1. Y. Jeong, J. Ma, ICU doc.: IEEE 802.11-04/1027r0 Clock Synchronization Issue in the IEEE 802.11 TGs September 2004 Slide 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. Y. Jeong, J. Ma, ICU doc.: IEEE 802.11-04/1027r0 Clock Synchronization Issue in the IEEE 802.11 TGs September 2004 Slide 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. Y. Jeong, J. Ma, ICU doc.: IEEE 802.11-04/1027r0 Clock Synchronization Issue in the IEEE 802.11 TGs September 2004 Slide 3 An Example Configuration for WLAN Mesh Mesh Portal Mesh AP Mesh Point Mesh ClientBSS(*Infrastructure Mode) WLAN Mesh

4. Y. Jeong, J. Ma, ICU doc.: IEEE 802.11-04/1027r0 Clock Synchronization Issue in the IEEE 802.11 TGs September 2004 Slide 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. Y. Jeong, J. Ma, ICU doc.: IEEE 802.11-04/1027r0 Clock Synchronization Issue in the IEEE 802.11 TGs September 2004 Slide 5 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. Beacon Interval Windows=[0,W] faster adopts 12:01 12:00  12:01 slower not adopts 12:0112:02

6. Y. Jeong, J. Ma, ICU doc.: IEEE 802.11-04/1027r0 Clock Synchronization Issue in the IEEE 802.11 TGs September 2004 Slide 6 What is the IEEE802.11’s state? Unidirectional clocks Equal beacon opportunity Single beacon per interval Beacon contention (collision) Not scalable Not consider multi-hop configuration

7. Y. Jeong, J. Ma, ICU doc.: IEEE 802.11-04/1027r0 Clock Synchronization Issue in the IEEE 802.11 TGs September 2004 Slide 7 How to Fix it? Desired properties: – simple, efficient, and compatible with current 802.11 TSF. – scalability – multi-hop configuration – mobility

8. Y. Jeong, J. Ma, ICU doc.: IEEE 802.11-04/1027r0 Clock Synchronization Issue in the IEEE 802.11 TGs September 2004 Slide 8 Backup Slides Simple Clock Structure What means the out of Synchronization? The Maximum Clock Drift of 802.11 TSF @ OFDM system Prob(Fastest station sends a beacon) Examples of Out of Synchronization References

9. Y. Jeong, J. Ma, ICU doc.: IEEE 802.11-04/1027r0 Clock Synchronization Issue in the IEEE 802.11 TGs September 2004 Slide 9 Simple Clock Structure (64bit Time Stamp)

10. Y. Jeong, J. Ma, ICU doc.: IEEE 802.11-04/1027r0 Clock Synchronization Issue in the IEEE 802.11 TGs September 2004 Slide 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. Y. Jeong, J. Ma, ICU doc.: IEEE 802.11-04/1027r0 Clock Synchronization Issue in the IEEE 802.11 TGs September 2004 Slide 11 The Maximum Clock Drift of 802.11 TSF @ OFDM system w = 30, b =4, d = 0.01%

12. Y. Jeong, J. Ma, ICU doc.: IEEE 802.11-04/1027r0 Clock Synchronization Issue in the IEEE 802.11 TGs September 2004 Slide 12 Prob(Fastest station sends a beacon)

13. Y. Jeong, J. Ma, ICU doc.: IEEE 802.11-04/1027r0 Clock Synchronization Issue in the IEEE 802.11 TGs September 2004 Slide 13 Examples of Out of Synchronization A BCD …. A BC

14. Y. Jeong, J. Ma, ICU doc.: IEEE 802.11-04/1027r0 Clock Synchronization Issue in the IEEE 802.11 TGs September 2004 Slide 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