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QLoad Element for OBSS Sharing

This presentation discusses the basics of how the proposed QLoad Element can be used in a sharing scheme for OBSS traffic. It covers fields such as overlap, QLoad mean, QLoad standard deviation, QAP ID, channel priority, and distance.

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QLoad Element for OBSS Sharing

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  1. Considerations for OBSS Sharing using QLoad Element Authors: Date: 2009, May 3 Graham Smith, DSP Group

  2. Abstract The proposed QLoad Element (as per 09/0496r2) contains information on the traffic of each QAP in the OBSS graph. This presentation discusses the basics of how the data could be used in a sharing scheme. Graham Smith, DSP Group

  3. Extended QLoad Element – 09/0496r2 Graham Smith, DSP Group

  4. QLoad Element Fields • Overlap • Number of APs that are sharing this channel and are overlapping • QLoad MEAN and STDEV • The mean and standard deviation of the total traffic presented to the QAP by TSPECs from STAs associated to that QAP • QAP ID • First octet = random number (0 to 255) • Second octet = octet 6 of MAC Address • Once selected, QAP retains this ID • Chosen so that it is still possible to know which specific QAP this is • QAPs need recognize their own QLoad • Channel Priority • Used only if QAP is operating with HCCA, indicates HCCA Supervisor • Distance • Distance is set to 0 for Self • If the QAP that corresponds to ID, MEAN and STDEV values is directly visible to the QAP Self, then “Distance” is set to 1 • If the QAP that corresponds to ID, MEAN and STDEV values is 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 Graham Smith, DSP Group

  5. Traffic Information Every QAP knows the traffic requirements of every other QAP in the OBSS Graph, and its ‘distance’ Composite Traffic can be calculated: MEAN µtot = ΣMEANi STDEV σtot = sqrt(Σσi2) Total Traffic Requirement can be estimated: • MAX traffic = µtot + 2 σtot • 90% Traffic = µtot + 1.3 σtot • 80% Traffic = µtot + 0.83σtot • Other? Total traffic for a particular QAP can be ‘loaded’ according to the “Distance” of each other QAP Total Traffic Requirement limit is also affected by • EDCA Overhead Contention overhead reduces the total traffic bandwidth. Important as number of streams increases (see next slide) • HCCA Allocation LimitNeed to allow bandwidth for non-QoS traffic, say only 90% of total bandwidth should be reserved Graham Smith, DSP Group

  6. EDCA Overhead – Capacity drops with # streams As number of video streams increases, the contention also increases. In order to keep latency low the capacity of the Channel is decreased. Maximum throughput on (shared) channel decreases as number of video streams increases Limits to ensure low loss: 1 stream @ 33Mbps 4 Streams, 27.5Mbps total 8 Streams 23.3Mbps total HENCE: Total Allocation MUST take account of the number of streams Note: This is also for Admission Control on each QAP NOTE: Above graph is simulation for independent streams. Downlink streams from QAP may be better due to queuing at the AP Graham Smith, DSP Group

  7. EDCA Bandwidth Overhead • Due to contention, the required bandwidth for EDCA is larger than the sum of the streams Approximation based upon simulation on previous slide: • EDCA Bandwidth Factor = 1 + 0.05 N (approx; keep it simple) • Where N = Number of streams • 4 streams Effective Bandwidth Factor = 1.2Therefore four 5.5Mbps streams will require 1.2 x 4 x 5.5 = 26.5Mbps NOTES: • This is only true if traffic is near saturation point; but with overlaps it may be always safe to assume it is • If all traffic is downlink contention may be less as AP controls the queue; but as traffic is coming from the overlapping networks, again it may be safer to assume this formula. (The formula for EDCA Overhead may need to be looked at further) Graham Smith, DSP Group

  8. Q Load Element with Priority Stream Fields Number of EDCA Priority Streams added to QLoad Element so that QAP can calculate EDCA overhead. Graham Smith, DSP Group

