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Transcript of Doc.: IEEE 802.11-09/0757-00-00aa Submission Apr 2009 Graham Smith, DSP GroupSlide 1 Proposed...
Apr 2009
Graham Smith, DSP Group
Slide 1
doc.: IEEE 802.11-09/0757-00-00aa
Submission
Proposed Overlapping BSS Solution
Date: 2009, July 10
Name Affiliations Address Phone email Graham Smith DSP Group 2491 Sunrise Blvd,
#100, Rancho Cordova, CA 95742
916 851 9191 X209
Dan Dillon DSP Group 2491 Sunrise Blvd, #100, Rancho Cordova, CA 95742
916 851 9191 X205
John Janecek DSP Group 2491 Sunrise Blvd, #100, Rancho Cordova, CA 95742
916 851 9191 X208
Authors:
Apr 2009
Graham Smith, DSP Group
Slide 2
doc.: IEEE 802.11-09/0757-00-00aa
Submission
Abstract08/0457r4 and 08/1260r1 examined the OBSS problem and outlined
possible solutions – “QLoad” introduced
08/1250r0, 09/0285r0, and 08/1470r4 looked at the OBSS scenarios, estimated worse case overlaps and ran simulations using Channel Selection so as to size the problem.
09/0230r0 and 09/0476r1 gave the details of the revised OBSS proposal with use of CHP bit and HCCA Supervisor
09/0496r2 examined video stream statistics
09/0497r2 extended the video stream statistics to QLoad
09/0660r3 examined using 11s MCCA for HCCA OBSS
09/0662r2 introduced OBSS Sharing with Access Fraction
09/0666r2 considered HCCAOP Advertisement for sharing
This presentation presents the proposed solution
Apr 2009
Graham Smith, DSP Group
Slide 3
doc.: IEEE 802.11-09/0757-00-00aa
Submission
Objectives of OBSS Proposal
Provide the means for:1. Meaningful Channel Selection
2. Co-operation between Admission Control QAPs
3. Co-operation between HCCA and Admission Control QAPs
4. Co-operation between HCCA QAPs
Apr 2009
Graham Smith, DSP Group
Slide 4
doc.: IEEE 802.11-09/0757-00-00aa
Submission Slide 4
Definitions and OBSS GraphLength(OBSS graph) – longest shortest path between any two APs in the OBSS graph
OBSS Extent is related to the length(OBSS graph) (used in 08/0285r0)_Size(OBSS graph)– number of nodes (APs) in the OBSS graphOBSS Solution Minimum Requirement (accepted in Los Angeles, Jan ’09)
length(OBSS graph) <= 2 and the size(OBSS graph) <=3
Overlap - Number of overlapping BSSs that are sharing this channel– Overlap is simple for a QAP to report– Overlap” does not by itself indicate the OBSS Size or Length
BUT, There is a direct relationship between OBSS Length AND:
– the probability of Overlap2– the number of Channels – and the number of other APs within radio rangee.g. If Probability of Overlap2 <1% then OBSS length<=2 and OBSSsize <=3
FOR MORE COMPLETE EXPLANATION, SEE SLIDES 33 - 38
Slide 4
Apr 2009
Graham Smith, DSP Group
Slide 5
doc.: IEEE 802.11-09/0757-00-00aa
Submission
Outline of Proposal
1. New IE “QLoad”• Used for Channel Selection and Channel Sharing
2. TSPEC Requirement Request Response• Used by QAP to STA to indicate or confirm their TSPECs
3. Recommendations to avoid/minimize OBSS problem– Channel selection based upon:
• QAP Overlap
• QLoad
• Received Signal Strength
– Channel width selection 40/20MHz
Apr 2009
Graham Smith, DSP Group
Slide 6
doc.: IEEE 802.11-09/0757-00-00aa
Submission
ElementID
LengthOverlap
And Prority
QAP (Self)
ID
QAP ID
Qload
1 1 2 2 6 12 VAR
Overlap and Priority
ReservedOverlap (4 bits)
Etc.For all QAPs
In OBSS Graph
QAP ID
Random Value
1
Octet 6 of MAC Address
1
QLOAD ELEMENT
1
QAPPriority
Streams6
b0
Number AC3 streams Number AC2 streams
QAP Priority Streams
QAPPriority
Streams
Qload
b0 b13 b15
STDEV
b4
b14
Distance
1
Access Fraction
Access Fraction
bits
Access Fraction
8 6
bits 4 4
2
Access Factor Integral
Access Factor Fraction
Qload(Self)
STDEV and Distance
MEAN HCCA Peak
2 2 2octets
CHPbit
b7
PROPOSED “QLOAD” ELEMENT
NOTE: CHP bit not used if HCCAOP Advertisementis used.
