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TBTT Announce in DTIM Beacons

TBTT Announce in DTIM Beacons

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TBTT Announce in DTIM Beacons

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  1. TBTT Announce in DTIM Beacons Authors: Date: 2010-01-19 Michael Bahr, Siemens AG

  2. Abstract • This submission provides the motivation and description for the inclusion of the TBTT in DTIM beacons of mesh STAs with MCCA enabled. This is proposed in order to allow correct alignment of MCCAOP reservation maps between neighboring mesh stations. • This submission is related to the normative text in document 11-10/00xxr0. Michael Bahr, Siemens AG

  3. DTIM Beacon Transmission Times • TBTT is only Target Beacon Transmission Time • Actual Transmission Time of a Beacon (BTT) might be later • TBTT  BTT • Beacon Offset = BTT - TBTT Michael Bahr, Siemens AG

  4. MCCAOP Reservations in DTIM Beacons • MCCAOP reservations of neighbors (MSTA A) are with respect to the DTIM Beacon Reception time at the recipient of the DTIM Beacon (MSTA B) • Changing Beacon Offset of DTIM Beacon receptions will shift the MCCAOP reservations of MSTA A at MSTA B (that receives the DTIM Beacon) • Problems: • MSTA B might think there are MCCAOP Reservation conflicts  unnecessary teardown and alternative setup • matching of MCCAOP Reservations as the same MCCAOP reservation might not be possible • there might be real collisions because MSTA B consideres the medium unreserved Michael Bahr, Siemens AG

  5. Shifted MCCAOP Reservations • Cause: • TBTT of neighbors is not known • originator of MCCAOP Advertisements takes TBTT as reference point, but recipient of MCCAOP Advertisements has to take reception time of DTIM Beacon as reference point • Beacon timing element is of no help, because it distributes Beacon reception times • Solution: • include TBTT in DTIM Beacons • recipient of DTIM beacon can shift MCCAOP Advertisements to the same reference time as originator (MSTAs are synchronized by Neighbor Offset Protocol) • will be also useful together with MCCAOP Advertisments frame and Beacon Timing element Michael Bahr, Siemens AG

  6. TBTT Announce Element • Length = 4 octets • covers at least one beacon interval • beacon interval is 2 octets in TUs (1024 µs) • = 216 * 210 = 226 4 octets • TBTT • last 4 octets of target beacon transmission time of this beacon in µs • in DTIM beacons if MCCAActivated = true • might be bound to existence of MCCAOP Advertisements element • might be used in other contexts as well (e.g. MBCA) Michael Bahr, Siemens AG

  7. References • IEEE 802.11s, Draft Standard, WLAN Mesh Networking, December 2009 • 11-10/0118r0: Michael Bahr: “TBTT Announce in DTIM Beacons”, January 2010 Michael Bahr, Siemens AG