  9. Basic General Sharing Considerations • Total Traffic Requirement of all the sharing QAPs can be determined • Calculate Total Traffic100%, 90%, 80%, Other? • EDCA contention overhead should be estimated • Based upon QLoads of QAPs with “Distance = 1”, i.e. directly competing for the air time • Allocation limit? • Definitely required for HCCA. E.g. Do not allocation over, say, 90% to allow for other traffic • For EDCA, lower priority traffic should still get bandwidth but could increase the EDCA Bandwidth Factor Graham Smith, DSP Group

  10. Number of Sharing QAPs How many sharing QAPs need to be assumed in order to evaluate a proposed Sharing Scheme? BASED UPON 08/1470r4, “TYPICAL WORSE CASE” IS; • OBSS length = 2 • OBSS size = 3 Note: This assumes 20/40MHz channel rules are adopted Graham Smith, DSP Group

  11. Sharing Scenario Note, the typical case is no or just 1 overlap BUT The typical “Worse case” scenario for an Overlap = 2 is shown below QAP A (If HCCA, CHP = 1) Overlap = 2, QAPs B and C are “Distance = 1” QAP B Overlap = 1, QAP A is “Distance = 1” QAP C is “Distance = 2” QAP C Overlap = 1, QAP A is “Distance = 1” QAP B is “Distance = 2” Graham Smith, DSP Group

  12. EXAMPLE - 11n Basic Video Data Rates Basic Video Data Rates, in Mbps, for MPEG2 Basic Video Data Rates, fraction of bandwidth for 80Mbps Bandwidth (11n) MPEG 2 Graham Smith, DSP Group

  13. Example - EDCA Example QLoads for each QAP Now what? OK if 80% Traffic is <1? Graham Smith, DSP Group

  14. Example - HCCA All are OK, But should still have a “Rule” If Total Traffic less than 1 Then OK for QAP A to allocate TXOPs based on 100%, 90% 80% for each QAP? Traffic Requirement per QAP Exceeds 90% allocation rule? Needs further Explanation (See next slide) Graham Smith, DSP Group

  15. HCCA Time Allocation • Note that in example the sum of the allocated time (TXOPs) is 91%. This appears to contradict the requirement that at least 10% of the bandwidth is not scheduled. Note that the 1.1 factor was applied to check that the sharing was OK but this is based upon the total composite traffic whereas the sum of the individual QAP TXOPs is a linear sum. In fact the TXOPs can be allocated as calculated because it is assured that the actual total TXOP times will not exceed 90% over time. In HCCA the TXOP is terminated as soon as no more packets are ready to be sent, so although the total TXOP allocation could be 100% it will never actually exceed 90%. Graham Smith, DSP Group

  16. Results • In example shown, if either the 90% or 80% service is considered acceptable then traffic can be allocated without any “Sharing Rule”, except for QAP A • If 90% or 80% is acceptable, then simple Sharing rule could be based on thisIn our example, only QAP A, using EDCA, is ‘in trouble’ • Problem is that this actually means dropping a video, not good. • If QAP A goes ahead as is, will it adversely affect the others? On the face of it, NO, but it will/may affect its own quality • Why should QAP A be punished? Is the idea of EDCA Overhead correct? • Real Answer is that QAPs should select partners based upon QLoads in addition to value of Overlap, if so this situatin may be avoided in practice • Need a set of rules, however, just to play safe – IDEAS? Graham Smith, DSP Group

  17. Conclusions • QLoad element provides information to allow each QAP to calculate • EDCA Overhead • Need to establish acceptable formula to estimate this • Total Traffic Requirement • Any point on the CDF curve, e.g. 100%, 90% and 80% • Could use an additional “Factor” for QAPs at Distances >1What is that Factor? • Need to Choose “Acceptable” Service, e.g. 90% or 80%? • Need to establish Recommended Sharing Rules • HCCA QAPs can determine their total TXOP time to ensure less than 90% of time is allocated • Need to include text to explain how this can be done BUT QLOAD ELEMENT DOES SEEM TO PROVIDE ALL THE REQUIRED INFORMATION Graham Smith, DSP Group

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