1. Time Features Date: 2009-05-11 Authors: May 2009 Month Year
doc.: IEEE yy/xxxxr0
May 2009
Time Features
Date:
Authors:
Allan Thomson, Cisco Systems
John Doe, Some Company

2. Month Year
doc.: IEEE yy/xxxxr0
May 2009
Abstract
TGp LB144 CID#110 raised an issue around the introduction of time advertisement features in TGp that overlaps somewhat with similar features in TGv and TGu
This presentation attempts to provide an overview of each of the features to facilitate further discussion on how to possibly align these features
Allan Thomson, Cisco Systems
John Doe, Some Company

3. May 2009
Summary
TGv – Provides general scheme to distribute external time reference (UTC) wrt TSF to non-AP STAs
Motivated by timestamping of events but is a general scheme
TGu – Provides scheme to distribute external timestamp (UTC & Time Zone) for local time at the AP
TGp – Provides scheme to distribute external time reference wrt TSF between STAs
Allan Thomson, Cisco Systems

4. TGp – Additional Information
May 2009
TGp – Additional Information
Courtesy Richard Roy:
“In a mobile environment where one might want to coordinate switching of physical channels so that the STAs were on the same RF channel at the same time, one could use GPS UTC time as the “external clock” that everyone would use for aligning channel switches.  Now imagine that NOT all STAs have a source of UTC time.  How should they get such a reference.  Answer: the TA frame with a TIE provides, in a single frame, information from a STA with UTC time to a STA w/o UTC time that allows the second STA to align an “external” clock to UTC and then can proceed to engage in aligned channel switches.  Note the big difference between this mechanism and the one in TGu.
The extension of 1) is for deployment of a TDMA scheme for channel assignments, eg. a logical MESH channel mapped to a physical channel which is a time slot on a given RF carrier … you get the idea.  Note that in highly mobile environments, alignment (i.e., TSF synchronization) using beacons is not viable for many reasons.  An external well-defined source of time is required.
Higher layer security mechanisms often use accurate time to prevent replay and other similar attacks on the system.  The higher layer protocols the ITS community is developing (IEEE 1609, ETSI TC ITS WG5) are examples of such use of accurate time.
Generally, any scenario one can dream up that makes use of/relies on “aligned time sources/clocks” on distinct (i.e., physically separate) physical devices (STAs)  can make use of the TA frame to distribute time to those STAs that don’t have  the agreed upon external time reference. “
Allan Thomson, Cisco Systems

5. May 2009
TGv – UTC TSF Offset
Provides an offset between the AP’s TSF counter and UTC to non-AP STAs
Distributed via an IE in Probe Response and Beacon frames
Two fields UTC TSF Offset & UTC TSF Offset Accuracy
UTC Time is represented as follows:
Same format is used by TGu
Allan Thomson, Cisco Systems

6. TGu – AP Date, Time and Time Zone Information
May 2009
TGu – AP Date, Time and Time Zone Information
Provides the AP Date, Time and Time Zone for the AP to non-AP STAs
Distributed via an IE in Native GAS Query Response
Two fields Timestamp & Time Zone
Timestamp uses TGv format without millisecond granularity
Time Zone specified using
Allan Thomson, Cisco Systems

7. TGp – Timing Advertisement
May 2009
TGp – Timing Advertisement
Provides a method for a STA to synchronize to an external time standard distributed by another STA
Introduces a new management frame, Timing Advertisement, frame containing the following elements
Allan Thomson, Cisco Systems

8. TGp – Timing Advertisement Cont’d
May 2009
TGp – Timing Advertisement Cont’d
Timing IE defined as:
Timing Capabilities defined as a set of bits that indicate whether the external time source is available…etc
TTOE (TSF Timer Offset Estimate)
TTOE-STD (TTOE Standard Deviation)
Allan Thomson, Cisco Systems

9. Observations May 2009 TGv TGu TGp On Demand Distribution
Periodic Update of time
Distribution of time from AP to non-AP STA
Distribution of time from any STA to any STA
Distribution of external time pre-association
Distribution of external time post-association
Defines offset between TSF and external time
Defines time zone
Granularity
mSec
Sec
nSec
Allan Thomson, Cisco Systems

10. May 2009
Straw Poll
Do you agree the time advertisement features in TGv, TGu and TGp should be aligned?
Yes (all or part of): 18
No: 1
Don’t Care: 4
Allan Thomson, Cisco Systems

11. Solutions Proposed Solution #1: Combine TGv &TGu
May 2009
Solutions
Proposed Solution #1: Combine TGv &TGu
Proposed Solution #2: Modify TGv and combine with TGu
Proposed Solution #3: Add TGp missing items to Solution #1 or Solution #2
Allan Thomson, Cisco Systems

