1. Overlapping BSS Proposed Solution
April 2008
doc.: IEEE yy/xxxxr0
April 2008
Overlapping BSS Proposed Solution
Date:
Authors:
Graham Smith, DSP Group
Graham Smith, DSP Group

2. Abstract The problem of OBSS is quantified and examined
April 2008
doc.: IEEE yy/xxxxr0
April 2008
Abstract
The problem of OBSS is quantified and examined
A solution for OBSS is presented and discussed
A set of recommendations is given.
Graham Smith, DSP Group
Graham Smith, DSP Group

3. OBSS – Estimation of Size of Problem
April 2008
OBSS – Estimation of Size of Problem
Floor Plan of Apartments
Each Apartment
26 x 40 feet,
about 1000 square feet
Imagine similar floors above and below this one.
Indoor propagation loss formula used:
Lp = – log F + 40 log d + WAF (p) + FAF (q) F in MHz, d in feet
At shorter distances, the Free Space formula dominates,
Lp =– log F + 20 log d + WAF (p) + FAF (q)
The predicted propagation loss is the higher of the two.
Each wall (WAF) and floor (FAF) between apartments is assumed to be 10dB penetration loss (fireproof). Ceiling height is assumed to be 10 feet.
Graham Smith, DSP Group

4. Received Signal Strengths
April 2008
Received Signal Strengths 1
Inside same apartment 2
Next door (one each side) x2 3
Two away (one each side) x2 4
Three away (one each side) x2 5
Opposite 6
Opposite, across one (one each side) x2 7
Opposite, across two (one each side) x2 8
Opposite, across three (one each side) x2 9
Directly up and down x2 10
Up or down, neighbor (one each side) x4 11
Up or down, two away (one each side) x4 12
Up or down, three away (one each side) x4 13
Opposite, up and down x2 14
Opposite, up and down, two across x4 15
Opposite, up and down, one across x4 16
Opposite, up and down, three across x4 17
Two floors directly up and down x2
30dB power control
Graham Smith, DSP Group

5. Number of OBSS – DFS and TPC
April 2008
Number of OBSS – DFS and TPC
Table 1 – Theoretical OBSS for Apartments sq. ft.
Frequency Band
Number of Interfering Networks
Interfering Networks per 20MHz Channel
Interfering Networks per 40MHz Channel
2.4GHz 31 10
5GHz 27
0-1 3
Ideal DFS reduces problem significantly!
5GHz for Home!
Received signal strength within each apartment is high, better than -40dBm.
Theoretically, therefore, the power could be reduced by 30dB
with no deterioration in the throughput. Solves OBSS!
Table 2 – Theoretical OBSS with 30dB Power Reduction
Frequency Band
Number of Interfering Networks with 30dB power reduction
Interfering Networks per 20MHz Channel with 30dB power reduction
Interfering Networks per 40MHz Channel with 30dB power reduction
2.4GHz 8 3
5GHz 4
Graham Smith, DSP Group

6. Effects of OBSS - 1 April 2008 # Network A OBSS Network B Effect
Result 1
Legacy
Traffic simply competes
Reduced bandwidth in each network
No lost packets
Not recommended for streaming 2
EDCA
Higher priority traffic in Network A will drive down traffic in Network B
AC_VO and AC_VI traffic dominates. Could be OK for streaming traffic but no admission policy
Network A “wins” 3
Traffic competes on a priority basis. Networks compete on an ‘equal’ basis
No real protection for streaming traffic in either network
Graham Smith, DSP Group

7. April 2008
Effects of OBSS - 2 #
Network A
OBSS
Network B
Effect
Result 4
Admission Control
Legacy
Higher priority traffic in Network A will drive down traffic in Network B
AC_VO and AC_VI traffic dominates. Could be OK for streaming traffic
Network B bandwidth can be drastically reduced 5
EDCA
Traffic competes on a priority basis.
Admission Control in Network cannot control traffic in Network B
No protection for admitted traffic in Network A 6
Admission
Control
Traffic competes on a priority basis.
Admission Control in either Network cannot control traffic in other Network
No protection for admitted traffic in either Network
These cases are cause for concern, Admission Control is intended
to provide QoS ‘protection’, and it breaks down in OBSS!
Graham Smith, DSP Group

8. Effects of OBSS - 3 April 2008 7 HCCA Legacy
Scheduled TXOPs in Network A also apply CFP to Network B.
Full protection for scheduled traffic in Network A
Network B bandwidth reduced 8
EDCA 9
Admission Control
Scheduled TXOPs in Network A also apply CFP to Network B
Admitted traffic Network B is lower priority than scheduled traffic in Network A
Both Networks using TSPECS 10
Each HCCA AP will admit streams and allocate time to them BUT each AP and STA will obey the TXOP allocation of the other.
No guarantee that each Network can allocate time when it needs to. ,
Reduced protection for scheduled traffic in either network.
Graham Smith, DSP Group

9. April 2008
OBSS and QoS
For non-QoS (non-real time streaming) applications OBSS is simply a sharing or reduced bandwidth per network – Not a significant problem
OBSS is a significant problem ONLY when QoS is used AND when some ‘guaranteed performance’ is at stake
Graham Smith, DSP Group

10. Conclusions: OBSS – EDCA on EDCA
April 2008
OBSS – EDCA on EDCA
Table clearly shows that OBSS is a problem for when it is intended to be used for applications that require QoS.
EDCA does not address the problem at all.
EDCA Admission Control only solves the bandwidth allocation problem within its own network and does not address OBSS.
HCCA does overcome OBSS problems in all but the case where two HCCA networks overlap.
Conclusions:
EDCA is not providing QoS in OBSS situation and any higher bandwidth streaming application is not protected
HCCA does solve the OBSS situation for all cases except when overlapping HCCA networks
Graham Smith, DSP Group

11. Solving OBSS Channel Selection is first important step
April 2008
Solving OBSS
Channel Selection is first important step
TPC is probably difficult to assume
If so, it could be assumed that the OBSS situation could be eliminated or limited to a maximum of two QAPs
Objectives:
Two HCCA networks can co-operate
HCCA and Admission Control QAPs can co-operate
Two Admission Control QAPs co-operate (Note: Still not protected against EDCA OBSS)
Graham Smith, DSP Group

12. QAP = HCCA or EDCA Admission Control
April 2008
Basic Methodology
QAP = HCCA or EDCA Admission Control
QAP builds knowledge of expected QoS load, “QLoad”
QAP advertises “QLoad” element
Channel selection to try to avoid another QAP
If must share, selection based upon relative “QLoads”
QAPs negotiate bandwidth sharing
HCCA QAPs harmonize TXOPs
Graham Smith, DSP Group

13. EDCA Admission Control only QAPs are now co-operating.
April 2008
OBSS – Basic Steps
When QSTAs associate, they send their TSPEC(s) corresponding to their expected requirements
Using the TSPECs, QAP ‘A’ builds knowledge of the QoS demands of its network, we shall call this the “Q Load”
Another QAP ‘B’, looking for a spare channel or whether to share, would interrogate QAP ‘A’ to establish the Q Load ‘A’. Based on this QAP ‘B’ can make a decision on whether to stay or not
Assuming that QAP ‘B’ does stay, then it determines its own Q Load ‘B’
QAP ‘A’ and QAP ‘B’ now negotiate the bandwidth, based upon their Q Loads
EDCA Admission Control only QAPs are now co-operating.
Note, however, that they still do not have protection against legacy EDCA networks.
If a successful outcome then HCCA networks proceed to step 7. If not, then QAP B must leave to seek another Channel.
QAP ‘A’ and QAP ‘B’ harmonize such that they schedule TXOPs correctly with respect to both networks
Each step will now be examined in more detail.
Graham Smith, DSP Group

14. OBSS - TSPEC Exchange April 2008
Figure 10 – TSPEC Element
On association, a QSTA sends its TSPEC, QAP knows the STA’s requirement (s).
The TSPEC has Inactivity Interval set to 0 (needs to be included for Admission Control)
Causes the TSPEC to expire instantly, once accepted.
QAP recognizes this as a special case and know that the intention is for the QSTA to inform the QAP of its expected load
Note that the QAP must remember the allocation required
for all the ‘sign on’ TSPECs and respond accordingly
Graham Smith, DSP Group

