1. doc.: IEEE 802.11-09/0757-00-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 1 Proposed Overlapping BSS Solution Date: 2009, July 10 Authors:

2. doc.: IEEE 802.11-09/0757-00-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 2 Abstract 08/0457r4 and 08/1260r1 examined the OBSS problem and outlined possible solutions – “QLoad” introduced 08/1250r0, 09/0285r0, and 08/1470r4 looked at the OBSS scenarios, estimated worse case overlaps and ran simulations using Channel Selection so as to size the problem. 09/0230r0 and 09/0476r1 gave the details of the revised OBSS proposal with use of CHP bit and HCCA Supervisor 09/0496r2 examined video stream statistics 09/0497r2 extended the video stream statistics to QLoad 09/0660r3 examined using 11s MCCA for HCCA OBSS 09/0662r2 introduced OBSS Sharing with Access Fraction 09/0666r2 considered HCCAOP Advertisement for sharing This presentation presents the proposed solution

3. doc.: IEEE 802.11-09/0757-00-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 3 Objectives of OBSS Proposal Provide the means for: 1.Meaningful Channel Selection 2.Co-operation between Admission Control QAPs 3.Co-operation between HCCA and Admission Control QAPs 4.Co-operation between HCCA QAPs

4. doc.: IEEE 802.11-09/0757-00-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 4 Definitions and OBSS Graph Length(OBSS graph) – longest shortest path between any two APs in the OBSS graph OBSS Extent is related to the length(OBSS graph) (used in 08/0285r0)_ Size(OBSS graph) – number of nodes (APs) in the OBSS graph OBSS Solution Minimum Requirement (accepted in Los Angeles, Jan ’09) length(OBSS graph) <= 2 and the size(OBSS graph) <=3 Overlap - Number of overlapping BSSs that are sharing this channel –Overlap is simple for a QAP to report –Overlap” does not by itself indicate the OBSS Size or Length BUT, There is a direct relationship between OBSS Length AND: –the probability of Overlap2 –the number of Channels –and the number of other APs within radio range e.g. If Probability of Overlap2 <1% then OBSS length<=2 and OBSSsize <=3 FOR MORE COMPLETE EXPLANATION, SEE SLIDES 33 - 38 Slide 4

5. doc.: IEEE 802.11-09/0757-00-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 5 Outline of Proposal 1.New IE “QLoad” Used for Channel Selection and Channel Sharing 2.TSPEC Requirement Request Response Used by QAP to STA to indicate or confirm their TSPECs 3.Recommendations to avoid/minimize OBSS problem –Channel selection based upon: QAP Overlap QLoad Received Signal Strength –Channel width selection 40/20MHz

6. doc.: IEEE 802.11-09/0757-00-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 6 PROPOSED “QLOAD” ELEMENT NOTE: CHP bit not used if HCCAOP Advertisement is used.

7. doc.: IEEE 802.11-09/0757-00-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 7 Overlap QAP indicates the number of other QAPs with which it is sharing and indicates the size of the OBSS graph: –Zero indicates QAP has no other QAPs on the same channel within range –1 indicates already sharing with one other QAP –2 indicates already sharing with two other QAPs –etc The QAP is advertising the overlap to other QAPs who may be considering sharing. This parameter should be included in the Channel Selection procedure in order to select the best channel (08/1470r4) Note: See also “Backup” slides 33 – 38 for further information

8. doc.: IEEE 802.11-09/0757-00-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 8 Distance Distance is set to 0 for Self If QAP ID Directly visible to the QAP Self, then “Distance” is set to 1 If not directly visible to the QAP Self, then “Distance” is set to 1 plus the value reported for that QAP ID in the QAP that is directly visible Any QAP with Distance” > 2 is not recorded in QLoad Element

9. doc.: IEEE 802.11-09/0757-00-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 9 QAP ID First octet = random number (0 to 255) Second octet = octet 6 of MAC Address Once established, QAP ID is not changed Enables a QAP to indentify its QLoad in other QLoad elements

