1. Considerations for OBSS Sharing using QLoad Element
Apr 2009
doc.: IEEE /0xxx
Apr 2009
Considerations for OBSS Sharing using QLoad Element
Date: 2009, April 30
Authors:
Graham Smith, DSP Group
Graham Smith, DSP Group

2. Apr 2009
doc.: IEEE /0xxx
Apr 2009
Abstract
The proposed QLoad Element (as per 09/0496r0) 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
Graham Smith, DSP Group

3. Proposed Extended QLoad Element
Apr 2009
Proposed Extended QLoad Element
Graham Smith, DSP Group

4. QLoad Element Fields Apr 2009 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
Visible Bit
If the QAP that corresponds to ID, MEAN and STDEV values is directly visible to the QAP Self, then this is set to 1
Visibility bit set to 1 for Self
Channel Priority
Used only if QAP is operating with HCCA, indicates HCCA Supervisor
QAP Priority Streams
Number of streams on ACs 2 and 3 per QAP. Used so that the contention overhead can be estimated.
Graham Smith, DSP Group

5. Traffic Information Apr 2009
Every QAP knows the traffic requirements of every other QAP in the OBSS Graph, even those that are not directly visible
Total 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 σtot
80% Traffic = µtot σtot
Other?
Total Traffic Requirement limit is also affected by
EDCA Overhead Contention overhead reduces the total traffic bandwidth. Important as number of streams increases
HCCA Allocation Limit Need 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
Apr 2009
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 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 for independent streams.
Downlink streams from QAP may be better due to queuing at the AP
Graham Smith, DSP Group

7. EDCA Bandwidth Overhead
Apr 2009
EDCA Bandwidth Overhead
Due to contention, the required bandwidth for EDCA is larger than the sum of the streams
Approximation based upon example on previous slide:
EDCA Bandwidth Factor = N (approx; keep it simple)
Where N = Number of streams
4 streams Effective Bandwidth Factor = 1.2 Therefore 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. EDCA and HCCA Sharing Considerations
Apr 2009
EDCA and HCCA Sharing Considerations
When considering defined methods or recommendations for Sharing to ensure quality of service, the following points apply:
EDCA
Contention based scheme, requires certain overhead to ensure throughput and latency requirement
EDCA Admission Control does attempt to provide a “guarantee” of QoS
Used bandwidth is always related to the required bandwidth
HCCA
Scheduled scheme, sharing requires knowledge of required durations of TXOPs for each QAP for partitioning of time slots
HCCA attempts to “guarantee” QoS.
TXOPs are terminated as soon as traffic sent for that slot, i.e. unused bandwidth is returned
Graham Smith, DSP Group

9. Basic General Sharing Considerations
Apr 2009
Basic General Sharing Considerations
Total Traffic Requirement of all the sharing QAPs can be determined
Calculate Total Traffic 100%, 90%, 80%, Other?
EDCA contention should be estimated
Based upon QLoads of QAPs that are “Visible”, 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. Apr 2009
Number of Sharing QAPs
How many sharing QAPs need to be assumed in order to evaluate a proposed Sharing Scheme? (see 08/1470r4)
Detached, Terraced and Town House scenarios (12, 16, 24 APs in range respectively)
Limited to just 1 overlap, or 2 sharing QAPs for 9 Channels
Single Apartment Blocks (28 APs in range)
98.3% Prob of OBSS length <= 2 for 9 Channels
98.8% Prob of OBSS length <= 2 for 11 Channels
OBSS length <= 2 for 17 Channels
Double Apartment Blocks (53 APs in range)
If 20/40MHz selection scheme adopted, then OBSSlength<=2
Graham Smith, DSP Group

11. Sharing Scenario Apr 2009 The typical case is no or just 1 overlaps
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 “Visible”
QAP B
Overlap = 1, QAPs A is “Visible”
QAP C
Overlap = 1, QAP A is “Visible”
Graham Smith, DSP Group

12. EXAMPLE - 11n Apr 2009 Basic Video Data Rates
Codec
SDTV
DVD
HDTV
MPEG2
1-5Mbps
8Mbps
15-23Mbps
MPEG4
1-2Mbps
2-3Mbps
8-12Mbps
Basic Video Data Rates, in Mbps, for MPEG2
MEAN
MAX
STDEV
DVD 5 8
1.5
SDTV 3 1
HDTV 15 20
2.5
Basic Video Data Rates, fraction of bandwidth for 80Mbps Bandwidth (11n)
MPEG 2
MEAN
MAX
STDEV
DVD
0.0625
0.1
SDTV
0.0375
0.0125
HDTV
0.1875
0.25
Graham Smith, DSP Group

13. Example - EDCA Apr 2009 Example QLoads for each QAP QAP # Streams Mean
STDEV
Visibility
COMMENTS A B C 3
0.31
0.04 1
2 DVD, 1 HDTVstreams
0.11
0.02
3 SDTV Streams
0.29
1 HDTV 1 DVD 1SDTV
TOTAL
0.71
0.06
EDCA Total Traffic Requirements
QAP
Streams
EDCA O/H
100%
90%
80% A 9
1.45
1.21
1.15
1.11 B 6
1.3
1.08
1.03
0.99 C
Now what?
OK if 80% Traffic is <1?
Graham Smith, DSP Group

14. (Peak) Allocation per QAP
Apr 2009
Example - HCCA
HCCA Total Traffic Requirements
QAP
HCCA O/H
100%
90%
80% A
1.1
0.92
0.87
0.84 B C
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
QAP
Mean
STDEV
(Peak) Allocation per QAP
100%
90%
80% A
0.31
0.04
0.39
0.37
0.35 B
0.11
0.02
0.16
0.14
0.13 C
0.29
0.36
0.34
0.32
Totals
91%
84%
Needs further
explanation
Graham Smith, DSP Group

15. Apr 2009
HCCA Time Allocation
Note that in example the sum of the allocated time (TXOPs) is 91%. This contradicts the requirement that at least 10% of the bandwidth is not scheduled. The 1.1 factor was applied in order to check if the sharing was OK but this is based upon the total traffic whereas the sum of the individual QAP traffic is not the statistical sum, but a linear sum. Hence, in practice, each QAP needs to adjust its TXOP allocation to keep the sum not more than 90%. They can do this because they know all the details. In this particular case they can calculate the individual and total time allocations, and then reduce their TXOP allocation to comply.
This procedure needs to be included in the specification.
Graham Smith, DSP Group

16. Apr 2009
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 Example “Rule”: “If QAP calculates that the calculated 80% Total Bandwidth Requirement is >1 then each QAP must reduce its bandwidth allocation so that the Total 80% Bandwidth Requirement is <=1”
In our example, only QAP A is more affected
Problem is that this actually means dropping a video, not good.
If QAP A goes ahead as is, will it adversely affect the others?
Why should QAP A be punished? Is the idea of EDCA Overhead correct?
Real Answer is that QAPs will select partners based upon QLoads in addition to value of Overlap
Need a set of rules, however, just to play safe
Graham Smith, DSP Group

17. Apr 2009
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%
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
Graham Smith, DSP Group