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Overlapping BSS Proposed Solution

Overlapping BSS Proposed Solution. Authors:. Date: 2008-04-21. Abstract. The problem of OBSS is quantified and examined A solution for OBSS is presented and discussed A set of recommendations is given. OBSS – Estimation of Size of Problem. Floor Plan of Apartments. Each Apartment

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Overlapping BSS Proposed Solution

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  1. Overlapping BSS Proposed Solution Authors: Date: 2008-04-21 Graham Smith, DSP Group

  2. Abstract The problem of OBSS is quantified and examined A solution for OBSS is presented and discussed A set of recommendations is given. Graham Smith, DSP Group

  3. OBSS – Estimation of Size of Problem • Floor Plan of Apartments • Each Apartment • 26 x 40 feet, • about 1000 square feet Imagine similar floors above and below this one. Indoor propagation loss formula used: Lp = – 69 + 20 log F + 40 log d + WAF (p) + FAF (q) F in MHz, d in feet At shorter distances, the Free Space formula dominates, Lp =– 38 + 20 log F + 20 log d + WAF (p) + FAF (q) The predicted propagation loss is the higher of the two. Each wall (WAF) and floor (FAF) between apartments is assumed to be 10dB penetration loss (fireproof). Ceiling height is assumed to be 10 feet. Graham Smith, DSP Group

  4. Received Signal Strengths 30dB power control Graham Smith, DSP Group

  5. Number of OBSS – DFS and TPC • Table 1 – Theoretical OBSS for Apartments - 1000 sq. ft. • Ideal DFS reduces problem significantly! 5GHz for Home! • Received signal strength within each apartment is high, better than -40dBm. • Theoretically, therefore, the power could be reduced by 30dB • with no deterioration in the throughput. Solves OBSS! • Table 2 – Theoretical OBSS with 30dB Power Reduction Graham Smith, DSP Group

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

  7. 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

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

  9. OBSS and QoS • For non-QoS (non-real time streaming) applications OBSS is simply a sharing or reduced bandwidth per network – Not a significant problem • OBSS is a significant problem ONLY when QoS is used AND when some ‘guaranteed performance’ is at stake Graham Smith, DSP Group

  10. OBSS – EDCA on EDCA • Table clearly shows that OBSS is a problem for 802.11 when it is intended to be used for applications that require QoS. • EDCA does not address the problem at all. • 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. Conclusions: • EDCA is not providing QoS in OBSS situation and any higher bandwidth streaming application is not protected • HCCA does solve the OBSS situation for all cases except when overlapping HCCA networks Graham Smith, DSP Group

  11. Solving OBSS • Channel Selection is first important step • TPC is probably difficult to assume • Can we assume OBSS is limited to a maximum of two QAPs? • Ideal Objectives: • HCCA and Admission Control QAPs can co-operate • Admission Control QAPs co-operate(Note: Still not protected against EDCA OBSS) • Two (or Three) HCCA networks can co-operate Graham Smith, DSP Group

  12. Basic Methodology QAP = HCCA or EDCA Admission Control • QAP builds knowledge of expected QoS load, “QLoad” • QAP advertises “QLoad” element • Channel selection to try to avoid another QAP • If must share, selection based upon relative “QLoads” • QAPs negotiate bandwidth sharing This covers OBSS for One HCCA and one (or more) EDCA Adm Cntrl APs And Two (or more) EDCA Admission Control QAPs Graham Smith, DSP Group

  13. OBSS – Basic Steps • When QSTAs associate, they send their TSPEC(s) corresponding to their expected requirements • This is ideal: alternatives are that the AP sends requests for ‘candidate’ TSPECS, and/or AP builds up Q Load as ADDTSs arrive. • Using the TSPECs, QAP ‘A’ builds knowledge of the QoS demands of its network, we shall call this the “Q Load” • Another QAP ‘B’, looking for a spare channel or whether to share, would interrogate QAP ‘A’ to establish the Q Load ‘A’. Based on this QAP ‘B’ can make a decision on whether to stay or not • Assuming that QAP ‘B’ does stay, then it determines its own Q Load ‘B’ • QAP ‘A’ and QAP ‘B’ now need to negotiate the bandwidth, based upon their Q Loads EDCA Admission Control only QAPs are now co-operating. HCCA and EDCA Admission Control are now co-operating Graham Smith, DSP Group

