1. Improved TSF Information
January 2003
IEEE /100r1
Improved TSF Information
Date: 2005-SEP-22
Authors:
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Joe Kwak, InterDigital
Submission

2. Improved TSF Information
January 2003
IEEE /100r1
doc: IEEE /0436r0
May 2005
Improved TSF Information
Joe Kwak
InterDigital
Submission
Joe Kwak, InterDigital

3. OUTLINE TSF LB73 comments concerning aging and clock drift
Specified accuracy means TSF information has limited useful life due to clock drift error.
Need to modify TSF Information to improve its lifetime and usefulness:
Add measurement time timestamp to track the age of the reported TSF offset
Improve usefulness of TSF Info by including Drift Rate obtained by simple measurement.
Details for Drift Measurement and Aging Calculations
Joe Kwak, InterDigital
Submission

4. LB73 TSF Comments
This presentation provides a proposal for improved TSF Information for TGK.
There are 5 comments from 4 commenters concerned about TSF offset aging and clock drift problems evident in the current TGk draft.
The approach described here satisfies these LB73 comments: #875, 1205, 1383, 1407, and 1508.
Joe Kwak, InterDigital
Submission

5. Problem with Accuracy/Aging
Current TSF Information ignores aging and indicates that AP sending Neighbor Report must guarantee +/-1.5 TU accuracy.
Error budget indicates that the entire +/-1.5 TU accuracy is allocated to transmission delays and rounding. Better allocation is:
+/- 0.5 TU is initial measurement and rounding error.
+/- 1.0 TU allocation for clock drift error.
This means that any TSF offset is useable by the reporting AP for only 20.5 seconds since the TSF offset was measured.
The Neighbor Report TSF Information must contain a timestamp so the recipient will know when the information becomes invalid due to clock drift.
Clock drift rate measurement info in the Neighbor Report can significantly extend the usefulness of reported TSF Information.
A practical timestamp/drift rate addition to TSF Information is needed.
Joe Kwak, InterDigital
Submission

6. Clock Drift Effects and Aging of TBTT TU
Each AP is equipped with a clock with +/-25ppm accuracy.
This means that max drift between two APs is +/-50ppm.
+/-50ppm means 50usec/sec time drift max between APs.
This is equivalent to 1 TU in 20.5 seconds.
Lets call 20.5 seconds the Drift Period (DP) for 1 TU drift.
1 DP after a beacon measurement, the TBTT receive window start time must be decremented by one TU to ensure receiving the next predicted neighbor beacon.
Since the drift is +/-, the time window width to guarantee inclusion of neighbor TBTT increments by 2 TUs.
Each DP, the TBTT receive window start time must be decremented by one TU and the receive window width must be incremented by two TUs.
Joe Kwak, InterDigital
Submission

7. Worst Case Aging of TSF Offset
Reported Neighbor
TSF Offset = XTUs
Start of receive
window for TBTT
TBTT Uncertainty Window
Parent AP TU time-->
For t < 1DP
X-1.5
Width = 3
For 1DP < t < 2DP
X-2.5
Width = 5
Width = 17
Width = 31
For 7DP < t < 8DP
For 14DP < t < 15DP
X-15.5
X-8.5
Where t is time since beacon measurement AND drift rate is unknown
Joe Kwak, InterDigital
Submission

8. Add Aging Information to TSF Information
To make TSF Info practical, we should add an Aging Information element to indicate the required window size to guarantee inclusion of the Neighbor TBTT.
A 5 bit window size will specify up to 31 TU (31.7 msec) window, for beacon measurements up to 5 minutes old.
The larger the TBTT window, the older the measurement for that TSF Info entry.
TSF info entries need to be updated for aging by a STA every 30 Target Beacon Intervals or just prior to using the information, whichever occurs first.
To be able to correctly update the TSF Information for aging, STAs need to know when last update was done.
Last Update Time (3 octets in TSF TU units) needs to be added to TSF Information.
Joe Kwak, InterDigital
Submission

9. Add Drift Rate to TSF Information
Accurately measuring drift rate between two APs is trivial, requiring only 2 Beacon Reports measured some time apart, the longer the time between measurements, the more accurate the drift rate result.
Beacon Report provides Parent TSF (PTSF) and Target TSF (TTSF) for the instant of beacon transmission. The difference is the TSF offset (TSFO) reported in the neighbor report.
The difference between the TSF offsets of two measurements is the Drift for the time period between those measurements.
For Beacon Reports #1 and #2, calculate drift rate as follows:
1. TSFO1 = PTSF1 - TTSF1
2. TSFO2 = PTSF2 - TTSF2
3. DriftRateusec/usec = (TSFO2-TSFO1) / (PTSF2 - PTSF1)
4. DriftRateppm = DriftRateusec/usec * 10E6usec/sec
Drift Rate measurement in eight steps (3 bits) is easy to do and greatly extends the useful life of the TSF Offset info in the Neighbor Report entry.
Joe Kwak, InterDigital
Submission

10. Drift Rate Definition
Define 8 steps for Drift Rate with 20.5 sec Drift Period (DP):
Default Drift Rate value is 7 when Drift Rate is not available or is unknown.
Joe Kwak, InterDigital
Submission

11. Summary and Conclusion
As specified, TSF Information doesn’t work.
We need to change information to include Aging Information:
TU Timestamp
Measured Drift Rate
TGK needs to vote to approve these changes to the TGk draft.
Joe Kwak, InterDigital
Submission

12. Motion To Improve TSF Information
Move to instruct editor to incorporate normative text in 05/0997r0 into the next version of thebTGk draft specification.
Moved by: Joe Kwak
Seconded by: _______________
Vote YES _______
Vote NO _______
ABSTAIN _______
Vote Passes/Fails at ___%
Joe Kwak, InterDigital
Submission

13. Joe Kwak, InterDigital
Submission