10. doc.: IEEE 802.11-09/0757-00-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 10 QLoad Self There are three methods for the QAP to build QLoad Self: 1.QSTAs in the BSS may send a TSPEC (ADDTS) with Inactivity Interval set to 0 (or 1) for instant timeout By sending in a TSPEC the STA has the QAP commit, in advance, medium time for the STA 2.QAP notes and adjusts for new TSPECs from QSTAs If accepted, “QLoad Self”, and also “QLoad Total” are adjusted only when the QSTA submits the ADDTS Chance that ADDTS is denied as QSTA did not reserve medium time in advance 3.In response to TSPEC Requirements Request QAP request STAs to indicate or confirm their TSPECs Used by QAP to ‘clear house’ or initially set up Q Load. The QAP is advertising its own potential QoS load to other QAPs who may be considering sharing

11. doc.: IEEE 802.11-09/0757-00-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 11 TSPEC Requirement Request Response Request from QAP to a particular STA Two types of Request: 1.Send All TSPECs (ID 0) –Effectively all previous (if any) TSPECs are deleted, need to set them up again 2.Confirm TSPECs (ID 1) –Confirm which TSPECs are still required –TSID plus Direction defines TSPEC

12. doc.: IEEE 802.11-09/0757-00-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 12 QLoad MEAN and STDEV MEAN and STDEV is estimated from the individual TSPECs: MEAN µ = ΣMEANi STDEVσ = 0.25 sqrt{Σ(MAXi – MINi) 2 } MEAN µ tot = ΣMEANi STDEV σ tot = sqrt(Σσ i 2 ) Total Traffic Requirement can be estimated: 1.MAX traffic = µ tot + 2 σ tot 2.90% Traffic = µ tot + 1.3 σ tot 3.80% Traffic = µ tot + 0.83σ tot

13. doc.: IEEE 802.11-09/0757-00-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 13 QAP Priority Streams Number of EDCA Priority Streams, AC_VO and AC_VI Used to estimate “EDCA Bandwidth Factor” EDCA Bandwidth Factor = 1 + 0.05 N (approx; keep it simple, see 09/0497) –Where N = Number of streams –Example: 4 streams Effective Bandwidth Factor = 1.2 Four 5.5Mbps streams will require 1.2 x 4 x 5.5 = 26.5Mbps

14. doc.: IEEE 802.11-09/0757-00-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 14 Access Fraction and Access Factor Access Fraction Total actual admitted time and/or scheduled time expressed as a fraction of 32us/sec rounded down to 1/256 Access Factor –Total Traffic Requirement of self plus all other visible QAPs. Expressed as a fraction that may be greater than 1 –Calculated as follows: Sum the individual QLoads of all QAPs in the QLoad element as a composite stream Calculate the EDCA Bandwidth Factor from the total number of Priority Streams in the visible QAPs (Distance 0 and 1) Multiply the two to obtain the “Access Fraction”.

15. doc.: IEEE 802.11-09/0757-00-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 15 ACCESS FACTOR FIELD Medium Time and TXOPs are all measured in 32us/sec Access Factor can be > 1 To express in 1 octet –2 bits for Integral (whole number) –6 bits for the decimal fraction, expressed as a fraction rounded down to 1/64 Example: Sum = 74268 in 32us/sec = 2.376576 seconds Hence, octet would be 10 01100 [2 and 24/64 = 2.375] Maximum value would be 3.98

16. doc.: IEEE 802.11-09/0757-00-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 16 Sharing If the Access Factor is >1, then there is a potential over- allocation –Hopefully QAPs should avoid this in the Channel selection process Sharing Scheme –QAPs should examine their QLoad Element in order to determine the maximum “Access Factor” being reported. This maximum value is then used to determine the allocation limit for that QAP in order not to cause over-allocation in other QAPs that are overlapping, –Using the Access Fractions (actual “live” traffic), Access Factor and QLoad self, a decision can be made whether to admit a new request. –Rules could be recommended in informative text.