Apr 2009
Graham Smith, DSP Group
Slide 7
doc.: IEEE 802.11-09/0757-00-00aa
Submission
Overlap• QAP indicates the number of other QAPs with which it is sharing
and indicates the size of the OBSS graph:– Zero indicates QAP has no other QAPs on the same channel within range
– 1 indicates already sharing with one other QAP
– 2 indicates already sharing with two other QAPs
– etc
The QAP is advertising the overlap to other QAPs who may be considering sharing.
This parameter should be included in the Channel Selection procedure in order to select the best channel (08/1470r4)
Note: See also “Backup” slides 33 – 38 for further information
Apr 2009
Graham Smith, DSP Group
Slide 8
doc.: IEEE 802.11-09/0757-00-00aa
Submission
Distance
• Distance is set to 0 for Self
• If QAP ID Directly visible to the QAP Self, then “Distance” is set to 1
• If 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
Apr 2009
Graham Smith, DSP Group
Slide 9
doc.: IEEE 802.11-09/0757-00-00aa
Submission
QAP ID
• First octet = random number (0 to 255)
• Second octet = octet 6 of MAC Address
• Once established, QAP ID is not changed
• Enables a QAP to indentify its QLoad in other QLoad elements
Apr 2009
Graham Smith, DSP Group
Slide 10
doc.: IEEE 802.11-09/0757-00-00aa
Submission
QLoad SelfThere are three methods for the QAP to build QLoad Self:1. QSTAs in the BSS may send a TSPEC (ADDTS) with Inactivity
Interval set to 0 (or 1) for instant timeout• By sending in a TSPEC the STA has the QAP commit, in advance,
medium time for the STA
2. QAP notes and adjusts for new TSPECs from QSTAs• If accepted, “QLoad Self”, and also “QLoad Total” are adjusted only
when the QSTA submits the ADDTS• Chance that ADDTS is denied as QSTA did not reserve medium time in
advance
3. In response to TSPEC Requirements Request• QAP request STAs to indicate or confirm their TSPECs• Used by QAP to ‘clear house’ or initially set up Q Load.
The QAP is advertising its own potential QoS load to other QAPs who may be considering sharing
Apr 2009
Graham Smith, DSP Group
Slide 11
doc.: IEEE 802.11-09/0757-00-00aa
Submission
TSPEC Requirement Request Response
Request from QAP to a particular STA
Two types of Request:
1. Send All TSPECs (ID 0)– Effectively all previous
(if any) TSPECs are deleted, need to set them up again
2. Confirm TSPECs (ID 1)– Confirm which
TSPECs are still required
– TSID plus Direction defines TSPEC
CATEGORYCODE
ACTIONCODE
DIALOGTOKEN
OPTIONAL SUB
ELEMENTS
REQUESTID LENGTH
1 1 1
1 1
VAR
1 1 1
0 = Send All TSPECs
1 = Confirm TSPECS # of TSPECs TSID+Dir TSID+Dir TSID+Dir
Not present
RESPONSEID LENGTH
1 1 1 1 10 = Accepted will send all TSPECs
1 = Confirmed TSPECS # of TSPECs TSID+Dir TSID+Dir TSID+Dir
Not present
TSPEC Requirement Request Response Action Frame
Apr 2009
Graham Smith, DSP Group
Slide 12
doc.: IEEE 802.11-09/0757-00-00aa
Submission
QLoad MEAN and STDEV
MEAN and STDEV is estimated from the individual TSPECs:
MEAN µ = ΣMEANi
STDEV σ = 0.25 sqrt{Σ(MAXi – MINi)2}
MEAN µtot = ΣMEANiSTDEV σtot = sqrt(Σσi
2)
Total Traffic Requirement can be estimated:1. MAX traffic = µtot + 2 σtot
2. 90% Traffic = µtot + 1.3 σtot
3. 80% Traffic = µtot + 0.83σtot
Apr 2009
Graham Smith, DSP Group
Slide 13
doc.: IEEE 802.11-09/0757-00-00aa
Submission
QAP Priority Streams
• Number of EDCA Priority Streams, AC_VO and AC_VI
• Used to estimate “EDCA Bandwidth Factor” • EDCA Bandwidth Factor = 1 + 0.05 N (approx; keep it
simple, see 09/0497)– Where N = Number of streams– Example:
4 streams Effective Bandwidth Factor = 1.2Four 5.5Mbps streams will require 1.2 x 4 x 5.5 = 26.5Mbps
Apr 2009
Graham Smith, DSP Group
Slide 14
doc.: IEEE 802.11-09/0757-00-00aa
Submission
Access Fraction and Access Factor
• Access Fraction• Total actual admitted time and/or scheduled time expressed as a
fraction of 32us/sec rounded down to 1/256
• Access Factor– Total Traffic Requirement of self plus all other visible QAPs.
