Y. Jeong, J. Ma, ICU doc.: IEEE 802.11-04/1027r0 Clock Synchronization Issue in the IEEE 802.11 TGs...
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September 2004 Slide 1 Y. Jeong, J. Ma, ICU doc.: IEEE 802.11-04/1027r0 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, [email protected]}
Transcript of Y. Jeong, J. Ma, ICU doc.: IEEE 802.11-04/1027r0 Clock Synchronization Issue in the IEEE 802.11 TGs...
September 2004
Slide 1 Y. Jeong, J. Ma, ICU
doc.: IEEE 802.11-04/1027r0
Clock Synchronization Issue in the IEEE 802.11 TGs
Clock Synchronization Issue in the IEEE 802.11 TGs
Berlin, Germany12-19, September
Yeonkwon Jeong, Joongsoo Ma
Mobile Multimedia Research CenterInformation and Communications University, KOREA
{ykwjeong, [email protected]}
September 2004
Slide 2 Y. Jeong, J. Ma, ICU
doc.: IEEE 802.11-04/1027r0
Clock Synchronization Issue in the IEEE 802.11 TGs
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
September 2004
Slide 3 Y. Jeong, J. Ma, ICU
doc.: IEEE 802.11-04/1027r0
Clock Synchronization Issue in the IEEE 802.11 TGs
An Example Configurationfor WLAN Mesh
Mesh Portal
Mesh AP
Mesh Point
Mesh Client BSS(*Infrastructure Mode)WLAN Mesh
September 2004
Slide 4 Y. Jeong, J. Ma, ICU
doc.: IEEE 802.11-04/1027r0
Clock Synchronization Issue in the IEEE 802.11 TGs
Why the Clock Synchronization is needed?
• Frequency Hopping Spread Spectrum
• QoS
•New Applications : Multi-Channel MAC Protocols, Location based Service, etc.
• Power Save Mode
September 2004
Slide 5 Y. Jeong, J. Ma, ICU
doc.: IEEE 802.11-04/1027r0
Clock Synchronization Issue in the IEEE 802.11 TGs
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 12:0012:01
slower not adopts
12:01 12:02
September 2004
Slide 6 Y. Jeong, J. Ma, ICU
doc.: IEEE 802.11-04/1027r0
Clock Synchronization Issue in the IEEE 802.11 TGs
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
September 2004
Slide 7 Y. Jeong, J. Ma, ICU
doc.: IEEE 802.11-04/1027r0
Clock Synchronization Issue in the IEEE 802.11 TGs
How to Fix it?
• Desired properties: – simple, efficient, and compatible with current
802.11 TSF.– scalability– multi-hop configuration– mobility
September 2004
Slide 8 Y. Jeong, J. Ma, ICU
doc.: IEEE 802.11-04/1027r0
Clock Synchronization Issue in the IEEE 802.11 TGs
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
September 2004
Slide 9 Y. Jeong, J. Ma, ICU
doc.: IEEE 802.11-04/1027r0
Clock Synchronization Issue in the IEEE 802.11 TGs
Simple Clock Structure
(64bit Time Stamp)
September 2004
Slide 10 Y. Jeong, J. Ma, ICU
doc.: IEEE 802.11-04/1027r0
Clock Synchronization Issue in the IEEE 802.11 TGs
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.
September 2004
Slide 11 Y. Jeong, J. Ma, ICU
doc.: IEEE 802.11-04/1027r0
Clock Synchronization Issue in the IEEE 802.11 TGs
The Maximum Clock Driftof 802.11 TSF @ OFDM system
w = 30,
b =4,
d = 0.01%
September 2004
Slide 12 Y. Jeong, J. Ma, ICU
doc.: IEEE 802.11-04/1027r0
Clock Synchronization Issue in the IEEE 802.11 TGs
Prob(Fastest station sends a beacon)
September 2004
Slide 13 Y. Jeong, J. Ma, ICU
doc.: IEEE 802.11-04/1027r0
Clock Synchronization Issue in the IEEE 802.11 TGs
Examples of Out of Synchronization
A B C D
…. ….
A B C
September 2004
Slide 14 Y. Jeong, J. Ma, ICU
doc.: IEEE 802.11-04/1027r0
Clock Synchronization Issue in the IEEE 802.11 TGs
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
