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Time Features

Time Features. Authors:. Date: 2009-05-11. 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

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Time Features

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  1. Time Features Authors: Date: 2009-05-11 Allan Thomson, Cisco Systems

  2. 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

  3. 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 • 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. 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 TGv – UTC TSF Offset Allan Thomson, Cisco Systems

  6. 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 TGu – AP Date, Time and Time Zone Information Allan Thomson, Cisco Systems

  7. 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 • 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 Allan Thomson, Cisco Systems

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

  11. 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 • 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 • 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. 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. Straw Poll • If you agreed that time advertisement should be aligned what is your preferred solution? • Option #1: • Option #2: • Option #3: • Other: Allan Thomson, Cisco Systems

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