17. doc.: IEEE 802.11-09/0757-00-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 17 HCCA Peak The total HCCA TXOP requirement for the QAP, expressed in 32us/sec. –The sum of all the HCCA Peak values is the “HCCA Access Factor” –If HCCA Access Factor > 1sec then potential for TXOP over- allocation –HCCA TXOPs can sum to “1” independent of EDCA Medium Time allocations, as TXOPs terminate immediately when no more data

18. doc.: IEEE 802.11-09/0757-00-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 18 Medium Time Allocations - Sharing It is important to understand how the AP allocates the actual Medium Times in responses to TSPECs and checks that it has not exceeded its ‘limit’ 1.In response to each TSPEC the AP allocates the Medium Time or TXOP (HCCA) that corresponds to the peak traffic 2.When allocating an additional Medium Time or TXOP, the AP must calculate what the composite stream would be, and check that this composite medium time does not exceed the limit 3.It is this composite time, that is advertised in the Access Fraction –The actual sum of the Medium Times and TXOPs will be greater than the composite time, but EDCA only uses what it needs, and hence the statistical nature of the streams causes the composite time to be the maximum of what is actually being used. Similarly HCCA TXOPs terminate when no more data. –Allocated HCCA TXOPs cannot exceed “1”

19. doc.: IEEE 802.11-09/0757-00-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 19

20. doc.: IEEE 802.11-09/0757-00-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 20 CHP bit CHP bit is used for overlapping HCCA QAPs If CHP = 1, then the QAP is the Supervisor responsible for handing off the TXOP to each QAP Rules are required for a QAP to set CHP = 1

21. doc.: IEEE 802.11-09/0757-00-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 21 Rules for Setting CHP bit and Sharing When a HCCA QAP is searching for a channel, it should do so in the following order: 1.Set CHP (Channel Priority) to 0 2.If finds a clear Channel, set CHP to 1 3.If no clear channel, then may share with a)Any legacy AP: Set CHP to 1 b)An Admission Control QAP, overlap 0 or 1: Resulting HCCA QAP overlap being 1:1, or 1:2Set CHP to 1 (see Note 1) c)An HCCA QAP with CHP = 1 CHP stays at 0 (see Note 1) 4.If conditions as per 3 cannot be found, the QAP may share with an HCCA QAP, with CHP=0 and Overlap = 1, CHP stays as 0 BUT the following must occur: a)The QAP that now has Overlap = 2 shall negotiate with the QAP that has CHP=1 to take over the Supervisor role. The new QAP must wait until it sees directly the QAP with CHP=1 5.If an HCCA QAP cannot find a channel that meets the above rules, it must fall back to Admission Control (see note 3) NOTES: 1.If 3b) or 3c) or 4), check that “QLoad” is such that the two can share 2.Restriction on sharing with HCCA would not be applicable in practice if 17 or more channels are available. HCCA QAP should be able to find channel that meets requirement, See Slides 24 – 29.

22. doc.: IEEE 802.11-09/0757-00-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 22 Harmonizing HCCA When sharing use Fixed Time Slot –Each AP (HC) knows how much of the Time Slot it can use: HCCA Peaks, Access Factor (and HCCA Access Factor) –Supervisor AP (CHP=1) hands off to the other QAP(s) using Wireless DS QoS CF-Poll (Null Data) for AP-AP communication Supervisor QAP (CHP = 1) controls the 10ms slot timing Supervisor QAP sends message to other QAPs (CHP = 0) indicating time to start of their respective TXOP periods. Uses Wireless DS (AP to AP), QoS CF-Poll (null data) 09/0230r0 describes rules for when Supervisor QAP goes away and when two QAPs have CHP set to 1 “Supervisor Claim” and “Is Supervisor There” packets used. “Supervisor Claim” also used when condition #4 is met. QAP with the higher Overlap takes over.