12. Proposed Solution #1: Combine TGv &TGu
May 2009
Proposed Solution #1: Combine TGv &TGu
TGv - Add Time Zone field to UTC TSF Offset IE
TGu - Remove AP Date, Time & Time Zone from Native GAS Query
TGp – leave as is
Continue to use Probe Response and Beacon frames to communicate time advertisement
Allan Thomson, Cisco Systems

13. Proposed Solution #2: Modify TGv & combine with TGu
May 2009
Proposed Solution #2: Modify TGv & combine with TGu
TGv - Add Time Zone field to UTC TSF Offset IE
TGv – Remove IE from Beacon and add to (Re)association response frames
TGu - Remove AP Date, Time & Time Zone from Native GAS Query
TGp – leave as is
Use Probe Response & (Re)Association Response frames to communicate time advertisement
Allan Thomson, Cisco Systems

14. May 2009
Proposed Solution #3: Add TGp missing items to Solution #1 & Solution #2
TGp – Remove Timing Advertisement frame
TGp - Add new combined UTC TSF Offset (and associated frames) including Timing Capabilities to TGp
TGv – Remove UTC TSF Offset
Allan Thomson, Cisco Systems

15. Proposed Solution #4: Combine TGp, TGv & TGu fields into a single IE
May 2009
Proposed Solution #4: Combine TGp, TGv & TGu fields into a single IE
TGp – Keep Timing Advertisement frame
TGp/TGv/TGu - Combine new combined UTC TSF Offset including Timing Capabilities from TGp
TGv – Refer to new IE
TGu – Remove Native GAS Query Response, Rely on TGv frames
Allan Thomson, Cisco Systems

16. May 2009
Straw Poll
If you agreed that time advertisement should be aligned what is your preferred solution?
Solution #1: 0
Solution #2: 0
Solution #3: 0
Solution #4: 14
Other: 2
Allan Thomson, Cisco Systems

17. May 2009
Straw Poll Summary
Do you agree the time advertisement features in TGv, TGu and TGp should be aligned?
Yes (all or part of): 18
No: 1
Don’t Care: 4
If you agreed that time advertisement should be aligned what is your preferred solution?
Solution #1: 0
Solution #2: 0
Solution #3: 0
Solution #4: 14
Other: 2
Allan Thomson, Cisco Systems

18. Proposed Solution #4 Summary
May 2009
Proposed Solution #4 Summary
Strategy:
Step 1: TGp used as the baseline for these changes
Step 2: TGv builds on TGp
Step 3: TGu builds on TGv and TGp
Step 1: TGp 09/0482r4 (Group Reviewed Tue PM)
Define TGp Time Advertisement IE to enable 3 amendment adoption
Retain TGp Timing Advertisement frame use
Step 2: TGv 09/0592r0 (Group Reviewed Tue Eve)
Add TGv specific updates to Time Advertisement IE
Update TGv to refer to new combined IE
Retain TGv Probe Response and Beacon frame use
Step 3: TGu 09/0593r0 (Group Reviewed Wed AM)
Update TGu to refer to new combined IE
Change TGu from GAS Query Response to use Probe Response frame
Allan Thomson, Cisco Systems

19. May 2009
TGp Details
Retains the Timing Advertisement frame that includes the Timestamp and Time Advertisement information element
Time Advertisement information element is basis for all 3 amendments
Timing capabilities field defines how the remaining contents are defined
0 = Arbitrary time base (no time value or time error included)
1 =
Time Value field specifies the time offset estimate between UTC UT0 and the Timestamp transmitted in the Timing Advertisement frame
Time Error field specifies the standard deviation of time error
2 to 255 – Reserved
Nanoseconds are units of standard deviation of time error
Element ID
Length
Timing
Capabilities
Time Value
Time Error
Octets: 1 9 5
Allan Thomson, Cisco Systems

20. May 2009
TGv Details
Continued support of time advertisement in the Beacon frame and Probe Response frame
Defines new Timing Capabilities field value (2) in Time Advertisement IE
Value 2 =
Time Value field defines as UTC UT0 estimate is referenced to the TSF timer at TSF time 0
Time Error field same definition as p
Time Value as per previous TGv that defines UTC down to milliseconds granularity
Adds Time Zone information element to probes and beacons
Allan Thomson, Cisco Systems

21. TGu Details Eliminate GAS Query Response use of AP Date, Time…etc
May 2009
TGu Details
Eliminate GAS Query Response use of AP Date, Time…etc
Instead use TGv approach of Probe Response frames with Time Advertisement IE
Allan Thomson, Cisco Systems