1. OBSS Sharing with Access Fraction
May 2009
doc.: IEEE /0xxx
May 2009
OBSS Sharing with Access Fraction
Date:
Authors:
Graham Smith, DSP Group
Graham Smith, DSP Group

2. May 2009
doc.: IEEE /0xxx
May 2009
Abstract
Presentation 09/0660r1 described the HCCAOP scheme for HCCA QAPs OBSS based upon the MCCAOP Advertisement Element used in 11s. In that element fields for Access Fraction and Access Fraction Limit are used. 09/0660r1 proposed adding these fields to the QLoad Element proposed for OBSS in order to provide a method for sharing between overlapping QAPs.
This presentation studies how these fields may be used for Sharing.
Graham Smith, DSP Group
Graham Smith, DSP Group

3. OBSS The OSQAP proposal introduced the QLoad Element QLoad Element
May 2009
OBSS
The OSQAP proposal introduced the QLoad Element QLoad Element
EDCA Admission Control and HCCA QAPs
This element has evolved over several presentations and the latest form is given overleaf.
Graham Smith, DSP Group

4. May 2009
Graham Smith, DSP Group

5. QLoad Element Fields May 2009 Overlap
Number of APs that are sharing this channel and are overlapping
Access Fraction (see 09/0660r0)
Access Fraction: Total actual admitted time and/or scheduled time expressed as a fraction of 32us/sec rounded down to 1/16
Access Fraction Limit: Maximum admitted time and/or scheduled time that this QAP may allocate expressed as a fraction of 32us/sec rounded down to 1/16
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 (see 09/0496r2 and 09/0497r2)
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
Distance
Distance is set to 0 for Self
QAP ID Directly visible to the QAP Self, then “Distance” is set to 1
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
Any QAP with Distance” > 2 is not recorded in QLoad Element
Graham Smith, DSP Group

6. Using the Access Fraction Fields In QLoad Element
May 2009
Using the Access Fraction Fields In QLoad Element
Access Fraction (as per MCCAOP)
Total actual admitted time and/or scheduled time
Use is straightforward. QAP reports what it is doing at that moment.
Access Fraction Limit (AFL):
Maximum admitted time and/or scheduled time
Use is clear. How it is set, is not
Observations:
The AFL refers to the QAP itself and hence sets the maximum allocation for that QAP.
For the AFL to be meaningful for all QAPs in the OBSS, they must all use the same “rule” for setting AFL
If we had a “rule”, then the advertising of the AFL would be solely to act as a check that the QAP was applying the rule correctly
Could “the rule” have variations? If so, how does the OBSS know which “rule”?
Graham Smith, DSP Group

7. QLoads All QAPs report their QLoads in the form of :
May 2009
QLoads
All QAPs report their QLoads in the form of :
MEAN and STDEV This allows the summing of the QLoads to take account of the distributed nature of the traffic
Total Traffic Requirement can be estimated (see 09/0497r2)
MEAN µtot = ΣMEANi
STDEV σtot = sqrt(Σσi2)
100% MAX traffic = µtot + 2 σtot
90% Traffic = µtot σtot
80% Traffic = µtot σtot
In addition EDCA overhead is required
N (N = # of directly competing streams, i.e. Distance =1)
Graham Smith, DSP Group

8. Sharing Method by way of example
May 2009
Sharing Method by way of example
Example used is 4 QAPs with 3 Video streams as per below
QAP
TRAFFIC A
2 DVD streams 1 HDTV B
3 SDTV Streams C
1 HDTV 1 DVD 1 SDTV D
2 DVD 1 SDTV
Video Streams, Fractions of Bandwidth
Bandwidth 80
Mbps
MEAN
MAX
STDEV
DVD
0.069
0.100
0.016
SDTV
0.050
0.075
0.013
HDTV
0.188
0.250
0.031
Video Streams, Mbps
MEAN
MAX
STDEV
DVD 6 8
1.25
SDTV 4 1
HDTV 15 20
2.5
Graham Smith, DSP Group

9. Summary of QLoad Elements
May 2009
Summary of QLoad Elements
QAP
# Streams
SELF
Distance
OTHER Distance <3
TOTAL
Mean
STDEV A B C D 3
0.33
0.04 1 2
(3)
0.46
0.78
0.06
0.15
0.02
0.82
0.97
0.31
0.66
0.05
0.19
0.03
0.64
# streams
EDCA
QAP
(Direct)
Factor A 6
1.3 B 9
1.45 C D
Graham Smith, DSP Group

10. May 2009
Traffic Requirements ignoring EDCA
QAP
SELF
TOTAL (visible)
100%
90%
80% A
0.40
0.37
0.36
0.90
0.86
0.83 B
0.19
0.18
0.17
1.09
1.05
1.02 C
0.38
0.35
0.34 D
0.24
0.22
0.21
0.74
0.71
0.69
Traffic Requirements with EDCA Factor
QAP
SELF
TOTAL (visible)
100%
90%
80% A
0.52
0.49
0.46
1.17
1.11
1.08 B
0.28
0.26
0.24
1.59
1.52
1.48 C
0.55
0.51 D
0.31
0.29
0.27
0.97
0.92
0.89
Example chosen to deliberately cause “Overload” so as to look at “Sharing”
As can be seen, the EDCA Factor does have a significant effect
100, 90 and 80% does not seem, in this case, to make a significant difference
Graham Smith, DSP Group

11. May 2009
SHARING PROCEDURE
Intention is to reduce the worse “Total” to unity. In this example this refers to QAPs B and C. The following procedure achieves this.
Each QAP calculates its 100% traffic Self Traffic = Mean + 2 x Stdev e.g. B = .19
Each QAP calculates the 100% traffic for all QAPs in QLoad Element Total Traffic = ΣMEANi + 2 x sqrt(ΣStdevi2) e.g. B = 1.09
Each QAP then applies its EDCA Factor (Total Traffic) x ( N) e.g. B = This Value, “Access Factor (AF)”, must be advertised in the QLoad Element
In order to make the Total Traffic at QAP B unity, then:
The Self Traffic that each QAP can allocate is reduced by largest value of “AF” in its QLoad Element
Hence, because of “EDCA Factor” AND “Access Factor”
QAP A Self Traffic = Allocation Limit = 0.25
QAP B Self Traffic = Allocation Limit = 0.12
QAP C Self Traffic = Allocation Limit = 0.24
QAP D Self Traffic = Allocation Limit = 0.15
THEREFORE THE “ACCESS FRACTION LIMIT” FIELD IN “QLOAD ELEMENT”
IS MODIFIED TO BE “ACCESS FACTOR”
Graham Smith, DSP Group

12. “Access Factor” in QLoad Element
May 2009
“Access Factor” in QLoad Element
In the QLoad Element proposed in slide 4, the “Access Fraction Limit” is replaced with “Access Factor”
“Access Factor” is the total traffic requirement for all overlapping QAPs, derived from the sum of the traffic of all QAPs in the QLoad Element. The Access Factor also should include the “EDCA Factor” to account for the overhead required due to multiple priority streams.
QAPs should examine their QLoad Element in order to determine the maximum “Access Factor” being reported. This maximum value is then used to determine the allocation limit for that QAP in order not to cause over-allocation in other QAPs that are overlapping.
The procedure for calculating Access Factor, as per previous slide, shall be detailed in the Specification
Graham Smith, DSP Group

13. May 2009
Conclusion
By including the “Access Factor” field in the QLoad Element, it is possible for all overlapping QAPs to allocate streams such that they do not cause over-allocation in other overlapping QAPs
The method by which the “Access Factor” may be used to prevent over-allocation has been described.
Graham Smith, DSP Group