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Dynamic Frequency Selection (DFS) in an Independent BSS (IBSS)

Dynamic Frequency Selection (DFS) in an Independent BSS (IBSS). S. Black, S. Gray, NOKIA Research Center S. Choi, A. Soomro, PHILIPS Research, USA. Outline. Frame Definitions The IBSS DFS Protocol Starting and Joining an IBSS The DFS Owner The IBSS STA Protocol The Channel Maps

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Dynamic Frequency Selection (DFS) in an Independent BSS (IBSS)

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  1. Dynamic Frequency Selection(DFS)in an Independent BSS(IBSS) S. Black, S. Gray,NOKIA Research CenterS. Choi, A. Soomro,PHILIPS Research, USA S. Black, S. Choi, S. Gray & A. Soomro, Nokia Research Center & Philips Research, USA

  2. Outline • Frame Definitions • The IBSS DFS Protocol • Starting and Joining an IBSS • The DFS Owner • The IBSS STA Protocol • The Channel Maps • Global & Local S. Black, S. Choi, S. Gray & A. Soomro, Nokia Research Center & Philips Research, USA

  3. Frame Definitions • Measurement Interval Control Frame • IBSS Channel Switch Announcement Frame • IBSS DFS Element Format • Channel Map Field • Channel Characteristics Sub-Field S. Black, S. Choi, S. Gray & A. Soomro, Nokia Research Center & Philips Research, USA

  4. Frame Control Duration RA BSSID QuietPeriod FCS Type = 1SubType = 3 BroadcastAddress Slot Times 0 Measurement Interval ControlFrame 2 Length 2 6 6 2 4 S. Black, S. Choi, S. Gray & A. Soomro, Nokia Research Center & Philips Research, USA

  5. IBSS Channel Switch Announcement Frame Channel Number Category Code Action Code Activation Delay Dialog Token 4 2 0 Length 1 1 1 1 1 S. Black, S. Choi, S. Gray & A. Soomro, Nokia Research Center & Philips Research, USA

  6. ElementID Length DFS Owner DFS Interval DFS Recovery Interval DFS Count Channel Map IBSS DFS Element Format 1 Length 1 6 2 1 2 2*n S. Black, S. Choi, S. Gray & A. Soomro, Nokia Research Center & Philips Research, USA

  7. IBSS DFS Element Definitions • DFS Owner - The IEEE MAC Address of the current DFS Owner • DFS Interval - Maximum time on a channel without re-evaluating the channel interference - STATIC • DFS Recovery Interval - Interval between end of DFS Interval and switching to a new channel during DFS Recovery Procedure - STATIC • DFS Count - No. of beacon intervals remaining until the start of the DECISION process • Decrements each beacon interval until it reaches Zero. • Channel Maps - PAIRS: Chan. No., Characteristics S. Black, S. Choi, S. Gray & A. Soomro, Nokia Research Center & Philips Research, USA

  8. Channel Number A Channel Number B Channel Characteristics A Channel Characteristics B 1 6 5 4 3 2 0 7 RSSI 0(lsb) Local Periodic ForeignPLCP RSSI 2(msb) RSSI 1 Un-Measured BSS Channel Maps ... ... 1 Length (octets) 1 Bit Sub-field Channel Characteristics S. Black, S. Choi, S. Gray & A. Soomro, Nokia Research Center & Philips Research, USA

  9. The IBSS DFS Protocol • Starting an IBSS • Scan channels and find a channel with RSSI  1 • If not found, select “Minimum” Channel • BSS Supported Channels, DFS Interval and DFS Recovery Interval specified by STA issuing MLME-START primitive • This STA  Initial DFS Owner • Channel Map contains results of the channel scan S. Black, S. Choi, S. Gray & A. Soomro, Nokia Research Center & Philips Research, USA

  10. The IBSS DFS Protocol • Joining an IBSS • Adopt IBSS DFS parameters from IBSS DFS Element in Beacon or Probe Response Management Frame • Store Channel Maps • Local – based on local measurement • Global – which DFS owner determined • More details follows S. Black, S. Choi, S. Gray & A. Soomro, Nokia Research Center & Philips Research, USA

  11. IBSS DFS Related Timing Beacon DFS Interval Channel Switch Announcement Period DFS Count  0 Measurement Period Beacon Time This STA  DFS Owner Channel Switch Announcement DFS ownership is retained till the end of DFS Interval DFS ownership may change according to rules specified Channel Switch Instant S. Black, S. Choi, S. Gray & A. Soomro, Nokia Research Center & Philips Research, USA

  12. The DFS Owner • Creates Revised Global Channel Map • Decides and announces next operating channel before DFS Interval expires • The STA that transmits first beacon in a channel becomes the DFS owner • Initiates measurements when DFS Count = 0 • Announces Channel Switch at the end of measurements. May send this more than once • Channel Switch to occur before DFS Interval expiration S. Black, S. Choi, S. Gray & A. Soomro, Nokia Research Center & Philips Research, USA

  13. The DFS Owner • Iff DFS owner: can use Measurement Interval frame to quiet and assess current channel S. Black, S. Choi, S. Gray & A. Soomro, Nokia Research Center & Philips Research, USA

  14. The DFS Owner Recovery • When the DFS owner fails … • No Channel Switch Announcement received within a DFS Interval • For example, if the owner leaves IBSS • The DFS Owner Recovery procedure begins • The STA to transmit first beacon after DFS Interval expiry becomes the DFS owner • Immediate transmission of Channel Switch Announcement based on revised global channel map - Derived from locally stored maps S. Black, S. Choi, S. Gray & A. Soomro, Nokia Research Center & Philips Research, USA

  15. The DFS Owner Recovery • Initial DFS Count = DFS Recovery Interval • Initial Channel Switch Count = Initial DFS count • Channel Switch expedited ! S. Black, S. Choi, S. Gray & A. Soomro, Nokia Research Center & Philips Research, USA

  16. The IBSS STA Protocol • All STAs adopt the latest IBSS DFS Element information superceding the previously received information. • STAs include IBSS DFS element in Beacons and Probe Responses that they send. • Decrement DFS Count before transmission • Channel Switch announcement relayed before DFS Interval expiration • Channel Count decremented S. Black, S. Choi, S. Gray & A. Soomro, Nokia Research Center & Philips Research, USA

  17. Channel Maps • A STA maintains • Local Channel Maps (Own) • Global Channel Maps • Local Maps of five individual STAs (min) • When re-transmitting maps • Replace global map with the local map if Periodicity, Foreign PLCP, BSS flag or MSB of RSSI differs • Maximum of three maps to be replaced • When receiving local maps, replace the oldest local map, if storage full S. Black, S. Choi, S. Gray & A. Soomro, Nokia Research Center & Philips Research, USA

  18. Channel Maps • Initial Global Map created by the STA initiating IBSS • Revised Global Maps created by STAs: • Use previous Global Channel Map - UNLESS • Unmeasured in Global Map - Local Map exists • In any Local Map: Either Periodicity, BSS or Foreign PLCP is ‘1’ or RSSI > Global Map Value • For all the local channel maps (min. of 2): Either Periodicity, BSS, Foreign PLCP is ‘0’ while Global Map had ‘1’ or, for all, RSSI < Global Map Value S. Black, S. Choi, S. Gray & A. Soomro, Nokia Research Center & Philips Research, USA

  19. Channel Maps • Form Revised Global Channel Map by • Logical OR of Periodicity, Foreign PLCP or BSS in each local channel map • Maximum RSSI value in local channel maps • All Unmeasured  Unmeasured • From Global maps (excluding unmeasured maps):Select channel with the minimum value in six LSBs of Channel Characteristics! S. Black, S. Choi, S. Gray & A. Soomro, Nokia Research Center & Philips Research, USA

  20. Conclusions • Distributed ownership • Global channel view considered • A robust DFS for IBSS • An essential element of 802.11h standard S. Black, S. Choi, S. Gray & A. Soomro, Nokia Research Center & Philips Research, USA

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