23. doc.: IEEE 802.11-09/0757-00-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 23 AP to 2 APs

24. doc.: IEEE 802.11-09/0757-00-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 24 Wireless DS QoS CF-Poll (Null Data) for AP to AP Communication AP to AP QoS CF-Poll Address Fields Function To DS From DS Address 1 Address 2 Address 3 Address 4 Wireless DS11RA = QAP B TA = QAP A DA =QAP B SA =QAP A AP to AP QoS CF-Poll Frame Type and Sub-type Type value b3 b2 Type Description Subtype value b7 b6 b5 b4 Subtype Description 10Data1110QoS CF-Poll (no data) Applicable Data Frame Bits 0-3Bit 4Bits 5-6Bit 7Bits 8-15 QoS CF PollTIDEOSP = 1 ACK PolicyAgg (11n)TXOP Limit QoS Control Field Use TID field as identifier

25. doc.: IEEE 802.11-09/0757-00-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 25 WDS QoS CF Polls To Supervisor from QAP with CHP=0 From Supervisor QAP with CHP=1 ACTION Bits 0-3Bit 4Bits 5-6Bit 7Bits 8-15 Indication from Supervisor to another QAP of Time to start TXOP (HCCA sharing) 11111100Time to start of TXOP in units of 32us Supervisor Claim, CHP = 1000110000 ACTIONBits 0-3Bit 4Bits 5-6Bit 7Bits 8-15 CHP is set to 0000010000 Is Supervisor There?001010000 * May not be required unless a more strict control of QLoad is needed

26. doc.: IEEE 802.11-09/0757-00-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 26

27. doc.: IEEE 802.11-09/0757-00-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 27 PROPOSED “HCCAOP ADVERTISEMENT” ELEMENT

28. doc.: IEEE 802.11-09/0757-00-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 28 HCCAOP Advertisement Element HCCA Advertisement Element informs ONLY TXOPs THAT ARE ACTIVE (except for HCCA Access Factor) Access Fraction Fields –HCCA Access Fraction: Total actual scheduled time expressed as a fraction of 32us/sec rounded down to 1/256 –HCCA Access Factor: Total HCCA Peak of all QAPs Distance 0, 1 and 2 2 bits for Integral (whole number) 6 bits for the decimal fraction, expressed as a fraction rounded down to 1/64 TXOP Reservation –Duration: In units of 32us –Service Interval (ms) –Offset: Beginning of first TXOP after a Beacon, relative to beginning of each scheduled Beacon, in units of 32us Interfering Times Report –Includes all QAPs that have a “Distance” of 1 and 2 in the QLoad Element

29. doc.: IEEE 802.11-09/0757-00-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 29 HCCAOP Advertisement Scheme HCCA QAPs need to schedule TXOPs that do not interfere with the other HCCA QAPs in the OBSS Graph. This is achieved by the HCCAOP Advertisement Element which lists all the TXOPs that have been already scheduled by the QAPs up to a “Distance” of 2 (see 09/0662) HCCA QAP looks at the HCCAOP Advertisement to select a TXOP that does not interfere with an existing TXOP in place QAP must check that allocating a new TXOP will not cause the total TXOPs of QAPs in the QLoad Element to exceed 1 sec/sec. –Uses HCCA Peak, HCCA Access Fraction, HCCA Access Factor

30. doc.: IEEE 802.11-09/0757-00-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 30 CHP bit Scheme, versus HCCAOP Advertisement CHP bit –Very efficient, Supervisor hands off the TXOPs using Wireless DS frames –No clock drift problem –Requires a Supervisor and selection procedure –Limited to OBSS length 2 and size 3 Supervisor must be at Distance 1 from other 2 QAPs In practice, will meet all scenarios if 20/40MHz channel selection is adopted (08/1470) HCCAOP Advertisement –Simple scheme and builds upon other work (11s) Especially if OBSS length 2 and size 3 –Is expandable to any OBSS Graph –Requires each QAP to convert slots to local time –Does suffer from Clock Drift –Can be inefficient

31. doc.: IEEE 802.11-09/0757-00-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 31

32. doc.: IEEE 802.11-09/0757-00-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 32 Poll #1 Which HCCA Scheme is preferred: 1.CHP bit Supervisor? 2.HCCAOP Advertisement?

33. doc.: IEEE 802.11-09/0757-00-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 33 Poll #2 “It is recommended that the OBSS proposal, as described in 09/xxxxr0 be adopted in principle and further recommends that normative and informative text should now be written” Y/N/A

34. doc.: IEEE 802.11-09/0757-00-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 34 BACKGROUND SLIDES OBSS Requirements Channel Selection Proposal Summary Poll

35. doc.: IEEE 802.11-09/0757-00-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 35

36. doc.: IEEE 802.11-09/0757-00-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 36 OBSS Requirement (from 09/0054r2) What is an OBSS graph? OBSS edge -- Any two APs operating in the same channel and can hear each other (either directly or via a STA associated to one of the APs) OBSS Graph – is a graph where APs are nodes of the graph and the edges are OBSS edges and every AP with in the OBSS graph can be connected via one or more OBSS APs to every other AP in the OBSS graph Length(OBSS graph) – longest shortest path between any two APs in the OBSS graph Size(OBSS graph) – number of nodes (APs) in the OBSS graph OBSS Solution Requirement (accepted in Los Angeles, Jan ’09) – if length(OBSS graph) <= 2 and the size(OBSS graph) <=3, enable the OBSS QAP solution otherwise (a) backoff to legacy (non.11aa) mode or (b) use a different solution Note: 08/0285r0 showed OBSSsizes up to 8 were likely with 10 Channels in dense apartment block scenario and argued size of 3 was not sufficient. 08/1470r4 also confirmed this with 9 and 11 channels. Slide 36

37. doc.: IEEE 802.11-09/0757-00-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 37 OBSS Size, Length and Overlap OBSS Size or Length difficult for an AP to directly indicate “Overlap” is simple for an AP to directly indicate “Overlap” does not by itself indicate the OBSS Size or Length

38. doc.: IEEE 802.11-09/0757-00-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 38 OBSS Size, Length and Overlap OBSS Length = 2 OBSS Size = 3 Overlaps A = 2, B = 1, C = 1 OBSS Length = 1 OBSS Size = 3 Overlaps A = 2, B = 2, C = 2 OBSS Length = 3 OBSS Size = 4 Overlaps A = 2, B = 2, C = 1, D = 1 Overlap <=2 But Length and Size above “spec” √ X √

39. doc.: IEEE 802.11-09/0757-00-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 39 OBSS Size, Length and Overlap Overlap of 2 does not directly indicate OBSS length BUT there is a direct relationship between OBSS Length AND: –the probability of Overlap2 –the number of Channels –and the number of other APs within radio range For OBSS length >2, there must be, at least: two Overlaps2 within the overlap area AND they must be on the same channel Calculation of Probability of this happening # of Channels = N; Prob of APs with Overlap2 = n; # of overlapping APs = M Probability of at least 2 Overlap2’s being in overlap area (binomial): Probability of no Overlap2 P0 = (1-n)^M Probability of one Overlap2P1 = M.n (1-n)^(M-1) Probability of two or moreP2 = 1 – P1 – P0 Probability of selecting same channel Probability of not selecting a certain channel Pc0 = (1-1/N)^n Probability of not selecting a certain channel just once Pc1 = n/N (1-1/N)^(n-1) Probability of selecting a certain channel at least 2 timesPc2 = 1 – P1 – P0 Probability of OBSS Length>2 = P2 x Pc2

