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 introduced 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. May 2009
OBSS
The OSQAP proposal introduced the QLoad Element for 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 09/0660) based on MCCAOP Advertisment
Access Fraction: Total actual admitted time and/or scheduled time expressed as a fraction of 32us/sec rounded down to 1/256
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 1/256)
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
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 clear – how to use for Sharing?
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”?
Want to look deeper into how to use this for Sharing
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 65
Mbps
MEAN
MAX
STDEV
DVD
0.085
0.123
0.019
SDTV
0.062
0.092
0.015
HDTV
0.231
0.308
0.038
Video Streams, Mbps
MEAN
MAX
STDEV
DVD 6 8
1.25
SDTV 4 1
HDTV 15 20
2.5
Graham Smith, DSP Group

9. May 2009
Summing of Times
At first sight, the “total” Requirement will be the sum of all the QLoads of the QAPs in the QLoad Element “A + B + C + D”
BUT, in this example, QAP D can re-use the time of QAP A (and vice versa) as they are at Distance 3 from each other
QAP B has QAP D in its QLoad Element, at Distance 2 and can see that QAP A does not have QAP D in its QLoad Element. Therefore, QAP B knows that QAP D and QAP A are at Distance 3 and re-use can happen. Therefore the Total Requirement is “B + C + (A or D)” (whichever is the larger)
In our example, QLoad shows QAP and Distances as:
A0 B1 C2
B0 A1 C1 D2
C0 B1 D1 A2
D0 C1 B2
QAP B looks for D in QAP A QLoad
Not present, so knows A and D can re-use time
Hence, B takes the greater Self QLoad of A and D
Graham Smith, DSP Group

10. Summary of QLoad Elements
May 2009
Summary of QLoad Elements
QAP
# Streams
SELF
Distance
OTHER
TOTAL
Mean
STDEV A B C D 3
0.40
0.05 1 2
(3)
0.56
0.96
0.07
0.18
0.03
0.78
0.38
0.58
0.23
0.79
0.06
B + C + A
A>D
# streams
EDCA
QAP
(Direct)
Factor A 6
1.3 B 9
1.45 C D
Graham Smith, DSP Group

11. May 2009
Traffic Requirements ignoring EDCA
QAP
SELF
TOTAL (visible)
100%
90%
80% A
0.49
0.46
0.44
1.10
1.05
1.02 B
0.24
0.22
0.21 C
0.47
0.41 D
0.29
0.27
0.26
0.92
0.87
0.84
Traffic Requirements with EDCA Factor
QAP
SELF
TOTAL (visible)
100%
90%
80% A
0.64
0.60
0.57
1.43
1.37
1.33 B
0.34
0.32
0.30
1.60
1.53
1.48 C
0.68
0.63 D
0.38
0.35
0.33
1.19
1.13
1.10
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

12. 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
Each QAP examines its QLoad Element for QAPs at Distance 2 and then checks to see if that QAP is not present in any QAP at Distance 1. The larger of QLoads is then used.
Each QAP calculates the 100% traffic for all QAPs in QLoad Element, taking account of re-use
Each QAP then applies its EDCA Factor 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”, in our example
QAP A Self Traffic = Allocation Limit = 0.31
QAP B Self Traffic = Allocation Limit = 0.15
QAP C Self Traffic = Allocation Limit = 0.29
QAP D Self Traffic = Allocation Limit = 0.18
THEREFORE THE “ACCESS FRACTION LIMIT” FIELD IN “QLOAD ELEMENT”
IS MODIFIED TO BE “ACCESS FACTOR”
Graham Smith, DSP Group

13. ACCESS FACTOR FIELD Medium Time and TXOPs are all measured in 32us/sec
May 2009
ACCESS FACTOR FIELD
Medium Time and TXOPs are all measured in 32us/sec
Access Factor can be > 1
32us/sec = 31,250 per second which is 15 bits
Use 2 octets for Access Factor gives maximum = Is this enough? Too precise?
To express in 1 octet
2 bits for Integral (whole number)
6 bits for the decimal fraction, expressed as a fraction rounded down to 1/64
Example: Sum = = seconds
Hence, octet would be [2 and 24/64 = 2.375]
Maximum value would be 3.98
Graham Smith, DSP Group

14. “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 Fraction” and “Access Factor” are 1 octet each
“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

15. May 2009
Graham Smith, DSP Group

16. Medium Time Allocations
May 2009
Medium Time Allocations
It is important to understand how the AP allocates the actual Medium Times in responses to TSPECs and checks that it has not exceeded its ‘limit’
Obviously, in response to each TSPEC the AP allocates the Medium Time that corresponds to the peak traffic
When allocating an additional Medium Time, the AP must calculate what the composite stream would be, and check that this composite medium time does not exceed the limit
It is this composite time, that is advertised in the Access Fraction
Note that the actual sum of the Medium Times will be greater than the composite time, but EDCA only uses what it needs, and hence the statistical nature of the streams causes the composite time to be the maximum of what is actually being used.
Graham Smith, DSP Group

17. 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
The calculation of Access Factor must include the method of determining re-use of QAPs at Distance 3
How to use the Access Factor for Sharing is further discussed in 09/0666
Graham Smith, DSP Group