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Overlapping BSS Proposed Solution – “OSQAP”

Overlapping BSS Proposed Solution – “OSQAP”. Authors:. Date: 2009 -02-06. Abstract. Presentation 08/0457r4 examined the OBSS problem and outlined possible solutions Presentation 08/1260r1 further expanded on a solution, “OSQAP”

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Overlapping BSS Proposed Solution – “OSQAP”

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  1. Overlapping BSS Proposed Solution – “OSQAP” Authors: Date: 2009 -02-06 Graham Smith, DSP Group

  2. Abstract Presentation 08/0457r4 examined the OBSS problem and outlined possible solutions Presentation 08/1260r1 further expanded on a solution, “OSQAP” Presentation 08/1470r3 looked at the OBSS scenarios, estimated worse case overlaps and ran simulations using Channel Selection so as to size the problem. This presentation uses the results from 08/1470r3 to propose a comprehensive solution “OSQAP” - OBSS Solution for QAPs Graham Smith, DSP Group

  3. OBSS and QoS A major assumption of this solution is: OBSS solution is required for QoS applications • The following Table was first presented in 08/457 and explain why OBSS is a significant problem ONLY when QoS is used AND when some ‘guaranteed performance’ is at stake Graham Smith, DSP Group

  4. Effects of OBSS - 1 Graham Smith, DSP Group

  5. Effects of OBSS - 2 These cases are cause for concern, Admission Control is intended to provide QoS ‘protection’, and it breaks down in OBSS! Graham Smith, DSP Group

  6. Effects of OBSS - 3 Graham Smith, DSP Group

  7. OBSS – Existing Problem for QoS • OBSS is a problem for 802.11 when it is intended to be used for applications that require QoS. • EDCA does not address the OBSS problem • EDCA Admission Control only solves the bandwidth allocation problem within its own network and does not address OBSS. • HCCA does overcome OBSS problems in all but the case where two HCCA networks overlap BUT it steps on overlapping EDCA Admission Control networks Graham Smith, DSP Group

  8. OBSS Solution • Objectives: • Admission Control QAPs co-operate • HCCA and Admission Control QAPs co-operate • HCCA networks co-operate • Channel Selection • Shown in 08/1470 to be extremely important Following few slides are the Summary of 08/1470: 08/1470 includes the results of a Channel Selection Simulation Program that was applied to various overlapping scenarios Graham Smith, DSP Group

  9. Zero or One overlap is almost guaranteed for 20MHz channels NOTE: If APs in apartments set their PHY rate to 54Mbps only (11a/g), the overlaps decrease to Single Apartment Block – 18 (Less than Town Houses) Double apartment Block – 29 (Similar to Single Apartment blocks) Graham Smith, DSP Group

  10. Identifying Hidden APs QAPs advertise their overlaps and use this information in the Channel Selection • We see following overlap situations; QAP A:QAP B: QAP C • 0 No sharing • 1:1 QAP A sees QAP B who sees only QAP A • 1:2 QAP A knows that QAP B sees an AP that is hidden from QAP A • 2:1:1 QAP A sees two APs who do not see each other • 2:2:1 QAP A knows that QAP B sees an AP that is hidden from QAP A • 2:2:2 Could be that all three QAPs see each otherbut not sure. (Note: Can be sure by looking at the QLoads) • 3:1:1:1 QAP A sees three other QAPs, but knows that they do not see each other • *3* Any other combination with a 3 implies hidden APs Can we use this in any meaningful way? Graham Smith, DSP Group

  11. Channels Conclusions – 40/20MHz Channels Use of 11 and 9 Channels • For “House” scenarios, 40MHz channels should be able to operate OK. • For “Apartment” Scenarios, 40MHz channels can cause concern CONCLUSION • 40MHz channels should drop back to 20MHz when the overlapping situation becomes excessive. • We need a workable method for 11n APs to decide when keeping to 40MHz channel is not the right thing to do (for their own sake as well as others) Graham Smith, DSP Group

