Doc.: IEEE 802.11-07/2684r0 Submission November 2007October 2007 Graham Smith, DSP GroupSlide 1 Overlapping BSS Proposed Solution Date: 2007-10-29 Authors:

November 2007October 2007

Graham Smith, DSP Group

Slide 1

doc.: IEEE 802.11-07/2684r0

Submission

Overlapping BSS Proposed Solution

Date: 2007-10-29

Name Affiliations Address Phone email Graham Smith DSP Group 4097 Sunrise Blvd,

#100, Rancho Cordova, CA 95742

916 851 9191 X209

[email protected]

Authors:



Slide 2

doc.: IEEE 802.11-07/2684r0

Submission

Abstract

The problem of OBSS is quantified and examined

A solution for OBSS is presented and discussed

A set of recommendations is given.



Slide 3

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Submission

OBSS – Estimation of Size of Problem•Floor Plan of Apartments

AP

40ft

40ft

26ft

1 223 3 44

5 6 7 88 7 6 Each Apartment•26 x 40 feet, about 1000 square feet

Imagine similar floors above and below this one.

Indoor propagation loss formula used:Lp = – 69 + 20 log F + 40 log d + WAF (p) + FAF (q) F in MHz, d in feetAt shorter distances, the Free Space formula dominates, Lp =– 38 + 20 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.



Slide 4

doc.: IEEE 802.11-07/2684r0

Submission

Received Signal StrengthsAppartments, 1000 sq ft

2.4GHz

-120

-100

-80

-60

-40

-20

0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

Sig

nal S

tren

gth

, d

Bm

-90dBm

Appartments, 1000 sq ft5GHz

-120

-100

-80

-60

-40

-20

0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

Sig

nal S

tren

gth

, d

Bm

-90dBm

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



Slide 5

doc.: IEEE 802.11-07/2684r0

Submission

Number of OBSS – DFS and TPC•Table 1 – Theoretical OBSS for Apartments - 1000 sq. ft.

• Ideal DFS reduces problem significantly!

•Table 2 – Theoretical OBSS with 30dB Power Reduction

• 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!

Frequency Band Number of Interfering Networks

Interfering Networks per 20MHz Channel

Interfering Networks per 40MHz Channel

2.4GHz 31 10 31

5GHz 27 0-1 3

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 3

5GHz 4 0 0

5GHz for Home!



Slide 6

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Submission

Effects of OBSS - 1

# Network A OBSS Network B Effect Result

1 Legacy Legacy Traffic simply competes Reduced bandwidth in each network

No lost packets Not recommended for

streaming

2 EDCA 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 but no admission policy

Network A “wins”

3 EDCA EDCA Traffic competes on a priority basis. Networks compete on an ‘equal’ basis

Reduced bandwidth in each network

No real protection for streaming traffic in either network



Slide 7

doc.: IEEE 802.11-07/2684r0

Submission

Effects of OBSS - 2

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 Admission Control

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

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

# Network A OBSS Network B

Effect Result

These cases are cause for concern, Admission Control is the highest QoS presently certified and it breaks down in OBSS!



Slide 8

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Submission

Effects of OBSS - 3

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 HCCA EDCA Scheduled TXOPs in Network A also apply CFP to Network B.



9 HCCA 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 HCCA HCCA 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.



Slide 9

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Submission

OBSS – EDCA on EDCA

• Table clearly shows that OBSS is a problem for 802.11 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:1. EDCA is not providing QoS in OBSS situation and any higher

bandwidth streaming application is not protected 2. If we wish to solve OBSS problem then the use of HCCA would

seem to be mandatory and we need to look into solving the OBSS situation for two HCCA networks (at the same time solving it for Admission Control)



Slide 10

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Submission

Solving OBSS• One clear recommendation would be to initiate

mandatory certifications for DCF and TPC

• If so, it could be assumed that the OBSS situation could be eliminated or limited to a maximum of two QAPs

• Investigation carried out that shows how:– Two HCCA networks could co-operate

– HCCA and Admission Control QAPs could co-operate

– Two Admission Control QAPs co-operateNote: Still not protected against EDCA OBSS



Slide 11

doc.: IEEE 802.11-07/2684r0

Submission

OBSS – Basic Starting Point

1. When QSTAs associate, they send their TSPEC(s) corresponding to their expected requirements

2. Using the TSPECs, QAP ‘A’ builds knowledge of the QoS demands of its network, we shall call this the “Q Load”

3. 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

4. Assuming that QAP ‘B’ does stay, then it determines its own Q Load ‘B’5. 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.

6. If a successful outcome then HCCA networks proceed to step 7. If not, then QAP B must leave to seek another Channel.

7. 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.



Slide 12

doc.: IEEE 802.11-07/2684r0

Submission

OBSS - TSPEC Exchange

•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 added for Admission Control)• Causes the TSPEC to expire instantly, once accepted.• QAP could recognize 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



Slide 13

doc.: IEEE 802.11-07/2684r0

Submission

QAP Q Load Reporting

QBSS Load element Format

Not adequate for purpose

2

Slot and Priority

1

AllocatedScheduled

1

AllocatedAdmitted

Q Load

11

b0 b6 b7

Slot Time in 8us Channel Priority

Slot and Priority Octet

Q LOAD Element

CHP = 1 HigherCHP = 0 Lower

Propose to add or replace similar new Element – “Q Load Element”

Scheduled Slot fieldBase timing for the Scheduled Service Intervals that the HC is usingAllocated Admitted fieldAmount of medium time that has been approved for EDCA Admission Control Allocated Scheduled Total of Scheduled TXOPs that has been approved for HCCA STAs

