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Empirical Formula for EDCA Bandwidth Factor

This presentation investigates the medium access overhead and derives empirical formulas to estimate the EDCA Bandwidth Factor. A simulation program is used to measure throughput and delays for multiple streams.

jameswoods
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Empirical Formula for EDCA Bandwidth Factor

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  1. Empirical Formula for EDCA Bandwidth Factor Authors: Date: 2010, March 12 Graham Smith, DSP Group

  2. Abstract The Medium Time returned for an EDCA Admission Control TSPEC includes the packet length, SIFS and ACK. It does not include the overhead required for medium access. This presentation: • Investigates the medium access overhead with respect to the number of VO and VI streams • Investigates equal streams, mixed streams, equal and mixed data rates, PHY rates, aggregations • Derives ‘simple’ empirical formulas and rules to estimate EDCA Bandwidth Factor Graham Smith, DSP Group

  3. Medium Time • Medium Time = Surplus Bandwidth Allowance * pps * medium time per frame exchange where: • pps = ceiling( (Mean Data Rate / 8) / Nominal MSDU Size ); • medium time per frame exchange = duration (Nominal MSDU Size, Minimum PHY Rate) + SIFS + ACK duration; • duration() is the PLME-TXTIME primitive defined in the standard that returns the duration of a packet based on its payload size and the PHY data rate employed • Note that it represents the time that the packet is on the air, and does not include the time between packets, the Medium Access Time • The Medium Access Time includes SIFS, AIFSN and the Contention Window value Graham Smith, DSP Group

  4. Medium Access Time • The Medium Access Time includes SIFS, AIFSN and the Contention Window value • As the number of streams increases, each stream is held up in the contention window while another stream is transmitting. • The total time required for EDCA streams is therefore greater than the Medium Time and it varies with the number of streams. • This total required time compared to the Medium Time is termed the EDCA Overhead Factor. Graham Smith, DSP Group

  5. Investigation into EDCA Overhead Factor • A simulation program was written to measure the throughput and delays for multiple streams. The following can be set for each stream: • EDCA parameters • Packet Size • Data rate, Mbps • PHY Rate, Mbps • The outputs are: • Packets in, Packets out • Actual data rate • Maximum and Average Packet Delay Graham Smith, DSP Group

  6. Method • For a set number of streams, the data rate was varied and the average packet delay noted. • For the values to be acceptable, or a PASS, the criteria used was: Average Delay < 2 times the SI of the packet • Where SI = reciprocal of Packets per Second = 1/pps • And pps = data rate/packet length Graham Smith, DSP Group

  7. Simulation Example 10 Video Streams @ 54Mbps Graham Smith, DSP Group

  8. Methodology • For a set number of streams, the data rate was varied until the average delay for every stream was acceptable • The total Medium time was then noted, in µsecs • Medium Time Factor = 1sec/Medium Time (overhead) • For VI the packet size was set to 1316Bytes • The VO packet size was set to 160Bytes and the data rate to 64 kbps • 11a/g assumed to be the same (6us extension on OFDM to make SIFS 16us)) • 11n 2SS with Aggregations of 0, 8k, 16k, 32k and 64k Graham Smith, DSP Group

  9. Example Results for equal streams Results taken for 54, 36, 24, 18, 12 and 6Mbps Graham Smith, DSP Group

  10. Results for Equal VI streams Graham Smith, DSP Group

  11. EDCA Overhead Formula 11a/g AC_VI • The results were examined and the following formula were derived for estimating the EDCA Overhead Factor, F for 11a/g For PHY Rate R, and number of equal streams S • 1 stream F = 1.01 + 0.004R • 2 streams F = 1.04 + 0.003R • 3 to 10 streams F = 1.1 + 0.007S + .0025R • >10 streams, S = 10 Graham Smith, DSP Group

  12. Comparison of Results vs. FormulaAC_VI Equal Streams Formula result is within 1% of simulated Graham Smith, DSP Group

  13. AC_VO EDCA Overhead Factorusing F formulas, Equal Streams 11a/g Graham Smith, DSP Group

  14. AC_VO Simulations Example with 8 bi-directional voice streams plus one video stream. Video stream data rate is increased until delay limit is reached. AP has 8 x Mbps Graham Smith, DSP Group

  15. VO Results • Simulations were carried out using bi-directional voice packets of 160 Bytes at 64kbps (20ms SI) • The results were examined and the following formula were derived for estimating the EDCA Overhead Factor, F for 11a/g For PHY Rate R, and number of streams S • 1 to 10 stream F = 1.175 + 0.007S + 0.0025R • >10 streams, S = 10 Graham Smith, DSP Group

  16. Comparisons of Simulations vs FormulaAC_VO NOTE: Number of streams is number of Bi-Directional streams Formula provides results for all PHY Rates, including 18Mbps Graham Smith, DSP Group

  17. AC_VO EDCA Overhead Factorusing F formula, 11a/g Graham Smith, DSP Group

  18. VO VI Mix • Measurements taken with 1, 2, 3 and 4 video streams • Overhead factor does not change with number of video streams RULE: Just count the AC_VO streams Graham Smith, DSP Group

