doc.: ieee 802.11-07/2684r0 submission november 2007october 2007 graham smith, dsp groupslide 1...
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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
Authors:
November 2007October 2007
Graham Smith, DSP Group
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.
November 2007October 2007
Graham Smith, DSP Group
Slide 3
doc.: IEEE 802.11-07/2684r0
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.
November 2007October 2007
Graham Smith, DSP Group
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
November 2007October 2007
Graham Smith, DSP Group
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!
November 2007October 2007
Graham Smith, DSP Group
Slide 6
doc.: IEEE 802.11-07/2684r0
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
November 2007October 2007
Graham Smith, DSP Group
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!
November 2007October 2007
Graham Smith, DSP Group
Slide 8
doc.: IEEE 802.11-07/2684r0
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.
Full protection for scheduled traffic in Network A
Network B bandwidth reduced
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
Full protection for scheduled traffic in Network A
Network B bandwidth reduced
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.
November 2007October 2007
Graham Smith, DSP Group
Slide 9
doc.: IEEE 802.11-07/2684r0
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)
November 2007October 2007
Graham Smith, DSP Group
Slide 10
doc.: IEEE 802.11-07/2684r0
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
November 2007October 2007
Graham Smith, DSP Group
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.
November 2007October 2007
Graham Smith, DSP Group
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
November 2007October 2007
Graham Smith, DSP Group
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
November 2007October 2007
Graham Smith, DSP Group
Slide 14
doc.: IEEE 802.11-07/2684r0
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
November 2007October 2007
Graham Smith, DSP Group
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
November 2007October 2007
Graham Smith, DSP Group
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
November 2007October 2007
Graham Smith, DSP Group
Slide 17
doc.: IEEE 802.11-07/2684r0
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
November 2007October 2007
Graham Smith, DSP Group
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
November 2007October 2007
Graham Smith, DSP Group
Slide 19
doc.: IEEE 802.11-07/2684r0
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
November 2007October 2007
Graham Smith, DSP Group
Slide 20
doc.: IEEE 802.11-07/2684r0
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
November 2007October 2007
Graham Smith, DSP Group
Slide 21
doc.: IEEE 802.11-07/2684r0
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.
November 2007October 2007
Graham Smith, DSP Group
Slide 22
doc.: IEEE 802.11-07/2684r0
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
November 2007October 2007
Graham Smith, DSP Group
Slide 23
doc.: IEEE 802.11-07/2684r0
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”
November 2007October 2007
Graham Smith, DSP Group
Slide 24
doc.: IEEE 802.11-07/2684r0
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
November 2007October 2007
Graham Smith, DSP Group
Slide 25
doc.: IEEE 802.11-07/2684r0
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.
November 2007October 2007
Graham Smith, DSP Group
Slide 26
doc.: IEEE 802.11-07/2684r0
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.
November 2007October 2007
Graham Smith, DSP Group
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
November 2007October 2007
Graham Smith, DSP Group
Slide 28
doc.: IEEE 802.11-07/2684r0
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?