1. doc.: IEEE 802.11-09/0757-01-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 1 Proposed Overlapping BSS Solution Date: 2009, July 15 Authors:

2. doc.: IEEE 802.11-09/0757-01-00aa 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 fields 09/0660r3 examined using 11s MCCA for HCCA OBSS 09/0662r2 introduced OBSS Sharing with Access Fraction 09/0666r2 considered HCCAOP Advertisement Element for sharing and TXOP avoidance This presentation presents the proposed solution.

3. doc.: IEEE 802.11-09/0757-01-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 3 Changes from 09/0757r0 TSPEC Requirement Request now just requests a re-issue of TSPECs., i.e. TSPEC Requirement Response deleted (Slide 12) HCCAOP Advertisement Element used throughput and CHP deleted QAP ID now a two octet random number “Sharing” description re-written for clarification (slide 19) Interfering Times Report in HCCAOP Advertisement Element now only includes times from Self Times Reports from direct neighbor QAPs (Distance 1) Description of use of HCCAOP Advertisement re-written for clarification (Slide 22)

4. doc.: IEEE 802.11-09/0757-01-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 4 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

5. doc.: IEEE 802.11-09/0757-01-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 5 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 OBSS size <=3 FOR MORE COMPLETE EXPLANATION, SEE BACKUP SLIDES Slide 5

6. doc.: IEEE 802.11-09/0757-01-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 6 Outline of Proposal 1.New IE “QLoad” Used for Channel Selection and Channel Sharing 2.New Action Frame “TSPEC Requirement Request” Used by QAP to STA to indicate or confirm their TSPECs 3.New IE “HCCAOP Advertisement Element” Used by HCCA QAPs to avoid the TXOPs of overlapping HCCA QAPs 4.Recommendations to avoid/minimize OBSS problem –Channel selection based upon information in the QLoad Element: Overlap QLoad Access Factor –Channel width selection 40/20MHz, based upon Overlap –How to use the fields in the QLoad Element for Sharing and to prevent over-allocation

7. doc.: IEEE 802.11-09/0757-01-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 7 PROPOSED “QLOAD” ELEMENT

8. doc.: IEEE 802.11-09/0757-01-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 8 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 for further information

9. doc.: IEEE 802.11-09/0757-01-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 9 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

10. doc.: IEEE 802.11-09/0757-01-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 10 QAP ID 2 Octet Random Number Once established, QAP ID is not changed Enables a QAP to indentify its own QLoad in other QLoad elements

11. doc.: IEEE 802.11-09/0757-01-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 11 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 confirm (re-send) 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

12. doc.: IEEE 802.11-09/0757-01-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 12 TSPEC Requirement Request Request from QAP to a particular STA 1.Send All TSPECs (ID 1) –Effectively all previous (if any) TSPECs are deleted, need to set them up again

13. doc.: IEEE 802.11-09/0757-01-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 13 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

14. doc.: IEEE 802.11-09/0757-01-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 14 QAP Priority Streams Number of EDCA Voice and Video Priority Streams using 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, based upon the default AC_VI settings) –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 Note: The EDCA Bandwidth Factor is advisory and more work may be required in order to describe how a QAP should derive and apply it. For example if QAPs were not using the default EDCA parameters.

15. doc.: IEEE 802.11-09/0757-01-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 15 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 The Access Fraction is the total composite stream that the AP has allocated at any one time. Access Factor –Total Traffic Requirement in 32us/sec. 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”.

16. doc.: IEEE 802.11-09/0757-01-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 16 ACCESS FACTOR FIELD QLoads, 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

17. doc.: IEEE 802.11-09/0757-01-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 17 HCCA Peak –The total HCCA TXOP requirement for the QAP, expressed in 32us/sec. “HCCA Access Factor” –The sum of all the “HCCA Peak” values in the QLoad Element is the “HCCA Access Factor” (which is provided in the HCCAOP Advertisement Element) –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 802.11-09/0757-01-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 18 Sharing Basis 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.

