1. HCCAOP Scheme, Efficiency and Sharing
May 2009
doc.: IEEE /0xxx
May 2009
HCCAOP Scheme, Efficiency and Sharing
Date:
Authors:
Graham Smith, DSP Group
Graham Smith, DSP Group

2. May 2009
doc.: IEEE /0xxx
May 2009
Abstract
Presentation 09/0660 introduced the HCCAOP scheme for HCCA QAPs OBSS based upon MCCAOP from 11s
Presentation 09/0662 took the idea of Access Fraction and introduced Access Fraction and Access Factor into the QLoad Element
That presentation concluded that more work was required before it was possible to make a firm decision if HCCAOP should replace the CHP scheme that was previously proposed
The areas of discussion are HCCA Access Fraction and TXOP allocation
Graham Smith, DSP Group
Graham Smith, DSP Group

3. HCCAOP Scheme based upon MCCAOP
May 2009
HCCAOP Scheme based upon MCCAOP
HCCA QAPs need to schedule TXOPs that do not interfere with the other HCCA QAPs in the OBSS Graph.
This is achieved by the HCCAOP Advertisement Element which lists all the TXOPs that have been already scheduled by the QAPs up to a “Distance” of 2 (see 09/0662)
HCCA QAP looks at the HCCAOP Advertisement to select a TXOP that does not interfere with an existing TXOP in place and that does not exceed the Access Fraction Limit.
Graham Smith, DSP Group

4. HCCAOP Advertisement Element based closely on MCCAOP from 11s
May 2009
HCCAOP Advertisement Element based closely on MCCAOP from 11s
QAPs with Distance <3 in the QLoad Element are included in the
Interfering Times Report
QAPs allocate new TXOPs by inspecting the HCCAOP Advertisement Element, checking no overlap and does not exceed
HCCA Access Limit
Graham Smith, DSP Group

5. HCCAOP Advertisement Element
May 2009
HCCAOP Advertisement Element
HCCA Advertisement Element informs ONLY TXOPs THAT ARE ACTIVE
Access Fraction Fields (as per MCCAOP Advertisement)
HCCA Access Fraction: Total actual scheduled time expressed as a fraction of 32us/sec rounded down to 1/256
HCCA Access Fraction Limit: Maximum scheduled time that this QAP may allocate expressed as a fraction of 32us/sec rounded down to 1/16
TXOP Reservation
Duration: In units of 32us
Service Interval (ms)
Offset: Beginning of first TXOP after a Beacon, relative to beginning of each scheduled Beacon, in units of 32us
Interfering Times Report
Includes all QAPs that have a “Distance” of 1 and 2 in the QLoad Element
Graham Smith, DSP Group

6. May 2009
Possible Problems
Relative Clock drift will cause delays in TXOPs, but not a big problem. Suggest that synchronizing the Beacons is not worth the effort (even if it is possible) see 09/0660.
Check that “HCCA Access Fraction” and “HCCA Action Fraction Limit” is correct and provides useful information in addition to the “Access Fraction” and “Access Factor” Fields proposed for the QLoad Element.
Look at TXOP scheduling with respect to different Service Periods, and see how to make it as efficient as possible.
We will look at 2 and 3
Graham Smith, DSP Group

7. May 2009
HCCA Access Fraction
Graham Smith, DSP Group

8. May 2009
Example
Example used is 4 QAPs with 3 Video streams as per below
Same example as used in 09/0662
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. 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
Traffic Requirements
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
Before any EDCA Overhead
A>D hence A, B and C Totals are A + B + C
With D re-using A’s time
Graham Smith, DSP Group

10. EDCA and HCCA Traffic May 2009 EDCA HCCA
The traffic requirements and limits that can be calculated using the QLoad Element are ideal for EDCA. In EDCA Admission Control the Used Time is calculated over one second periods and hence the statistical nature of the streams is accounted for in the method used to calculate the traffic allocations.
Unlike HCCA there is no concept of actual contention free periods and hence the medium time allocations can be based upon the statistical calculations as previously described.
Hence the actual Medium Time allocations can represent the ‘peak’ traffic demand
The EDCA overhead included in the Access Factor, is the “inefficient” payment, i.e. the bandwidth loss
HCCA
Like Medium Time allocations, the TXOPs also need to represent the peak traffic*. Unlike EDCA there is a basic concept of actual time allocation and hence the sum of these will be linear, not statistical. Obviously these cannot be greater than 100% in overlapping APs or else the Service Periods cannot be met
The need to allocate and reserve time periods (TXOPs) that meet the peak requirement is the “inefficient” payment for HCCA, but HCCA does terminate the TXOP as soon as traffic is completed in that service period.
* Could allocate below peak, say 90%, of the peak (see later)
Graham Smith, DSP Group