  12. Channel Search Conclusions – Overlap Indication • Channel Search algorithm is slightly better if the overlaps are used (as against just number of APs on same channel) • QAPs should indicate their overlap situation • QAPs know if “hidden” QAPs are on same channel, if they know the overlap figures Conclusion: Worthwhile that a QAP indicates its overlap situation Graham Smith, DSP Group

  13. “OSQAP” • Based upon 08/1470 and upon other work in the task group, • The original “OSQAP” proposal as per 08/0457 and 08/1260 has been modified Graham Smith, DSP Group

  14. Basic Methodology QAP = HCCA or EDCA Admission Control • QAP, proposed addition of “QLoad Element” • Overlap information • QoS loads • Channel selection • If sharing, selection can be based upon: • Number of other APs on same channel • Overlap situation • Relative “QLoads” • QAPs sort out bandwidth sharing Furthermore • Sharing HCCA QAPs must coordinate TXOPs Graham Smith, DSP Group

  15. QLoad Element • Propose new Element “QLoad” for an QAP Indicates the following: • “QLoad Self” • Total potential QoS traffic for this QAP • “QLoad Total” • Total potential QoS traffic for this QAP AND the other QAPs with which it is sharing the channel • “Overlap Count” • Number of QAPs that are within range of this QAP and that are sharing this channel Graham Smith, DSP Group

  16. Potential QoS Requirements – “QLoad Self” - TSPEC • QSTA can send a TSPEC with Inactivity Interval set to 0 (or 1 if 0 is ‘reserved/ignore’) Causes the TSPEC to expire instantly, once accepted: • QAP recognizes this as a special case and knows that the intention is for the QSTA to inform the QAP of its expected load • QSTA knows if its TSPEC will be accepted later TSPEC Element Note that the QAP must remember the allocation required Using these TSPECs and/or actual TSPECsQSTA can send at any time, preferably at Association Graham Smith, DSP Group

  17. QAP ‘Q Load Element’ - New • The Q Load Element is used to • Aid in Channel Selection • Determine the ‘sharing’ between overlapping QAPs QLoad Self Potential QoS traffic for this QAP in units of 32 µsec periods per second (as per Medium Time) QLoad Total Potential QoS traffic for sharing QAPs, in units of 32 µsec periods per second NOTE: If QLoad Total>Q Load Self, indicates sharing Overlap Number of APs that are sharing this channel and are overlapping Channel Priority Used only if QAP is operating with HCCA, indicates HCCA Supervisor. Graham Smith, DSP Group

  18. Suggested General Scheme • Monitor the QLoad Element(s) in the Beacon(s) overheard from each overlapping QAP • Intended for EDCA Admission Control and HCCA overlaps with EDCA Admission Control • (Overlapping HCCA requires further addition) Graham Smith, DSP Group

  19. Using the QLoad Element QLoad Rules: QAPs keep a note of the QLoad Elements of their overlapping QAPs • For QAPs that are directly overlapping, “QLoad Total”MUST be the SAME for each QAP • If sharing with only one other, then “QLoad Total” is sum of the two “QLoad Self” • If QAP is not sharing, then “QLoad Self” = “QLoad Total” • If a QAP increases or decreases its QLoad Self it must adjust its QLoad Total accordingly • If any overlapping QAP increases or decreases its “QLoad Total”, then the other QAPs follow and set their “QLoad Totals” to be the same • If a QAP sees that an overlapping QAP is no longer overlapping, then it reduces its own “QLoad Total” by the “QLoad Self” of the QAP that has disappeared Hopefully these become clearer from the following examples Graham Smith, DSP Group

  20. Example #1 Graham Smith, DSP Group

  21. Example #1 Extended 110%? More later Adding new QAPs is straightforward using Rules #1 and #2 Adding to QLoad Self is straightforward using Rules #1 and #2 Also note that each QAP is aware of the hidden QAPs Overlaps are: A = 2:2:1; B = 2:2:1; C = 1:2; D = 1:2 Graham Smith, DSP Group

  22. Example #2 – Reduction in QLoad Self(e.g. A QSTA disassociates) Any QAP can decrease its QLoad Self and the corresponding reduction in ALL the QLoad Totals will follow This means that a QAP must keep a note of the QLoad Elements of its overlapping QAPs Graham Smith, DSP Group