Also could be used in 11r Fast Handoff avoiding need to pre-register



Slide 14

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Submission

Channel Priority – Finding a Clear Channel

When a QAP is searching for a channel, it should do so in the following order:

1. Set its CHP to 1

2. Check no other AP present

3. Check no other QAP present

4. 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, then5. Check that no other QAP is within

range of its network QSTAs using Beacon Request Report6. If positive, and decides to stay, set CHP to 0

If 4, and QAP chooses to share, sets CHP to 0



Slide 15

doc.: IEEE 802.11-07/2684r0

Submission

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

This is enough for WMM-Admission Control QAPs, HCCA QAPS need to Harmonize



Slide 16

doc.: IEEE 802.11-07/2684r0

Submission

Harmonizing HCCA QAPs Explanation of Scheduling of TXOPs

Schedule for QSTAs

TXOPi

TXOPj

TXOPk

TXOPi

TXOPj

TXOPk

TXOPi

TXOPj

TXOPk

SI SI SI

TXOPi

TXOP

j

TXOPk

TXOPi

TXOPj

TXOPk

TXOPi

TXOPj

TXOPk

SI SI SI

A - Schedule for QSTAs I, j and k – maximum TXOPs

B - Schedule for QSTAs I, j and k – actual TXOPs

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



Slide 17

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Submission

Fixed Slot time 10ms

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 20ms

VoiceG711,G729,G723.1

30ms 30ms

VoiceG726-32

10ms 10ms

VideoSDTV, HDTV

0ms 16ms

10ms fixed Slot

Video Video Video Video Video

Slot = 10ms Slot = 10ms Slot = 10ms Slot = 10ms Slot = 10ms

Voice 20ms Voice 30ms

Scheduled 20ms and 30ms Voice with Video streams

SI = 10ms

SI = 20ms

SI = 30ms



Slide 18

doc.: IEEE 802.11-07/2684r0

Submission

Harmonize Slot times

QoS (+) CF Poll Frame sent by HC

034567815

TIDEOSP

ACKPolicy

TXOP limit

Re

serv

ed

QoS Control FieldAt the beginning of the Slot,QAP sets bit 7

The suggested procedure (see next slide)1. 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.

2. 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

3. QAP B starts its Slot time and TXOPs

Simple and straightforward



Slide 19

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Submission

Service Interval Harmonization

QAP A QAP A

Allocated ScheduledΣTXOPmaxA

Allocated ScheduledΣTXOPmax

QAP B hears QoS Poll

And checks bit 7

Waits ΣTXOPmaxA

QAP BCHP=0

Allocated Scheduled

ΣTXOPmaxB

QAP A hears QoS Poll

And checks bit 7

Waits ΣTXOPmaxBThen can check that QAP B TXOPs are

completed

SI SI

SI SI

QAP ACHP=1

Service Interval Harmonization Example

QAP A sends QoS Poll to itself with bit

7 set

QAP B hears QoS Poll

And checks bit 7

Allocated Scheduled

ΣTXOPmaxB

Waits ΣTXOPmaxA



Slide 20

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Submission

OBSS Proposed Procedure Summary

1. Before seeking a channel, a QAP sets its CHP to 1 in the Q LOAD element.

2. 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

3. 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.

4. If no other QAP is reported, then the QAP may choose that channel. 5. If another QAP is reported, then the respective Q LOADs are examined

and a decision made as to share or not.6. If the decision is to share, then the CHP is set to 0.7. 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.

8. 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



Slide 21

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Submission

OBSS Proposed Procedure SummaryHCCA QAPs need to harmonize their SIs.9. Each HCCA QAP indicates its start of the Slot Time by setting

bit 7 in the QoS CF Poll.10. 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.



Slide 22

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Submission

Beacon Report ExchangesExample of Exchanges

QAP B has overlapping QSTA from QAP A

Network A Network BQAP ACHP = 1 Beacon Report Requestto all QSTAs- Same Channel- Active Probe

QSTA (A)

Probe Request - Active Probe

Probe Response- Q LOAD QAP B

- Maintains CHP = 1

QAP BCHP = 1

Beacon Report- Q LOAD QAP B- Notes another QAP on

same channel

QAP A makes decision to shareCHP = 0 OBSS Beacon Report Request for QAP B- Same Channel- Active Probe- IE Type ‘OBSS”

Probe Request - Active Probe- Type OBSS- Q LOAD QAP A Probe Response

- Q LOAD QAP B

- Maintains CHP = 1- Sees QAP A has CHP = 0- Knows sharing in effect- Calculates available bandwidth

Beacon Report- Type OBSS- Q LOAD QAP B

- Notes QAP B aknowledged- Calculates available bandwidth

OBSS Beacon Request

• Provides other QAP the Q Load element

• Informs CHP



Slide 23

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Submission

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.

Additions to the Standard are proposed:• “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”



Slide 24

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Submission

Hidden QAPs

QAP A

QAP B

QAP A

QAP B

QAP A

QAP B

Case 1Overlapping QSTAs from both networks

Case 2QAP B has Overlapping QSTAs from Network A

Case 3QAP A has Overlapping QSTAs from Network B

Hidden QAPs, OBSS

QAP A

QAP B

Case 4QSTAs from Networks A and B are within range of each other

• 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



Slide 25

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Submission

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.



Slide 26

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Submission

Harmonizing SI – Indirect MethodQAP 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.



Slide 27

doc.: IEEE 802.11-07/2684r0

Submission

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



Slide 28

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Submission

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 OBSSNote: 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?

Documents

Doc.: IEEE 802.11-07/2684r0 Submission November 2007October 2007 Graham Smith, DSP GroupSlide 1 Overlapping BSS Proposed Solution Date: 2007-10-29 Authors: