculates a random delay uniformly distributed in the range between zero and 2·aCWmin·aSlotTime. (The aCWmin and aSlotTime parameters are specified in Table 1.) • The station waits for the period of the random delay. • If a beacon arrives before the random delay timer has expired, the station cancels the pending beacon transmission and the remaining random delay. • When the random delay timer expires, the station transmits a beacon with a timestamp equal to the value of the station’s TSF timer1. • Upon receiving a beacon, a station sets its TSF timer to the timestamp of the beacon if the value of the timestamp is later than the station’s TSF timer2.

  6. Unidirectional clocks Equal beacon opportunity Single beacon per interval Beacon contention (collision) Not scalable Not consider multi-hop configuration What is the IEEE802.11’s state?

  7. How to Fix it? • Desired properties: • simple, efficient, and compatible with current 802.11 TSF. • scalability • multi-hop configuration • mobility

  8. Backup Slides • Simple Clock Structure • What means the out of Synchronization? • The Maximum Clock Driftof 802.11 TSF @ OFDM system • Prob(Fastest station sends a beacon) • Examples of Out of Synchronization • References

  9. Simple Clock Structure (64bit Time Stamp)

  10. What means the out of Synchronization? • Definition • Two clocks are out of synchronization if their times are different by more than Δ. Where let Δ be the maximum clock difference tolerable by power management and FHSS. • Example • The IEEE 802.11 specifications require clock accuracy to be within +/-0.01%. • For the length of a beacon interval, T=0.1s, two clocks with a difference of d in accuracy will drift away from each other by d*T=0.01%*0.1s=10μs. Thus, with Δ=224 μs, d=0.01%, T=0.1s, we have τ=[Δ/(d*T)]=23.

  11. The Maximum Clock Driftof 802.11 TSF @ OFDM system w = 30, b =4, d = 0.01%

  12. Prob(Fastest station sends a beacon)

  13. B C A B C D A …. …. Examples of Out of Synchronization

  14. References • Lifei Huang, and Ten-Hwang Lai, “On the Scalability of IEEE 802.11 Ad Hoc Networks”, MobiHOC 2002 • Ten-Hwang Lai, and Dong Zhou, “Efficient and Scalable IEEE 802.11 Ad-Hoc-Mode Timing Synchronization Function”, Proc. of AINA03 • Jang-Ping Sheu, Chih-Min Chao, and Ching-Wen Sun, “A Clock Synchronization Algorithm for Multi-Hop Wireless Ad Hoc Networks”, Proc. of ICDCS04

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