  23. Example #3 – QAP D Disappears Note: Although the Overlap information is not directly used, it can be used to confirm the required action. The main purpose, however, is to aid Channel Selection. Graham Smith, DSP Group

  24. Example #4 – QAP A Disappears Graham Smith, DSP Group

  25. Probability of Using 40MHz Channel • Sharing a 40MHz channel is not as efficient as two independent 20MHz channels (assumption) • Graph above shows universal use of 40MHz channels is not always possible or a good thing • Need a scheme that causes APs to scale back to 20MHz channels when overlapping occurs. Graham Smith, DSP Group

  26. 20/40MHz Operation 20/40 BSS Operation* Co-Existence with neighboring BSSs is managed through a number of mechanisms, including: • Overlapping BSS scanning and careful channel selection on initial BSS setup to avoid channels already in use by other BSSs. • Changing channels or operating width after BSS setup if a new BSS is detected operating on the secondary channel Also • 40MHz Intolerance bit can be set by any STA, (7.3.2.61) but only for 2.4GHz • 20/40 Intolerant Channel Report Element (7.3.2.59) • Overlapping BSS Scan Parameters element (7.3.2.60) In high OBSS conditions, networks should fall back to 20MHz operation NEED TO MAKE SOME RULES? * Reference: “Next Generation Wireless LANS: Throughput, Robustness and Reliability in 802.11n”, Eldad Perahia and Robert Stacey, Cambridge University Press 2008. Graham Smith, DSP Group

  27. 40/20MHz Channels – when to switch Assuming that an independent 20MHz channel is better than a shared 40MHz channel, then: Proposed Rules (these agree with the “Co-Existence with neighbors”) • If an 11n QAP cannot find a free channel using 40MHz, then it must 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 • 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 Note: The 11n AP could simply switch to using the primary channel, and the new AP settles on the secondary channel. Graham Smith, DSP Group

  28. QLoad Total > 100% If QLoad Total >100% then simple rule is: Rule #5 • “If QLoad Total = X, where X > 100%, then each QAP effectively reduces its QLoad Self by 100/Xi.e. Each QAP should not allocate above (QLoad Self x 100/X)” • Note: The advertised value of “QLoad Self” does not change in the QLoad Element Graham Smith, DSP Group

  29. Beacons or Direct Communication? • The preceding slides show a scheme based upon simple monitoring of Beacons of the overlapping QAPs, for EDCA Admission Control and mixed EDCA Admission Control and (single) HCCA • An alternative is to have the QAP s communicate directly to each other using the Wireless DS QoS CF-Poll (Null Data) as proposed in 08/0457 and 08/1260 • It is now suggested that simple Beacon monitoring should be sufficient (Note: This is a natural break, could stop here and seek views. Next part deals with HCCA) • Note: QAP communication using Wireless DS QoS CF-Poll (Null Data) is proposed for HCCA QAPs that are sharing. This is next part of presentation Graham Smith, DSP Group

  30. HCCA Considerations • HCCA QAPs have to co-operate more tightly as the TXOP allocation schedules need to be aligned • Propose use of CHP bit (Channel Priority) in the QLoad Element Graham Smith, DSP Group

  31. Channel Priority HCCA – Finding a Channel When a HCCA QAP is searching for a channel, it should do so in the following order: • Set CHP (Channel Priority) to 0 • If finds a clear Channel, set CHP to 1 • If no clear channel, then may share with • Any legacy AP: Set CHP to 1 • An Admission Control QAP, overlap 0 or 1: Resulting HCCA QAP overlap being 1:1, or 1:2 Set CHP to 1 (see Note 1) • An HCCA QAP with CHP = 1 CHP stays at 0 (see Notes 1 & 2) • If an HCCA QAP cannot find a channel that meets the rules, it must fall back to Admission Control NOTES: • If 3b) or 3c), check that “QLoad Total” is such that the two can share • An HCCA QAP may not share with an HCCA QAP which has CHP = 0, unless it also is sharing directly with the corresponding HCCA QAP that has CHP = 1 Graham Smith, DSP Group

  32. Harmonizing HCCA • When sharing use Fixed Time Slot • Each AP (HC) knows how much of the Time Slot it can use. • AP to AP Schedule control • Supervisor AP (CHP=1) hands off to the other QAP(s) Graham Smith, DSP Group

  33. Wireless DS QoS CF-Poll (Null Data) for AP to AP Communication AP to AP QoS CF-Poll Address Fields AP to AP QoS CF-Poll Frame Type and Sub-type QoS Control Field Use TID field as identifier Graham Smith, DSP Group

  34. Fixed Slot time 10ms Min and Max Service Intervals for Voice and Video 10ms fixed Slot Graham Smith, DSP Group

  35. TXOP Scheduling QAP A Acts as “Supervisor” (CHP=1) • QAP A controls the 10ms slot timing • QAP A sends message to QAP B indicating end of TXOPs for this Time Slot, and time to start of QAP B TXOP periods. • Uses Wireless DS (AP to AP), QoS CF-Poll (null data) Graham Smith, DSP Group

  36. AP to AP Poll Graham Smith, DSP Group

  37. AP to 2 APs Graham Smith, DSP Group

  38. WDS QoS CF Polls To Supervisor from QAP with CHP=0 From Supervisor QAP with CHP=1 * May not be required unless a more strict control of QLoad is needed Graham Smith, DSP Group

  39. Supervisor QAP goes away RULE • If QAP with CHP = 0, does not hear 3? Beacons from Supervisor, then following: • QAP with CHP = 0 sends WDS QoS Poll “Is Supervisor There?” • Retry limit of 3? • IF Supervisor is there, responds with “Supervisor Claim” • If no response • If no other QAP on this channel then • QAP sets CHP = 1 • If another QAP (CHP = 0) on this channel • Carry out procedure for higher claim • Sends “Supervisor Claim” Graham Smith, DSP Group

  40. Two QAPs with CHP = 1 • HCCA QAP checks if other is also HCCA • If other is not, then sends “Supervisor Claim” WDS QoS Poll • Other QAP should ACK and then send “CHP set to 0” WDS QoS Poll • QAPs compare received TSF Timer in received Beacon to own TSF timer • QAP that has higher TSF timer sends “Supervisor Claim” WDS QoS Poll • Other QAP should ACK and then send “CHP set to 0” WDS QoS Poll This exchange could be used at other times to confirm OBSS sharing Graham Smith, DSP Group

  41. Can Two APs have CHP = 1? • Although unlikely, we shall assume that a QAP with CHP = 1, picks up Beacons from another QAP, on the same channel, with CHP = 1. How do the QAPs sort this out? • Proposal: • Higher TSF timer • When QAP establishes its BSS it sets TSF timer to zero • QAP with higher TSF value is the “Supervisor”(One could have faster clock, but seems fair) • Use WDS QoS Poll interchange to confirm Graham Smith, DSP Group

  42. What if a Third HCCA QAP comes along? √ OK X NO Not seen as restrictive as probability of zero or one overlap is very high in practice X NO Graham Smith, DSP Group

  43. Proposal • Add new QLOAD Element • Channel Priority CHP • Slot time concept for HCCA • Use of TSPEC with Inactivity Interval set to 0 or 1 to build “QLoad” • Monitoring of QLoad Element of overlapping QAPs to determine sharing loads For HCCA QAPs • Rules and procedures for Channel Selection and setting of CHP • Rules and Procedures for Sharing Graham Smith, DSP Group

  44. OBSS Summary • EDCA Admission Control networks can share • An HCCA and one or more EDCA Admission Control Networks can share • Two (three) HCCA networks can share • HCCA networks will drop back to Admission Control when sharing not possible with other HCCA networks Proposed additions to the Standard are : • “Q LOAD Element” • Rules for indicating overlap and QLoad sharing • Description of Channel Selection • Rules for 20/40MHz channels • Fixed 10ms Slot time for HCCA QAPs that share • Use of Wireless DS QoS CF Polls (null data) for HCCA TXOP scheduling Graham Smith, DSP Group

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