  19. Mixed AC_VI Data Rates, 11a/gSame PHY Rate • For a fixed PHY Rate, with fixed number of streams, • select one data rate, vary other for Delay Limit • E.g. 24Mbps, 2 streams • 10 and 7.25Mbps • 12 and 5.1Mbps • 14 and 2.6Mbps • 15 and 1.2Mbps • Repeated this for more streams and other PHY Rates. • By examining results, established following “Rule” S equivalent, Sequ = 0.5 x Total Medium Time of all streams Medium Time of Stream with least Medium Time MaxSequ = 10 Enter Sequinto the formula to obtain F for fixed R Formulas for 1 and 2 streams is modified for use when Sequ is a ‘fraction’ For 1< Sequ<2 F = 0.97 + 0.04S + .004R For 2< Sequ<3 F = 0.97 + 0.04S + .003R Graham Smith, DSP Group

  20. Mixed PHY Rates, 11a/g • Simulated several mixes of PHY Rates and data rates • Investigation resulted in the following Rules, similar to Mixed Data Rates Sequ = 0.5 x Total Medium Time of all streams Medium Time of ‘lowest stream’ R = PHY Rate of the ‘lowest stream’ (‘lowest stream’ is stream with shortest Medium Time) Hence, use Sequ and Rlowest in the F formula Graham Smith, DSP Group

  21. Summary 11a/g EDCA Overhead Factor F AC_VI Basic Formula • 1 stream F = 1.01 + 0.004RFor 1< Sequ<2 = 0.97 + 0.04S + .004R • 2 streams F = 1.04 + 0.003RFor 2< Sequ<3 = 0.97 + 0.04S + .003R • 3 to 10 streams F = 1.1 + 0.007S + .0025R • >10 streams, S = 10 For Mixed Data Rates and PHY Rates Sequ = 0.5 x Total Medium Time of all streams Medium Time of stream with least Medium Time R = PHY Rate of the stream with least Medium Time AC_VO Basic Formula • 1 to 10 stream F = 1.175 + 0.007S + 0.0025R • >10 streams, S = 10 For Mixed VO and VI Just count the AC_VO streams For mixed PHY Rates R = Highest PHY Rate of the VO streams Graham Smith, DSP Group

  22. Break for Questions on 11a/g, then tackle EDCA Overhead Factor for 11n This gets complicated!! Graham Smith, DSP Group

  23. EDCA Overhead Factor 11n • The 11n networks investigated were: • 2SS • No Aggregation • A-MPDUs for 64k, 32k, 16k, 8k • In case of aggregation, Medium Time was calculated as the ‘on-air time’ including BA • Implicit BA was assumed Note: As far as I can tell, there is no set method for the STA to indicate it is using A-MPDUs and whether 64, 32, 16 or 8k and this makes it difficult for an AP to calculate the correct Medium Time. Proposals are being considered within Wi-Fi Alliance but these will not be discussed here. In the following slides, it is assumed that the Medium Time is correctly calculated knowing the aggregation. Graham Smith, DSP Group

  24. 11n Same procedure as for 11a/g, For No Aggregation and 64k, 32k, 16k and 8k Aggregation • Equal VI streams for different PHY rates • Calculate F formulas • VO bi-directional streams with VI stream(s) • Calculate F formula • Investigate Mixed Aggregation Graham Smith, DSP Group

  25. 11n No Aggregation VI - Results For PHY Rate R, and number of equal streams S • 1 stream F = 1.05 + 0.0026R • 2 streams F = 1.076 + 0.002R • 3 to 10 streams F = 1.12 + 0.0073S + .0015R • >10 streams, S = 10 Graham Smith, DSP Group

  26. AC_VI EDCA Overhead Factorusing F formulas, Equal Streams 11nNo Aggregation Graham Smith, DSP Group

  27. 11n No Aggregation VO - Results For PHY Rate R, and number of streams S • 1 to 10 stream F = 1.21 + 0.004S + 0.0016R • >10 streams, S = 10 Graham Smith, DSP Group

  28. AC_V0 EDCA Overhead Factorusing F formula, 11nNo Aggregation Graham Smith, DSP Group

  29. 11n AggregationEqual Streams, same PHY Rate • Individual formulas calculated for each Aggregation • Then a ‘combined formula’ derived • 1 streamF = 1 + 0.0075R/AFor mixed streams= 0.94 + .06S + 0.0075R/A (1<Sequ<2) • 2 streams F = 1.05 + 0.0044R/AFor mixed streams= 0.94 + .06S + .0044R/A (2<Sequ<3) • 3 to 10 streamsF = 1.1 + 0.007S + .0041R/A • >10 streams, S = 10 Where R = PHY Rate S = number of streams A = 8, 16, 32 or 64 (corresponding to 64k, 32k, 16k, 8k) Graham Smith, DSP Group

  30. 11n Aggregation F formula ResultsEqual Streams VI Packet Duration > 10ms Packet Duration > 10ms Graham Smith, DSP Group

