1. Applicability of 11s MCCA to HCCA OBSS
May 2009
doc.: IEEE /0xxx
May 2009
Applicability of 11s MCCA to HCCA OBSS
Date:
Authors:
Graham Smith, DSP Group
Graham Smith, DSP Group

2. May 2009
doc.: IEEE /0xxx
May 2009
Abstract
This presentation looks at MCCAOPs as proposed in 11s and then discusses how a similar scheme could be used in the OBSS proposal for HCCA sharing.
We then compare this scheme with the present OSQAP proposal
Finally work areas proposed in order to decide if this scheme should be used.
Graham Smith, DSP Group
Graham Smith, DSP Group

3. May 2009
MCCAOP Basic Scheme
Mesh STA scans channel for neighborhood mesh STAs supporting MCCA
MCCA Advertisement Element includes Reservation Fields for Self and Interfering (Neighbor’s MCCAOPs that do not involve the STA itself)
Build a map of neighborhood MCCAOP reservations by listening to the MCCA Advertisement Element
Choose start and duration times so no overlaps with Neighbor or Interfering times
Check that new MCCAOP does not exceed the maximum access fraction limit
Graham Smith, DSP Group

4. MCCAOP Reservations Time reservations are defined as Duration,
May 2009
MCCAOP Reservations
Time reservations are defined as
Duration,
Periodicity: Number of MCCAOPs in mesh DTIM interval (sub-Intervals)
Offset: Beginning of MCCAOP relative to beginning of each sub-interval
Graham Smith, DSP Group

5. TX-RX and Interfering Times Reports
May 2009
TX-RX and Interfering Times Reports
TX-RX Times Report
MCCAOP Reservations for the mesh STA itself
Interfering Times report
A mesh STA reports MCCAOP Reservations that its neighbors have advertised in their TX-RX Reservation Reports in which it is not involved itself. The Interfering Times are directly derived from neighbor peer mesh STAs’ TX-RX Times Report.
These times shall not be used for a new MCCAOP with the reporting mesh STA as they may experience interference
Mesh STAs know the absolute timing from the TSF and TBTT of the neighbor mesh STAs
Graham Smith, DSP Group

6. MCCAOP Advertisements Element
May 2009
MCCAOP Advertisements Element
Current value of MCCA access fraction
at the mesh STA.
Maximum MCCA access fraction allowed at the mesh STA.
Graham Smith, DSP Group

7. Observations, Questions
May 2009
Observations, Questions
Periodicities must be in harmony to avoid clashes*:
If Periodicities are 1, 2, 4, 8 etc. then STA can calculate times that will not interfere
If Periodicities are, say, 1, 2, 3, 4, 5,etc. Then impossible to calculate times that will always be free.
Scheme is “First-Come-First-Served” with “Access Limit”
Time is reserved even if not used for actual traffic
The Access Fraction and Access Fraction Limit is per STA but not clear if it is an agreed/imposed value for the neighborhood and interfering.
Time reservations are for traffic between the STAs
MCCAOP Times can be re-used after 4 hops (see example next slide)
Relies on STAs scheduling OPs in an efficient order to produce good bandwidth usage – pick sequential if possible? No mention on this*
STAs need to calculate the “Times” from neighbors with respect to their own timing
Does a STA report the times in the Interfering Time Report in terms of its local time? Can’t see it in the Spec but must happen for it to work*
STA needs to use TSF and TBTT information from neighbor to calculate actual Times
*NOTE: Not sure if this is realized in 11s
Graham Smith, DSP Group

8. May 2009
Graham Smith, DSP Group

9. May 2009
Applicability to OBSS
Graham Smith, DSP Group

10. May 2009
Graham Smith, DSP Group

11. Q Load Element proposed in 09/0497r2
May 2009
Q Load Element proposed in 09/0497r2
DISTANCE > 2
Can be ignored
Graham Smith, DSP Group

12. QLoad Element Fields May 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 (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
Channel Priority
Used only if QAP is operating with HCCA, indicates HCCA Supervisor
Can we use MCCAOP
Scheme
In place of this?
Graham Smith, DSP Group