11. HCCA allocates Peak Time
May 2009
HCCA allocates Peak Time
If all four QAPs were HCCA, example:
QAP A wants 2 DVD and 1 HDTV stream
Summing as a composite video = 0.49 (100%)
Summing the maximums = 0.55
Summing as 90% peaks =0.50
ACCESS FACTOR
For HCCA
Summary
QAP
SELF
Peak
Distance (From QLoad Element)
Interfering
TOTAL
MAX A B C D
0.55 1 2
(3)
0.80
1.35
0.28
1.08
0.52
0.83
0.34
1.14
Graham Smith, DSP Group

12. May 2009
HCCA Access Factor
Treat HCCA Access Factor same way as per QLoad Access Factor and reduce each QAP allocation accordingly:
QAP A Self TXOPs = TXOP Limit = 0.41
QAP B Self TXOPs = TXOP Limit = 0.20
QAP C Self TXOPs = TXOP Limit = 0.39
QAP D Self TXOPs = TXOP Limit = 0.25
The HCCA Access Limit sets the limit on the TXOPs that each QAP may set
Note: If 90% TXOPs used, the same TXOP Limit occurs:
QAP A Self TXOPs = TXOP Limit = 0.41
Hence whichever way the QAP wishes to allocate its TXOPs it is still limited to 41% of the time
Sum = 1.0
Graham Smith, DSP Group

13. How about Mixed EDCA and HCCA?
May 2009
How about Mixed EDCA and HCCA?
The QLOAD 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 – EDCA and HCCA.
Note that the Access Factor also includes the “EDCA Factor” which is based upon the “Priority Streams”, which will not include the HCCA streams
As the QAP grants Medium Times (EDCA) and TXOP times (HCCA), it must calculate the total composite time, and this must be less than the “Traffic Allowance”, otherwise it must not grant the time (if it wants to be a good neighbor)
HCCA QAPs may not allocate TXOPs that exceed the TXOP limit
Graham Smith, DSP Group

14. HCCA requires Max TXOP per QAP
May 2009
HCCA requires Max TXOP per QAP
The sum of the HCCA TXOPs cannot be derived from the Mean and STDEV fields as it is a linear sum.
Therefore we require each HCCA QAP to inform its total peak scheduled time in order to determine the HCCA Access Factor
Hence, proposed to add a field to each QAP in the QLoad Element
Could include in the HCCAOP Advertisement, but then would need to repeat the QAP ID and Distance fields (which are already present in the QLoad Element)
Only has a real value if the QAP is HCCA. Zero indicates “not HCCA”
BUT HCCA Access Factor appears in the HCCAOP Advertisement
HCCA Access Factor is the sum of the “HCCA Peak” values for all QAPs in the QLoad Element (taking account of re-use)
Graham Smith, DSP Group

15. May 2009
Added
Graham Smith, DSP Group

16. The “Access Factor” used by a QAP to calculate the “Traffic Limit” is
May 2009
The “Access Factor” used by a QAP to calculate the “Traffic Limit” is
the highest value seen in itself or any overlapping QAP.
Graham Smith, DSP Group

17. May 2009
TXOP Allocations
Graham Smith, DSP Group

18. Introduction and Observations
May 2009
Introduction and Observations
Assuming that the HCCA QAPs obey the HCCA Access Factor then total combined scheduled times for the overlapping QAPs will be =< 100%
So there should theoretically be room for everyone…BUT
The HCCAOP Advertisement will cause new schedules to avoid existing ones only if the Service Intervals are related
i.e. If Service Intervals are all 10ms, say, then QAPs can schedule so as to avoid each other.
If not on a common time slot, then impossible to schedule times that avoid each other, e.g. 10ms and 16ms will “beat” and collide at set intervals
Without some co-operation or ‘rule’, the existing schedules may not leave room for the new ones
Insufficient gaps to allow other QAPs to fit in between
Graham Smith, DSP Group

19. Introduction and Observations
May 2009
Introduction and Observations
When reporting TXOP schedules in the Interfering Times Report, the QAP must express them referenced to its own Beacon Timing
Otherwise the overlapping QAP cannot work out the schedules as it does not know the relative Beacon times
Clock Drift will cause the schedules to collide
When they collide, one TXOP will have to wait.
This does have consequences for Power Save
Should not introduce any excessive jitter or latency
Delay will be a few ms
Alternative is to synchronize the Beacons and Clocks and this is not easy
Graham Smith, DSP Group

20. Possible Solutions Introduce a 10ms common Slot Time
May 2009
Possible Solutions
Introduce a 10ms common Slot Time
Is compliant with all voice TSPECs (10, 20 and 30ms) as well as Video (0 to 16ms)
Introduce a scheme where each QAP lays claim to a specific section of the 10ms Slot
Allows efficient use of the time slot so that each QAP will have enough room
Needs co-operation between HCCA QAPs
We do know the maximum requirement for each QAP i.e. HCCA Peak in the QLoad Element
OR simply “Use HCCAOP to avoid” and accept what happens
Maybe a simple rule such as “always schedule directly after an existing schedule, wherever possible”
Graham Smith, DSP Group

21. Time Slot Allocation Scheme
May 2009
Time Slot Allocation Scheme
In the QLoad element, the following information is available for all QAPs at Distance up to 2:
QAP ID
HCCA Peak
Using our Example
A, B and C know their respective allocations, and these make up 100%. D should re-use some of A’s time allocation
Hence, A, B and C could allocate the time between them (e.g. using QAP ID to settle order)
Problem is what does D do? It must re-use the time allocation of A, but how does it know?
Possible Solution, using our example
From A’s point of view, QAP order, say, is C, A, B
From D’s point of view, QAP order, say, is D, C, B
A sees D in B’s QLoad, so knows that it is at Distance 3 and can re-use. Hence A can compare with D to see who is the larger; D can do the same, and, in this case, the order will be C, A, B. D knows it takes A’s slot.
Need to establish the base timing, and in this case that would be C.
Graham Smith, DSP Group

22. Simple Allocation Using HCCAOP Advertisement (HCCAOP Ad) Problems
May 2009
Simple Allocation
Using HCCAOP Advertisement (HCCAOP Ad)
If no previous TXOP, schedule as soon as possible (Offset = 0)
Otherwise, schedule a new TXOP directly after an existing TXOP (self or other), if gaps exist, schedule in gap if room
Problems
Ideally QAP D must somehow choose to re-use A time, and vice versa, otherwise 100% cannot be achieved
If gap exists because say a DVD was stopped, then filling it with say an SDTV will cause an unusable gap
Graham Smith, DSP Group

23. OPTIONS May 2009 Controlled Allocation System
Introduce ‘Order’ Rule based on QAP ID with special rules for re-use (largest Peak QLoad for QAPs at Distance 3)
No need to Times Reports in HCCAOP Advertisement
Use the Handoff Scheme with CHP bit
Limits on who can share but does allocate efficiently.
Requires Supervisor and AP to AP communications
Use HCCAOP Advertisement to avoid others.
Accept that the “example” is a very unlikely scenario (according to 08/1470r4) even more so for HCCA QAPs.
QAPs at Distance 3 are extremely unlikely, so why bother about them?
Using HCCAOP Advertisement to avoid others should work in practice because HCCA QAPs will tend to avoid each other and 100% capacity will be very rare, i.e. Option 3
Only problem becomes ‘spare gaps’
Do we need to add any rules about scheduling close to existing TXOPs or using gaps?
PROPOSE THIS SIMPLEST OPTION AS IT SHOULD WORK IN PRACTICE
Graham Smith, DSP Group

24. PROPOSED “QLOAD” ELEMENT
May 2009
PROPOSED “QLOAD” ELEMENT
Graham Smith, DSP Group

25. PROPOSED “HCCAOP ADVERTISEMENT” ELEMENT
May 2009
PROPOSED “HCCAOP ADVERTISEMENT” ELEMENT
Graham Smith, DSP Group