  14. If two HCCA APs • For two HCCA, QAP ‘A’ and QAP ‘B’ must harmonize such that they schedule TXOPs correctly with respect to both networks • Requires changes OPTION: Could consider that if 2 HCCA APs are forced to “Share”, then second AP to join must fall-back to EDCA Admission Control Each step for Steps 1-6 will now be examined in more detail. Graham Smith, DSP Group

  15. OBSS - TSPEC Exchange • Figure 10 – TSPEC Element • On association, a QSTA sends its TSPEC, QAP knows the STA’s requirement (s). • The TSPEC has Inactivity Interval set to 0 (needs to be included for Admission Control) • Causes the TSPEC to expire instantly, once accepted. • QAP recognizes this as a special case and know that the intention is for the QSTA to inform the QAP of its expected load Note that the QAP must remember the allocation required for all the ‘sign on’ TSPECs and respond accordingly Graham Smith, DSP Group

  16. QAP ‘Q Load’ Reporting QBSS Load element Format Not adequate for purpose Propose to add or replace similar new Element – “Q Load Element” Scheduled Slot field Base timing for the Scheduled Service Intervals that the HC is using (see later) QLoad Self Potential QoS traffic for this QAP QLoad Total Potential QoS traffic for sharing QAPs. If Total>Self, indicates sharing Note: Also could be used in Fast Handoff avoiding need to pre-register Graham Smith, DSP Group

  17. Channel Priority – Finding a Clear Channel When a QAP is searching for a channel, it should do so in the following order: • Set CHP (Channel Priority) to 0 • Check no other AP present • Check no other QAP present • If another QAP present, then check QAP Q Load is small enough such that the two can share • If QAP finds no other QAP present, then set CHP = 1 • Check that no other QAP is within range of its network QSTAs using Beacon Request ReportDo we need to do this? • If positive, and decides to stay, keep CHP to 0 • If QAP chooses to share, sets CHP to 0 • NOTE: Non QAPS would/should also try to avoid QAPs. Graham Smith, DSP Group

  18. Possible Beacon Report Exchanges for Hidden QAP OBSS Beacon Request • Provides other QAP the Q Load element • Informs CHP Graham Smith, DSP Group

  19. QAPs Negotiate • Basic options for sharing ‘rules’ are: • First Come, First served (FCFS).TSPECs are accepted, HCCA and EDCA, in the order they appear. Both QAPs must know the prevailing total Q Load so as not to over-allocate. • Negotiated Bandwidth • First AP (CHP=1) keeps its bandwidth, second gets what’s left • Simple Proportion (SPNB)Based upon the potential Q-Load of each QAP, the bandwidth is proportioned up between them accordingly. This way, each QAP knows its modified maximum bandwidth allocation • On-Demand Negotiated Bandwidth (ODNB)Basically, when a QAP receives an ADDTS request, that, if accepted, would take the QAP over the SPNB allocation, it must get permission from the other QAP to accept it. Preferred Methods (they are easy) • This is enough for OBSS situations: • EDCA-Admission Control QAPs, • One HCCA QAP and EDCA-Admission Control QAPs • If more than one HCCA QAP they need to Harmonize Graham Smith, DSP Group

  20. Bandwidth Sharing Questions to be considered: • Do we need an exchange so as to check that the two QAPs agree? • Should we mandate the sharing rules? • Suggest that in order to answer these, first see how it could be done and then return to these questions. • Proposal for AP to AP exchange Graham Smith, DSP Group

