1. Empirical Formula for EDCA Bandwidth Factor
March 2010
doc.: IEEE /0304r0
March 2010
Empirical Formula for EDCA Bandwidth Factor
Date: 2010, March 12
Authors:
Graham Smith, DSP Group
Graham Smith, DSP Group

2. March 2010
doc.: IEEE /0304r0
March 2010
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
Graham Smith, DSP Group

3. March 2010
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. March 2010
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
March 2010
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. March 2010
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 March 2010 10 Video Streams @ 54Mbps
Graham Smith, DSP Group

8. March 2010
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 1470Bytes
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
March 2010
Example Results for equal streams 36
Mbps
VI Streams
Mbps/stream
Medium Time
O/H Factor 1
23.65
853628
1.17 2 24 12
866261
1.15 3
23.19
7.73
837025
1.19 4
22.68
5.67
818617
1.22 5
22.5
4.5
812120
1.23 6
22.32
3.72
805623
1.24 7
22.19
3.17
800931
1.25 8
22.08
2.76
796960 9
22.05
2.45
795878
1.26 10 22
2.2
794073
Results taken for 54, 36, 24, 18, 12 and 6Mbps
Graham Smith, DSP Group

10. Results for Equal VI streams
March 2010
Results for Equal VI streams
Graham Smith, DSP Group

11. EDCA Overhead Formula 11a/g AC_VI
March 2010
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 = R
2 streams F = R
3 to 10 streams F = S R
>10 streams, S = 10
Graham Smith, DSP Group

12. Comparison of Results vs. Formula AC_VI
March 2010
Comparison of Results vs. Formula AC_VI
Formula result is within 1% of simulated
Graham Smith, DSP Group

13. AC_VO EDCA Overhead Factor using F formulas, Equal Streams 11a/g
March 2010
AC_VO EDCA Overhead Factor using F formulas, Equal Streams 11a/g
Streams 54 36 24 18 12 6 1
1.23
1.15
1.11
1.08
1.06
1.03 2
1.20
1.09 3
1.26
1.21
1.18
1.17
1.14 4
1.22
1.19
1.16 5
1.27
1.28 7
1.24 8
1.29
1.25 9
1.30 10
1.31
Graham Smith, DSP Group

14. AC_VO Simulations March 2010
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. March 2010
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 = S R
>10 streams, S = 10
Graham Smith, DSP Group

16. Comparisons of Simulations vs Formula AC_VO
March 2010
Comparisons of Simulations vs Formula AC_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 Factor using F formula, 11a/g
March 2010
AC_VO EDCA Overhead Factor using F formula, 11a/g
Bi- Streams 54 36 24 18 12 6 1
1.32
1.27
1.24
1.23
1.21
1.20 2
1.28
1.25
1.22 3
1.33
1.29
1.26 4
1.34 5
1.35
1.30
1.31 7
1.36 8
1.37 9 10
1.38
Graham Smith, DSP Group

18. VO VI Mix Measurements taken with 1, 2, 3 and 4 video streams
March 2010
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/g Same PHY Rate
March 2010
Mixed AC_VI Data Rates, 11a/g Same 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
Max Sequ = 10
Enter Sequ into the formula to obtain F for fixed R
Formulas for 1 and 2 streams is modified for a ‘fraction’
For 1< Sequ<2 F = S R
For 2< Sequ<3 F = S R
Graham Smith, DSP Group

20. March 2010
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 = 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
March 2010
Summary 11a/g EDCA Overhead Factor F
AC_VI Basic Formula
1 stream F = R For 1< Sequ<2 = S R
2 streams F = R For 2< Sequ<3 = S R
3 to 10 streams F = S R
>10 streams, S = 10
For Mixed Data Rates and PHY Rates
Sequ = 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 = S R
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. EDCA Overhead Factor for 11n
March 2010
Break for Questions on 11a/g, then tackle
EDCA Overhead Factor for 11n
This is a BIG subject!!
Graham Smith, DSP Group

23. EDCA Overhead Factor 11n The 11n networks investigated were:
March 2010
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 correct.
Graham Smith, DSP Group

24. 11n Same procedure as for 11a/g,
March 2010
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
March 2010
11n No Aggregation VI - Results
For PHY Rate R, and number of equal streams S
1 stream F = R
2 streams F = R
3 to 10 streams F = S R
>10 streams, S = 10
Graham Smith, DSP Group

26. March 2010
AC_VI EDCA Overhead Factor using F formulas, Equal Streams 11n No Aggregation
Streams
130
117
104 78 52 39 26 13 1
1.39
1.35
1.32
1.25
1.19
1.15
1.12
1.08 2
1.34
1.31
1.28
1.23
1.18
1.13
1.10 3
1.30
1.26
1.22
1.20
1.16 4
1.27
1.21
1.17 5
1.33 6
1.36
1.24 7
1.37
1.29 8 9
1.38 10
Graham Smith, DSP Group

27. 11n No Aggregation VO - Results
March 2010
11n No Aggregation VO - Results
For PHY Rate R, and number of streams S
1 to 10 stream F = S R
>10 streams, S = 10
Graham Smith, DSP Group

28. AC_V0 EDCA Overhead Factor using F formula, 11n No Aggregation
March 2010
AC_V0 EDCA Overhead Factor using F formula, 11n No Aggregation
Bi-Dir
Streams
130
117
104 78 52 39 26 13 1
1.42
1.40
1.38
1.34
1.30
1.28
1.26
1.23 2
1.43
1.41
1.24 3
1.39
1.35
1.31 4
1.29
1.27
1.25 5
1.44 6
1.36
1.32 7
1.45 8
1.37
1.33 9 10
1.46
Graham Smith, DSP Group

29. 11n Aggregation Equal Streams, same PHY Rate
March 2010
11n Aggregation Equal Streams, same PHY Rate
Individual formulas calculated for each Aggregation
Then a ‘combined formula’ derived
1 stream F = R/A For mixed streams = S R/A (1<Sequ<2)
2 streams F = R/A For mixed streams = S R/A (2<Sequ<3)
3 to 10 streams F = S R/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 Results
March 2010
11n Aggregation F formula Results
Packet Duration > 10ms
Packet Duration > 10ms
Graham Smith, DSP Group

31. 11n Aggregation F formula Results
March 2010
11n Aggregation F formula Results
Graham Smith, DSP Group

32. 11n Aggregation Results by Formula
March 2010
11n Aggregation Results by Formula
Left to Right
In each A Block
130 -> 13Mbps
64k 32k 16k 8k
130M 13M
Graham Smith, DSP Group

33. Simulation Example March 2010 O/H Factor Two 8k Streams
Eight 64k Streams
Graham Smith, DSP Group

34. Mixed Aggregation March 2010
Same PHY Rate, same data rate per stream, mixed aggregation, example Note: 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
March 2010
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 March 2010 Sequ = 0.5 x MT Total/MTmin
Factor is equivalent to
For Sequ F = Sequ R/A (accounts for fractions)
For Sequ F = Sequ R/A (accounts for fractions)
For Sequ F = S R/A
THIS IS SAME RULE FOR Sequ AS PER 11a/g
Graham Smith, DSP Group

37. 16k + 8k Example March 2010 Rule goes wrong here
And gets worse for 32k and 64k
Need to account for aggregation somehow
Graham Smith, DSP Group

38. 64k + 8k March 2010 78Mbps One 64k and one 8k Stream Data Rate, Mbps
FACTOR
64k 8k
Tot
56.2 5
61.2
1.15
49.3 10
59.3
1.18
43.5 15
58.5 35 20 55
1.24 28 25 53
1.27 22 30 52
1.28
13.6
48.6
1.34
8.3 40
48.3
4.3 45
1.30
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
March 2010
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
March 2010
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 = [ (Amax/Amin)] r
Graham Smith, DSP Group

41. Results March 2010 Mixed Aggregation formula
Equal stream, Sequ formula
See over
Graham Smith, DSP Group

42. 11n Aggregation Formula March 2010 r = MTmin/MTtot
Note: Could use this formula for same aggregation, using
r = MTmin/MTtot
Not so accurate, but is
‘safe’ here
Graham Smith, DSP Group

43. March 2010
11n Aggregation Formula
r = Total MT of lowest aggregation streams (total Medium Time)
Then, for mixed aggregation streams, at all PHY Rates:
F = [ (Amax/Amin)] r
Graham Smith, DSP Group

44. Examples using Aggregation Formula
March 2010
Examples using Aggregation Formula
Reasonable results,
Graham Smith, DSP Group

45. SUMMARY 11n VO Bi-directional streams (no aggregation)
March 2010
SUMMARY 11n
VO Bi-directional streams (no aggregation)
1 to 10 stream F = S R (>10 streams, S = 10)
Equal VI streams, same aggregation
For Streams F = S R/A
For Streams F = S R/A
For Streams F = S R/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)
Mixed streams, same Aggregation
S = Sequ = 0.5 x MT Total/MTmin
Mixed Streams, mixed Aggregation
F = [ (Amax/Amin)] r
Where r = Total MT of Amin aggregation streams Total Medium Time
Graham Smith, DSP Group

46. March 2010
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 the details of individual streams, and this is probably too much data
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 F = 1.30
At the moment the QLoad reports, number of streams and total medium times
Could be based upon each AP reporting its Overhead Factor and then combine them
Graham Smith, DSP Group