13. HCCA Sharing based closely upon MCCAOP Scheme
May 2009
HCCA Sharing based closely upon MCCAOP Scheme
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

14. HCCAOP Advertisement Element
May 2009
HCCAOP Advertisement Element
HCCA Access Fraction (HAF) HAF = Sum of TXOPs in 32us/sec
HCCA Access Fraction Field Current value of HAF at the QAP rounded down (floor) to the nearest multiple of (1/256).
HCCA Access Fraction Limit field Maximum HAF expressed in units of (1/16) allowed at the QAP. This number is always a multiple of (1/16)
Note: See more on this later.
Interfering Times Report
All QAPs that have a “Distance” of 1 and 2 in the QLoad Element
TXOP Reservation Note: The HCCA Schedule Element is used for scheduled power save and hence not directly applicable
Duration: In units of 32us
Service Interval (may specify 10ms basic slot)
Offset: Beginning of first TXOP after a Beacon, relative to beginning of each scheduled Beacon, in units of 32us
NOTE: HCCA Advertisement Element informs ONLY TXOPs THAT ARE ACTIVE.
Graham Smith, DSP Group

15. Comments on this Scheduling Scheme
May 2009
Comments on this Scheduling Scheme
First-Come-First-Served
QAP cannot allocate a TXOP if time not available No concept of sharing
No concept of efficiently allocating TXOPs, e.g. allocate in blocks and have a ‘common’ time slot idea
Need to establish a procedure to set the HAF Limit so as to create a “Sharing” method?
Clock Drift
Clock drift between QAPs will cause clashes to occur.
11s uses Neighbor Offset Protocol and Beacon Timing Elements. Constant monitoring of Beacon timing. TSF timer is suspended to match. Scheme is basically designed to avoid Beacon clash.
Graham Smith, DSP Group

16. Setting the Access Limit and Sharing
May 2009
Setting the Access Limit and Sharing
09/0497r2 Sharing described how the summing of the individual streams can be achieved using the QLoad values. Based upon the QLoad it would be possible to set an “Access Limit” if there is over-allocation. This could then be translated into requirements for each QAP.
By this means the HAF and HAF Limit could be calculated and advertised in the HCCA Advertisement.
Also looks useful for EDCA Admission Control and hence should go into the QLoad Element
Graham Smith, DSP Group

17. Access Fraction Field in QLoad Element
May 2009
Access Fraction Field in QLoad Element
Add 1 octet to QLoad Element
Access Fraction: 4 bits Total actual admitted time and/or scheduled time
expressed as a fraction of 32us/sec rounded down to 1/16
Access Fraction Limit: 4 bits Maximum admitted time and/or scheduled time that this QAP may allocate
Calculated based upon:
sum of MEAN and STDEV for ALL QAPs in QLoad Element
EDCA only, include factor for number of Priority Streams for Distances 0 and 1
Scaled if result is greater than 100%
Notes:
HCCA Advertisement Element could repeat this or omit it
Needs agreement that Sharing rules should apply, or at least be explained how to do it.
Graham Smith, DSP Group

18. May 2009
New Field added
Graham Smith, DSP Group

19. Clock Drift Mitigation?
May 2009
Clock Drift Mitigation?
Two Options
Let it happen
When say TXOP A finishes late, next TXOP B just get delayed Delay would build up in subsequent slots until they exchange positions and TXOP B occurs first
No real problem but does introduce some inefficiency if scheduled power save is being used
Synchronize
Monitoring of TSF and TBTT possible for neighbors but complicated through the QAPs in the Interference Times Report.
Beacon Timing Report Element only reports neighbors
Conclusion - Let it happen
Graham Smith, DSP Group

20. Before comparison - Refresher on CHP scheme
May 2009
Before comparison - Refresher on CHP scheme
Graham Smith, DSP Group

21. Harmonizing HCCA When sharing use Fixed Time Slot
May 2009
Harmonizing HCCA
When sharing use Fixed Time Slot
Each AP (HC) knows how much of the Time Slot it can use: QLoad Self
Supervisor AP (CHP=1) hands off to the other QAP(s) using Wireless DS QoS CF-Poll (Null Data) for AP-AP communication
Supervisor QAP (CHP = 1) controls the 10ms slot timing
Supervisor QAP sends message to other QAPs (CHP = 0) indicating time to start of their respective TXOP periods.
Uses Wireless DS (AP to AP), QoS CF-Poll (null data)
09/0230r0 describes rules for when Supervisor QAP goes away and when two QAPs have CHP set to 1
“Supervisor Claim” and “Is Supervisor There” packets used.
Graham Smith, DSP Group

22. May 2009
AP to 2 APs
Graham Smith, DSP Group

23. Rules for Setting CHP bit and Sharing (per 09/230r0)
May 2009
Rules for Setting CHP bit and Sharing (per 09/230r0)
When a HCCA QAP is searching for a channel, it should do so in the following order:
Set CHP (Channel Priority) to 0
If finds a clear Channel, set CHP to 1
If no clear channel, then may share with
Any legacy AP: Set CHP to 1
An Admission Control QAP, overlap 0 or 1: Resulting HCCA QAP overlap being 1:1, or 1:2 Set CHP to 1 (see Note 1)
An HCCA QAP with CHP = 1 CHP stays at 0 (see Notes 1 & 2)
If an HCCA QAP cannot find a channel that meets the rules, it must fall back to Admission Control (see note 3)
NOTES:
If 3b) or 3c), check that “QLoad Total” is such that the two can share
An HCCA QAP may not share with an HCCA QAP which has CHP = 0, unless it is also sharing directly with the corresponding HCCA QAP that has CHP = 1
Restriction on sharing with HCCA would not be applicable in practice if 17 or more channels are available. HCCA QAP should be able to find channel that meets requirement, See Slides 24 – 29.
Graham Smith, DSP Group

24. May 2009 WDS QoS CF Polls To Supervisor from QAP with CHP=0 ACTION
Bits 0-3
Bit 4
Bits 5-6
Bit 7
Bits 8-15
CHP is set to 0
0000 1 00
Is Supervisor There?
0010
From Supervisor QAP with CHP=1
ACTION
Bits 0-3
Bit 4
Bits 5-6
Bit 7
Bits 8-15
Indication from Supervisor to another QAP of Time to start TXOP (HCCA sharing)
1111 1 10
Time to start of TXOP in units of 32us
Supervisor Claim, CHP = 1
0001 00
Graham Smith, DSP Group

25. Quick Summary May 2009 CHP scheme HCCAOP scheme
Supervisor controls the timing of a 10ms Slot and passes control over to each QAP in turn
Uses the “total QLoad” to allocate time within a 10ms Slot. QAPs then free to allocate individually within that sub slot. Produces efficient use of the bandwidth.
Idea of one network being ‘controlled’ by another, is unique within
Easy to comply with Sharing Rules (Access Fraction Limit)
Has limitations with Supervisor visible to other QAPs. Does not cater for Distances > 1.
HCCAOP scheme
Each QAP determines its own TXOP schedules by looking for unused time between Beacons. Requires time conversion in order to determine the available slots
Clock drift will cause delays in TXOPs, but not a big problem
If common time slot concept used, together with QAPs allocating or reserving blocks based on “Total QLoad”, then can approach efficiency of CHP – is this practical? needs work
Compliance with Sharing rules (Access Fraction Limit) needs more work
Caters for “Distances” >1
Graham Smith, DSP Group

26. May 2009
Conclusion
Before final decision, need to look at HCCAOP Scheme closely with respect to the sharing and efficient selection of TXOPs by the individual QAPs.
Need to prepare submissions on:
Access Fraction in QLoad Element
How to use it for Sharing
HCCAOP Advertisement Element
Use of HCCA Access Fraction
How to make it more efficient and compliant with HCCA scheduling
Graham Smith, DSP Group