  31. 11n Aggregation F formula ResultsEqual Streams VI Graham Smith, DSP Group

  32. 11n Aggregation Results by Formula Left to Right In each A Block 130 -> 13Mbps 64k32k16k 8k 130M 13M Graham Smith, DSP Group

  33. Simulation Example O/H Factor Eight 64k Streams Two 8k Streams Graham Smith, DSP Group

  34. Mixed Aggregation • Same PHY Rate, same data rate per stream, mixed aggregation, exampleNote: actual readings (not formula) Compare: Two 8k streams is better throughput than one 8k and one 64k! Also, note that the O/H Factor is constant for the mix AND the value is higher than 8k O/H, even for 9 and 10 streams. Repeating for two and three 8k streams, similar O/H Factor results Graham Smith, DSP Group

  35. More “Strange” results with Aggregation 1.27 Factor as per previous slide Note that the total throughput is often better with lower Aggregation!! AND The Overhead Factor is worse For higher aggregation EXCEPT when 8k portion is low Graham Smith, DSP Group

  36. 8k + 8k Example • Sequ = 0.5 x MT Total/MTmin • Factor is equivalent to • For 1 - 2 SequF= 0.94 + .06Sequ + 0.0075R/A (accounts for fractions) • For 2 - 3 Sequ F = 0.94 + .06Sequ + .0044R/A (accounts for fractions) • For 3 - 10 Sequ F = 1.1 + 0.007S + .0041R/A • THIS IS SAME RULE FOR Sequ AS PER 11a/g Graham Smith, DSP Group

  37. 16k + 8k Example Rule goes wrong here And gets worse for 32k and 64k Need to account for aggregation somehow Graham Smith, DSP Group

  38. 64k + 8k Dominated by 64k but Factor is high for 64k Equal streams, F = 1.27 as per slide 34 Much worse than 10+ at 8k (1.21) Why is this worse? When equal, 8 x as many 8k packets as 64k packets, but 64k packets are 8x longer than 8k packets Graham Smith, DSP Group

  39. EDCA Overhead Factor and Aggregation • When equal streams and fixed PHY Rate, the formulas are straightforward • Mixing data rates with same aggregation, the “rule’ derived for 11a/g holds • Mixing aggregation, unable to derive a ‘rule’ based upon the “equal stream’ formulas • Therefore, new formulas derived Graham Smith, DSP Group

  40. Mixed Aggregation and Data Rate • Looking (real hard) at the results, the following empirical formula results: • Specify a new parameter, r r = MT of lowest aggregation stream (total Medium Time) • Then, for mixed aggregation streams: F = 1.12 + [0.005 + 0.028 (Amax/Amin)] r Graham Smith, DSP Group

  41. Results Mixed Aggregation formula Equal stream, Sequ formula See over Graham Smith, DSP Group

  42. 11n Aggregation Formula Question: Could we use the same formula for same aggregation, using r = MTmin/MTtot ? Not so accurate, but is ‘safe’ here Graham Smith, DSP Group

  43. 11n Aggregation Formula Proposed formula is: For r = Total MT of lowest aggregation streams (total Medium Time) • Then, for mixed aggregation streams, at all PHY Rates: F = 1.12 + [0.005 + 0.028 (Amax/Amin)] r Graham Smith, DSP Group

  44. Examples using Aggregation Formula Reasonable results, Graham Smith, DSP Group

  45. SUMMARY 11n R = PHY Rate, S = number of streams, A = 8, 16, 32 or 64 (corresponding to 8k, 16k, 32k, 64k) Equal VI Streams, no aggregation • 1 stream F = 1.05 + 0.0026R • 2 streams F = 1.076 + 0.002R • 3 to 10 streams F = 1.12 + 0.0073S + .0015R (>10 streams, S = 10) VO Bi-directional streams (no aggregation) • 1 to 10 stream F = 1.21 + 0.004S + 0.0016R (>10 streams, S = 10) Equal VI streams, same aggregation • For 1 - 2 StreamsF = 0.94 + .06S + 0.0075R/A • For 2 - 3 StreamsF= 0.94 + .06S + .0044R/A • For 3 - 10 Streams F = 1.1 + 0.007S + .0041R/A (>10 streams, S = 10) Mixed streams, same Aggregation • S = Sequ = 0.5 x MT Total/MTmin Mixed Streams, mixed Aggregation F = 1.12 + [0.005 + 0.028 (Amax/Amin)] r Where r = Total MT of Amin aggregation streams Total Medium Time Graham Smith, DSP Group

  46. Next Steps • These empirical formulas could be used by the AP to estimate the required bandwidth for EDCA Admission control, as the AP will have all the information required • For OBSS, to estimate the combined Overhead Factor, each AP would need to know more details of individual streams, such as the max PHY Rate, and this maybe too much data for QLoad? • Next step is to use these results to look at the minimum amount of data required in the OBSS QLoad, so as to make a reasonable estimate • This could be as simple as always assume a fixed Overhead, e.g. F = 1.30 • Could be based upon each AP reporting its Overhead Factor and then combine them somehow • At the moment the QLoad reports, number of streams and total medium times • Could be a simplified ‘safe’, formula Graham Smith, DSP Group

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