40. doc.: IEEE 802.11-09/0757-00-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 40 OBSS Size, Length and Overlap Example: Double Apartment, 53 APs in range: –17 CH, (N=17, M=53) Probability of Overlap2 = 0.73% Probability of OBSS length > 2 = 0 –16 CH, (N=16, M=53) Probability of Overlap2 = 1.88% Probability of OBSS length > 2 = 0.08% –15 CH, (N=15, M=53) Probability of Overlap2 = 3.9% Probability of OBSS length > 2 = 1.4% Hence, in this case, 53 APs in range, for 99% service, at least 16CH are required 100% service for 17 or more Channels

41. doc.: IEEE 802.11-09/0757-00-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 41 Overlap Field Channel Selection simulations run as described in 08/1470r4 For Double Apartment scenario, (53 QAPs in range): –9CH maximum value of Overlap = 5 –8CHmaximum value of Overlap = 6 –7CHmaximum value of Overlap = 7 –3CHmaximum value of Overlap = 8 Hence, Overlap Field size made 4bits (0 - 15)

42. doc.: IEEE 802.11-09/0757-00-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 42

43. doc.: IEEE 802.11-09/0757-00-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 43 1 - Channel Selection Channel Selection Procedure: 1.Select Channel(s) with least number of APs 2.If more than one channel, select channel with least “overlaps” 3.If more than one channel, select lowest “QLoad Total” in “QLoad Element” Results dependant upon number of available channels –(see 08/1479r4 and 09/0285r0) 2.4GHz Band 3 CH maximum 5GHz Band20MHz USA 24 CH, Europe 19 CH 40MHz USA 11 CH, Europe 9 CH

44. doc.: IEEE 802.11-09/0757-00-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 44 Channel Selection Analysis (08/1470r4) Double Apartment 100% occupancy 53 overlapping apartments 17 CH (20MHz Channels) 99.3% probability of 0 or 1 channel overlap *Zero chance of length > 2 or size > 3 (<1 occurrence of 2 overlaps in 100 apartments) 9 CH (40MHz Channels) *Zero chance of length < 2 many cases of size > 3 Hence need to drop back to 20MHz and increase number of available channels BUT Many APs will use 40MHz channels 2.4GHz Band hopeless!

45. doc.: IEEE 802.11-09/0757-00-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 45 Channel Selection Analysis Summary If 17CH or greater, then Channel Selection can ensure OBSSlength <=2 in all scenarios examined With Channel Selection, Networks using 40MHz channels will have high percentage of no OBSS for all scenarios except dense apartments Channel Section is improved if ‘overlap selection’ is included

46. doc.: IEEE 802.11-09/0757-00-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 46 40/20MHz Channels If a QAP wanted to determine when to use 40MHz or 20MHz channel, then following procedure could be used: 1.If an 11n QAP cannot find a free channel using 40MHz, switch to using 20MHz If it still cannot find a clear channel, then it can settle on a 40MHz channel (secondary?) Rule #2 then comes into play 2.If an 11n QAP, using 40MHz, finds itself overlapping with more than one other QAP (20 or 40MHz) then it must switch to using 20MHz ( It may decide to search again using 40MHz, and then rule 1 applies Notes: 1.The primary intention is to avoid OBSSlengths > 2 2.It is assumed that it is in the QAP’s own interest to use an independent 20MHz channel rather than share a 40MHz channel

47. doc.: IEEE 802.11-09/0757-00-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 47 Channel Selection Summary Using suggested selection scheme: –17 available channels required to ensure OBSSlength <=2 and OBSSsize <=3 in most extreme scenario examined –Only applicable to 5GHz Band, 2.4GHz is a “lost cause” 20/40MHz 1.40MHz channels is fine for many scenarios 2.Suggested procedure for 40MHz channels to drop back to 20MHz when overlap and sharing exists in order to prevent excessive OBSS lengths