Expressed as a fraction that may be greater than 1– Calculated as follows:
• Sum the individual QLoads of all QAPs in the QLoad element as a composite stream
• Calculate the EDCA Bandwidth Factor from the total number of Priority Streams in the visible QAPs (Distance 0 and 1)
• Multiply the two to obtain the “Access Fraction” .
Apr 2009
Graham Smith, DSP Group
Slide 15
doc.: IEEE 802.11-09/0757-00-00aa
Submission
ACCESS FACTOR FIELD
• Medium Time and TXOPs are all measured in 32us/sec
• Access Factor can be > 1
• To express in 1 octet– 2 bits for Integral (whole number)
– 6 bits for the decimal fraction, expressed as a fraction rounded down to 1/64• Example: Sum = 74268 in 32us/sec = 2.376576 seconds
• Hence, octet would be 10 01100 [2 and 24/64 = 2.375]
• Maximum value would be 3.98
Apr 2009
Graham Smith, DSP Group
Slide 16
doc.: IEEE 802.11-09/0757-00-00aa
Submission
Sharing
• If the Access Factor is >1, then there is a potential over-allocation – Hopefully QAPs should avoid this in the Channel selection
process
• Sharing Scheme– QAPs should examine their QLoad Element in order to determine
the maximum “Access Factor” being reported. This maximum value is then used to determine the allocation limit for that QAP in order not to cause over-allocation in other QAPs that are overlapping,
– Using the Access Fractions (actual “live” traffic), Access Factor and QLoad self, a decision can be made whether to admit a new request.
– Rules could be recommended in informative text.
Apr 2009
Graham Smith, DSP Group
Slide 17
doc.: IEEE 802.11-09/0757-00-00aa
Submission
HCCA Peak
• The total HCCA TXOP requirement for the QAP, expressed in 32us/sec.
– The sum of all the HCCA Peak values is the “HCCA Access Factor”
– If HCCA Access Factor > 1sec then potential for TXOP over-allocation
– HCCA TXOPs can sum to “1” independent of EDCA Medium Time allocations, as TXOPs terminate immediately when no more data
Apr 2009
Graham Smith, DSP Group
Slide 18
doc.: IEEE 802.11-09/0757-00-00aa
Submission
Medium Time Allocations - Sharing
It is important to understand how the AP allocates the actual Medium Times in responses to TSPECs and checks that it has not exceeded its ‘limit’
1. In response to each TSPEC the AP allocates the Medium Time or TXOP (HCCA) that corresponds to the peak traffic
2. When allocating an additional Medium Time or TXOP, the AP must calculate what the composite stream would be, and check that this composite medium time does not exceed the limit
3. It is this composite time, that is advertised in the Access Fraction– The actual sum of the Medium Times and TXOPs will be greater than the
composite time, but EDCA only uses what it needs, and hence the statistical nature of the streams causes the composite time to be the maximum of what is actually being used. Similarly HCCA TXOPs terminate when no more data.
– Allocated HCCA TXOPs cannot exceed “1”
Apr 2009
Graham Smith, DSP Group
Slide 19
doc.: IEEE 802.11-09/0757-00-00aa
Submission
Apr 2009
Graham Smith, DSP Group
Slide 20
doc.: IEEE 802.11-09/0757-00-00aa
Submission
CHP bit
• CHP bit is used for overlapping HCCA QAPs
• If CHP = 1, then the QAP is the Supervisor responsible for handing off the TXOP to each QAP
• Rules are required for a QAP to set CHP = 1
Apr 2009
Graham Smith, DSP Group
Slide 21
doc.: IEEE 802.11-09/0757-00-00aa
Submission
Rules for Setting CHP bit and SharingWhen a HCCA QAP is searching for a channel, it should do so in the following order:1. Set CHP (Channel Priority) to 02. If finds a clear Channel, set CHP to 13. If no clear channel, then may share with
a) Any legacy AP: Set CHP to 1b) 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)c) An HCCA QAP with CHP = 1 CHP stays at 0 (see Note 1)
4. If conditions as per 3 cannot be found, the QAP may share with an HCCA QAP, with CHP=0 and Overlap = 1, CHP stays as 0 BUT the following must occur:a) The QAP that now has Overlap = 2 shall negotiate with the QAP that has CHP=1 to take over the
Supervisor role. The new QAP must wait until it sees directly the QAP with CHP=15. If an HCCA QAP cannot find a channel that meets the above rules, it must fall back to
Admission Control (see note 3)
NOTES:1. If 3b) or 3c) or 4), check that “QLoad” is such that the two can share2. 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.
Apr 2009
Graham Smith, DSP Group
Slide 22
doc.: IEEE 802.11-09/0757-00-00aa
Submission
Harmonizing HCCA
• When sharing use Fixed Time Slot– Each AP (HC) knows how much of the Time Slot it can use:
• HCCA Peaks, Access Factor (and HCCA Access Factor)– 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.• “Supervisor Claim” also used when condition #4 is met. QAP with the
higher Overlap takes over.
Apr 2009
Graham Smith, DSP Group
Slide 23
doc.: IEEE 802.11-09/0757-00-00aa
Submission
AP to 2 APs
A QLOAD B QLOAD
Slot Slot
A Actual B Actual A Actual B Actual
QAP A is the “Supervisor” and effectively in charge of the 10ms Slot and handing off to QAPs B and C
QAP A knows the 10 ms slot timing
QAP A sends QoS CF Polls to QAPs B and CQAP A sends QoS CF Polls to QAPs B and CTID= 1111 NAV=0
TXOP Limit = Tb and Tc
Tb
C QLoad
A BC
Tc
Tb
Tc
C Actual C Actual
Apr 2009
Graham Smith, DSP Group
Slide 24
doc.: IEEE 802.11-09/0757-00-00aa
Submission
Wireless DS QoS CF-Poll (Null Data) for AP to AP Communication
AP to AP QoS CF-Poll Address Fields
FunctionTo DS
From DS
Address 1
Address 2
Address 3
Address 4
Wireless DS 1 1 RA= QAP B
TA= QAP A
DA=QAP B
SA=QAP A
AP to AP QoS CF-Poll Frame Type and Sub-type
Type valueb3 b2
Type Description
Subtype value b7 b6 b5 b4
Subtype Description
10 Data 1110 QoS CF-Poll (no data)
Applicable Data Frame
Bits 0-3 Bit 4 Bits 5-6 Bit 7 Bits 8-15
QoS CF Poll TID EOSP= 1
ACK Policy Agg (11n) TXOP Limit
QoS Control Field
Use TID field as identifier
Apr 2009
Graham Smith, DSP Group
Slide 25
doc.: IEEE 802.11-09/0757-00-00aa
Submission
WDS QoS CF Polls To Supervisor from QAP with CHP=0
From Supervisor QAP with CHP=1
ACTIONBits 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 0 Time to start of TXOP in units of 32us
Supervisor Claim, CHP = 1 0001 1 00 0 0
ACTION Bits 0-3 Bit 4 Bits 5-6 Bit 7 Bits 8-15
CHP is set to 0 0000 1 00 0 0
Is Supervisor There? 0010 1 00 0 0
* May not be required unless a more strict control of QLoad is needed
Apr 2009
Graham Smith, DSP Group
Slide 26
doc.: IEEE 802.11-09/0757-00-00aa
Submission
Apr 2009
Graham Smith, DSP Group
Slide 27
doc.: IEEE 802.11-09/0757-00-00aa
Submission
Element ID
LengthHCCA
InfoSelf Times
ReportInterfering Times
Report
HCCA Access Fraction
Self Times Report Present
Interfering Times Report
Present
Octets 1 1 2 Variable Variable
1 166
Number of Reported TXOP Reservations
TXOP Reservation 1
TXOP Reservation N
Duration Service Interval Offset
Octets 1
1 1 2
4 4
Self Times Report
Number of Reported TXOP Reservations
TXOP Reservation1
TXOP Reservation N
Octets 1 4 4
Interfering Times Report
HCCA Access Factor Fraction
HCCA Access Factor Integral
2Bits
PROPOSED “HCCAOP ADVERTISEMENT” ELEMENT
Apr 2009
Graham Smith, DSP Group
Slide 28
doc.: IEEE 802.11-09/0757-00-00aa
Submission
HCCAOP Advertisement Element• HCCA Advertisement Element informs ONLY TXOPs THAT ARE
ACTIVE (except for HCCA Access Factor)• Access Fraction Fields
– HCCA Access Fraction: Total actual scheduled time expressed as a fraction of 32us/sec rounded down to 1/256
– HCCA Access Factor: Total HCCA Peak of all QAPs Distance 0, 1 and 22 bits for Integral (whole number)6 bits for the decimal fraction, expressed as a fraction rounded down to 1/64
• 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
Apr 2009
Graham Smith, DSP Group
Slide 29
doc.: IEEE 802.11-09/0757-00-00aa
Submission
HCCAOP Advertisement Scheme
• 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
• QAP must check that allocating a new TXOP will not cause the total TXOPs of QAPs in the QLoad Element to exceed 1 sec/sec.– Uses HCCA Peak, HCCA Access Fraction, HCCA Access Factor
Apr 2009
Graham Smith, DSP Group
Slide 30
doc.: IEEE 802.11-09/0757-00-00aa
Submission
CHP bit Scheme, versus HCCAOP Advertisement
• CHP bit– Very efficient, Supervisor hands off the TXOPs using Wireless DS frames– No clock drift problem– Requires a Supervisor and selection procedure– Limited to OBSS length 2 and size 3
• Supervisor must be at Distance 1 from other 2 QAPs• In practice, will meet all scenarios if 20/40MHz channel selection is adopted
(08/1470)
• HCCAOP Advertisement– Simple scheme and builds upon other work (11s)
• Especially if OBSS length 2 and size 3
– Is expandable to any OBSS Graph– Requires each QAP to convert slots to local time– Does suffer from Clock Drift– Can be inefficient
Apr 2009
Graham Smith, DSP Group
Slide 31
doc.: IEEE 802.11-09/0757-00-00aa
Submission
Apr 2009
Graham Smith, DSP Group
Slide 32
doc.: IEEE 802.11-09/0757-00-00aa
Submission
Poll #1
Which HCCA Scheme is preferred:
1. CHP bit Supervisor?
2. HCCAOP Advertisement?
Apr 2009
Graham Smith, DSP Group
Slide 33
doc.: IEEE 802.11-09/0757-00-00aa
Submission
Poll #2“It is recommended that the OBSS proposal, as described
in 09/xxxxr0 be adopted in principle and further recommends that normative and informative text should now be written”
Y/N/A
Apr 2009
Graham Smith, DSP Group
Slide 34
doc.: IEEE 802.11-09/0757-00-00aa
Submission
BACKGROUND SLIDES• OBSS Requirements• Channel Selection• Proposal Summary
• Poll
Apr 2009
Graham Smith, DSP Group
Slide 35
doc.: IEEE 802.11-09/0757-00-00aa
Submission
Apr 2009
Graham Smith, DSP Group
Slide 36
doc.: IEEE 802.11-09/0757-00-00aa
Submission Slide 36
OBSS Requirement (from 09/0054r2)What is an OBSS graph? OBSS edge -- Any two APs operating in the same channel and can hear each other (either directly or via a STA associated to one of the APs)
OBSS Graph – is a graph where APs are nodes of the graph and the edges are OBSS edges and every AP with in the OBSS graph can be connected via one or more OBSS APs to every other AP in the OBSS graph
Length(OBSS graph) – longest shortest path between any two APs in the OBSS graph
Size(OBSS graph) – number of nodes (APs) in the OBSS graph
OBSS Solution Requirement (accepted in Los Angeles, Jan ’09)
– if length(OBSS graph) <= 2 and the size(OBSS graph) <=3 , enable the OBSS QAP solution otherwise (a) backoff to legacy (non .11aa) mode or (b) use a different solution
Note: 08/0285r0 showed OBSSsizes up to 8 were likely with 10 Channels in dense apartment block scenario and argued size of 3 was not sufficient. 08/1470r4 also confirmed this with 9 and 11 channels.
Slide 36
Apr 2009
Graham Smith, DSP Group
Slide 37
doc.: IEEE 802.11-09/0757-00-00aa
Submission
OBSS Size, Length and Overlap
• OBSS Size or Length difficult for an AP to directly indicate
• “Overlap” is simple for an AP to directly indicate
• “Overlap” does not by itself indicate the OBSS Size or Length
Apr 2009
Graham Smith, DSP Group
Slide 38
doc.: IEEE 802.11-09/0757-00-00aa
Submission
OBSS Size, Length and Overlap
A
B
C
A B
C
OBSS Length = 2OBSS Size = 3
Overlaps A = 2, B = 1, C = 1
OBSS Length = 1OBSS Size = 3
Overlaps A = 2, B = 2, C = 2
A
B
C
D
OBSS Length = 3OBSS Size = 4
Overlaps A = 2, B = 2, C = 1, D = 1
Overlap <=2 ButLength and Size above “spec”
√
X
√
Apr 2009
Graham Smith, DSP Group
Slide 39
doc.: IEEE 802.11-09/0757-00-00aa
Submission
OBSS Size, Length and OverlapOverlap of 2 does not directly indicate OBSS lengthBUT there is a direct relationship between OBSS Length AND:
– the probability of Overlap2– the number of Channels – and the number of other APs within radio range
For OBSS length >2, there must be, at least: • two Overlaps2 within the overlap area
AND• they must be on the same channel
Calculation of Probability of this happening# of Channels = N; Prob of APs with Overlap2 = n; # of overlapping APs = MProbability of at least 2 Overlap2’s being in overlap area (binomial):Probability of no Overlap2 P0 = (1-n)^MProbability of one Overlap2 P1 = M.n (1-n)^(M-1)Probability of two or more P2 = 1 – P1 – P0Probability of selecting same channel Probability of not selecting a certain channel Pc0 = (1-1/N)^nProbability of not selecting a certain channel just once Pc1 = n/N (1-1/N)^(n-1)Probability of selecting a certain channel at least 2 times Pc2 = 1 – P1 – P0
Probability of OBSS Length>2 = P2 x Pc2
Apr 2009
Graham Smith, DSP Group
Slide 40
doc.: IEEE 802.11-09/0757-00-00aa
Submission
OBSS Size, Length and Overlap
Example:Double Apartment, 53 APs in range:
– 17 CH, (N=17, M=53)Probability of Overlap2 = 0.73% Probability of OBSS length > 2 = 0
– 16 CH, (N=16, M=53) Probability of Overlap2 = 1.88% Probability of OBSS length > 2 = 0.08%
– 15 CH, (N=15, M=53) Probability of Overlap2 = 3.9% Probability of OBSS length > 2 = 1.4%
• Hence, in this case, 53 APs in range, for 99% service, at least 16CH are required
• 100% service for 17 or more Channels
Apr 2009
Graham Smith, DSP Group
Slide 41
doc.: IEEE 802.11-09/0757-00-00aa
Submission
Overlap Field
• Channel Selection simulations run as described in 08/1470r4
• For Double Apartment scenario, (53 QAPs in range):– 9CH maximum value of Overlap = 5
– 8CH maximum value of Overlap = 6
– 7CH maximum value of Overlap = 7
– 3CH maximum value of Overlap = 8
• Hence, Overlap Field size made 4bits (0 - 15)
Apr 2009
Graham Smith, DSP Group
Slide 42
doc.: IEEE 802.11-09/0757-00-00aa
Submission
Apr 2009
Graham Smith, DSP Group
Slide 43
doc.: IEEE 802.11-09/0757-00-00aa
Submission
1 - Channel Selection
Channel Selection Procedure:1. Select Channel(s) with least number of APs2. If more than one channel, select channel with least
“overlaps”3. If more than one channel, select lowest “QLoad Total” in
“QLoad Element”
Results dependant upon number of available channels – (see 08/1479r4 and 09/0285r0)
• 2.4GHz Band 3 CH maximum• 5GHz Band 20MHz USA 24 CH, Europe 19 CH
40MHz USA 11 CH, Europe 9 CH
Apr 2009
Graham Smith, DSP Group
Slide 44
doc.: IEEE 802.11-09/0757-00-00aa
Submission
Channel Selection Analysis (08/1470r4) Probability of Zero or One overlap
100% Assignment
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
24 22 19 17 11 9 3
Channels
Pro
bab
lili
ty
Detached Houses, 12 overlaps
Terrace Houses, 16 overlaps
Town Houses, 24 overlaps
Single Apartments, 28 overlaps
Double Apartments, 53 overlaps
Probability of Zero Overlaps with 40MHz Channels
0
0.2
0.4
0.6
0.8
1
11 9 3
Number of Channels
Pro
ba
bili
ty
Detached Houses, 12 overlaps
Terrace Houses, 16 overlaps
Town Houses, 24 overlaps
Single apartment, 28 overlaps
Double apartment, 53 overlaps
Double Apartment • 100% occupancy• 53 overlapping apartments17 CH (20MHz Channels)99.3% probability of 0 or 1 channel overlap*Zero chance of length > 2 or size > 3(<1 occurrence of 2 overlaps in 100 apartments)
9 CH (40MHz Channels)*Zero chance of length < 2 many cases of size > 3Hence need to drop back to 20MHz and increase number of available channelsBUTMany APs will use 40MHz channels 2.4GHz Band
hopeless!
Apr 2009
Graham Smith, DSP Group
Slide 45
doc.: IEEE 802.11-09/0757-00-00aa
Submission
Channel Selection Analysis Summary
• If 17CH or greater, then Channel Selection can ensure OBSSlength <=2 in all scenarios examined
• With Channel Selection, Networks using 40MHz channels will have high percentage of no OBSS for all scenarios except dense apartments
• Channel Section is improved if ‘overlap selection’ is included
Apr 2009
Graham Smith, DSP Group
Slide 46
doc.: IEEE 802.11-09/0757-00-00aa
Submission
40/20MHz ChannelsIf a QAP wanted to determine when to use 40MHz or 20MHz
channel, then following procedure could be used:
1. If an 11n QAP cannot find a free channel using 40MHz, switch to using 20MHz • If it still cannot find a clear channel, then it can settle on a 40MHz
channel (secondary?)• Rule #2 then comes into play
2. If an 11n QAP, using 40MHz, finds itself overlapping with more than one other QAP (20 or 40MHz) then it must switch to using 20MHz (• It may decide to search again using 40MHz, and then rule 1 applies
Notes: 1. The primary intention is to avoid OBSSlengths > 22. It is assumed that it is in the QAP’s own interest to use an
independent 20MHz channel rather than share a 40MHz channel
Apr 2009
Graham Smith, DSP Group
Slide 47
doc.: IEEE 802.11-09/0757-00-00aa
Submission
Channel Selection Summary
• Using suggested selection scheme:– 17 available channels required to ensure OBSSlength <=2 and
OBSSsize <=3 in most extreme scenario examined
– Only applicable to 5GHz Band, 2.4GHz is a “lost cause”
• 20/40MHz 1. 40MHz channels is fine for many scenarios
2. Suggested procedure for 40MHz channels to drop back to 20MHz when overlap and sharing exists in order to prevent excessive OBSS lengths