19. doc.: IEEE 802.11-09/0757-01-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 19 Medium Time, TXOP Allocations - Sharing It is important to understand how the AP allocates the actual Medium Times and TXOPs in responses to TSPECs and checks that it has not exceeded its ‘limit’ 1.In response to each TSPEC the AP would normally allocate the Medium Time or TXOP (HCCA) that corresponds to the peak traffic. 2.When allocating a Medium Time or TXOP, the AP must calculate what the composite stream would be for that AP, and check that this composite medium time does not exceed the limit. The limit is the defined as follows: –If the Access Factor is <=1, an AP may allocate up to its advertised Self QLoad (composite stream calculated as MAX traffic = µ tot + 2 σ tot ) –If the Access Factor is >1, an AP may only allocate up Self QLoad/Access Factor 3.Before allocating an HCCA TXOP, the AP must check the “HCCA Access Factor” and check that: If the HCCA Access Factor is <=1, an AP may allocate up to its advertised “HCCA Peak” If the HCCA Access Factor is >1, an AP may only allocate up HCCA Peak/HCCA Access Factor

20. doc.: IEEE 802.11-09/0757-01-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 20 PROPOSED “HCCAOP ADVERTISEMENT” ELEMENT

21. doc.: IEEE 802.11-09/0757-01-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 21 HCCAOP Advertisement Element HCCAOP Advertisement Element must be provided by an HCCA QAP and also by an EDCA QAP that is a direct neighbor of an HCCA QAP. –A non-HCCA QAP will set “HCCA Access Factor”, HCCA Access Fraction” and “Self Times Report Present” to zero 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: Sum of “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 the TXOPs of only QAPs at a “Distance” of 1, i.e. are directly visible

22. doc.: IEEE 802.11-09/0757-01-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 22 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 1 (see 09/0662) HCCA QAP looks at the HCCAOP Advertisement of direct neighbor QAPs (Distance 1), in order to select a TXOP time that does not interfere with any TXOP being advertised in either the Self or Interfering Times Reports –Note that the neighboring QAPs will include the times for the QAPs at distance 2 in its Interfering Times Report, as well as any existing TXOP times for the QAP looking for the “space” 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. –See Slide 19 All times in the HCCAOP Advertisement Element are expressed in the TSF of the QAP that is transmitting the element –QAPs need to monitor the TSF of their neighbors so as to keep an up to date TSF Offset value.

23. doc.: IEEE 802.11-09/0757-01-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 23 Poll “Would you be interested in normative and informative text corresponding to the OBSS proposal as described in 09/757r1?” Y/N/A

24. doc.: IEEE 802.11-09/0757-01-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 24 BACKGROUND SLIDES OBSS Requirements Channel Selection Proposal Summary

25. doc.: IEEE 802.11-09/0757-01-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 25

26. doc.: IEEE 802.11-09/0757-01-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 26 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 26

27. doc.: IEEE 802.11-09/0757-01-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 27 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

28. doc.: IEEE 802.11-09/0757-01-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 28 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 √

29. doc.: IEEE 802.11-09/0757-01-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 29 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

30. doc.: IEEE 802.11-09/0757-01-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 30 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

31. doc.: IEEE 802.11-09/0757-01-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 31 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)

32. doc.: IEEE 802.11-09/0757-01-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 32

33. doc.: IEEE 802.11-09/0757-01-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 33 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” being advertised in the QLoad Element 3.If more than one channel, select lowest “Access Factor” 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

34. doc.: IEEE 802.11-09/0757-01-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 34 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!

35. doc.: IEEE 802.11-09/0757-01-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 35 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

36. doc.: IEEE 802.11-09/0757-01-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 36 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, then a)It may chose to share a 40MHz channel, or b)Switch to 20MHz operation and search again 2.If an 11n QAP, using 40MHz, finds itself overlapping such that Overlaps of 2 are present in the QLoad Element, then It should switch to 20MHz operation and search again Notes: 1.The primary intention is to avoid OBSSlengths > 2 which will cause excessive OBSS sizes 2.It is in the QAP’s own interest to use an independent 20MHz channel rather than share a 40MHz channel when there is an Overlap of 2 or more present.

37. doc.: IEEE 802.11-09/0757-01-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 37 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