  21. Harmonizing HCCA • Fixed Time Slot Idea • Approaches a TDMA method • Each AP (HC) knows how much of the Time Slot it can use. • Suggested ideas: • Listen to each others QoS Polls, knowing the QLoad, QAP can know when to start it own Polls • AP to AP Schedule control – This is the preferred approach Graham Smith, DSP Group

  22. Explanation of Ideal Scheduling of TXOPs Harmonizing HCCA QAPs - FTS Schedule for QSTAs • Desirable that the start times of the TXOPs are maintained at the same interval. • This enables the QSTA use efficient S-APSD, • Maintain the minimum service interval (SI) requirement as per the TSPEC Graham Smith, DSP Group

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

  24. QoS Polling Example - Ideal QLOAD Sharing Accepted TSPECS Actual TXOPs QoS Polls set up TXOP If early end, QoS poll terminate TXOP Graham Smith, DSP Group

  25. Timing Options Problem: How does QAP B know when to start its TXOPs? • If situation static, then QAP B could listen to QAP A Polls and “learn” the timing • What happens when streams disappear or new streams arrive? • 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) This option #2 is now proposed Graham Smith, DSP Group

  26. AP to AP Poll Graham Smith, DSP Group

  27. Wireless DS QoS CF-Poll (Null Data) 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 T is time to start of “other” AP TXOPs period Graham Smith, DSP Group

  28. What if QAP B does not hear the Poll? • Desirable to avoid ACKs to the QoS Polls • Conditions are static, hence if QAP B misses a particular poll, then can safely assume absolute start time is same as last. Using QAP A to indicate the start time is to overcome long term clock drifts. Graham Smith, DSP Group

  29. What if QAP A or QAP B get a new QSTA? New QSTA on QAP A : • QAP A gets ADDTS request (zero Inactivity Time) • If available space, QAP A simply increases the QLoad values • QAP B sees the new QLoads and changes the Total QLoad value • Alternative is that QAP A sends QAP B a QoS CF Poll, with TID field set to 1100 (say) and TXOP limit contains addition. New QSTA on QAP B : • QAP B gets ADDTS request (zero Inactivity Time) • QAP B sends QoS CF Poll to QAP A • TID field = 1010 (special case “Request additional QLoad”) • NAV = 0, TXOP = duration of new TXOP request • QAP A responds with QoS CF Poll to QAP B • ApprovedTID Field = 1010 NAV = 0, TXOP = duration of new TXOP request • DeniedTID Field = 1011 NAV = 0, TXOP = 0 • QLoad values updated QAP A adjusts the time of start of QAP B TXOP period. Graham Smith, DSP Group

  30. Use of AP to AP QoS CF Pollfor Sharing • QAP B sees QAP A has available bandwidth. • QAP B builds its QLoad, as STAs associate on this channel. • QAP B decides if wants to apply to share – if so • QAP B sends QoS CF Poll to QAP A • TID Field = 1000 NAV = 0, TXOP = QLoad • Approved: QAP A responds:TID Field = 1000 NAV = 0, TXOP = QLoad • Denied: QAP A respondsTID Field = 1001 NAV = 0, TXOP = 0 Use this to formulate a “mandated” sharing rule? Graham Smith, DSP Group

  31. What if a Third QAP comes along? Is this too restrictive? How good do we believe DFS will work? Probably OK for 5GHz Graham Smith, DSP Group

  32. Third QAP • Sees one AP with CHP = 1 and one AP with CHP = 0 • Looks at Total QLoad determines available space and decides if wants to consider to apply • QAP C builds its QLoad, as STAs associate on this channel. • QAP C decides if wants to apply to share – if so • QAP C sends QoS CF Poll to QAP A • TID Field = 1000 NAV = 0, TXOP = QLoad • Approved: QAP A responds:TID Field = 1000 NAV = 0, TXOP = QLoad • Denied: QAP A responds:TID Field = 1001 NAV = 0, TXOP = 0 • QAP A is now responsible for “Handoff” QoS Polls to both QAPs • Keeps B where it is and fits C in between. Seems to work Could be extended for more QAPs but must all hear each other Is this restriction OK? Graham Smith, DSP Group

