Overlapping BSS Proposed Solution

1. Overlapping BSS Proposed Solution Authors: Date: 2008-04-02 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 • If so, it could be assumed that the OBSS situation could be eliminated or limited to a maximum of two QAPs • Objectives: • Two HCCA networks can co-operate • HCCA and Admission Control QAPs can co-operate • Two Admission Control QAPs co-operate(Note: Still not protected against EDCA OBSS) 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 • HCCA QAPs harmonize TXOPs Graham Smith, DSP Group

13. OBSS – Basic Steps • When QSTAs associate, they send their TSPEC(s) corresponding to their expected requirements • 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 negotiate the bandwidth, based upon their Q Loads EDCA Admission Control only QAPs are now co-operating. Note, however, that they still do not have protection against legacy EDCA networks. • If a successful outcome then HCCA networks proceed to step 7. If not, then QAP B must leave to seek another Channel. • QAP ‘A’ and QAP ‘B’ harmonize such that they schedule TXOPs correctly with respect to both networks Each step will now be examined in more detail. Graham Smith, DSP Group

14. 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

15. 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) Allocated Admitted field Amount of medium time that has been approved for EDCA Admission Control Allocated Scheduled Total of Scheduled TXOPs that has been approved for HCCA STAs Note: Also could be used in Fast Handoff avoiding need to pre-register Graham Smith, DSP Group

16. 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 1 • 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 selects its channel based upon 1 or 2, then • Check that no other QAP is within range of its network QSTAs using Beacon Request Report • If positive, and decides to stay, set CHP to 0 • If 4, and QAP chooses to share, sets CHP to 0 • NOTE: Non QAPS would also try to avoid QAPs. Graham Smith, DSP Group

17. 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 • 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 Method (it’s easy) This is enough for WMM-Admission Control QAPs, HCCA QAPS need to Harmonize Graham Smith, DSP Group

18. Explanation of Scheduling of TXOPs Harmonizing HCCA QAPs 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

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

20. Harmonize Slot times QoS (+) CF Poll Frame sent by HC At the beginning of the Slot, QAP sets bit 7 • The suggested procedure (see next slide) • At the beginning of the Slot, the QAP A sets bit 7. • This could be included in the first TXOP or, • if there is no TXOP at that time then the QAP simply sends a QoS Poll to itself. • QAP B waits the maximum duration of the TXOPs sequence • Period indicated in the Allocated Scheduled field in the Q LOAD element for QAP A • QAP B starts its Slot time and TXOPs • Simple and straightforward Graham Smith, DSP Group

21. Service Interval Harmonization Graham Smith, DSP Group

22. OBSS Proposed Procedure Summary • Before seeking a channel, a QAP sets its CHP to 1 in the Q LOAD element. • QSTAs send their expected TSPECs as they associate, with Inactivity Period set to 0, and the QAP calculates its values for the Q LOAD Element • A QAP should try to find a channel that has no other QAP present, by first listening for another Beacon, and then issuing a standard Beacon Request (see figure). An extension to this is that the Beacon Request and resulting Probe Request is tailored to seek out the Q Load Element. • If no other QAP is reported, then the QAP may choose that channel. • If another QAP is reported, then the respective Q LOADs are examined and a decision made as to share or not. • If the decision is to share, then the CHP is set to 0. • If the QAPs are not hidden then the condition of sharing is recognized (CHP 0 and 1) and each QAP calculates its available schedule time based on Simple Proportion. • If the QAPs are hidden then the OBSS Beacon Request /Report is used such that each QAP knows the Q LOAD of the other and of the decision to share (CHP 0 and 1) EDCA Admission Control QAPs can now proceed Graham Smith, DSP Group

23. OBSS Proposed Procedure Summary HCCA QAPs need to harmonize their SIs. • Each HCCA QAP indicates its start of the Slot Time by setting bit 7 in the QoS CF Poll. • If QAPs are not hidden, the Slot Times are harmonized using the Start of Slot Time indication and the Allocated Scheduled information in the Q LOAD. Proposal for Hidden APs (to be discussed) • If QAPs are hidden, if they experience scheduling problems to specific QSTAs, they adjust their respective Slot Times (TSF Timer), at DTIM intervals, by 0.5ms in a positive or negative direction as per the CHP setting. Graham Smith, DSP Group

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

25. OBSS Summary • Two HCCA networks could share • Two EDCA Admission Control networks could share • An HCCA and an EDCA Admission Control Network could share • An EDCA Admission Control and an EDCA network would still not share. Proposed additions to the Standard are : • “Q LOAD Element” for HCCA and EDCA Admission Control QAPs • “OBSS” Beacon Request Report • Fixed 10ms Slot time • Use of bit 7 in QoS CF Poll to indicate start of Slot Time We now consider “Hidden –AP” Graham Smith, DSP Group

26. 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

27. 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

28. 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. Graham Smith, DSP Group

29. 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 • The same procedures as previously described can be used Graham Smith, DSP Group

30. Recommendations • Recommendations: • “Q LOAD Element” for HCCA and EDCA Admission Control QAPs • “OBSS” Beacon Request Report • Fixed 10ms Slot time for HCCA • Use of bit 7 in QoS CF Poll to indicate start of Slot Time • Addition of recommended practices for OBSSNote: Wi-Fi Alliance could then devise tests to certify the behavior (this is important) • What to do about TPC? Treat as a separate subject? • Support for this approach? • Should we go ahead to write normative text based on this approach? Graham Smith, DSP Group