15. QAP ‘Q Load’ Reporting April 2008 Not adequate for purpose
QBSS Load element Format
Not adequate for purpose
Propose to add or replace similar new Element – “Q Load Element”
Scheduled Slot field
Base timing for the Scheduled Service Intervals that the HC is using (see later)
Allocated Admitted field
Amount of medium time that has been approved for EDCA Admission Control
Allocated Scheduled
Total of Scheduled TXOPs that has been approved for HCCA STAs
Note: Also could be used in Fast Handoff avoiding need to pre-register
Graham Smith, DSP Group

16. Channel Priority – Finding a Clear Channel
April 2008
Channel Priority – Finding a Clear Channel
When a QAP is searching for a channel, it should do so in the following order:
Set CHP (Channel Priority) to 1
Check no other AP present
Check no other QAP present
If another QAP present, then check QAP Q Load is small enough such that the two can share
If QAP selects its channel based upon 1 or 2, then
Check that no other QAP is within range of its network QSTAs using Beacon Request Report
If positive, and decides to stay, set CHP to 0
If 4, and QAP chooses to share, sets CHP to 0
NOTE: Non QAPS would also try to avoid QAPs.
Graham Smith, DSP Group

17. Basic options for sharing ‘rules’ are:
April 2008
QAPs Negotiate
Basic options for sharing ‘rules’ are:
First Come, First served (FCFS). TSPECs are accepted, HCCA and EDCA, in the order they appear. Both QAPs must know the prevailing total Q Load so as not to over-allocate.
Negotiated Bandwidth
Simple Proportion (SPNB) Based upon the potential Q-Load of each QAP, the bandwidth is proportioned up between them accordingly. This way, each QAP knows its modified maximum bandwidth allocation
On-Demand Negotiated Bandwidth (ODNB) Basically, when a QAP receives an ADDTS request, that, if accepted, would take the QAP over the SPNB allocation, it must get permission from the other QAP to accept it.
Preferred Method
(it’s easy)
This is enough for WMM-Admission Control QAPs, HCCA QAPS need to Harmonize
Graham Smith, DSP Group

18. Harmonizing HCCA QAPs Explanation of Scheduling of TXOPs April 2008
Schedule for QSTAs
Desirable that the start times of the TXOPs are maintained at the same interval.
This enables the QSTA use efficient S-APSD,
Maintain the minimum service interval (SI) requirement as per the TSPEC
Graham Smith, DSP Group

19. Fixed Slot time 10ms April 2008
Min and Max Service Intervals for Voice and Video
Category
Minimum Service Interval
Maximum Service Interval
Voice
G711, G729, AMR-NB, AMR-WB, iLBC, EVRC, VMR-WB
20ms
G711,G729,G723.1
30ms
G726-32
10ms
Video
SDTV, HDTV
0ms
16ms
10ms fixed Slot
Graham Smith, DSP Group

20. Harmonize Slot times April 2008 At the beginning of the Slot,
QoS (+) CF Poll Frame sent by HC
At the beginning of the Slot,
QAP sets bit 7
The suggested procedure (see next slide)
At the beginning of the Slot, the QAP A sets bit 7.
This could be included in the first TXOP or,
if there is no TXOP at that time then the QAP simply sends a QoS Poll to itself.
QAP B waits the maximum duration of the TXOPs sequence
Period indicated in the Allocated Scheduled field in the Q LOAD element for QAP A
QAP B starts its Slot time and TXOPs
Simple and straightforward
Graham Smith, DSP Group

21. Service Interval Harmonization
April 2008
Service Interval Harmonization
Graham Smith, DSP Group

22. OBSS Proposed Procedure Summary
April 2008
OBSS Proposed Procedure Summary
Before seeking a channel, a QAP sets its CHP to 1 in the Q LOAD element.
QSTAs send their expected TSPECs as they associate, with Inactivity Period set to 0, and the QAP calculates its values for the Q LOAD Element
A QAP should try to find a channel that has no other QAP present, by first listening for another Beacon, and then issuing a standard Beacon Request (see figure). An extension to this is that the Beacon Request and resulting Probe Request is tailored to seek out the Q Load Element.
If no other QAP is reported, then the QAP may choose that channel.
If another QAP is reported, then the respective Q LOADs are examined and a decision made as to share or not.
If the decision is to share, then the CHP is set to 0.
If the QAPs are not hidden then the condition of sharing is recognized (CHP 0 and 1) and each QAP calculates its available schedule time based on Simple Proportion.
If the QAPs are hidden then the OBSS Beacon Request /Report is used such that each QAP knows the Q LOAD of the other and of the decision to share (CHP 0 and 1)
EDCA Admission Control QAPs can now proceed
Graham Smith, DSP Group

23. OBSS Proposed Procedure Summary
April 2008
OBSS Proposed Procedure Summary
HCCA QAPs need to harmonize their SIs.
Each HCCA QAP indicates its start of the Slot Time by setting bit 7 in the QoS CF Poll.
If QAPs are not hidden, the Slot Times are harmonized using the Start of Slot Time indication and the Allocated Scheduled information in the Q LOAD.
Proposal for Hidden APs (to be discussed)
If QAPs are hidden, if they experience scheduling problems to specific QSTAs, they adjust their respective Slot Times (TSF Timer), at DTIM intervals, by 0.5ms in a positive or negative direction as per the CHP setting.
Graham Smith, DSP Group

24. Beacon Report Exchanges
April 2008
Beacon Report Exchanges
OBSS Beacon Request
Provides other QAP the Q Load element
Informs CHP
Graham Smith, DSP Group

25. OBSS Summary Two HCCA networks could share
April 2008
OBSS Summary
Two HCCA networks could share
Two EDCA Admission Control networks could share
An HCCA and an EDCA Admission Control Network could share
An EDCA Admission Control and an EDCA network would still not share.
Proposed additions to the Standard are :
“Q LOAD Element” for HCCA and EDCA Admission Control QAPs
“OBSS” Beacon Request Report
Fixed 10ms Slot time
Use of bit 7 in QoS CF Poll to indicate start of Slot Time
We now consider “Hidden –AP”
Graham Smith, DSP Group

26. April 2008
Hidden QAPs
If QAP stays after Beacon Report, set CHP to 0 and sends OBSS Beacon Request
QAP B now knows of QAP A and its Q Load
QAP ‘A’ and QAP ‘B’ calculate their maximum allocated bandwidth, based upon their Q Loads and the SPNB method.
QAP A and QAP B must now harmonize their Scheduled Allocations
Graham Smith, DSP Group

27. Harmonizing SI – Direct Method
April 2008
Harmonizing SI – Direct Method
Direct Method (as per non-hidden QAPs)
Could be possible using a common STA
BUT
The QSTA may be in power save mode
If the first TXOP has been granted then the QSTA is prevented from transmitting, so sending the timer onto the other QAP is not possible
The only legitimate transmission from a STA to an AP outside its network, is the Probe Request
It is not advisable, or even allowed to change a scheduled time by too much.
Graham Smith, DSP Group

28. Harmonizing SI – Indirect Method
April 2008
Harmonizing SI – Indirect Method
QAP A CHP = 0; QAP B CHP = 1
QAP A determines that a scheduled stream to a particular QSTA is blocked and suspects that it is due to scheduling from the QAP B. In this case, QAP A shifts its TSF timer, at DTIM, in the positive direction by 5% of the slot time, i.e. 500us.
Similarly, QAP B determines that a scheduled stream to a QSTA is blocked and suspects that it is due to scheduling from the QAP A. In this case, QAP B shifts its TSF timer, at DTIM, in the negative direction by 5% of the slot time, i.e. 500us.
Graham Smith, DSP Group

29. 802.11n - 40MHz OBSS 40MHz Channels MUST use the OBSS proposals to:
April 2008
802.11n - 40MHz OBSS
40MHz Channels
Easy/intuitive to see how two 40MHz overlapping networks will be less efficient than separate, independent 20MHz channels.
MUST use the OBSS proposals to:
Try to find clear channel
If not clear, look for 20MHz channel
MUST introduce procedures for preventing or controlling OBSS and usage of 40MHz channels
The same procedures as previously described can be used
Graham Smith, DSP Group

30. Recommendations Recommendations: Support for this approach?
April 2008
Recommendations
Recommendations:
“Q LOAD Element” for HCCA and EDCA Admission Control QAPs
“OBSS” Beacon Request Report
Fixed 10ms Slot time for HCCA
Use of bit 7 in QoS CF Poll to indicate start of Slot Time
Addition of recommended practices for OBSS Note: Wi-Fi Alliance could then devise tests to certify the behavior (this is important)
What to do about TPC? Treat as a separate subject?
Support for this approach?
Should we go ahead to write normative text based on this approach?
Graham Smith, DSP Group