  33. Questions • Is it reasonable to ask QSTAs to ‘sign on’ with the sample TSPECS? • Do we need a request from the QAP for “sample TSPECS”? • The QAP could simply build up the QLOAD as time goes on • Procedure on Slide 29 for new STAs also applies for new TSPECS • Do we need to mandate sharing rules? • Which one should be used? • Use AP to AP QoS Polls to exchange, similar to slide 29, 30 • What about Admission Control QAPs, what do they need? • Still need/use CHP for Supervisor function • Procedure on Slide 29 for new STAs then used for new TSPECS • Procedure on Slide 30 for sharing then applies • Mixed OBSS? • If Admission Control QAP is Supervisor, then only one HCCA QAP may share? • Could consider HCCA taking over as Supervisor (probably contentious) • If HCCA is Master then more than one Admission Control could share? • When should an HCCA be forced to become Admission Control? Graham Smith, DSP Group

  34. OBSS Summary • Two (three) HCCA networks could share • Two EDCA Admission Control networks could share • An HCCA and one or more EDCA Admission Control Network(s) could share Proposed additions to the Standard are : • “Q LOAD Element” for HCCA and EDCA Admission Control QAPs • Rules and procedures for sharing • Admission Control only • HCCA only • Mixed Admission Control and HCCA • Fixed 10ms Slot time for HCCA • Use of Wireless DS QoS CF Polls (null data) for inter-AP negotiation and HCCA TXOP scheduling • Possibly “OBSS” Beacon Request Report (hidden APs) Graham Smith, DSP Group

  35. Hidden QAPs • If QAP stays after Beacon Report, set CHP to 0 and sends OBSS Beacon Request • QAP B now knows of QAP A and its Q Load • QAP ‘A’ and QAP ‘B’ calculate their maximum allocated bandwidth, based upon their Q Loads and the SPNB method. • QAP A and QAP B must now harmonize their Scheduled Allocations Graham Smith, DSP Group

  36. Harmonizing SI – Direct Method • Direct Method (as per non-hidden QAPs) • Could be possible using a common STA BUT • The QSTA may be in power save mode • If the first TXOP has been granted then the QSTA is prevented from transmitting, so sending the timer onto the other QAP is not possible • The only legitimate transmission from a STA to an AP outside its network, is the Probe Request • It is not advisable, or even allowed to change a scheduled time by too much. Graham Smith, DSP Group

  37. Harmonizing SI – Indirect Method QAP A CHP = 0; QAP B CHP = 1 • QAP A determines that a scheduled stream to a particular QSTA is blocked and suspects that it is due to scheduling from the QAP B. In this case, QAP A shifts its TSF timer, at DTIM, in the positive direction by 5% of the slot time, i.e. 500us. • Similarly, QAP B determines that a scheduled stream to a QSTA is blocked and suspects that it is due to scheduling from the QAP A. In this case, QAP B shifts its TSF timer, at DTIM, in the negative direction by 5% of the slot time, i.e. 500us. Can’t think of anything better, do we even need to consider this? Graham Smith, DSP Group

  38. 802.11n - 40MHz OBSS • 40MHz Channels • Easy/intuitive to see how two 40MHz overlapping networks will be less efficient than separate, independent 20MHz channels. • MUST use the OBSS proposals to: • Try to find clear channel • If not clear, look for 20MHz channel • MUST introduce procedures for preventing or controlling OBSS and usage of 40MHz channels • Need to see if the same procedures as previously described can be used Graham Smith, DSP Group

  39. Summary • Support for this approach? • Should we go ahead to • Brainstorm more? OR • Write draft normative text based on this approach? Then discuss further. Graham Smith, DSP Group

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